Biodiversity changes and ecosystem functioning

Biodiversity changes

The most fundamental meaning of biodiversity is probably the concept of species richness, i.e., the number of species which occur at a given site, or within a region or an ecosystem. Higher diversity has often been seen as an advantage for an ecosystem to better cope with environmental fluctuations (insurance hypothesis). A large species pool might increase the resilience of an ecosystem to changes in environmental conditions or to anthropogenic impacts. The value of biodiversity as an indication of environment health and for the functioning of ecosystems has been recognised not only by scientists but also by decision-makers and the general public. The maintenance of high diversity is often seen as something positive to aim for; and to “halt the loss of biodiversity” has become a major political aim. It is now clear that marine ecosystems are at risk, especially those receiving the most sustained and unrelenting pressure from human activities, such as estuaries, intertidal shores and coastal waters. The composition of species within marine communities can change in three main ways: • species may be lost (extinctions); • species may be added (invasions or speciation); • species’ relative abundances can change (rare species become abundant, abundant species become rare). Although the number of rare species comprise the majority of the taxa in a biologically diverse region, they do not play a quantitatively important role in the structuring of the community. However, when species are removed or added to a community, and thus the biodiversity changes, the energy flow, predator-prey interactions or food web-related processes may change dramatically. As a consequence, the productivity of the seas is directly affected. Structurally complex habitats are becoming rarer across European marine environments. Habitat heterogeneity is another important factor when describing biodiversity. Presently, a gradual transition from very complex to simpler habitats has been observed. MarBEF explored the numerous ways in which habitat loss can affect marine species diversity, and thus community structure, and some examples are presented in this booklet. The loss of habitat structure is generally thought to lead to lower abundance (biomass) of key species and often to a decline in species richness. However, experiments in different coastal areas of Europe, performed within MarBEF, have shown that the removal of key species does not always affect the stability of the ecosystem and that effects depend on where, when and what species are removed. For example, an invading species may replace a resident species which plays an ecologically important role for ecosystem structure and functioning. The ecosystem may continue to function and provide similar services, but not necessarily in the same way as before. MarBEF also showed that, despite increasing pressure from overfishing, habitat destruction and pollution, species richness appears to be on the increase in many coastal and marine European waters. This is due to the establishment of non-indigenous species, especially of warm-water 56 affinity, and to a