Trait-based indices to assess benthic vulnerability to trawling and model loss of ecosystem functions

The physical impact of bottom towed fishing gears does not only reduce the abundance and biomass of species, but also alter the overall species composition and, through this, the functioning of benthic communities. The vulnerability of a species is determined by its individual combination of morphological, behavioural and life history traits. In turn, ecosystem functions are most affected when those species identified as vulnerable, contribute disproportionately to that function. On the basis of this paradigm, trait-based indices of physical resistance (RI) and recovery potential (RPI) were developed and combined into an overall vulnerability index on a species level, the RRI or Resistance and Recovery Potential Index. The developed indices can be used to explore how resistance and recovery potential of benthic communities change over different levels of trawling. Furthermore, the RRI allows for dividing the benthic community into groups expressing different levels of vulnerability that can be linked to ecosystem functions to explore functional vulnerability to trawling. The RRI index futher opens up the possibility for scenario modelling by simulating the extinction or loss of vulnerable species and its effects on functions. This may be of particular interest in data poor case studies that lack trawling gradient data, or to explore the consequences of potential increases in fishing effort. The validity of the trait-based RRI index was tested by comparing individual species’ RRI scores to empirically observed responses over a trawling gradient. RRI score and observed responses (regression slopes) were significantly correlated providing support for the rationality of the approach. Moreover, further analysis of the data evidenced clear increases of resistance and resilience indices over the trawling gradient, demonstrating that communities lost vulnerable species with increasing trawling. When exploring the effects of trawling on the bioturbation, as a chosen ecosystem function, we found it to be disproportionately affected though the loss of vulnerable species. The proposed indices provide new insights into the link of species vulnerability and function. Such information is of vital interest to environmental managers focused on preserving ecosystem functions and services in the face of anthropogenic global change.