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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; clonal fish; DNA fingerprinting; ecological genetics; generalist model; genetics; gynogenetic; landscape succession; niche relationships; morphology; oxygen stress; Phoxinus eos-neogaeus; unisexual., INTRODUCTION The literature concerning unisexual vertebrates has expanded rapidly over the last few decades in response to the growing number of discoveries of unisexual taxa (Dawley 1989) and their usefulness [...]

I combined long-term (10 yr) descriptive and short-term experimental studies in a headwater stream in northern Minnesota to assess: (1) the effect of annual variation in stream discharge and spatial proximity of beaver (Castor canadensis) ponds on lotic fish abundance and (2) the subsequent influence of discharge and fish predation on lotic invertebrate colonization. Considerable annual variation in fish density occurred in the stream over the 10-yr period, particularly in pool habitats. Increased fish density was associated with increased stream discharge and creation of beaver ponds downstream from the study site. Weir traps used to monitor directional (upstream vs. downstream) fish movement during the last 4 yr of the study indicated annual changes in fish density were associated with the amount of fish dispersal occurring along the stream segment. Downstream fish movement, out of an upstream beaver pond occurred primarily during periods of elevated stream discharge. Upstream movement, out of a downstream beaver pond, occurred over a broader range of discharge conditions. A controlled, 'split-stream,' experiment examining the effect of very low vs. elevated discharge on upstream fish movement indicated, however, that upstream movement of fish out of beaver ponds was also reduced by very low discharge conditions. Movement data for individual fish species revealed considerable variation among the taxa in the tendency for downstream vs. upstream movement, due to variation in the morphology of upstream vs. downstream beaver ponds and its subsequent effect on the composition of fish dispersing from these source areas. Most fish movement occurred over relatively brief time periods, suggesting life history and developmental processes were critical in influencing the timing of dispersal. Size structure of fishes captured in the stream indicated predominantly older age classes (>age I) of fish were dispersing along the stream. However, based on the occurrence of age 0 individuals only 1 of 12 species, the creek chub (Semotilus atromaculatus), routinely reproduced in the stream. Experiments conducted in an artificial stream located below one of the beaver ponds indicated discharge and fish predation have potentially strong and interactive effects on invertebrate colonization in stream ecosystems. Differences in colonization of riffles and pools under low vs. elevated discharge and fish vs. no-fish treatments suggested, however, that the interactive effect of these factors on invertebrate colonization was variable over even small spatial scales. Elevated discharge increased invertebrate colonization in riffles but decreased invertebrate colonization in pools. Contrary to intuitive expectations, fish predation reduced invertebrate colonization more under elevated than low discharge conditions, particularly in pool habitats. Taken together, these results suggest: (1) beaver ponds act as reproductive 'sources' for fish on the landscape, while adjacent stream environments act as potential reproductive 'sinks,' (2) large-scale spatial relationships between beaver ponds and streams, along with the influence of discharge on the permeability of the boundaries between these habitats, are critical in controlling fish dispersal between ponds and streams and the subsequent abundance and composition of fish in lotic ecosystems, and (3) fish predation and discharge have potentially cascading effects on invertebrate colonization in lotic ecosystems. Key words: beaver ponds; boundary processes; Castor canadensis; colonization; fish assemblages; fish dispersal; invertebrates; landscape ecology; lotic ecosystems; Minnesota; predator-prey interactions; stream discharge; trophic-level interactions., INTRODUCTION Many aquatic vertebrates exhibit complex life cycles, with various life history stages using widely separated habitats (Sale 1978, 1979, Wilbur 1980, Werner and Gilliam 1984). Future empirical and theoretical [...]

More than 40 years ago, conservationist Aldo Leopold articulated the need for a 'land ethic,' or self-imposed limitation on the degree to which a person will modify the landscape, to [...]

The genetic composition, habitat use and morphological variation of a Phoxinus eos-neogaeus unisexual hybrid complex and its sexually reproducing progenitor species inhabiting beaver-modified drainages in Minnesota was studied. A single genotype may actually respond to environmental variation with as much phenotypic variation as a genetically variable sexual population.

ISAAC J. SCHLOSSER [1,3] LARRY W. KALLEMEYN [2] Abstract. Beavers are increasingly viewed as 'ecological engineers,' having broad effects on physical, chemical, and biological attributes of north-temperate landscapes. We examine [...]

ISAAC J. SCHLOSSER [1,3] JAMES D. JOHNSON [1,4] W. LADD KNOTEK [1,5] MARGARET LAPINSKA [2] Abstract. We examine the influence of biological interactions and interannual variation in climate on the [...]

