Pollinator effectiveness varies with experimental shifts in flowering time

Abstract

The earlier flowering times exhibited by many plant species are a conspicuous sign of climate change. Altered phenologies have caused concern that species could suffer population declines if they flower at times when effective pollinators are unavailable. For two perennial wildflowers, Tradescantia ohiensis and Asclepias incarnata, we used an experimental approach to explore how changing phenology affects the taxonomic composition of the pollinator assemblage and the effectiveness of individual pollinator taxa. After finding in the previous year that fruit set varied with flowering time, we manipulated flowering onset in greenhouses, placed plants in the field over the span of five weeks, and measured pollinator effectiveness as the number of seeds produced after a single visit to a flower. The average effectiveness of pollinators and the expected rates of pollination success were lower for plants of both species flowering earlier than for plants flowering at historical times, suggesting there could be reproductive costs to earlier flowering. Whereas for A. incarnata, differences in average seed set among weeks were due primarily to changes in the composition of the pollinator assemblage, the differences for T. ohiensis were driven by the combined effects of compositional changes and increases over time in the effectiveness of some pollinator taxa. Both species face the possibility of temporal mismatch between the availability of the most effective pollinators and the onset of flowering, and changes in the effectiveness of individual pollinator taxa through time may add an unexpected element to the reproductive consequences of such mismatches.

What drives selection on flowering time? An experimental manipulation of the inherent correlation between genotype and environment

“…Second, decreasing total seed mass with later JDF may have resulted from a temporal decline in pollinator service if, for example, the composition of the pollinator community or pollinator visitation rates varied across the season (e.g., Forrest et al. ; Rafferty and Ives , ). We did not conduct pollinator observations, but nor did we notice any remarkable change in the pollinator community during daily site visits.…”

“…, path d ). JDF can influence phenological matching with effective pollinators (e.g., Rafferty and Ives ), seed predators and herbivores (e.g., Pilson ), and abiotic environmental stress (e.g., drought, Franks et al. ; frost, Inouye ), and can thereby affect reproductive output (Fig.…”

Physiological effects of climate warming on flowering plants and insect pollinators and potential consequences for their interactions

“…However, most plants have compensatory traits that can provide reproductive assurance in the absence of pollinator visitation and thus do not fall into this category (Bond, 1994). The consequences of a shortened period of pollinator availability could be similar to those of reduced phenological overlap, and individual plants that flower outside of the window of overlap with effective pollinators could have reduced reproductive output (Hegland et al, 2009; Rafferty and Ives, 2012). …”

Individual and combined effects of two types of phenological shifts on predator–prey interactions

“…), herbivory (Liu et al. ), interspecific competition (Shorrocks and Bingley , Hernandez and Chalcraft , Rudolf and Singh ), and pollination (Rafferty and Ives ). Similarly, variation in the degree of temporal clustering (i.e., shifts in synchrony) of a phenological event can affect intraspecific interactions including cannibalism (Hopper et al.…”