J . Zool., Lond. (1974) 172,289-302

The structure and diversity of the animal communities in a broadland reedswamp

C . F. MASON AND R. J. BRYANT School of Biological Sciences, University of East Anglia, Norwich

(Accepted 5 September 1973)

(With 1 figure in the text)

The fauna of a Phragmites reedswamp at Alderfen Broad, Norfolk, is described in terms of structure and diversity. Three transects were laid parallel to the water's edge at 1 m (TI), 10 m (7'2) and 25 m (T3) from the reedswamp/open water interface. Twenty samples were taken randomly along each transect in July and all the macroinvertebrates in the samples were collected. The pH at each site was measured. The pH fell from 8.4 at TI to 6.4 at T3.

The fauna of the three transects was analysed by recurrent groups and by plotting the log abundance against the frequency for each species and dividing the resulting graph into units in terms of abundance and frequency. Both methods gave very similar results but the recurrent groups analysis only gave information on the most frequent species. The recurrent groups of the three transects had no common elements. The abundance/frequency analysis is simple to perform and could be of considerable use in evaluating sites for conservation purposes.

Transect T2 had the greatest species diversity, equitability and evenness, T3 the least. Reasons for this are discussed.

Contents

Introduction. . . . . . . . Study area . . . . . . . . Methods . . . . . . . . Results . . . . . . . .

Fauna of the reedbed .. Structure of the communities Species diversity . . . .

Discussion . . . . . . . . References . . . . . . . . Appendix . . . . . . . .

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Page 289 290 291 291 29 1 292 296 297 299 301

Introduction Reedswamps are amongst the most productive plant communities in temperate regions

(Westlake, 1963) and often cover extensive areas. Little work has been done, however, on the animal communities within reedswamps, largely because of sampling and taxonomic difficulties.

Before a useful understanding of the functional interrelationships of an animal com- munity can be obtained, it is essential to know the structure of that community. The work presented here is a preliminary description of the structure and diversity of the animal communities within a reedswamp at Alderfen Broad, Norfolk.

289

290 C . F . M A S O N A N D R . J . B R Y A N T

Study area Alderfen Broad (National Grid Ref. TG354196) is a small, shallow eutrophic lake

(area 5.3 ha, depth 0.8 m) overlying fen peat and fed by a small inlet. It was formed by the flooding of peat diggings during medieval times (Lambert et al., 1960). The Broad used to have a rich and productive flora of floating and submerged macrophytes, but these dis- appeared in the early part of the last decade. The benthic fauna is now also poor, and consists almost entirely of oligochaetes and chironomids. The eastern edge of the Broad (a length of some 550m) is bordered by reedswamp while the remainder is surrounded by alder carr.

I

FIG. 1. Sketch map showing the position of the transects in the reedswamp of Alderfen Broad (inset shows Alderfen Broad).

The reedswamp covers an area of approximately 1-6 ha and is dominated by Phragmites communis Trin, with an average density of 250 stems/m2. Along the water’s edge is a band of Typha angustifolia L., in places 1 m broad, but usually intermixed with Phragmites. Scirpus lacustris L. and Epilobium hirsutum L. also occur in patches at the water’s edge.

The reedswamp has been unmanaged for some years and contains a varied flora. It was previously cut in rotation for reed. The wetter parts (up to 10m from the open water) contain Bidens cernua L., Mentha aquatica L., Solanum dulcamara L., Peucadanum palustre (L.). Moench, Lycopus europaeus L., Galium aparine L. and Carex appropinquata Schumacher and as the swamp becomes drier (10 m from open water) Thelypteris palustris Schott., Festuca pratensis Huds and Rumex palustris Sm. appear. At the upper end of the

STRUCTURE OF A REEDSWAMP F A U N A 29 1

reedbed (20 m from open water), Iris pseudacorus L., Rubus fruticosus agg., Urtica dioica L., Valeriana dioica L., Cirsitim palustre (L.) Scop. and Agrostis tenuis Sibth. occur. There are scattered, small trees (2 m high) of Ahus glutinosa (L.)., Gaertn., Salix cinerea L. and Betula pendula Roth., and Alnus reaches a height of 6 m at distances greater than 30 m from the water, on the upper edge of the swamp.

