The relative timing of species interactions depends on both the phenologies and ontogenies of the interacting species. While the phenologies of many species are shifting earlier in the calendar due to climate change, specific species often show considerable variation in the degree and direction of these calendar shifts. These species-specific responses to environmental changes have the potential to create phenological shifts in species interactions. While there is increasing evidence for phenological shifts in the relative timing of species interactions, the fitness consequences of these shifts are not well understood. Here, we describe the results of an experiment in which monarch butterflies were introduced to native milkweed hosts (A. fascicularis) at two host plant densities (1 plant per larvae and 3 plants per larvae) with different degrees of phenological shift (at 7 intervals) throughout the season. The aim of this experiment was to determine how variation in the timing of monarch oviposition relative to milkweed ontogeny and density affects the developmental success of monarch caterpillars. Specifically, this experiment sought to determine if there is an optimum time for monarch oviposition relative to milkweed development and if so, the fitness consequences of shifting away from this optimal timing.
Results/Conclusions
We found an unexpected pattern suggesting two separate “windows of opportunity” for larval success during the season. The first window of opportunity corresponded to a period of rapidly increasing plant height, leaf toughness, latex volumes and trichome densities in the early summer. The second window of opportunity occurred in the late summer and corresponded to a period of encroaching senescence with high leaf toughness, declining trichome densities and declining latex volumes. These two windows of opportunity were separated by a period in the middle of the summer with consistently low hatching success, poor larval growth and high predator densities. Contrary to expectations, larval success was not influenced by host plant density. These windows of opportunity appear to be delimited by a complex combination of both abiotic and biotic effects. These narrow windows of opportunity suggest that the timing of milkweed-monarch interactions has strong consequences for larval success, and this two-peaked “phenology-ontogeny fitness landscape” may explain recent phenological shifts in the timing of monarch activity observed in the California Central Valley. More generally, these results suggest that phenological shifts could have important fitness consequences for species interactions, with implications for both population dynamics and evolutionary responses to climate change.