Showing papers in "American Midland Naturalist in 1985"

Resource Partitioning by Stream-dwelling Crayfish: The Influence of Body Size

Abstract: The crayfishes Orconectes luteus and 0. punctimanus are sympatric in over 200 km of the Jacks Fork and Current rivers in S-central Missouri. The combinations of substrate particle size, current velocity, depth and macrophyte cover used by youngof-the-year and adults of each species tended to be distinct. Orconectes punctimanus was more restricted in its use of habitats. Young-of-the-year were strongly associated with macrophyte cover and shallow water, and adults with larger substrate particles; both life stages preferred slow current velocities. Orconectes luteus was more evenly distributed over most of the habitats. Young-of-the-year used current velocities and substrate particle sizes somewhat more in proportion to their availability than did adults, which were concentrated within the higher ranges of both of these variables. Laboratory choice experiments involving combinations of species and size classes indicated that crayfish dominance was a function of body size and that habitat use by one size class could be modified when larger individuals of the same or a different species were present. The young of 0. punctimanus hatched earlier and grew faster than did those of 0. luteus. Orconectes punctimanus therefore maintained a distinct size advantage throughout its life cycle. Orconectes punctimanus probably inhabited its preferred set of microhabitat conditions and excluded 0. luteus from these areas. Because much of the remaining stream habitat was unsuitable for 0. punctimanus, competitive exclusion did not occur. Other factors, most notably fish predation, probably complement physical factors and species interactions in determining the distributional patterns of these crayfishes. INTRODUCTION The crayfishes of any particular aquatic habitat generally are limited to one or very few species, even if many species occur in the geographical region (e.g., Williams, 1954). Closely related species commonly exhibit disjunct distributions (see Bovbjerg, 1970, for a review) and an introduced species may rapidly replace one that occurred naturally (Capelli, 1982). The mechanisms involved in displacement or in maintaining species separation are not well understood, but crayfish behavior often gives insight into probable causes. It was well documented by Bovbjerg (1953, 1956), Penn and Fitzpatrick (1963) and Capelli (1982) that crayfish are aggressive, and Capelli and Munjal (1982) recorded interspecific encounters leading to attack in laboratory experiments. Intraspecific attacks are probably common either during the molting period, or by adults on juveniles after production of a maternal pheromone ceases (Little, 1976). Crayfish are mobile, and migrate (Momot, 1966; Flint, 1977), and disperse from areas of high density to areas of low density (Bovbjerg, 1964). Such behaviors have led several investigators to conclude that disjunct crayfish distributions were due to competitive exclusion (Aiken, 1965; Fitzpatrick, 1967; Crocker and Barr, 1968; Capelli and Magnuson, 1983). There are also situations where closely related species coexist in what appears to be a common habitat. Given their aggressive tendencies, different species existing in proximity is more surprising than is the exclusion of one species by another. The question why some crayfish are sympatric and others are not was the impetus for this study. In the Current and Jacks Fork rivers in S-central Missouri, Orconectes luteus (Creaser) and 0. punctimanus (Creaser) coexist in over 200 km of stream. The objectives 1 The Unit is sponsored by the U.S. Fish and Wildlife Service, the Missouri Department of Conservation and the University of Missouri.

Pollinator Foraging, Experimental Nectar-robbing and Plant Fitness in Impatiens capensis

Abstract: Bombus vagans workers were observed collecting nectar from two populations of Impatiens capensis. Approximately one half of the flowers in one population had their standing crops of nectar removed in a manner simulating nectar-robbing by bumblebees. The other population served as a control. Bombus vagans employed arearestricted searching behavior in both populations. The increased frequency of longdistance bee flights associated with the abundant low-quality reward blossoms in the nectar-robbed patch served to increase neighborhood size relative to the control area. Pollen was transported greater distances because of nectar thievery, thus making it more likely that the main purpose (i. e., genetic variability via outbreeding) of the chasmogamous flowers of I. capensis was achieved. Because seed set was statistically indistinguishable for robbed and unrobbed blossoms, nectar-robbing may thus actually enhance female fitness in L capensis.

Plant succession following the Mount St. Helens volcanic eruption : facilitation by a burrowing rodent, Thomomys talpoides.

