Uncovering structural features that underlie coexistence in an invaded woody plant community with interaction networks at multiple life stages

Understanding the patterns of competitive and facilitative interactions
within and among species in plant communities is a central goal of plant
ecology, because these patterns determine species coexistence and
community dynamics. Network theory provides tools that allow these
patterns to be quantified, and can provide greater understanding of
important community properties, including community stability, than can
documenting pairwise species interactions. I characterized the
interactions of multiple, co-occurring invasive and native species in an
old field woody plant community to build plant interaction networks at two
different life stages. With the goal of identifying structural features
that may operate to maintain species coexistence, I characterized the
architecture of these networks at multiple scales: the entire network, the
substructures that compose the network, and species' roles within
substructures. I found that species-level pairwise interactions alone did
not provide an accurate or sufficiently detailed picture of community
structure. Rather, using a network approach, I identified substructures
that have the potential to promote and hinder species coexistence in
interactions among seedlings. Characterizing the nuances of network
substructures was illuminating, as the size of the substructures and the
pattern of interaction intensities within substructures influence the
expected effects on species coexistence. Including interactions at
multiple life stages was also important; the seedling species that
benefited most from the nested structure of facilitative interactions with
adults occupied subordinate roles in substructures with other seedlings.
This role reversal at different life stages is a potential factor
promoting coexistence in the community. Last, the network framework was
useful for comparing species' roles between native and invasive
members of the community, and the three invasive species in this system
had different, life-stage dependent strategies in interactions with
co-occurring plants. Synthesis. The interplay of network architecture and
substructures within plant communities and among plants at different life
stages is important for understanding species coexistence. In the plant
community characterized in this study, there were several features that
may promote coexistence, and these features were not observable in
interactions within a single life stage or when considering pairwise
interactions independently.