Significance of intraspecific variation for decomposition processes

Abstract

Over the last decades a growing awareness for the role of biodiversity in an ecological context has grown. Species richness and functional diversity have been found to influence ecosystem processes like production and decomposition. In the context of decomposition mainly the diversity of the decaying litter has been in the focus of interest – and (like for other studies on diversity) mainly interspecific diversity. As the concept of biodiversity shifted from "number of species" towards “functional diversity”, intraspecific variation was more and more recognized as a potentially significant level of biodiversity. However, very little is known about the influence of intraspecific variation and diversity in the context of decomposition and up to now, nothing is known regarding the ecological role of this level of diversity in a detritivore species in the context of decomposition. Consequently, the aim of this thesis was to study the influence of intraspecific variation in a detritivore species on consumption and on decomposition of different litter mixes. To test if an increased diversity within such a detritivore species results in a higher consumption, isopods of the species Porcellio scaber from several European populations were combined into artificial groups of three different diversity-levels. Groups were tested with newly developed genetic markers to confirm differences in diversity and fed on different leaf litter species and litter mixtures with different levels of interspecific diversity in a microcosm-experiment. A positive effect of intraspecific diversity on litter consumption only occurred in single litter treatments, but – other than expected – not in litter mixtures. However, these effects were considerably lower than the also measured “mixed litter effects” (effects of increased interspecific litter diversity). In addition to the results (due to the developed genetic marker) some insights about the variation and population differentiation of the widely distributed Isopod species P. scaber were be gained. To test the effect of an (expected) decreased diversity on consumption, two populations of the Isopod Trachelipus ratzeburgii (Brandt 1833, Isopoda, Oniscidea) living on two different altitudes of the same mountain were fed on different leaf litter species and litter mixtures – hypothesizing that the fluctuation in environmental parameters like temperature at high altitude reduced the diversity in a high-altitude population. In addition, genetic variation within and differentiation between both populations was tested and stress resistance (via HSP70-expression) was measured to see if a reduced variation goes along with an increased level of stress resistance due to positive selection towards this trait. However, results show that the chosen populations did not differ in variation (along with being not differentiated) and had no differences in stress resistance. They also did not differ in absolute consumption, despite a strong difference in size, as the isopods from high altitude were much smaller than those from the low altitude population. The reasons for this difference remain unknown, but these findings show a high plasticity for this species. As consumption rate is only one element of the direct and indirect influences detritivores have on decomposition, the variation in digestion of P. scaber was measured in an additional approach. Animals from several populations were fed with standardized food, whereafter the chemical structure of the feces was compared using Pyrolysis-GC-MS. The isopods showed (also in comparison to specimen of other Oniscidea species) a substantial amount of variation in the chemical fingerprint of the feces, whereas the differences could only partly be explained by genetic proximity. Other factors, such as the microbiome of the digestive tract, appear to have an additional influence. In summary, the results provide indications of an importance of intraspecific variation and diversity for decomposition processes, but its influence was (with respect to consumption of leaf litter) considerably less strong compared to the influence of the interspecific variation of the leaf litter. Nevertheless, the results show in many respects that intraspecific variation (and plasticity) should not be neglected in the context of decomposition.