Abstract: The 18 May 1980 eruption of Mount St. Helens in the Cascade Range (USA) initiated plant succession over a broad region. Northern pocket gophers Thomomys talpoides survived in widespread subalpine and montane sites where up to 25 cm of tephra had buried dormant or only recently snow-free vegetation. Burrowing and consequent construction of mounds by this herbivorous rodent has both modified the physical structure of the tephra and led to the placement of pre-eruption soil material on the tephra surface. We compared the composition of the plant community and the pattern of seedling establishment on and off gopher mounds. More plant species and individuals survived burial on mounds than at off-mound sites. Seedlings are also most diverse on mounds. We conclude that pocket gophers, through their soil-disturbing activities, can strongly affect plant population dynamics in volcanically disturbed areas, and thus may be an important agent in determining the pathway of succession. Limited observations suggest that differential herbivory may further affect plant community composition. Possible mechanisms that enhance plant survivorship and seedling establishment on mounds are discussed.

An Evaluation of Host Fish Suitability for Glochidia of Villosa vanuxemi and V. nebulosa (Pelecypoda: Unionidae)

Abstract: Fish species congeneric with previously identified hosts, as well as exotic fishes (Xiphophorus variatus and Tilapia aurea) and the mosquitofish (Gambusia affinis), were exposed to glochidia of Villosa vanuxemi or V. nebulosa to determine whether phylogenetically similar fishes can serve as hosts. Glochidia of V. vanuxemi metamor- phosed on black (Cottus baileyi), mottled (C. bairdi) and slimy (C. cognatus) sculpins, and glochidia of V. nebulosa metamorphosed on spotted (Micropterus punctulatus), largemouth (M. salmoides) and Suwannee (M. notius) basses and the mosquitofish. Exotic fishes were unsuitable hosts. A review of previous in vivo and in vitro studies suggests that chemical components of the blood serum in fishes, as yet unidentified, dictate host suitability to specific glochidia.

Distribution of Midwestern Fishes on Multivariate Environmental Gradients, with Emphasis on Notropis lutrensis

Abstract: The abundance of Notropis lutrensis, and presence-absence of eight midwestern fishes are quantified on environmental gradients identified by principal components analysis of conditions at 101 field localities in eight states at which collections were made in one 35-day span of 1978. Although all species included in the analysis have large geographic ranges, they differ in their patterns of distribution, ranging from "rare-specialists" to "common-generalists" in this analysis, and in distribution across the suites of environmental conditions which are considered to define multivariate environmental space. The environments of the species identified by principal components analysis agreed with and supported the relatively subjective autecological descriptions of species' habitats found in standard reference works, providing mutual support for both approaches to evaluating ecology of fishes.

Growth of Native Plant Species on Recent Volcanic Substrates from Mount St. Helens

Abstract: Subalpine vegetation on Mount St. Helens is recovering from the volcanic eruptions of May 1980. The recovery rate depends on the nature and intensity of destruction suffered by the vegetation at a particular site. New volcanic substrates were collected annually from 1980 through 1983 and bioassays were used to analyze their ability to support plant growth. The importance of soil structure and fertility was also investigated. Lack of soil nutrients appears to be a major factor limiting recovery. Soil weathering, the influx of airborne nutrients and the burrowing of small mammals enhance fertility. Some species now grow as well on these substrates as they do on an infertile loam. Despite the ability of some species to grow on these substrates, many areas of the mountain were devoid of vegetation as of October 1984. Immigration, a function of isolation, rather than soil development, may be the key to revegetation of totally devastated habitats.

Consumption of Leaf Litter by Lumbricus terrestris (Oligochaeta) on a Michigan Woodland Floodplain

