Climate-driven decoupling of a coevolved mutualism between Joshua trees (Yucca brevifolia and Y. jaegeriana, Asparagaceae) and pollinating yucca moths (Tegeticula synthetica and T. antithetica, Prodoxidae)
Environmental change can disrupt ecologically important mutualisms, if coadapted partners respond to changes differently---but modeling differential responses by specialized mutualists can be challenging. Here, we use TARDIS (Temporal Analysis of Reproduction Distributed in Space) to examine how annual weather variation drives flowering in Joshua trees (Yucca brevifolia and Y. jaegeriana; Asparagaceae), and subsequent fruit production that relies on obligate yucca moth pollinators (Tegeticula antithetica and T. synthetica; Prodoxidae). We used more than 16,000 observations of Joshua trees and corresponding weather records, covering an 18-year period, to train Bayesian additive regression tree models of flowering and fruiting activity. The models find flowering and fruiting respond differently to temperature and inter-annual precipitation variation, and a hindcast for the years 1900-2025 reveals that years with conditions favoring flowering have become slightly more frequent, while conditions favoring fruiting--and moth activity--have been stable or declining. Trends in this predicted mismatch between Joshua tree flowering and yucca moth activity vary with latitude and elevation, but do not differ in regions identified as climate refugia. Overall, our results suggest climate change to date has had pervasive, if so far small, impacts on both these specialized mutualists.