Relative Effects of Biotic and Abiotic Processes: A Test of the Biotic–Abiotic Constraining Hypothesis as Applied to Cutthroat Trout

Abstract

The Biotic–Abiotic Constraining Hypothesis (BACH) suggests that biotic interactions can have an overriding negative influence on a fish species, even when abiotic conditions are suitable. The abundance of a fish species is predicted to be largely regulated by abiotic habitat characteristics when densities of predators or competitors are low. However, when predator or competitor densities are high, the abundance of the fish species is suppressed by biotic interactions regardless of environmental conditions. We used data from the Salt River basin (Idaho–Wyoming) to investigate whether the BACH could be used to explain patterns in the density of cutthroat trout Oncorhynchus clarkii across a watershed. Using a combination of principal components and nonlinear multiple-regression analyses, we determined that the BACH was useful for explaining patterns in cutthroat trout density. When brown trout Salmo trutta and brook trout Salvelinus fontinalis densities were low, cutthroat trout density was highly variable and was best explained by habitat characteristics. High cutthroat trout densities were found in high-gradient reaches with little fine substrate and a low proportion of deep-pool habitat (i.e., reaches with a diversity of pools, riffles, and runs). When brown trout and brook trout densities were high, cutthroat trout density was always low despite favorable habitat conditions. Brown trout density was highest in low-elevation reaches with warm water temperatures and abundant instream cover, whereas brook trout density was highest in high-elevation reaches with low water temperatures. This study suggests that the influence of brown trout and brook trout densities overrode the influence exerted by habitat conditions on cutthroat trout density in the Salt River basin and that the BACH is useful for explaining biotic and abiotic influences on lotic cutthroat trout populations.