PS 16-225 - Environmental and biological factors surrounding the interaction between a Joshua tree, its pollinating yucca moth, and their associated arthropod community.

Abstract: Joshua trees, long-lived, monocot yucca trees endemic to the Mojave Desert of the U.S. Southwest, are iconic as one of the most prevalent woody plants in their range. They can play a pivotal role in shaping ecosystems where their populations are dense. The Joshua tree depends on a mutualistic interaction with specialist moth pollinators of the genus Tegeticula, which rely on the Joshua tree for oviposition. This Joshua tree-moth interaction has been well-documented as an example of a highly coevolved, obligately specialized mutualism. However, much of the basic natural history of this system has not been formally recorded. Furthermore, the phenology and fitness of both partners may be linked to changing environmental cues which remain undetermined. To shed light on ecological factors affecting this interaction, we measured environmental parameters, Joshua tree arthropod visitation, and indicators of Joshua tree fitness along an elevational gradient over the course of two Joshua tree flowering seasons.

During the 2022 and 2023 spring flowering seasons, we located eastern Joshua trees (Yucca jaegeriana) at sites along an elevational gradient in the Arizona Joshua Tree Forest of NW Arizona. We designed a trap method which allowed us to document the occurrence of the specialist pollinator Tegeticula antithetica, and of other arthropods visiting eastern Joshua tree flowers. For each tree, we also measured temperature and humidity, recorded aspects of tree density and reproductive ecology, and evaluated seed production and germination.

We detected high variability in arthropod diversity on traps across our sites, and in the occurrence of T. antithetica. Additionally, we reared and identified several larvae interacting within Joshua tree seed pods. We found indications of potential site-level differences in our environmental data and in our indicators of Joshua tree reproduction as measured by the number of inflorescences and seed pod production per tree at each site. Due to the high variability of Joshua tree populations and environmental heterogeneity, multi-year studies of this system may be required to understand the natural history, phenology, and specific risks facing the Joshua tree and moth interaction. We plan to continue our studies over multiple flowering seasons.

Here, we present the findings from our first two years of studying the relationship between moth activity, environmental parameters, and seed production.