The key role of honey mesquite (Prosopis glandulosa) in promoting the survival of desert woodrats (Neotoma lepida) in Death Valley, California

Background/Question/Methods

Death Valley, California is one of the hottest, driest places on Earth. Summer daytime temperatures are regularly over 50˚C, and precipitation averages <5cm per year. Despite these harsh conditions, Death Valley supports a dense population of desert woodrats (Neotoma lepida). Unlike other populations of N. lepida, those on the valley floor are entirely dependent on a single species of plant, honey mesquite (Prosopis glandulosa) for survival. Mesquite is their sole source of food, water, and protection from heat and predators. Previous work indicated that there is significant heterogeneity in the survivorship of woodrats across our study site. Given the importance of mesquite to woodrats, we hypothesized that the observed heterogeneity was related to physical and/or physiological differences in the Prosopisplants. Here, we analyze mesquite plant characteristics to determine which factors contribute to differential woodrat survivorship. We trapped and marked over 500 individual woodrats from 2004-2008. We then assessed physical traits of the mesquite (area and volume, spine length and spine density) and physiological traits of the leaves and stems (%C, %N, fiber, phenolics, and water content), as well as a metric for predation risk. We constructed linear models and used AIC as a metric of model comparison.

Results/Conclusions

Surprisingly, most mesquite characteristics measured were not related to woodrat persistence (a proxy for survivorship) across the site. Three mesquite characteristics were significantly negatively correlated with woodrat persistence: phenolic content of leaves (spring season), spine length, and spine density. The model with the lowest AIC included both spine length and spine density as predictors of woodrat persistence. We hypothesize that mesquite complexes with thicker, denser spines impede the woodrats’ ability to forage efficiently, exposing them to thermal stress and predators for longer periods of time. An important variable that we were not able to measure was the thermal microenvironment of individual mesquite plants. Previous work in this system showed that woodrat dens buffer summer daytime temperatures by as much as 20˚C, when temperatures outside of the woodrat dens are above lethal. This may be the key to understanding variation in woodrat persistence not explained by other variables. Among the variables that we measured, the length and density of mesquite spines were the most important factors contributing to differential survivorship of woodrats in this extreme environment.