The floor of the reedswamp is densely carpeted in places with the mosses Sphagnum spp. Acrocladium cuspidatum (Hedw.). Limb. and Mnium afine Bland. Open, muddy puddles within the swamp support Lemna minor L.

During the summer, standing water penetrates up to 1.5 m into the reedbed. Moving landwards the swamp becomes progressively drier, but there are puddles of water up to 20 m from the open water. Beyond this, the water table is still just below the surface of the ground and boot-prints rapidly fill with water. The swamp averages 30 m broad from the open water to the land-ward edge. Arable farmland slopes down to the swamp and the two are separated by a dyke 0-5 m high.

Methods Three transects, parallel to the water’s edge, were laid out at three sites within the reedswamp.

Their position is shown in Fig. 1. They were:- Transect I (TZ), 1 m in from the interface between swamp and open water, in standing water

overlying organic mud and detritus. Transect 2 (T2), 10 m from the water’s edge, in an area of soft organic mud and detritus, with

puddles of standing water and many mosses. Transect 3 (T3), 25 m from the water’s edge, 5 m from the upper edge of the reedbed, in an

area of soft mud and detritus, with no surface water. The transects were 100 m long, marked at 10 m intervals. They were further divided by eye

into 0.5 m intervals, but were not physically marked as reedswamp is extremely sensitive to damage. Twenty samples of mud and detritus were taken at random intervals along each transect, using a table of random numbers, during the first week in July 1971. Samples were taken with a pond-net (mesh size 20 mesheslinch), and each consisted of 2 1. of material.

In the laboratory each sample was sieved (sieve aperture 0-25 mm) to remove silt and sorted in water in a white tray under strong light. All animals were removed. Flatworms and leeches were identified immediately while all other animals were preserved in 70 % alcohol for later identification.

Samples of water were taken from each transect for the determination of pH. The water from TI and T2 was taken from the standing water on the site, that from T3 was subsurface water which rapidly filled a hole left on taking a net sample.

Results Fauna of the reedbed

A total of 4359 animals was collected from TI ; 1252 animals from T2; and 1152 animals from T3. A complete list of organisms identified, together with authorities, is given in Appendix I. Difficulties in identification were encountered with the Oligochaeta, Ostracoda, Chironomidae larvae and Hydrachnellae, and these groups are given the same “weighting” as “species” in the ensuing analysis. A series of 130 oligochaetes were later identified as Lumbriculus variegatus and Rhynchelmis limosella in proportions of 7.6 : 1 respectively over all transects. A single Stylaria lacustris was found in T1. Identification of five Hydrachnellae included Eylais hamata and three Arrenurus sp.

292 C. F . MASON A N D R . J . BRYANT

Treating the difficult groups as “species” as above, 60 species were obtained from TI; 74 from T2; and 51 from T3. The distribution of the major groups of organisms over the three transects, together with the pH is given in Table I. The pH dropped as the distance from the water increased from pH 8.4 at TI to 6.4 at T3. Similarly the Hemiptera decreased in importance from TI to T3. The Coleoptera and Crustacea were numerically most important in the central area of the reedbed (T2) while the Oligochaeta and Diptera

TABLE I ThepH of the water andpercentage abundance of the major groups of organisms

at three sites in a Phragmites reedswamp

Site TI T2 T3

Distance from open water (m)

Oligochaeta Mollusca Crustacea Hemiptera Coleoptera Diptera Other groups

PH 1 8.4 1.3 3.6

13.5 50.3

0.3 19.9 11.0

10 7.3

13.9 4.8

21.1 0.2

35.8 20.6 3.6

25 6.4 2.9 2-9 3.1 0

19.3 56.6

I .7

formed an increasingly larger proportion of the fauna at greater distances from the open water. Of 110 species in TI and T2, 28 (25.5 %) were common to both transects. Of 103 species in TI and T3, only 16 (15.5 %) were common to both, and of a total of 87 species in T2 and T3, 39 (44.8 %) occurred in both transects. There is thus a greater uniformity in the fauna of the upper part of the reedswamp.

