An alignment of diversity and energy models of ecosystems

Cousins, Stephen H (1983). An alignment of diversity and energy models of ecosystems. PhD thesis The Open University.



In this work the problem of incompatibility between diversity and energy models of ecosystems is investigated. Compatible models are developed by using species density measures for species diversity measurement and by adopting a new model of ecosystem energetics, the trophic continuum, based on plant structure, animal size and a detritus cascade. A combined, compatible measure is developed. This is the species density of a size class of a taxon. In generating the trophic continuum model an extensive critique of the trophic level concept is undertaken.

Four published papers form the basis of the thesis and these are linked by a commentary. The commentary details how the papers are related. Paper 1 examines the properties of sample size independent measures of species diversity and concludes that sample size dependent measures, such as species density, are preferable for compatibility with energy models. Paper 2 develops a new model of ecosystem energetics, the trophic continuum, applicable to large (that is, many species) ecosystems. The model is defined by organisms harvesting resources from an environment. Thus the model has strong affinities with optimal foraging theory. This is developed in appendix 2. Animal size classes, a classification of plant products and of detritus, determine the resource state boundaries used in the trophic continuum. Paper 3 combines the approaches developed in papers 1 and 2 to examine the species density distribution of birds of different size classes using data from the UK atlas of breeding birds. This work shows that large birds are predominately found in the north of Britain and so has implications for the structure of food webs in different geographical areas. Paper 4 considers species density and species size distributions over a hypothesised gradient of environment patch size. It examines the distribution of birds and snail species of an urban area. Average species size declined towards the centre of the area as predicted. Papers 1,3 and 4 adopt the species density measure at different scales.

In the commentary, possible counter examples to the trophic continuum are discussed, including the phenomenon of parasitism and the group behaviour of animals. Significant differences in the species diversity relationships between predators and prey compared to parasites and hosts are identified. Future research is indicated by appendices containing currently unpublished papers on Bergmann's rule as applied to an avifauna (appendix 1) and a mathematical model of the trophic continuum for large ecosystems (appendix 2). A poster paper illustrating the seed size distribution of part of the UK flora (appendix 3) shows biogeographical changes in one aspect of the plant structure sector of the trophic continuum model.

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