Negative density dependence promotes persistence of a globally rare yet locally abundant plant species (Oenothera coloradensis)

Abstract

AbstractIdentifying the mechanisms underlying the persistence of rare species has long been a motivating question 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 majority of the species found on Earth are rare. Consequently, several mechanisms have been proposed to explain their persistence, including negative density dependence, demographic compensation, vital rate buffering, asynchronous responses of sub-populations to environmental heterogeneity, and fine-scale source-sink dynamics. Persistence of seeds in a seed bank, which is often ignored in models of population dynamics, can also buffer small populations against collapse.We used integral projection models (IPMs) to examine the population dynamics ofOenothera coloradensis, a rare, monocarpic perennial forb, and determine whether any of five proposed demographic mechanisms for rare species persistence contribute to the long-term viability of two populations. We also evaluated how including a discrete seed bank stage changed population models for this species.Including a seed bank stage in population models had a significant positive impact on modeledO. coloradensispopulation growth rate. Using IPMs that included a discrete seedbank state, we found that negative density-dependence was the only supported mechanism for the persistence of this rare species.Synthesis: IPMs of two populations of the rare speciesO. coloradensisemphasize 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 in a spatially heterogeneous environment enables this species to persist, allowing it to sidestep the demographic and genetic challenges of small population size that rare species typically face. These results emphasize that globally rare species can employ many different strategies for persistence, including the somewhat counter-intuitive phenomenon of local abundance. This reinforces the need for customized management and conservation strategies that mirror the diversity of mechanisms that allow rare species persistence.