Commonness and rarity in plants with special reference to the Sheffield flora Part I: The identity, distribution and habitat characteristics of the common and rare species

Biological Consert'ation 36 (1986) 199 252

Commonness and Rarity in Plants with Special Reference to the Sheffield Flora Part I: The Identity, Distribution and Habitat Characteristics of the Common and Rare Species

J. G. Hodgson Unit of Comparative Plant Ecology (NERC), Department of Botany, The University, Sheffield S10 2TN, Great Britain

ABSTRACT By reJ~,rence to extensive eegetation surveys the habitats o f common and rare herbaceous plants and small ( < 1 m) woody species bathe been compared f o r the Sh~f~eld region o f Central England. Common species are ]requent at sites of high ]~'rtility, are strongly represented in habitats which are heavily disturbed or o f recent origin and generally hat,e a wide geographical and ecological range in the Shejfield region and within Britain as a whole. M a n y o f the rare species are restricted to the Iowland halJof the region and most o f the species which have become extinctjrom the region in the recent past were associated with lowland agricultural areas where the pressures o f changing land use are greatest. Rare species are concentrated within less fertile sites, particularly those with calcareous soils, and they are often present in species-rich vegetation in ancient habitats. Many rare species have a narrow ecological range, some appear restricted to ~intermediate' habitats (either ecoclines or ecotones) and a high proportion are near the northern limit of their geographical range. No single ecological characteristic is totally diagnostic o f either common or rare species. Most o f the differences between common and rare species in the Shejfield region may be interpreted in terms of the azailability o f suitable habitats within the region with common species occupying common, and rare species less common, habitats. 199 Biol. Conserv. 0006-3207/86/$03.50 ~? Elsevier Applied Science Publishers Ltd, Englan~ 1986. Printed in Great Britain

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I N T R O D U C T I ON As noted by Rabinowitz (1981), m a n y studies of rarity have dealt with only a small proportion of the multiplicity of relevant factors. In most cases they have involved few species. Furthermore, analyses of commonness and rarity are generally treated as separate problems. This is unfortunate since the a m o u n t of a given species, regardless of whether it is c o m m o n or rare, is likely simply to reflect the balance between attributes causing an increase and those causing a decrease in abundance. Thus comparative studies o f c o m m o n and rare species appear to provide a promising approach towards an understanding o f the reasons for both commonness and rarity. Accordingly, a study of c o m m o n and rare species has been carried out within the Sheffield region, an area of 3000 km 2 in central England. The aims of this study were to (1) identify which higher plant species within the Sheffield flora are c o m m o n and which are rare; (2) examine the extent to which c o m m o n and rare species differ in their distribution and ecology; (3) review the species characteristics associated with commonness and rarity; and (4) identify the principal mechanisms which determine commonness and rarity within the Sheffield flora. This paper is an introduction to the study, using as its base vegetation surveys which have been carried out within the Sheffield region and which enable features of the distribution and ecology of all herbaceous species and small shrubs within the study area to be described. Following a brief description of the Sheffield region, this paper compares the ecology and distribution of c o m m o n and rare species within the local flora. Subsequent papers will consider the ecological and evolutionary mechanisms regulating abundance in the modern flora.

THE S T U D Y A R E A

Climate, topography and geology The climate, geology and land use of the study area (Fig. 1) have been described in detail by Linton (1956), Edwards (1966) and Anderson & Shimwell (1981) and a summary of this information is presented in Fig. 1 and Tables 1 and 2. Higher rainfall and lower temperatures occur in the west of the region, with climate becoming progressively drier and warmer

Commonness and rarity in plants

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Fig. 1. The Survey area. Squares indicate the position of towns or cities (remaining data from the Geological Survey of Great Britain and Ordnance Survey Relief and Rainfall maps). Scale 1.5mm = l km. The major geological formations and constituent rock types are from left to right:

Upland portion o[ region

Carboniferous

Carboniferous Limestone ( C L ) ~ a r b o n i f e r o u s and dolomitised limestone + shales, and intrusive and contemporaneous igneous rocks. Millstone Grit (MG) gritstone, sandstones, shales. Lower Coal Measures (LCM) sandstones, shales.

Lowland portion q/ region

Permo-triassic

Middle (MCM) and Upper Coal Measures (UCM) sandstones, shales. (Magnesian Limestone (ML) -magnesian limestone, ] permian marls and sand. ~ Bunter Sandstone (BS) sandstone and pebble beds. Keuper Marl (KM) keuper marl and waterstones, (, green beds, skerry.

The pattern of deposition of the various types of drift (alluvium, river terrace, sand and gravel, boulder clay, head and peat) on each geological stratum is on too small a scale for inclusion. However, an exception is made for the area of alluvium, included within a broken line at the north-east boundary of the region. This is associated with formerly extensive and floristical[y important areas of wetland. The isohyet of 825 mm annual rainfall is indicated by a string of open circles. To the left of it land is mainly above 200 m: to the right it is mainly below this value. Bu and Fi indicate the position of Buxton and Finningley relative to the survey area (see Table 1).

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to the east (Table 1). The major geological strata outcrop as bands that run n o r t h - s o u t h through the survey area with the older rocks and higher land both associated with the western part of the region and the younger rocks and lower altitudes found in eastern areas. Since discontinuities in climate, geology and topography follow a similar pattern, the Sheffield region may be subdivided into two approximately equal portions (Table 2), the upland area [Carboniferous Limestone (CL), Millstone Grit (MG), Lower Coal Measures (LCM)] where a majority of land is above 200 m and rainfall generally exceeds 850 mm per annum; and the drier lowland region [Middle and Upper Coal Measures ( M + U C M ) , Magnesian Limestone (ML), Bunter Sandstone (BS) and Keuper Marl (KM)]. The lowland is also generally associated with warmer air temperatures. Both regions have geological similarities, each including one calcareous (CL, ML), together with one separate (MG, BS) and one shared [Coal Measures (CM)] stratum that is non-calcareous. The one minor anomaly is the Keuper Marl associated with the lowland only and having no comparable major stratum within the upland region. A wide range of soil conditions is associated with each geological stratum (Fig. 1). From a plant nutritional standpoint, many of the soils of the calcareous strata are very different from those of non-calcareous strata. In particular the natural soils of non-calcareous strata tend to be acidic while those of calcareous strata often contain free calcium carbonate and are of relatively high pH, as is illustrated for the soils associated with semi-natural grasslands of the region (Lloyd et al., 1971). L a n d use

A majority of the region is subjected to agricultural management (Table 2). Urban development and forestry account for most of the land not utilised for farming. The qowland' area has a potential quality of land for agriculture of Grade 2 or 3 (Table 2) while in the ~upland' the less productive Grades of 4 and 5 prevail. This difference in land quality is reflected in the type of farming, the lowland area being predominantly arable and the upland consisting mainly of grassland. The fact that 671!i; of the more fertile qowland' region is farmed compared with 79 ','ill of the ~upland' area reflects the competition which exists within the 'lowland' area between contending types of land use. In Britain urban development is greater within areas of higher agricultural potential (Engledow & Amey, 1980) and notwithstanding the large

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quantity of urban land associated with the Lower Coal Measures, this national trend is also shown within the Sheffield region. Thus the higher levels of agricultural management and urban development are associated with the lowland portion of the Sheffield region. The difference between the two regions is further accentuated by the presence within the upland region of part of the Peak District National Park, set up in 1950.

