Disturbances, biodiversity changes and ecosystem stability

Disturbance by 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.

Biodiversity and ecosystem stability

Combined effect of species loss and disturbances

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.

Impacts of biodiversity change on ecosystem stability

There is concern about the potential effect of biodiversity loss on the functioning of ecosystems and their services to society. A key consideration is how much biodiversity can improve the stability of ecosystems. More stable ecosystems are more reliable providers of ecosystem services such as fish catches and stabilisation of coastal habitats.

In this study, the relationship between biodiversity and stability (as temporal variability) of marine benthos was investigated using existing datasets and by taking new samples at fifty rocky shores throughout Europe.

The overall outcome of the analysis of the existing datasets indicated a negative (although weak) relationship between diversity and stability. These relationships were observed at small and large scales, but there was variation in the outcome depending on which habitats and locations were considered.

In the sampling programme employed by BIOFUSE scientists, which was focused on emergent rock on rocky shores, there were generally no relationships observed. However, at small scales (areas of less than a metre), they observed a positive relationship between diversity and stability of the suite of species present. The relationships varied among regions.

Outcomes from both approaches led to similar results for rocky shores. Therefore, if sufficient datasets exist, meta-analysis of those datasets can provide a cost-effective alternative to collecting new data on diversity stability relationships.

Human disturbance and the stability of rocky shore assemblages

The structure and functioning of marine ecosystems is threatened by a range of human activities. The impact of these activities on the stability of ecosystems is not clear. The BIOFUSE project combined and reanalysed the results of a large number of experimental studies on impacts of disturbances on the stability of assemblages of animals and seaweeds on rocky shores.

Only some types of human disturbance have strong effects on the stability of rocky shore assemblages. Overall, the results of this study indicated that some types of disturbance, such as loss of large seaweeds and nutrient enrichment, did not influence stability. Other sources of disturbance (such as removal of organisms) can reduce the stability of intertidal assemblages.

An increase in the severity of disturbances in intertidal habitats is predicted as a consequence of increased frequency of extreme meteorological events (i.e. sea-storms and hurricanes). Similarly, the introduction of exotic species is increasing rapidly with the intensification of global trading. Management initiatives should focus their attention on responses to climate change and on reducing the impact of invasive species on rocky shore assemblages.