Discussion
Global change is influencing plant community structure by leading to shifts in species dominance and competitive hierarchies. Determining the co-occurring density-independent and dependent mechanisms underlying these changes is critical to accurately predict net outcomes for community structure and biodiversity maintenance over long time scales. While this concept is not new to community ecology, few studies (to our knowledge) have fully parameterized the density-dependent and independent components of species changes over long time scales under multiple scenarios of global change.
Our work highlights the importance of density-dependent mechanisms, including shifts in intra- and interspecific competitive strengths, in driving long-term changes in the abundance of species groups under global change. We show that these competitive interactions can shift with the environment, in our system primarily with N, and drive non-linear species responses across environmental gradients further influencing community structure. Furthermore, despite significant shifts in community structure under global change, community stability can persist, or even increase, given that the dominant species maintains negative density-dependence.
Overall, our results provide a clearer understanding of how global change can lead to either community reshuffling and varying degrees of diversity decline through reduced evenness of species groups, and that only considering density-independent responses to the environment fails to or only partially explains these outcomes. It is well supported that that direct (i.e. density-independent) species responses to the environment insufficiently predict how communities will be restructured under global change (Suttle et al. 2007; Liancourt et al.2013; Alexander et al. 2015). Yet many species distribution modeling (SDM) approaches continue to utilize only climatic or environmental constraints when predicting future species distributions (Davis et al. 1998; Swab et al. 2015; Roe et al.2021). Our work suggests that while this approach may inform changes for some species groups under certain global change scenarios, such as dominant species responses to N, it is limited in understanding community-level responses to multiple global change drivers, which is critically needed for the maintenance of biodiversity (CITE).
Finally, our approach of utilizing species groups based on dominance rather than estimating species specific patterns proved highly useful for predicting changes in community structure over time. Initial abundance rather than the functional mechanisms of a species were shown to be a strong predictor of species losses under atmospheric N deposition across ecosystem types (Suding et al. 2005). In addition, a recent study of plant responses to climate change in the Arctic tundra suggest that commonness itself may be a strong predictor of response types, as rates of change in taxa over time were related to the baseline commonness of species early in the experiment (Postet al. 2021). Rarity and dominance within a community are only informative at local to regional scales; these designations are based on the species pool in which the species exists, as well as the mechanisms driving its local dominance vs. rarity, making global scaling difficult. In addition, what formally defines dominant vs rare species in a spatiotemporal setting is still somewhat elusive, and considering both the relative abundance and frequency of a species as well as its ecological impacts on the community is critical (Avolio et al.2019). Despite these caveats, understanding how and why patterns of dominance will shift with global change is of critical importance for predicting novel community assemblages and corresponding changes in community diversity and stability.