Competitive Asymmetry and Coexistence in Size-Structured Populations of Brook Trout and Spring Salamanders

Abstract

The question of coexistence of competing species has generated much of the basic theory of community ecology. The question becomes more complex when competition occurs between size-structured populations of predators in which the nature of the interaction extends beyond exploitative competition. I conducted a field experiment examining the interactions between fingerling brook trout, Salvelinus fontinalis (total length 78-101 mm), and larval spring salamanders, Gyrinophilus porphyriticus (snoutvent length 35.7-62.4 mm), in an array of replicate experimental streams. Brook trout affected both the growth and survival of larval spring salamanders, reducing survival by 50 % and growth by > 90 % for mass and 44 % for SVL. Brook trout also altered the habitat use of larval G. porphyriticus. G. porphyriticus had no effect on any of the responses of fingerling S. fontinalis. These results parallel those found in an earlier experiment on interactions between small adult brook trout and larval spring salamanders. Interactions between size-structured populations of G. porphyriticus and S. fontinalis are characterized by strong asymmetry in favor of S. fontinalis across a wide range of relative body sizes. This calls into question the mechanisms allowing continued coexistence of G. porphyriticus and S. fontinalis, and in general, the persistence of stream salamanders with predatory fish. Persistence is likely a complex function of the interactions between the life history and local demography of both G. porphyriticus and S. fontinalis, coupled with characteristics of the local environment, rather than the result of classic mechanisms of species coexistence such as niche divergence or alpha selection. Such complex mechanisms of species coexistence are likely more common in situations involving competition between size-structured populations than single factor explanations based on models of exploitative competition between competitors of fixed body sizes and life stages