Invertebrate Community Structure Along a Habitat-Patch Size Gradient Within a Bog Pool Complex
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Invertebrate community structure along a habitat-patch size gradient within a bog pool complex by Naomi M. Towers A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the University of Edinburgh 2004 Abstract This thesis characterises species richness and community structure over a habitat- patch size gradient of a typical bog-pool complex, investigating the effect of pool size on aquatic invertebrate communities. In this study, twenty-two pools were surveyed ranging in area from 8.6 m2 to 280.9 m2 within a single complex at Forsinard in the north of Scotland. Three different sampling methods were used: baited and unbaited activity traps and a sediment sampler. Univariate and multivariate methods were used to investigate the effects of pool size and pool location within the complex on species richness and community structure. The research expands our knowledge of peatland pool invertebrates by providing a comprehensive survey of the aquatic invertebrate fauna representative of the Flow Country of northern Scotland. Two IUCN British Red Data Book species were recorded: the Northern Damselfly, Coenagrion hastulatum (Charpentier), and the cased caddisfly, Nemotaulius punctatolineatus (Retzius). Three species of aquatic Coleoptera were collected that have Nationally Notable status according to Ball (1986): Dytiscus lapponicus Gyllenhal, Ilybius aenescens Thomson and Gyrinus minutus Fabricius. All these species are typical of, and often restricted to, this habitat type. The three different sampling methods differed in their sampling efficiency and each gave a different species spectrum. A distinct seasonal change in the samples was also observed. The number of taxa caught per standardised sampling unit (taxon density) was investigated over the pool size gradient. Relationships between taxon density and area were weak or non-existent in both unbaited activity traps and sediment samples. However, the number of beetle species caught in baited activity traps increased significantly with pool size, indicating that the total number of beetle species per pool may also increase over the size gradient. Ratios of the number of predator taxa to prey (non-predator) taxa for each pool ranged from 0.34 to 0.78 with a mean of 0.49 and were not affected by pool area or total taxonomic richness. Taxa displayed a positive abundance-occupancy relationship and the possible underlying mechanisms involved in creating this pattern are discussed. Multivariate techniques showed that pool area, depth, and distance from the centre of the pool complex (periferality) all had a small but significant affect on community composition and that between certain taxa there were distinctly different optima along the pool size gradient. These results are discussed in the context of species area theory. ii Declaration This thesis has been composed by myself and the work it describes has been carried out by myself except where specific reference is made to other individuals. Naomi M. Towers iii Acknowledgements I am indebted to Dr Colin Legg for his uncompromising support and impeccable professional advice during a very difficult period of my Ph.D. and give him my most heartfelt thanks for the time he generously made available. I would like to thank both Colin and Dr Mark Huxham for their expert advice on my analyses, stimulating discussions and highly valuable comments on drafts of this thesis. I am extremely grateful to them both for their support and encouragement throughout. I thank all those who helped me in the field: Simon Drew, Kevin Hall, Sally Greenwood, Francois Edwards, Peter Mahoney, Claire Fennell, Jill Lancaster and Lisa Belyea. I also thank everybody at the RSPB in Forsinard who allowed me to carry out my research on their reserve, and for their valuable advice on the local area. I must thank all my former departmental and lab mates who, well, provided me with many amusing memories if nothing else … and for their support and advice of course, which was sometimes very good: Kevin Hall, Alasdair Hardie, Simon Drew, Francois Edwards, Guy Woodward, Alasdair Hamilton and David Oldmeadow. I must also thank Rosie Fisher, my flatmate, for uncomplainingly bringing me tea in the mornings and reliably cooking me dinner during the last couple of months of my write-up, and for listening to my endless rants about my data without falling asleep, too often. Lastly, for their patience and support, I would also like to thank all my friends and colleagues whom I totally neglected over the last two or three months as ‘the fear’ set in. iv Table of Contents Title page………………………….………………………………………..…………………….……i Abstract………………………………………...……………………….………………………….….ii Declaration.……………………………………..…………………………………………………….iii Acknowledgements…………………………….…………………………………………………….iv Table of contents………………………………….…………………………………………………..v 1 INTRODUCTION............................................................................... 1 1.1 POOL AREA AND INVERTEBRATE ASSEMBLAGES .......................... 1 1.2 THESIS AIMS............................................................................................... 4 1.3 THESIS STRUCTURE.................................................................................. 5 1.4 REFERENCES .............................................................................................. 8 2 SITE DESCRIPTION......................................................................... 9 2.1 THE FLOW COUNTRY OF CAITHNESS AND SUTHERLAND............. 9 2.2 FORSINARD............................................................................................... 10 2.2.1 Physical and chemical properties of the Forsinard pools............................. 13 2.3 REFERENCES ............................................................................................ 16 3 PEATLAND POOL INVERTEBRATE ASSEMBLAGES ................ 17 3.1 INTRODUCTION ....................................................................................... 17 3.2 METHODS .................................................................................................. 19 3.2.1 Invertebrate sampling and identification ..................................................... 19 3.2.2 Statistical analysis........................................................................................ 23 3.3 RESULTS .................................................................................................... 24 3.3.1 Sampling method efficiency ........................................................................ 27 3.3.2 Seasonal variation ........................................................................................ 28 3.4 DISCUSSION.............................................................................................. 30 3.4.1 Species assemblages .................................................................................... 30 3.4.2 Sampling methods........................................................................................ 32 3.4.3 Seasonal variations....................................................................................... 36 3.4.4 Conclusions.................................................................................................. 36 3.5 REFERENCES ............................................................................................ 38 4 THE RELATIONSHIP BETWEEN TAXONOMIC RICHNESS AND POOL AREA................................................................................... 40 4.1 INTRODUCTION ....................................................................................... 41 4.1.1 Species richness and species density ........................................................... 50 v 4.2 METHODS .................................................................................................. 52 4.2.1 Activity Traps .............................................................................................. 53 4.2.2 Sediment samples......................................................................................... 54 4.2.3 Taxonomic and trophic groups .................................................................... 56 4.2.4 Random sampling ........................................................................................ 57 4.2.5 Statistical analysis........................................................................................ 57 4.2.6 Sample effort curves .................................................................................... 58 4.3 RESULTS .................................................................................................... 59 4.3.1 Number of all taxa (taxon density) .............................................................. 59 4.3.2 Taxonomic and trophic groups .................................................................... 63 4.3.3 Sample effort curves .................................................................................... 68 4.4 DISCUSSION.............................................................................................. 72 4.4.1 Number of all taxa per pool ......................................................................... 72 4.4.2 Taxonomic and trophic groups .................................................................... 76 4.4.3 Sample-effort curves.................................................................................... 79 4.4.3 Conclusions.................................................................................................. 80 4.5 REFERENCES ...........................................................................................