Disturbances, biodiversity changes and ecosystem stability

Increased river outflow

Climate models predict increasing variance in rainfall, with increased frequency of droughts paralleled by unusual amounts of rainfall and floods. In anticipation of this, the Mediterranean region is now being subjected to extensive river damming, which can have far reaching impacts on coastal food webs. For instance, the diets of the five most abundant flat fish species of the Gulf of Lions and their prey depend on river inputs. The common sole largely profits from the contributions from terrestrial organic matter, via their main prey: deposit-feeding polychaete worms. Therefore inland climate changes may affect coastal marine food webs, through variation in river flow.

Combined effect

Climate change scenarios predict an increase in physical stress (e.g. by storms) and organic matter. Local activities cause the loss of some of the key species in the ecosystems such as large seaweeds, seagrasses and burrowing worms. It is not yet known how these different impacts might combine to affect ecosystem processes.

MARBEF scientists working on the BIOFUSE project used simple experiments to compare the effect of loss of a key species on a number of marine ecosystems, which were also subjected to an experimental disturbance. The goal was to find out whether the effects of biodiversity loss are the same across different habitats and locations.

The loss of key species affected many, but not all, ecosystems. The influence of loss of species and disturbance varied among habitats and locations. In only a few cases there were complex combined effects of these two impacts. There rarely was any influence on ecosystem functioning, which suggests a widespread capacity of ecosystems to compensate for the loss of a single species, even key species. This is good news with respect to these habitats, but the results showed variation between locations, something which is reflected in the EU Marine Strategy Framework Directive where there is emphasis on regional focus.

Species abundance and ecosystem functioning

Many species are being reduced in abundance or driven to local extinction by human activities. Although changing biodiversity clearly has consequences of the functioning of ecosystems, the relative importance of different kinds of changes is not clear.

MarBEF scientists on the BIOFUSE project used an experimental system of intertidal communities of algae and invertebrates to assess the effects of changes in key species on the functioning of the selected ecosystem. The results showed that changes in the abundance of certain species were more important than changes in the variety of species. The key result was that, although the effects of changes in diversity vary according to the habitat and location, the effects of changes in species abundance are much more consistent. Therefore alteration of key species abundances affects ecosystem functioning more than changes in species diversity. This outcome emphasises the importance of preserving not only particular species but also the relative abundances with which species populate our marine coastal environments.