Nonnative trout impact an alpine-nesting bird by altering aquatic-insect subsidies

Area of Science:

• Ecology
• Invasive Species Biology
• Conservation Biology

Background:

• Adjacent ecosystems are linked by resource subsidies, where more productive donor systems support consumers in less productive recipient systems.
• Introduced species can have direct impacts, but their indirect effects on cross-boundary subsidies are less understood.
• Gray-crowned Rosy-Finches (Leucosticte tephrocotis dawsoni) forage on aquatic insects emerging from Sierra Nevada lakes.

Purpose of the Study:

• To investigate the indirect effects of introduced trout on aquatic insect subsidies.
• To determine how introduced trout alter the foraging behavior of Gray-crowned Rosy-Finches in response to these subsidies.
• To assess the conservation implications of nonnative fish introductions on recipient species.

Main Methods:

• Compared aquatic insect abundance and Rosy-Finch foraging at fish-containing versus fishless alpine lakes in California.
• Quantified mayfly emergence as a key aquatic insect subsidy.
• Observed Rosy-Finch aggregative responses to insect availability.

Main Results:

• Fish-containing lakes had 98% fewer mayflies than fishless lakes, indicating trout outcompeted Rosy-Finches for this resource.
• Rosy-Finches showed a strong aggregative response, with 5.9 times more birds at fishless lakes.
• Introduced trout reduced both the magnitude of the insect subsidy and the strength of the trophic interaction.

Conclusions:

• Introduction of nonnative trout significantly diminishes aquatic insect subsidies vital for terrestrial consumers like Rosy-Finches.
• Reduced subsidies, particularly during the breeding season, may negatively impact Rosy-Finch populations.
• Conservation and restoration plans for alpine lake ecosystems should incorporate the recipient terrestrial communities dependent on aquatic subsidies.