Habitats of the British amphibians (3): river valley marshes

Biologic¢llConservation18 (1980) 281 287

HABITATS OF THE BRITISH AMPHIBIANS VALLEY MARSHES

(3): RIVER

TREVOR J. C. BEEBEE

School of Biology, University o f Sussex, Falmer, Brighton, Sussex, BNI 9QG, Great Britain

ABSTRACT

An attempt to assess the distribution and abundance of the common British amphibians in low-lying river valley marshes in part o f Sussex is reported. These areas contain large amounts o f freshwater habitat, mainly in the Jbrm o f drainage dykes which intersect otherwise continuous tracts o f (predominantly)pasture land. Indigenous amphibians were, however, rare: Triturus vulgaris was well-spread across some o f the areas examined but population densities were very low; Rana temporaria was very infrequent, and a single specimen ofTriturus helveticus was seen. T. cristatus and Bufo bufo were not encountered at all. On the other hand, the introduced f r o g Rana ridibunda was common and widespread in one sector of the marshes. Possible explanations f o r these observations are discussed.

INTRODUCTION

This paper reports part of a series of investigations into the use of various habitat types by amphibians in a defined area of Sussex, England. Previous studies have involved chalk downland (Beebee, 1977) and suburban parks and gardens (Beebee, 1979). The present work was carried out in the low-lying alluvial flood plains of the rivers Adur (east bank) and Ouse (west bank), as indicated in detail elsewhere (Beebee, 1977). The terrain is flat and intersected by numerous drainage dykes which ultimately outflow into the respective rivers through tidal sluices. True marsh is therefore now virtually absent, and the fields defined by the dykes are used for either arable or livestock farming. In general the study areas resemble the larger marshes further east in Sussex and Kent (Pevensey, Romney) and the fen district of the East Midlands and East Anglia. Within the study zone, four more or less discrete blocks of dyke habitat occur: two on the Adur (henceforth referred to as A1 and A2) and 281 Biol. Conserv. 0006-3207/80/0018-0281/$02"25 © Applied Science Publishers Ltd~ England, 1980 Printed in Great Britain

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TREVOR J. C. BEEBEE

tWO on the Ouse (O1 and 02). A1 is very small (about 50,000m 2) and near Shoreham; A2 is further north and larger, about 4 km 2. O1 is the largest block of all, between Newhaven and Lewes (about 13 km 2) and 0 2 is a narrow belt north of Lewes (about 2 km2). In total, dyke habitat constitutes only about 6-7 ~ of the area of Sussex under study. Also included in this survey were a small number of spring-line ponds along the boundaries of the chalk downs and alluvial plains. METHODS Because the marsh dykes are so extensive it was possible to examine only a sample of the total freshwaters available. Fifty sections, including all eight of the spring-line ponds present, were selected using a 1 : 25,000 ordnance map. In the case of the 42 dykes, this choice was entirely at random with the proviso that all 4 blocks of habitat should be about evenly covered. Each pond and dyke was visited twice; on the first occasion water samples were taken and a stretch about 50 m long netted from one end to the other to determine species of vertebrates present. Width and depth were measured, and major floral components noted. Terrestrial habitat in the vicinity of the dyke was recorded as well as the presence or absence of detectable water flow. Water samples were used later (within a few hours) for pH and conductivity measurements (Beebee & Griffin, 1977; Beebee, 1979). The second visits were by night with a torch, to extend observations on the fauna of the dykes. Fieldwork was carried out between mid-March and midMay, during which time spawn and tadpoles (anurans) or adults (urodeles and sometimes anurans) should be readily detectable in freshwaters if any species are present in the area. RESULTS Nature o f the habitat

General physical characteristics of the dyke and pond habitats are summarised in Table 1.70 % of the sites visited were essentially stagnant, nearly 20 % exhibited TABLE 1 PHYSICALCHARACTERISTICSOFDYKEHABITAT Data refer to the numbers of dykes in each category Flow

Dry None Slight Fast

Width (m)

6 35 5 4

0 1.2 5 >1-2-2.4 24 >2-4-3.6 7 >3.6 (ponds) 8

Depth (cm)

0-30 14 >30-6018 >60 12

pH

<6.0 6.1 7.0 7.1 8.0 >8.0

Ionic score (parts/ lO6)

0 7 34 3

0-600 8 600-120026 1200 1800 8 1800 2400 1 > 2400 1

'Slight' flow was defined by aquatic plants being orientated in the same direction; "fast" flow by conspicuous surface rippling.