Abstract: The consumption rate of leaf litter by Lumbricus terrestris (Oligochaeta) was studied from April through May 1981, on a Michigan woodland floodplain. Feeding chambers containing earthworms and leaf litter, which excluded soil microfauna, were placed on the floodplain. After 4 weeks, earthworms consumed leaves equivalent in amount to 93.87o of the total annual leaf fall. The mean consumption rate was 11.0 mg dry weight leaves g-1 live weight earthworms d-1 for a 4-week period with a maximum rate of 14.3 mg. The consumption rate of leaf litter by L. terrestris reported herein was similar to rates reported in other studies. INTRODUCTION Few studies have been conducted on the role of soil macroinvertebrates in leaf litter breakdown in wetland areas, particularly floodplains. It has been hypothesized that a woodland floodplain may serve as a temporary storage area for detritus such as leaf litter before it enters the stream or river (Merritt and Lawson, 1979; Cummins, 1980). In a Michigan woodland floodplain, Merritt and Lawson (1979) found that several groups of macroinvertebrates were involved in leaf litter breakdown (e.g., Gastropoda, Isopoda, Insecta, Diplopoda); however, earthworms appeared to be the most important group. The importance of earthworms in leaf litter degradation is welldocumented for several types of forest and agricultural lands (Kurcheva, 1960; Raw, 1962; Edwards and Heath, 1963; Van Rhee, 1963; Satchell, 1967; Edwards and Lofty, 1972; Lofty, 1974); however, less is known about their importance in floodplain soils. The objective of this study was to determine the rate at which Lumbricus terrestris (Linn.) consumed leaf litter in a Michigan woodland floodplain. MATERIALS AND METHODS Research was conducted on a point bar of the Augusta Creek floodplain located in the W. K. Kellogg Research Forest (Kalamazoo Co.), southwestern Michigan. At this location, Augusta Creek is a third-order channel (ca. 10 m width, 0.7 m depth). The floodplain is heavily forested and contains numerous species. Some of the major tree and shrub species are black ash (Fraxinus nigra Marsh.), green ash (F. pennsylvania Marsh.), basswood (Tilia americana L.), dogwood (Cornus spp.) and black cherry (Prunus serotina Ehrh.). The dominant lowland herbaceous vegetation consists of jewelweed (Impatiens capensis Meerb.), wood nettle (Laporta canadensis (L.) Wedd.), running strawberry (Euonymus obovatus Nutt.), skunk cabbage (Symplocarpus foetidus (L.) Nutt.) and iris sp. The soil of the point bar is poorly drained Adrian muck (Terric Medisaprist) which has high soil moisture and organic content most of the year. Green ash leaves were collected from the ground in early spring, air-dried and weighed in 10-g packs. A 3 m2 experimental plot, dominated by skunk cabbage, was chosen for its homogenous composition. In each of six rows, six holes were dug (27 cm apart, 18 cm deep) in the plot. The excavated soil was hand-sorted to remove large debris and steam heated to 55 C for 30 min to eliminate soil fauna. Thirty-six perforated plastic feeding chambers (22 cm high x 20 cm diam), each lined with 50 ,tm

Sugar Maple-Environment Relationships in the River Hills and Central Ozark Mountains of Missouri

Abstract: Sugar maple is becoming more important in oak-hickory forests near the relatively xeric western margin of the eastern deciduous forest. To identify site characteristics conducive to sugar maple in these forests, vegetation, environmental and disturbance history data were obtained for 115 oak-hickory stands in the Missouri river hills and central Ozark Mountains. Detrended Correspondence Analysis ordination of the data revealed clear vegetational gradients in importance of sugar maple and oak species. Sugar maple was most important on soils with high pH and water-holding capacity, whereas white and black oak and flowering dogwood were more common on drier sites with more acidic soils. Topographic exposure, fire and grazing were negatively correlated with sugar maple importance and positively correlated with importance of several oak species. Sapling data suggested that oak species are regenerating successfully only on exposed dry sites despite the ubiquitous presence of canopy oaks capable of producing large seed crops. Sugar maple reproduction, on the other hand, is abundant in most areas where a source of seed is present.

Patterns of territoriality and mating success in the shite-tailed skimmer Plathemes lydia (Odonata: Anisoptera).

Abstract: We studied the behavioral ecology of Plathemis lydia Drury (Odonata: Anisoptera) in central coastal California during summer 1983. Oviposition peaked in the early afternoon, was fairly synchronous, and varied spatially, with females avoiding a sandbar and preferring the deepest section of the pond. Only 79% of males known to be alive visited the pond on any particular day. Those that did come were territorial, nonterritorial "poachers," or often both within the same day. Territories were always defended individually and individual males were territorial for only a few hours in any one day, even though mating success was considerably higher for territory holders. Daily mating success of males was estimated based on the time, place and duration of territorial behavior. Estimated daily mating success correlated significantly with wing condition and body length. The components of mating success we measured all correlated with one another; thus, we found no evidence that tradeoffs occurred either among components of daily mating success or between estimated daily mating success and survivorship. INTRODUCTION Territoriality in odonates has been known for nearly a century (see references in Jacobs, 1955) and has now been documented in a variety of both dragonflies (subfamily Anisoptera) and damselflies (Zygoptera) (e.g., Waage, 1979a; Parr, 1980; Sherman 1983a). Here we describe patterns of territoriality and mating in the white-tailed skimmer Plathemis lydia Drury. This species has previously been studied by Jacobs (1955) in Indiana and Campanella and Wolf (1974) in New York. Our goals are to describe (1) patterns of male territoriality; (2) mating patterns of males and females, including correlates of mating success in males, and (3) contrasts between our population and previously studied populations of Plathemis lydia. Plathemis lydia is a medium-sized, sexually dimorphic dragonfly occupying pond habitats in the United States and southern Canada (Needham and Westfall, 1955; Campanella and Wolf, 1974). Each day during the breeding season males go to ponds, set up territories, and compete for access to females which visit ponds to copulate and oviposit. An arriving female is usually pursued by one or more males attempting to seize her and copulate. Following successful copulations a female oviposits by hovering just above the surface while dipping her abdomen into the water to wash off eggs, which are laid at the rate of about 23 eggs per second (McVey, 1984). Meanwhile, the male mate hovers approximately 0.5 m above the female guarding heragainst intruding males who themselves try to copulate with the female ("noncontact guarding," Parker, 'Present address: Department of Zoology, University of California, Berkeley 94720.

