Resource subsidies

Food web studies have added significantly to our understanding of the magnitude of various energy pathways and how these pathways differ in importance with environmental context, particularly landscape setting. Energy subsidies to fluvial ecosystems that can be extremely important to ecosystem metabolism include leaf litter, the infall of terrestrial invertebrates, and the carcasses and reproductive products of migrating fishes. Salmonids in small streams can obtain half or more of their energy from the consumption of terrestrial insects, suggesting that stream-side forest management may be an important consideration in salmon management (Wipfli 1997). These exchanges can be bidirectional, as exemplified by the dependence of spiders and lizards in riparian zones on the emergence of aquatic insects (Sabo and Power 2002).

Evidence from a variety of ecosystems reveals that energy subsidies are an important and widespread facet of food web ecology (Polis et al. 1997). Exclusion of leaf litter inputs to an Appalachian stream altered basal resources and organic matter flow to predators in the study by Hall et al. (2000), described above. The input of terrestrial insects to fishes can be an important energy subsidy to top consumers, as Nakano et al. (1999) demonstrated by placing a fine-mesh net over a forest stream in Japan. The exclusion of terrestrial invertebrates resulted in greater fish predation on benthic aquatic invertebrates, triggering a trophic cascade and an increase in periphyton biomass. Further, the biomass of herbivorous invertebrates and periphy-ton did not differ between treatments with or without fish when terrestrial invertebrate inputs were allowed, suggesting that the supply of arthropods from land normally prevented strong top-down control (Baxter et al. 2005). The complicated consequences for food web interactions due to external subsidies is also evident in a competitive shift in feeding by native Dolly Varden charr (Salvelinus malma) in northern Japan in response to nonnative rainbow trout (Baxter et al. 2004). Trout were better able to consume the terrestrial invertebrates that fell into the stream, forcing charr to feed on herbivorous invertebrates and causing an increase in periphyton biomass. However, the biomass of adult aquatic insects emerging from the stream decreased, and this in turn reduced the density of spiders in the riparian forest.

For migratory and mobile species, stable isotope analysis has been useful in determining the location where C was assimilated, thus providing insight into the magnitude of cross-system subsidies. Differences in the S13C of muscle tissue in the migratory Semaprochilodus insignis established that C produced in blackwater systems contributed to fish stocks harvested from white-water systems of the Amazon (Benedito-Cecilio and Araujo-Lima 2002). Because epilithic algae, detritus, and algal filaments varied in abundance among benthic habitats and streams in the headwaters of the Eel River of northern California, Finlay et al. (2002) were able to use C isotope ratios to assess habitat use by different consumers. The S13C values of collector-gatherers and scrapers indicated a reliance on algae from local sources within their riffle or shallow pool habitats, whereas filter feeders derived more C from upstream shallow pools. Algal production from shallow pools was the dominant resource base for vertebrate predators in late summer regardless of the habitat where they were collected. The drift of pool insects into riffles, rather than movement of trout among habitats, was the presumed mechanism, illuminating between-habitat subsidies parallel to cross-ecosystem subsidies.

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