Reverberating effects of resource exchanges in stream-riparian food webs.

Fluxes of materials or organisms across ecological boundaries, often termed "resource subsidies," directly affect recipient food webs. Few studies have addressed how such direct responses in one ecosystem may, in turn, influence the fluxes of materials or organisms to other habitats or the potential for feedback relationships to occur among ecosystems. As part of a large-scale, multi-year experiment, we evaluated the hypothesis that the input of a marine-derived subsidy results in a complex array of resource exchanges (i.e., inputs, outputs, feedbacks) between stream and riparian ecosystems as responses disperse across ecological boundaries. Moreover, we evaluated how the physical properties of resource subsidies mediated complex responses by contrasting carcasses with a pelletized salmon treatment. We found that salmon carcasses altered stream-riparian food webs by directly subsidizing multiple aquatic and terrestrial organisms (e.g., benthic insect larvae, fishes, and terrestrial flies). Such responses further influenced food webs along indirect pathways, some of which spanned land and water (e.g., subsidized fishes reduced aquatic insect emergence, with consequences for spiders and bats). Subsidy-mediated feedbacks manifested when carcasses were removed to riparian habitats where they were colonized by carrion flies, some of which fell into the stream and acted as another prey subsidy for fishes. As the effects of salmon subsidies propagated through the stream-riparian food web, the sign of consumer responses was not always positive and appeared to be determined by the outcome of trophic interactions, such that localized trophic interactions within one ecosystem mediated the export of organisms to others.

The impact of protein quality on stable nitrogen isotope ratio discrimination and assimilated diet estimation.

Accurately predicting isotopic discrimination is central to estimating assimilated diets of wild animals when using stable isotopes. Current mixing models assume that the stable N isotope ratio (delta(15)N) discrimination (Delta(15)N) for each food in a mixed diet is constant and independent of other foods being consumed. Thus, the discrimination value for the mixed diet is the combined, weighted average for each food when consumed as the sole diet. However, if protein quality is a major determinant of Delta(15)N, discrimination values for mixed diets may be higher or lower than the weighted average and will reflect the protein quality of the entire diet and not that of the individual foods. This potential difference occurs because the protein quality of a mixed diet depends on whether, and to what extent, the profiles and amounts of essential amino acids in the individual foods are complementary or non-complementary to each other in meeting the animal's requirement. We tested these ideas by determining the Delta(15)N of several common foods (corn, wheat, alfalfa, soybean, and fish meal) with known amino acid profiles when fed singly and in combination to laboratory rats. Discrimination values for the mixed diets often differed from the weighted averages for the individual foods and depended on the degree of complementation. Delta(15)N for mixed diets ranged from 1.1 per thousand lower than the weighted average for foods with complementary amino acid profiles to 0.4 per thousand higher for foods with non-complementary amino acid profiles. These differences led to underestimates as high as 44% and overestimates as high as 36% of the relative proportions of fish meal and soybean meal N, respectively, in the assimilated mixed diets. We conclude that using isotopes to estimate assimilated diets is more complex than often appreciated and will require developing more biologically based, time-sensitive models.