Microclimate modification by riparian vegetation affects the structure and resource limitation of arthropod communities

Ground and surface water hydrology often governs the abundance and type of vegetation present in dryland riparian ecosystems, particularly along rivers that have both perennial and intermittent reaches. But how these differences in riparian vegetation affect riparian arthropod communities is not well understood. I conducted a replicated field experiment at three sites along the San Pedro River in central Arizona, United States, which varied in ground and surface water hydrology and in riparian vegetation. The most mesic site was comprised of cottonwood-willow gallery forest and had more canopy cover, higher absolute humidity, and lower maximum air temperature than the most arid site, which was comprised of mesquite and saltcedar scrubland. Experimental treatments added resources (water, sugar, and a no resource control) to assess water and energy limitation of ground-dwelling arthropods over a 4-week period. I found that arthropod community structure differed between the three study sites, and that these changes were strongly associated with differences in microclimate. The most mesic site had 5.5 times greater arthropod biomass than the most arid site. I also found that the effect size of water supplementation on cricket abundance and the effect size of sugar supplementation on ant abundance were related to microclimate variables. Water effects on cricket abundance were larger, and sugar effects on ant abundance smaller, as general aridity increased. However, I found no relationship between the magnitude of water or sugar effects on total arthropod community abundance and microclimate variables. Nevertheless, these results indicate that groundwater depth can indirectly affect the abundance, community structure, and water vs. energy limitation of riparian arthropod communities via groundwater effects on riparian vegetation, and riparian vegetation effects on microclimate. Increases in river drying events due to climate change and increased water withdrawals will likely lead to changes in riparian vegetation in arid systems, and here I show that these changes could affect riparian animal communities as well.