Ecological network structure in response to community assembly processes over evolutionary time

The dynamical structure of ecological communities results from
interactions among taxa that change with shifts in species composition in
space and time. However, our ability to study the interplay of ecological
and evolutionary processes on community assembly remains relatively
unexplored due to the difficulty of measuring community structure over
long temporal scales. Here, we made use of a geological chronosequence
across the Hawaiian Islands, representing 50 years to 4.15 million years
of ecosystem development, to sample 11 communities of arthropods and their
associated plant taxa using semi-quantitative DNA metabarcoding. We then
examined how ecological communities changed with community age by
calculating quantitative network statistics for bipartite networks of
arthropod-plant associations. The average number of interactions per
species (linkage density), ratio of plant to arthropod species
(vulnerability), and uniformity of energy flow (interaction evenness)
increased significantly in concert with community age. The index of
specialization H2’ has a curvilinear relationship with community age. Our
analyses suggest that younger communities are characterized by fewer but
stronger interactions, while biotic associations become more even and
diverse as communities mature. These shifts in structure became especially
prominent on East Maui (~0.5 my) and older volcanos, after enough time had
elapsed for adaptation and specialization to act on
populations in situ. Such natural progression of specialization
during community assembly is likely impeded by the rapid infiltration of
non-native species, with special risk to younger or more recently
disturbed communities that are composed of fewer specialized
relationships.