Plants and animals shrinking with climate change?

a juvenile bufo bufo
The common toad is one of many organisms shown to decrease in body size when the climate warms. Credit: T.J. Blackwell

Life on earth may be shrinking in size as a result of climate change, according to a recent study published in Nature Climate Change. From tortoises to sheep, to trees and grass, University of Singapore biologists Jennifer Sheridan and David Bickford have compiled fossil and experimental evidence showing that many species adapt to climate change by decreasing in body size.

This is not Alice in Wonderland – the adaptation evolves over the course of generations, not within given individuals. Still, species with short generation times can evolve quickly and some have already begun to show growth changes associated with climate trends. For example, over the course of a 23 year study of the common toad in southern England, British ecologist C.J. Reading showed that female body size decreased as winters became more mild from 1983 to 2005. Dr. Reading, who published his findings in Oecologia in 2007, argues that the mild winters disrupted the metabolic hibernation cycles within these cold-blooded creatures. As in all cold-blooded animals, the common toad’s metabolism speeds up in warm temperatures and slows down in cool temperatures. So, during mild winters, a toad burns through its fat reserves faster than during a colder winter, depleting its energy reserves available for springtime growth.

Some plant species may also experience stunted growth from indirect effects of climate warming, like changes in water and nutrient availability. Increased flooding, for example, washes nutrients from forest floors and prevents plants from growing to full capacity. Drought, on the other hand, decreases plant respiration and growth. In response, animals that feed on these shrinking plants will need to compensate either by eating larger quantities of the plant, or by succumbing to the shrinking trend by evolving to be smaller themselves. Since animals with small bodies often give birth to small offspring, Sheridan and Bickford argue that this trend may be exacerbated through time in a positive feedback loop.

How low will we go? The extent of growth stuntedness will vary across species and habitats, and will unfold in a web of complicated ecological adjustments. Fossil evidence from the last great warming period on earth, which occurred about 56 million years ago, indicates that beetles, ants, and cicadas shrank by 50 – 75% over the course of 20,000 years. This likely bares little on the fate of 21st century animals, since warming is happening at a faster rate today than it did back then. However, it does at least offer compelling evidence that warming-shrinking trends have unfolded in the past.

But how low will we go, as humans? Sheridan and Bickford argue that ecological shrinking could impact human nutrition by limiting important crop and protein sources. This may or may not ultimately affect the way human body size evolves. Meanwhile, if you do find yourself falling down a rabbit hole with the opportunity to eat size-reducing cake, it may be nutritionally beneficial to go for it. These things are difficult to predict.

Posted by Laura Poppick, Assistant Editor of Maine Climate News