THE FLORA

History The early history of the Sheffield region is very similar to that of the British Isles as a whole and a flora similar to that of the present day is of comparatively recent origin, dating from the Pleistocene (c. 1000000 BP) onwards (West, 1969). Early development was marked by ecological discontinuity brought about by the ice-ages (ending at c. 10300 BP) which fundamentally reshaped the British flora (Godwin, 1975). Inundation in c. 7000 B P of the land bridge joining England to the European mainland, and forming the main route into Britain for colonising species, effectively isolated the British flora. During the earlier stages of its development we may suppose that the Sheffield flora was mainly a product of climate, geology, land form and species immigration. More recently, however, the effects of man have been strongly superimposed upon these factors. In particular the transition from a hunting to a pastoral economy in c. 5000BP (Armstrong, 1956) represented a first stage in the formation of the modern Sheffield landscape and flora. Different habitats were exploited at different rates. In consequence deforestation was well advanced by AngloSaxon times--c. 1100 BP (Maxwell, 1956) while many areas of lowland mire (see Fig. 1) were still undrained at the beginning of the 17th century (Cory, 1972). Later still the transition from dependence upon agriculture to an industrially based economy located in the lowland part of the region initiated further vegetation changes. Following the Industrial Revolution of the 18th century, landscape and flora have been directly affected by industry, e.g., the utilisation of land for urban and industrial development, coal-mining, various types of spoilage and environmental pollution. However, arguably the most significant changes are those

206

J. G. Hodgson

which have occurred in the countryside in response to the consumer demands of a large industrially based population, which, in the case of the parish of Sheffield, grew twentyfold between 1750 and 1901 (Pollard & Hunt, 1956). Unfortunately for the aboriginal flora this has generally resulted in the development of communities very dissimilar from those occurring formerly. For example, the afforestation carried out since the inception of the Forestry Commission in 1919 has tended to replace the dwindling relicts of deciduous woodland with plantations characterised by exotic conifers and a high level of mechanised disturbance. In agriculture some important changes occurred in the 19th century. For example, the consumer demands of the new urban population caused a rapid increase in both agricultural prices and the number of Enclosure Acts (Chambers, 1966). Nevertheless, most developments in British agriculture were rather slow to take place despite the potential for increased mechanisation+ and even in 1940 two-thirds of the country's food supply was imported (Lloyd & Wibberley, 1977). However, the ending by Parliament in 1939 of all covenants forbidding ploughing of old grassland heralded the beginning of agricultural expansion (Duffey et al., 1974) and since the war there has been more or less continuous growth in output (Lloyd & Wibberley, 1977; Engledow & Amey, 1980). This period is described in a local context for the lowland portion of the region by Chambers (1966) as 'a new chapter in the Agricultural Revolution. It is characterised by a great ploughing campaign, especially for barley, the putting of field to field and farm to farm, which is giving a touch of the prairie to the historical Midland landscape . . . . Agricultural history is being made faster than we can record it." Today, despite an increased population, the United Kingdom is close to agricultural self-sufficiency, assuming a continued supply of fuel and fertilisers and changes in dietary habits (Engledow & Amey, 1980). This achievement, which has involved the creation and maintenance, particularly within the lowland regions, of very productive systems that are regularly disturbed, has clearly had profound effects on both the local and, as described by Ratcliffe (1984), the national floras. Thus native plant species of the Sheffield region now occur within a landscape where very little of the existing vegetation can be considered as unaltered by man and where recent changes have resulted in the formation of a range of fertile and/or frequently disturbed habitats, many of which probably have no historical equivalent. A majority of these plants have already survived within a changing landscape for many

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thousands of years, some doubtless by processes of genetic specialisation at the level of the species or populations, others perhaps by exploiting local environmentally constant sites, as has been suggested by Pigott & Walters (1954). During this period and in their earlier history many species will have acquired characteristics which will crucially affect their contribution to the contemporary flora. Present constitution

Recent vegetation surveys (Table 3) reveal that there are 708 native, or probably native, and 192 apparently naturalised, herbaceous species and small ( < 1 m) shrubs growing in the Sheffield area. In addition, 15 native and 10 alien hybrid taxa (henceforth, for convenience, also described as species) are sufficiently well-established and reproductively successful for inclusion in the study. A further 60 native and 52 alien tall ( > 1 m) woody species plus a large but uncertain number of hybrids also occur within the region. The latter have been excluded from this study since the l m 2 sample area used in the surveys (Table 3), while appropriate for the herbaceous vegetation of the region, is too small to describe the composition of vegetation of tall woody species. T H E A B U N D A N C E OF NATIVE SPECIES Rare and common species in the total flora

Various characteristics may be used to define rarity and commonness in space and time (Margules & Usher, 1981). Rabinowitz (1981), for example, suggests that geographical range, habitat specificity and population size should be considered. In the present study, however, this was not practicable because of the large number of species involved and the small proportion of their geographical ranges considered. Here the native species of each of the six major habitats described in the next section were simply classified according to the proportion of the 5 km square subdivisions of the survey area in which they were recorded during vegetation surveys (Surveys I1 and III, Table 3). As illustrated in Fig. 2, species were said to be 'restricted' where they were found in less than 20 O~/oof the squares, and 'widespread' species were those occurring in more than this number. For many analyses the 'restricted' grouping was subdivided into 'uncommon' (6-20 'Yo)and 'rare' (1-5 %) by further reference to the survey of rare plant species and

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communities (Survey III). The 'widespread' category is split using the data obtained from the survey of major habitat types (Survey II). The sampling procedures of this survey were not designed to assess in absolute terms the level of abundance of each species within the region and the proportion of samples in which each species is recorded gives only a value of approximate abundance within each habitat. Therefore, the less frequently recorded two-thirds of the "widespread' grouping has been arbitrarily assigned to the "frequent' category and the more c o m m o n onethird to the 'common' grouping. To this latter category are also added a few exceptionally abundant and ecologically wide-ranging species which occur sufficiently often to be considered 'common' but which are even more frequent in another habitat (see Fig. 2). Using these procedures separately for each of the habitats described in the next section, 150 species over all were classified as c o m m o n , 283 as frequent, 168 as u n c o m m o n and 122 as rare. Rare and common species in particular habitats

The ecology of c o m m o n species was examined by reference to the survey of major habitat types (Survey If) only. The combined results of Surveys II and III (survey of rarer species and communities) were used for the remaining species, Each native and alien species was classified according to the habitat in which it was most commonly recorded in the surveys. A very simplified system was used, consisting of only six broad habitat groupings: (1) 'Aquatic'--sites with the water surface above ground level at the time of vegetation sampling (May October). (2) 'Mire'--includes also canal, ditch, pond and marshy river banks. (3) 'Arable'. (4) 'Open habitats'---comprises both skeletal habitats (cliffs, rock outcrops, scree and walls) and some disturbed ones, e.g., paths. (5) 'Grassland'--includes pastures, meadows, road verges, etc. and also heathland and tall herb communities. (6) 'Woodland'--includes plantations, scrub and hedgerows. The distinction between Groups 4 and 5 is somewhat arbitrary and represents an attempt to separate plants of discontinuous herbaceous vegetation (Group 4) from those usually associated with more continuous vegetation cover (Group 5).

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The major habitats of the Sheffield region contribute unequally to the local flora (Table 4). Thus, at all levels of abundance, the aquatic, arable and woodland categories include fewer species than mire, open habitats and grassland. Habitats also differ in the extent to which their component species are common or rare in the region as a whole (Table 4). Thus wetlands (aquatic +mire) have the smallest proportion of common species and the greatest proportion of rarities while arable land has many common and few rare species. Grassland, open habitats and woodland fall between the two extremes. Rare and common species on different geological strata

The proportion of the flora found on each geological stratum is illustrated in Fig. 3. Common species are, with few exceptions, found on all geological strata while rarer species show restriction with respect to geology. The total number of species on each stratum may be ranked in the order Magnesian Limestone > Coal Measures > Bunter Sandstone > Carboniferous Limestone = Millstone Grit. Since common species are widely distributed this relationship reflects differences in the proportion of rarer species present. Several mathematical equations have been devised to relate number of species to area (e.g. Williams, 1943; Preston, 1962). None of the resulting species area curves is universally applicable (Margules & Usher, 1981) and in any event there are insufficient data in Fig. 3 to test for a general relationship of this type in the present study. The stratum with the greatest number of species, the Magnesian Limestone, contributes only about one-third the area of the largest stratum, the Coal Measures (Table 2). We may conclude, therefore, that the contribution of a geological stratum to the total number of species in the flora is related to factors other than its geographical extent within the survey area. Fig. 3. The proportion of the extant native flora of the Sheffield region associated with each geological stratum. The numbers encircled within each histogram denote the proportion of the species associated with the habitat that are found on the stratum. The number above and to the right of each histogram ranks the stratum in order of the number of species present with tied values indicated additionally by a +. C, F, U and R refer to the common, frequent, uncommon and rare abundance categories respectively and CL, ML, MG, CM and BS refer to the Carboniferous Limestone, Magnesian Limestone, Millstone Grit, Coal Measures and Bunter Sandstone. The data for the two calcareous strata are shaded.

