Evolutionary ecology axiom overturned: No advantage for 'gleaners' over 'exploiters'

New research has overturned the basic assumption in ecology and evolution of a trade-off between food availability to an organism and how efficiently it can process it. 

In a paper published in the Proceedings of the National Academy of Sciences (PNAS) Oct. 6, researchers Thomas Kiørboea and Mridul Thomas show that there is no such trade-off. 

It is not true, the researchers found, that organisms that do poorly in a low-resource environment do better in a climate rich with resources. Instead, looking at data from more than 500 species, the authors found that organisms that were more successful in finding food in a low resource environment were also more successful in finding food in a high resource environment.

This relationship held across many different types and sizes of species. The researchers looked at data for organisms that ranged in size from unicellular to large mammals, and both terrestrial and aquatic organisms.

Previously, it was assumed that organisms that could rapidly search over a large area for food (called gleaners) survived better in low-resource areas, and that organisms that could process resources rapidly (called exploiters) would do better in resource-rich areas. This was assumed to account for biodiversity and the coexistence of species.

In particular, the authors looked in detail at whether the assumption held up with eukaryotic heterotrophic organisms. Eukaryotes are organisms whose cells contain a clearly defined nucleus, and heterotrophic means that the organism feeds on matter outside itself.

The trade-off concept is important in understanding the diversity and coexistence of species, and how species will respond to environmental changes.

The authors suggest that other types of trade-offs could be involved, such as how fast an organism can grow and how well it can survive its predators.

They state that their findings provide evidence for "a dominant slow-fast gradient in organismal strategies," or what they term the "pace of life." In other words there is a trade-off between how fast an organism grows and its longevity.

They write: "In other words, traits and behaviors that favor success in the short term come at the cost of longevity and success in future reproductive possibilities. Strong environmental variation that keeps population sizes low favors 'fast' life strategies that can rapidly reproduce and colonize ephemeral habitats. 'Slow' traits such as high competitive ability are favored in stable environments, where population densities are large and density-dependent effects are strong."

The researchers note that their findings also relate to plants, as plants that grow more slowly are better able to avoid predators.

In a lengthy section, the authors answer four possible objections to their findings: (1) that trade-offs can apply within species but not among different species, (2) that differences in resource acquisition can obscure underlying trade-offs, (3) that their assumption that increases in maximum ingestion correlate with increase in maximum processing is wrong, and (4) that their analyses do not account for relatedness.

They conclude, however, that "the gleaner-exploiter trade-off does not exist." Instead, they suggest, further research into the trade-off between foraging and the risk of predators.