Effects of Introduced Trout on Ecosystem Subsidy
and Amphibian Decline
In the western United States, thousands of historically fishless mountain lakes have been stocked with trout (primarily Oncorhynchus, Salmo, and Salvelinus spp.) since the late 1800's to increase recreational fishing opportunities (Bahls 1992). The resulting fisheries foster recreational use of wilderness and national forests, however, widespread fish introductions have been implicated in the decline of several amphibian species (e.g., Knapp and Matthews 2000, Orizaola and Brana 2006, Hartel et al. 2007) and have been found to alter the abundance and composition of aquatic insects (e.g., Carlisle and Hawkins 1998, Knapp et al. 2001, Progar and Moldenke 2002, Baxter et al. 2004). Aquatic insects and amphibians are major prey items for bats, birds, snakes, and other terrestrial predators.
We conducted a four year, whole-lake replicated experiment to test the effects of introduced trout at scales relevant to the ecosystem subsidy of lake basins. Twelve lakes subject to three different fish treatments (continued fish stocking, cessation of stocking, and fish removal) were compared to four fishless reference lakes (map). Amphibians and snakes were monitored via visual surveys and mark-recapture, aquatic insects were sampled using benthic sweeps and emergence traps, birds were sampled using double-observer point counts, and bat activity ws measured with Anabat bat detecters.
Trout and Cascadae frog densities were similar among the 12 treatment lakes in 2003, prior to manipulation. Fish removals dramatically increased densities of Cascades frogs, in contrast to continued low densities of frogs at the stocked and stocking-suspension lakes. There was a significant treatment effect when we compared the densities of both frogs and larvae across treatments. In addition, we found a strong treatment by year effect on frog density. Post-treatment (2004-2006) frog densities at the removal lakes were greater than all years’ densities at both the stocked and stocking suspension lakes. By 2006 frog densities at the fish-removal lakes were not significantly different from those at the reference lakes. Stocked and stocking-suspension lakes had lower densities of larvae for all years compared with the reference lakes.
Aquatic insects are major components of both freshwater communities and adjacent terrestrial habitats. Larval insects serve as prey for larger aquatic insects, amphibians and fishes, and the winged adult stages feed terrestrial predators such as birds, bats and spiders (Power and Rainey 2000, Nakano and Murakami 2001, Sanzone et al. 2003) . Sport fish such as rainbow trout, and brook trout are often top predators in aquatic systems and feed heavily on larval and emerging insects (Wellborn et al. 1996, Carlisle and Hawkins 1998) . Where trout are introduced, a proportion of the biomass that formerly would have emerged and supported terrestrial life is diverted to trout (Progar and Moldenke 2002). When we removed trout from four lakes, abundances of mayflies, caddisflies, and insect predators and overall insect biomass emerging from the lakes increased compard to lakes stocked with fish. Fish density was a more important predictor of aquatic insect emergence than habitat complexity. Correlative evidence suggests indirect negative effects of trout on bird abundance and activity of large-bodied bats.
"Hyperpredation" refers to the indirect interactions between non-indigenous and native prey via a shared predator (Smith and Quin, 1996; Courchamp et al., 1999) and occurs when a non-indigenous prey species indirectly facilitates the decline of a native prey species by enabling a shared predator to increase in abundance (Smith and Quin, 1996). The shared predator often moves into the habitat of the indigenous prey by following the expansion of the non-indigenous prey (Courchamp et al., 2000). We hyphothisized that introduced trout are facilitating the expantion of the aquatic garter snake (Thamnophis atratus) into these high lake environments (a species normally found at lower elevations).
We evaluated this hypothesis by comparing the diet, distribution, and density of aquatic garter snake with the common garter snake (T. sirtalis), which naturally occurs at high lakes. In terms of distribution, aquatic garter snakes were found in association with trout but were not associated with amphibian presence. In contrast, common garter snake were found more often where amphibians were present but were not associated with trout. In terms of diet, aquatic garter snakes ate fish and amphibians at nearly equal rates, where common garter snakes only ate amphibians. Additionally, We found fewer Cascades frogs in trout-containing sub-basins where we also found aquatic garter snakes compared, to trout-containing sub-basins where we did not find aquatic garter snakes. Multiple regression results showed a significant negative relationship between relative abundance of Cascades frog and presence of aquatic garter snakes (more).
Given the worldwide practice of stocking fish into aquatic habitats, it is important to more fully understand the consequences of the practice on food web structure and ecosystem functioning. Documented food-web consequences include increased top-down effects, simplified food web structure, and changes in habitat coupling (reviewed by Eby et al., 2006). Our evidence of an indirect bottom-up food-web effect of trout introductions via hyperpredation suggests that there are likely more unforeseen consequences to be studied and addressed so that land managers can incorporate the range of ecosystem-level effects into conservation goals and decisions.
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Pope, K. L. In Press. Assessing changes in amphibian population dynamics following experimental manipulations of introduced fish. Conservation Biology.
Pope, K. L., J. Piovia-Scott, and S. P. Lawler. In prep. Changes in Aquatic Insect Emergence in Response to Whole-lake Experimental Manipulations of Introduced Trout.
This research was conducted by Karen Pope for her Ph D at UC Davis.