Feral horses are associated with a decline in a rare semi-arid grassland plant

摘要

Physaria lesicii (Rollins) O'Kane & Al-Shehbaz is a globally rare plant known from only three populations in the Pryor Mountain Wild Horse Range in Montana, USA, an area subjected to grazing by feral horses for at least 100 years. The purpose of this study was to determine effects of trampling by feral horses on this rare plant. I measured density of P. lesicii plants in 33 paired on- and off-trail sample plots at the one of three known populations in a semi-arid habitat where horse trailing was prevalent. I also determined the strength of the association between the presence of presumed nurse plants and P. lesicii at two different populations. Density of P. lesicii plants was approximately four times as great in grassland than on adjacent trails. There was a significant association between the presence of potential nurse plants (canopy cover) and P. lesicii density only at the lower, presumably more stressful site. These results indicate that feral horse trampling has resulted in a significant decline in the number of P. lesicii plants where horses are most abundant and suggest that loss of nurse plants may partly explain this decline.