Predicting patterns of species turnover in response to nutrient addition and herbivory

Background/Question/Methods

Global change frequently leads to increased nutrient availability and alterations to food web structure.  These drivers can alter species turnover in space and time.  However, we have a poor understanding of controls on the magnitude and direction of compositional shifts, and whether sites that are susceptible to one driver are equally susceptible to another.  Furthermore, theory predicts that the response of a site should be affected by its species pool and background levels of fertility, but predictions have not been rigorously tested.  Here we address these questions using data on changes in species composition from a globally-distributed experiment manipulating nutrient availability and herbivory (NutNet).  Specifically, we ask: Does fertilization and removal of vertebrate herbivores lead to biotic homogenization? How do fertilization and reduced herbivory affect compositional stability? Are similarity and stability responses to fertilization and herbivory correlated across sites? And what factors explain variation in responses across sites?  Using mixed-effects models, we analyzed the effects of nutrient addition and reduced herbivory on three variables related to species turnover: spatial similarity (i.e. mean similarity within treatments), spatial turnover (i.e. mean similarity between treatments), and temporal turnover (i.e. mean similarity between year zero and year three of the experiment within treatments).

Results/Conclusions

We found no evidence of either nutrient addition or removal of vertebrate herbivores leading to widespread homogenization.  However, nutrient addition did cause greater species turnover in time, and the interaction between nutrients and herbivore removal led to greater species turnover in space.  Within sites, responses of spatial heterogeneity to nutrient and herbivory manipulations were uncorrelated.  However, for both spatial and temporal turnover, sites’ responses to herbivory and nutrient addition were correlated positively, suggesting that some sites were generally more susceptible to disturbance.  Adding data on species pool size and site soil fertility as covariates significantly improved model fits, suggesting that both of these factors modify the degree to which a site will experience species turnover in response to global change factors.