The interplay of density and insects as drivers of milkweed population dynamics

Background/Question/Methods

Common plant species may be currently experiencing low density or increased rarity due to human activity (e.g. habitat fragmentation), and it is unknown whether they may be at risk demographically to changes in density. The effects of plant density and animal interactions on population dynamics have been examined separately. However, a simultaneous examination of the effects of density and multiple interspecific interactions on plant populations is required to enhance our understanding of the drivers of plant population dynamics in order to effectively manage species threatened by habitat loss and shrinking population density. Milkweeds (Asclepias spp.) provide an exceptional study system to investigate the joint effects of plant-insect interactions and density on population dynamics because their interactions with herbivores and pollinators are well-studied, and they occur in habitats frequently developed for agriculture and show declining abundance in agricultural settings. We used published data to parameterize matrix population models in order to explore the effect of density on Asclepias population growth. Then we designed a greenhouse experiment to examine the effects of density, herbivory, and pollination on individual plant growth, survival, and reproduction.

Results/Conclusions

Deterministic stage-based matrix population models were parameterized for a sparse (6 plants/m²) and a dense (20 plants/m²) population of A. tuberosa. Although survival of flowering plants was similar between populations (0.88 for dense populations and 0.85 for sparse populations), per capita reproduction in the sparse population was nearly double that observed in the dense population. However, pre-reproductive plant survival and probability of transition to flowering plants were lower in the sparse population. Deterministic lambda for the dense population was 0.94 and for the sparse population was lower, 0.90. Our experimental data will enable us to explicitly model the effects of density, herbivory, and pollination on Asclepias population dynamics. This research will provide a basis for understanding the importance of insect pollinators and herbivores on plant population dynamics in the context of changing plant population densities in an era of global change and will provide a critical foundation for linking individual plant-level performance to population effects in order to connect management actions with conservation outcomes for adaptive management.