What predicts the use by brook trout (Salvelinus fontinalis) of terrestrial invertebrate subsidies in headwater streams?

Abstract

Summary Spatial subsidies are important resources for organisms in receiving habitats, particularly when production in those habitats is low. Terrestrial invertebrates provide a critical subsidy for trout, including eastern brook trout (Salvelinus fontinalis), but we have limited understanding of what causes input and use of these subsidies to vary among streams. We predicted that forest successional stage would be an especially important driver of variation in terrestrial invertebrate subsidies to brook trout in headwater streams due to differences in terrestrial invertebrate biomass in early and late successional habitats. Specifically, we expected biomass of aerial invertebrates, those capable of dispersal to the stream, to be greater in early successional habitat than late successional habitat due to the nutrient‐rich, herbaceous vegetation typical of early successional habitat. We measured aerial terrestrial invertebrate biomass in early and late successional habitats, input to streams and use by resident brook trout in 12 first‐ and second‐order catchments in northern New Hampshire, U.S.A. The study catchments represented a range of early successional habitat coverage (0–51.5%). We also measured a suite of reach‐scale variables that might influence terrestrial invertebrate input and use by brook trout, including riparian forest conditions and benthic invertebrate biomass. Within study catchments, aerial terrestrial invertebrate biomass and abundance were significantly higher in early successional habitats than late successional habitats. However, terrestrial invertebrate input to streams and use by brook trout were unrelated to per cent early successional habitat in the catchment, and to other catchment and riparian forest characteristics. These results indicate that the management for upland early successional habitat has little effect on terrestrial invertebrate subsidies to headwater streams and fish. Surprisingly, benthic invertebrate biomass was the one significant predictor of per cent terrestrial invertebrates in brook trout diets. Use of terrestrial invertebrate subsidies declined with increasing benthic invertebrate biomass, suggesting that productivity in the aquatic environment influences the degree to which brook trout use terrestrial subsidies. Although subsidy inputs are controlled by the donor system, this study shows that use of these subsidies by consumers can be determined by conditions in the recipient habitat.