C - Biodiversity
The function of ecosystems is largely determined by the taxonomic composition and diversity of their constituting species assemblages, often involving complex interactions between trophic levels as well as with abiotic factors, especially climate and soils. Many of these functions, so-called ecosystem services, are critical to human survival. Important services are for example the provision of food, the mitigation of floods and droughts, the purification of water and air, the preservation of soil fertility, and the pollination and pest control of crops. Yet the ecological understanding of the underlying role of biodiversity in providing these services is quite limited. Species that play central roles in a given ecosystem and are not “backed up” by other ecologically similar species are so-called keystone components. These are sometimes easily identified, but are just as often hidden among the high number of other species, especially in tropical forests. In other cases, however, many functionally roughly equivalent species are present, whose individual role is uncertain. Such “redundant” species have been hypothesized to provide an insurance of ecosystems against exogenic disturbances, but empirical evidence for this is still weak. Untangling the relative role of individual species, i.e., biodiversity-functioning relationships, and the response of ecosystems to man-made and natural disturbances is therefore one of the great challenges of biological research. Human impact on ecosystems takes place at multiple spatial scales, from global climate change, regional conversion of natural to managed ecosystem, to local disturbance by agricultural management practices.
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