Botanical aspects of conservation management of chalk grasslands

Botanical Aspects of Conservation Management of Chalk Grasslands T. C. E. WELLS Lowland Grassland and Grass Heaths Division, The Nature Conservancy, Monks Wood Experimental Station, Abbots Ripton, Huntingdon, England

ABSTRACT

grassland, with their wealth of plant and animal communities, form the main components of the chalk The biological interest of chalk grassland is discussed ecosystems, but in this paper I shall be concerned with under three headings: the richness of the flora and fauna, the presence o f more than 60 species of plants in England alone the conservation management of the grasslands. The primary interest of chalk grassland to the conwhich are restricted to the chalk or other calcareous grasslands, and the concept that chalk grassland is a well-defined servationist is biological, although archaeological ecosystem. Management problems which arise mostly as a (Bowen, 1961; Whittington, 1962), geological, and result of the cessation of grazing are discussed. The effect of aesthetic, considerations are also of some interest and grazing and mowing on the flowering of the species, and on the establishment of annuals from seed, are noted. In general, have in the past influenced the selection of National grazing or mowing at the time when the dominant grass is Nature Reserves on the chalk in England. growing most rapidly is likely to be the most effective form o f management; but this may adversely affect other species (both plant and animal), and indeed the management o f each site requires individual consideration.

INTRODUCTION

Chalk deposits, laid down during the Cretaceous period, are found in many countries of northwestern Europe and are particularly widespread in southern England and north-west France. The soils developed above the chalk rock have a high content of free calcium carbonate and exchangeable calcium, with a highly alkaline reaction (pu range of 7.0-8.4 at more than 200 sites examined in England). Locally, superficial deposits of sand and gravel, glacial deposits of the clay-with-flints type, and surface leaching, may create soils with an acid reaction. The precise chemical and physical nature of the soil varies with slope and aspect (Perring, 1959), but in spite of these variations the soils have a high degree of uniformity and are usually described either as protorendzinas and rendzinas (Kubiena, 1953) or as rendzinas and calcimorphic loams (Avery, 1958). Partly as a product of the special physical and chemical properties of the soil, and partly as a result of land management, a characteristic association of plants and animals has developed on the chalk, which has for many years attracted the attention of the ecologist and conservationist. Woodland, scrub, and 36

THE BIOLOGICALINTERESTOF CHALKGRASSLANDS (i) Richness o f the Flora and Fauna o f Chalk Grasslands Lousley (1950) estimates that more than half of the 1,550 British species of seed plants and ferns are found on calcareous soils, there being an unusual proportion of rare species among this number. On the chalk, more than 300 species of flowering plants have been recorded, and descriptive accounts of chalk grassland by Tansley & Adamson (1926), Hope-Simpson (1941), Thomas et al., (1957), and others, provides lists of species commonly associated with this vegetation type. On the mainland of Europe, accounts of chalk grasslands in France by Liger (1959, 1961)and Rose (1965), and in Denmark by B6cher et al. (1946), clearly show that floristic richnessis a widespread characteristic, although the relative abundance of species, and actual species composition, may change with the geographical position of a particular grassland. Thus as one moves eastwards across Europe, chalk grasslands come to contain a higher proportion of the continental and southern continental elements in the flora, while conversly in the western chalk grassland in England, species of the oceanic western European element, such as Thesium humifusum, become more plentiful. The species composition of chalk grassland appears superficially to be remarkably constant, but closer examination reveals that the range of variation is high--a feature which is probably associated partly with land-use history and partly with climate. It has

Biological Conservation, Vol. 2. No. 1, October 1969-- 0 Elsevier Publishing Company Ltd, England--Printed in Great Britain

Wells: Botanical Aspects of Conservation Management of Chalk Grasslands been clearly demonstrated by Morris (1967) that considerable changes in the fauna of chalk grassland may be produced by relatively short periods of sheep grazing, or by enclosing previously grazed grassland; but changes in floristic composition do not occur with such rapidity. Information on the fauna of chalk grassland is sparse, but Duffey & Morris (1966) have shown that the number of species of insects associated with chalk grassland and chalk scrub is high, which supports the view commonly held that chalk grassland is one of the richest (in terms of numbers of animal as well as plant species) formations in Britain. (ii) Species Restricted to Chalk Grasslands or to These

37

the ecology of chalk grassland, for a comprehensive analysis of the chalk ecosystem. Of perhaps the greatest importance is the evidence from archaeological and land-use studies (see Bowen, 1961, for references) which suggest that some areas of chalk grassland in England may have been grassland for many centuries, possibly millennia: if this is true, they could be regarded as some of the oldest relatively stabilized ecosystems in western Europe.

