Direct and indirect effects of ants on seed predation in moth/yucca mutualisms

Abstract

Strong indirect effects are important in community and ecosystem processes, particularly in aquatic communities where predation and competition are the dominant processes. In this study, we examine an obligate pollination/seed predation mutualism between moths and yuccas and the balance of direct and indirect effects of ants, aphids, and cheater moths on this mutualism. Cheater moths (Tegeticula corruptrix) do not pollinate yuccas but oviposit into developing fruit, where their larvae consume yucca seeds. The overall effect of ants and aphids on yuccas was positive. Ants and aphids had no negative impact on the number of fruit retained per plant, and they had a positive effect on the average number of viable seeds per fruit. Yuccas with high ant and aphid densities had a 60% increase in viable, uneaten seeds. This increase in seed production was largely due to the negative interactions between ants and T. corruptrix. The reduction of cheater ovipositions saved yuccas almost 46 seeds per fruit, plus an additional 26 seeds from the reduction of cheater larvae. Aphids also had an additional indirect benefit, because ants were less likely to damage yucca fruit when aphids were present. This study shows that strong positive interactions, such as the aphid/ant mutualism on yuccas, can also generate strong indirect effects on another positive interaction, the yucca/yucca moth mutualism.