Influence of grazed riparian vegetation on aquatic insects in Western Mongolia
Mongolia has undergone major changes in pastoral practices over the past three decades. Increased grazing has damaged riparian vegetation within some watersheds. Riparian zones in western Mongolia are narrow and landscape vegetation is sparse, thus loss of riparian vegetation along western streams may impair stream insects by limiting dispersal and removing habitat of adults. The objective of this study was to determine the relationship between riparian condition and emerging insect abundance. Most aquatic insects live out their immature stages in water and then emerge from the water as adults, linking stream fauna to terrestrial landscapes. The surface floating pupal exuviae (SFPE) collection method was used to estimate abundance of emerging non-biting midges, a representative group of aquatic insects. The SFPE method is standardized as a catch per unit effort allowing for comparison of samples across sites. Riparian condition was measured using modified line-intercept and Daubenmire frame methods. Watershed and riparian coverage were estimated following the EPA Rapid Bioassessment Protocol. Thirty-three landscape, watershed, and riparian variables and a total of 12,661 midge exuviae were analyzed from 34 streams. Stepwise multiple regression using backward elimination was used in building a model to explain variation in the relative abundance of emerging midges.
Heavy grazing was observed at a majority of sites as evidenced by defoliated forbs and short, broken grass. In many cases grazing was observed while sampling. Constraining the multiple regression analysis to selection of 7 variables produced the best model, although it was relatively weak with an R² of 0.46 (p<0.05). The strongest predictor of variation in midge abundance was the estimate of forb coverage along the shore. According to the model, midge abundance increased with increasing shoreline forbs, but decreased with increasing bare ground along the shore and increasing percent of boulder and rocky substrate in the stream. Abundance also increased with increased grass height along the stream. The influence of rocky substrate is indicative of naturally occurring variation in stream substrate among the study sites, but the other variables clearly show the influence of changing riparian condition on the abundance of emerging aquatic insects. This study provides preliminary evidence that grazing impacts riparian condition and subsequently affects aquatic insects in Mongolian streams. Further research directly linking riparian range condition with the ecology of aquatic insects is recommended to better understand this relationship in grassland streams.