Replacement of a native salmonid by a nonnative salmonid in the western United States: Changes in trout production and consequences for stream-riparian food webs

Frequently nonnative species invade habitats occupied by a similar, even closely related, native species, but the consequences of these types of invasions are rarely studied. Throughout western North America nonnative brook trout Salvelinus fontinalis are assumed to be an "analog" of the native cutthroat trout Oncorhynchus clarkii they replace. My dissertation consists of four studies designed to test this assumption by comparing production of brook trout and cutthroat trout and their effects on stream-riparian food webs via a combination of comparative and experimental studies conducted in Idaho and Colorado. In a range-wide comparison, I observed that when in sympatry brook trout exhibited 3.1 times greater density and 2.5 times greater biomass than cutthroat trout, but no difference in production (chapter one). In allopatry, density and biomass was 1.5 times greater and production 1.9 times higher for brook trout than cutthroat trout. To account for environmental factors that may be driving the patterns observed in chapter one, I conducted a comparison of five paired streams with either brook trout or cutthroat trout in allopatry and I found brook trout exhibited 2.4 times greater density, 1.7 times greater biomass, and 2.5 times greater annual production than the cutthroat trout they replace (chapter two). In a comparative study of twenty paired streams split between the northern and southern Rocky Mountains, I observed emergence was 24% lower from streams with brook trout in allopatry and there was a positive relationship between riparian spiders and emergence insects. From these models, I predicted that brook trout replacement would result in a 6-20% reduction in spider counts (chapter three). Similarly, in a large-scale field experiment, brook trout reduced emerging insects by 58% (chapter four). I did not measure an overall effect of brook trout on benthic insects, periphyton, or riparian spiders. This research represents some of the first tests toward understanding the ecological consequences when brook trout replace cutthroat trout and highlights the need for more studies that examine the broader consequences of a nonnative species replacing a closely related native species.