How seasonal and annual variation in demography influence populations of a Neotropical milkweed, Asclepias curassavica
Kellett, Kimberly Mary
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Temporal variation is generally examined only at the annual scale in demographic studies. Seasonal variation in demography therefore plays an unclear, but likely important role in the population dynamics and evolution of many species, especially for those with year-round reproduction. To examine the role of seasonal variation to population dynamics and evolution, I use demographic models constructed from data collected every three months for three years from populations of a continuously flowering, long-lived Neotropical milkweed, Asclepias curassavica, in Monteverde, Costa Rica. Seasonal variation in demography contributed over twice as much to temporal variation in population growth than annual variation, and seasonal and annual variation in population growth occurred via different demographic pathways. Reproductive costs and payoffs varied over time for these populations, but without a predictable seasonal pattern. Models suggested this variation is integral to the evolution of reproductive delay in this species. The lack of seasonal pattern in the variation of reproductive costs may contribute to the year-round flowering strategy of this species. Response of population growth to precipitation was season-specific during the length of our study. Three seasons responded positively to decreased precipitation, while one responded negatively, and the precipitation measure that best predicted population growth varied among season. Including season-specific responses influenced our predictions for population growth under climate change, and our most realistic model predicted a modest increase in population growth for this species in the future. This dissertation emphasizes the importance of seasonal variation in demography, and encourages further exploration of seasonal variation in the demography of tropical and temperate species.