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Relationships with geology are less clearly defined in comparisons involving particular habitats. The number of species in each habitat on a particular stratum is to some extent related to the total size of the flora on that stratum. For example, the Magnesian Limestone, which includes the greatest number of species, is ranked in the top three strata in terms of size of flora in all six major habitats (Fig. 3). However, the Carboniferous Limestone presents a major anomaly. Already in Fig. 3 this stratum has been ranked equal last in total species number. Despite this, the Carboniferous Limestone is the most species-rich stratum for three habitats (open habitats, grassland and woodland). It is also the least species-rich for the remaining three (aquatic, mire and arable). Furthermore, the Carboniferous Limestone also scores best in terms of floristic uniqueness. Of the 10 ".,; of the flora confined to a single stratum 41 ~,, are restricted to the Carboniferous Limestone, over twice as many as that found on any other stratum. Rare and common species in different regions Occurrence on calcareous and non-calcareous strata With the exception of the Keuper Marl, the geological strata can be readily subdivided into a calcareous (Carboniferous and Magnesian Limestone) and a non-calcareous grouping (Millstone Grit, Coal Measures and Bunter Sandstone) (see page 204). The distribution of the flora on each type of stratum is given in Table 5. Each grouping includes at least 90 '~'/;of the total flora. Thus over 80 ~'~,of species are c o m m o n to both groupings. The non-calcareous strata include only a 2°/0 greater proportion of the total flora than calcareous strata despite their occupying 76 '~/,, of the region (Table 2). A greater number of less c o m m o n species (common species generally occur on all geological strata. Fig. 3) from aquatic, mire and arable land are recorded from non-calcareous strata. Although their surface area is small (231~'~,;of the region Table 2), calcareous strata include a larger proportion of open habitat, grassland and woodland species. Occurrence within the upland and lowland portions of the region The upland and lowland areas, defined on page 204, each constitute approximately one-half of the Sheffield region (Table 2). Only 11 "j/oand 10'~, of the extant flora are unique to the lowland and to the upland regions respectively (Table 5). Thus 79 i',, of the total native flora including

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TABLE 5 A Comparison of the Percentage of Species Associated with the Upland and Lowland Areas and with the Calcareous and Non-calcareous Strata of the Sheffield Region. The geological strata are classified as follows: Calcareous Non-calcareous

Upland

Lowland

Both

Carboniferous Limestone Millstone Grit

Magnesian Limestone Bunter Sandstone

-Coal Measures (lower = upland; middle and upper = lowland)

The effects of geology and topography are not independent of each other. However, in the case of some of the results, those identified by an asterisk, there is evidence of a consistent and over-riding effect of a single factor. Thus, for example, the greater number of aquatic species found on non-calcareous strata is a consistent feature both of the whole Sheffield region (this table) and of, taken separately, the upland (values for Millstone Grit exceed those for Carboniferous Limestone--see Fig. 3) and lowland areas (values for Bunter Sandstone exceed those for Magnesian Limestone)

Calcareous

Non-calcareous

Upland

Lowland

90 74 83 97 94* 96* 96*

92 100" 96 99 89 90 87

89 70 88 81 92 95* 96

90* 92* 91 98* 88 88 94

38 11 24 54 44 63 50

84* 100 87* 77* 100 63 67

34 0 32 54 44 38 33

90* 100" 84* 92* 89 100" 67*

76 50 38 74 79 85 67

89* 100" I00" 89* 83 96 75

77 83 83* 63 72 84 75*

77 67 33 89* 78* 88* 57

Extant native.flora All habitats Aquatic Mire Arable Open habitats Grassland Woodland

Extinct nati~'e.flora All habitats Aquatic Mire Arable Open habitats Grassland Woodland

A lien .flora All habitats Aquatic Mire Arable Open habitats Grassland Woodland

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virtually all common species are found in both areas (Table 5). Despite their tendency to occur in only one region, equal numbers of rare species are found in each area. However, a greater number of less common aquatic, mire and arable species are located in the lowland area while more of the rare species from open habitats, grassland and woodland are found within the upland area. Thus the results for upland and lowland areas parallel to some extent those of the previous section. For three habitats (aquatic, mire and arable) a larger proportion of the flora is present in the lowland region and on non-calcareous strata, while in the remainder (open habitats, grassland and woodland) more species occur in the upland and on calcareous strata. However, it must be stressed that the effects of stratum type and topography are by no means independent of each other (see Table 5) and that this pattern is to some extent an oversimplification. Rare and common species classified with respect to plant families Although the flora contains a disproportionately large number of Dicotyledons compared with Monocotyledons and Pteridophyta, each of these three major taxonomic groupings has a similar proportion of common and rare species (Table 6). In contrast, the major ( > 20 native species represented) angiospermous families of the region differ markedly in this respect (Table 6). The three major monocotyledonous families exemplify the two extremes of present-day abundance within the Sheffield flora. The Orchidaceae and Cyperaceae have the smallest percentages of common, and the greatest percentages of rare, species while the Gramineae include many common but few rare species. Of the remaining (dicotyledonous) families of less extreme abundance, the Compositae and Labiatae are those whose component species have the greatest tendency to be common and the Leguminosae, Rosaceae and Umbelliferae are those with the greatest tendency towards rarity. The Scrophulariaceae is exceptional as it includes both few common and few rare species. A C O M P A R I S O N OF THE ECOLOGY AND G E O G R A P H I C A L DISTRIBUTION OF C O M M O N A N D RARE NATIVE SPECIES Fertility of the habitat The common and the rare abundance categories both encompass a diverse range of ecologies and each includes species exploiting productive

218

J. G. H o d g s o n

conditions and others confined to infertile environments. However, the level of fertility is universally recognised as an important factor affecting the distribution of plant species, and the possibility that c o m m o n and rare species may be characteristically associated with different levels of fertility deserves consideration. Estimates of maximum potential growth-rate (Rma×) indicate that many species from fertile habitats have the potential to grow rapidly while those from unproductive habitats have a low Rma x (Grime & Hunt, 1975). Unfortunately, while values are available for a majority of c o m m o n species (Grime & Hunt, 1975), R~a × has been measured for very few of the rarer species of the Sheffield region. However, as shown in Table 7, the Rma x of a species shows a predictable relationship to the Rma x values of the species with which it is most consistently associated in the field. This provides the basis of the indirect estimate of Rma x which has been used to estimate the levels of habitat fertility with which species from each abundance grouping are associated (Table 8). With the exception of the arable category, where all species are associated with fertile environments, the results in Table 8 show the same distinctive pattern in all habitats. In each, a greater proportion of c o m m o n than of rare species is associated with productive conditions. TABLE 7 Comparison of Rmax Measured under Growth-room Conditions with that Estimated from the R ...... of C o m m o n l y Associated Species (This analysis used the growth rate data of Grime & Hunt (1975). R .... wflues have been subdivided into four approximately equal classes, 1,2, 3 and 4, corresponding to growth rates of < 1 0 0 , 1.01-1.25, 1.26 1.50, >1.50 week --1, respectively. For each species the average value for its five most consistent associates was calculated and compared with the direct measurement obtained under growth-room conditions)

Rma,: class o f g~s.so c ia t e d sp ec ies Measured R ...... of species < 1-25 > 1.25

I 2 3-4 42 19 21 33 Z2 = 1 0 . 3 8 ( P < 0 . 0 1 )

Commonness and rarity in plants

219

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220

J. G. Hodgson

Thus the data indicate an association between rarity and habitat infertility. The level of habitat disturbance

