Abiotic influences on the behaviour of rodents, ants, and plants affect an ant-seed mutualism

Deserts are open environments characterized by striking shifts in temperature and light regimes. We hypothesized that the abiotic environment mediates the interaction between an ant-dispersed plant, Datura wrightii (Solanaceae), ant mutualists, and rodent seed predators in the Sonoran Desert. Field experiments contrasting diurnal and nocturnal seed collection rates in the presence of ants only, vertebrates only, and in the presence of both groups indicated that 85% of seed collection by mammalian seed predators occurred at night (between 1900 and 0700). Seed collection by ants, in contrast, was similar between day and night, although seed collection decreased during very hot days and very bright nights. The total number of seeds collected by both groups foraging separately exceeded the number removed when both groups shared access to seed depots, suggesting that ants and rodents compete for seeds. However, D. wrightii plants dehisced 86% of their fruits between 0700 and 1900, increasing the likelihood of seed collection by ant mutualists rather than rodent seed predators. Dehiscence was sensitive to environmental cues: greenhouse plants kept at constant temperature and humidity dehisced 47% of their fruits between 0700 and 1900. Additional field experiments demonstrated that seed-collecting ants transported seeds considerable distances to their nests, microsites that can be rich in nutrients. The mean (± SE) seed dispersal distance was 6.1 ± 0.5 m, the longest mean dispersal distance yet reported for an ant-dispersed seed.