Dear Adaptation Mavens,

I’ve been reading up on climate change and what effects it may have, and there’s something that’s got me a little puzzled. Lots of stories talk about how species are shifting towards the poles or higher up on mountains—does this mean that eventually there will be hardly any species left near the equator? And how the heck can plants shift their range at all since they can’t really move?

Confused About Range Shifts

Dear CARS,

An excellent pair of questions. The general concept of shifting species ranges is simple—species can only survive in conditions that are climatically suitable for them, so if their “climate envelope” moves, they’ll move with it—but there are a number of subtleties that are often lost in discussions of this topic.

I’ll start with your second question. Species range shifts don’t happen because a bunch of individuals decide to pick up and move. As you so cleverly point out, if that was how range shifts happened, plants would be stuck. Yet we know from the fossil record and historic observations that plant species ranges can in fact expand, contract, and shift thousands of kilometers over time. So how do species range shifts happen? It has to do with how well individuals survive and reproduce in different areas. If all the individuals at the southern end of a species’ range die, then the southern boundary of that species’ range will shift northward. If the individuals at the northern end reproduce well, some individual seeds or offspring will end up even further north, and if enough of them survive the species range will expand northward. So even if individuals within a species can’t move, the species as a whole still can.

But what about that equatorial species vacuum you ask about? It’s unlikely for several reasons.  First, not all species ranges will shift to higher latitudes or altitudes. During past periods of climate change, even some pretty rapid climate change, species responded in all kinds of ways. Some went extinct, some stayed put and did fine, some had range shifts that tracked climate shifts, and some even expanded their ranges. Thus while some species ranges will certainly shift as a result of climate change (many already have), plenty of species ranges won’t change that much (at least not as a result of climate change itself). Species that do fine without range shifts may simply be able tolerate a broad range of climatic conditions, or to adapt evolutionarily or physiologically to new climatic conditions. And climate isn’t always what’s limiting a species’ distribution: biological factors like competition or facilitation by other species, diseases or predation can be what matters most, or even non-biological factors like the right soil type or water chemistry. For example, while several species of small mammals in the Sierra Nevada mountains have shifted their ranges upward over the past century, some have actually expanded downslope rather up, perhaps because competitors that once kept them out have moved. The same is true for some of the Sierra’s plant species.

In past periods of climate change, both range shifts and evolutionary adaptation have helped some species survive (others weren’t so lucky!). These processes will likely be important during the current period of climate change as well. We can help species help themselves by maintaining large populations and high genetic diversity to support evolutionary adaptation, and by limiting impediments to dispersal (roads, cities, sea walls, etc…) and survival (pollution, disease, overharvest, competition for resources, etc…) to support range shifts. And of course limiting climate change itself by reducing greenhouse gas emissions will be helpful in all kinds of ways…

Ever yours,

The Mavens

P.S. For all you hardcore Mavens fans out there: You can get a 20% discount on our book, Climate Savvy, through the end of January. Details on cake virtual library page.