Timing of invasive species removal influences nonnative biotic resistance and trajectories of community reassembly

As biological invasions increasingly threaten biodiversity, the removal of
invasive nonnative species emerges as a possibility to recover the
structure and function of native communities. Yet, we have limited
knowledge of how communities assemble after nonnative removals. Since most
ecosystems are invaded by multiple nonnative species, the impact of their
removal likely depends on the interactions among nonnative species which,
in turn, are contingent on the environmental context in which they
occur.  We evaluated the community assembly after the targeted
removal of two highly invasive shrubs, Sweetbriar rose (Rosa rubiginosa)
and Scotch broom (Cytisus scoparius). The removal was performed at two
different times in the growing season (early or late removal) in field and
mesocosm communities. In search of general patterns across species, we
modeled species responses as a function of their origin (i.e., native /
nonnative) and functional traits. We found evidence for negative and
asymmetric interactions between dominant invasive species that translated
into changes in the abundances of the rest of the species in the
community. Depending on the identity of the removed species, the removal
of invasive species affected community assembly by promoting other
nonnative species or hindering the performance of native species. These
effects were modulated by the timing of removal and did not depend on leaf
or seed traits. Synthesis: Accounting for nonnative interactions and their
temporal dependency should improve our inferences about assembly
processes and the effectiveness of nonnative removal aimed at
reducing the accumulation of nonnatives.