Food or sex: which would you choose? Pollinator-prey conflict and reproductive assurance in New Zealand Drosera

Abstract

This is the first study to test the possibility that autonomous selfing in carnivorous plants functions as a mechanism to reduce the pollinator-prey conflict by reducing pollen-limitation. Carnivorous plants can rely on insects both for pollen supply and nutrition and past studies suggest that pollinator-prey conflict will occur when flowers and traps compete for this shared resource, resulting in either a pollen or prey-limitation. The extent of either limitation will initially be governed by insect availability in a given environment. Conflict is most likely to occur in resource and insect-limited environments. When pollinators are trapped as prey (pollinator-prey overlap), pollen-limitations can be exacerbated. I additionally propose that conflict can occur when pollinating prey visit flowers instead of traps exacerbating prey-limitations. Trade-offs in the selection on traits that attract insects as pollinators and prey may occur when plant fitness is limited by both pollen and prey. To reduce pollen-limitation, traits that reduce pollinator dependency should be selected and, in the event of pollinator-prey overlap, the selection of trap-flower separation may provide protection of pollinators. I quantify pollen-limitation and pollinator-prey overlap for two New Zealand Drosera species, Drosera arcturi and D. spatulata to explore the influence of various trap morphologies on prey types captured and pollinator-prey overlap. Only D. arcturi caught insects from pollinating families (96% of pollinators were from families also caught as prey) although both Drosera species were outcross pollen-limited. I examined the role self-fertilisation to reduce pollen-limitation in these and three other Drosera species (D. arcturi, D. spatulata and D. stenopetala in alpine habitats and D. auriculata and D. peltata in lowland habitats) including the costs of inbreeding depression in the three alpine Drosera species. I found that all Drosera species examined were fully self-compatible and that selfing reduced pollinator-prey conflict (mediated by pollen-limitation) by reducing pollinator dependency. The three alpine Drosera species autonomously selfed when outcrossing was unavailable providing reproductive assurance when pollinators are scarce. Furthermore, measures of cumulative lifetime inbreeding depression found no inbreeding depression for any species. To relieve outcross pollen-limitations, traits that increase floral attraction may also be selected. I therefore test whether the pollinator attraction trait of flower height and the pollinator protection trait of trap-flower separation increase outcrossing success for two Drosera species. Within species patterns were not detected, but the species that exhibited taller flowers further from its traps, D. spatulata, experienced greater outcrossing success than D. arcturi with shorter flowers closer to its traps. Since pollinator dependency is low for both species, I argue that the increased overlap and decreased separation in D. arcturi indicates the importance of capturing pollinators as prey, whereas the lack of overlap and increased trap-flower separation in D. spatulata suggests that there are some benefits to outcrossing that I did not detect. To reduce prey limitations, traits that increase trap attraction/effectiveness should be selected. It is likely, however, that both pollen and prey limitations can occur simultaneously, in which case traits that relieve both limitations may be selected (e.g. floral attraction of pollinating prey). I test whether resource allocation to flowers alleviates both pollen and prey limitations and found that flowering indeed increases per plant prey capture for Drosera arcturi. Though selfing reduces pollen-limitations, continuing to invest in floral attraction may also benefit prey capture, relieving pollinator-prey conflict mediated by both pollen and prey-limitations. This study is the first to explore the role of floral traits in relieving prey-limitations in addition to demonstrating that selfing in carnivorous plants not only reduces pollinator dependency, but also influences the consequences of pollinator-prey overlap. This highlights the necessity of examining breeding systems in all future studies of pollinator-prey conflict.