Study Notes on Trout and Amphibian Interactions via Hyperpredation

Evidence of Indirect Impacts of Introduced Trout on Native Amphibians via Facilitation of a Shared Predator

Authors and Affiliations

  • Karen L. Pope: USDA Forest Service, Pacific Southwest Research Station, Redwood Sciences Laboratory, Arcata, CA, USA
  • Justin M. Garwood: UC Davis, Davis, CA, USA
  • Hartwell H. Welsh Jr.: USDA Forest Service, Pacific Southwest Research Station, Redwood Sciences Laboratory, Arcata, CA, USA
  • Sharon P. Lawler: UC Davis, Davis, CA, USA

Article Information

  • Date Received: 7 June 2007
  • Date Revised: 7 March 2008
  • Date Accepted: 11 March 2008
  • Available Online: 16 May 2008
Keywords
  • Hyperpredation
  • Indirect effects
  • Amphibian declines
  • Predator-prey interaction
  • Non-native
  • Invasive species

Abstract

Hyperpredation is a phenomenon where non-native prey species support the existence of invasive predators, which subsequently suppress native prey populations. While direct impacts of introduced fish on amphibian populations are well-documented, the role of fish facilitating shared predators has yet to be explored. This paper presents evidence showing that introduced trout have indirect effects on native amphibians via predation by snakes, specifically the Pacific coast aquatic garter snake (Thamnophis atratus).

Key Findings

  1. Diet and Distribution of Snakes:

    • Analysis shows that T. atratus consumed both trout and amphibians, whereas the common garter snake (Thamnophis sirtalis) preyed only on amphibians.
    • The distribution and density of T. atratus correlated strongly with the presence of introduced trout, rather than native amphibians.
    • High populations of T. atratus were observed even in the absence of native amphibians, suggesting that they can thrive due to the alternate prey provided by trout.
    • The presence of T. atratus in lake basins was associated with a reduction in native frog populations, specifically the Cascades frog (Rana cascadae).

  2. Hyperpredation Hypothesis:

    • The study expands on traditional hyperpredation definitions by demonstrating interactions where two prey species (trout and amphibians) have competitive influences on each other, suggesting a complex food-web interaction.
    • There is concern regarding how fish stocking practices influence ecosystem structure and functions, emphasizing the need for comprehensive management strategies.

1. Introduction

Predators can dramatically influence the abundance of their prey, producing effects that can range from negligible to severe. Strong top-down effects are commonly seen when alternative prey sources increase the density of generalist predators, leading to further declines in local prey species. This dynamic is termed apparent competition or indirect amensalism.

1.1 Hyperpredation

Hyperpredation, as defined by Smith and Quin (1996) and expanded by Courchamp et al. (1999), relates to indirect interactions between non-indigenous and native prey facilitated by shared predators, which enable non-indigenous prey to indirectly decline native species through predator population growth.

1.2 Research Focus

This study redirects the analysis of introduced fish's impacts, particularly focusing on how the introduction of trout serves as an alternate prey source facilitating the Pacific coast aquatic garter snake's expansion into higher elevations, affecting native amphibian populations negatively.

2. Materials and Methods

Three datasets were collected from the Klamath Mountains from 1999-2006, focusing on three study types:

  • Landscape Survey: Documenting the distribution and relative abundances of introduced trout, amphibians, and garter snakes across lentic water bodies.
  • Basin Study: Assessing garter snake diets and densities in various lake basins, including natural fishless environments and those with trout present or removed.
  • Case Study: A detailed examination of garter snake and frog distributions in a specific sub-watershed.
2.1 Landscape Survey
  • This survey involved sampling 728 water bodies, investigating trout presence via timed gill nets, and using visual encounter surveys (VES) to document amphibian and snake populations.
  • Five amphibian species were frequently encountered, including R. cascadae and T. atratus.
2.2 Basin Study
  • Implemented between 2003-2006, focused on sampling T. atratus diet and density in 16 Trinity Alps lake basins across different trout stocking statuses. Mark-recapture methods aimed to provide robust density estimates.
2.3 Case Study
  • Conducted in Deep Creek, involving mark-recapture methods for both garter snakes and R. cascadae over four years, with techniques developed for ensuring accurate diet assessments through stomach content analysis.

3. Analysis

3.1 Diet
  • Diet analysis aimed to differentiate between T. atratus and T. sirtalis, with key findings revealing 33% of T. atratus stomachs contained R. cascadae, indicating direct predation pressures.
  • Cumulative prey curves provided evidence of sufficient sample sizes to validate the dietary composition conclusions.
3.2 Distribution
  • The occurrence probability of both garter snake species was studied via generalized additive models. Results showed T. atratus was significantly associated with trout presence while T. sirtalis correlated positively with native amphibian sightings.
3.3 Density
  • Estimation of snake densities used mark-recapture data, showing a notable correlation between T. atratus relative abundance and trout density, whereas T. sirtalis presence was linked to amphibian abundances.

4. Results

Results indicated clear dietary distinctions between snake species, distribution patterns associated with prey type, and density measures reinforcing the hypothesis of hyperpredation affecting native frog populations through garter snake predation facilitated by trout.

4.1 Diet
  • The dietary habits illustrated a significant reliance of T. atratus on both fish and amphibians, contrasting with T. sirtalis.
4.2 Distribution
  • T. atratus located predominantly in areas with trout while T. sirtalis preferred areas associated with amphibian presence.
4.3 Density
  • Density estimates reflected that T. atratus had a greater mean density compared to T. sirtalis underlined by the presence of trout.

5. Discussion

The study concludes with strong evidence supporting the hyperpredation phenomenon manifesting through the introduced trout influencing local food-web dynamics.

  • The introduction of salmonids provides valuable yet indirect predation pressure down the line, significantly affecting R. cascadae populations and necessitating a reevaluation of fish stocking policies in California's wilderness areas to better align with conservation efforts sensitive to amphibian populations.

6. Conclusions

The findings highlight crucial ecological relationships and inform conservation strategies, suggesting land managers consider indirect impacts of species introductions when making stocking decisions.