The structure of the communities The sampling procedure gave information on the numerical abundance of each species

in each transect and on the frequency of occurrence of each species (i.e. the number of samples in which the species occurred). Using these two parameters the structure of the communities was analysed in the following way :- (1) For each transect the total number of individuals (n) of each species was determined. (2) For each transect the total number of individuals ( N ) was determined (i.e. Zn). (3) A relative abundance index was then calculated as

nx1000 N

(4) A graph of log relative abundance against frequency of occurrence was plotted for each transect. The resulting graph was then arbitrarily divided in the following way:

(5) Vertical lines were drawn at frequencies of 33.3 % and 66.7 %. This subdivided the community into three equal levels of frequency. A-species occurring in >67 % of samples B-species occurring in 33-67 % of samples C-species occurring in (33 ”/, of samples.

STRUCTURE OF A REEDSWAMP F A U N A 29 3

T A B L E I1 The structure of the animal communities of the Alderfen reedswamp determined from the abundancelfrequency

characteristics. The relative abundance is given in brackets

Group TI T2

A1

A2

A3

A4

BI

B2

B3

B4

CI

c 2

c 3

Sigara (nymphs) (430.83) Chironomidae (1 54.85)

Planorbis albus Asellus aquaticus Asellus meridianus Ostracoda Sigara dorsalis Sigara falleni Cyrnus flavidus Chaoborus crystallinus

(16.75) (32.12) (34.64) (11.93) (35.10) (3212) (49.32) (44.05)

Eurycercus lamellatus (6.19) Caenis horaria (8.95)

(0)

(0)

Oligochaeta (13.81)

Erpobdella octaculata Bithynia tentaculata Physa fontinalis Sphaerium coreum Gammarus pulex Caenis robusta Agrypnia pagetana Mystacides longicornis Gyrinus substriatus Hydrachnellae

(4. I 3) (459) (3.44) (2.79) (4.1 3) (3.67) (4-36) (3.90) (2.29) (7.57)

(0)

(0)

Daphnia cucullata (49.32)

Planaria torva Erpobdella testacea Glossiphonia complanata Vafvata piscinalis Ceriodaphnia reticulata Daphnia longispina Sida crysfallina Cloeon dipterum

(1.61) (2.98) (1.61) (3.44) (1.38) (5.28) (1.15) (1-84)

Oligochaeta (138.98) Helodidae (Cyphon)

larvae ( 1 63.74) Ptychoptera albimana (1 11.82)

Planorbis albus (1 9.17) Asellus aquaticus (67.09) Asellus meridianus (67.09) Gammarus pulex (59.90) Hydroporus angustatus (31.95) Hydroporus Iineatus (61.50) Chironomidae (42.33)

(0)

(0)

Hydroporus palustris ( 1 1.98) Hygrotus inaequilis (13.58) Peltodytes caesus (14.38) Bezzia spp. (11.98)

Glossiphonia complanafa (799) Cyphon ochraceous (7.99)

Daphnia cucullata Theobaldia annulata

Polycelis tenuis Haemopis sanguisuga Helobdella stagnalis Bithynia tentaculata Lymnuea palustris Physa fontinalis Planorbis contortus Planorbis vortex

(0)

(0)

(14.38) (14.38)

(5.59) (1.60) (2.40) (5.59) (1.60) (1.60) (8.79) (1.60)

T3

Oligochaeta (1 58.85) Cyphon larvae (137.15)

(180.56) Pericoma nubila Ptychoptera albimana (151.91) Chironomidae (1 21.53)

Prionocera turcica (75.52)

Planorbis albus Asellus aquaticus Asellus meridianus

Gammarus pulex Diva amphibia

Erpobdella octaculata Erpobdella testacea Glossiphonia complanata Lymnaea palustris Planorbis contortus Planorbis Iaevis Planorbis vortex Zonitoides nitidus

(0)

(0)

(1 1.28) (15.63) (10.42)

(0)

(0)

(1 1.28) ( I 3.02)

(3.47) (8.68) (3.47) (2.60) (3.47) (0.74) (2.60) (6.08)

