Robbing begets robbing: How additional interactions can change the outcome of a pollination mutualism
Nectar robbing is a common form of cheating in pollination mutualisms. The effects of primary nectar robbing (the chewing of holes or slits through flowers) on plant female fitness have been well described in the literature, and the outcome of the interaction varies. That is, third party interactions can mediate the cost of this behavior. However, the presence of primary robbers is known to facilitate secondary nectar robbing (i.e., secondary use of existing holes). The effects of secondary nectar robbing are largely unknown. Because secondary nectar robbing is a facilitated behavior, we can expect community context to be equally, if not more important. In Ipomopsis aggregata (Polemonaceae), primary nectar robbing reduces fruit and seed set because hummingbird pollinators avoid flowers robbed by bumble bees. We asked: does secondary robbing affect Ipomopsis fitness above and beyond the effect of primary nectar robbing? We manipulated plants in the field at the Rocky Mountain Biological Lab in Gothic, CO to experience either no robbing, primary robbing only, or primary and secondary robbing (n=40 plants per treatment). To determine whether secondary robbing imparts a cost on plant fitness, we measured pollen receipt, fruits produced, and seeds produced.
Plants exposed to secondary nectar robbing produced significantly fewer fruits, seeds, and seeds per fruit than plants that experienced primary robbing only. Our results show that secondary-robbed plants received significantly fewer pollen grains compared to plants that did not experience nectar robbing. Interestingly, our results showed no effect of primary nectar robbing in some cases, suggesting that facilitation of secondary robbing by primary robbers may change the net outcome of the interactions from neutral to negative. Previous studies have shown that primary robbing reduces plant fitness in Ipomopsis, but did not tease apart effects of primary and secondary robbing. Our results demonstrate the importance of doing so, as it may affect the direction and strength of these interactions. Future work will investigate the facilitative interaction between primary and secondary robbers, in light of the fact that many floral visitors change their strategy from legitimate foraging to secondary nectar robbing when given the opportunity. This study is the first to isolate the effects of secondary robbing from primary robbing. Our results illustrate the need to study pollination mutualisms at the community level, rather than focusing only on pair-wise interactions.