River food chains lead to riparian bats and birds in two mid‐order rivers

Abstract

River regulation can modify natural flow regimes with deleterious effects on aquatic communities. While the effects of flow manipulation on the physical environment and populations and assemblages of aquatic organisms have been described thoroughly, how and to what extent river regulation influences ecosystem processes like food web architecture is less studied. Emergent aquatic insect prey can provide an important food resource to riparian consumers like birds and bats with concomitant consequences for nutrient cycling through aquatic-terrestrial food webs, thus potentially increasing the spatial influence of river regulation into the riparian zone and beyond. We used naturally abundant stable isotopes of carbon and nitrogen to compare food web architecture (trophic position and reliance on an aquatic nutritional pathway) leading to birds and bats between a regulated river, the Tuolumne River downstream of Hetch Hetchy Reservoir, and an adjacent unregulated river, the Merced River, located in Yosemite National Park on the west slope of the Sierra Nevada, California, USA. We found that both birds and bats derived >50% of their nutrition from food webs originating in photosynthesis by algae. In addition, birds and bats occupied a similar trophic position to predatory fish in other systems. Both birds and bats seemed to rely more strongly on an aquatic nutritional pathway during the dryer year of our study period, underscoring the potential importance of emergent aquatic prey as a water subsidy in dry systems and in dry years. In the Tuolumne River, reservoir managers strive to simulate characteristics of the natural flow regime, including seasonal scouring flows and prolonged floodplain inundation. Although we found no conclusive evidence of an effect of river regulation on food web responses, our study suggests that nutrient cycling through aquatic-terrestrial food webs expands the potential influence of river regulation to organisms and ecosystems typically characterized as terrestrial.

Low flows in a regulated river system are associated with reduced breeding bird abundance and diversity in Yosemite National Park, California

“…In a separate study, we found that birds in our study areas rely heavily on an aquatic‐derived food chain (B. K. Jackson et al, 2020), and when food quantity or quality is reduced, birds will breed at lower densities (Rolando, Caprio, Rinaldi, & Ellena, 2007). Breeding birds rely heavily on invertebrate prey (Martin, 1995; Trevelline et al, 2018), and aquatic insect emergence is often highest in the spring when terrestrial invertebrate prey availability is low (Nakano & Murakami, 2001).…”

“…During the breeding season, many birds rely heavily on invertebrate prey, including terrestrial invertebrates and emergent aquatic insects, to meet their energetic and nutritional needs, as well as the needs of their rapidly growing young (Martin, 1995; Trevelline et al, 2018). Some riparian birds derive more than half of their energetic needs from aquatic food chains, and this percentage can increase in dry years (B. K. Jackson et al, 2020). Additionally, altered flow regimes that result in reduced water availability can promote differential investment in plant growth (e.g., investment in roots at the expense of fruits and seeds), that reduces plant food availability to birds directly, as well as indirectly through reductions in food to invertebrate herbivores, the invertebrate predators that prey on them, and ultimately breeding birds (Bazzaz, Chiariello, Coley, & Pitelka, 1987; Eziz et al, 2017; Rai et al, 2018).…”

Bat Use of Hollows in California’s Old-Growth Redwood Forests: From DNA to Ecology

“…Particularly, we expected to find an association between roost use and distance to streams based on the results of previous studies e.g., [ 11 , 51 ]. While bats often forage on clearing and road edges, their use of streams is higher due to the increased density of insect prey [ 52 ] and the availability of drinking water [ 53 ]. However, congruent with our study, there has not always been a positive correlation between roosts and the distance to water e.g., [ 53 , 54 ].…”

Biological Diversity: Current Status and Conservation Policies

“…Avian abundances along the rivers are positively correlated with vegetation and stream size (Ivicheva et al, 2019). Both water birds and terrestrial birds benefit from riparian nutritional pathways along the rivers (Jackson et al, 2020). Therefore, habitat monitoring along rivers is crucial for the conservation of avian abundance and diversity.…”