Understanding stability in mutualisms: Can extrinsic factors balance the yucca-yucca moth interaction?

Mutualisms form an integral part of many communities and play an important role in the maintenance and promotion of biodiversity. The inherent conflict of interest between mutualists suggests that partners should reduce the cost of the interaction whenever possible. As a result, defense mechanisms may evolve to prevent overexploitation by both partners. Selection for the evolution of defenses intrinsic to the interaction may be unnecessary, however, under particular ecological conditions. I examined the interaction between the pollinating yucca moth Tegeticula cassandra and its host plant Yucca filamentosa to determine whether extrinsic factors are important in promoting stability in this mutualism. Yuccas can selectively abscise fruit with high egg loads for moth species that damage ovules during oviposition into the locule. Female T. cassandra, however, lay eggs less than a millimeter beneath the tissue surface and fail to damage ovules, circumventing this mechanism of selective abscission. Consequently, yuccas are unable to regulate egg loads by T. cassandra, and moths are free to lay many eggs. Despite this, only 5-11% of T. cassandra eggs survive to exit the fruit as mature larvae. I tested the hypothesis that T. cassandra survivorship is regulated by the extrinsic factors dehydration and predation. While exclusion of potential predators seemed to have no effect on survivorship, moth eggs in flowers with experimentally increased vapor pressure had significantly greater survivorship compared to controls. Survivorship of a closely related locule-ovipositing species was unaffected by either predator exclusion or changes in vapor pressure. The results suggest that the stability of mutualisms can depend at least in part on extrinsic factors.