Invertebrate Colonization of Submerged Wood in a Cypress-Tupelo Swamp and Blackwater Stream

Abstract: We examined the effects of location within a swamp-stream ecosystem on colonization rate and community structure of the macroinvertebrate assemblages of submerged wood. Uniform-sized sections of freshly cut wood from water tupelo trees (Nyssa aquatica L.) were suspended below floating platforms at swamp and stream sites for periods of 1-8 weeks. The location of wood substratum affected community structure to a large degree and patterns of colonization to a slight extent. Logs in the swamp-tributary site held almost three times as many individuals and twice as many taxa as did logs in the swamp and outflow stream sites. Stream sites, however, were similar in proportions of various functional and taxonomic groups. Except for the abundance of true midges (Chironomidae), the densities of most major taxonomic groups were significantly affected by location within the swamp-stream ecosystem. Recruitment of individuals and species were extremely rapid and reached a rough steady state at most sites within 1 week. Filter-feeding taxa were numerically dominant early but soon were subordinate to gatherer and scraper functional feeding groups. This trend resulted primarily from the progressive increase in abundances of riffle beetles and mayflies. Current velocity, seston particle size, dispersal capacity and competition for space may be important factors affecting community structure and colonization patterns in this aquatic ecosystem.

Litter-fall patterns within a floodplain forest.

Abstract: Litter-fall patterns were examined for 3 years within a floodplain forest on the upper coastal plain of South Carolina. Average annual leaf fall and total litter fall ranged from 415-538 g-m-2.year-1 and 570-667 g-m-2-year-1, respectively, across the floodplain habitat. These values were similar to those of other alluvial habitats and exceeded the litter-fall totals reported for nonwetland, warm-temperate, deciduous forests. Leaf fall increased across the floodplain from stream bank to upland forest. Leaf fall varied from 73-84%o of total litter fall which exceeds the worldwide average of 70%o. Woody litter fall decreased across the floodplain whereas reproductive inputs remained similar. Nutrient return (Ca> N > K> Mg) in leaf fall was relatively high, particularly for the less mobile elements (Ca, Mg). Calcium and Mg inputs in leaf fall increased from bank to upland terrace. Leaf fall inputs of N and K were reduced on the sandy upland where soil leaching potential is greatest. Translocation may minimize nutrient return of N and K on the upland, whereas fluvial subsidies of these nutrients would compensate for potential losses within the alluvial floodplain habitat. Our results support the contention that leaf fall and nutrient return in litter are higher in forested wetlands receiving fluvial subsidies than in most nonwetland, deciduous forests. INTRODUCTION The available information on litter-fall inputs to terrestrial ecosystems indicates geographic patterns which are correlated with particular environmental factors (Bray and Gorham, 1964; Jordan, 1971; Jordan and Murphy, 1978; Meentemeyer et al., 1982). Litter production rates generally increase latitudinally from Arctic to tropics in conjunction with total light availability during the growing season (Jordan and Murphy, 1978) and total actual evapotranspiration (Meentemeyer et al., 1982). Leaf fall accounts for approximately 70% of total litter fall in different ecosystems worldwide (Meentemeyer et al., 1982). These overall patterns have usually been derived from a single year's estimates and therefore preclude considerations of annual variations in litter-fall rates and composition (Sykes and Bunce, 1970). Litter-fall patterns in wetland forests are usually not included in geographical determinations. However, recent evidence indicates that litter production in forested wetlands often exceeds corresponding values for nonwetland habitats (Bell et al., 1978; Brinson et al., 1980; Gomez and Day, 1982). Many wetlands receive fluvial inputs of water, oxygen and nutrients which can increase nutrient cycling and productivity (Brinson et al., 1980). Primary productivity and litter production are often higher in flowingwater than still-water wetlands which lack these fluvial inputs (Schlesinger, 1978; Brinson et al., 1980; Gomez and Day, 1982; but see Brown and Peterson, 1983). The extent and timing of flooding may thus affect wetland litter production and the import-export dynamics (Post and de la Cruz, 1977) of allochthonous litter sources within watershed units. The present study was conducted to determine litter-fall patterns in a floodplain forest surrounding a blackwater stream on the coastal plain of South Carolina. We were interested in assessing spatial and temporal changes in litter-fall components across the floodplain from stream bank to adjacent upland terrace forest. The study was continued for several years to determine the annual variations in litter-fall production at different points along the flood frequency gradient. METHODS The study was conducted in a floodplain along Lower Three Runs Creek (LTRC)

