Demographics of Riparian Lizards in the Chiricahua Mountains in Relation to Water Availability and Emerging Aquatic Insects as a Potential Food Source

Abstract

Severe drought driven by climate change and water use by humans are causing formerly perennial streams to flow intermittently, presenting an unprecedented level of disturbance. The loss of emerging aquatic insects as potential prey items could negatively impact riparian and terrestrial species, including lizards. Because lizards play important roles in riparian food webs (e.g. predators, nutrient cycling), it is crucial to understand the cascading effects of stream drying on lizard communities. We hypothesized that perennial streams provide aquatic subsidies to riparian lizards, reducing competition and opening niches. We predicted that lizard abundances would be greater, and that individuals within a species would grow larger and faster, along perennial streams compared to ephemeral streams. We studied three paired 100-meter perennial and ephemeral reaches with similar microhabitat but differing water availability in the Chiricahua Mountains of southeastern Arizona. We measured individual growth rates during a 2-month mark-recapture study of Yarrow’s spiny lizards (Sceloporus jarrovii), striped plateau lizards (Sceloporus virgatus), and ornate tree lizards (Urosaurus ornatus). We used emergence traps to quantify the availability of aquatic prey. Aquatic insects were collected in high abundances, suggesting a potential food source for lizards along perennial streams that may be unavailable along ephemeral streams. When considering mass at first capture, we found that S. jarrovii were larger at perennial versus ephemeral reaches. However, this pattern did not hold true for S. virgatus. Additionally, we failed to detect differences in abundances between paired perennial and ephemeral reaches for either S. jarrovii or S. virgatus. Low sample sizes prevented us from performing any statistical analyses for Urosaurus ornatus and on the mark-recapture data for S. jarrovii and S. virgatus. Although more research is needed to confirm these results, they indicate that emerging aquatic insects may be an important resource to riparian lizard species in arid environments. Future research should quantify trophic links between lizards and potential aquatic subsidies.