Fulgorid bugs congregate to feed on young palm fruit in a Costa Rican rainforest. Credit: Samraat Pawar

A team of life scientists at UCLA, including SFI External Professor Van Savage, has demonstrated that the relationship between animals' body size and their feeding rate -- the overall amount of food they consume per unit of time -- is largely determined by the properties of the space in which they search for their food. The work is published May 30 in Nature.

An animal searching for food in a three-dimensional space, like the ocean or sky, is likely to consume much more than a similarly sized animal searching in a flat, two-dimensional space, like a savannah or a sea bed, they found.

"Surprisingly, the spatial dimension of the search space turns out to be the most important property," says Savage, an assistant professor of ecology and evolutionary biology, and of biomathematics, at UCLA. "Yet up until now, work on food webs and predator–prey relationships has almost universally assumed that feeding rate increases with body size in a way that is independent of dimensionality."

Animals, of course, cannot simply go to the market when they are hungry. Instead, they must search for their food, which can also move in space. Cheetahs, for instance, scan the savannah for gazelle in two dimensions (left-to-right, forward-to-backward), while sharks search the ocean and some birds, such as flycatchers, scour the skies in 3-D (with an added up-and-down dimension).

"Would you rather search for food in two dimensions or three?" Savage asks. "That is, would you rather search for food just left-to-right and forward-to-backward on the floor of a room, or would you also want to search up-and-down, from floor to ceiling? When I quiz people, including scientists, most say they would rather search in two dimensions because it would be easier to find food. But we found that in nature the third dimension usually adds a huge number of extra resources. Ultimately, searching in this extra dimension provides many more chances to find food."

The researchers developed a new mathematical model that predicted that feeding rates increase more quickly with body size in three dimensions than in two. The model helps explain why huge whales can subsist on tiny krill in three dimensions -- but likely could not in two dimensions, if they had evolved to live on land.

To test their ideas and model predictions, the researchers compiled and analyzed the largest-ever database on relationships between feeding rates and body size. They were surprised to discover how well the data fit their predictions.

The study also predicts that the stronger feeding interactions in three dimensions -- that is, the tendency for animals to consume more than they would in two dimensions -- will lead more often to boom-and-bust cycles in the abundance of species, analogous to large fluctuations seen in stocks or housing prices. These booms and busts make species more prone to extinction and therefore "could have profound consequences for understanding and preserving biodiversity in different habitats and for the planet as a whole," Savage says.

The scientists are currently looking at the effects of temperature -- another major driver of feeding rates -- and studying how to combine that with the results of the current study on feeding rates and dimensionality.

The ultimate goal of the research, the scientists say, is predicting how climate change will affect biodiversity levels and, thus for humans, the availability of goods and services, such as food and plants, fungi or bacteria used to make pharmaceuticals. The new findings suggest there may be crucial differences in how climate change will affect ecosystems that are more two dimensional, such as land and water surfaces, and those that are three dimensional, such as the open ocean or air.

Savage's research group utilizes perspectives from physics and applied mathematics to study biological problems. "Having been trained in theoretical particle physics, I automatically consider dimensionality as part of any problem, but I was impressed by how large the effect is here," Savage says.

Two co-authors on the paper, Samraat Pawar and Tony Dell, are past participants in SFI's Complex Systems Summer School.

The research is funded by the National Science Foundation.

Read the Nature paper (May 30, 2012)