Temperature sets the bounds on marine species’ distributions

People often think about species’ geographic ranges as static, that they primarily result from processes of dispersal and range expansion and contraction happening over evolutionary and geological timescales

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The paper in Nature Ecology & Evolution is here: http://go.nature.com/2ldX9Hs

As part of our Reef Life Survey citizen science program (www.reeflifesurvey.com), we’ve been doing dive surveys of marine species around the world for over a decade – and nothing seems further from the truth for most of the common species we see. Sure, there are clear biogeographic patterns in species composition, but most species are relatively widespread and share common north-south range boundaries which appear to be associated with particular temperatures, rather than dispersal barriers. This of course makes sense, given that most marine species have a widely dispersing larval stage and do not have mountain ranges or large rivers blocking their paths. Marine species range edges can be highly dynamic, and can contract and expand as edge populations are lost or gained due to changing environmental conditions.

In this paper we looked at the seasonal temperature extremes at the range edges for more than 1,700 of the most common fish and invertebrate species found on coral and rocky reefs all over the world. It was quite amazing how similar these temperature limits were between species which are unrelated and found in completely different parts of the world. For example, a mollusc found in temperate Australia is found at locations with up to a similar maximum temperature to that of a fish in temperate northern America. Based on our observations, it looks like there are some relatively consistent physiological constraints on marine species which limit their ranges to particular temperature windows (or realised thermal niches). These will clearly continue to influence the distribution of marine species as the seas warm with climate change.

The idea that some species will adapt to warmer conditions better than others has also been common in the climate change literature. If marine species have been following their preferred thermal niche as isotherms have changed in the sea over evolutionary time scales, how much adaptation should we be expecting to happen in the future? These species have always experienced hot conditions at their warm range edge, with extreme seasons and years usually much warmer than will occur through climate change over the next century.

Instead, what we see in the shared upper thermal limits of numerous marine species possibly represents the limit to adaptive capacity of species (as distinct from local populations). Rather than stay put and adapt, most marine species will probably just continue to ‘move’, and not just through expanding cold range edges, but also losing warm range edge populations. The ones that won’t be able to, due to extreme isolation and poor dispersal capability, are the species we should be focussing our thermal acclimation and adaptation studies on. It seems clear than thermal physiology will be critical for determining the future distribution of life in the sea (as it is for contemporary distributions). But laboratory studies of thermal tolerance need to move beyond single generations to better inform us of future consequences of warmer seas. Even the most realistic experiments will likely fall short of characterising the combined influences of temperature on reproduction, larval development and performance, recruitment dynamics, growth and individual fitness, and many other important aspects of life history. So perhaps we can learn most about where species will be in future by looking at where they are now.

Rick Stuart-Smith

Research Fellow, University of Tasmania