Any factor, whether mechanical or environmental, which results in the destruction of biomass is regarded as a form of disturbance by Grime (1979) and this definition is accepted here also. The degree of habitat disturbance is, like the level of habitat fertility, an important determinant of species distribution and abundance. Most disturbed habitats of the Sheffield region are characterised by the presence of species with a short life-span and abundant seed production. The level of disturbance in habitats or vegetation containing c o m m o n or rare species has been estimated on the basis of the presence or absence of such indicator species. Major habitats differ markedly in their proportion of species that are indicative of disturbance (ephemeral and facultatively monocarpic species) (Table 9). However, within each habitat the percentage contribution of such species is very similar whether the extant, extinct or alien flora is considered. The vegetation in each habitat has its own specific level of 'ephemerality" and commonness appears to be directly related to it (Table 9). Thus the habitats with the greatest proportion of facultatively monocarpic species include the greatest proportion of both c o m m o n monocarpic and c o m m o n polycarpic species. There is also the obverse tendency for rare species to be associated with habitats of low 'ephemerality' but the results are not statistically significant at the 5 "~, level (Table 9). This difference between c o m m o n and rare species is evident also in Table 8, which shows that more c o m m o n than rare species are associated with vegetation containing on average at least two monocarpic species per metre square. Thus c o m m o n species appear to be more frequently associated than rare species with disturbed environments containing ephemerals. C o m m o n and rare species occur in rather different types of disturbed situation. Calculations reveal that of the c o m m o n grassland species of disturbed habitats recognised in Table 8, 62 '~/oare associated with fertile habitats compared with only 29'~'i, of the corresponding group of rare species and a similar analysis for open habitats gives a value of 54 ~0 for c o m m o n and 20 ~,i for rare species. These results reflect the fact that many c o m m o n species are particularly associated with habitats such as

22

C o m m o n n e s s and rarity in plants

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J. G. Ho~,~son

intensively managed agricultural land, gardens and urban wasteland, all of which are both disturbed and fertile, whereas those rare species which co-exist with ephemeral species tend to occur in disturbed habitats of lower fertility, e.g. rock outcrops, screes and quarries.

Species richness of vegetation Recent work has greatly increased our knowledge of the mechanisms that may enable species of contrasted ecology and competitive ability to coexist (e.g. Grime, 1973: Grubb, 1977, 1982: Richerson & Lum, 1980: Tilman, 1982: Rorison et al., 1983: Hillier, 1984). Nevertheless, the complex mechanisms that control species richness within plant communities remain poorly understood. It is difficult, for example, to define the niche width of species, one potential determinant of fine-scale species richness (i.e. within the plant community: alpha diversity, sensu Whittaker, 1972). Likewise the role of environmental controls operating at the scale of the habitat mosaic (beta diversity) cannot be readily quantified. However, one characteristic which relates to both types of diversity appears particularly relevant to this study. This is that speciesrich herbaceous plant communities tend to be associated with intermediate levels of environmental stress and/or disturbance (Grime, 1973). Presumably stemming from this relationship, a consistent feature of species-rich herbaceous vegetation is that it has an intermediate value for the seasonal maximum of above-ground plant material (shoot biomass + litter) both in the Sheffield region (Al-Mufti et al., 1977) and elsewhere (Fox, 1981 : Wheeler & Giller, 1982). It is of interest, therefore, to determine whether rare and c o m m o n species differ in the species richness of their habitats, particularly since species richness is highly valued as a criterion for wildlife conservation (Margules & Usher, 1981). Species richness is in part a function of area (Margules & Usher, 1981). Consequently, the comparison of the species density of vegetation samples containing rare species from Survey IlI with that for more representative vegetation collected during Survey II has been based throughout on l m 2 sample areas, This size has proved particularly suitable for studies of species densities within the Sheffield region (Grime, 1973: Al-Mufti et al., 1977). The results presented in Tables 10 and 11 show that, at surface (0 3 cm) soil pH values exceeding 4.5, vegetation containing rare species has a greater number of species per square metre than is characteristic of the

Commonness and rarity in plants

223

corresponding c o m m o n l y occurring vegetation types. In contrast, the few rare species occurring on acidic soils appear to be restricted to speciespoor vegetation (Tables I0 and 11). These findings are generally consistent with the hypothesis that rare species are particularly associated with sites of intermediate environmental stress and/or disturbance as proposed by Grime (1973) and intermediate standing crop as indicated by, for example, A1-Mufti et al. (1977). This ecological characterisation of communities with rare species must, however, be regarded as tentative, particularly since much of the vegetation with rare species includes fewer than 20 species m 2, a value used to define species richness in Grime (1973). Habitat range The extent to which species occur in several habitats It has been argued (e.g. Sukopp, 1976) that species with a wide ecological amplitude are likely to be c o m m o n and those with a narrow amplitude rare. Unfortunately there are insufficient data for rare species to allow a comparison o f the ecological amplitude of c o m m o n and rare species and this important characteristic has been assessed only for some of the region's most abundant species. This involved scoring the number of major habitats in which each species is classified as common. In the analysis presented in Fig, 4 an additional point has been given to any species equally c o m m o n on acidic and calcareous soils. Thus 7 is the m a x i m u m value that a plant with a wide ecological amplitude could achieve and a c o m m o n species with a narrow range would have a value of 1. The results of this analysis (Fig. 4) suggest that c o m m o n species differ in their ecological range at least 9¢ithin the Sheffield region. Certain species (e.g. Agrostis stolonifera, Poa trit'ialis and Urtica dioica) ave found under a wide range of environmental conditions. Other species (e.g. Dactylis glomerata and Festuca rubra), recorded equally frequently in Survey II, have a narrower habitat range and owe their abundance to their consistent appearance in some o f the most abundant vegetation types of the region. Occurrence in rarer habitats A species will be rare if the environmental conditions which it exploits are also rare in space or time. In such circumstances rare species are also likely to be associated with an unusually high proportion of ecologically

J. G. Hodgson

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TABLE I 1 A S u m m a r y o f t h e D i f f e r e n c e s m Floristic C o m p o s i t i o n b e t w e e n C o m m o n l y O c c u r r i n g V e g e t a t i o n a n d V e g e t a t i o n c o n t a i n i n g R a r e Species 1 Difference m mean total number of spp. m 2 I I Percentage difference m mean total number of spp. m 2 Ill Difference in mean number of c o m m o n spp. m - 2 IV Difference in proportion of c o m m o n spp. m 2 V Difference in number of frequent + restricted spp. m 2 VI Difference in proportion of frequent + restricted spp.

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I

No. common spp.

I1

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17

(a) Habitats used for agricuhure and forestry I. Meadows and pastures 75.6(+33) i. Enclosed pH > 4-5 +3'2(+13) it. Unenclosed pH >4.5 [-1"5( 25) pH <4-5

+11"8(+23) +1'9(+13) +2"6(+•2)

+4.6(+/9) +4"4(+14) +0"2(+•4)]

2-1(+10)

+5.1(+22)

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#6"71+5#)

3. Broadleaved woodland (not obviously plantation) pH >4"5

+3.6 ( + 4 1 )

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pH >4.5

+2.2 ( + 5 9 )

pH >4-5 pH_<4'5

-~ 5.3 (+55) -0.5( 9)

- 1 . 0 (+17) +2.2 (+25)

+3"3 (+15) +0.7(+22)

3. Wasteland. etc. i. Wasteland it. Road verges

pH > 4.5 pH >4.5

+2.7 ( + 171 +2.1 (+15~

+ 1.2 ( + 14) +1.8t+18)

+ 2 ' 5 ( + 12) +2.6(+16)

4. Mire~ Wasteland Riverbanks

pH >4.5

+2.4 ( + 15')

0.2 ( + 7)

5. Skeletal habitats i. Cliffs it. Outcrops iii. Scree iv. Walls

pH pH pH pH

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and rarity in plants

Commonness

227

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Fig. 4. An estimate of ecological amplitude for the most commonly recorded species in the Sheffield region. (Data abstracted from Survey II: Grime et al., in press.) The method of calculating the number of habitats in which a species is common is described on page 223. Species abbreviated as follows: Ac Ae As Df Dg Ea Fo Fr

