Islands have been widely recognized as natural laboratories in ecological and evolutionary studies (1, 2). Compared to mainland species, species endemic to islands (i.e., island endemics) may only occupy a small part of their fundamental niche due to dispersal limitation caused by surrounding water (3). When being introduced to new areas, island endemics very likely expand their realized niche as a consequence of the increased availability of suitable environments (3). Understanding such niche changes of island endemics is not only important for predicting their future distribution in areas where they were introduced but also for anticipating the response of other species with limited distributions to ongoing climate change.
Stroud (4) heuristically compared niche breadth of island endemics between their native and introduced range using data we compiled for Liu et al. (5). He found a much lower breadth ratio (the ln-transformed ratio of the breadth of the native niche to that of the introduced niche) for island endemics than mainland (continental) species and concluded that island endemics experienced much larger niche expansion during invasions than mainland species. However, we noticed that all of the island endemics in this dataset are amphibians and reptiles, after excluding two species from Australia. Hence, to control for the influence of focal invaders (5), we compared niche dynamics of species from islands and mainlands only for the subset of 71 amphibians and reptiles, using the same Bayesian models as in Liu et al. (5).
Our results are consistent with Stroud (4). Island endemics demonstrated a very low breadth ratio (−0.72; hereafter, mean effect sizes are represented by the mean of Bayesian posterior values), meaning that their introduced niche was around two times larger than the native niche. However, the breadth ratio of mainland species was 0.57, indicating a much larger native than introduced niche. In contrast with mainland species showing large niche unfilling (0.40) and low niche expansion (0.14), island endemics showed low niche unfilling (0.05) but large niche expansion (0.48), suggesting that they already occupied most of their native niche and a considerable extent of the novel environmental space in new ranges.
Together with Stroud (4), we reveal the very high potential of island endemics to expand their niche in new areas, at least for amphibians and reptiles (3). Future studies are needed to investigate the generality of this finding for other taxonomic groups. Islands are frequently considered to be invasion hotspots [cf. the island susceptibility hypothesis (6, 7)], but little attention has been paid to species introduced from islands. Given the next decades will probably witness increasing invasion risks of island endemics across the globe (8), partly caused by an increasing number of assisted migrations of island endemics threatened by climate change (9), we need to better understand niche changes and associated spatial expansions of introduced island endemics.
Acknowledgments
We are grateful to James T. Stroud for providing data on the classification of island endemics.
Footnotes
The authors declare no competing interest.
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