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BRITISH AMPHIBIANS IN VALLEY MARSHES

visible water flow and 12 ~ were quite dry (usually due to extensive growth of Phragmites). Widths of dykes varied between 1 and 4 m, with almost 70 ~ in the range 1.2-2.4 m. Depths were from 14 cm to well over 1 m at the time of sampling. Although levels do drop during the summer, fluctuations are not usually severe and, for example, most dykes held water throughout the very dry summer of 1976. pH distribution was as might be expected in areas receiving run-off from adjacent chalk strata, and was similar to that seen in dewponds and garden ponds in the study zone (Beebee, 1977, 1979). Ionic scores, however, were significantly higher than those observed in garden ponds. The few very high recordings ( > 1500 parts/106, and up to 14,000 parts/106) were in sites clearly contaminated by brackish water, but it was notable that the majority of dykes and ponds were in a high range. Many of these were in and around the spring-line, and it seems likely that this water source contains large quantities of ions before entry into the marsh dykes. Ditches furthest away from the chalk hills (especially those in 02) had the lowest ionic scores, all less than 500 parts/106. By comparison, more than 90 ~o of garden ponds had ionic scores of < 600 parts/106 (Beebee, 1979). Table 2 gives an indication of the vegetation structure of the marshlands. Most of the terrestrial habitat surrounding the dykes and ponds remains as pasture; there has as yet been little change towards arable farming, unlike the situation in the fens TABLE 2 VEGETATION STRUCTURESON THE MARSHES

Terrestrial habitat Pasture alone Pasture and arable Arable alone Other

Aquatic habitat °/o cooer 33 5 1 5

0-25 > 25 50 > 50- 75 >75 100

Major types oJ flora 15 8 5 16

Grass mats Filamentous algae

22 19

Phragmites Lemna minor Callit riche Rorippa

18 9 4 9

Others

7

Data refer to the numbers of dykes and ponds (total) in each category, or to the number of dykes with a particular species. 'Other' terrestrial habitat includes copse-woodland and garden edge; "Other' dominant aquatic plants occurring only in one sample dyke included Lemna trisulca, Myriophyllum, Chara, Ranunculus.

where this trend has virtually reached completion and livestock farming is now almost non-existent (Cooke & Ferguson, 1976). Both cattle and sheep are still plentiful on the Sussex marshes in summer, while the fields usually lie fallow during the winter months. Trees and scrub are not common on the marshes (dykes replace hedges as field boundaries), though occasional outgrowths of hawthorn Crataegus monogyna in particular occur along the ditches but are heavily persecuted by farmers. The terrestrial scene is therefore of rather a uniform flat grassland, with vegetation less than 15 cm high in winter and rarely more than 30 cm (due to grazing pressure) in summer.

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TREVOR J. C. BEEBEE

Within the dykes there is enormous variation in the extent of vegetation and species present, between different dykes, different years and different times of year. Generally, the ditches resemble dewponds in that most of them have either very little or a lot of vegetation. Most abundant types in early spring are floating mats of terrestrial grasses, filamentous algae such as Spirogyra, and dead reed Phragmites stems from the previous summer. The majority of dykes even at this time of year contain more than one such abundant type.