294 C. F. MASON A N D R. J . BRYANT

TABLE I I-continued

Group TI T2 T3

c 3 Notoneeta glauca -contd . Callicorixa praeusta

Phryganea grandis Trianodes conspersa Sigara distincta

c 4 Glossiphonia heteroclita Haemopis sanguisuga Helobdella stagnalis Aeroloxus Iacustris Lymnaea peregra Planorbis crista Planorbis laevis Planorbis Ieucostoma Planorbis vortex Pisidium casertanum Daphnia pulex Diaphanosoma

brachyurum Pleuroxus trigonellus Copepoda Coenagrion puella lschnura elegans Sialis lutaria Corixa punctata Limnephilus sp. Phryganea striata Hyphydrus ovatus Helius sp. Dixa amphibia Tinca tinca

(1.61) Valvata cristata (2.06) Pisidium casertanum (1 . I 5 ) Eurycercus lamellatus (1.38) Agrypnia pagetana (1.1 5) Bidessus unistriatus

Coelostoma orbicularia Cymbiodyta marginella Dytiscidae (larvae) Enochrus coarctus Enochrus quadripunetatus Hydrobius fuscipes Cyphon variabilis Hydrochus brevis Hygrorus

Rantus grapii Prionocera turea Tipula luteipennis Pericoma nubila Odontomyia viridula Ephydridae sp.

parallelogrammus

(0.23) (0.23) (0.92) (0.69) (0.92) (0.92) (0.92) (023) (0.69) (0.46) (0.69)

(023) (023) (0.23) (0.92) (0.69) (0.46) (0.23) (0.23) (0.92) (0.69) (0.23) (0.23) (069)

Planaria torva Clossiphonia heteroclita Acroloxus lacustris Bythinia leachi Lymnaea peregra Planorbis Iaevis Segmentina nitida Copepoda Nepa einerea Sigara (nymph) Saldula sp. Mystacides longicornis Phryganea striata Chrysomeiidae larvae Curculionidae sp. Hydrochus angustatus Hydroporus larvae Hydroporus gyllenhalii Hydroporus pictus Hydroporus ruffrons Hydroporus tristis Hygrobia hermanni Hyphydrus ovatus Laccobius biguttatus Chaoborus crystallinus Ceratopogon sp. Tabanus sp. Hydrachnellae

(1.60) Cyphon variabilis (3.99) Dytiscidae larvae (2.40) Hydrobius sp. larvae (3.19) Hydrochus brevis (3.1 9) Hydroporus angustatus (1.60) Hydroporus lineatus (7.99) Hydroporus palustris (6.39) Hydroporus pictus (1.60) Hygrotus inaequalis (2.40) Hygrotus (5.59) parallelogrammus (3.99) Laccobius biguttatus (3.1 9) Leistus fulvibarbis

Helius sp. (6.39) Tipula luteipennis (1.60) Ephydridae sp. (1.60) Bezzia sp. (7.99) Diptera cyclorapha (3.29) (3.19) (3.19)

(0.80) (0.80) (0.80) (0.80) (0.80) (0.80) (0.80) (0.80) (0.80) (0.80) (0.80) (0.80) (0.80) (0.80) (0.80) (080) (0.80) (0.80) (0.80) (0.80) (0.80) (0.80) (0.80) (0.80) (0.80) (0.80) (0.80) (0.80)

Planaria torva Bythinia tentaculata Cymbiodyta marginella Cyphon oehraceous Curculionidae Hydrochus angustatus Hydroporus larvae Hygrotus larvae Peltodytes caesus Staphylinidae Phalacrocera replica ta Ceratopogon sp. Eristalis sp. Hydrachnellae

(2.60) (2.60) (3.47) (1.74) (3.47) (4.34) (6.08) (6.08) (1.74)

(6.94) (1.74) (1.74) (2.60) (4.34)

(4.34)

(1.74) (6.94)

(0.87) (0.87) (0.87) (0.87) (0.87) (0.87) (0.87) (0.87) (0.87) (0.87) (0.87) (0.87) (0.87) (0-87)

STRUCTURE O F A REEDSWAMP F A U N A 295

(6) Horizontal lines were drawn to subdivide the community into four levels of abundance: 1. Relative abundance >lo0 2. Relative abundance 10-100 3. Relative abundance 1-10 4. Relative abundance < I .

Thus the community was divided into 12 components into which species could be placed according to their abundance and frequency. These ranged from species which were abundant and occurred in many samples (AZ) to those which were rare and occurred in few samples (C4). The structure of the communities of the reedswamp at the three sites is given in Table 11.

