Costs of two non-mutualistic species in a yucca/yucca moth mutualism

Abstract

Mutualisms often involve significant costs for participants. Costs are inflicted by mutualists themselves, as well as by associated, non-mutualistic species. These costs are rarely quantified, however, particularly the ones extrinsic to the pairwise interaction. We compare costs inflicted by an obligate mutualist pollinator and two common exploiters of an Arizona yucca over a 2-year period. The magnitude of seed damage from seed and fruit-feeding beetle larvae (Carpophilus longus, Nitidulidae) was similar to damage from the seed-eating larvae of Yucca schottii's pollinator moth Tegeticula yuccasella (Prodoxidae), averaging about 15 seeds destroyed per fruit in each case. The two seed predators usually fed within the same fruits, although rarely side by side. In contrast, the presence of fruit-galling moth larvae (Prodoxusy-inversus, Prodoxidae) appeared to benefit the yucca: individual Tegeticula destroyed only half as many seeds in galled fruits as they did in ungalled fruits. We discuss three general implications of these results. Firstly, the costs of non-mutualists to the two mutualistic partners are not necessarily parallel. Secondly, measurable costs of non-mutualists do not necessarily translate into an impact on the success of the mutualism itself, because they may be incurred after mutualistic activities take place. Finally, the costs of mutualists to each other can differ substantially depending on the presence or absence of non-mutualistic species.

Community Context of an Obligate Mutualism: Pollinator and Florivore Effects on Yucca Filamentosa

“…Most of these species feed on or within the inflorescence scape, flowers, or fruits and have the potential to impact both yucca and pollinator fitness. For example, Bronstein and Ziv (1997) demonstrated that feeding by a bogus yucca moth species benefited the plant by reducing the number of yucca seeds eaten by pollinator larvae. In contrast, the beetle Carpophilus longus fed on seeds and added to the overall seed mortality in fruits.…”

Characterizing the interaction between the bogus yucca moth and yuccas: do bogus yucca moths impact yucca reproductive success?

“…For another Prodoxus species, P. y-inversus, Bronstein and Ziv (1997) demonstrated that the presence of larvae in the fruit wall had a positive effect on Y. schottii. Although the mechanism is unknown, fruit with larvae of P. y-inversus had significantly reduced seed destruction by pollinator larvae.…”

Facilitated exploitation of pollination mutualisms: fitness consequences for plants

“…Mutualistic interactions are rarely pairwise, as most mutualists interact with multiple partner species (Stanton ), and the ecological and evolutionary consequences of interacting with single vs. multiple partners have been investigated in depth (Bronstein ). Species engaged in mutualism also commonly interact with multiple exploiter species sequentially and/or simultaneously (Bronstein & Ziv ; Vannette, Gauthier & Fukami ). However, there has been minimal recognition of this phenomenon, nor measurement of its effects.…”

“…; Sachs ). Exploitation is ubiquitous, and many mutualisms are exploited by multiple species simultaneously (Bronstein & Ziv ; Vannette, Gauthier & Fukami ). However, it is most often studied by examining consequences to a mutualist of interacting with a single exploiter.…”