Vertebrate fauna of the dykes Table 3 records the numbers of dykes in which various amphibian species were found and relates these to the occurrence of the most c o m m o n vertebrates in the dykes: sticklebacks Gasterosteus aculeatus and Pygosteus pungitius. The latter T A B L E 3" VERTEBRATEFAUNA OF THE MARSH DYKES No. offish per 50 m dyke or per pondcatch

Fish species Gasterosteus Pygosteus aculeatus pungitius

Amphibians present Triturus Triturus helveticus vulgaris

Rana temporaria

Rana ridibunda ° 3

0

39

22

0

2

0

1 5

2

12

l(P)

l(P)

0

l(P)

t%20 >20

3 0

6 4

0 0

2(G) 0

I(G) 0

2(P) 0

Data refer to the numbers of ponds or dykes in each category. Letters in brackets refer to the fish species cohabitating with the amphibian (P = Pygosteus, G = Gasterosteus). aRecords in this column are out of a total of only 13 samples in the area thus far colonised by R. ridibunda.

species was especially abundant, and out of the 44 sites investigated which held water, at least 26 held fish of one sort or other. In addition to sticklebacks, eels Anguilla anguilla were seen in two dykes and small roach Rutilus rutilus in a further three. A single female Triturus helveticus was caught in one dyke, the only record of this species (section A 1). T. vulgaris was found in three dykes and two spring-line pools, in areas A1 and O1; again all records are for single specimens. Although measurements were not made, it was evident that these newts were all very small specimens when compared with those normally encountered in other habitats. In only one ditch, in area O1, was evidence of Rana temporaria seen; 15 clumps of spawn were present at a site centrally placed in the alluvial plain. The same dyke also yielded one of the smooth newt records, but was not different in any readily visible way from m a n y other ditches where these species were not found. Set in pasture, it was about 2.5 m wide and 25cm deep with grass mats covering some 90~o of the surface.

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The scarcity of indigenous amphibians in the dykes confirmed observations, mainly in area O1, made during less systematic netting over the last 10 years. During this time, Triturus vulgaris has been caught very occasionally and spawn of Rana temporaria observed (in groups of 1--4clumps) in only three instances. A few clumps of spawn were present in a shallow pool adjacent to the marshes in area A2, but otherwise this extensive dyke system and area 02 failed to produce a single amphibian record. On the other hand, the alien Rana ridibunda is present in considerable numbers over a substantial area of O 1, into which a few specimens were introduced in 1973/74 (J. Robinson, pers. comm.). Of the 13 dykes sampled in the relevant area, adult or juvenile marsh frogs were present in six. It should be noted, however, that this species is particularly easy to record; males croak loudly at night, and adults bask in the sun at the edges of dykes during the day, jumping in conspicuously when approached. Nevertheless, there can be little doubt that Rana ridibunda is much more successful in this habitat than any of the indigenous British amphibians. Though the records are few, the data of Table 3 do not suggest that the distribution of sticklebacks relates in any way to that of any species of amphibians in the dykes.

DISCUSSION

At first sight the river valley marshes of Sussex seem to offer an ideal habitat for amphibians. Expanses of pasture are intersected by innumerable dykes containing adequate and permanent freshwater. Pasture has been thought of as much better terrestrial habitat for amphibians than arable land (e.g. Cooke & Ferguson, 1976) and suitable breeding sites could scarcely be more abundant: the areas studied in this survey included more than 100 km of dykes within less than 20 km 2 of farmland, and assuming the sample examined was representative (albeit less than 2 ~o of the total), nearly 40 ~o of these fell within the optimum range of depths for breeding sites of the indigenous anurans (8 35cm, Cooke, 1975; Cooke & Ferguson, 1976). Even more should be suitable for newts, yet the native species were either absent or rare on the marshes whereas an introduced frog was doing well. It is pertinent to ask why this should be so. Some possible reasons can be readily discarded: pesticides are used scarcely if at all on these marshes; direct human predation is minimal; and although the dykes are occasionally recut by machines, this happens to any particular ditch only once every 1lY20 years (only two of the randomly-selected dykes had been dredged the previous autumn, and this frequency of renovation was confirmed by local farmers). None of these explanations would, in any case, account for the recent spectacular success of Rana ridibunda on the marshes.

