Inverse priority effects: The order and timing of removal of invasive species influence community reassembly

1. An ongoing restoration challenge is to recover native communities after
the removal of invasive species. Because priority effects (i.e., the order
and timing of species arrival) can strongly determine the trajectory of
community assembly, their intentional manipulation is gaining attention to
manage invasive plants and achieve restoration goals. Yet, ecologists and
conservationists rarely consider how the order and timing of species
removal inverse priority effect may impact future plant communities. 2.
Here, we evaluated the dependence of community reassembly on inverse
priority effects by experimentally removing the target invasives
Sweetbriar rose (Rosa rubiginosa) and Scotch broom (Cytisus scoparius) in
field and mesocosm communities. We manipulated removal order
(rose-before-broom vs. broom-before-rose) and timing (simultaneously early
vs. simultaneously late in the season). We performed a Hierarchical
Modeling of Species Community to assess differences in community structure
in response to order and timing of removal, and to evaluate whether
species origin and leaf and seed traits were associated with species
responses. 3. We found that the order of removal was as important as
timing driving community reassembly. Simultaneous removal favoured
nonnatives, more so when performed early. Sequential removals led to
contrasting communities. Rose-before-broom removal also favoured nonnative
grasses at expense of native species, whereas the inverse order produced
small changes in communities. In general, species with high specific leaf
area were boosted, regardless of their seed size. 4. Synthesis and
applications. Inverse priority effects are neglected mechanisms that can
drive variability in the reassembly of plant communities and can
potentially upgrade invasive species management. These historical
contingencies suggest the existence of an optimal order of removal that
facilitates the recovery of the native community. We found that
simultaneous removal promoted secondary invasions to a greater extent than
sequential removals. Furthermore, removal order affected post-removal
community structure. In our system, we suggest removing the rose before
the broom to hinder nonnatives and pave the way for restoration of native
communities. Our results show that manipulation of the order and timing of
removal can help to achieve restoration goals.