FISHECO · Fish community structure and ecosystem properties in a global change context

Fish community structure and ecosystem properties in a global change context

Executive Summary: The human-induced collapse of species has triggered a sixth mass extinction crisis worldwide. This ongoing biotic impoverishment may, in turn, markedly alter key ecosystem processes, such as productivity, nutrient cycling, and bioerosion, with undisputed consequences on ecosystem services that humanity needs to prosper. Beyond the loss of species, the loss of particular functions appears to be the main threat imperiling ecosystem processes and services. When several species perform similar functions, this functional redundancy may insure against the loss of ecosystem functioning following declines in species diversity. The critical issue is whether the extraordinary species diversity on Earth matters for ecosystem functioning or whether a smaller proportion of species is enough to perform most of the key functions. This debate, at the core of ecological science, is even more vigorous in ecosystems under multiple pressures that experience unprecedented biodiversity erosion like coastal ecosystems where fish populations are declining worldwide and even more particularly in European waters be they coastal lagoons, estuaries, the Atlantic Ocean, the Mediterranean Sea and coral reefs. Beyond aesthetic and moral arguments, the importance of conserving the whole coastal fish biodiversity for maintaining the breadth of potential functions performed in marine ecosystems is still under scrutiny. Indeed, fishes play a remarkable diversity of functions that sustain essential ecosystem processes (e.g. trophic control, bioerosion, nutrient cycling) with some species performing unique roles, apparently irreplaceable. We had two main objectives in this research program with the common goal to bring a fresh view on species and their assemblages using the framework of functional ecology and extensive databases on marine fishes. The first objective was to evaluate the apparent level of functional insurance provided by the high biodiversity of fishes in coastal waters and on coral reefs. The key question was: do we need all this biodiversity to perform key functions or is the system buffered against species loss? Our results indicate that, even in tropical systems like coral reefs, we can no longer assume that the erosion of species diversity can be discounted by the high probability of functional redundancy since species tend to disproportionally pack into a few particular functions instead of spreading evenly and insuring most functions, leaving a high functional vulnerability with many functions performed by one species only. Moreover we show that rare species, those that tend to go extinct more rapidly in face of various pressures (fisheries, pollution, habitat degradation), support those vulnerable functions. Taken together we point out that the promised benefits from massive levels of species richness may not be as strong as we have though before and that ecosystem functioning remains fragile to biodiversity erosion even that of rare species. This kind of assessment may guide future conservation strategies towards critical functions and may call for a new generation of experiments that will specifically address the role of rare species in ecosystem functioning. The second objective was to evaluate the ability of functional diversity, assessed through the diversity of species traits within assemblages used as a proxy for the diversity of functions, to unravel the severity and type of human-mediated disturbance. Traditionally, the assessment of human impacts on ecosystems is based on the decline of populations or on the loss of vulnerable habitats and species. However, the level of functional diversity within communities may provide advanced warning of disturbance to ecosystems since they do not need species loss to be reactive. Instead functional diversity react as soon as some traits are depleted in the system under pressure (e.g. fish with large body size under fishing pressure). We extracted the effect of human activities on the taxonomic and functional diversity of fish communities while teasing apart the influence of biogeography and habitat across the Pacific. We found that species richness of fish communities decreased by 11.7% along the whole human density gradient while functional diversity dropped by 46.6%. Our results call for caution when using species richness as a benchmark for measuring the status of ecosystems since it appears less responsive to variation in human densities than its functional counterpart that is early and drastically depleted by human activities, potentially imperiling the functioning of ecosystems. More practically our study call for the implementation of trait- or function-based indicators in the assessment of disturbance or ecosystem health. Our findings, beyond the most conceptual and theoretical breakthroughs, have immediate socio-economic impacts at the EU level. We first show that the conservation of biodiversity on coastal areas has more to offer than protecting a natural heritage; it guarantees to sustain the breadth of key functions necessary for providing goods and services upon which human welfare depends on a long-term basis. More practically, in line with IUCN (http://www.iucn.org/) objectives but for different reasons, we suggest to focus on rare or threatened species that perform unique and irreplaceable functions in coastal and coral reef ecosystems. This change of paradigm in conservation science, from a taxonomic towards a functional orientation, is entering socio-political agendas and urges managers of protected areas as well as conservation agencies to also think in terms of functions beyond the mere level of species diversity. The other face of the same coin is to develop and apply indicators that take into account the diversity of functions within ecosystems to assess their health and their need of restoration. For instance this new generation of indicators, based on species traits as surrogates for species functions, may serve European strategies like the Marine Strategy Framework Directive (MSFD), the water framework directive (WFD), the Common Fishery Policy (CFP), the European Regional Sea Conventions like the Barcelona Convention, the OSPAR Convention, as well as the Action Plans by HELCOM and BSC.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз