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2 Polish Journal of Ecology

2 Ulrich W.

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Relative abundance distributions of species : the need to have a new look at them

Ulrich W.

Polish Journal of Ecology

2001

Vol. 49, nr 4

391-405

This paper shows that recent models of relative abundances (RADs) like the log-normal model or sequential breakage or nich apportionment models are not able to describe and explain RADs found in natural communities because they are derived from a classical niche concept and assume strong past or present interspecific competition. None of them refers especially to temporal variability and functional niche dimensions. The present paper identifie three basic features of natural communities (unimodal species-weight distributions, abundance-weight distributions with more or less marked upper boundaries, and species density fluctuations that can be modelled by four different random processes). Modelling communities with these basic features resulted in RADs that only in part could be described by classical models but that had shapes often found in sampling from larger natural communities. No single distribution like the canonical log-normal appeared that may serve as a general null-model but RAD and evolutionary strategy (r- or K selection) seem to be related. The shape of relative abundance distributions was found to depend on the number of species even if all parameter setting of the generating distributions were identical. This indicates that classical evenness indices (that assume independence of species number) might not be appropriate to compare communities with different numbers of species. It appeared that RAD and body weight related community patterns have to be studied together.

The density, weight and the biomass, weight distribution can be generated by the species, weight distribution together with density fluctuations : evidence from model species distributions in the Hymenoptera

Ulrich W.

Polish Journal of Ecology

1999

Vol. 47, no 1

87-101

This paper tests the hypothesis that the density - weight distribution of species assemblages may be generated alone by underlying weight dependent density fluctuations by constructing several model species - weight and density - weight distributions. The simulations revealed that indeed even a slight dependence of density fluctuation on body weight generated typical density - weight distributions with upper density boundaries and triangular shapes. Other ecological explanations, such as metabolic constrants or arguments based on fractal geometry, may therefore not be necessary to explain the pattern. In a second step of analysis the density - weight relationship was combined with various species - weight distributions (computed over log[2] weight classe) to show that in nearly all parameter settings steadily rising biomass distributions with a decline only towards the largest weight classes result. The generality of the equal biomass hypothesis is therefore rejected. A general model is developed to explain several of the patterns in density - weight and species - weight plots assuming only underlying weight dependent speciation and extinction rates and weight dependent density fluctuations.