Limiting the costs of mutualism: multiple modes of interaction between yuccas and yucca moths

In pollination-seed predation mutualisms between yuccas and yucca moths, conflicts of interest exist for yuccas, because benefits of increased pollination may be outweighed by increased seed consumption. These conflicts raise the problem of what limits seed consumption, and ultimately what limits or regulates moth populations. Although the current hypothesis is that yuccas should selectively abscise flowers with high numbers of yucca-moth eggs, within-inflorescence selective abscission occurs in only one out of the three moth-yucca systems that we studied. It occurs only when oviposition directly damages developing ovules, and does not, therefore, provide a general explanation for the resolution of moth-yucca conflicts. Within-locule egg mortality provides an alternative and stronger mechanism for limiting seed damage, and generating density-dependent mortality for yucca-moth populations. However, the most important feature of moth-yucca systems is that they are diverse, encompassing multiple modes of interaction, each with different consequences for limiting and regulating yucca moths.