COS 113-6 - The importance of negative density dependence as a persistence mechanism in a globally rare yet locally abundant plant (Oenoethera coloradensis)

Abstract: Identifying mechanisms that allow for the long-term persistence of rare species has long been a focus for ecologists. Classical theory implies that community dynamics should be driven by common species, and that natural selection should not allow small populations of rare species to persist. Yet, a large portion of species worldwide are rare. Consequently, several demographic mechanisms have been proposed to explain their persistence, including negative density dependence, demographic compensation, demographic buffering, asynchronous responses of subpopulations to environmental heterogeneity, and fine-scale source-sink dynamics. Persistence of seeds in a seed bank, a process often ignored in population ecology, can also buffer populations of rare species against collapse.

We used three years of individual vital rate data from two populations of the rare, monocarpic perennial forb Oenothera coloradensis to create size-based integral projection models (IPMs) with a seed bank stage. We used these IPMs to examine the population dynamics of this species and to determine whether any of the proposed mechanisms for rare species persistence contribute to the long-term viability of the two monitored populations. We also evaluated whether including a discrete seed bank stage altered models of O. coloradensis population dynamics. Due to the inherent variability of the riparian habitat where this species occurs, we predicted that asynchronous subpopulation responses and environmental heterogeneity would be important for O. coloradensis persistence. We also predicted that negative density dependence is important, because of wave-like abundance patterns that have been observed in this species.

We found that including a seed bank stage in population models had a significant impact on modeled O. coloradensis population growth rate. IPMs that included a discrete seed bank state show that negative density-dependence is acting to maintain positive growth rates in both populations. Surprisingly, we did not identify support for any other proposed mechanisms of rare species persistence.

Our analyses of two populations of the rare species O. coloradensis emphasize the importance of including cryptic life stages such as seed banks in demographic models, but fail to provide strong support for most of the proposed mechanisms of rare species persistence. We propose that high micro-site abundance enables this species to persist, allowing it to sidestep the demographic and genetic challenges of small population size that typically accompany rarity. These results support the idea that globally rare species can be locally abundant, and reinforce that uniform strategies of management and conservation will not equally benefit all rare species.