Seasonal changes in fecundity and size at first reproduction in an Indiana Population of the mosquitofish Gambusia affinis

Abstract: A natural population of the mosquitofish Gambusia affinis (Poeciliidae) was studied in Hovey Lake, Indiana. Winter mortality reduced the population to low levels, and population density increased over summer to high levels. Fecundity, adjusted for total length (TL) of the female, was high early in the breeding season, then declined to low levels as the population increased. The mean TL of gravid females declined during the same period, as a result of two factors: (1) large overwintering females died early in the breeding season; (2) young-of-the-year females began breeding at smaller TL later in the breeding season. The pattern of seasonal change in fecundity fits the predictions of r-K selection theory; the pattern of TL at first reproduction does not.

Life History Characteristics of Populations of the Long-toed Salamander (Ambystoma macrodactylum) from Different Altitudes

Abstract: Life history characteristics of seven populations of the long-toed salamander (Ambystoma macrodactylum columbianum) were examined for differences in relation to altitude from 1976-1978. Populations were located in northeastern Oregon and western Idaho, and varied in elevation from 360-2470 m. Populations below 2100 m bred in the spring, whereas animals from sites above 2100 m bred during midsummer. Egg size differed significantly among the three primary study sites. Females from low elevations had significantly more eggs than those from mid and high elevations. Mean size at hatching was significantly larger in the primary high-elevation populations. In spite of lower water temperatures, larval growth rate at high elevation sites was faster than at lower elevations. Larvae from populations below 2100 m metamorphosed during their 1st summer, whereas larvae from high elevation sites metamorphosed in August and September, presumably during their 3rd or 4th summer. Larvae from the high-elevation sites metamorphosed at 47 mm snout-vent length, whereas those from lower elevations metamorphosed at a significantly shorter length, 35-40 snout-vent length. Size at maturity for both sexes was ca. 50 mm snout-vent length regardless of elevation.

Post-Fire Succession in Californian Coastal Sage Scrub: the Role of Continual Basal Sprouting.

Abstract: Dominant shrub species of coastal sage scrub in coastal southern California are able to produce shoots from their base on a continual basis in the absence of fire or other major defoliation. As a result, each shrub becomes a population of mixed-aged branches (ramets) and extends its duration in the canopy beyond the age of any of its above-ground phytomass, reduces the incidence of senescence, and permits an individual (genet) to survive during long fire-free intervals. A computer simulation of succession in coastal sage scrub under differing fire intervals shows that continual basal sprouting may be significant in influencing the long-term composition of the vegetation.

Effects on the forest floor of whole-tree harvesting in an Appalachian oak forest.

Abstract: The size of the forest floor nutrient pool and degree of mineralization from this pool (as measured by nutrient concentrations in the soil solution) were compared between a clear-cut and an adjacent forested site. Forest floor dry weights and nutrient contents were monitored for 28 months following the whole-tree harvest. Soil solution nutrient concentrations, temperature at a depth of 2.5 cm, and percent moisture in the top 10 cm of mineral soil were also monitored following cutting. Immediately following cutting, the forest floor was significantly heavier in the cut area due to logging residue, but after 28 months the clear-cut area forest floor had a dry weight of 10,500 kg ha-' compared to 15,400 kg ha-' in the uncut area. Following cutting, concentrations of NH4 + -N, N03--N and K were higher in the soil solution of the cut area, whereas concentrations of P, Ca and Mg were similar in both areas. The higher concentrations in the clear-cut were attributed to more rapid decay of logging slash and forest floor material and reduced vegetative uptake.

