Invertebrates play important roles in most food webs because they consume basal resources and are consumed by larger species. Southeastern Arizona has a high level of invertebrate diversity, especially in riparian areas along streams. However, terrestrial invertebrate communities may differ between perennial (i.e., wet) and ephemeral (i.e., dry) stream beds due to differing levels of moisture, resource availability, and habitat complexity. We surveyed riparian invertebrates along three perennial and three ephemeral 100-meter stream reaches in the Chiricahua Mountains of southeast Arizona, midday, pre-monsoon season in June 2017. In each of the six 100-meter reaches, we used beating sheets to collect terrestrial invertebrates from leaf litter and riparian vegetation in 5 x 5 meter grids at 25, 50 and 75 meters, as well as three lateral distances (5, 10 and 15 meters) away from the streams. Based on these distances, there were 18 collection grids per reach, resulting in a total of 108 possible grids across 6 study reaches. We predicted that: 1) a greater abundance and variety of invertebrates would be found along perennial streams compared to ephemeral streams and 2) a greater abundance and diversity of invertebrates would be found closer to the stream bed than further away laterally.
Results/Conclusions
Data analyses shows some support for our predictions, but there was high variability among reaches. We found 10 different invertebrate orders across the six reaches: Araneae, Hemiptera, Hymenoptera, Ledioptera, Coleoptera, Orthoptera, Scorpiones, Isopoda, Neuroptera, and Diptera. Community analyses are ongoing, but it appeared some orders exhibit preferences for perennial versus ephemeral streams or for different distances laterally from the stream bed. At both perennial and ephemeral sites, the richness of invertebrate orders and total abundance of invertebrates generally were higher closer to the stream bed as opposed to farther away. An average of 5 invertebrates per grid was found closest to the stream bed, decreasing in grids farthest from the stream bed. Overall abundance of invertebrates was marginally higher in perennial streams compared to ephemeral streams. However, one of our three ephemeral streams had very high abundances of invertebrates at 10 and 15 meters from the stream bed, which obscured overall patterns. Including a larger number of replicate perennial and ephemeral stream reaches could help clarify our findings. We hope our results will help predict how riparian invertebrate communities might change if drought and water withdrawals cause perennial streams to become ephemeral.