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T R E V O R J. C. BEEBEE

Although the dykes appeared fundamentally suitable for amphibians, the ionic scores did tend to be much higher than in the well-used garden and park ponds and conceivably this (or the presence of high concentrations of particular ions) could mitigate against successful larval development. Again, however, the success of Rana ridibunda requires some special explanation on such a hypothesis. Also, there is considerable evidence that the indigenous species prospered earlier this century on the fens and in some Sussex dyke systems (Smith, 1951 ; Menzies, 1962; Cooke & Ferguson, 1976). Toads Bufo bufo can certainly breed successfully at ionic scores of up to 10,000 parts/106 (Mathias, 1971 ; Hardy, 1974) and at least some of the other common species are probably similar in this respect. On the other hand, Triturus helveticus much prefers pools of low ionic score and its scarcity in the dykes may relate to this preference (Cooke & Frazer, 1976). The fact that fish and eels are widespread in the ditches may in particular be significant in relation to population densities of newts (Beebee, 1979). The abundance of Rana ridibunda today and common frogs and toads in similar places in the past suggests that such predators are probably not responsible for the scarcity of the native anurans. Nor can predation of the indigenous species by the insurgent marsh frogs be a major factor, because as yet the latter species has only spread over a part of the study area. Indeed, the native amphibians were apparently at their rarest in areas A2 and 02, neither of which is occupied by Rana ridibunda. Terrestrial habitat structure is known to be an important factor in amphibian distribution (Beebee, 1977) and the history of the marshes was investigated with this in mind. The Domesday Book reveals that 1000 years ago the villages of Rodmell and Pidinghoe, in area O1 and now separated from the sea by several kilometres of alluvial plain, were thriving fishing ports. Little change had probably occurred by 1587, at which time a report on coastline conditions prepared in anticipation of a Spanish invasion described the Sussex seashore as a wasteland of sandy marshes. Reclamation and drainage leading to the present situation began in Sussex, as in the fens, during the 17th century (described by Brandon, 1974). Thus earlier than 3-400 years ago the alluvial plains were subject to regular inundations by seawater and must have been totally inhospitable to amphibians. Even today the rivers Ouse and Adur are tidal up to well north of the study area and are banked above the fields to prevent flooding. Subsequent to this establishment of truly freshwater habitat, amphibians no doubt invaded the areas and are known to have been common until around the 1930s. Cooke & Ferguson (1976) have implicated the switch from pasture to arable farming as a major cause of amphibian declines on the fens; in Sussex, however, this change has not happened, yet the amphibians are nevertheless absent. One possible explanation is that even in the case of livestock farming, the intensity of grazing and land utilisation has increased over the last 40 years to the detriment of the amphibia. Photographs of the fens taken before 1940, and indicating the presence of rough pasture with areas of scrub and tall vegetation, lend some support to this possibility (Wentworth-Day, 1954; Darby, 1956). Analogous pictures of Sussex marshes could not be found, the earliest being post-war and

BRITISH AMPHIBIANS IN VALLEY MARSHES

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subsequent to a major intensification of farming effort: the impression they give is of a landscape similar to that seen today (Meynell, 1947). Rana ridibunda, a species which spends all its life in or near the Water, would not be expected to respond to such changes in terrestrial habitat and its recent success therefore becomes explicable. In conclusion, an important inference of this work is that although not quite as inhospitable as arable fields, pasture does not constitute good amphibian habitat in the absence of some secondary vegetation such as scrub or hedgerows. It may well be significant that four of the six newt records came from the tiny marsh area A 1, in which the pasture is very rough with much scrub vegetation and probably representing a relic of a more general pre-war situation. Of the native species, only Rana temporaria and Triturus vulgaris seem able to survive on the marshes generally, and then only in small numbers. T. helveticus and T. cristatus are very rare or absent. Small numbers of Bufo buJo have been seen in dykes at the very edge of other marsh systems during the past 10 years, but clearly this too is exceptional and none was observed during this study. The immense botanical and entomological value of the marsh dykes unfortunately does not, under the present regime of land use, extend to the indigenous amphibia.

ACKNOWLEDGEMENTS

I would like to thank Dr A. Cooke and Mrs M. Beebee for their invaluable assistance with this work, which was carried out on behalf of the Conservation Committee of the British Herpetological Society.

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