The persistence of small populations remains a puzzle for ecology and conservation. Especially interesting is how naturally small, isolated populations are able to persist in the face of multiple environmental forces that create fluctuating conditions and should, theory predicts, lead to high probabilities of extirpation. We used a combination of long‐term census data and a five‐year demographic study of a naturally rare, endemic plant, Yermo xanthocephalus, to evaluate the importance of several possible mechanisms for small population persistence: negative density dependence, vital rate buffering, demographic compensation, asynchrony in dynamics among sub‐populations, and source–sink dynamics. These non‐exclusive explanations for population persistence all have been shown to operate in some systems, but have rarely if ever been simultaneously examined for the same population or species. We hypothesized that asynchrony in dynamics and demographic compensation would be more powerful than the other three mechanisms. We found partial support for our hypothesis: density dependence, asynchrony among population segments, and source–sink patterns appear to be the most important mechanisms maintaining population viability in this species. Importantly, these processes all appear to operate strongly at very fine spatial scales for Yermo, allowing the only two, extremely small, populations to persist. We also found considerable differences in the results of our census and demographic analyses. In general, we estimated substantially greater chances of population survival from the census data than from the shorter‐term demographic studies. In part, this difference is due to drier than average climate conditions during the years of the demographic work. These results emphasize that while demographic information is necessary to understand various components of population dynamics, longer term studies, even if much less detailed, can be more powerful in uncovering some mechanisms that may be critical in stabilizing population numbers, especially in variable environments.

Abstract. In many ecosystems, foundational species create spatial patterns that structure a broader community. It is unclear, however, how robust these patterns are across large areas and strong environmental gradients, and how the landscape-level consequences of these patterns may vary. We investigated the robustness of non-random patterning in the dispersion of the western harvester ant (Pogonomyrmex occidentalis), a widely recognized ecosystem engineer of western North America. We used remote imagery to characterize the spatial structure and densities of western harvester ant mounds at sites spanning their range within the sagebrush steppe and short-grass prairie areas of Wyoming (581 3 450 km area). We found that ant mound densities varied substantially across the study region, but that mounds were strongly and consistently overdispersed (regularly patterned) across both climatic gradients and mound densities. Precipitation was the only abiotic factor that significantly affected either density or pattern, with stronger patterning among mounds at drier sites. This robustness in ecological patterning is likely to have strong effects on community function; mound dispersion increased the fraction of the landscape within typical ant foraging distances up to 30% over what density alone would predict. We estimated how patterning can modify one key ant effect at a landscape level by combining mound dispersion data with information from a seed removal experiment. Randomization tests based on these results showed that in a representative area, overdispersion could increase the mean landscape-wide seed removal rate by 16%, and decrease its spatial variance by 50%. Western harvester ants are known to affect multiple aspects of community function and structure at a relatively fine scale, and our results show that their spatial dispersion may therefore influence many features of interspecific interactions and community dynamics.

2021. A critical comparison of integral projection and matrix projection models for demographic analysis. Ecological Monographs 91(2):e01447.

Predicting how environmental factors affect the distribution of species is a fundamental goal of conservation biology. Conservation biologists rely on species distribution and abundance models to identify key habitat characteristics for species. Occupancy modeling is frequently promoted as a practical alternative to use of abundance in identifying habitat quality. While occupancy and abundance are potentially governed by different limiting factors operating at different scales, few studies have directly compared predictive models for these approaches in the same system. We evaluated how much occupancy and abundance are driven by the same environmental factors for a species of conservation concern, the greater short‐horned lizard (Phrynosoma hernandesi). Occupancy was most strongly dictated by precipitation, temperature, and density of ant mounds. While these factors were also in the best‐supported predictive models for lizard abundance, the magnitude of the effects varied, with the sign of the effect changing for temperature and precipitation. These discrepancies show that while occupancy modeling can be an efficient approach for conservation planning, predictors of occupancy probability should not automatically be equated with predictors of population abundance. Understanding the differences in factors that control occupancy versus abundance can help us to identify habitat requirements and mitigate the loss of threatened species.

Thousands of microbial taxa in the soil form symbioses with host plants, and due to their contribution to plant performance, these microbes are often considered an extension of the host genome. Given microbial effects on host performance, it is important to understand factors that govern microbial community assembly. Host developmental stage could affect rhizosphere microbial diversity while, alternatively, microbial assemblages could change simply as a consequence of time and the opportunity for microbial succession. Previous studies suggest that rhizosphere microbial assemblages shift across plant developmental stages, but time since germination is confounded with developmental stage. We asked how elapsed time and potential microbial succession relative to host development affected microbial diversity in the rhizosphere using monogenic flowering-time mutants of Arabidopsis thaliana. Under our experimental design, different developmental stages were present among host genotypes after the same amount of time following germination, e.g. at 76 days following germination some host genotypes were flowering while others were fruiting or senescing. We found that elapsed time was a strong predictor of microbial diversity whereas there were few differences among developmental stages. Our results support the idea that time and, likely, microbial succession more strongly affect microbial community assembly than host developmental stage.