Demographic Analysis of the Endangered Plant Pectis imberbis Highlights Tradeoffs between Deer Browse and Interspecific Competition

Abstract

Pectis imberbis is an endangered plant found in the Madrean Archipelago ecoregion of southern Arizona. Numerous, potentially interacting stressors, such as drought, shifting fire regimes, invasive species, and grazing by domesticated and wild ungulates, affect this species and region. We used Integral Projection Models (IPMs) to describe dynamics of the largest documented P. imberbis population. While collecting census data, we quantified impacts of species interacting with P. imberbis, including pollinators, Coues deer (Odocoileus virginianus couesi), and co-occurring vegetation, and evaluated their impact on P. imberbis population vital rates. Despite a large overall size (N = 794), the growth rate for this population was below replacement level (λ = 0.9519). Browse by Coues deer negatively impacted demographic vital rates. Woody species and perennial grasses afforded protection against browse, especially for small individuals, but in the absence of browse, coincidence with heterospecific vegetation decreased growth, survival, and reproduction of P. imberbis. We observed 12 taxa of invertebrates interacting with P. imberbis flowers, including native bees, flies, and wasps. Seed production by bagged flowers indicated that flowers are self-compatible. In summary, we detected a unique interplay between deer, which negatively impacted P. imberbis populations via browse, and vegetation, which reduced incidence of browse, while simultaneously depressing P. imberbis vital rates when no browse occurred. Findings suggest that shifts in community dynamics, such as introduction of invasive grasses or fluctuations in deer populations, could impact P. imberbis population trajectories. Management actions, including exclusion of herbivores and removal of invasive grasses, should be considered to aid species recovery.