MANAGEMENT OF CHALK GRASSLAND

(a) Effects Following Cessation of Grazing Chalk grassland is a biotic plagioclimax maintained Ten species of flowering plants are completely by grazing animals of which the principal kinds are restricted to the chalk formation in England. Six of sheep, cattle, and rabbits. The type and race of these (Iberis amara, Ajuga chamaepitys, Phyteuma animal that was used or allowed at any particular tenerum, Ophrys fuciflora, Orchis purpurea, and time was determined by farming traditions and econOrchis simia) are placed by Matthews (1955) in the omic conditions. The main conservation problem on continental southern element of the flora, while four chalk grasslands is caused by cessation of grazing. In (Bunium bulbocastanum, Seseli libanotis, Gentianella the absence of grazing, grasses, which usually make germanica, and Orchis militaris) are assigned to the up more than 60 per cent of the total herbage, increase continental element of the flora. Although all of these in leaf area to such an extent that they compete with species are rare in England, many are common on the the dicotyledons--principally for light, but also in chalk in France. The high proportion of species in some situations for water and nutrients. This leads to this group which occur in Kentish and other eastern the elimination of the low-growing perennials and of chalk grassland in England, strongly suggests that all annuals from the sward within a short period climatic factors are limiting their northern and western (usually within 4 years), although some broad-leafed range in the British Isles. This explanation has been herbs, particularly Cirsium acaule and Poterium put forward by Good (1928) and supported by Rose sanguisorba, are able to compete successfully in all & Gemu (1964). but the densest grasslands. Litter formed by the grasses More than 50 species of higher plants are confined plays a significant role in this process of eliminating to the chalk and other calcareous grasslands (e.g. on their competitors, the rate of breakdown being of the oolitic limestone, carboniferous limestone, etc.) particular importance. It is of more than passing interest in England, many of these being among the rarest that the grasses Bromus erectus and Brachypodium British plants. Examples are: Salvia pratensis, pinnatum, whose growth is responsible for the fastest Teucrium botrys, Cirsium tuberosum, Ophrys sphegodes, rate of loss of dicotyledonous species from ungrazed and Pulsatilla vulgaris. grassland in northwestern Europe, also produce large The phytogeographical significance of the chalk quantities of leaf-litter which are resistent to breakflora is further increased by the presence of 50 species, down by other organisms. some of which are widespread on the chalk while Eventually scrub, principally of Hawthorn others are rare, which reach their northern limit in the (Crataegus monogyna) but in some places of Dogwood British Isles on the carboniferous or magnesian lime- (Cornus sanguinea), Privet (Ligustrum vulgare), stone. Their presence on these different geological Juniper (Juniperus communis), Hazel (Corylus formations of varying chemical composition, makes avellana), Ash (Fraxinus excelsior), and Blackthorn them ideally suitable for studies of the factors con- (Prunus spinosa), may invade the grassland and begin trolling the distribution of plants. the succession to woodland. Although the changes which occur in ungrazed grassland are well known in (iii) Analysis of the Ecosystem general terms, there are surprisingly few quantitative Chalk grassland may be considered as a well-defined data available on the rate of accumulation of litter ecosystem with a wide geographic range, which is and its effect on the flowering of perennials or annuals, linked biologically with other calcareous grassland although the observations of Morris (1969) on the ecosystems. This makes it possible to use data obtained faunas of ungrazed grassland provide some data for from studies in many countries~ on different aspects of selected species of invertebrates,

and Other Calcareous Grasslands

38

Biological Conservation

Information obtained from a cutting experiment on the Barton Hills in Bedfordshire suggests that the time of year when grazing ceases, and the depth of soil on the slope, may both be important factors in determining the initial rate of accumulation of litter. In this experiment, a heavily-grazed chalk grassland in which Bromus erectus was the dominant was enclosed in May and sampled by cutting small areas 55, 139, and 325, days after enclosure. Samples were taken from the grassland over 3 soil depths, the results being given in Table I.

after 2 years of excluding sheep. After 4 years of enclosure, flowering in most species was greatly reduced, so that it was only the more vigorous competitors, such as Poterium sanguisorba and Filipendula vulgaris, that were able to produce a few flowers. Wells (1968) has also shown that the number of flowers of Pulsatilla vulgaris can be greatly reduced by enclosure.

