Biodiversity and ecosystem functioning in stream detritivore systems: patterns and mechanisms. Cheryl Inglis Thesis submitted to The University of Sheffield for the degree of Doctor of Philosophy Department of Animal and Plant Sciences July 2003 Acknowledgements. The research reported in this thesis was funded by the NERC. Facilities for the research were provided by the Department of Animal and Plant Sciences at The University of Sheffield, for which I extend my thanks to the head of department, Malcolm Press. Many thanks go to my supervisors, Lorraine Maltby and Phil Warren for their excellent supervision and support throughout. Thank you also to Andrew MacColl and Owen Petchey for advice on statistics, and to Tony Sugden and Alison Blake for assisting with field work. Special thanks go to Naomi, Paul, Jo, Rob and Jenny for accompanying me on field trips and for their support, and also to Jim, Nick, Isabel, Fiona, Kevin, Andrew and Thibaud for their friendship. Finally, I would like to thank my family for their constant encouragement, and Christophe for his unending support and patience. Biodiversity and ecosystem functioning in stream detritivore systems: patterns and mechanisms. Cheryl Inglis. Summary Global changes in biodiversity have prompted ecologists to examine the relationship between biodiversity and the functioning of ecosystems. The occurrence and form of such a relationship, and the mechanisms driving it, are critical for predicting the effects of biodiversity loss. The aim of this study was to address whether species diversity is important for ecosystem function in streams, focusing on the relationship between the diversity of leaf-shredding macroinvertebrates and detritus processing. An initial field study revealed that there was no simple positive relationship between shredder diversity and leaf processing at a regional scale (i.e. between streams), but when combined with other biotic and abiotic factors did contribute significantly to explaining leaf-processing rate. The presence of particular species also appeared to be important for leaf processing. A second field study tested whether an increased variety of leaf types might lead to increased detritus processing in high, compared to low, diversity shredder communities. Again, particular species, or species combinations, appeared to be important in processing a mixed leaf resource, but shredder diversity was unimportant. Artificial streams were then used to examine experimentally the effects of shredder identity and species number on detritus processing. Once more shredder diversity had little effect on processing rates, while shredder identity was important, on single and mixed leaf resources. A final experiment quantified the extent of resource-use complementarity among a set of shredder species, and then tested whether differences in complementarity among species pairs influenced leaf-processing rates. Although shredders did show differences in their leaf diets, experimental increases in complementarity did not translate into positive effects on leaf processing rates. Both field and experimental studies provided little clear evidence for a positive diversity-function relationship, or the mechanisms proposed to underpin it. Results imply that the consequences of biodiversity loss for stream ecosystem function are dependent on which species are lost. ii Thesis contents Acknowledgements................................................................................................ i Summary.................................................................................................................ii Thesis contents.......................................................................................................iii 1. General introduction......................................................................................... 1 1.1. Introduction................................................................................................. 1 1.1.1. The diversity-ecosystem function issue............................................ 1 1.1.2. The development of hypotheses........................................................2 1.2. Biodiversity-ecosystem functioning experiments......................................... 4 1.2.1. Measures of diversity, response variables and types of system used.................................................................................................4 1.2.2. The general patternsobserved...........................................................5 1.3. Species identity and composition effects on ecosystem function................ 7 1.3.1. Species identity effects......................................................................7 1.3.2. Species composition effects.............................................................. 8 1.4. Mechanisms that drive positive biodiversity-ecosystem function relationships........................................................................................................9 1.4.1. The selection effect........................................................................... 9 1.4.2. 'Complementarity effects’.................................................................. 11 1.4.2.1. The niche-differentiation effect........................................... 11 1.4.2.2. Facilitation.......................................................................... 12 1.4.2.3. Functional groups and complementarity............................ 12 1.4.2.4. Distinguishing between different mechanisms................... 14 1.5. Limitations of the current evidence..............................................................16 1.5.1. The mechanisms...............................................................................16 1.5.2. Relevance of experiments to natural systems.................................. 18 1.5.2.1. Relevance of the selection effect in natural communities... 18 1.5.2.2. Scale of experiments......................................................... 19 1.5.3. Generalising across ecosystem types.............................................. 21 1.6. Biodiversity and ecosystem function in streams......................................... 23 1.6.1. Detritus processing in streams..........................................................23 1.6.2. Relationship between shredders and detritus processing................. 25 1.7. Aims............................................................................................................ 29 iii 2. The importance of macroinvertebrate species richness, identity and community composition for detritus processing in natural streams........................ 33 2.1. Introduction................................................................................................. 33 2.1.1. Factors that influence detritus processing in streams......................34 2.1.1.1. Environmental factors........................................................ 34 2.1.1.2. Biotic factors...................................................................... 35 2.1.2. Influence of individual species on detritusprocessing ...................... 36 2.1.3. Influence of community composition on detritus processing............ 37 2.1.4. Aims..................................................................................................38 2.2. Methods....................................................................................................... 39 2.2.1. Study sites........................................................................................ 39 2.2.2. Quantifying leaf decomposition........................................................ 44 2.2.3. Physico-chemical measurements.....................................................46 2.2.4. Quantifying biotic variables...............................................................46 2.2.5. Statistical analyses...........................................................................47 2.3. Results.........................................................................................................49 2.3.1. Variation in environmental and biotic factors.................................... 49 2.3.2. Factors influencing leaf mass loss.....................................................51 2.3.2.1. Whole macroinvertebrate community.................................51 2.3.2.2. Shredder community.......................................................... 54 2.3.2.3. Non-shredder community...................................................55 2.3.2.4. All components of the macroinvertebrate community........ 55 2.3.2.5. General patterns................................................................ 56 2.3.3. Influence of individual shredder species on leaf mass loss............... 56 2.3.3.1. Species presence/absence................................................56 2.3.3.2. Biomass............................................................................. 56 2.3.3.3. Abundance.........................................................................58 2.3.4. Influence of community composition on leaf mass loss................... 62 2.3.4.1. Whole community.............................................................. 62 2.3.4.2. Shredder community..........................................................64 2.4. Discussion...................................................................................................66 2.4.1. Influence of species richness on leaf processing..............................66 2.4.2. Influenceof individual species on leaf processing...........................