Agrostis eapillaris Arrhenatherum elatius Agrostis stolon(l?ra Deschampsia ftexuosa Daetylis glomerata Epilobium angust(lblium Festuea ovina Festuea rubra

HI Hm Pa Pp Pt Rf Ta Ud

Holeus lanatus Holcus mollis Poa annua Poa pratensis Poa trilialis Rubus lrutieosus agg. Taraxacum agg. Urtica dioiea

specialised and hence rarer species. This possibility has been examined by comparing characteristics of vegetation containing rare species from Survey III with data for the more commonly occurring vegetation types sampled during Survey II. The results presented in Tables 10 and 11 show that vegetation containing rare species is characterised by an unusually high proportion of other less common species. This finding is consistent with the prediction that rare species tend to be restricted to less common environmental conditions. As we might expect, common species are particularly well represented in commonly occurring vegetation types (Tables l0 and 11). Nevertheless they also contribute on average over 50 "~ of the species total in vegetation samples containing rare species and

228

J. G. Hodgson

their number per metre square is not depressed to any appreciable extent in such circumstances (Table 11). Thus c o m m o n species as a group appear to be both the most consistent and important exploiters of both c o m m o n and less frequently occurring habitats. Occurrence in 'intermediate" habitats

The concept that rare species are associated with intermediate or boundary communities is well established, at least with respect to the 'limes divergens' (ecocline) boundary of Van Leeuwen (1966). British examples illustrating this relationship include the nationally important Teesdale rarities of N England (Bellamy et al., 1969) and the locally important, highly species-rich vegetation on the Carboniferous Limestone of the Peak District that marks the transition from herbaceous vegetation to woodland (Shimwell, 1971). However, the extent to which rare species as a group are more restricted to intermediate habitats than c o m m o n species is not known and will therefore be considered here. The vegetation samples associated with each of the six major habitats recognised on page 210 include two types of c o m m o n species. These are (a) typical species, i.e. those classified by the procedures of pages 208 10 as c o m m o n in the habitats under consideration and (b) atypical species which are rare in the habitat under consideration but c o m m o n in one or other of the remaining five major habitats. The contribution of atypical species has been calculated for the total complement of quadrats for each habitat in Survey II to provide an index of vegetation intermediacy. Subsequently each estimate for commonly occurring vegetation was compared with identically processed data for equivalent vegetation containing rare species derived from Survey III. Ideally, vegetation characteristic of the diffuse boundaries of the 'limes divergens' type would have been separated from the 'limes convergens' (ecotone) which has a well-demarcated margin (Van Leeuwen, 1966) and a more uncertain association with rare species. Unfortunately, insufficient is known about the communities of the region to distinguish fully between these two types. The results in Fig. 5 indicate that habitats differ considerably in vegetation 'intermediacy' and some habitats, e.g. aquatic and river banks, are even in their commonly occurring variant highly 'intermediate'. In open or disturbed habitats vegetation containing rare species differs little from commonly occurring vegetation in the proportion of atypical species present (Fig. 5). However, in the remaining group rare species tend

A Open and Disturbed Habitats ,

,

Remaining Habitats [ - -

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Fig. 5. A comparison of vegetation samples with and without rare species with respect to the proportion of atypical species. Comparisons are drawn for a wide range of different habitats. The proportion of atypical species (species not primarily found in the habitat in which they are recorded) is calculated as described on page 228. The percentage of atypical species (an estimate of habitat intermediacy) is presented under (A) for a range of commonly occurring habitats. There follows an examination (B) of the extent to which vegetation containing rare species differs from that of comparable commonly occurring habitats. The habitats are abbreviated by an encircled symbol as follows: Open and disturbed habitats A, outcrops, non-calcareous rock: B, arable: C, cliffs, calcareous rock; D, walls: E, scree: F, cliffs, non-calcareous rock: G, outcrops, calcareous rock: H, riverbanks. Remaining habitats--l, enclosed meadows and pastures, pH_>4.5: 2, unenclosed pasture, p H < 4 . 5 ; 3, woodland, pH <4,5: 4, road verges, pH>_4-5: 5, unenclosed pasture, p H < 4 . 5 : 6, wasteland, pH>_4-5: 7, wasteland, p H < 4 . 5 : 8. woodland, pH _> 4.5; 9, mire pH < 4.5: 10, mire, pH > 4.5; 11, aquatic, pH _> 4-5. The major habitat under which each may be classified is as follows: @, Aquatic (Aq); @, Arable (Ar): @, Grassland (Gr): ~ , Mire (M): O, Open Habitats (OH): @, Woodland (Wd). If the percentage of atypical species classified under a single other major habitatis > 10, this major habitat, in its abbreviated form, is also appended. [] valuefor habitat + rare species: ~ values identical: --~ the proportion of atypical species from a single major habitat increases by > 10 ~!~,of the total number of species when rare species present: this major habitat is also identified next to the square:-:-~,,, increase or decrease less than this amount.

230

J. G. Hodgson

to be associated with a higher than normal level of habitat intermediacy. Base-rich woodland and mire habitats with rare species appear intermediate with grassland while base-rich wasteland and road verges, both essentially grassland habitats, show intermediacy with woodland and open habitats respectively. Intermediate habitats of this type may include an unusually diverse selection of environmental conditions conferring beta diversity (sensu Whittaker, 1972). This, together with the narrow ecological amplitude suggested for rare species on page 227, may help to explain the greater species richness of vegetation containing rare species described on pages 222-3. The restriction of species either to acidic or to calcareous substrata

The natural soils of the non-calcareous strata of the Sheffield region tend to be infertile and acidic while the calcareous strata include a range of soils that are also infertile but which have a high pH and in many cases free calcium carbonate (see page 204). M a n y factors distinguish these two types of habitat and their combined effect on species distribution within the Sheffield region is considerable (Balme, 1953; Grime & Hodgson, 1969: Rorison, 1973). Features of soil chemistry are generally considered to be the most important of these factors. The simplest and perhaps most widely used predictor of chemical status and the general availability of nutrient and toxic ions is soil pH, a character which has been used in other local studies of the calcicole calcifuge problem (e.g. Grime & Hodgson, 1969; Grime & Lloyd, 1973). An attempt was therefore made to determine whether c o m m o n and rare species differed in their degree of restriction to infertile calcareous or acidic soils. Species from less fertile habitats (as defined on page 218) were included in the ~restricted to acidic soils' grouping if 75 "J~, of their records in the vegetation surveys were associated with a surface (0 -3 cm) soil pH less than or equal to 5.0.* Those species wtth ' 90/o o J of their survey localities situated on limestone and 75 ''/ /o of their quadrat samples with a pH of greater or equal to 6.5 were classified as 'restricted to calcareous soils'. There is no consistent difference in the extent to which rare and c o m m o n species are restricted to acidic soils (Table 12). In contrast, a greater proportion of rare than of c o m m o n species show a particular association with calcareous soils. * For uniformity of treatment this cut-off point was used throughout even though data on the solubility of toxic ions given in Hem (1970) suggest that a higher wflue would be more appropriate for wetland species.

231

Commonness and rarity in plants

TABLE 12 The Percentageof Species from DifferentAbundance Groupings that are Restrictedeither to Calcareous or to Acidic Soils. (The "restrictedto calcareous soils' and 'restricted to acidic soils' categories are defined on page 230. The aquatic and arable categories have been omitted from this analysis) "Restricted to calcareous soils' C o m m on

Wetland habitats Mire Dryland habitats Open habitats Grassland Woodland

A lien

Rare

'Restricted to acidic soils' Common

A lien

Rare

0

0

13

5

0

7

7 4 0

14 4 8

56 40 38

4 9 15

0 4 0

4 12 0

Species at the limits of their geographical distribution Many less c o m m o n species have a restricted geographical distribution in Britain while c o m m o n species tend to be widespread (Petting & Walters, 1976). To quantify this difference for the Sheffield flora the proportion of c o m m o n and of rare species that are close to the b o u n d a r y of their British distribution in the Sheffield region was determined by reference to Perring & Walters (1976). While a disjunct geographical distribution may result from a variety of factors (Pigott, 1970), climate is generally regarded as the most important (e.g. Salisbury, 1932; Pigott, 1970) and to minimise any effects on distribution pattern of recent habitat destruction old records as well as more recent ones have been used to define geographical range. Few c o m m o n but m a n y of the rare species of the Sheffield flora are at the geographical limit of their British distribution (Table 13). In both the lowland and the upland regions most geographically restricted species are at the northern b o u n d a r y of their British distribution (Table 13).

