Abstract
Global warming can disrupt mutualistic interactions between solitary bees and plants when increasing temperature differentially changes the timing of interacting partners. One possible scenario is for insect phenology to advance more rapidly than plant phenology. However, empirical evidence for fitness consequences due to temporal mismatches is lacking for pollinators and it remains unknown if bees have developed strategies to mitigate fitness losses following temporal mismatches. We tested the effect of temporal mismatches on the fitness of three spring-emerging solitary bee species, including one pollen specialist. Using flight cages, we simulated (i) a perfect synchronization (from a bee perspective): bees and flowers occur simultaneously, (ii) a mismatch of 3days and (iii) a mismatch of 6days, with bees occurring earlier than flowers in the latter two cases. A mismatch of 6days caused severe fitness losses in all three bee species, as few bees survived without flowers. Females showed strongly reduced activity and reproductive output compared to synchronized bees. Fitness consequences of a 3-day mismatch were species-specific. Both the early-spring species Osmia cornuta and the mid-spring species Osmia bicornis produced the same number of brood cells after a mismatch of 3days as under perfect synchronization. However, O.cornuta decreased the number of female offspring, whereas O.bicornis spread the brood cells over fewer nests, which may increase offspring mortality, e.g. due to parasitoids. The late-spring specialist Osmia brevicornis produced fewer brood cells even after a mismatch of 3days. Additionally, our results suggest that fitness losses after temporal mismatches are higher during warm than cold springs, as the naturally occurring temperature variability revealed that warm temperatures during starvation decreased the survival rate of O.bicornis. We conclude that short temporal mismatches can cause clear fitness losses in solitary bees. Although our results suggest that bees have evolved species-specific strategies to mitigate fitness losses after temporal mismatches, the bees were not able to completely compensate for impacts on their fitness after temporal mismatches with their food resources.
Highlights
Species interactions depend on synchronization of the partner species; a mismatch in their timing results in the disruption of the interaction (Miller-Rushing, Hoye, Inouye, & Post, 2010)
Activity was lower in the second half of adult life compared to the first half of adult life in all treatments in O. cornuta and O. bicornis, but we found a significant interaction between time of activity and treatment for these two species
Ours is the first study of how temporal mismatches in bee–plant interactions can affect the fitness of solitary bees
Summary
Species interactions depend on synchronization of the partner species; a mismatch in their timing results in the disruption of the interaction (Miller-Rushing, Hoye, Inouye, & Post, 2010). Most solitary bee species that emerge in early spring overwinter as already full-fledged adults, but still inside their brood cells These bees could respond quickly to a brief period of warm weather in spring, potentially leading to temporal mismatches with their host plants. The few studies available on adult food limitation in pollinating insects examined bumblebees and butterflies in the laboratory (Boggs & Ross, 1993; Murphy, Launer, & Ehrlich, 1983; Vesterlund & Sorvari, 2014) They indicated that fecundity and/or longevity are reduced, implying severe fitness losses for these species. We chose two polylectic and one oligolectic bee species that emerge between early and late spring We measured their survival rates, their activity over their lifetimes, the number of brood cells and nests produced, and the sex of their offspring. Our results suggest that fitness losses after temporal mismatches are higher during warm than during cold springs
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