Despite the claims of a few grandstanders, it’s clear that our planet’s thermostat is dialing up. Some plants and animals have already responded to this change by modifying their distributions away from the Equator, towards the cooler poles, and by shifting the timing of their breeding or flowering cycles. But beyond their location and behaviours, the physical stature of some organisms is also affected by climate change — which could play havoc with ecosystems and even global food security.
The more we can predict and prepare for such changes, the better we will be able to mitigate their effects, argue Jennifer Sheridan of the University of Alabama in Tuscaloosa and David Bickford of the National University of Singapore. In a Perspective published today in Nature Climate Change, the conservation biologists outline the evidence for past and present downsizing of organisms in response to warming environments and how these changes could affect ecosystems as well as human health.
Fossil evidence suggests that over the last 65 million years or so, organisms ranging from diatoms to ground squirrels have shrunk when the Earth has warmed. Similarly, over the past century, various plant species, and land and ocean animals have scaled down as temperatures increased. The trend holds up in experimentally controlled environments as well — a variety of plants and their fruits shrink as temperatures grow, as do fish, beetles, marine invertebrates and salamanders.
For cold-blooded organisms, whose internal temperatures are inherently connected to the environment, higher temperatures could lead to a higher metabolic rate, say the authors. Unless these animals can compensate with more food or limited activity, it’s possible that their revved up metabolism would lead to quicker development and shrinking body sizes.
Temperature-linked changes in precipitation also affect the size of organisms. Higher temperatures lead to drier environments, and Sheridan and Bickford suggest that reductions in size will be most pronounced in areas where global warming causes reduced precipitation as well. Tropical trees, toads and mammals are known to grow slower during drought years and under experimental drying conditions.
Other environmental changes will also affect life on earth. As the atmosphere loads up with carbon dioxide, so do the planet’s oceans, which raises the acidity of the water. Higher acidity reduces the rate at which organisms like corals and oysters can form their shells. The result, the authors say, is that these ocean creatures shrink. The growth of red algae and phytoplankton are also hampered by the lower pH.
The authors note that the warming-shrinking trend does not apply to every organism, such as those with longer generation times or some at higher latitudes. This variation exacerbates the problem. If all organisms in a given ecosystem shrank on scale with one another, smaller predators could eat smaller prey that eat smaller plants, and all would be fed. But that is not what ecologists are observing. Organisms change with variable intensity depending on their lineage, size and location, and ecosystems are likely to be thrown off balance.
Sheridan and Bickford want researchers to identify the mechanisms for the observed size changes and examine the mixed effects on organisms across ecosystems. That work would begin with better descriptions of the warming-shrinking trend.
A perfect spot to measure changes in size across a broad array of species is a museum, they suggest. Museum specimens can be examined for free or cheaply and can help scientists quantify the size-change trend. This work could be complemented with controlled environment settings in which researchers can manipulate the temperature and moisture of growing conditions.
The changes associated with global warming are unlikely to benefit humans, say the authors, so we need to better understand past and present changes to predict what will come. “Being able to predict change is critical in creating strategies that reduce negative effects and guide positive courses of action,” the authors write.
Image Caption: The California Ground Squirrel is one of many species that grow to smaller sizes in warmer periods
Image Credit: Howard Cheng via Flickr under Creative Commons.