Many of the earth’s ecosystems are experiencing large species losses due to human impacts such as habitat destruction and fragmentation, climate change, species invasions, pollution, and overfishing. Due to the complex interactions between species in food webs the extinction of one species could lead to a cascade of further extinctions and hence cause dramatic changes in species composition and ecosystem processes. The complexity of ecological systems makes it difficult to study them empirically. The systems often consist of large species numbers with lots of interactions between species. Investigating ecological communities within a theoretical approach, using mathematical models and computer simulations, is an alternative or a complement to experimental studies. This thesis is a collection of theoretical studies. We use model food webs in order to explore how biodiversity (species number) affects the response of communities to species loss (Paper I-III) and to environmental variability (Paper IV).
In paper I and II we investigate the risk of secondary extinctions following deletion of one species. It is shown that resistance against additional species extinctions increases with redundancy (number of species per functional group) (Paper I) in the absence of competition between basal species but decreases with redundancy in the presence of competition between basal species (Paper II). It is further shown that food webs with low redundancy run the risk of losing a greater proportion of species following a species deletion in a deterministic environment but when demographic stochasticity is included the benefits of redundancy are largely lost (Paper II). This finding implies that in the construction of nature reserves the advantages of redundancy for conservation of communities may be lost if the reserves are small in size. Additionally, food webs show higher risks of further extinctions after the loss of basal species and herbivores than after the loss of top predators (Paper I and II).
Secondary extinctions caused by a primary extinction and mediated through direct and indirect effects, are likely to occur with a time delay since the manifestation of indirect effects can take long time to appear. In paper III we show that the loss of a top predator leads to a significantly earlier onset of secondary extinctions in model communities than does the loss of a species from other trophic levels. If local secondary extinctions occur early they are less likely to be balanced by immigration of species from local communities nearby implying that secondary extinctions caused by the loss of top predators are less likely to be balanced by dispersal than secondary extinctions caused by the loss of other species. As top predators are vulnerable to human-induced disturbances on ecosystems in the first place, our results suggest that conservation of top predators should be a priority. Moreover, in most cases time to secondary extinction is shown to increase with species richness indicating the decay of ecological communities to be slower in species-rich than in species-poor communities.
Apart from the human-induced disturbances that often force species towards extinction the environment is also, to a smaller or larger extent, varying over time in a natural way. Such environmental stochasticity influences the dynamics of populations. In paper IV we compare the responses of food webs of different sizes to environmental stochasticity. Species-rich webs are found to be more sensitive to environmental stochasticity. Particularly, species-rich webs lose a greater proportion of species than species-poor webs and they also begin losing species faster than species-poor webs. However, once one species is lost time to final extinction is longer in species-rich webs than in species-poor webs. We also find that the results differ depending on whether species respond similarly to environmental fluctuations or whether they are totally uncorrelated in their response. For a given species richness, communities with uncorrelated species responses run a considerable higher risk of losing a fixed proportion of species compared with communities with correlated species responses.