Composition, Classification and Species Response Patterns of Remnant Tallgrass Prairies in Texas

Abstract: Thirty-five remnant upland grasslands within the Blackland, San Antonio, Fayette and Upper Coastal prairies were sampled for species frequency, foliage cover and richness. PCA ordination and cluster analysis were used to ordinate and classify these communities. The majority of these grasslands are dominated by Schizachyrium scoparium with Sorghastrum nutans the second most important species. Andropogon gerardii and Bouteloua curtipendula increase in importance with increasing soil clay content, organic matter and pH and decreasing total annual precipitation. Paspalum plicatulum becomes an important secondary species in communities over acid Alfisols in the Fayette Prairie and acid Alfisols and Vertisols of the Upper Coastal Prairie. High precipitation ( > 90 cm) areas over Alfisols at the northern end of the Blackland Prairie support a unique grassland dominated by Sporobolus silveanus with Carex meadii as an important secondary species. In this same area on Vertisols communities are found that are dominated by Tripsacum dactyloides and Panicum virgatum. Species respond independently and continuously along soil and climatic gradients except where locally sharp transitions occur between Alfisols and Vertisols. Species diversity did not vary significantly among communities. Soil pH was positively correlated with species richness, which was related to slightly higher richness of stands on Alfisols whici. have low pH, and slightly higher richness of Coastal Prairie stands which also have generally lower pH values. INTRODUCTION The Blackland, Fort Worth, San Antonio, Fayette and Upper Coastal prairies occupy a N-S strip comprising approximately 8 million ha within E central Texas (Godfrey et al., 1973) (Fig. 1). Native vegetation of these grasslands has largely been destroyed by cultivation, overgrazing and urbanization. Although several authors have described these prairies (Dyksterhuis, 1946; Launchbaugh, 1955; Collins et al., 1975; Butler, 1979; Diamond, 1983; Smeins and Diamond, 1983), no attempt has been made to compare patterns of community composition and species diversity among them. The Upper Coastal Prairie has traditionally been separated from other Texas prairies (Tharp, 1926; Gould, 1975), but lack of quantitative data makes an evaluation of the degree of relationship among the Coastal Prairie and other Texas prairies impossible. This study provides quantitative data on vegetation and soils of uplands within the Blackland, Fayette, San Antonio and Upper Coastal prairies. These data allow detailed analysis of Texas grasslands to: (1) determine relationships among communities; (2) evaluate species-to-species and species-to-environment relationships; (3) clarify classification, and (4) evaluate patterns in species diversity. STUDY AREA Location and extent. -The grasslands studied are located in a N-S strip approximately 300 km wide and 100 km long in E and central Texas (Fig. 1). To the N the Blackland Prairie borders on the Red River, whereas to the S the Coastal Prairie grades into the Gulf Coast marshes. The eastern border is defined by the Post Oak Savannah, while the western border is defined by the western Cross Timbers in the N, the Edwards Pla-

Responses of Understory Tree Seedlings to Trenching

Abstract: The relative importance of aboveground and belowground competition to growth and establishment of understory tree seedlings was examined in the Piedmont region of North Carolina. First-year seedlings of three species-red maple (Acer rubrum L.), flowering dogwood (Cornus florida L.) and sourwood Oxydendrum arboreum (L.) DC]-were grown in trenched and untrenched plots under a forest canopy and within a windthrow. Survival and changes in height and leaf area were recorded. Nearly all maple seedlings survived. Those in windthrow plots were significantly larger than those in forest plots. Trenching had little significant effect on maple. Dogwood mortality was greater in untrenched than in trenched plots. Light (as estimated by canopy photographs) was significantly correlated with dogwood seedling size only in trenched plots. Availability of soil resourses, especially water, moderated the positive effect of decreased aboveground competition. Although all sourwood seedlings eventually died, those within the windthrow survived longer than those within the forest. Seedlings of this species are extremely scarce and apparently require both high light and mineral soil for successful establishment.

Bumblebee Foraging at a "Hummingbird" Flower: Reward Economics and Floral Choice

Abstract: For a brief period in 1981 Bombus appositus queens visited Ipomopsis aggregata, a hummingbird-pollinated species with floral characteristics typical of that pollination syndrome. This behavior was not observed in other years. Despite the fact that Ipomopsis lacks floral features associated with bee visitation, bumblebees foraged on Ipomopsis at a rate only somewhat slower than that for a typical bee flower. Because of the deep corolla, bumblebees are able to probe only a limited distance into Ipomopsis flowers but in 1981 nectar was more accessible due to a higher standing crop than in other years. While foraging on Ipomopsis, bees were able to obtain a net energetic profit which was similar to that obtained from their primary forage species. These observations indicate that floral choice is governed primarily by profitability-which can overcome innate preferences for bee flowers-and that the features of hummingbird flowers pose no absolute barrier to bee visitation. Bees apparently sample a wide spectrum of available flower types and will cross pollination syndrome lines if the reward economics are favorable.

