Tradeoffs are an important concept in many fields of study, including economics (“opportunity costs”), architecture (functionality vs. aesthetics), and medicine (side effects vs. efficacy of treatment). In ecology, tradeoffs are hypothesized to be important in maintaining biodiversity in ecosystems, but how evolution maintains these tradeoffs is poorly understood. In a suite of Sonoran Desert winter annual plants, species exhibit a tradeoff between growth rate and stress tolerance: species either grow quickly or tolerate stress, but not both. In a new study published in The American Naturalist, Sonoran Desert researchers test whether natural selection favored plant species with both high growth rate and stress tolerance in the field. If it does, then why this combination doesn’t exist in nature becomes a mystery. The presumption must be that selection for high values of one trait leads to reductions in the values of others.
The researchers study four species that vary in growth rate and measure how the growth rate and stress tolerance of individuals of each species relate to their success. They find that species with high values of both growth rate and stress tolerance are favored, which presents a conundrum in that this combination of high-trait values would be favorable in this ecosystem, but is essentially absent. Their data suggest that the inter-species tradeoff reflects some fundamental constraint preventing the formation of species with high values of both traits and thus preventing the evolution of a single species that could competitively exclude other species from the community. Whatever this constraint may be, it is essential to maintaining species diversity in this ecosystem. Study authors are Sarah Kimball and Travis E. Huxman with UC Irvine’s Center for Environmental Biology; Jennifer Gremer and D. Lawrence Venable with the University of Arizona; and Amy L. Angert with the University of British Columbia.