Context‐dependent mutualisms in the Joshua tree–yucca moth system shift along a climate gradient

Changing climate patterns can affect the geographic distribution of species through effects on species interactions. Iconic Joshua trees are limited to a narrow range of climate conditions, and climate change is expected to shift suitable habitat to higher elevations and latitudes than their current geographic distribution. As such, the survival of the species requires colonization of new habitats. However, Joshua trees form an obligate mutualistic relationship with yucca moths that pollinate the flowers but whose offspring then consume a portion of the developing seeds. It is not known whether the yucca moths will move in synchrony with Joshua trees, or how changing environmental conditions may affect the net benefits and costs of the yucca moth–Joshua tree interaction. To evaluate the spectrum of conditional outcomes of the interaction, we examined a range of performance measures and abiotic factors across the elevation range that spans the distribution of Joshua trees in Joshua Tree National Park. We found a strong concordance between tree size, moth and tree abundance, and reproductive success, with peak performance of both partners at intermediate elevation. Within sites, larger trees produced more flowers, attracted more pollinators, and had greater seed set. We found that the conditional outcomes of the interaction varied predictably along the gradient: Seed set, as well as seed predation, was greatest at intermediate elevations where trees and pollinators were both at high abundance. At range margins, the proportion of infertile seeds increased, possibly because low pollinator abundance led to pollen limitation. The reproductive success of Joshua trees is tightly linked to pollinator abundance, and the conditional outcomes (magnitude of the fitness benefit) of the mutualism change depending on where it occurs on the elevation gradient.