As Weather Warms, Some Animals and Plants Get Smaller

Whether it’s the polar bear or the petite house sparrow, many of Earth’s species seem to be shrinking in size, a new study reports; its authors believe that is likely a result of global warming.  Other experts disagree, noting that the conclusion goes too far, and that global warming should not be blamed for what could be natural changes.  The research was published in the journal Nature Climate Change. 

The study found that 38 of 85 animal and plant species showed a definite reduction in size over decades, including a type of Scottish sheep that is now five percent smaller than a quarter century ago.  Those studies examined species over different time periods and in diverse numbers.  According to the study, species that are getting smaller include cotton, corn, strawberries, bay scallops, shrimp, crayfish, carp, Atlantic salmon, herring, frogs, toads, iguanas, hooded robins, red-billed gulls, California squirrels, lynx and wood rats.  The study notes that the house sparrow’s weight has dropped by one-seventh between 1950 and 1990.  A bird known as the graceful warbler showed a 26 percent weight loss during the same timeframe.

“There is a trend in a number of organisms across the board from plants to big vertebrates getting smaller,” said study co-author Jennifer Sheridan, a biology researcher at the University of Alabama.  “The theory is as things get warmer they don’t need to grow as large.”  The majority of these animals are cold-blooded, so the warmer the weather the faster their metabolism and the more calories they burn, according to Sheridan.  A biological law, called Bergmann’s rule, says that as the weather gets colder, animals get bigger.  This is the unwritten flip side of it, Sheridan said.

Yoram Yom-Tov, a zoologist at Tel Aviv University whose studies Sheridan used in her research, agreed that many species are shrinking, and noted that global warming isn’t the only reason.  “Changes in body size are a normal phenomenon,” Yom-Tov said.  “When conditions are favorable, they increase in size or reproduce at higher rates, and when conditions are deteriorating, they do the opposite.  I think that most species will adapt to climate change and survive.  No need for alarm.”

Many scientists believe that the study confirms that climate change is shrinking many plant and animal species and is likely to have a negative impact on human nutrition in the future.  Warmer temperatures and increasing variability in rainfall are affecting the size of all species in the ecosystem from microscopic sea organisms to land-based predators.  “Our study suggests that ectotherms (cold-blooded animals like toads, turtles, and snakes that rely on environmental heat sources) are already changing a lot,” said David Bickford from the National University of Singapore and the study’s co-author.  “What was most surprising to me was that it was such a uniform signal across all these different organisms,” Bickford said.

Sheridan and Bickford examined fossil records, which they found to be clear-cut: past eras of rising temperatures saw both marine and land organisms becoming progressively smaller.  During a time of warming 55 million years ago — often viewed as an analogue for current climate change — beetles, bees, spiders, wasps and ants shrank by 50 to 75 percent over a period of several thousand years.  Mammals such as squirrels and wood rats also shrank by about 40 percent.

Because warming is occurring at unprecedented rates, “Many organisms may not respond or adapt quickly enough”, especially those with long generation times, according to Sheridan and Bickford.  “We do not yet know the exact mechanisms involved, or why some organisms are getting smaller while others are unaffected.  Until we understand more, we could be risking negative consequences that we can’t yet quantify.”

Stanford biologist Terry Root, an expert in climate change, said the study’s conclusions “seem kind of far-fetched.”

Writing for the Nature.com news blog,  Susan Young says that “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.”