Long-term yield from harvester ant colonies: implications for horned lizard foraging strategy

For a long-term, prudent foraging strategy to evolve via natural selection at the level of the individual, three conditions must be met: (1) the forager must use resources from a discrete subpopulation, (2) use of that subpopulation must be virtually exclusive, and (3) the resource population must respond in such a way that a long-term strategy actually provides a greater economic benefit to the forager than does a short-term strategy. In this study, the third condition is tested for a predator (the horned lizard: Phrynosoma cornutum) that has been hypothesized to employ a long-term strategy when harvesting worker ants from colonies of the ant Pogonomyrmex desertorum. Five artificial harvesting experiments were undertaken to determine if, at some point, an increased harvesting intensity will lead to a decreased long-term yield. Such a result would indicate that it is economically advantageous for horned lizards to employ a strategy in which harvest intensity is reduced in order to promote the long-term availability of ants. However, in none of the five experiments was this result obtained. It is therefore concluded that evolution of a long-term strategy in horned lizards is unlikely. Failure to obtain the required result is attributed to three characteristics of P. Desertorum colonies. (1) Because colonies cease surface activity in response to depredation of surface-active workers, and increase in foraging effort (above some threshold) by horned lizards will not lead to an increase in losses from preyed-upon colonies. (2) Because colonies have an unharvestable reserve consisting of the queen, immature workers, and colony stores, they can, for a limited time, continue to produce surface-active workers (the resource used by horned lizards), even if all surface-active workers have been previously harvested. (3) In the short term, colonies are a finite resource; it should matter little whether the resource is harvested in one intense bout or in a series of less intense bouts.