Timing is everything: flowering phenology influences pollinator-mediated indirect interactions between native and exotic plants

Background/Question/Methods

Native and exotic forbs share pollinators, resulting in fitness consequences for plants when pollinator-mediated competition or facilitation influences seed production. The strength of such pollinator-mediated interactions depends on the duration of time such an indirect interaction can occur; i.e., degree of phenological overlap. However, phenological overlap may increase or decrease with climate change, if species respond individualistically. In fact, non-native plants are thought to have advanced their phenology more than natives, potentially altering the consequences of this indirect interaction for natives.  To determine how individualistic changes in flowering phenologies might affect plant-plant interactions through shared pollinators, we manipulated the flowering phenology of a ubiquitous exotic plant of western Washington prairies, Hypochaeris radicata, relative to seven native perennial forb species.  Specifically, we created three H. radicata treatments--“early” (flowering pots placed in native plots ~2 weeks before local H. radicata peak bloom), “peak”, and “late” (~2 weeks after peak), and quantified pollinator visitation and seed set of native forbs. To assess the importance of native phenological status to the interaction, we also quantified seed production for early, peak, and late-blooming individuals of each of the seven native forbs in control and treatment plots.

Results/Conclusions

Native phenological status (early, peak, or late-blooming within the native population) impacted seed production for four native forbs (Camassia quamash, Ranunculus occidentalis, Microseris laciniata, Campanula rotundifolia), with early blooming individuals suffering reduced seedset compared with peak and late bloomers. This pattern occurred for both spring-flowering and summer-flowering native species. We found evidence for pollinator-mediated competition between H. radicata and four of the native forbs. Seed production decreased for these forbs in the presence of H. radicata, but the effect size depended on H. radicata flowering phenology treatment. Composition and abundance of pollinators visiting natives also varied with exotic phenology treatment and with native phenology status. Thus, the influence of indirect competition for pollination services by H. radicata depended on its phenological stage relative to the native. Our study provides evidence that the contribution of species’ phenologies to their indirect interactions could become increasingly important as climate changes and phenologies shift individualistically.