Mammal Diversity Aided In Survival Over Deep Time
April 24, 2012

Mammal Diversity Aided In Survival Over Deep Time

Lawrence LeBlond for

In a first of its kind study, researchers from Vanderbilt University found that mammals´ best defense to adapting to climate change was diversity, and families with higher taxonomic diversity were better able to survive ongoing environmental changes.

Larisa R. G. DeSantis, Assistant Professor of Earth and Environmental Sciences at Vanderbilt, led researchers in studying how North American mammals adapted to climate change over a 56-million-year period, beginning with the Eocene and ending with the Pleistocene extinction 12,000 years ago, when the continent´s megafauna (mammoths, saber-tooth tigers, giant sloths, etc) vanished.

DeSantis noted that it was important to first understand how mammals responded to climate change in the deep past, before predicting how they will respond to future climate change. “It is particularly important to establish a baseline that shows how they adapted before humans came on the scene to complicate the picture,” she said.

An established baseline may help researchers determine how mammals respond to climate changes because, historically, mammals have shown a strong ability to adapt to environmental changes, which, in turn, makes it difficult to formulate accurate predictions of how mammals will react to climate change in the future.

What DeSantis and colleagues found, are that mammals have demonstrated the ability to dramatically alter both their size and their diets in the face of environmental changes. Mammals also have the mobility to move as the environment shifts, and also regulate their internal temperature to better adapt to changing temperatures.

For their study, the Vanderbilt researchers tracked the ranges and diversity of families (collections of genera of animals with common attributes) of mammals that inhabited the continental US during the 56-million-year epoch.

The fossil record of mammals in the US for the study period is considered to be fairly complete. However, researchers note that it can be difficult to distinguish between closely related species based on fossil remains, and it can sometimes be just as difficult to separate members of different genera from one another. Therefore, DeSantis and her team performed analyses on mammals at the family level. In total, the researchers analyzed 35 different mammalian families, including Bovidae, Equidae and Mammutidae.

The current study, published April 23 in the journal PLoS One, builds on previous work to determine whether relative range size is conserved at the family level for North American mammals, including those that did not survive the terminal Pleistocene extinction (also known as the Quaternary, or Ice Age Extinction).

“We add to the families studied by Hadly et al. in the contiguous United States, and extend their work in deep time to include epoch-scale time bins for the Eocene through the Pleistocene,” said the authors.

The analysis of mammalian range changes through “deep time” can clarify how climate and environmental changes affect these families and the possible influence of these variables on range conservation.

Furthermore, the team examined if “particular niche characteristics influence range changes at the family level from the Pliocene to the Pleistocene.”

Specifically, the researchers wanted to find whether mammal families that went locally extinct in North America during the Pleistocene were already on the decline, whether maximum body size of family members affect changes in range size, and whether particular orders and/or functional groups of mammals (e.g., ungulates, carnivorans) were more successful at increasing their ranges since the Pliocene.

“If body size or other intrinsic life history characteristics are more susceptible to climate change, we expect that families containing megafauna would respond similarly,” the authors wrote. “Likewise, higher level categorizations such as taxonomic orders or functional groups may also reflect similar responses to climatic changes.”

Similar to past studies that focused on the Pleistocene and Holocene, the authors examined absolute range size (additionally standardizing ranges by using percent range area occupied) and centroids of mammalian taxa. The team also analyzed relative range sizes in accordance with past studies to control for sampling biases across epochs.

During any given epoch from the Eocene to the Pleistocene, the greater the diversity among genera or species the greater percent range area occupied, per family.

“Diversity is good. Families with higher taxonomic diversity (and potentially greater niche diversity) have larger range sizes,” DeSantis told RedOrbit. “Changes in diversity occur proportionally.

DeSantis and her colleagues found that horses, members of the Equidae family, were among the most widely distributed families from the Eocene to the Pliocene epochs (and remained highly dominant in the Pleistocene). In contrast, families with more restricted ranges maintained lower range areas.

The results demonstrate that mammals maintained similar niches through deep time and is consistent with the idea that family members may inherit their ranges from ancestral species. The idea that niches are conserved over time is a fundamental assumption of models that predict current responses of mammals to climate change, the authors note.

“These data clearly show that most families were extremely resilient to climate and environmental change over deep time,” said DeSantis.

Although the extent of family ranges remained relatively constant, the study found that these ranges moved south and east from the Eocene to the Pleistocene.

“Mammals have been responding to changes in climate over deep time and show a southeastern shift in the center point of their ranges over the past 56 million years,” DeSantis told RedOrbit. “This is likely due to overall cooling since the Eocene, even when accounting for sampling biases in the Eocene and southern immigrants from the Great Biotic Interchange in more recent epochs,” she added.

However, southeastern movement of ranges from the Pliocene to the Pleistocene may also be complicated by the arrival of South American animals when the Isthmus of Panama was formed, triggering an enormous exchange of species that has been labeled “The Great American Interchange” or (Great Biotic Interchange). As a result, some of the southern movement of families´ ranges may have been due to the influx of mammals from South America, the researchers cautioned.

The study also looked for evidence that families containing megafauna and other species that went extinct during the Quaternary may have been in decline beforehand, but failed to produce any solid evidence for any such “extinction prone” families.

DeSantis expected to see differences between families containing megafauna and those composed of smaller animals if in fact climate change was the culprit. However, the fact that they didn´t find such evidence cannot completely rule out this possibility.

“It is important to consider both taxonomic diversity and the diversity of life history characteristics when predicting geographic ranges at higher taxonomic levels,” the authors wrote. “For example, families that contain more species or more species with a greater diversity of ecological niches may prove more resilient to climatic changes then families with more specialized and/or overlapping life history variables.”

“Therefore, deep time ecological data has the potential to provide valuable insight to understanding mammalian responses to future climate change,” they conclude.

With record extinctions of mammals occurring over the past 400 years, diversity will play a major role in mammals survival of future climate change. The International Union for the Conservation of Nature (IUCN) predicted in a 2008 report that one in four species of land mammals in the world faces extinction.

DeSantis´ latest research was supported by funds from Vanderbilt University.