Population viability analysis and the effects of light availability and litter on populations of Cirsium pitcheri, a rare, monocarpic perennial of Great Lakes shorelines

Abstract

An understanding of plant population dynamics in response to natural and anthropogenic influences is critical for rare species management. We present a long-term study and population viability analysis of a federally threatened monocarpic, Cirsium pitcheri, endemic to the Great Lakes, in northern Michigan. Population growth rates of C. pitcheri were highly variable; only four of the 14year-pairs studied during the 19yr study period indicated a growing population (λ>1). Elasticity analysis showed population growth was sensitive to seedling recruitment. We manipulated light and litter levels in the growth chamber to simulate natural succession or invasive plants and to determine their effects on seedlings and ultimately, population performance. The absence of shading and litter increased seedling emergence from 8% to 42%, but no simulated population had less than a 5% risk of extinction over a 25-year period. These populations are, strictly speaking, non-viable. Population viability and conservation of this species are linked to recruitment, including seedling success. Control of plant and animal invasive species and addition of greenhouse transplants to increase recruitment may help reverse population declines.