How exactly plant communities will respond to global change drivers (GCDs) remains uncertain; however, a recent study has shown that community composition is shifting in response to GCDs – the composition of control plots is increasingly different from treated plots over time. Surprisingly, this change is not associated with a systematic increase or decrease in species richness, begging the question, what about these communities is changing. Besides changes in richness, the three other potential ways a community can change are shifts in evenness, reordering (changes in rank), and turnover (gain and/or losses of species). Here, we explore these four aspects of community change in response to GCDs utilizing CoRRE, a database of 101 resource manipulation experiments. To be included in CoRRE an experiment had to manipulate a plant resource (water, light, nutrients, or carbon dioxide), but could have also manipulated other factors, such as temperature or herbivory. For each dataset between all consecutive time periods we measured changes in evenness and richness, reordering (changes in species ranks) and turnover (species gain and losses, separately).
Results/Conclusions
We used a multiple regression across all datasets to explain variation in composition change (measured as the Euclidean distance between the centroids of consecutive years in multivariate community space) with changes in richness, evenness, rank and gains and losses as explanatory variables. Across all datasets we were able to explain approximately half the variation in community composition change (R2 = 0.503, p < 0.001). We found changes in rank explained the most variation in community change (partial R2 = 0.32, P < 0.001), followed by gains (partial R2 = 0.09, p < 0.001), losses (partial R2 = 0.07, p < 0.001), changes in richness (partial R2 = 0.02, p = 0.03), and lastly changes in evenness (partial R2 = 0.01; p = 0.71). Next, we investigated whether the drivers of community change varied over time, utilizing a subset of CoRRE experiments that ran 10 years or longer (n=24). Overall, we found few temporal trends, i.e., as the experiment progressed the rate of gains or losses did not increase but generally remained the same. This research shows the importance of reordering, a drastically overlooked community component, whereby the predominate aspect of community change is the extant species rearranging, with species either gaining or loosing dominance.