Asymmetric competition, habitat selection, and niche overlap in juvenile salmonids

The relationship among asymmetric competition, habitat selection, and niche overlap is poorly understood. I conducted a response surface design experiment in artificial stream, channels using juvenile coho salmon (Oncorhynchus kisutch) and steelhead trout (O. mykiss). to explore the density- and scale-dependent relationship among asymmetric competition, habitat selection, and niche overlap. In these sympatric populations, juvenile coho are larger, occupy mutually preferred deep, low-Velocity pools, and displace steelhead into shallow, high-velocity riffles. I measured habitat selection over a wide range of species' densities under two competitive scenarios: coho larger than steelhead (natural size asymmetry), and species size matched (natural size asymmetry removed). When coho had their natural size advantage, interspecific competition was. strongly asymmetric, and habitat selection by both species depended primarily. on coho density. Removing the size advantage of coho reduced by half the per capita effect of coho on steelhead habitat selection, which was then affected similarly by intra- and interspecific competition. However, habitat selection by coho still depended more on intra- than interspecific competition. The effect of asymmetric. competition on niche overlap was density- and scale-dependent. Asymmetric competition reduced niche overlap nonsignificantly across all densities at the habitat type scale (pools, riffles). At the microhabitat scale (velocity, distance, depth), asymmetric competition reduced niche overlap at low, but not high, densities. Because I manipulated the competitive asymmetry. in a single sympatric species pair, this experiment alters a competitive interaction while controlling for evolutionary history and thus provides a direct test of the effect of asymmetric competition on habitat selection and niche overlap. Combining a response surface design with different measures of niche overlap reveals how this effect can be both density- and scale-dependent.