The indirect role of species‐level factors in biological invasions

Abstract

AbstractAimBiological invasions are driven by factors related to species, locations and events. Species‐level factors (namely functional traits) often distinguish successful invaders from other groups of species in trait‐only analyses. However, traits are routinely found to be much less important than event‐level (e.g. propagule pressure) or location‐level (e.g. abiotic or congeneric effects) factors when included in the same model. We hypothesized that this rift exists largely because traits have (often overlooked) indirect effects that affect invasion success by influencing other mechanisms.LocationGlobal.MethodsWe developed a conceptual path diagram hypothesizing indirect effects on invasion success of species traits directly affecting (1) native range size, (2) propagule pressure, and (3) climate matching between native and non‐native ranges. We used structural equation modelling to evaluate our hypothesis using open‐access datasets on: (1) the naturalization success of non‐native Pinus in New Zealand (McGregor et al., 2012, Diversity and Distributions, 18, 1013–1023), and (2) the global establishment success of non‐native reptiles (Mahoney et al., 2015, Diversity and Distributions, 21, 64–74). Both these studies found little to no direct importance of traits on invasion success.ResultsOur analyses revealed indirect effects of species traits and native range size on invasion success. Traits indirectly affected Pinus naturalization success and reptile establishment success by affecting introduction effort and native range size. Consistent with the original analyses, traits had little measurable direct effect on invasion success.Main conclusionsSpecies‐level factors have both direct and indirect roles in biological invasions. Consideration of the indirect effects of functional traits may help to reconcile disparities in findings on the relative role of traits versus other contributors to the invasion process.