Trade-offs between attracting pollinators and ant bodyguards with nectar in Turnera spp
Often species use the services of multiple mutualists; however we lack a substantive understanding of how traits that mediate mutualisms are influenced by multiple partners. For example, many plants simultaneously attract insect pollinators with floral nectar and ant “bodyguards” with extrafloral nectar (EFN). If nectar production is costly, investment in one mutualism could trade off with investment in the other mutualism. We sought to investigate such multiple mutualistic effects using three species of the neotropical perennial weed, Turnera (T. ulmifolia, T. subulata, T. joelii). We confirmed that removal increases investment in nectar and then mimicked the consumptive effects of visiting mutualists by removing floral nectar and EFN daily for 50 days in a full factorial design under glasshouse conditions. We then measured the effect of treatment on floral nectar and EFN sugar production to quantify resource-allocation trade-offs between the two nectar rewards. We also measured seed set across treatments to look for direct fitness costs of nectar production. To supplement our glasshouse study, we experimentally manipulated floral nectar and EFN production in the field and measured various fitness/ performance parameters (i.e. florivory, herbivory, seed set) to quantify ecological costs of resource-allocation trade-offs between floral nectar and EFN production.
For T. ulmifolia and T. subulata, but not T. joelii, we found evidence for a resource-allocation trade-off between attracting pollinators and ant bodyguards; greater production of one nectar type decreased production of the other nectar type. This is the first demonstration of a trade-off between floral nectar and EFN. In T. subulata, greater production of floral or EFN directly negatively impacted fitness by reducing seed set, although only marginally for EFN. In a field population of T. ulmifolia, adding floral nectar to flowers tended to increase seed set, and this was not the result of increased resource availability from the added nectar. Additionally, decreasing EFN production increased florivory and such flowers were significantly less likely to set fruit. Thus, visitation by pollinators may negatively impact ant defense of the plant, and visitation of ant bodyguards may negatively impact pollination as a result of the trade-off between floral nectar and EFN production. This represents an ecological cost of nectar production in addition to the direct cost to production of the other nectar and seed set. Overall, our results suggest that multiple mutualistic effects play a role in nectar production which can lead to non-additive interactions between pollination and ant defense mutualisms.