Competition, trophic interactions and the impact of habitat fragmentation on species richness

Abstract

The world is experiencing a rate of species extinction that is unprecedented in human history. The most important driving force of this global biodiversity loss is considered to be human induced alteration of ecosystems, leading to the destruction of species habitats. Quite some modeling studies have been done in order to gain insight in the impact of habitat fragmentation on the abundance of species that follow a trade-off between competitive strength and colonizing ability. Others have studied the impact of habitat fragmentation on species that interact with each other trophically. Almost no research has however been done that includes both a trade-off and trophic interactions. This is surprising because both concepts are highly compatible. It is probable that trophic interactions and competitive coexistence made possible via a trade-off, co-occurs within many ecosystems. Such systems will probably respond in a different way to habitat fragmentation than is predicted by models that include a only a trade-off or only trophic interactions. In this thesis results are presented and discussed of models that incorporate a group of resource and a group of consumer species. The species within both groups compete and coexist via a trade-off between competitive strength and colonizing ability. Resource and consumer species influence each other by trophic interactions, i.e. donor control and top-down control. Among other things, the results of these models show that the impact of habitat fragmentation on resource species depends strongly on the strength of trophic interactions. When trophic interactions are strong, habitat fragmentation may lead to an increased species richness of resource species and favors competitively strong resource species, that are poor colonizers. When trophic interactions are weak, the impact of habitat fragmentation on resource species is opposite. Also the impact of the degree wherein areas of different size are isolated may have a very different impact on resource species richness, depending on the strength of trophic interactions. An increased migration between areas of different size will lead to the loss of resource species when trophic interactions are strong. Again the opposite is true when trophic interactions are weak. Conventional conservation policies that try to deal with habitat fragmentation, are often based on a theoretical view of ecosystems that does not include competition nor trophic interactions. If the theoretical description of ecosystems presented in the thesis is correct, other conservation policies than the conventional would probably be the most effective.