Abstract
Species invasion represents one of the major drivers of biodiversity
change globally, yet there is widespread confusion about the nature of
non-indigenous species (NIS) impact. This stems from differing notions
of what constitutes invasive species ‘impact’ and the scales at which it
should be assessed. At local scales, the mechanisms of impact on
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 caused by
modifications that filter out other species. I developed a statistical
test to distinguish these four mechanisms based on community
rank-abundance curves and then created a scale-independent standardized
impact score. Using an example long-term dataset, that has high native
plant diversity and an abundance gradient of the invasive vine,
Vincetoxicum rossicum, I show that impact resulted in either targeted
extirpations or widespread biodiversity loss. 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.