Abstract
Species invasion represents one of the major drivers of biodiversity
change globally, yet there is widespread scientific and popular
confusion and controversy about the nature of non-indigenous species
(NIS) impact. This confusion stems from differing notions and
understanding of what constitutes invasive species ‘impact’ and the
scales at which it should be assessed. I argue that the proximate
mechanisms determining invasive species impact happen at smaller scales
where species interact, and by understanding these mechanisms, we can
scale up to a broader understanding of how invasive species impact
biodiversity. The mechanisms of NIS impact on potential competitors can
be classified into four scenarios: 1) minimal impact from NIS inhabiting
unique niche space; 2) neutral impact spread across the community and
proportional to NIS abundance; 3) targeted impact on a small number of
competitors with overlapping niches; and 4) pervasive impact that is
disproportionate to NIS abundance and ostensibly caused by ecosystem
modification that filters out other species. I develop a statistical
test to distinguish these four mechanisms based on community
rank-abundance curves. Using an example dataset from plant communities
invaded by the dominant invasive vine, Vincetoxicum rossicum, I show
that in long-term plots that had high native plant diversity and where
V. rossicum increased, impact resulted in either targeted extirpations
(scenario 3) or widespread biodiversity loss (scenario 4). Regardless of
whether NIS impact is neutral, targeted or pervasive, the net outcome
will be the homogenization of ecosystems and reduced biodiversity at
larger scales, perhaps reducing ecosystem resilience.