Niche relationships of clonal and sexual fish in a heterogenous landscape

We examined spatial, physiological, and morphological niche relationships in a Phoxinus eos-neogaeus gynogenetic complex of cyprinid fish in five drainages influenced by beaver (Castor canadensis) activity along the 294-[km.sup.2] Kabetogama Peninsula in Voyageurs National Park of northern Minnesota. Assessment of clonal variation using "in gel" DNA fingerprinting of gynogens from three drainages indicated that all drainages contained only a single clone. When environments associated with beaver pond succession were pooled across drainages, P. eos was more abundant in active beaver ponds, while gynogens were at higher frequencies in collapsed pond and stream environments. Gynogen frequency for different successional environments classified according to drainage indicated strong variation among drainages, especially in pond environments, but a consistent pattern of increased gynogen frequency in collapsed ponds or streams. Intensive sampling within one drainage across successional beaver pond sites, physical habitats, and gradients of oxygen concentration indicated that P. eos, P. neogaeus, and gynogens were all most abundant in the highly oxygenated littoral environment of an upland pond. P. eos abundance and its relative frequency in the gynogenetic complex decreased, while gynogen frequency increased, in a poorly oxygenated collapsed beaver pond and beaver meadow. Fish transplant experiments revealed that gynogens survived longer and had greater physiological tolerance to oxygen stress than sexual progenitors, especially P. eos. Principal component analysis of 11 growth-related morphometric characteristics revealed morphological intermediacy of the hybrid gynogens between the sexual progenitors, with factors related to trophic morphology, especially mouth width and mouth length, segregating the different forms. A comparison of morphological relationships in a beaver pond with a gynogen frequency of 25-35% to those of a pond with a gynogen frequency of only 6-7% suggested that there was a release in the trophic morphology of P. eos in response to reduced gynogen abundance. The temporal and spatial unpredictability of environmental conditions in the north temperate landscape inhabited by these fish has apparently resulted in selection for a physiologically flexible, "general purpose" clonal genotype, which occupies a broad ecological niche and may make establishment of additional clones in the same ecosystem difficult. Key words: beaver; Castor canadensis;