It’s becoming a smaller world, after all

The geophysical community is well-versed in the effects of climate change on the cryosphere, hydrosphere, and atmosphere. But higher global mean temperatures, increased acidification of oceans, and changes in precipitation promise to produce interesting effects on the biosphere, too. One of the most notable is a tendency for plant and animal species to shrink in size.

In a recent Nature Climate Change article by Jennifer Sheridan and David Bickford of the National University of Singapore, the authors review the literature and highlight some of the key hypotheses and observations of why bigger isn’t better in the warming natural world.

As increasing levels of atmospheric carbon dioxide further acidify the oceans, phytoplankton growth rates are expected to fall. That trend, in turn, is expected to have a huge impact on all marine life. Although land plants would be expected to get larger in response to rising CO₂ levels, such conditions as temperature, humidity, and nutrient availability can affect their ability to process CO₂. In the past century, many species have shown a negative correlation between growth and temperature, and a positive correlation between growth and precipitation or seed mass. When plants, which form the bottom link in most food chains, get smaller, those organisms higher up will struggle.

Smaller body sizes of living things on land may be caused by changes in water and nutrient levels. Areas drier than normal can lead to decreased plant respiration and soil nitrogen loss. Areas wetter than normal can lose nutrients due to leaching. All those effects result in smaller plants and, therefore, smaller herbivores.

Faster metabolisms, smaller animals

It’s not just about the food chain. Ectothermic organisms, which stay warm by absorbing heat from their surroundings, scale their metabolic rate with temperature. If, by 2100, Earth’s ambient temperature rises by 1.1–6.4 °C, metabolic rates could increase by 10–75, say Bickford, Sheridan, and their collaborators. Unless ectothermic organisms can adapt, they’ll have to get smaller.

Size reduction has happened before. During the Paleocene–Eocene Thermal Maximum , temperatures increased by 3–7 °C and precipitation decreased by 40%. Fossil-bearing rocks from this time period show smaller burrowing invertebrates than there were before. California squirrels, gophers, and woodrats, as well as algae such as diatoms, also shrank during warm periods, as indicated in the fossil records. When ambient temperatures are increased and water limited, experiments show decreased plant biomass and decreased body size in many birds, mammals, and amphibians.

It’s not surprising that environmental stress causes individual organisms to be smaller. And smaller individuals tend to have lower birth rates, produce yet smaller offspring, and be more susceptible to disease. Those trends have already been seen in organisms that have relatively short time spans between generations, but further studies are needed to compare size differences between those organisms and ones whose generations span longer time periods. Organisms with narrow thermal tolerances and relatively limited population are at the highest risk: If the climate changes too fast for organisms to adapt, they could become extinct.

To understand the mechanisms for those changes, Sheridan and Bickford propose that future research should include field and lab experiments as well as large-scale comparative studies of organisms already experiencing size change. Examining museum collections could help to further quantify size-change trends. And exceptions to the size-reduction trend should be considered: Certain secondary consumers from high latitudes benefit from increased growing or feeding seasons, for example.

Most of the ramifications of global climate change are as yet unknown. Almost certainly, however, plants and animals will become smaller in size, which could result in the extinction of some species. To mitigate climate change and conserve biodiversity, we need to better understand and predict the processes involved in global warming. For many species, smaller may not be better for evolutionary success.

Rachel Berkowitz