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.