Well-seasoned demography: The importance of intra-annual variation in populations of a Neotropical milkweed

Background/Question/Methods

Most plant species exhibit annual patterns in reproduction, survival, and size transitions, with seed production and growth occurring during favorable months and death or dormancy during harsher months. In the Neotropics, seasonal changes in precipitation, light penetration, pollinator activity, herbivory, and other factors impact plant fitness, and so investment in reproduction, growth, and survival should have different seasonal consequences for fitness. I investigated the role of seasonality and associated factors on tropical plant demography using a multi-season census of a continuously flowering Neotropical milkweed, Asclepias curassavica. I established 22 demographic plots in three natural populations of A. curassavica  in Monteverde, Costa Rica. I conducted population censuses and measured demographic traits in all individuals within plots every three months starting in June 2011. I also estimated pollinator activity via presence of pollinia, and assessed herbivory levels at each census.

Results/Conclusions

Populations grew (λ=1.04) during the wet season and decreased (λ=0.99) during the dry season. Though adult plant survivorship was higher during the dry (0.78 ± 0.028) than the wet season (0.76 ± 0.027), seedling recruitment was 2.4 times higher in the wet than in the dry season. Fruit production per individual was higher during late wet (0.97 ± 16) than early wet (0.32 ± 0.07) or dry (0.51 ± 0.08) season, although pollinators were most active during dry season and flower production did not differ among seasons. Larger plants were more likely to survive, and growth increased survival probability.  During the dry season, adults had more shoots on average (3.7 ± 0.24) than during wet season (2.7 ± 0.14), perhaps contributing to a greater rate of survival.

This research indicates that tropical seasons can impact fitness in a continuously reproducing species, and that plant investment in survival versus reproduction may have different consequences depending on season. The wet season tended to be more important for sexual reproduction, while individuals were more likely to add shoots during the dry season.  Seasonal censuses will continue in order to assess the year-to-year consistency of these seasonal trends and their evolutionary consequences.