Effects of temperature and body mass on soil communities

The Earth is undergoing a climate change with predicted increases in temperature by up to 6° C until 2100. How this warming affects soil food webs is of fundamental interest for mankind as it may influence global food production. Due to the complexity of soil systems and species’ interactions, simplifications are required in search for general patterns. One simplification used in this thesis is the categorization of species into con- sumer types such as carnivores, herbivores and detritivores, as physiological traits such as assimilation efficiencies and respiration rates are thought to differ between consumer types. As an additional explanatory parameter in all chapters, I used body mass which was shown to be of high importance for food-web structure and dynamics due to its influence on respiration rates, species abundances, consumption rates and interference competition. The aim of this thesis was to investigate the impact of environmental warming on soil communities starting with physiological reactions of respiration rates and assimilation efficiencies which influence the individual’s consumption rates (Chapter 2). Therefore, I performed a metastudy of published studies on respiration rates and assimilation efficiencies to investigate how the influence of temperature and body mass differs between consumer types. Based on that, I calculated maintenance consumption rates (i.e. amount of energy required to balance life maintenance) in dependence on temperature, body mass and consumer types by dividing respiration rates by assimilation efficiencies. The scaling of respiration rates and assimilation efficiencies with temperature and body mass differed between consumer types with the strongest impact of temperature on carnivores and the strongest body-mass effect for herbivores. Considering assimilation efficiencies, I found a temperature effect on herbivores and a body-mass effect on detritivores. The resulting maintenance consumption rates increased with temperature and body mass for all consumer types with the strongest increase with temperature for carnivores whereas the body-mass effect was most pronounced for detritivores. Therefore, climate change will have profound energetic consequences for natural communities by increasing turnover rates at the detritivore level due to their accelerated consumption rates and by strongly increasing consumption rates of carnivores. Comparison with experimentally measured consumption rates showed that calculated maintenance consumption rates increased less under warming for lower trophic levels. Therefore, they should be able to increase their biomass under warming. In contrast, calculated maintenance consumption of carnivores increased stronger under environmental warming than realized consumption rates which should leave them struggling to consume enough energy for maintenance and increase their risk of extinction. In a next step, I used a functional-response approach to investigate how consumption rates of differently-sized predators are affected by intraspecific interference competition (Chapter 3). Generally, I expected warming to increase the speed of movement, encounter rates and in consequence interference among predator individuals. This expectation was supported by the results obtained for the larger predator, whereas the opposite pattern characterized the interference behaviour of the smaller predator. The explanation I propose is based on the differing sensitivity to warming of respiration rates of both species. As expected, increasing temperature led to stronger interference competition of the larger species which exhibited a weaker increase in their respiration rates with increasing temperature. However, the stronger increase in the respiration rates of the smaller predator had to be compensated by increased searching activity for prey, which did not leave time for increasing interference. These results contribute to my previous findings of the strong susceptibility of carnivores to environmental warming. Also, generalizations of how interference competition responds to warming should take the species’ metabolic response to temperature in dependence on its body mass into account. Finally, I raised the complexity of the system to a soil community spanning four trophic levels and introducing a second climate-change factor, soil dryness (Chapter 4). In order to have a system mimicking a natural community under controlled climatic conditions, I transferred soil cores with their natural pore structure and a natural microorganism community into the laboratory. The community investigated consisted of fungi, springtails (collembolans), mites and geophilids with maize litter as resource. As body-mass structure is of high importance for communities, I incorporated a body-size aspect for the higher trophic levels by using two differently-sized collembolan species which were preyed on by a small and a large predator species. My results show that predicting the outcome of climate change is far from trivial and emphasize the importance of taking multiple climate change factors into account. For a climate change scenario with increasing temperature and soil dryness I found that consumption rates increased, thus climate change amplified the negative influence of the consumer population on the resource. However, trophic cascades may neutralize this negative influence of increased consumption rates under climate change. Of high importance for carbon cycling are increased decomposition rates resulting in accelerated nutrient turnover. Investigation of body-mass effects showed that for geophilid’s the consumption rates decreased with increasing body mass. I presume this to be caused by decreasing capture efficiency as the experimental habitat structure was more supportive of smaller individuals. Most parts of this thesis only include one climate change factor, temperature, due to it’s high importance for all biological interactions. However, climate change contains far more factors which may influence a species’ physiology and interactions. In Chapter 4, I could not only show the importance of taking multiple climate change factors into account but also experienced the difficulties in doing so. As it is impossible to quantify every single interaction in natural communities due to the high complexity, I tried to find general patterns in this thesis by starting with a simple system and increasing complexity. The findings of this thesis can now be incorporated into theoretical-modelling approaches on the impact of climate change on populations and food-web stability. Also, they provide important insights for nature conservation strategies as I could show the outcome of environmental warming to differ between trophic levels.