Resurgence of Harrisia portoricensis (Cactaceae) on Desecheo Island after the removal of invasive vertebrates: management implications

Abstract

Desecheo Island hosts a natural population of the higo chumbo cactus Harrisia portoricensis, listed as threatened under the US Endangered Species Act. The species was extirpated from mainland Puerto Rico and is restricted to the offshore islands of Mona, Monito and Desecheo. Herbivory by goats Capra hircus, rhesus macaques Macaca mulatta, and black rats Rattus rattus introduced to Desecheo Island have likely contributed to a population decline, with only a few individuals of higo chumbo reported in 2003. As part of a restoration program, goats have been eradicated, macaques are considered functionally extirpated, and actions to remove invasive rodents were completed in 2016 and confirmed as successful in 2017. Systematic monitoring was implemented between 2010 and 2013, including widespread searches for H. portoricensis, collecting data on population structure, number of individuals, height, number of branches and evidence of phenological events. After 4 yr of continuous monitoring, 72 individual plants were identified, which exhibited increased height and branching throughout the study years. No seedlings were observed and only a few juveniles recorded, suggesting a resurgence from suppressed adults with limited recruitment. Long-term monitoring is critical to understanding the population dynamics of this species on Desecheo Island. Efforts to safeguard this species on the island require completion of the invasive vertebrate removals and would benefit from a seed banking program and manual establishment of new populations.