Community context of an obligate mutualism:: Pollinator and florivore effects on Yucca filamentosa

Obligate pollination mutualisms have been central to our understanding of the ecology and evolution of mutualisms. Although usually viewed as pairwise interactions, obligate mutualists also interact with other community members that may impact the mutualism. In this study, we examined the community context of the obligate inutualism between the plant Yucca filamentosa and its yucca moth pollinator Tegeticula cassandra to determine the relationships among plant traits, the community assemblage, and fruit set. In addition to attracting its pollinator, Y. filamentosa in central Florida also attracts two insect florivores: the hemipteran Leptoglossus phyllopus and the beetle Hymenorus densus. We Surveyed flowering Y. filamentosa in 2001 and 2002 to determine the abundance of the three insect species throughout the flowering period. We also measured fruit set and four plant traits thought to be important in attracting the moth pollinator. Path analysis revealed that T. cassandra was the most important determinant of relative fruit set for both years. Peak pollinator abundance was significantly earlier than peak flowering. Consequently, we found a negative relationship between plant flowering date and pollinator abundance. Furthermore. taller plants also attracted more pollinators. Leptoglossus phyllopus had a significantly negative impact on relative fruit set in 2001 but not in 2002, whereas H. densus had no effect in either year. Abundance of the two florivores was more closely tied to peak flowering of Y. filamentosa than was the peak abundance of the pollinator moths. Although both florivore species reached high densities on Y. filamentosa, their effect on relative fruit set through floral abscission may be partially masked by the characteristically high abscission rates found in yuccas. As a result, florivores may be feeding on flowers that are already destined to be abscised. Similarly, pollinator moths may be preadapted to handle the effects of florivores because females lay eggs across multiple plants and flowers. Hence, the obligate mutualism between yuccas and yucca moths may be somewhat buffered from the effects of local community members depending on whether florivores substantially increase floral abscission rates from year to year.