|Abstract or Summary
- Riparian areas are eco-tones where aquatically- and terrestrially-derived insect biomass is exchanged between habitats, presenting consumers with new sources of energy, and resulting in a reciprocal subsidy. The relative contribution of energy exchange and the
resulting impacts on vertebrate riparian consumers, such as fish or birds, remains poorly understood. We explored this reciprocal exchange within Honeygrove watershed--an alder dominated riparian system within the Oregon Coast Range. Diet samples were collected from birds and fish along with a suite of insect samples during the summer and fall of 2003 and spring 2004. We detected seasonal differences in the abundance and biomass of terrestrial and aquatic insects available to riparian consumers. Spring provided the most adult aquatic insect biomass, and biomass was similar in summer; the fall emergence was an order of magnitude less than the other seasons. Prey sources differed between salmonids. Salmonid diet varied in biomass consumption by season and prey type. Coho salmon (Oncorhynchus kitsutch) on average, consumed more benthic aquatic biomass than adult aquatic insect biomass regardless of season. Despite the availability of externally derived prey, this species depended more on stream-derived resources in
summer and fall but not during spring. In all sampling seasons, co-occurring cutthroat
trout (Oncorhynchus clarkii), consumed more terrestrial invertebrate biomass than
aquatic biomass, on average. Only in summer, cutthroat trout ate more adult aquatic than benthic aquatic biomass. In fall, their consumption of adult and benthic insect biomass was equal. During spring, cutthroat trout consumption, on average, consisted of more benthic aquatic biomass than adult aquatic biomass. Bird diet samples obtained from commonly encountered species such as Swainson’s thrush, Song Sparrow, and Pacificslope
Flycatcher, showed more terrestrial derived than aquatic prey during the summer
sampling season. These data provided evidence of a reciprocal subsidy occurring in the Honeygrove watershed. There is potentially a seasonal synchrony between the two habitats such that when prey availability is low in one habitat, it is subsidized by the other’s high productivity.