Island Plant Species Distributions Contracted at the Cooler Edge Compared to Mainland

Continental islands have long been used as ecological models for understanding species assembly dynamics in isolated habitat fragments. But competition or colonisation constraints might be different to mainland populations, manifesting as expanded or contracted ranges across a geographic distribution of islands in comparison to a mainland population range. Here, we demonstrate that plants on coastal islands do not experience ecological release due to lack of competition, but rather a contracted range at the cool edge in a cross-continental dataset of 843 small coastal islands spanning contrasting environments fringing the Australian coast. We found the cool edge of species ranges across their distribution of small islands averaged 2.2°C warmer in mean annual temperature, or about 4-500 km nearer the equator. The tendency not to colonise islands at the cool edge suggests species may struggle to track their niche poleward as the climate shifts over fragments of habitat on the mainland.