Diet plasticity is a common behavior exhibited by piscivores to sustain predator biomass when preferred prey biomass is reduced. Invasive piscivore diet plasticity could complicate suppression success; thus, understanding invasive predator consumption is insightful to meeting conservation targets. Here, we determine if diet plasticity exists in an invasive apex piscivore and whether plasticity could influence native species recovery benchmarks and invasive species suppression goals. We compared diet and stable isotope signatures of invasive lake trout and native Yellowstone cutthroat trout (cutthroat trout) from Yellowstone Lake, Wyoming, U.S.A. as a function of no, low-, moderate-, and high-lake trout density states. Lake trout exhibited plasticity in relation to their density; consumption of cutthroat trout decreased 5-fold (diet proportion from 0.89 to 0.18) from low- to high-density state. During the high-density state, lake trout switched to amphipods, which were also consumed by cutthroat trout, resulting in high diet overlap (Schoener’s index value, D = 0.68) between the species. As suppression reduced lake trout densities (moderate-density state), more cutthroat trout were consumed (proportion of cutthroat trout = 0.42), and diet overlap was released between the species (D = 0.30). A shift in lake trout δ13C signatures from the high- to the moderate-density state also corroborated increased consumption of cutthroat trout and lake trout diet plasticity. Observed declines in lake trout are not commensurate with expected cutthroat trout recovery due to lake trout diet plasticity. The abundance of the native species in need of conservation may take longer to recover due to the diet plasticity of the invasive species. The changes observed in diet, diet overlap, and isotopes associated with predator suppression provides more insight into conservation and suppression dynamics than using predator and prey biomass alone. By understanding these dynamics, we can better prepare conservation programs for potential feedbacks caused by invasive species suppression.
Invasive species require management to mitigate their harmful effects on native biodiversity and ecosystemprocesses. However, such management can also have negative, unintended consequences on non-target taxa,ecosystem processes, and food web dynamics. In Yellowstone Lake, invasive lake trout (Salvelinus namaycush)have caused a decline in the native Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri) population. Tosuppress the invader, lake trout carcasses are deposited on the species’ spawning sites, causing embryo mortalityby reducing dissolved oxygen as they decay. The non-target effects of carcass deposition are unknown, but benthicinvertebrates may be sensitive to reductions in dissolved oxygen. Benthic invertebrate taxa have varying hypoxiatolerances; caddisflies of the family Limnephilidae are hypoxia sensitive whereas the amphipods Gammarus lacustrisand Hyalella azteca are hypoxia tolerant. Both taxa are widespread and abundant in Yellowstone Lake andcomprise a large proportion of fish diets, and changes in their abundances could therefore alter food web dynamics.We conducted an in situ experiment to determine if carcass deposition causes mortality in these two benthic invertebratetaxa. The probability of mortality for caddisflies was 3.15 times higher in carcass treatments compared tocontrols, while amphipod mortality did not change in response to carcass treatment. Amphipods, which contributemost significantly to fish diets, are unlikely to be reduced in response to carcass deposition, which is confined to asmall fraction of the lake where lake trout spawn, limiting the possibility for lake-wide effects. We conclude thatcarcass deposition is unlikely to alter the availability of invertebrates as a food source for fish in Yellowstone Lake.
Invasive species can have negative effects on native biodiversity and ecosystem function, and suppression is often required to minimize the effects. However, management actions to suppress invasive species may cause negative, unintended effects on non‐target taxa. Across the United States, lake trout (Salvelinus namaycush) are invasive in many freshwater ecosystems, reducing native fish abundance and diversity through predation and competition. In an integrated pest management approach, lake trout embryos in Yellowstone Lake, Wyoming, are suppressed by depositing lake trout carcasses onto spawning sites; the carcasses reduce dissolved oxygen concentrations as they decay, causing embryo mortality. We conducted a field experiment during one ice‐free season at four sites in Yellowstone Lake to investigate the non‐target effects of carcass treatment on benthic invertebrates, which could have consequences for native fish diets. While overall invertebrate density and biomass did not respond to carcass treatment, Chironomidae midges and Sphaeriidae fingernail clams decreased in abundance. Carcass treatment altered invertebrate community structure based on density, but not biomass. Carcass treatment to suppress invasive fish embryos has spatially localized, non‐target effects on some benthic invertebrate taxa. Given the small spatial extent of carcass treatment within the lake, we conclude it is unlikely that carcass treatment will alter food availability for native fishes.
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