Experimental shifts in exotic flowering phenology produce strong indirect effects on native plant reproductive success

Abstract

1. By causing phenological shifts that vary among species, climate change is altering time envelopes for species interactions, often with unexpected demographic consequences. Indirect interactions, like apparent competition and apparent facilitation, are especially likely to change in duration because they involve multiple interactors, increasing the likelihood of asynchronous phenological shifts by at least one interactor. Thus, we might observe ecological surprises if intermediaries of indirectly interacting species change their mediating behaviour. 2. We explored this possibility in a plant-pollinator community that is likely to experience asynchronous phenological shifts. We advanced and delayed the flowering phenology of two ubiquitous exotic plants of western Washington prairies, Hypochaeris radicata and Cytisus scoparius, relative to seven native perennial forb species whose phenologies remained unmanipulated. These species interact indirectly through shared pollinators, whose foraging behaviour influences plant reproductive success. We quantified impacts of experimental phenological shifts on seedset, pollinator visitation rates and visiting pollinator composition relative to an unmanipulated control. We first verified that unmanipulated indirect interactions between native and exotic plants were strong, ranging from facilitative to competitive. 3. Seedset of native plants was strongly affected by changes in exotic flowering phenology, but the magnitude and direction of effects were not predicted by the nature of the original indirect interaction (facilitative vs. neutral vs. competitive) or the change in interaction duration. The relationship between pollinator visitation and seedset changed for most species, though changes in pollinator visitation rate and pollinator composition were not as widespread as effects on native seedset. 4. Synthesis. Changes in pollinator foraging behaviour in response to changes in available floral resources are probably responsible for the unexpected effects we observed. Asynchronous phenological shifts have the potential to produce large and unexpected effects on reproductive success via indirect interactions.

Stage-mediated priority effects and season lengths shape long-term competition dynamics

“…Stage differences are also not the only mechanism that could cause trait-mediated priority effects. For instance, priority effects in plants could arise from light competition (Schwinning and Weiner 1998), plant-soil feedbacks (Kardol et al 2007; Van der Putten et al 2013) or plant-pollinator interactions (Waters et al 2020); arrival time of competing predators or parasites can also alter defensive traits of common prey or host, subsequently affecting competition (Olito and Fukami 2009; Clay et al 2019) and sometimes generate late arriver advantage (Hoverman and Relyea 2008; Clay et al 2020). All of these alternative mechanisms could lead to a different competition-phenology function (different shape or direction of curves in Figure 2A) that often is poorly documented or unknown in natural systems.…”

Bridging Theory and Experiments of Priority Effects

“…In dragonfly and damselfly communities, priority effects can be driven by intraguild predation because early arrivers gain a size advantage that allows them to prey on smaller late arrivers [22,32]. In plants, priority effects can arise from a range of different mechanisms, including competition for light [33] or pollinators [34], and plant-soil feedback (e.g., via shifts in microbiome or allelopathy) [35]. In host-pathogen systems, priority effects between two parasites co-infecting the same host can be mediated via an immune response of the host (e.g., cross-immunity) [36][37][38].…”

Pollinator-Mediated Indirect Effects on Plant Fecundity Revealed by Network Indices

“…Such potential facilitative interactions were linked to species with long flowering, which are generalized and share pollinators with several other species in this community (Lázaro et al 2020). It is still unclear how flowering phenology mediates indirect interactions via shared pollinators because flowering overlap can lead to both competition and facilitation depending on the case (Waters et al 2020). Overall, trait patterns indicate that facilitation occurs between generalized species in this community.…”