Temporal‐ and density‐dependent impacts of an invasive plant on pollinators and pollination services to a native plant

Abstract

Abstract. Pollinators and pollination services are under threat globally, and invasive plants have been implicated in their decline. Results of previous studies suggest that consequences of invasion for pollinators and plant-pollinator interactions are context specific. Investigating factors such as the density of an invasive plant and its phenology may provide a nuanced understanding of invasive species impacts. We conducted a 2-yr study in Montana to investigate how local pollinator abundance, richness, community composition, and visitation patterns varied with invasive Centaura stoebe density and phenology, and whether C. stoebe altered the reproduction of a co-flowering native plant, Heterotheca villosa, through changes in pollinator visitation. In an observational study, we found that during its peak bloom in August, Centaurea stoebe provided abundant floral resources to late-season pollinators. However, prior to C. stoebe bloom, native floral density and pollinator abundance and richness of these plots were lower compared to plots where C. stoebe was low or absent. Pollinator community composition in plots without C. stoebe was different compared to plots with C. stoebe (both high and low C. stoebe density), and these differences in pollinator composition strongly depended on the time of season. In an experimental study, we found that there was little evidence of competition between C. stoebe and H. villosa for pollinators at low relative densities of C. stoebe. Using experimental pollen supplementation, we observed no evidence of pollen limitation of seed set in H. villosa with increasing density of experimentally added C. stoebe. Our results suggest that the impact of an invasive plant on pollinators and plant-pollinator interactions depends on the relative density of the invasive plant and the timing of its bloom. Differences in pollinator visitation patterns over the growing season suggest that although C. stoebe provides abundant resources to lateseason pollinators, displacement of native plants at high C. stoebe density may indirectly harm pollinators that are active before C. stoebe blooms or that prefer native plants. Based on our results, restricting C. stoebe to low densities may help mitigate negative repercussions to native plant reproduction and may even be beneficial to some pollinators.

Temporal scale‐dependence of plant–pollinator networks

“…For example, mean annual temperature might dictate not only which species potentially can interact but also the length and the synchrony of the periods in which these interactions are realized. This perspective may also help to predict the consequences of climate change (Park and Mazer ), species invasions (Herron‐Sweet et al , Arroyo‐Correa et al ) or habitat degradation and restoration (Burkle et al , Ponisio et al ).…”

“…Additionally, even temporally co‐occurring species may switch interaction partners over time, thereby rewiring the links within the network (Poisot et al ). Understanding the influence of temporal dynamics – such as species turnover and link rewiring – on network structure at multiple temporal scales might be particularly important considering various drivers that alter the timing of species interactions, such as climate change (Park and Mazer ), habitat modification (Burkle et al , Ponisio et al ) and species invasions (Herron‐Sweet et al , Arroyo‐Correa et al ).…”

Drought and increased CO2 alter floral visual and olfactory traits with context‐dependent effects on pollinator visitation

“…Insect pollination is known to increase the reproductive output for three of the forb species. Heterotheca villosa and P. recta are self-compatible, but produce fewer seeds in the absence of insect visitors (McIver & Erickson, 2012;Herron-Sweet et al, 2016). Campanula rotundifolia is self-incompatible and relies on bees for pollination (Shetler, 1979).…”

Risks to pollinators and pollination from invasive alien species

“…interactions. Invasive alien species can alter the flow of energy and nutrients within an ecosystem 4 , and disrupt mutualisms including those underpinning crop and wild plant reproduction [7][8][9] . Strongly interacting alien invaders can also establish novel selection pressures within a community that can modify evolutionary trajectories and adversely affect species with low genetic diversity and/or small effective population sizes [10][11][12] .…”

“…Whether regional facilitation or local competition predominates may depend on the spatial scale of the alien plant invasion, and the differing foraging ranges and ecology of pollinators in the species pool 53,[58][59][60] . Overall, the impact of alien plant invasions on native plant pollination and reproductive success is greater if, relative to the native flora, the alien produces higher densities of flowers, they are phylogenetically related, or they possess similar phenology and anatomy of floral displays 9,19,59,61 . Aside from fundamental competition for pollinators, there may also be native pollen loss and pick up of foreign pollen during visits to alien flowers.…”