SPECIES O F C H A N G E D A B U N D A N C E

Categories of changed abundance There have been marked alterations in land use within historically recent times (pages 205-8). An inevitable consequence of this is that many

232

J. G, Hodgson

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Commonness and rarity in plants

233

species will have changed considerably in their abundance, some increasing, others decreasing. Rare species of decreasing abundance may differ considerably from those of stable (or even increasing) abundance in terms of their reasons for rarity (Stebbins, 1942). Differences in the reasons for commonness between increasing and more stable (or decreasing) c o m m o n species may also be predicted. It is therefore unfortunate that the flora of the Sheffield region over the last century has not been sufficiently well documented to assess accurately which species have increased, which have decreased and which have maintained a comparatively stable level of abundance. It is, however, possible to identify by reference to survey data and to Lees (1888, 1941), Linton (1903), Howitt & Howitt (1963), Clapham (1969) and Perring & Walters (1976), two groupings of greatly changed abundance, namely aliens (species formerly absent) and extinct (species formerly present). The alien grouping consists predominantly of species of garden origin (Table 4), although some natural colonists (native British species outside their natural distributional range) are included. A few colonists, e.g. Corrigiola littoralis and Dryopteris rillarii, are nationally rare (Petting & Walters, 1976). Most are associated with dryland habitats (Table 4) and the Cruciferae, Scrophulariaceae, Leguminosae and Compositae are particularly well represented (Table 6). Extinctions appear to have occurred in all habitats but have been most pronounced in the arable and wetland habitats (Table 4). There have also been species losses from all the major families except the Gramineae and Rosaceae, with the Umbelliferae and Leguminosae being the worst affected (Table 6). A comparison of aliens with common species

While the alien flora includes only species that have increased, the c o m m o n grouping is less consistent in this respect and even includes one species, Vaccinium myrtillus, that has become extinct on one geological stratum. We may suspect, therefore, that attributes allowing species to increase within the modern flora may be even more recognisable within the alien than the c o m m o n grouping. Accordingly, the distribution and ecology of alien species have been compared with those of the c o m m o n species in a manner similar to that used in the comparison between c o m m o n and rare species (pages 217-31). Comparisons of the type used to estimate species richness (pages 222 3) and habitat specificity (pages 223-30) are, however, excluded because the alien grouping, unlike the

A. ESTABLISHED ALIENS All Hebit(Ifs

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Fig. 6. The distribution of A, the established alien and B, the native extinct, floras on the different geological strata of the Sheffield region. To facilitate the visual recognition of trends related to region or rock type, the results for each habitat are presented in two histograms separating the calcareous (on the left, shaded) from the non-calcareous strata. In each histogram the upland stratum (CL or MG) is positioned first and the lowland stratum (ML or BS) last. The strata are ranked according to the number of species present with tied values indicated additionally by a + . For the extinct category (B) broken-line shading is used to indicate that the recorded locality for species is close to a geological boundary and cannot be referred with certainty to one stratum. (Under these circumstances half of a species was ascribed to each of the two strata.)

Commonness and rarity in plants

235

B EXTINCTFLORA

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236

J. G. Hodg, son

c o m m o n and rare categories, includes species of widely differing abundance. The results from such analyses would reflect to a disproportionate extent the characteristics of the more abundant aliens. The further problem, that newly arrived species among the aliens may appear ecologically and geographically restricted simply by chance, limits still further the ecological usefulness of the data for aliens from Surveys II and Ill. Aliens are more c o m m o n l y found on calcareous soils and less frequently on acidic ones (Table 12) but there is no consistent tendency for the total alien flora to be associated with a particular stratum type or topographical area (Table 5, Fig. 6A). In fact the ranking of the individual geological strata in terms of the number of alien species present, C M = M L > BS > M G > CL (Fig. 6A), is very similar to that for the proportion of the stratum that has undergone urban development (CM > M L > BS > M G > CL, Table 2) and presumably also to the density of gardens (see Table 4). Thus the geographical distribution of aliens appears to be still centred on the sites of their initial introduction and therefore aliens are unlikely to be identical to increasing c o m m o n species in all aspects of their ecological distribution. Aliens are, however, similar to c o m m o n species in some very important respects. They tend to occur in disturbed conditions and are even more restricted than c o m m o n species to habitats of high fertility (Table 8).

A comparison of recently extinct with rare species The rare species grouping, like that for c o m m o n species, is heterogeneous. It includes many rare species which have decreased, some with a more stable distribution, e.g. T r o l l i u s c u r o p a e u s , and a few which have increased because of the activities of man, e.g. R u m e x m a r i t i m u s . Thus ecological characteristics that will cause a species to be infrequent within the region might be expected to be even more conspicuous within the recently extinct grouping. Unfortunately, there are no data comparable to those for rare species describing the ecological characteristics of extinct species. Consequently, only the geographical distribution of the two groups may be compared. The proportion of extinct species at the geographical limit of their distribution is similar to that for rare species (Table 13) except that even more are at the northern limit of their distribution. Recent extinctions have occurred principally m the h)wland area, particularly on noncalcareous strata (Table 5 and Fig. 6B). The pattern of extinctions is very

Commonness and rarity in plants

237

different from the existing distribution of rare species (Fig. 3, pages 213-14) and suggests that rare species growing on non-calcareous strata within the lowland region may be particularly at risk of extinction.

The importance of changes in the availability of habitats The marked changes in land use in the Sheffield region within the recent historical past (pages 205 8) have resulted in the creation of a range of new, m a n - m a d e habitats and a corresponding diminution in the extent and range of more ancient vegetation. Therefore the possibility will be considered that species able to exploit these recently made habitats are likely to be increasing while those associated only with older habitats have decreased. With the emphasis on the reasons for increasing abundance, the distribution of aliens, the most wide-ranging c o m m o n species (those that are c o m m o n in more than one habitat), species classified as c o m m o n in only one major habitat and less c o m m o n species were examined in a wide range of c o m m o n habitats by reference to Survey II. Aliens are most frequently recorded in m a n y o f the most recently man-made, base-rich habitats, e.g. arable land and demolition sites (Fig. 7A). They are also well represented in habitats associated with natural or man-made transport systems (e.g. river banks and paths), but less frequent in older environments, e.g. scree and unenclosed pasture, as well as a few manmade ones, e.g. broad-leaved plantations. Species that are c o m m o n in more than one habitat (indicative of a wide ecological amplitude) have a similar pattern of distribution to that of aliens. The relative abundance of the two groupings is positively correlated and both are presumably increasing. The less c o m m o n species, and indeed species c o m m o n in one major habitat only, tend to be less frequent in man-made habitats. Since their relative abundance is negatively correlated with that of alien species and of species c o m m o n in more than one major habitat, both groups may include a majority of decreasing species. Whether the findings for baserich habitats are also relevant to acidic habitats is uncertain. No statistically significant correlations were detected using the much smaller data set (Fig. 7B). Considering now the reasons for decreasing abundance and rarity, rare plants too can occur in comparatively recent vegetation (Merton, 1970; Wells et al., 1976). There is, however, evidence for grassland and woodland suggesting that some plants are indicator species of sites that

J.G. Hodgson

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have not been drastically altered for at least one, and often several, hundred years (e.g. Pigott, 1969, 1981; Rackham, 1976; Peterken, 1974; Wells et al., 1976) and the distribution of some species may have been determined even earlier by glacial phenomena (Pigott & Walters, 1954). Unfortunately, the reasons for this restriction of species to more ancient habitats is poorly understood. Not only does the degree of restriction differ in different parts of the country (Peterken, 1974) but it has been considered, for example, to arise from such diverse causes as failure to produce appreciable quantities of viable seed under normal climatic conditions (Pigott & Huntley, 1981), the infrequent availability of suitable sites for seedling establishment (Ward, 1981) and lack of species mobility (Peterken, 1974; Rackham, 1976: Hodgson, 1982). Nevertheless, this characteristic is too central to studies of decreasing abundance to be ignored. In the absence of an adequate historical documentation of sites it is not possible to assess directly the extent to which the rare or the common species of the Sheffield region are confined to ancient habitats. However, a restricted species would be expected (a) to Fig. 7. The degree of co-occurrence of c o m m o n , alien and less c o m m o n species in a range of (A) base-rich and (B) acidic habitats. The values used were calculated for each habitat as follows: relative proportion of species c o m m o n in > 1 major habitats = (no. of spp. c o m m o n in > 1 major habitat) no. of c o m m o n spp.

