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Abstract

Energy development, in combination with other environmental stressors, poses a persistent threat to rare species endemic to the energy-producing regions of the Western United States. Demographic analyses of monitored populations can provide key information on the natural dynamics of threatened plant and animal populations, and how they might be affected by ongoing and future development. In the Uinta Basin in Utah and Colorado, Graham’s beardtongue (Penstemon grahamii) and White River beardtongue (Penstemon scariosus var. albifluvis) are two rare endemic wildflowers that persist on oil shale habitats heavily impacted by current energy exploration and development, and slated for expanded traditional drilling and oil shale development. We described demographic characteristics and population viability for two populations of each species that have been monitored since 2004. First, we measured population size, survival rates, transitions between life stages, and recruitment using individually marked plants at the four study areas. Then, we used matrix population models to determine stochastic population growth rates (λ) and the probability that each population would persist 50 years into the future, given current conditions. The two P. grahamii study plots had small populations averaging 70 adult plants, and relatively constant and high survival in both vegetative and flowering plants. The two P. scariosusvar. albifluvis study plots had populations that averaged 120 adult plants, with high and stable survival in flowering plants and variable survival in vegetative plants. Recruitment of new seedlings into all populations was low and variable, with most recruitment occurring in one or two years. Both P. grahamii populations had λ near 1.0 (stable). OneP. scariosus var. albifluvis population appeared to be declining (λ=0.97), while the other was increasing (λ=1.16). Our analyses reveal populations that appear relatively stable, but that are susceptible to declines now and into the future. Increases in environmental variability, deterministic changes in habitat conditions or stressors, or a single catastrophic event could all have immediately deleterious impacts on the long-term growth trajectory of these populations.

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