Unexpected costs of extended phenotypes: nest features determine the effect of fires on leaf cutter ant's demography.

A key principle of the extended phenotype concept is that the benefit of the structures that an animal builds exceeds its cost. However, some contexts may enhance the costs of structures that often represent a benefit, reversing their adaptive nature. In leaf-cutting ant nests, thatched mounds are extended phenotypes that offer a stable microclimate for the growth of the fungus culture. We hypothesized that fires will affect the species that build external, easily flammable thatch mounds (Acromyrmex lobicornis) more than colonies that build subterranean nests in the less-flammable bare ground (Amoimyrmex striatus). We use a stochastic matrix demographic model parameterized with 4 years of data in pre- and post-fire scenarios. Before fires, Ac. lobicornis showed higher stochastic population rate (λs) than Am. striatus. However, fire frequency every 2 years completely reversed this trend, showing population decline only in Ac. lobicornis. Small nests were the stage that most contributed to λs and the most sensitive in all the species and fire scenarios. This illustrates a novel effect of disturbances; the reversion of the adaptive nature of extended phenotypes, which may have strong consequences on population dynamics and assemblage structure through the invert of dominance relationships.