Food habits of species of Ephemerellidae (Insecta: Ephemeroptera) in streams of Oregon

Abstract: Data on food habits were obtained for 20 species in the mayfly family Ephemerellidae. Diet of a species often varied with site, habitat and size, but such variation did not completely mask differences among species. Adaptive radiation in diet is relatively great in this family, but species showed no distinct groupings when classified either on a functional basis or by traditional trophic categories.

Spatio-Temporal Patterns of Foraging Success for Fishes in an Illinois Stream

Abstract: Stomach contents of 10 fish species were examined to evaluate several scales of spatio-temporal variation in foraging success within an Illinois stream. Most species fed primarily on aquatic or terrestrial invertebrates, but one species (Pimephales notatus) specialized on fine detritus. Fish ate a wide variety of invertebrate taxa, and diet overlap based on prey taxa was extensive among fish species and age classes. Foraging success, measured by number and size of prey eaten, varied among seasons, years, substrate composition and amounts of woody debris. However, no factor influenced foraging success of all species, and no two species responded identically to all factors. Fish generally fed most successfully in June, and least successfully in October. Some fish ate more or larger prey in a study area with gravel-cobble substrates than in a silt-sand area, and many ate more or larger prey in sites with woody debris than in sites from which debris was removed. In addition, aquatic invertebrates increased in abundance in silt-sand substrates when fish were excluded, but did not increase in gravel-cobble substrates, suggesting a greater potential for resource depression in the silt-sand area. Stream fishes apparently face continuous variation in food availability at several temporal and spatial scales. Because changes in availability are often unpredictable and influence each species differently, the competitive environment of stream fishes may be in a state of constant flux. Anthropogenic modifications of streams may create conditions that favor fishes such as certain cyprinids which have highly opportunistic feeding habits. INTRODUCTION Successful foraging is essential to reproduction of most animals; thus natural selection should favor individuals which are most efficient at obtaining food. Efficiency in foraging probably evolves in response to temporal and spatial variation in foraging success, which can result from inherent fluctuations in resource availability or from resource depression (sensu Charnov et al., 1976) due to coexploiters. Resource depression may, in turn, generate strong competitive interactions and lead to the evolution of food partitioning. Among vertebrates, selective advantages of rapid growth and efficient foraging are perhaps best known for fishes. For example, rapid growth of fish to a relatively large size may be adaptive through increased gamete production (Bagenal, 1978) or immunity from predation (Werner et al., 1983a) or success in dominance disputes. Thus, spatio-temporal patterns of' fish foraging success may reveal information that is useful in understanding their population dynamics. Aquatic or terrestrial invertebrates are the primary food resource of most fishes in small streams of the midwestern United States, whereas detritus, algae and other fish are less commonly consumed (Pflieger, 1975; Smith, 1979; Trautman, 1981). Although diets of most fish species are dominated by only one or two of these food resources, opportunistic feeding is widespread, and seasonal shifts in diet composition are common (Starrett, 1950; Whitaker, 1977). Furthermore, foraging rates often exhibit strong seasonal patterns (Mathur, 1977; Allan, 1981; Angermeier, 1982), which are probably induced by cycles of insect emergence and recruitment (Hynes, 1961; Resh, 1977). In addition to temporal variation, fish foraging patterns should also reflect spatial dynamics of invertebrate availability. For example, production and abundance of stream inverte'Present address: Virginia Cooperative Fishery Research Unit, Department of Fisheries and Wildlife Sciences, Virginia Polytechnic Institute and State University, Blacksburg 24061.

Influence of Soil Texture, Moisture and Temperature on Nest-site Selection and Burrowing by the Pine Vole, Microtus pinetorum

Abstract: The influence of soil temperature on nest-site selection by the semifossorial pine vole (Microtus pinetorum) was examined by presenting the voles with a dichotomous choice in soil temperature (19 C vs. 25 C, 30 C or 35 C). In this instance, the number of nests constructed in heated soil decreased with increasing soil temperature; no subsurface nests were built in soil maintained at 30 C or 35 C. These findings may be related to an inability of these voles to effectively regulate body temperature in a heated subsurface environment. When presented with a choice among three soil types consisting of a base mixture of loam and peat moss (3:1 volume/volume), or the base mixture to which was added gravel (3:2 v/v) or stone (1:1 v/v), a significantly larger number of voles selected the loam/peat moss mixture in which to construct subsurface nests and tunnel systems than the other two soil mixtures. The soil/stone mixture was always avoided completely. Similarly, voles confined to the soil/stone mixture, and subsequently released, re-established themselves in an alternate soil type. The majority of voles selected the loam/peat moss mixture. Lastly, when a moisture gradient was imposed on each of the three soil types, voles continued to select the loam/peat moss mixture, but significantly more voles were located in an area of intermediate soil moisture content than in the wet or dry regions of each soil type. These results are discussed with regard to both the energetic costs of burrowing and the water conservation requirements of this species;