(b) Effects of Graz&g The management of chalk grassland by grazing sheep and cattle has previously been discussed in TABLE 1 general terms (Wells, 1965). Livestock have three Dead Plant Material (in gm/m 2 at 80°C) in Samples main effects on the vegetation: they selectively deTaken from Ungrazed Grassland 55, 139, and 325, Days foliate the grassland, they trample the vegetation, After Enclosure over 3 Depths of Soil and they return nutrients to the ecosystem in the form of dung and urine. It is well known that sheep Days after cessation of Soil depth (cm) are more selective grazing animals than cattle, and grazing 5 13 i8 that differences in the method by which sheep and cattle bite vegetation may produce grasslands which 55 (8 July) 2.98 2"12 1.49 are structurally different. In general, at high stocking139 (1 October) 4.97 2.13 5.00 325 (4 April) 20'73 2 4 - 8 7 44"14 rates sheep produce a short, evenly cropped turf, whereas cattle produce a mosaic of tufts and closelygrazed grassland. However, some cattle--for example Death of the above-ground parts of plants in chalk Ayrshires grazing on chalk grassland in Wiltshire-grassland following enclosure was very limited, death have been observed to produce a species-rich turf of the lower leaves of grasses during the winter months which is evenly grazed, and which is similar to that being responsible for most of the litter accumulated in produced by sheep. the following spring. It was noticeable that forbs contributed little to the litter in the following spring, The value of general statements concerning the which suggested that their decomposition and break- grazing effects of different animals is limited, for as down occured more readily than in the case of grasses. Arnold (1964) has shown, grazing is a complex activity This would accord with the structure of grasses, which with many variables of which only a few can be conconsist in large part of cutinized epidermis and trolled at any one time. Nevertheless, experiments siliceous material that is resistant to decay. The pro- along these lines have been attempted by agriculductivity of grassland on the deeper soils was higher turalists, primarily with sheep and cattle on grassthan on the thin soils, which led to corresponding lands in Britain, though nearly all of them have been differences in litter production. on sown grassland, and little attention has been paid The enclosure of previously heavily-grazed down- to the effects of grazing on semi-natural pastures land resulted, in the first year of enclosure, in a profuse except in hilly and mountainous districts where flowering of most species, which contrasted strongly 'marginal land' is frequent. However, an exception is with the behaviour of plants in the grazed areas. How- provided by the replicated experiment done on chalk ever, the enclosure of previously grazed grassland for grassland at Hurley, Berkshire, which had the object a period as short as 3-4 weeks in May, June, or July, of investigating the effect of different systems of was often long enough to enable many species, management on the botanical composition of the such as Campanula rotundiJolia and Helianthemum sward, using cattle. In a 4-years experiment in which chamaeeistus, to flower. Morris (1967) has shown the treatments as fundamentally different as continuous importance of this brief respite from grazing to those grazing, frequent grazing, and rotational grazing, were species of insects which pass part of their life-cycle in compared, Kydd (1964) showed that changes in flowers. ground-cover under the different treatments were small, The beneficial effect of enclosure on the flowering of and there was no profound change in botanical many species may continue into the second year of composition during the 4 years. However, the status enclosure, although on the Barton Hills there were of species in the sward did alter during this period. approximately two-and-a-half times as many flowers in Holcus lanatus, which is tolerant of heavy grazing, the grazed areas as there were within the enclosure increased in all the grazed plots in 4 years. In the