We used descriptive and experimental approaches on Gould Creek, a first-order tributary of the Mississippi River near Lake Itasca, Minnesota to assess the influence of (1) flow regime on the colonization dynamics and abundance of invertebrates and cyprinids, and (2) cyprinid predation on invertebrates and fishes. Stream flow varied annually during the 3-yr(1984-1986) period. One dry year(1984), with few periods of elevated flow, was followed by two wet years (1985-1986), with prolonged elevated (nonscouring) discharge. The density of benthic and drifting invertebrates increased dramatically with elevated flow. Benthic riffle invertebrates in particular increased, from a maximum of 9000 individuals/[m.sup.2] in 1984 to 91000 individuals/[m.sup.2] in 1985 and 51000 individuals/[m.sup.2] in 1986. Larval Hydropsychidae and Simuliidae were the primary groups increasing in abundance during elevated flow. To assess directly the influence of flow regime on benthic insect densities, flow was manipulated in six subsections of a riffle and colonization of natural rock substrates monitored. Total insect abundance was higher under elevated (nonscouring) vs. low flow within 6-8 d; after 24 d insect abundance was three times as high under elevated flow. Insect families responded differently to elevated flow, with the largest increase resulting from a pulse of colonization by larval Hydrospsychidae in 6-8 d. Cyprinid density in Gould Creek also increased with elevated flow from 0.1-0.3 cyprinids/[m.sup.2] in 1984 to 1-2 cyprinids/[m.sup.2] in 1985 and 1986. The increased fish density was most pronounced during spawning periods in spring (May-June) and consisted primarily of older individuals. All cyprinids, except the creek chub (Semotilus atromaculatus), decreased in abundance 4-5 wk after spring colonization, regardless of flow conditions and invertebrate abundance. All cyprinids in Gould Creek selected pool habitats. Experiments in an artificial stream on Gould Creek indicated cyprinids influenced invertebrate abundance, but the effect of cyprinid predation was variable among habitats. Invertebrate abundance decreased most in structurally complex pools but exhibited little response to cyprinid predation in shallow riffle and raceway habitats. Because predation intensity varied among habitats, pool-dwelling invertebrates such as Chrinomidae and Crustacea decreased more in the presence of cyprinid predation than riffle-dwelling Hydropsychidae and Simuliidae. However, if Simuliidae occurred in pool habitats, they were strongly selected by cyprinids, resulting in a significant depression in prey size in pools. Experiments in the artificial stream indicated creek chubs preyed on adult cyprinids, but larger species (adults 70-80 mm) were less susceptible to predation than smaller species (adults 50-60 mm). However, even taxa with small adult size were preyed on at a low rate, and all cyprinids strongly selected pools, with creek chubs having minimal effect on habitat use. These results suggest that (1) the hydrologic regime has broad and pronounced effects on the colonization dynamics and abundance of invertebrates and fishes in headwater streams, and (2) cyprinid predation was weaker but variable effects on the abundance of stream organisms. Predation intensity varies (a) over short temporal scales, because of the dynamic nature of flow regime and the rapid colonizing ability but short post-spawning persistence of cyprinids, (b) over small spatial scales, because of increased abundance of cyprinids in pool vs. riffle habitats, (c) between invertebrate and vertebrate trophic levels because the creek chub is a relatively ineffective piscivore, and (d) between small and large fish because many minnows have a size refuge from creek chubs.

We sampled riffle and pool habitats of small streams in Minnesota, Illinois, and Panama to examine variation in species-area relationships within and between the respective fish faunas. For six of the seven steams studied, habitat volume was a better predictor of species richness than was habitat area, and number of individuals was a better predictor of species richness than habitat volume. Slopes of species-volume relationships were similar among regions, but the number of species per unit volume was greater in Panama. Multiple regression analyses indicated that knowledge of habitat complexity and volume did not enhance appreciably the capability of linear models to predict species richness from number of individuals in the sample. These results support the hypothesis that species-area relationships may often be epiphenomena stemming from the more comprehensive community 'samples' intercepted by larger habitat patches. Although number of individuals was the best single predictor of species richness, habitat structure and type clearly influenced species' distributions in some streams, thereby indicating that species-area relationships were not strictly sampling phenomena. An index of habitat complexity based on depth, current, and bottom type was correlated with species richness in two Panama streams. Also, the abundance of individual species was more likely to be correlated with habitat volume in Panama than in Illinois or Minnesota, and species relative abundances were more similar between years in Panama than in Illinois or Minnesota streams, especially in pools. These patterns suggest that in streams subject to strong seasonal and annual environmental variation, habitat features are poorer predictors of fish distribution and abundance than in streams subject to less environmental variability. We speculate that annual variability in reproductive success and harsh winters interact to maintain imbalance between the fish assemblages and their habitat in Minnesota. Weak relationships between species richness and habitat volume or complexity may be indicative of population variability and the predominance of extinction/recolonization processes in community organization.