| dc.description.abstracteng | Biodiversity is declining rapidly throughout the world. The rate of extinctions matches historical extinction waves and is likely to accelerate even further since two main drivers of biodiversity loss, global warming and nutrient enrichment, are predicted to increase in their impact over the coming decades. These extinctions are not only irreversible, diminishing the astounding diversity of life on our planet, but they also pose a direct threat to the health and wellbeing of humans. Thus, understanding the drivers and consequences of extinctions in existing species communities, and how they are affected by climate change and nutrient enrichment, is of vital importance.
In this thesis I examine different aspects of extinctions in food webs, representations of feeding interactions among species in a community. This accounts for the fact that, in nature, species interact and whatever affects one species also affects other species in the same community. Thus, species interactions have to be considered when investigating extinctions. In this thesis I used a dynamic food web model which, based on biological meaningful parameters such as growth, consumption and metabolism, tracks the flow of energy through complex food web structures.
To be able to assess species extinctions in changing environments I established a basic extinction risk for species in recent food webs. I found that the extinction risk of species is mainly governed by energy availability and dynamic stability of the population. Consequently, species that are at a high trophic levels, have few prey species that are of similar size bear, the highest intrinsic risk of extinction. I found that the time to extinction for species is determined by its body size and that small species will go extinct earlier than large ones. This implies that in the field the extinctions of small species will be detected earlier, even if the large species are already doomed to die and will go extinct as well.
Furhermore, I investigated trophic cascades, which describe the well known positive indirect influence a top species has on the populations of a species two feeding links down by controlling the population of the intermediate species. Many studies on trophic cascades are on isolated food chains since the manipulation of entire species communities is difficult. I compared the top species influence in an isolated chain and in a chain that was embedded in a food web. In isolated three-species food chains the impact of the top on the basal species was always positive, because it controls the population of the intermediate species. Whereas, when embedded in a food web, the top species impact was much more variable and could be both positive and negative. Its strength was determined by the body mass and abundance of both the top species and the basal species. At the example of trophic cascades, this demonstrates that accounting for species interactions in a food web context is important for accurately assessing the indirect effects of one species on another.
Warming directly affects growth, metabolism, feeding and death of organisms. Enrichment increases the energy availability in the food web. I account for the temperature dependency of biological rates and show that warming and enrichment interactively affect the dynamics of populations. At low temperatures, increasing energy input leads to increased population oscillations and therefore species extinctions. Warming decreases the flux of energy to higher trophic levels and thus counteracts the destabilizing effect of enrichment. However, with increasing temperatures the metabolism of species increases faster than their ability to consume food. This leads to extinctions due to starvation. These extinctions at high temperatures can be counteracted by enrichment induced higher energy levels. In combination, these two main drivers of global change and biodiversity loss can have both positive and negative effects on population stability.
In food webs, predator-prey body mass ratios are an important determinant of species persistence. Large body mass ratios decrease the strength of the interaction and increase species persistence. I found that body-mass ratios, temperature and fertility interactively affect food web connectance, species persistence and link distributions. Body-mass ratios influence the effects of temperature on persistence and determines the interactive effects of temperature and fertility. In food webs in which predator and prey species are roughly of equal size species persistence is increased by warming and reduced by enrichment. In food webs in which predators are substantially larger than their prey persistence is reduced by warming, but can be reduced or increased by enrichment. At high body mass ratios and high temperatures specialist consumers with only few feeding links are more likely to survive than generalist consumers. This influences food web connectance, which is reduced under these conditions. This study shows that the multi-faceted interactions of temperature, fertility and body mass ratios trigger varied responses in complex food webs. However, knowledge of the interactions is a first important step towards disentangling these effects.
This thesis elucidates different aspects of extinctions in food webs. It shows that accounting for the complexity of species interactions is necessary to fully apprehend the dynamics within food webs. Looking at undisturbed food webs and ones affected by warming and enrichment helps grasp the effects of global change on species communities. This thesis also identifies the complex interactive effects of drivers of species extinctions. It contributes to the sorely needed understanding of extinctions in complex species communities. | de |