Topology and stability of complex foodwebs

Abstract

Since the early twentieth century, different general laws have been investigated to understand mechanisms driving stability in natural ecosystems, but until today the mechanisms are still generally unexplored. The main goal for ecology is to understand mechanisms driving food web dynamics, to counteract the hazard of global species loss. The studies presented in this thesis investigate the general scaling of different strucural food web (e.g. diversity, connectance, vulnerabilty) and species properties (e.g. body mass, trophic level), and how these properties influence secondary extinctions in food webs. The backbone of this thesis is a database of food webs , including information about predator–prey interactions, the metabolic type, and the species’ body mass. The relationship between diversity and topology is widely discussed, especially the hypothesis that a constant number of species per link leads to a decreasing connectance with increasing number of species. The alternative to this idea has been the ’constant connectance hypothesis’, where connectance is constant with increasing number of species. As part of my thesis (Chapter 2), I analysed the scaling of topological properties based on my compiled database and found power–law scaling relationships with diversity and complexity for most properties. Also, connectance tends to decrease with increasing number of species. The results illustrate the lack of universal constants in food web ecology as a function of diversity and complexity. Furthermore, common measures of bio–complexity (e.g. the fractions of top, intermediate and basal species, and the average trophic level) have been reinvestigated, as scale–dependent on diversity and connectance to. Interestingly, the scale dependence is partly significantly different between ecosystem types. A lot of species’ characteristics depend on body mass (eg. predator–prey interactions, metabolism, mobility) thus nominating body mass as the most important species attribute. Chapter 3 illustrates the distribution of mean population body masses in communities for different ecosystem types. The body masses are often roughly log–normally (terrestrial and stream ecosystems) or multi–modally (lake and marine ecosystems) distributed, and most networks exhibit exponential cumulative degree distributions. An exception are stream networks which most often possess uniform degree distributions. Furthermore, with increasing body mass vulnerability decreases in 70% of the food webs and generality increases in 80% of the food webs. Facing paradigms developed by Elton, I analysed the relationship of predator mass to prey mass and trophic level and the relationship between predator–prey body–mass ratio (hereafter: mass ratio) and trophic level (Chaper 4). In 1927, Elton suggested that (i) the mean prey mass increases with predator mass, (ii) the predators become larger in size with increasing trophic level, and (iii) the mass ratio is constant across trophic levels. After analysing the data base, the result supports the paradigms (i) and (ii). However, consistant with theoretical derivations, I found a systematic decrease in mass ratios with the trophic level of the predator. This result indicates the general pattern that on average predators at the top of the food webs are more similar in size to their prey than those closer to the base. Food–web stability is critically dependent on species loss. In two subsequent projects (Chapter 5, 6), I applied a bioenergetic model approach to simulate species loss in a set of (Chapter 5) 1000 model food webs and (Chapter 6) 30 empirical food webs randomly chosen from the food web data base . I analysed the stability of model food webs in respect of effects of topological, size–based, and dynamical properties. Stabiltiy has been messured as the number of secondary extinctions after removing one species from the network. The results show that food–web robustness is affected by factors from all three groups. However, the most striking effect was related to the body mass–abundance relationship which points to the importance of body mass relationships for food web stability. Additionally to the network–related properties (e.g. diversity, connectance), I analysed species related properties (e.g. body mass, trophic level). Overall, ecosystem-types (lake, stream, marine, and terrestrial ecosystems) react in the same way to species loss. I found food webs with high diversity and a low standard deviation of vulnerability were less affected by secondary extinctions. At the species level, consistent with classical conservation biology findings, I found that the loss of large–bodied top predators increases the extinction–risk for all others species in the ecosystem. The work presented here contributes to the understanding of underlying mechanisms and dynamics between interacting species in ecosystems. It illustrates differences between ecosystem types, where ”streams tend to be different than other ecosystems”. Overall, the studies show how energy fluxes can contribute to the stability of natural communities, how topological properties influence the interplay between animal populations and how complex communities react to species loss.