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The K e n d a l l Correlation Coefficient has been abbreviated to K. The habitats have been numbered as follows: Man-made habitats mostly 20th century origin-~Cinders * (1): Coal mine spoil (2): Demolition sites (3): Manure and sewage spoil (4): Quarry spoil (5): Soil heaps (6): Walls (7): Arable (8): Enclosed pasture (9): Broad-leaved plantation (10): Conifer plantation (11). Habitatsjbrming or adjacent to natural or man-made tran,ff~ort .s:vstems Aquatict (12):Riverbanks(13):Path(14):Roadverge(15).Long-establishedhabitats Mire(16): Lead mine spoil (17): Cliffs (18): Outcrops (19): Scree (20): Unenclosed pasture (21): Wasteland (22): Woodland (23): Hedgerows* (24). * = p a r t of category forming or adjacent to a transport system. + = part of category not part of a transport system.

240

J. G. Hodgson

occur in habitats where there is no proven evidence of recent modification by man and (b) to have been recorded from its sites for many years. These two characteristics will now be considered. The extent to which species are associated with habitats not recently modified by man was estimated by reference to maps of the first edition Ordnance Survey (surveyed c. 1840, reprinted by David & Charles) together with data from the vegetation surveys (Table 3), some works on local history (Farey, 1813: Howarth, 1889; Moss, 1913) and county floras. From these combined sources it is possible to ascertain that some rare species have colonised certain of the newer man-made 'post 1840' habitats, e.g. railway banks, gravel pits and new ponds. The species for which there is no evidence for a major change in land use at any of these sites since the first series Ordnance Survey maps (confined to 'pre-1840' habitats) will include species restricted to ancient habitats. The extent to which species are associated with sites in which they have long been known (characteristic (b) above) has been ascertained by reference to county floras and data from the vegetation surveys. From these sources it has been possible to identify the species which show correspondence at the level of parish between present-day and 19th century records. To a considerable extent results using these methods must be a function of the number of sites in which each species is recorded. Therefore these procedures require that species of similar abundance are compared and for this reason both rare native and rare alien species have been included but common species, none of which is restricted to 'pre-1840' habitats, have not. The results (Table 14) indicate that about half of the rare native species are confined to 'pre-1840' habitats. There is also a high degree of correspondence at the parish level between 19th century and present-day records for rare native species (Table 14). In contrast, the information in local floras and other sources indicates that 60'~5o of all alien species, irrespective of their present-day rarity, were first recorded from the Sheffield region within the last fifty years and few rare aliens were recorded during the 19th century (Table 14). Consequently, with the possible exception of the aliens of woodland, many of which have become naturalised in the ornamental woodlands of large country houses, rare aliens, unlike rare native species, show little evidence of a requirement for habitat continuity and are strongly represented in "post-1840' sites (Table 14).

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DISCUSSION Problems and conclusions

A wide range of differences between common and rare species of the Sheffield region has been described. These are summarised as a part of Table 15 and provide a potential basis for further exploration of the reasons for commonness and rarity. However, before attempting to use the data for such a purpose, some of the basic problems associated with acquiring and using a data base of this type should be considered. At all stages of the analysis the procedures adopted have necessitated assumptions, the most far-reaching of which are outlined below. (1) The flora of each habitat has been analysed independently to enable consistent differences between common and rare species to be identifed. While this approach appears valid, many will regard an ecological classification of species with respect to six major habitats (page 210) as unduly simplistic and inevitably some species will be mis-classified. (2) The decisions to subdivide common and frequent species and to allow common species to be assigned to more than one habitat but to restrict the remaining species to a single major habitat (pages 208-10) are arbitrary. Their use reflects the difficulty involved in classifying species into categories of greater abundance when information on the absolute number of sites and the size of populations is unavailable. In consequence, the measure of species abundance used here relates to compafative frequency within a major habitat rather than absolute abundance in the total landscape. This is perfectly appropriate for the types of analyses undertaken. However, because the major habitats differ considerably in surface area, it must be noted that commonness in the different habitats may be associated with very different levels of absolute abundance. (3) Unfortunately, the decreasing and the increasing components of the native flora could not be accurately identified. The use of a grouping composed mainly of garden species (aliens) for increasing species and the absence of ecological information on extinct species are particularly unsatisfactory. (4) Ecological comparisons between species of differing abundance have been based upon limited data sets. Less common species present a particular problem, since, because of their rarity, there is only a limited

Commonness and rarity in plants

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a m o u n t of field-based and even less laboratory-based information available for each. Consequently, a policy of using 'best estimates' of the ecological characteristics of c o m m o n and rare species has been adopted. Some estimates have assumed that the species whose ecology is most clearly defined (common species) can be used to predict the ecology of species of lesser abundance with which they are associated in the field. Others are based more directly on a habitat or distributional characteristics. None is as scientifically rigorous nor as accurate as might be wished. The estimates for rare species are particularly liable to error simply because, as already stated, they are based on a small volume of data. Ideally in a comparative study of the reasons for commonness and rarity, data concerning the distribution of each species in the field should be coupled with experimental studies of the biology and ecology of the species before a synthesis is attempted. Clearly this approach would have been necessary if the paper had represented an attempt at innovation by defining previously undescribed characteristics. However, this work deals, albeit in a simplistic manner, with a range of species attributes whose relationship to commonness and rarity is comparatively uncontroversial. Moreover, the conclusions are little affected by modifying the cut-off point for c o m m o n and rare species since the categories of intermediate abundance ('frequent' and 'uncommon') are also characterised by intermediate values for the various species attributes studied (Hodgsom unpublished data). Thus, the approach adopted, which attempts to provide a preliminary overview of reasons for commonness and rarity, is considered an acceptable compromise between the ideal, but excessively time-consuming, procedure described above and a more intensive study that encompasses few species. It must, however, be emphasised that the c o m m o n , frequent, u n c o m m o n , rare and extinct species of the Sheffield region are native to an average of 82 '.!0, 76 ,~i;, 67 70, 62 ~',, and 60 ,,/~,,of the European territories of Tutin et al. ( 1964- 80). Thus even the species classified as rare in this study are generally wide-ranging in Europe and this paper is not addressed to the group of rare species which is arguably of greatest conservation interest, namely, narrowly endemic species. There is an additional problem. For each generalisation presented in Table 15 there are exceptions. Consequently (as noted by Stebbins, 1980), there is no single ecological feature which is diagnostic of either c o m m o n or of rare species. Commonness and rarity therefore appear to be the

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product of a-multiplicity of ecological factors, some more important than others, with each species having some attributes favouring an increase in abundance and others a decrease. The balance between these two groups of characteristics will presumably determine the abundance of the species. Towards an understanding of the reasons for commonness and rarity

The characteristics of c o m m o n and rare species presented in Table 15 are consistent with the concept that the abundance of species may be explained solely by reference to the availability of suitable habitats. The attributes of commonly occurring species fit them to commonly occurring habitats and those of rare species tend to restrict them to less c o m m o n habitats (through an inability to exploit c o m m o n habitats). However, three pieces of data seem inconsistent with such an explanation: (1) The high percentage within the upland area of species apparently at the northern climatic limit of their distribution. While the disjunct geographical limit of the distribution of a species may result from a variety of factors (Pigott, 1970), climate is generally regarded as the most important (Salisbury, 1932; Pigott, 1970). This being so, it is surprising to observe in Table 13 that within the upland area the proportion of species at the southern (and eastern) limit of their British distribution (e.g. Rubus chamaemorus and Wahlcnbcrgia hederacea) is smaller than the proportion at their northern limit (e.g. Cirsium acaulon, a species whose distribution is considered restricted by climatic factors (Pigott, 1970)). (2) The disproportionately large number of species restricted to calcareous soils (see Table 12). This result is particularly anomalous bearing in mind the small surface area of calcareous strata within the region (Table 2). (3) The disproportionately large number of species restricted to less fertile habitats. By use of the procedure described on pages 217-19 it is estimated that 55'~;, of the native flora of the Sheffield region (excluding aquatics) are associated with less fertile habitats despite the fact that a majority of the region must, because of the effects of agriculture and urban development (see Table 2), be considered fertile.

