Effect of capitulum position and weevil infestation on seed production of threatened monocarpic perennial, Cirsium pitcheri

Abstract

Cirsium pitcheri is a federally threatened, monocarpic perennial thistle species endemic to the Great Lakes region of the United States and Canada. Human habitat disturbances, lake level fluctuations and climate change have resulted in extensive declines of C. pitcheri populations. Seed predation by non-native weevil, Larinus planus, has also further exacerbated C. pitcheri population declines and reduced its projected time to extinction to less than 20 years. Given the widespread presence of L. planus throughout C. pitcheri’s range, studies that investigate its impacts are essential for understanding and predicting changes in the life histories and demographic rates of C. pitcheri populations. We present a two-year field study and common garden experiment that examine the relationship between C. pitcheri whole plant fecundity and capitulum position and assess the impacts of L. planus on C. pitcheri seed production. This study is part of an on-going population viability analysis and aims to inform C. pitcheri conservation and restoration efforts. Our results demonstrate that C. pitcheri mature seed production differs among capitulum positions and that the overall reproductive effort of C. pitcheri is reduced by L. planus. Terminal capitula being the largest and first to develop in C. pitcheri, produced the heaviest and greatest number of mature seeds across all capitulum positions. Capitula with evidence of weevil infestation, however, produced 60% fewer mature seeds and 40% fewer unfilled seeds that were also lighter than those without weevils. Evidence of weevil infestation was also primarily concentrated in secondary and tertiary capitula, which are produced at the tips of first- and second-order branches respectively. Along with information on capitulum contributions to C. pitcheri whole plant fecundity, our report presents one of the first accounts for the position-dependent oviposition of non-native weevil, L. planus, and its direct effects on C. pitcheri seed production.