Woodrat herbivory influences saguaro (Carnegiea gigantea) reproductive output

Abstract

The saguaro (Carnegiea gigantea) is a keystone resource for Sonoran desert consumers of nectar, pollen, fruit, and cactus tissues. Saguaro tissue contains oxalic acid and is unavailable to most consumers. The white-throated woodrat (Neotoma albigula) is, however, able to consume foods with high oxalate content, and is strongly associated with desert succulents, primarily cacti of the genus Opuntia. N. albigula forages secondarily on saguaro tissues, reducing photosynthetic surface area and eliciting an energetically-demanding wound response that reduces energy stores available to fuel reproduction. We observed and quantified Neotoma herbivory on saguaros in a low desert environment. Evidence of Neotoma grazing was found on 44% of all saguaros surveyed, and 13% of all saguaros had >20% of their surface area affected by Neotoma grazing. Neotoma herbivory on saguaros was predicted by the number of nearby succulents, presence of Neotoma middens, and saguaro age. When comparing similarly sized plants, saguaros with high levels (>20% of surface) of herbivory produced fewer flowers and fruits than plants with no Neotoma herbivory. These findings suggest that periodic use of saguaros by N. albigula, such as during extended droughts with conditions unfavorable for Opuntia growth and establishment, may reduce long-term reproductive capacities in low-density saguaro populations.