Interspecific Competition and Qualitative Competitive Asymmetry between Two Benthic Stream Fish

Abstract

The mottled sculpin, Cottus bairdi, and the fantail darter, Etheostoma flallellare, are benthic fish common in small streams over much of eastern North America. Both juveniles and adults of these two species are closely associated in low elevation headwater streams in western Virginia. I explored the effects of competition between juveniles of the two species, and the effects of heterospecific adults on juvenile performance, in a controlled, replicated experiment in artificial streams. Effects of competition were determined for three measures of juvenile performance that can be potentially related to reproductive output, and hence, to population dynamics: survival, growth and relative condition. Juveniles of C. bairdi and E. flabellare competed at densities well within the range observed in their natal streams. Adult E. flabellare had a positive effect on the survival of juvenile C. bairdi, the origin of which is unknown. Adult C. bairdi had no effect on juvenile E. flabellare. The most interesting result was that competition between juveniles of the two species was characterized by a qualitative asymmetry; C. bairdi responded with a reduction in growth, both in total length and mass, while E. flabellare responded with a decrease in relative condition, but no decrease in growth. These differences in response were clearly reflected in mass/length regressions. Unless responding variables can be precisely translated into quantitative effects on population dynamics, such qualitative asymmetries in competitive interactions greatly complicate the determination of competitive symmetry and the possible prediction of competitive outcomes, especially in species where r is difficult to determine directly.