T A B L E 111 The recurrent groups, together with their associates, of the Alderfen reedswamp

TI T2 T3

Recurrent group Planorbis albus Eurycercus lamellatus Ostracoda Asellus aquaticus Asellus meridianus Caenis horaria Sigara (nymphs) Sigara dorsalis Sigara falleni Agrypnia pagetana Cyrnus Javidus Chaoborus crystallinus Chironomidae

Associates Oligochaeta Erpobdella octaculata Bythinia tentaculata Physa fontinatis Valvata piscinalis Sphaerium corneum Gammarus pulex Caenis robusta Mystacides Iongicornis Gyrinus substriatus Hy drachnellae

Oligochaeta Planorbis albus Aselfus aquaticus Asellus meridianus Gammarus pulex Cyphon larvae Hydroporus angustatus Hydroporus lineatus Ptychoptera albimana Chironomidae

Glossiphonia complanata Bythinia tentaculata Cyphon ochraceous Hydroporus palusiris Hygrotus inaequalis Bezzia sp.

Oligochaeta Cyphon larvae Prionocera turcica Ptychoptera albimana Pericoma nubila

Planorbis albus Asellus aquaticus Asellus meridianus Chironomidae

The communities were also analysed by the recurrent groups method of Fager (1957, 1968) in which only the frequency of occurrence of animals in samples is takeninto account. The index of affinity, which forms the basis for the derivation of recurrent groups, was that of Fager & McGowan (1963), i.e.

where N , is the total number of occurrences of species A , Nb is the total number of occur- rences of species B, J is the number of joint occurrences, and N,? Nb.

{J/(NaNb)'}- &(Nb)'

296 C. F . MASON A N D R . J . BRYANT

Only one recurrent group was obtained for each transect, and they are given, together with their associates, in Table 111.

The recurrent groups obtained were very similar to the groups A determined by the graphical method. Thus the recurrent group of TI contained groups AI-A3, plus Agrypnia pagetana of B3. The recurrent group of T2 contained AI+A2, and the recurrent group of T3 contained A1 +A2, except the Chironomidae.

Site 1 (TI) had the largest recurrent group and the greatest number of species in group A , whereas site 3 had the smallest. No species were common to all recurrent groups, although the Chironomidae occurred in Group A of all three sites. Planorbis albus, Asellus aquaticus and A . meridianus were common to the recurrent groups and to group A of TI and T2. Oligochaetes and the larvae of Cyphon and Ptychoptera albimana were common to the recurrent groups and group A of T2 and T3. Waterbugs (Sigara nymphs, S. dorsalis and S. falleni) were also of particular importance in the structure of the com- munity of TI, as were the Ostracoda, the caddis CyrnusJlavidus and the phantom midge larvae Chaoborus crystallinus. The beetles Hydroporus angustatus and H. lineatus, together with Gammarus pulex were important components of T2 and the dipterous larvae of Pericoma nubila and Prionocera turca in T3.

TABLE IV The species diversity, equitability and evenness of three communities within a

reedswamp

Species diversity, H’bitslindividual 4.18 4.54 3.40 Equitability (s’/s) 0.48 0.49 0.32 Evenness (H‘/H’,,,ax) 0.12 0.74 0.61

Species diversity The mean species diversity per individual of the three communities was calculated

using the formula C H’= - ( N logl&- Cni logldzi)

where c is a scale factor for conversion from base 10 to base 2 logs (= 3.321928), N is the total number of individuals and ni is the number of individuals in each species (Lloyd, Zar & Karr, 1968).

For this analysis the unidentified groups Oligochaeta, Ostracoda, Chironomidae and Hydrachnellae, together with the unidentified juvenile stages of Helodidae and Sigara, have been omitted.

The equitability of the three communities, that is the extent of numerical equality between the species (Lloyd & Ghelardi, 1964) was determined from the formula

N

S f E= -

where s is the number of species in the sample and s’ is the theoretical number of species fitting MacArthur’s broken-stick model and giving the same species diversity as s

S

STRUCTURE OF A REEDSWAMP F A U N A 297

(MacArthur, 1957; Lloyd & Ghelardi, 1964). The evenness of the three populations (Pielou, 1966~2, b) was calculated as

H'/H',ax where H',,, is the maximum diversity obtainable if all species in the samples were equally abundant.