Wells: Botanical Aspects of Conservation Management of Chalk Grasslands ungrazed controls, total grass cover increased from 37 per cent to 79 per cent in the 4 years, with a corresponding fall in the cover of herbs. Arrhenatherum elatius increased considerably, while Trifolium repens was eliminated from the ungrazed sward after 2 years. Cattle are being used increasingly on the chalk in southern England for grazing steep scarp slopes which have been ungrazed for 20 or more years. The immediate effect of this is beneficial in that the height of the sward is reduced and much of the accumulated litter is broken up. However, the heavy treading of cattle soon leads to localized areas of erosion, which are especially apparent where cattle congregate for shelter or for feed. Similarly, sheep can cause erosion on steep slopes when their numbers per hectare are high. The effect of sheep on chalk grassland has been studied experimentally since October 1964 at Aston Rowant National Nature Reserve, Oxon, on areas of coarse grassland which have been invaded by scrub. Border-Leicester x Cheviot sheep are grazed in paddocks at either 0.4 or 1-2 sheep per hectare for 51 months regularly each year during spring and winter. Changes in the structure and botanical corn-

39

position of the grassland are measured twice a year. Photographs from fixed points taken twice a year provide a pictorial record of changes in the structure of the grassland (Figs 1 and 2). After 4 years, the most outstanding result of the trial is the effect of grazing on the structure of the grassland. In plots in which Arrhenatherum elatius and Brachypodium sylvaticum were dominants, the average height of the vegetation has been reduced from 47 cm to ca 7 cm in plots grazed at 1.2 sheep per hectare, and to about 15 cm in plots grazed at 0.4 sheep per hectare. In plots in which Festuea ovina was the dominant, corresponding changes in height have been noted. Litter, which was considerable at the beginning of this experiment in all plots, has been broken down as the result of treading. Heavy grazing has caused a change in the habit of growth of Arrhenatherum from a coarse, tussock grass to one in which small green leaves are scattered in a predominantly F. ovina grassland. The effect on Brachypodium sylvaticum of heavy sheep-grazing has been less marked, as this unpalatable grass is only eaten at the end of the period of grazing when other herbage is not available. Even then, only the tips of the leaves are grazed. In plots containing a

Fig. 1. Aston Rowant, Oxon, England. Plot $4 immediately before grazing, 22 October 1964. Dense Arrhenatherum and Brachypodium sylvaticum grassland, with scrub of Viburnum, Cornus, and Crataegus,

40

Biological Conservation

Fig. 2. The same plot as shown in Fig. 1, but photographed on 15 June 1968 after grazing by Border Leicester × Cheviot Sheep at 1"2per hectare for 5½ months each year since 1964. Note the change in the structure of the grassland, and how the lower scrub has been eaten and has decreased in height but some of the taller bushes have grown unchecked.

mosaic of grassland communities, it is noticeable that areas of Festuca ovina grassland are grazed initially in preference to areas of Arrhenatherum, whereupon the latter are grazed in preference to Brachypodium sylvaticum. Considerable changes in the numbers of annuals, especially of Euphrasia nemorosa and Crepis capillaris, have occurred in the grazed plots. One hundred 2 0 c m 2 quadrats per acre were thrown at random in each plot and the presence (or absence) of each species in each quadrat was noted. These results are expressed as percentage frequency (the number of occurrences of an individual in 100 samples) and are shown in Table II. Although at the start of this experiment there were considerable differences in the numbers of annuals in each plot, evidently associated with the earlier history of the site, there has been a general increase in the numbers of annuals in the grazed plots when compared with changes in the control plots. The data also indicate that if a species is not present in a plot initially, its rate of spread is slow even though conditions for

TABLE II

Changes in the Frequency ofEuphrasia nemorosa during 1964-68 in the Control Plots (Ungrazed) and in Plots Grazed at 1.2 and 0.4 Sheep per Hectare Controls (ungrazed) Plot No. C1 C3"

1964 1965 1966 1967 1968

0 0 0 0 0

0 4 12 8 8

Grazed at 0.4 sheep~ha

Grazed at 1.2 sheep~ha

S1 $3 $5

$2 $4 $6

0 2 0 8 1 6 3 24 4 18

7 4 12 18 39

3 3 9 15 8

12 24 36 39 58

0 0 0 2 2

* Localized rabbit scratching occurred in this plot. establishment may be optimum. It is suggested that the short turf and areas of bare ground created by sheep treading and grazing, provide suitable habitats for the successful establishment from seed of annuals. Similar results have been obtained with Linum

