Allometric structure and topology of food webs: Energetic constraints conserve food-web structure across ecosystems and space

Natural ecosystems comprise an innumerable amount of different organisms. These organisms are not separated, they interact and depend on each other. Today’s ecosystems are facing an enormous decline in biodiversity due to human impacts with thus far unknown consequences. One key objective of ecological research is to understand the mechanisms generating and maintaining this incredible amount of diversity. However, comprehensive analyses of natural ecosystems are impeded by their complexity and diversity. Food webs, therefore, provide an excellent tool to analyze the complexity of ecosystems. They depict the system‘s diversity and species interactions in a condensed form. Furthermore, food-web structure can help to predict the interaction strengths between species and the energy pathways through the system. In my thesis, I use food web structure to analyze structural properties which separate food webs from other network types and furthermore I investigate generalities and differences of food-web structure across different ecosystems. One of the most important ecosystems is the soil ecosystem, as it provides the base for aboveground productivity. However, detailed soil food webs are scarce. In chapter 2, I assembled the complex food webs of 48 forest soil communities and analyzed if soil food webs differ in their topological parameters from those of other ecosystems. I found that soil food webs are characterized by a higher number of omnivorous and cannibalistic species. Moreover, they comprise more trophic chains and intraguild-predation motifs than food webs from other ecosystems. Finally, soil food webs showed high average and maximum trophic levels. These differences in network structure to other ecosystem types may be a result of ecosystem-specific constraints on hunting and feeding characteristics of the species that emerge as network parameters at the food-web level. Despite these differences, soil food webs showed the same scaling of their properties with connectance and size. In a second analysis of land-use effects, I found significant but only small differences of soil food web structure between different beech and coniferous forest types, which may be explained by generally strong selection effects of the soil that are independent of human land use. This study has unravelled systematic structures of soil food-webs, extending our mechanistic understanding how their environmental characteristics determine patterns at the community level. Additionally, I have shown that the general scaling laws also apply for soil food webs. In addition to purely topological properties, I analyzed another important aspect of food webs. The distributions of body masses and degrees across species are key determinants of food-web structure and dynamics. In chapter 3, I analyzed body masses of species and their systematic distributions across food-web structure. In particular, allometric degree distributions combine both aspects in the relationship between degrees and body masses. They are of critical importance for the stability of complex ecological networks. I used an entirely novel global body-mass database including food-web structures of four different ecosystem types to analyze body-mass distributions, cumulative degree distributions, and allometric degree distributions regarding differences among ecosystem types. My results demonstrate some general patterns across ecosystems: the body masses are either roughly log-normally (terrestrial and stream ecosystems) or multimodally (lake and marine ecosystems) distributed, and most networks exhibit exponential cumulative degree distributions except stream networks that most often possess uniform degree distributions. Additionally, with increasing species body masses we found significant decreases in vulnerability in 70% of the food webs and significant increases in generality in 80% of the food webs. Overall, these analyses document striking generalities in the body-mass and degree structure across ecosystem types as well as surprising exceptions (uniform degree distributions in stream ecosystems). This suggests general constraints of body masses on the link structure of natural food webs irrespective of ecosystem characteristics. While I revealed general patterns of food-web topology in chapter 2 and 3, I investigated the drivers of these general patterns in chapter 4. Therefore, I analyzed the influence of different external factors on community (beta diversity) and food-web structure. Two main theoretical bodies explain β-diversity, the niche theory and neutral theory. However, neutral theory predicts only distributions for trophically identical species, whereas influences of local niches or neutral effects on food-web structure as a crucial part of the multitrophic structure of ecosystems are not taken into account. In chapter 4, I therefore analyzed the effects of spatial distance and environmental dissimilarity on the species dissimilarity (beta diversity) and food web dissimilarity (structural dissimilarity) of multitrophic forest communities. I showed that the mechanisms proposed by neutral theory can adequately predict the beta diversity of multitrophic species communities. Furthermore, food-web structure was robust and affected neither by spatial distance (random dispersal, neutral theory) nor by environmental filtering (niche theory). I additionally analyzed model food webs (random and niche topology) and compared their dissimilarities to empirical food webs. The highest dissimilarity was reached by random food webs whereas niche model food webs were in between and the lowest distances were expressed by empirical food webs. Further, random food webs displayed the highest mean trophic level (115), while niche model food webs showed lower (5) and empirical food webs the lowest (4) mean trophic level values. Hence, food-web structure appears to be energetically optimized with local species adapted to energetic niches within the food web while species identity within these niches remains random. This suggests that different species could be adapted to the same energetic niches and, while following random drift, still assemble into similar food web structures. Altogether, the results of this thesis demonstrate the practicality of food-web structure in unravelling generalities across different ecosystems. Furthermore, food-web structure explains species distributions across the environment and provides additional important information on the ecosystem. The observed generalities indicate constraints on food-web structure. The allometric degree distributions demonsrate such constraints on food-web structure by distributing the links in dependence of the species body masses. Finally, my results from chapter 4 indicate that, additionally to global topological constraints, local communities have to meet certain energetic constraints to explain the similarity found across food webs.