Streams and the terrestrial areas that flank them, called riparian zones, are connected through fluxes of energy and nutrients (subsidies) such as leaf litter and insects. Emergent aquatic insects that become prey for terrestrial insectivores provide linkages between streams and riparian zones. Previous studies suggest that agriculture can affect insect abundance and diversity, but it remains unclear how aquatic insect subsidies to terrestrial habitats vary with land-use and affect diets of terrestrial consumers. We investigated insect abundance and spatial foraging habits of the Eastern Phoebe (Sayornis phoebe) near streams across an agricultural land-use gradient to address: 1) How do aquatic and terrestrial insect abundance and biomass vary across a land-use gradient? 2) How does land-use affect phoebe foraging patterns? and 3) How do phoebe diets vary across land-uses? We quantified abundance and biomass of emergent aquatic insects and terrestrial insects, and conducted phoebe foraging observations and stable isotope analysis (d2H) of aquatic insects, terrestrial insects, and phoebe chicks at seven stream sites.
Results/Conclusions
We found that phoebes foraged over the stream more frequently at sites with less tree cover, and d2H signatures suggested that phoebes foraged on a mixture of aquatic and terrestrial insects, but generally more terrestrial insects. Insect abundance was not correlated with percent agriculture; percent tree cover was significantly correlated with some insect taxa. Terrestrial but not aquatic insect biomass was positively correlated with percent agriculture. Phoebe d2H values were significantly higher (signaling increased terrestrial insect consumption) at sites with more tree cover, and also tended to decrease with increasing agricultural land-use, though the trend was only marginally significant. Understanding the habitat requirements of riparian consumers will aid land managers in designing appropriate agricultural riparian buffers. This study highlights the potential impacts of agricultural land-use on connections between stream and riparian food webs through changes to both prey availability and predator foraging preferences, with implications for directing land management.