Beaver invasion alters terrestrial subsidies to subantarctic stream food webs |
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Authors: | Christopher B Anderson and Amy D Rosemond |
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Institution: | (1) Odum School of Ecology, University of Georgia, Athens, GA 30602, USA;(2) Conservation and Society Group, Institute of Ecology and Biodiversity, Casilla 653, Santiago, Chile;(3) Sub-Antarctic Biocultural Conservation Program, University of North Texas, Denton, TX, USA;(4) Sub-Antarctic Biocultural Conservation Program, Universidad de Magallanes, Puerto Williams, Chile |
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Abstract: | North American beavers (Castor canadensis) were introduced to Tierra del Fuego Island in 1946 for their fur, and have since spread across the archipelago and onto
the South American mainland. We assessed the impact of invasive beavers on streams of these forested watersheds by quantifying
the trophic basis of production (TBP) and consumptive organic matter flows of benthic macroinvertebrate assemblages. TBP was
determined in two streams: clear- and black-water. Stable isotopes were used across four streams to further elucidate food
web structure and dominant pathways. TBP and stable isotopes showed that terrestrially derived organic matter (amorphous detritus,
leaves, and wood) supported a majority of secondary production in the benthic food webs at all sites (forested reaches, beaver
ponds, and sections downstream of ponds with foraged riparian zones). The magnitude of these flows was enhanced in beaver-modified
sites compared with forested habitats (4.0–5.3× increase g AFDM m−2 year−1 in pond habitats, 1.1–2.1× increase in downstream habitats). Diatoms were the only autochthonous resource identified in macroinvertebrate
guts, but their contribution to secondary production was small. Consumptive flows mirrored trends in TBP (i.e., dominance
of terrestrial sources and greater magnitude in beaver ponds). Collector–gatherer consumption of amorphous detrital material
dominated food web flows in all habitats, but was higher in beaver ponds relative to other habitats. Food web structure was
simplified in beaver ponds; only two of the five possible functional groups contributed >1% of total organic matter flow in
ponds (collector–gatherers and predators). Consumptive flows to predators increased in ponds, and stable isotopes of nitrogen
and carbon (δ15N and δ13C) corroborated a relatively greater importance of predators (greater trophic distance), as well as less diversity of basal
resources (less variation in δ13C) in ponds. Our findings indicate that invasive beaver’s engineering activities resulted in greater flows of terrestrial
organic matter subsidies to in-stream food webs, which had a relatively greater change in the clear-water than in the black-water
stream. Owing to the fact that these streams were naturally dependent on allochthonous resources for a majority of production
and material flows, changes wrought by beavers to streams in forested environments are probably less than in watersheds with
inherently greater dependence on autochthonous production such as the adjacent steppe biome. |
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