This is a re-post of the second in a four-part series on the relationship between global climate change and mass extinction. I plan to post on the IPCC’s recent announcement later this week.
Most people, by this time, probably know that global warming means extinction for some species; if the arctic sea ice completely melts, the fate of polar bears is fairly obvious. Yet the link between global warming and extinction risk is rarely so direct. Understanding how global warming can lead to extinction is important for conservation scientists, who may be able to discover ways to minimize the damage. For the warnings from scientists to make sense to the rest of us, we have to understand the anatomy of climate-related extinctions as well.
One possible source of confusion is the distinction between ultimate and proximate cause. Basically, if someone trips over the edge of cliff, the ultimate cause of death is bad footing but the proximate cause of death is the hard landing at the bottom. Climate change could become the ultimate cause of a lot of extinctions, but the proximate causes will mostly be something other than warm temperature. Because we don’t hear much about this distinction, the clear risk of extinction from climate change might look like a confusing swarm of multiple environmental problems.
Sometimes the proximate cause is obviously connected to climate change, as when organisms suffer from high temperatures, high carbon dioxide concentrations, or habitat destruction through the melting of polar ice.
Hot water, for example, bleaches coral. Bleached coral isn’t dead, but it no longer has the algae that give coral animals their color and their food. Coral can survive occasional bleaching, but if bleaching events last too long or are too frequent, the coral will starve to death. If water temperature is too high too often across a particular coral species’ range, that whole species goes extinct. Rising carbon dioxide concentrations are making the oceans more acidic, thinning some animals’ shells. If acidification continues, their shells will dissolve completely and they will die.
But there are other, less direct and less obvious, mechanisms as well.
Some species use changing day length as a kind of calendar to predict the seasons. Animals who cannot recalibrate their calendars to the new, earlier springs are starting to have trouble breeding. Their habitat is still intact, but their timing is off now. Other species might die out because an ecological partner, such as a favored food source or an insect pollinator, dies out, or because warming temperatures bring in new pests. For example, moose populations in some places are crashing because the winters don’t get cold enough anymore to kill ticks. Unlike whitetail deer, moose haven’t evolved ways to de-tick themselves efficiently and they’ve been found dead, bled to death through anemia by ticks.
Some of these changes are not just indirect, they’re counter-intuitive.
Some species respond to warming temperatures by moving to an even warmer area; the temperature itself isn’t a problem for them, but drought is, and the wetter refuge might happen to be warmer. Climate change can even cause extinction by making conditions easier. For example, red spruce trees grow mostly in cold, snowy areas, but they do not actually grow well there. That species would grow better if their habitat warmed up, but they won’t get to because warm weather would let faster-growing hardwoods come in and shade them out. There are mountains in New England with hardwoods at the base and red spruce forests near the top. As the climate warms, the hardwoods will creep upslope, pushing the spruce out. If this goes on long enough, the spruce, and the Bicknell’s thrush, which only breeds in spruce/fir forests, will be gone from New England. A warm enough planet will lose all its red spruce and all the other species dependent on it, forever.
Now, changes in climate have occurred before without causing mass extinction events. Species shift their ranges or adapt to new conditions, and life goes on. What makes this change different (other than humans being its accidental authors) is time–time and, to a lesser extent, space.
For a species to adapt takes time. Even insects, which sometimes do change very quickly, still can’t eat most exotic plants, even species first imported hundreds of years ago. For evolution, “fast” usually means thousands or tens of thousands of years. Even just shifting a geographical range can take centuries for some species, like nut-baring trees. Global warming now causes ecologically meaningful changes in only decades. In comparison to change this fast, evolution essentially stands still.
Even those species that can move quickly might not be able to. Many of today’s animals and plants are now stranded in isolated parks. The built-up world around the parks is now completely inhospitable to them. Some species survive only in a few parks and preserves. If those parks cease being appropriate habitat, where are these animals and plants going to go?
Climate change does not threaten all species equally. As we have seen, which species are at risk is a complex and sometimes surprising thing. Yet there is something that links most of the species in danger; they are specialists.
Rapid change favors generalists. This is the same principle whereby people in a rapidly changing economy try to learn to do a lot of different things. When society is stable, it makes sense to take the time to become an expert in one thing. The idea is to stand out from the crowd and find a niche that no one else can occupy. But in a rapidly changing job market, locking yourself in to just one career is economic suicide. Ecosystems work the same way.
Human activity as a whole favors generalists, because human activity causes rapid change. This is part of the reason why built-up areas sometimes have coyotes but almost never wolves; wolves mostly hunt mostly large animals, while coyotes eat almost anything. Starlings, house sparrows, pigeons, and rats are ubiquitous in American cities and towns because they are all generalists. Generalists are usually small and unassuming. Many people find them boring. But they do survive major extinction events, because they can handle change and instability. When the large, specialized dinosaurs died out, mammals and birds survived, in part because we were generalists at the time.
The current major extinction event follows the same pattern. Underneath the details of which species are at risk due to which proximate cause, there is the deep, ultimate cause of rapid environmental change. And we know the type of species that will be lost facing this sort of problem. We stand to lose the large, the distinctive, the unusual—we stand to lose the specialists, among them many of the animals we care most about.
We may be able to save some, through captive breeding programs, carefully focused legal protections, and other extraordinary efforts, but the problem is not particular to this or that species. The problem is systemic. The Earth is changing, and the new version simply has less room in it for biodiversity and for specialization. Like a wet sponge wrung, the excess is going to drop out.
If you don’t like it, then stop changing the Earth so fast.