The effects of river drying on a terrestrial arthropod community

Background/Question/Methods

Rivers around the world are drying with increasing frequency. However, little information is available on the effects of this drying on terrestrial animal communities. We added artificial pools near a drying section of the San Pedro River, located in southeastern Arizona, and maintained them as the river dried. Ground arthropods were sampled using pitfall traps near artificial pools and adjacent sites as the river dried over a 2 month period. 

Results/Conclusions

Order richness, family richness, and the abundance of most arthropods increased from May to June, but did not differ between pools and dry areas. However, the abundance of primary consumers differed substantially, increasing near pools but remaining low in dry areas (time x treatment F = 4.4; df = 3, 54; H-F p = 0.007). Orthopterans, primarily crickets, were an important contributor to this pattern (time x treatment F = 3.1; df = 3, 54; H-F p = 0.042). Higher trophic levels were primarily comprised of beetles, spiders, and ants. While beetles and spiders did not differ between treatments, they did appear to differ in their overall response as the river dried, with beetles increasing continuously, but spiders at first increasing and then slightly decreasing. The lack of treatment effects on higher trophic levels indicates a greater resistance to the effects of river drying. One plausible mechanism for this resistance is predatory switching from consumption of emergent insects and river water to moist terrestrial prey (e.g. crickets). This could contribute to the low orthopteran abundance we observed along dry river reaches without pools.