Disentangling the multiple dimensions of invasiveness
Considerable effort has gone into identifying characteristics of invasive species, yet a definitive list of traits remains elusive. Species can be invasive in different ways and different definitions of invasiveness are used. Plant traits associated with high abundance are likely to differ from those associated with fast spread rates and broad distributions, so species classified using different criteria may have distinct traits. Using evidence from ecological theory, literature reviews and quantitative models of alien plants’ biogeography, we contend that failure to distinguish among 15 forms of invasiveness may obscure invasive species’ traits, hindering accurate weed risk assessment.
A review of 24 prominent publications revealed that most invasive species classifications rely on combinations of four biogeographic criteria (local abundance, geographic range size, environmental range size, spread rate), plus impact. Supporting our assertion that species may therefore be invasive in 15 ways, all combinations of the four biogeographic criteria were used by at least one of the 112 studies that underlie a highly influential meta-analysis of invasive plant traits (van Kleunen et al. 2010 Ecology Letters). Notably, 14% of these studies defined invasiveness solely on high abundance while 44% did not include abundance as a criterion at all.
Using observational data from Victoria, Australia, we found that the maximum relative cover, range sizes and spread rates of 444 alien herbs were independent, apart from a correlation between species’ environmental and geographic range sizes. The probability of these 444 species being classified as invasive increased with species cover, but invasive and non-invasive species featured at the both ends of each biogeographic gradient. The Victorian data illustrates that species are classified as invasive for different reasons and that invasive species can possess distinct biogeographic characteristics.
Only comparing like with like should lead to more accurate conclusions about the traits that invasive species possess.