Assessing intensification-driven functional diversity loss in arable plants: A meta-community approach
Increasing land use intensification is driving biodiversity losses worldwide, which in turn can reduce the functionality of ecosystems. However, it is increasingly clear that not all species are equally important for ecosystem processes. Thus, whereas the loss of a functionally unique species will reduce the capacity of the community to perform some function, the loss of a functionally redundant species should have a much smaller impact. In this context, assessments of the changes in functional trait diversity –a proxy of the range of functions provided by a community– as species are lost appears as a promising tool to predict the impacts of land use intensification.
Here, we modified a recently developed method that compares the changes in functinal diversity (FD) caused by random losses of species with those expected under the most likely order of species loss, which is estimated using the meta-community nestedness matrix (Sasaki et al. 2014). Based on the differences in the rate of decrease in FD, we developed an index of functional vulnerability (FV) to species loss; FV values greater than 0 indicate that, under the most ecologically plausible order of species loss, FD is reduced faster than under random extinction order, whereas FV values smaller than 0 indicate the opposite result. We applied this method in weed communities from 78 agricultural fields in the area of Madrid (Spain), conforming a gradient of agricultural intensification at two different spatial scales: Field and Landscape. We calculated FD based on three life history traits important for plants (Height, SLA and Seed mass). We calculated FV at each field, and studied how it is affected by intensification at the two considered scales using GAM.
We found that the vulnerability of FD to species loss increased with agricultural intensification. Interestingly, only intensification at the field level showed a significant effect on FV (R2 = 31.7%), whereas the effect of intensification at the landscape level was much smaller and non-significant. Moreover, the GAM analysis revealed that the response of FV to intensification at the field level was markedly non-linear, with great increases at the first stages of intensification, and much smaller increases afterwards. Our results suggest that field-level agricultural intensification does not only reduce the taxonomical and functional diversity of weed communities (Guerrero et al. 2014), but also eliminate functionally redundant species, thus increasing their vulnerability to species loss.