Climate Change, Extinctions and Edges
Co-written with Robert K. Robbins, Curator of Lepidoptera in the Department of Entomology, Smithsonian Institution
A previous post talked about tipping points and how hard it is to predict when things will change to a point of no return so that we can try to avoid it. A tipping point evokes images of things turning over a threshold — or an edge. All that got us thinking about how climate change can have an effect on other types of thresholds and edges, especially related to wildlife.
We already know that when climate changes significantly, some species of plants and animals will adjust to changes and shift their distribution range, or modify the timing of life events such as flowering, flight, migration or reproduction in order to persist under such significantly altered conditions. However, we also know that others will go the way of the Dodo and Carolina Parakeet, forever gone except in paintings and museums, because they cannot handle the changes; they simply don’t have the “tools” for it. Preventing this dire consequence (extinction!) has been the focus of many efforts in the conservation community.
A lesser-known and more subtle consequence of climate change is that local colonies at the southern edge of a North American species’ distribution (i.e., the hottest location where they exist) may become locally extinct as temperatures rise. Think about it: They are at the hottest place they can survive. If it gets hotter, they potentially can go northward, but this option is unlikely if, for instance, they live on a mountain surrounded by inhospitably hot lowland habitats. These mountain dwellers would have little choice but to move to higher and cooler elevations.
A likely example of just such situation was recently published. The Coral Hairstreak (Satyrium titus) is a small butterfly (the size of your thumbnail) that is recognized by a series of conspicuous coral-red spots on its wings. It occurs widely in North America and is a common summer visitor at the flowers of Orange Milkweed. Colonies of this species were recently discovered in south-central New Mexico (Sacramento Mountains). They are at the southern edge of the Coral Hairstreak’s distribution, geographically isolated from the rest of the species by hot, low-elevation habitats. These Sacramento Mountain Coral Hairstreaks have evolved unique traits, which help them adapt to the local conditions of the only two mountaintops to which they are precariously restricted. As New Mexican Dick Holland, the scientist who authored the paper describing these butterflies, wrote, “a very tiny increase in global temperature shall push this taxon into thin air, and give it no place to live.”
One might wonder, “So what if a local colony of an otherwise common species disappears?” I mean, this butterfly can be found abundantly in many other places, albeit without the special unique traits that they have at those two mountaintops. And therein lies the subtlety: Colonies at the edge of their distributions may be biologically more significant than is apparent at first thought. The late Ernst Mayr, a noted evolutionary biologist, provided examples of small, isolated colonies at the periphery of a species’ distribution that evolved rapidly. These isolates are likely to go extinct eventually (e.g., birds on small peripheral islands unfortunately have been shown to have very high extinction rates), but occasionally, a small isolated population will evolve significant adaptations that allow them to persist under those extreme conditions. A classic example might be the iguanas in the Galapagos Islands, which evolved major adaptations allowing them to thrive in the marine environment, unlike iguanas in other parts of the world.
So when distinct isolated colonies of species at the southern edge of their distribution — such as the Sacramento Mountain Coral Hairstreaks — go extinct as a consequence of climate change, one detrimental effect (other than the obvious loss of local species diversity) will be the loss of essential combinations from the “genetic toolbox” that species could use to survive in an ever-changing world (as described at the beginning of this blog). More importantly, though, a potential source of the major kinds of novel changes on which evolutionary revolutions are based is also being snuffed out. Evolution and adaptation potential… gone!
So when you wonder, “So what if a local colony of an otherwise common species disappears?” the reasons that it matters may be subtle, but the consequences for the species may be oh-so-pernicious.