Climate change is causing Earth's animals to shrink

Earth's species are shrinking. As temperatures rise, plants and animals the world over are adapting, some more quickly than others, by becoming smaller — a trend that scientists say could have serious implications for ecological diversity the world over.

For decades, scientists have been uncovering evidence that Earth's flora and fauna are becoming smaller in response to rising temperatures. Now, researchers Jennifer Sheridan and David Bickford of The National University of Singapore have assembled this evidence into a comprehensive overview that they hope will allow us to better understand this trend. Their findings are published in the latest issue of Nature Climate Change.

"We have summarized the data and provided a review of what has been shown to be occurring over the past few decades, as well as similar trends from fossil record that also demonstrate a cohesive trend," explains Bickford. He continues:

We have also provided the theories that explain the data, a set of reasonable ecological and metabolic rules that help explain why we see this repeat trend of species getting smaller. Many studies are corroborating this general trend, and as more studies come out saying the same thing, we need to understand why this trend is happening and what it will mean for society.

As Bickford points out, there is substantial evidence that recent increases in global temperatures have led to significant physiological adaptations in species. A study published in The American Naturalist this September, for example, provides evidence that almost all cold-blooded animals (which, it bears mentioning, comprise the majority of animals on Earth) tend to reach a smaller adult size when reared at warmer temperatures. (Consider the fully grown frog specimens shown up top. The frog on the left was collected on Mount Kinabalu in Malaysia in the 1980s; the one on the right in 2008.)

According to Dr. Andrew Hirst, co-author of the study published in The American Naturalist, this effect arises when an animal's growth rate (how quickly the animal acquires body mass) and developmental rate (how quickly an individual reaches sexual maturity, for example) become decoupled.

"The consequences," explains Hirst, "are that at warmer temperatures a species grows faster but matures even faster still, resulting in them achieving a smaller adult size. Decoupling of these rates could have important consequences for individual species and ecosystems."

Sheridan and Bickford agree with Hirst's assessment, but note that the cause for concern isn't so much that species are shrinking, but rather that they're shrinking at different rates.

"This isn't a ubiquitous or general law of ecology," Bickford says, "but when some animals are affected and others aren't, that's when an imbalance will happen, and that's what we're most concerned about."

Consider what happens when a predator's prey shrink in size faster than the predator itself; to survive, the predator will have to either catch more of its smaller prey, or shrink in size itself. If it cannot do either of these things, the predator dies off.

Sheridan and Bickford compiled experimental data on numerous plant and animals species to demonstrate the effects of warming on species size. In one set of experiments, every 2 degrees of temperature increase was shown to correspond to a decrease of anywhere from 3-17 percent in various types of fruit size. In fish these percentages ranged from 6-22 percent.

So are we all doomed to an existence of small fruit and tiny fish? That remains unclear. The researchers readily admit that the long term effects of species shrinkage are difficult to predict, noting that "impacts could range from food resources becoming more limited (less food produced on the same amount of land) to wholesale biodiversity loss and eventual catastrophic cascades of ecosystem services."

"As temperatures change even more," they write, "these changes in body size might become much more pronounced – even having impacts for food security."

And while we haven't noticed any large-scale effects due to changes in animal size yet, Bickford and Sheridan say that it would behoove us to remain mindful of what more and more members of the scientific community are coming to recognize is another of climate change's impacts.

"No matter what else they're doing in the field, we want more ecologists to pay attention to body size and take these measurements," they explain.

Nature Climate Change (doi:10.1038/nclimate1259) via Reuters and The New York Times
Top image by David Bickford via The New York Times