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.

Fig. 1

Effectiveness (mean ± SE), as measured by seed set from a single visit to a single flower, by visitors to (a) Tradescantia ohiensis and (b) Asclepias incarnata. The SE values are presented for descriptive purposes only.

Fig. 2

Proportion of visits to plants with different weeks of flowering onset relative to the current date of first bloom (CDFB) for (a) Tradescantia ohiensis and (b) Asclepias incarnata. “Other” are taxa that made fewer than five visits. Flowering onset is described as A2, advanced by 2 weeks; A1, advanced by 1 week; D1, delayed by 1 week; D2, delayed by 2 weeks; D3, delayed by 3 weeks; and D4, delayed by 4 weeks.

Fig. 3

(a, b) Seed set (mean ± SE) from a single visit by visitors and hand or open pollinations over different weeks of flowering onset relative to the current date of first bloom (CDFB) for (a) Tradescantia ohiensis (diamonds, hand pollinations; solid squares, Augochlorella sp.; open squares, Dialictus sp. large; open triangles, Bombus spp.; solid circles, Ceratina dupla; open circles, Toxomerus geminatus) (asterisks indicate taxa that differ significantly [P < 0.05] from the hand-pollinated controls) and (b) Asclepias incarnata (diamonds, open pollinations; solid circles, Apis mellifera; solid squares, Polistes fuscatus; open squares, Polistes dominula; solid triangles, Bombus spp.; open circles, Vespula arenaria; open triangles, Chalicodoma sp., line at 0, ineffective visitors: Augochlorella sp. and Dialictus sp. large). (c, d) Observed and expected seed set of focal flowers on plants with different weeks of flowering onset relative to the CDFB for (c) T. ohiensis and (d) A. incarnata. Expected seed-set values are based on the summed product of each pollinator taxon’s effectiveness and the proportion of visits each pollinator taxon made each week (pollinator importance) and, for T. ohiensis, pollinator importance plus the effect of week. The SE values are presented for descriptive purposes only. Flowering onset is as described in Fig. 2.

Fig. 4

Visitation rate (mean ± SE) of pollinators that visited focal flowers and expected rate of pollination success (mean effectiveness of all visitors × number of visits per minute) for plants with different weeks of flowering onset relative to the current date of first bloom (CDFB) for (a) Tradescantia ohiensis and (b) Asclepias incarnata. The SE values are presented for descriptive purposes only. Flowering onset is as described in Fig. 2.