Chemical stress on marine primary producers: Impact on ecosystem functioning
The impact of biodiversity loss on the functioning of ecosystems has become a central issue within ecology. Anthropogenic stressors such as toxic chemicals, which represent potential drivers of biodiversity loss, are however rarely included into biodiversity-ecosystem functioning research. Instead, ecosystem functioning is commonly measured in randomly composed communities representing broad gradients of taxonomic richness. It remains unsure whether this approach is representative of biodiversity loss and diversity-functioning relations as induced by chemical stressors.
In benthic diatoms, the main primary producers of the muddy North Sea intertidal, we contrasted evenness gradients created by random community assembly with those induced by applying the herbicide atrazine. Next, we tested if changes in ecosystem functioning in both sets of experiments are comparable, i.e. if diversity-functioning relationships inferred from classic random-assembly approaches predict pesticide-driven diversity-functioning relationships. We measured three functions that are particularly relevant for mudflat ecosystems: biomass production, sediment stabilization (production of extracellular polymeric substances) and energy content (fatty acid profiles) of the diatoms. Last, we tested if differences between the two types of diversity-functioning relationships could be explained by selective atrazine effects, i.e. to what extent species-specific contributions to functioning and species-specific sensitivity to chemicals combine in shaping diversity-functioning relationships.
Evenness gradients induced by atrazine were narrower than those obtained with random community assembly. Diversity-functioning relations were 3 to 6 times steeper in stressed than in randomly assembled communities. The large discrepancy between random and stress-induced diversity-functioning relations could be related to physiological effects as well as selective biodiversity alterations by atrazine. The latter consisted of a selective removal of the functionally most important species (Nitzschia sp.), that contributed most to biomass, sediment stabilization and energy content of the community, but proved to be most sensitive to atrazine.
Overall, the classic approach of broad biodiversity gradients and random species removal overestimated biodiversity loss induced by chemical stress. On the other hand, it underestimated the resulting diversity-functioning relations, which were shaped by atrazine effects on diatom physiology and diversity. Stress sensitivity and contribution to ecosystem functioning at the species level should be considered for a more accurate prediction of biodiversity and ecosystem functioning under anthropogenic stress.