41

Wells: Botanical Aspects of Conservation Management of Chalk Grasslands catharticum (another small annual) in grazed downland on the Barton Hills, Bedfordshire. In plots grazed at 1.2 sheep per hectare, buds, leaves, and twigs, of Viburnum lantana (Wayfaring Tree), have been heavily browsed (Fig. 3), and this has controlled the further growth of this shrub although it has not so far killed many bushes. In plots grazed at 0-4 sheep per hectare, this Viburnum was still browsed, although grasses were never in short supply--a result which indicated that a selection for Viburnum was being made by the sheep. Other shrubs contributing to the scrub, such as Hawthorn, Dogwood, and species of Rosa, were only lightly browsed even in the heavily-grazed plots. Changes in the grazed plots of species other than annuals have been small, the results obtained being similar to those from the Barton Hills (unpublished results). These results suggest that chalk grassland is relatively stable when grazed, and that changes in botanical composition are only to be expected when grazing animals are excluded or excessively heavy grazing leads to erosion. The effect of grazing on flower production varies with the kind of grazing animal and the species of plant that is being grazed. In a long-term study of fluctuations in a population of the orchid Spiranthes spiralis (cf. Wells, 1967), the following damage to inflorescences of this plant caused by rabbits, cattle, and sheep, were noted:

Fig. 3. Aston Rowant, Oxon, 11 June 1965. Heavily grazed Viburnum lantana: twigs, buds, and leaves, have all been eaten. (Scale." the surveyors" pole is divided into 50-cm lengths.)

TABLE 111

Damage to the Inflorescences of Spiranthes spiralis during 1962-68 Caused by Herbivores

No. of inflorescences No. of damaged inflorescences Percentage suffering damage Grazing animal

1962

1963

1964 1965

122 12 10 R

449 313 3 94 1 30 None S

250 56 22 Bx

1966

1967

1968

773 66 9 Bx

640 98 15 Bx

557 90 16 Bx

* R = Rabbits, S = Sheep, B = Bullocks, x = in addition, a small amount of rabbit grazing. In a similar study of the Man Orchid (Aceras anthropophorum) and Musk Orchid (Herminium monorchis) in Bedfordshire, damage from rabbit grazing varied considerably between these two species on the same site (Table IV). Rather unexpectedly, the more conspicuous and taller Man Orchid received only 14 per cent damage in 1968 in contrast to the 62 per cent damage recorded to inflorescences of the smaller, inconspicuous Musk Orchid. This suggests that rabbits may be selectively

grazing the Musk Orchid, although other factors, such as the proximity of rabbit warrens to the orchid sites, may affect the degree of damage sustained by each species.

(c) Effects of Mowing Mowing of chalk grassland, as an alternative system of management to grazing, is an attractive proposition. The cost of fencing, provision of water, buying of sheep and cattle, and daily visits to livestock, is high,

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Biological Conservation TABLE 1V

Percentage Damage to Inflorescences of Man Orchid and Musk Orchid at a Site in Bedfordshire during 1966-68

INo. of inflorescences Man Orchid ~No. of damaged inflorescences Percentage damaged INo. of inflorescences Musk Orchid lNo. of damaged inflorescences Percentage damaged

whereas the only capital cost in mowing is the purchase of a mowing machine. Moreover, more easily than grazing, defoliation by mowing may be carried out at a precise time in relation to the growth-cycle of important species. Unfortunately, mowing is only possible on relatively gentle slopes at the present time, while the greater proportion of existing chalk grasslands are on steep scarp slopes. However, the recent development of cutting machines which float on a cushion of air (the Hovercraft principle), and which are capable of operating on uneven terrain on steep slopes, makes the feasibility of cutting in these conditions more likely in the future. The principal difference between mechanical cutting and grazing is that the mowing machine is nonselective in defoliating and removes all vegetation above a certain level. There is also no localized return of nutrients as dung and urine, although there is a uniform return of nutrients if the cut material is discharged by the machine in a finely-divided form. Although there have been many cutting experiments performed on leys, there have been few investigations of the effects of cutting on floristically complex grassland. Warne (1934), in a trial on the Wiltshire Downs, compared cutting at 3-weekly intervals with grazing by sheep--with and without the application of fertilizers. His main conclusion was that cutting without the application of fertilizers had the same effect as grazing by sheep, but that cutting with fertilizer application increased the proportion of gr asses at the expense of the forbs. In a cutting experiment which was laid down on the Barton Hills in 1963, in which 7 treatments were used (including testing of the effects of the season and intensity of cutting), cutting 3 times a year (in April, July, and September) produced a turf that was similar in botanical composition to adjacent downland grazed at about 1.2 sheep per hectare for 9 months of the year. Some control of the dominant grass, Bromus erectus, was achieved by cutting only once, in April, in contrast to cutting in July and September, although

