Population ecology of an invasion: demography, dispersal, and effects of nonnative brook trout on native cutthroat trout

Abstract

Invasions are population-level phenomena, but are seldom studied as such. To test for population-level mechanisms by which nonnative brook trout (Salvelinus foniinalis) replace native Colorado River cutthroat trout (Oncorhynchus clarki plenriticus), I conducted a four-year removal experiment and mark-recapture study in long segments of four Rocky Mountain headwater streams with sympatric trout populations. Two streams were at moderate (2500-2700 m) and two were at high (> 3150 m) elevation, and brook trout were removed annually from two streams (treatments), but not the other two (controls). At moderate elevation, age-0 and -1 cutthroat trout survived at rates 2-13 times higher on average where brook trout were removed, but age-0 cutthroat trout survived at rates nearly 10 times lower than age-0 brook trout where they were not removed. At high elevation, cutthroat trout recruitment failed despite brook trout removals apparently because of cold water temperatures, but adult cutthroat trout at all elevations survived at similar rates whether brook trout were removed or not (mean survival 44-47%). Brook trout reduced survival of juvenile cutthroat trout through biotic interactions, leading to population declines. Weirs operated in three streams indicated that cutthroat trout tended to emigrate downstream during summer, but removing brook trout did not change this behavior. In comparison, brook trout, primarily mature adults, immigrated upstream at high rates and could rapidly recolonize reaches where their populations had been removed. Immigration may help sustain invasions in harsh habitats where environmental conditions limit recruitment. Brook trout immigrated from a range of distances. Although 46-89% of brook trout recaptured in three streams moved < 50 m, 6-36% moved > 250 m, and some individuals moved up to 2 km within a season. Brook trout invasions appear to be driven by two simultaneous processes: a wave of local movement that exerts biotic pressure on the downstream limit of the cutthroat trout population, and jump dispersers moving upstream past the invasion front facilitating rapid population spread. Prevention or complete eradication are the best management options, but two within-year brook trout removals repeated in consecutive years may forestall extirpation of invaded cutthroat trout populations.