Fruit Production and Seed Dispersal of Opuntia phaeacantha (Cactaceae) in the Southwest Mojave Desert

Abstract: Mode of reproduction in the genus Opuntia varies among species but often includes both vegetative and sexual reproduction, with the latter often facilitated by animal seed dispersal. In this multi-year study, we examined fruit and seed production and seed dispersal in the Mojave prickly pear (Opuntia phaeacantha) at two sites in the southwest Mojave Desert. Between 2015–2020 we counted fruits on randomly selected cactus patches, quantified fruit losses from those patches over time, collected fruits, and extracted and counted seeds. To assess seed dispersal, we collected fresh mammal pellets in belt transects at one site from 2016–2018; pellets were examined for Opuntia seeds. To assess animal fruit consumption and removal, we installed camera traps at one site. We baited selected patches with fruits and photographed animals that consumed or removed fruits. Fruit numbers varied widely among years and sites, although fruit production was not significantly correlated with climatic variables. Fruit losses were high at both sites, occurring more slowly in years of high fruit production. Seeds per fruit also varied with means ranging from 65 to 125 seeds. Rabbit and deer (Odocoileus hemionus) pellets were abundant at one site where we found 0.02 seeds per rabbit pellet but none in deer pellets. Camera traps baited with fruits revealed that they disappeared more quickly from patch edges than from patch interiors. Desert cottontails (Sylvilagus audubonii) and California jackrabbits (Lepus californicus) dominated photographs at patch edges while nearly all interior photographs were of white-tailed antelope squirrels (Ammospermophilus leucurus). In summary, although the number of seeds produced by O. phaeacantha is highly variable, the total number per site is high in some years, and fruits are consumed and seeds spread by animal dispersers.