The results of these measures of diversity are given in Table IV. Site T2 had the greatest species diversity, and the greatest equitability and evenness, followed by T3.

Discussion A large amount of subjectivity must necessarily enter into any attempt to delimit

communities and to describe components of their structure. The method used here was subjective in that a plot of relative abundance against frequency of occurrence of each species in each community was arbitrarily divided to produce a number of groupings. The community of animals was divided into a structure of 12 parts. The most important animals in the community structure were considered to be those which were abundant and frequent ( A I and A2). There were a number of groups where animals may have been abundant but occurred in few samples (e.g. group B2) or were widely distributed but scarce (e.g. A3, B3). A final category was those species where only one or two individuals were collected (i.e. rare and infrequent, group C4), and this group probably contained a number of vagrants to the community.

Community delimitation by recurrent groups (Fager, 1957) which considers only the frequency of occurrence of the species, gave only one recurrent group for each site, The recurrent groups obtained were very similar to the groups AI-A3 obtained by the abundancelfrequency analysis. Had the whole reedswamp been treated as a single com- munity initially, more recurrent groups would have been generated, but nothing could have been said about their spatial distribution. The recurrent groups method gives no informa- tion on the less frequent species within the community.

The abundancelfrequency analysis divides the community into a number of components and groups together those organisms with similar characteristics in terms of abundance and frequency. The method gives some information on all species within the community, including rare species. Our method is extremely easy to perform and may be of especial use in evaluating sites for conservation purposes, where neither time nor manpower may be available for the usual detailed computations such as recurrent groups or association analysis.

The pH was the only physical parameter of the environment measured and this declined markedly from TI to T3. The pH at T1 would be constantly maintained by the calcareous water of the open Broad. During the summer at least, T3 would only be supplied with rain-water. Dvorak (1970b) noted a similar decline in pH from open water to the interior of a reedswamp. He also recorded very low oxygen concentrations within the reedswamp. Decomposition taking place at summer temperatures with low oxygen tensions would probably be at least partly responsible for the low pH. Both oxygen and pH must have a marked effect on the structure of the animal community.

The total density of all animals of site TI was higher than that at the other two sites, though this was largely due to the very large population of Sigara. Although the reed- swamp was a continuum, three rather different communities were recognized at the three

298 C. F . M A S O N A N D R. J . B R Y A N T

sites, no species being common to all recurrent groups. The fauna at TI contained a number of aquatic elements, for instance Corixidae, Leptoceridae and Cladocera, actively swimming forms with high oxygen requirements. Similarly, the Zygoptera and Ephemer- optera only occurred at TI. Conversely, the site T3 contained a number of terrestrial members of the fauna, such as the carabid beetle Leistus fulvibarbis, a species of staphylinid and the snail Zonitoides nitidus. However the terrestrial component of T3 is considerably less than the aquatic component of TI and this largely explains the greater quotient of similarity between T2 and T3 than between TI and T2.

Chironomids were numerous at all sites, but were unfortunately not identified further, so that any possible changes in the species composition across the swamp were not detected. Only two species of oligochaete were common in the reedswamp, Lumbriculus variegatus and Rhynchelmis limosella. Both species occurred throughout the reedswamp, but neither occur in the benthos of the open broad (personal obs.).

Asellus aquaticus and A . meridianus gradually declined in numbers across the reedswamp, but the population at all three sites consisted of equal numbers of both species. A com- parative study of their ecology in reedswamp would be interesting, especially in view of Williams’ (1963) conclusions that A . aquaticus is competitively superior to, and is gradually replacing A . meridianus.

The dipterous families Tipulidae, Ptychopteridae and Psychodidae were more common in the upper, drier parts of the swamp than elsewhere. The ecology of these groups is still very poorly known.