Commonness and rarity in plants

247

Possible explanations for these apparently anomalous statistics can be suggested. With reference to the first anomaly it should be noted that the moist tropics are considered to be the original home of the angiosperms (Cronquist, 1968; Takhtajan, 1969) and not only is the frost line a barrier to the spread of tropical species, but as one moves further towards the Poles the number of species of angiosperm per unit area progressively decreases (Cronquist, 1968; Rejmanek, 1976). Thus, the high proportion of species at the northern climatic limit of their distribution may have an evolutionary origin even though the climate of the Sheffield region is not extreme in a European context (see Wallen, 1970). The second anomaly may perhaps be similarly explained. The greater number of rare species restricted to calcareous rather than to acidic soils within the Sheffield flora is consistent with the greater number of calcicoles than calcifuges within the British flora. Grime (1979) considers that calcicoles may have evolved principally in semi-arid environments, where, as indicated by Margalef (1968) and Stebbins ( i 974), the rate of speciation is often high. The third anomaly, the low proportion of species associated with fertile habitats, may perhaps be partially explained by the widespread occurrence of dominant species in fertile habitats and the wide ecological range of many common species (see Fig. 4). However, the presence also of a large number of aliens, a group of species both predominantly from fertile habitats (Table 8) and recently introduced (page 240), suggests the need for a perspective that includes an appraisal of some of the episodes described on pages 205-8. Takhtajan (1980) states that 'the most primitive living flowering plants are rarely dominants and usually occupy modest niches, in the undergrowth of tropical forests, in the mountain mossy forests, etc.' This description strongly suggests that these primitive woody species may be classified as stress-tolerators (sensu Grime, 1979). If these 'archaic' species do accurately reflect the ecological strategy of their ancestors, the world flora is presumably derived from essentially stress-tolerant ancestral stock. Under such conditions the evolutionary history of angiosperms may have led to a preponderance of stress-tolerators within the world flora analogous to that of tropical species discussed above. The consistent presence of aliens in man-made environments throughout the world (Holm et al., 1979) may also be a symptom of a global scarcity of species adapted to the more fertile conditions often present in man-made environments, although the high effectiveness of 'supertramps' sensu

248

J. G. Hodgson

Diamond (1975) may also be important. This speculation suggests that the anomalously high proportion of species associated with less fertile habitats in the Sheffield region may be the product of the evolutionary history of higher plants as well as more recent historical factors. On the basis of the interpretation of the data presented here, it seems necessary to consider reasons for commonness and rarity in terms of not only the availability of habitats but also the recent history of the region and the evolutionary history of higher plants. To these ends the relative importance of the various environmental determinants of abundance in regulating present-day commonness and rarity within the Sheffield region has been assessed (Part II) and in a sequel (Part III) evolutionary and taxonomic aspects of the commonness-rarity problem have also been considered. A comparison of the Sheffield flora with that of Europe in general has also been undertaken (Part IV). This includes a section on endemism, which scarely occurs within the Sheffield flora but is prevalent elsewhere in Europe.

A C K N O W L E D G E M ENTS I should like to thank Professor J. P. Grime for reading the manuscript and for suggesting numerous improvements to it. I am also grateful to him and to Dr Hunt both for permission to utilise jointly collected survey data and for providing additional information on the ecology of relevant species. Too many other members of the Unit of Comparative Plant Ecology have over the years made a significant contribution to the work for all their names to be mentioned here. However, I should particularly like to thank S. R. Band, who participated in the fieldwork during Survey III, and Amanda K. Pearce and Joanna M. L. Mackey for assistance in the preparation of the data for this paper. Finally many amateur botanists have freely shared their knowledge of the local flora and I have relied heavily on their expertise. The work on rare species was instigated in 1971 during a three-year research fellowship within the Botany Department of the University of Sheffield. However, the majority of the study has been carried out in the Unit of Comparative Plant Ecology under NERC research contract F60/G2/I 1.

Commonness and rarity in plants

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REFERENCES A1-Mufti, M. M., Sydes, C. L., Furness, S. B., Grime, J. P. & Band, S. R. (1977). A quantitative analysis of shoot phenology and dominance in herbaceous vegetation. J. Ecol., 65, 759-91. Anderson, P. & Shimwell, D. (1981). Wildflowers and other plants of the Peak District, an ecologicalstudy. Ashbourne, Derbyshire, Moorland Publishing Co. Armstrong, A. L. (1956). Prehistory--Palaeolithic, Neolithic and Bronze Ages. (In Linton 1956, 90-110.) Balme, O. E, (1953). Edaphic and vegetational zoning on the Carboniferous Limestone of the Derbyshire Dales. J. Ecol., 41, 331-44. Bellamy, D. J., Bridgewater, P., Marshall, C. & Tickle, W. M. (1969). Status of the Teesdale rarities. Nature, Lond., 222, 238-43. Chambers, J. D. (1966). Economic development- Agriculture. (In Edwards 1966, 215 23.) Clapham, A. R. (ed.) (1969). Flora o[ Derbyshire. County Borough of Derby, Derby Museum and Art Gallery. Clapham, A. R., Tutin, T. G. & Warburg, E. F. (1962). Flora of the British Isles, 2nd edn. Cambridge, Cambridge University Press. Cory, V. (1972). Hatfield Chace and its reclamaHon. Harrogate, MAFF. Cronquist, A. (1968). The erolution and c lassOqcation ol~lqoweringplants. London and Edinburgh, Nelson. Diamond, J. M. (1975). Assembly of species communities. In Eeology and evolution of communities, ed. by M. L. Cody and J. M. Diamond, 342 444. Cambridge, Massachusetts, Harvard University Press. Duffey, E., Morris, M. G., Sheail, J., Ward, L. K., Wells, D. A. & Wells, T. C. E. (1974). Grassland ecology and wildlife management. London, Chapman and Hall. Edwards, K. E. (ed.) (1966). Nottingham and its region. British Association for the Advancement of Science, Nottingham. Derry & Sons Ltd. Engledow, F. & Amey, L. (1980). Britain'sJuture in farming." Principles o[poli~3' for British agriculture. Berkhamstead, Herts, Geographical Publications. Farey, J. (1813). General view of the agriculture of Derbyshire, 2. London, Macmillan. Fox, J. F. (1981). Intermediate levels of soil disturbance maximize alpine plant diversity. Nature, Lond., 293, 564-5. Godwin, H. (1975). The histoo' o[ the British )qora. A .[actual basis ,[or phytogeography, 2nd edn. Cambridge, Cambridge University Press. Grime, J. P. (1973). Competitive exclusion in herbaceous vegetation. Nature, Lond., 242, 344 7. Grime, J. P. (1979). Plant strategies and t'egetation proeesses. Chichester, John Wiley. Grime, J. P. & Hodgson, J. G. (1969). An investigation of the ecological significance of lime-chlorosis by means of large-scale comparative

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