1966

1967

1968

93 7 7 274 27 10

85 15 17 87 21 24

98 14 14 34 21 62

better control was obtained by cutting 3 times a year. A similar result has been obtained from another experiment at Knocking Hoe National Nature Reserve, where cutting in May, June, and July, of a sward dominated by Bromus erectus, has effectively reduced the competitive power of the Bromus without adversely affecting other species. In this experiment, in which a split-plot design was used, the return of nutrients in a dried, finely-ground form to half of each plot, while the cut material was removed from the other halves, has had no effect on either total drymatter production or species composition after 3 years. It is important for the establishment of annuals from seed that cut vegetation be either removed or returned in a finely-shredded form, so that areas of bare ground are available upon which seeds can fall and germinate.* With such return of cut materials the results are illustrated in Table V, which shows the performance of Linum catharticum at Knocking Hoe, Bedfordshire, in the control plots (not cut) and in plots which were cut once, twice, or three times, a year.

DISCUSSION

It is now widely accepted that if the biological interest of chalk grasslands is to be maintained, some form of management, employing grazing animals or a substitute for grazing such as mowing, must be used. This simple concept is readily appreciated; but the richness of the chalk flora and fauna, and the complex inter-relationships between the plants and insects and grazing animals, are not fully understood. Consequently any proposal for management must be based, at the present time, on very inadequate data obtained from observations and simple experiments. Data from four * On the other hand, according to the author (in litt.), if at all coarse 'material is left on the ground as cut, seeds of annuals are unable to reach the soil surface and germinate and, also, there is considerable shading of plants beneath the cut area'.--Ed.

Wells: Botanical Aspects of Conservation Management of Chalk Grasslands

43

TABLE V

Numbers of Linum catharticum Plants in 1.5 × 1.2 m Plots at Knocking Hoe, Bedfordshire. Uncut Controls are Compared with Plots Cut Once Annually (in May), Twice (in May and June), and Three Times (in May, June, and July). Each value is a mean of four replicates

Cut 1966, recorded 4 June, 1967 Cut 1967, recorded 4 July, 1968

Control (not cut)

Cut once

Cut twice

Cut thrice

1.6 0.6

12.7 44.3

72.2 120.1

89.3 128.6

experiments suggest that in grazed chalk grasslands the perennials (which make up the bulk of the vegetation) are relatively stable, and it is only the annuals which fluctuate greatly in quantity. Similarly, the evidence that is available from cutting experiments also suggests stability in cut grassland. However, in grassland which contains species of high phytogeographic interest, or species which for other reasons are 'rare', it may be necessary to limit grazing or cutting to periods when these species are not flowering, or to a stage in their life-cycle when they are not adversely affected by these treatments. The immediate problem is to obtain information leading to a management programme which will (a) prevent the continued invasion of undergrazed grassland by coarse grasses, and (b) increase the floristic diversity of previously ungrazed grassland. An important difference between ungrazed and grazed grassland is the long-term effect of these treatments on the botanical composition of the sward. In grasslands that are not grazed for ten ormore years, many species will be lost, and even if this grassland is eventually grazed, many of these species appear to be unable to re-establish themselves. On the other hand, although heavy grazing prevents many species from flowering, it rarely causes a reduction in the total number of species present in an area and, when the grazing pressure is eased, these species are able to flower and reproduce. Available evidence suggests that cutting or grazing at the period when the dominant grass is making its most rapid growth is likely to be most successful, although grazing at any time of the year is beneficial, as it prevents the build-up of additional litter and helps to break down litter which has already accumulated. It may be beneficial to burn off the accumulated litter in ungrazed grasslands before grazing, and this was done successfully on the Barton Hills in 1954; but a world of caution is needed here, as the effect on the fauna of this treatment has not been investigated, and furthermore it is known that Brachypodium pinnatum spreads rapidly in burnt grasslands,

