Dartmouth biology professor Ryan Calsbeek and postdoctoral researcher Robert M. Cox have recently discovered that intraspecific competition is a more important agent of phenotypic selection than predation for the Anolis sagrei lizard. Their findings were recently published in Nature.
Numerous studies have shown that both predation and competition influence the “adaptive landscape” of mainland A. sagrei. However, Calsbeek and Cox were the first to simultaneously evaluate both agents and compare their relative importance in driving natural selection.
They tested the relative significance of competition and predation by manipulating populations of A. sagrei lizards on several small islands in the Bahamas.
The researchers created three predatory treatments — no predators, avian predators only, and both avian and terrestrial predators — with two replicates of each treatment. Islands receiving the “no predators” treatment were enclosed with bird-proof netting while the “avian predators only” treatments were left open to predatory birds. To expose the lizards to both terrestrial and avian predators, predatory snakes were introduced to the islands receiving the third treatment.
To simultaneously evaluate the impact of intraspecific competition, each predatory treatment was distributed across a range of low to high population densities. Un-manipulated islands of A. sagrei lizards were also maintained as “natural reference population[s].”
Survival rates were lowest among lizards exposed to both terrestrial and avian predation. Among the other predatory treatments, survival rates were not significantly different from those in the control populations.
Notably, lizards exposed to both bird and snake predators were recaptured higher in the canopy than lizards in the other two predatory treatments were. Thus, increased terrestrial predation not only increases mortality rates, but also induces behavioral changes, specifically changes in perching habits. However, no phenotypic alterations were observed in these populations, which suggests that predation is not a significant agent of selection.
Contrastingly, the study revealed strong evidence of the role of intraspecific competition in shaping phenotypic selections. In high-density populations of lizards, there was strong selection for larger body size and longer limb length, both of which provide significant competitive advantage for lizards. Thus, among high-density populations of A. sagrei, strong competition induces selection on phenotypic traits that increase the lizards’ fitness, while lower population density reduces competition and relaxes selection pressure.
These findings are an important step forward in our understanding of the mechanisms that drive selection and shape “adaptive landscapes” in natural populations.