The very rich beetle assemblage (28 species) in the centre of the reedswamp (T2) is probably typical of such habitats. Macan (1949) recorded a rich beetle fauna in a Sphagnum bog bordering a moorland tarn and many species of beetles occur in the rice fields of southern France (Miss S . Macdonald, pers. comm.). There were seven species of Hydro- porus at T2 and a study of their methods of utilizing the habitat would prove very fruitful. A comparison of the present results with other studies is difficult because of the highly variable nature of reedswamp as a habitat, and because different studies examined different micro-habitats. For instance Arenkova (1965), Meschkat (1934) and Shcherbakov (1961) were mainly concerned with the fauna attached to reed-stems and largely ignored the benthos. In a partly submerged, partly emersal littoral swamp of Stratiotes aloides L. Higler (1968, 1969) found a zonation of caddis species from open water to the shore. Rudescu & Popescu-Marinescu (1970), working in Rumania, found that the fauna of stands of Phragmites communis was richer than that of Typha angustifolia and Scirpus lacustris. They also showed that fauna was richer and more productive in swamps that were cut for reed than in unmanaged swamps. The latter were also less productive of fish. A comparison of cut and uncut reedstands in Broadland would be interesting. The Rumanian reed-bed, however, extended for over 2000m from the water’s edge and had considerably more standing water than the Alderfen reed-bed.

The work most comparable with the present study is that of Dvorak (197Oa, b) who examined the fauna at a number of points through a littoral stand of Glyceria aquatica (L.) in Czechoslovakia. The Mollusca were equally important throughout the reed-bed, as in Alderfen Broad, but were much more numerous, accounting for some 50 % of the fauna. The larvae of Helodidae (Cyphon spp.) were abundant in the central part of the swamp, but not elsewhere, again resembling the pattern in Alderfen. Mosquito larvae were im- portant in the body of the Glyceria stand (cf. T3) and leeches and waterbugs at the site

STRUCTURE OF A REEDSWAMP F A U N A 299

near the water’s edge. Dvorak‘s study showed considerable seasonal variation in the structure of the community, particularly with respect to the temporary fauna (those species which spend only part of their life in the reedswamp). These seasonal changes could be most usefully followed up in further work on Alderfen Broad. For instance it is known that damselfly nymphs were far more numerous at TI at times of year other than July, the samples described here being taken just after the major hatch of damselflies.

The highest species diversity and the greatest equitability and evenness were recorded in the centre of the reed-bed (TZ). Species diversity, as determined by the information formula, has two components, species richness (the number of species present) and equitability or evenness of species abundances (Lloyd & Ghelardi, 1964). Equitability is to some extent dependent on the species count, at least for low species richness (Sanders, 1968; Sheldon, 1969), and Hurlbert (1971) considered that the number of organisms present (i.e. collection size N ) would affect the value of equitability. In the present study adjustment of the collection sizes of the three transects to equal 1000 individuals had negligible effect on the equitabilities calculated and no effect on their relative order. As the number of samples taken and their size were consistent for the three sites, the species diversities and equitabilities calculated can be usefully compared.

The species diversity and equitability of TI and T2 are very similar, but they are lower for T3. The reedswamp habitat of T3 is less structured than those of TI and T3 as there is no surface water component. The habitat at T3 is probably also less stable, being liable to dry out during the summer and to flood during the winter, making colonization by primarily aquatic or terrestrial animals difficult. It would be unwise, however, to place too much emphasis on species diversity indices, as some of the most numerous groups (Chironomidae, Oligochaeta, Sigara nymphs etc.) were not included in the analysis owing to difficulties in identification.

This study was restricted to one time of year only, early summer. Nothing can be said of the seasonal changes of the fauna and of the concurrent changes in species richness and equitability. These must obviously be essential considerations in order to obtain a complete understanding of the structure of the communities within reedswamps.

The work was supported financially by a Natural Environmental Research Council grant to Professor J. A. Kitching, whose encouragement at all times is gratefully acknowledged. The following people kindly helped with identifications :

Miss R. M. Badcock (Leptoceridae), J. Bass (bivalves), A. Brindle (Tipulidae and Ptychop- teridae), T. Gledhill (Hydrachnellae) and Dr M. Ladle (Oligochaeta).

Dr M. George and Dr R. Hornby of the Regional Office of the Nature Conservancy gave much advice in the early stages of the research, and the Norfolk Naturalists’ Trust granted permission to work at Alderfen. Dr R. James and Miss S. Macdonald made many useful comments on the manuscript.

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S T R U C T U R E OF A REEDSWAMP FAUNA 301

Appendix I List of animal species found in the reedswamp at Alderfen Broad

TURBELLARIA Planaria forva (0. F. Muller)

OLIGOCHAETA Lumbriculus variegatus (Muller) Stylaria lacustris (L.)