Although short-term mowing experiments have shown that coarse grasses can be controlled by these means, and no unduly adverse effects have been noted, the long-term effects may well be undesirable-especially the continual drain on nutrients from the ecosystem if cutting and removal of herbage is practised. However, the modern rotary machine floating on a cushion of air, which shreds and redistributes the cut material in small fragments on the grassland, may be acceptable, although long-term experiments are still desirable to settle this question. The scientifically based management of chalk grassland in the future depends principally on what Dr A. S. Watt has simply called 'observations on the biology of individual species'.

ACKNOWLEDGEMENTS

Dr E. Duffey, Dr M. Hooper, and Mr D. A. Wells, kindly criticized a draft of this paper and made many useful suggestions. Mr P. Wakeley was responsible for the photographic recording of the Aston Rowant experiment. References

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Biological Conservation

GOOD, R. D'O. (1928). Notes on a comparison of the angiosperm floras of Kent and pas de Calais. J. Bot., London, 66, 253-64. HOPE-SIMPSON, J. F. (1941). Studies of the vegetation of the English Chalk. VII. Bryophytes and lichens in chalk grassland with a comparison of their occurrence in other calcareous grasslands. J. Ecol., 29, 107-16. KUBIENA, W. L. (1953). The Soils o f Europe. Murby, London, 318 pp., illustrated. KYDD, D. D. (1964). The effect of different systems of cattle grazing on the botanical composition of permanent downland pasture. J. Ecol., 52, 139-49. LIGER, J. (1959). Remarques sur la v6g6tation des falaises de l'estuaire de la Seine. Revue Socs say. Hte Normandie, 13, 3-15. LIGER, J. (1961). V6g6tation des pentes crayeuses de la vall6e de la Varenne. Revue Socs say. Hte Normandie, 14, 53-73. LOUSLEY, J. E. (1950). Wild Flowers o f Chalk and Limestone. Collins, London, xvii + 254 pp., illustrated. MATTHEWS, J. R. (1955). Origin and Distribution o f the British Flora. Hutchinson, London, vi + 176 pp.

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Charles Darwin and Conservation in the Galfipagos Islands

Although a perusal of 'The Origin of Species' reveals how strongly influenced Darwin was by his study of oceanic islands, especially those of the Gal~.pagos Archipelago, some workers have questioned whether or not his visit to the Galfipagos in 1835 served as the initial catalyst for his views on the mutability of species and natural selection. For example, L. C. Eiseley has remarked (Scientific American, Feb. 1956), 'Whether his visit to the Galfipagos Islands was the single event that mainly led Darwin to the central conceptions of his evolutionary m e c h a n i s m . . , is a moot point upon which Darwin himself in later years shed no clear light.' However, Charles Darwin himself, in his Evolutionary Notebook, stated as early as 1837, 'In July opened first notebook on "transmutation of species". Had been greatly struck from about month of previous March on character of South American fossils, and species on Gal~pagos Archipelgo. These facts origin (especially latter), of all my views' (Francis Darwin, Ed., The Foundations o f the Origin o f Species, Cambridge, 1909).

Again, in his letter of 11 January 1844 to J. D. Hooker, Charles Darwin writes, 'I have been now ever since my return engaged in a very presumptuous work, and I know no one individual who would not say a very foolish one. I was so struck with the distribution of the Gal~pagos organisms, &c. &c., and with the character of the American fossil mammifers, &c. &c., that I determined to collect blindly every sort of fact, which could bear in any way on what are species . . . . At last gleams of light have come, and I am almost convinced (quite contrary to the opinion that I started with) that species are not (it is almost like confessing a murder) immutable' (Francis Darwin, Charles Darwin, His Life Told in an Autobiographical Chapter and in a selected series o f his published letters, Appleton, New York, 1892). This letter in fact probably represents the first announcement of the views that would later shake the world. Eiseley (Darwin's Century, Doubleday, Garden City, N.Y., 1958) also points out later the importance of the Galfipagos experience in the shaping of Darwin's ideas which finally led to the publication of the The Origin o f