Evidence of massive deviations in the climatic niches of over one thousand introduced plant species
Does a species retain its niche upon arrival onto a new continent? This question is central to our understanding of the ecological function of a species, and how ecological function changes when that species is introduced to a new range. Introduced species encounter unfamiliar biotic interactions and face novel environments in their new ranges, and can evolve rapidly in response to these foreign conditions. Thus evolutionary and ecological forces may cause the realized niche of an introduced species to diverge from the niche it occupied in its home range. How much and how often the niches of species change upon introduction remains the subject of considerable, active debate, and current evidence is inconclusive and often contradictory. In this study we use ordination techniques and ecological niche models to reconstruct changes in the realized climatic niches of over one thousand terrestrial plant species upon introduction to new continents. We use established as well as new techniques to characterize niche changes and to account for possible sources of sampling bias in over ten million species presence records across six continents.
At the time of this writing, we have concluded preliminary comparisons of the Eurasian and North American ranges of 131 terrestrial plant species out of the 1135 species for which we have data. These analyses suggest that the realized climatic niches of introduced species change dramatically upon introduction to a new range. In 118-122 (~90%) out of the 131 species studied to date, niche overlap between native and introduced populations fell below that expected among species (Schoener’s D < 0.4), suggesting extreme differentiation. Our results were highly sensitive to the method used to account for potential sampling biases. Nonetheless, trends emerged. Measures of niche overlap were consistently low, and tended to decrease further as we applied more rigorous methods to remove bias. Introduced populations occupied warmer, wetter, and presumably more productive climates than native populations of the same species, although this depended on the species’ home continent, indicating that northern range limits are more stable than southern limits. This suggests that introduced species may be better at overcoming range limits defined by species interactions rather than abiotic conditions. Overall we find evidence for major deviations in the native- and invasive-range climatic niches of an unprecedented number of terrestrial plant species.