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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.

Abundance, biomass and density boundaries in the Hymenoptera : analysis of the abundance - weight relationship and differences between forest and open landscape habitats

Ulrich W.

Polish Journal of Ecology

1999

Vol. 47, no 1

73-86

The density - weight distributions of the hymenopteran species in a beech forest on limestone and a dry meadow on limestone near Gottingen (FRG) were studied. Species and density distributions (logarithmic size classes) turned out to follow normal and log-normal distributions with the meadow having the more narrow shape. Upper weight classes accumulated more total biomass m^-2 than lower ones, resulting in a rejection of the equal biomass hypothesis in the case of the Hymenoptera. An analysis of the density - weight relationship revealed an upper density boundary for the hymenopteran species which can be defined by second order polynomial functions. Mean and upper densities of small hymenopteran species ranged well below their boundaries, with an asymptotic relationship between distance from the boundary and species weight. The area defined by the boundaries may mark the area of stability: exceeding the species specific boundary was always followed by a marked decline oreven a collapse of population density.