HIRUDINEA Erpobdella octaculata (L.) Glossiphonia complanata (L.) Haemopis sanguisuga (L.)

GASTROPODA Acroloxus lacustris (L.) Bithynia tentaculata (L.) Lymnaea peregra (Mull) Planorbis albus Mull Planorbis crista (L.) Planorbis leucostoma Millet Segmentina nitida (Mull) Valvata cristata (Mull)

BIVALVIA Pisidium casertanum (Poli)

CRUSTACEA Ceriodaphnia reticulata (Jurine) Daphnia longispina 0. F. Muller Diaphanosoma brachyurum Likven Pleuroxus trigonellus (0. F. Muller) Copepoda Asellus aquaticus (L.) Gammarus pulex (L.)

EPHEMEROPTERA Caenis horaria (L.) Cloeon dipterum (L.)

ODONATA Coenagrion puella (L.)

HEMIPTERA Callicorixa praeusfa (Fieb.) Nepa cinerea L. Saldula sp. Sigara dorsalis Leach

MEGALOPTERA Sialis lutaria (L.)

TRICHOPTERA Agrypnia pagetana Curtis Limnephilus sp. Phryganea grandis L. Trianodes conspersa (Rambur)

Polycelis tenuis Ijima

Rhynchelmis limosella Hoffmeister

Erpobdella testacea (Savigny) Glossiphonia heteroclita (L.) Helobdella stagnalis (L.)

Bifhynia leachi (Sheppard) Lymnaea palustris (Mull.) Physa fontinalis (L.) Planorbis contortus (L.) Planorbis laevis Alder Planorbis vortex (L.) Valvata piscinalis (Mull) Zonifoides nitidus (Mull)

Sphaerium corneum (L.1

Daphnia cucullata Sacs Daphnia pulex (De Geer) Eurycercus lamellatus (0. F. Miiller) Sida crystalha (0. F. Muller) Ostracoda Asellus meridianus Rac.

Caenis robusta Etn.

fschnura elegans (van der Linden)

Corixa punctata (Illig.) Notonecta glauca L. Sigara distincta (Fieb.) Sigara falleni (Fieb.)

Cyrnus jfavidus (McLachlan) Mystacides longicornis (L.) Phryganea striata L.

302 C . F. MASON A N D R . J. B R Y A N T

Appendix I-continued COLEOPTERA . Bidessus unistriatus (Schrank)

Coelostoma orbicularia (Fa br) . Cynzbiodyta marginella (Fabr). Cyphon variabilis (Thunberg) Enochrus quadripunctatus (Herbst) Hydrobius fuscipes (L.) Hydrochus brevis (Herbst) Hydroporus gyllenhalii Schioedte Hydroporus palustris (L.) Hydroporus rufifrons (0. F. Miiller) Hygrobia hermanni (Fabr.) Hygrotus parallelogrammus (Ahrens) Laccobius biguttatus (Gerhardt) Peltodytes caesus (Duftschmid) Staphylinidae sp.

DIPTERA Bezzia sp. Chaoborus crystallinus (Degeer) Diptera cyclorapha Ephydridae sp. Helius sp. Pericoma nubila (Meigen) Prionocera turcica (Fabr.) Tabanus sp. Tipula luteipennis Meigen

HYDRACHNELLAE Arrenurus sp.

PISCES Tinca tinca (L.)

Chrysomelidae sp. Curculionidae sp. Cyphon ochraceous Stephens Enochrus coarctus (Gradl) Gyrinus substriatus Stephens Hydrochus angustatus Germar Hydroporus angustatus Sturm Hydroporus lineatus (Fabr.) Hydroporus pictus (Fabr.) Hydroporus tristis (Paykull) Hygrotus inaequalis (Fabr.) Hyphydrus ovatus (L.) Leistus fulvibarbis Dejean Rantus grapii (Gyllenhal)

Ceratopogon s p . Chironomidae Dixa amphibia (Degeer) Eristalis sp. Odontomyia viridula (Fabr.) Phalacrocera replicata (L.) Ptychoprera albimana (Fabr.) Theobaldia annulata (Schrank)

Eylais hama ta