As climate changes, species with specific climatic requirements and that cannot adapt to changing conditions must either relocate to newly-suitable regions or perish. In order to prioritise species’ candidacy for Managed Relocation, we must answer two questions: 1. which species have distributions that are truly limited by their climatic requirements, and 2. which species will face difficulty in relocating themselves to future climatically-suitable regions unaided? We address these questions to investigate the level of need for Managed Relocation, using mammal and amphibian assemblages on the west coast of the USA.
Question 1, i.e. which species are actually limited by climate, is important because species distributions may be limited by non-climatic factors. Such species might currently occur within particular climatic conditions, without being dependent on them and so remain unthreatened by climate change. Alternatively, widespread species that occur across a range of habitats may actually have strong climatic requirements and thus be threatened by climate-change. We address this by i) creating a metric for the ‘uniqueness’ of the climate space a species occupies, thus indicating the strength of its climatic requirements, and ii) by estimating the relative roles of climate and habitat in determining a species’ distribution.
Question 2 addresses the effects of barriers such as geographic features (mountain ranges, deserts) and human landscape-alteration (urbanisation, intensive agriculture) on species’ likelihood of shifting their geographic range through dispersal. We propose ‘climate-paths’ as a new way of incorporating these effects into climate-change risk assessments. Climate-paths are the landscapes through which species must move in order to reach their future climatically-suitable range. By measuring the climate- and habitat-suitability of these landscapes, and their degree of fragmentation, we assess the relative likelihood of a species tracking its climate-niche without assistance.
Assessing the relative strength of species’ climatic associations (question 1) identifies a number of species that may be threatened by climate-change, including some that are fairly widespread and not currently considered threatened. Climate-path estimation (question 2) indicates that species have varying abilities to reach future ranges unaided. Geographic features appear to be the most frequent barriers to this process, but their effects are exacerbated by densely urbanised areas. We combine these results to identify species for which Managed Relocation is likely to be required. We then summarise the geographic regions in which both strong climatic effects on distributions, and frequent dispersal barriers, indicate that the need for Managed Relocation may be most urgent.