Factors Limiting the Upstream Distribution of the Sabine Map Turtle

Abstract: Several environmental variables and population densities of Graptemys ouachitensis sabinensis were measured in a transect along Whisky Chitto Creek, Louisi- ana, from downstream localities where turtles were abundant to upstream localities without turtles. Correlations among environmental variables and population density were calculated over six study sites and over 72 subdivisions of five of those sites. A causal model for the effects of the environmental variables on population density was hypothesized, and its consequences were investigated by path analysis. Algal density on logs, and basking site area emerged from this analysis as the most important prox- imate determinants of turtle density. Of the variables measured, stream width ap- peared to be the ultimate determinant of density, and thus of the upstream limit of the distribution, with positive indirect effects of width being somewhat greater than the direct effect. A discriminant function was used to classify patches of habitat as ei- ther favorable or poor, and the distribution of favorable patches along the transect was investigated. Patches of favorable habitat tend to become smaller and more iso- lated upstream, and use of these patches decreases with distance upstream. This in- formation supports the hypothesis that distributional termination results from in- creasingly difficult dispersal and progressively reduced dispersal pools upstream, as well as from changing environmental factors.

Holocene Vegetational Changes in the Tombigbee River Valley, Eastern Mississippi

Abstract: Stratigraphic studies of pollen from an oxbow on a Holocene terrace and from an upland swale near the Tombigbee River in eastern Mississippi provide a record of Holocene vegetational changes. From -11,000 to 7300 years B.P. the forests were dominated by oaks and hickories. Mesic forest species were common, at least locally. This suggests the existence of a significant moisture gradient between central Miss.-Ala. and the vicinity of Goshen Springs (Delcourt, 1980) in southern Alabama. Mesic taxa declined markedly beginning at about 9000 years B.P. Between 7300 and 3500 years B.P., water levels in the oxbow dropped. Regional forests were dominated by oak and hickory, and mesic taxa became uncommon. Nyssa was extremely common between 2400 and 500 years B.P.; this probably included black gum in the uplands and both black gum and tupelo gum in wet lowland sites. A regional increase in pine is evident beginning at about 2400 years B.P. This pine expansion is eventually associated with decreases in oak; hickory and Nyssa. We suggest that aboriginal land use and an increased frequency of fires caused the latter forest changes. An increase in Ambrosia in the uppermost levels marks European settlement. At this time cypress, wetland herbs and shrubs, and Orontium colonized the oxbow, perhaps because of generally higher water levels due to decreased evapotranspiration caused by land clearance.

Homing in and Ecology of the Southern Flying Squirrel Glaucomys volans in Southeastern Virginia

Abstract: A population of the southern flying squirrel Glaucomys volans in southeastern Virginia was examined for its homing ability. Nest boxes were used to capture the experimental animals and to evaluate homing. Squirrels returned home with high frequency from distances up to 1000 m. There was a significant association between low temperature and large aggregations of huddling squirrels in nest boxes. Holes used as feeding stations were never converted to primary nests.

Competition and coexistence of small mammals in an East Tennessee pine plantation

Abstract: Microhabitat use and niche characteristics of Peromyscus leucopus, Ochrotomys nuttalli and Blarina brevicauda were examined in a pine plantation on the Oak Ridge National Environmental Research Park. Although general microhabitat use was the same, niche parameters (such as niche breadth) for each species varied between two study grids, apparently in response to differing understory density. Microhabitat specialization is thus proposed to be a function of local microhabitat structure. Removal of the generalist species, P. leucopus, from one grid while maintaining the other grid as a control elicited a significant microhabitat shift and increase in niche breadth by O. nuttalli. Blarina brevicauda displayed a slight microhabitat shift and increased niche breadth. These results are inconsistent with the hypothesis that generalist species are poor competitors. Microhabitat heterogeneity created by plant succession and extrinsic disturbances such as tree blow-down are suggested to allow coexistence of these species by altering competitive abilities or microhabitat selection.