Associate Editor, Pieter De Frenne (Ghent University, Belgium) discusses assisted migration, climate change and the recent article by Brooker et al. Tiny niches and translocations: the challenge of identifying suitable recipient sites for small and immobile species.

One of the key outstanding issues in applied ecology is to better inform land managers and policy makers how to adapt to climate change. Many species are currently shifting their range towards the poles or towards higher elevations in mountains. Numerous other species, however, are essentially unable to move. On the one hand, their slow colonization rates may be inherent to their life history characteristics. For instance, as one of the more extreme examples, bluebells (Hyacinthoides non-scripta) move only at speeds of a few meters per 100 years (yes, per century!) due to their slow life cycle and the absence of specific adaptations for long-distance dispersal such as winged seeds. On the other hand, other species are potentially faster colonisers but are hampered by their inability to grow outside of the current habitat. In many parts of the world, biodiversity hotspots in protected areas, for instance, are small, scattered and surrounded by largely unsuitable habitats such as farmlands, cities or plantations.

Because current management practices may not suffice to avoid biodiversity loss in the face of climate change, assisted migration (also often referred to as assisted colonization) is an increasingly considered management option to adapt to climate change and to circumvent (some of) these issues. Assisted migration is the active translocation of populations, species or even entire ecosystems (e.g., soil plus vegetation) towards currently cooler sites (e.g. at higher elevation) where they do not yet occur now or in the recent past. Once a manager decides to go for this option, there is still an important question to address: where exactly to put the transplants? This question is further complicated if the natural habitat of an immobile species exhibits large microclimatic heterogeneity due to factors such as vegetation shading or topography. Such heterogeneity can easily result in temperature differences of several degrees within the same landscape, and thus mean the difference between life and death of the transplant.

In a recent Journal of Applied Ecology article, Rob Brooker from the James Hutton Institute in Aberdeen and colleagues addressed exactly this question, bearing microclimate in mind. Their study species is an arctic-alpine lichen, Flavocetraria nivalis, which has almost all of its British records within the Cairngorm Mountains of Scotland. Even though it has a very wide global distribution, occurring in the arctic-alpine regions of North America, Patagonia and Scandinavia (where it is very common and its ground cover percentage remained basically unchanged over the last 15 years, the populations in Scotland are at their warm range limit. Within Scotland, conditions are such that the lichen only reproduces vegetativelly, by means of thallus (vegetative tissue) transport, and does not produce spores that are capable of long-distance dispersal. Hence, the British populations are virtually unable to move, and most certainly not fast enough to track the contemporary shifting isotherms. All these factors together make the species an excellent candidate to consider for assisted migration towards currently microclimatically cooler sites.

To determine which locations are suitable for the species, the authors first undertook a survey of environmental conditions associated with Flavocetraria presence and absence across its natural UK range in the Cairngorms. The study is exceptional in that the authors subsequently combined species distribution modelling, actual microclimate measurements using miniature temperature sensors and a transplant experiment.

The authors find that survival of the transplants was best predicted when the modelling encompassed microclimatic temperature effects. The a priori model, based on current natural occurrence records within Scotland but without the microclimate included, did not produce satisfactory results and a large amount of the variation remained unexplained. These findings illustrate that in the absence of detailed microclimatic maps it is very difficult to predict where exactly to transplant individuals to maximize survival and transplantation success. Brooker and colleagues suggest solving this issue by either transplanting a large number of individuals, accepting important mortality losses, or by including expert judgement. Another solution to address this issue, potentially more effective in the longer term, is the creation of highly detailed microclimatic maps. For instance, Eric Meineri & Kristoffer Hylander recently created such a detailed microclimate map at 50 m resolution for the whole of Sweden. Such maps easily allow one to detect ‘cold spots’ and ‘hot spots’ in the landscape at a scale usable by land managers. As shown by Brooker et al. in the current study, such detailed maps are likely indispensable when planning assisted migration of slow-colonizing plants, lichens or other taxa.

In sum, the Brooker et al. paper in Journal of Applied Ecology gives us a fascinating insight into how translocations can be planned, managed and executed when populations, species or ecosystems are threatened by climate change. Meanwhile, it becomes increasingly evident that we need to quantify microclimates more often, and measure temperature and other climatic variables not only in weather stations at 2 m height in standardized, flat treeless areas but at spatial and temporal scales relevant for the species at threat.

Read the full article, Tiny niches and translocations: the challenge of identifying suitable recipient sites for small and immobile species in Journal of Applied Ecology.

Find out what the authors had to say about the study in this post.