Invasive species have severe impacts on native biodiversity, ecosystem functioning and health. A major goal of invasion biology is to predict the establishment of invasive species under novel environmental conditions outside of their native range. Much work has focussed on species distribution models (SDMs) to predict species’ potential distributions. These models make the strong assumption of niche conservatism, meaning that invasive populations occupy the same ecological niches as native populations. However, this assumption could be wrong either because the fundamental niche is larger than the realised niche of native populations or because species have adapted to novel environmental conditions in their introduced range. Both can lead to apparent niche shifts in the invaded range. To test the validity of these models’ assumption, we determined the frequency of climatic niche shifts among all 241 alien ant species. Large-scale assessments of niche shifts are rare, especially for animals. We tested niche conservatism using multivariate methods to quantify the n-dimensional hypervolume (a representation of niche space) that encompasses the climatic conditions in each species’ native and non-native ranges and compared the extent of niche shifts in invasive species (with ecological/economic impacts) and alien species (no known impacts).
Results/Conclusions
The amount of niche expansion in the non-native range was significantly higher in invasive species compared to alien species. Moreover, larger range size and establishments on multiple continents were linked to a greater extent of niche expansion. Niche expansion therefore may be key to the success of the most invasive species. Using first records of observation per country, we modelled the n-dimensional hypervolume of the occupied niche space for each species (for which data was available) over time. This allowed us to explore the temporal dynamics of niche shifts. We could distinguish between two types of dynamics (A) a gradual increase in niche space over time and (B) larger niche shifts, which show a large initial niche expansion that saturates over time. Type-B dynamics were more frequent in invasive species, and in those that had spread over multiple continents. Assessing temporal dynamics of niche shifts has allowed us to pinpoint differences between species in their potential to further expand their niches. Type-B species have likely filled their entire fundamental niche and this niche space can be used to model suitable environmental conditions. On the other hand, type-A species are less predictable because they are still expanding their niche.