title: Potential effects of short-term climate variation on shrubs, grasshoppers and lizards in the northern Great Basin desert scrub
Creators: name: Dugger, Philip J.
Publisher: name: Western Washington University
Year: 2012

description: <p>Analyzing trophic interactions among organisms may refine our ability to predict the impacts of climate change on organismal communities in an ecosystem or biome. The Great Basin desert scrub biome comprises relatively simple biotic communities in which bottom-up trophic processes should be comparatively easy to document, analyze and understand. Observing 1) the direct effects of abiotic factors (precipitation and temperature) on desert primary producers, primary consumers, and secondary consumers and 2) the indirect effects of abiotic factors on desert community members--as mediated by biotic effects--should enhance our understanding of community trophic dynamics and may improve the accuracy of biotic predictions for desert communities facing climate changes. Because the two ecologically dominant Great Basin shrub species, Artemisia tridentata and Sarcobatus vermiculatus, differ in root depth distributions and leaf morphology, I expected that differences in short-term responses to summer rainfall by these species may result in species-specific effects on higher trophic levels. To test the hypothesis that rain affects these perennial plants differently, I measured the short-term effects of simulated summer rain pulses (periodic watering over 18 d) on water content, nitrogen concentration, and carbon concentration in leaves of these two species. To study the climate-related consequences for representative and abundant primary consumers of these shrubs, I tested the hypothesis that year-to-year variation in grasshopper populations correlates with year-to-year variation in precipitation and temperatures. I analyzed the distribution and abundance of grasshoppers on and near these shrubs using data collected from a single Alvord Basin site over seven summers with respect to 1) seasonal and annual patterns of temperature and precipitation, and 2) predictions of among-summer variation in water content of foliage of A. tridentata and S. vermiculatus. I also examined the consequences for body condition of secondary consumers-males of the long-nosed leopard lizard, Gambelia wislizenii-with respect to variation among years in weather and grasshopper abundance. Using pre-treatment water content of leaves of A. tridentata as a covariate, I found a significant, direct effect of the watering treatment on water content of A. tridentata leaves; there was no apparent effect of watering on S. vermiculatus leaves. Analyses of leaf water content, nitrogen concentration, and carbon concentration revealed differences between A. tridentata and S. vermiculatus in rates of decline in leaf nutrient quality with the descent into the summer dry season. Insufficient data from single-factor analyses allowed weak inferences only about the effects of May weather on grasshopper abundance, but data were sufficient to infer that extremely low winter temperatures may directly reduce grasshopper abundance. Arthropod prey abundance, in turn, had a positive, direct effect on Gambelia wislizenii body condition; thus, bottom-up effects potentially extend to multiple higher trophic levels. Body condition of G. wislizenii, however, was inversely correlated with air temperatures in May, a result of either a direct effect on lizard physiology or indirect effect via plant quality and grasshopper numbers and sizes. Spearman rank correlation analysis revealed that grasshopper abundance was the best predictor of G. wislizenii body condition (rs = 0.901), but abiotic variables (i.e., winter and spring weather) were also strong predictors (rs = 0.890), thus illustrating the importance of considering multiple abiotic and biotic variables when predicting year-to-year differences in lizard fitness.</p>