Body Size Matters When It Comes to Extinction

They found that species of different sizes gravitate toward population states most stable against extinction. The states they derived in the model reproduce two oft-observed patterns in biology. The first, Damuth’s law, is an inverse relationship between body size and population density: the bigger the species, the fewer of individuals cohabitate in a given area. Within the NSM, this fewer/larger more/smaller pattern emerges because large species are most stable against starvation in small numbers, while small species can afford to reach larger population densities.

The second relationship, Cope’s rule, holds that terrestrial mammals tend to evolve toward larger body sizes. This NSM shows that, overall, larger animals with slower metabolisms are the most stable against extinction by starvation. It even predicts an energetically “ideal” mammal, robust in the face of starvation, which would be 2.5 times the size of an African elephant.

“As we incorporated more realism into how quickly organisms gain or lose body fat as they find or don’t find resources, the results of our model began aligning with large-scale ecological and evolutionary relationships. Most surprising was the observation that the NSM accurately predicts the maximum mammalian body size observed in the fossil record,” explains Yeakel. Though the model doesn’t account for predation, it does offer a dynamic and systematic framework for understanding how foragers survive on limited resources.

“The dynamics of foraging and the interaction of body size in foraging and resource availability, these are all rich problems for which there is beautiful phenomenology,” says Redner. “I hope some of this will have relevance in managing resources and ensuring species don’t go extinct.”