Species of the near future, like a new sweater you accidentally put through a tumble-dry cycle, may be smaller and less useful than you remember them. Organisms from polar bears to plants to farmed fish are already losing stature. As the world gets hotter and rainfall gets more sporadic, countless other species are expected to shrink, too--provided they don't disappear altogether.
The authors of a new paper in Nature Climate Change compiled data from dozens of studies on species size and climate change. They had reason to expect that many species would be shrinking in response to global warming. Animals and plants have already been observed breeding earlier, flowering sooner, and gradually shifting their ranges toward the poles (and cooler temperatures). Additionally, the fossil record tells us that invertebrates and small mammals shrank during ancient periods of warming. On those occasions, though, the thermostat was turned up gradually--nothing like today.
The review found that 38 species of animals and plants have recently shrunk. The shrinkage wasn't universal, though; an equal number of species have been observed not shrinking in recent years, or provided equivocal results. And a handful of species have actually increased in size (more on them in a moment).
The species seen shrinking include representatives from the mammals, birds, reptiles, amphibians, fish, and trees. There's reason to believe that these species are trendsetters, not outliers, and that more of the so-far unchanging species will soon follow in their footsteps (or hoofsteps or wakes). If climate change is to blame for miniaturizing animals and plants, then this effect should become more obvious as the earth gets warmer.
And according to experimental evidence--not just the circumstantial evidence of species happening to shrink during the past few decades--climate change is to blame. Controlled experiments have shown that increased temperatures cause plants, fish, marine invertebrates, beetles and salamanders to grow smaller. Other studies have found that reduced precipitation produces smaller mammals, frogs and toads, and tropical trees. In the ocean, acidification caused by increased carbon dioxide in the water is known to impede the growth of corals, scallops, and other animals that build calcified bodies for themselves. Acidification also slows the growth of phytoplankton, the microscopic plants that hold up the whole ocean ecosystem. (And, don't forget, ocean acidification creates ambidextrous fish.)
There are a few ways that warmer temperatures could shrink species. In "cold-blooded" or ectothermic animals, including fish, reptiles and amphibians, a higher temperature speeds up the metabolism. Unless animals can readily find more calories to consume, this means they'll burn through their fuel faster and have fewer resources left over for growing their bodies.
For animals that maintain their metabolic rate on their own, namely birds and mammals, shrinkage might have more to do with a lack of resources. If heat or drought kill (or shrink) the species that animals feed on, they'll have to compete more to stay alive. Undernourished animals, including humans, don't grow as big as well-nourished ones.
Some regions, rather than experiencing drought, are expected to become wetter with climate change. But even these areas will have more variation in rainfall than they're accustomed to--that is, there could be periods of drought in between the rainy periods. Less consistent rain will cause plants to shrink or die. Even though increased carbon in the atmosphere might be expected to give a boost to plant life (it's called the greenhouse effect, after all), plants can't take advantage of that extra carbon without sufficient water and nutrients and a comfortable temperature. The few plant species studied so far have shriveled, not grown, in response to climate change.
Over successive generations, evolution might favor smaller individuals within species: Those that need fewer resources might be more likely to survive. The survivors, then, would pass their small-bodied genes to their offspring, and the effects of climate change would be written into animals' genomes for good.
As for those few species that are growing instead of shrinking? They're mostly at high latitudes, where a slight increase in temperature is a welcome relief from the cold. As temperatures rise even more, those species might not be so happy with the change. The species that are most likely to benefit from increased temperatures, or at least not to be too bothered, are predators that can adjust their diet to a wide range of prey. Certain invasive species, which have already demonstrated their handiness at eating anything available to them and adapting rapidly to changing conditions, might also be unscathed.
The authors point out that if every species on the planet shrank at the same rate, we'd all be fine: Within each Polly-Pocket ecosystem, everything would stay in balance. But since species are responding differently to climate change, with some already shrinking and others not, we can expect to see food chains getting bunched up or broken. As those lower on the food chain grow smaller (or, failing to adapt, go extinct altogether), species that eat them will find less food in their meals than they're used to.
This applies to humans, too. Both farmed and wild fish have already been observed shrinking, and heat and drought threaten our crops. Meanwhile, of course, the human population is growing. As climate change leads to food shortages for various groups of people, and undernourished children fail to grow as large as their parents, then even the human species might start shrinking.
Sheridan, J., & Bickford, D. (2011). Shrinking body size as an ecological response to climate change Nature Climate Change DOI: 10.1038/nclimate1259