| Summary: | Vole and mouse population densities in the Kluane boreal forest (Yukon) vary noncyclically;
densities are usually low but unpredictable and ephemeral high densities
occasionally occur. Anecdotal observations suggest that vole and mouse densities could be
correlated to summer rainfall amounts. Small mammal populations in Kluane are suspected to
be food-limited, and food production is suspected to be rainfall-limited, since the Kluane
boreal forest experiences a water-deficit during the summer.
I tested the hypothesis that rainfall acts through a trophic chain to influence vole
numbers, and that vole numbers should increase with rainfall two-to-three fold as they do with
addition of sunflower seed. To simulate increased rainfall, I installed irrigation systems and
operated them for two summers on three areas of approximately 1.5 hectares of boreal forest
habitat. I monitored small mammals, mushrooms, understory vegetation, spruce trees and
forest-floor invertebrates. Three unirrigated areas of equal size were used as control grids:
treatment and control grids were paired within three different sites.
Mushrooms and one species of understory plant (Arctostaphylos uva-ursi) responded
significantly to the treatment. There was no clear treatment effect on spruce trees,
invertebrates, and shrews. Voles were generally more numerous on the treatment grids than
on the controls, but the difference was of the expected three-fold magnitude at only one site.
Overall, only one out of three sites supported the hypothesis that vole densities can be affected
by rainfall through a trophic chain.
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