BIODIV-LOG · A global approach towards linking climate and marine biodiversity to ecosystem functioning

A global approach towards linking climate and marine biodiversity to ecosystem functioning

1. Diversity of plankton : a global approach Various hypotheses have been proposed to explain why the diversity of so many taxonomic groups increases from the poles to the equator. We formulated a new theory based upon the concept of the niche sensu Hutchinson and the principle of competitive exclusion, which shows that the latitudinal diversity gradient in the marine environment may result from an interaction between the thermal tolerances of species and climatic variability. To test this theory, we constructed a bioclimatic model that creates pseudospecies from strict stenotherms to large eurytherms and subsequently allows them to colonize a global ocean provided they can tolerate fluctuations in temperature. We test 74 ecologically realistic scenarios that are then correlated with observed patterns of species richness for foraminifers and copepods, two important oceanic planktonic groups. We found that the model accounted for 96% of the latitudinal gradient in foraminifers and 85% for copepods. Our model both reveals how patterns of biodiversity may develop, and suggests why some taxonomic groups appear not to follow the general pattern. By showing the strong effect of temperature on biodiversity and revealing how it enables the development of a planetary gradient in marine biodiversity, our results offer a way to better understand why temperature is so often positively correlated with global patterns in species richness on a global scale. A promising approach to increasing the mechanistic understanding of the structure and function of zooplankton communities is to look systematically at zooplankton trait distributions along latitudinal gradients, associated with temperature and other physical parameters. Species are usually described by mean trait values without considering their individual variations even though this approach can hide functional variations along environmental gradients. In our study, we wanted to investigate the intraspecific body size variability and spatial structuring for marine copepod species with a circum-global distribution in relation to environmental conditions. We found that copepods show a strong geographic variation in intraspecific body size but the pattern does not follow a latitudinal cline as stated by the Bergmann’s rule. Instead, large individuals occur everywhere, whereas small individuals occur mainly in lower latitudes and in transition zones. Against a backdrop of variability, the latitudinal pattern of intraspecific body size variation follows environmental gradients, in particular SST and seasonal variability in primary production. This result corroborates with previous experimental work where food availability in conjunction with environmental temperature was found to influence body size of copepods. The high intra-specific variability in copepod body size on a global scale and its environmental correlates highlights the need to consider the temperature regime and food availability in the environment when body size data is used for modelling purposes. The next step will be to investigate spatial variations at the community level and link with taxonomic diversity on a global scale. 2. Diversity of plankton: a regional application With the onset of the Marine Strategy Framework Directive (MSFD) in 2010, each European Member State is requested to select and apply ecological indicators to evaluate the Good Environmental Status (GES) in their national waters. As a result of this need, scientists have been engaged to provide advice and develop appropriate indicators at the national level and also engage in improving the coherence of regional assessment as requested by the MSFD. OSPAR and other Regional Sea conventions are playing a key role as a platform for EU member states to coordinate their approaches in implementing the MSFD on a regional scale. Also, ICES working groups, such as WGBIODIV and WGECO, are actively involved in this process. So as to harmonise management strategies and indicators to monitor change across regional seas, robust and objective procedures are needed for selecting the most effective metrics to perform each specified MSFD indicator function. Within ICES WGBIODIV, we developed a set of 16 indicator performance evaluation criteria and applied a quantitative analytical procedure to these criteria so as to score the performance of 33 metrics proposed as potential OSPAR “common indicators”. Of all the metrics, only 8 met the combined benchmarks for overall performance, adequate spatial coverage, and being close to operational. This result suggests the need to identify current shortcomings in the common indicators and consequently, address these issues so that the indicators can be made operational for regional assessment. Indicators based on plankton taxonomic diversity can be used to quantify the state of pelagic habitats in relation to climate change and anthropogenic pressures. To date, a large number of diversity indices exist to characterize plankton community structure but there is no consensus on which indices are most appropriate to evaluate the health of pelagic habitats. For examples, which plankton diversity indices reflect the underlying structure of the biological community and are less sensitive to environmental variation, and do the various indices respond differently to changes in their natural environment? Using three study sites in French waters, we tested several diversity indices for their mathematical performance, ecological relevance, and links to environmental conditions. We obtained a complimentary set of diversity indices that characterize specific structural differences in plankton assemblages and respond mainly to anthropogenic pressures rather than environmental variability, as requested by the MSFD. This result will feed into the OSPAR ICG-COBAM expert group on Pelagic Habitats to extend indicator development up to the regional sale.

Data: CORDIS, © European Union