bluuebunny/crossref_metadata_2025 huggingface dataset:
Production Ecology of Grassland Plant Communities in Western North Dakota
Authors: Redmann Robert E.
Published: April 2006

Net primary productivity and distribution of mixed grassland plant communities in western North Dakota were studied. The semiarid continental climate supports several grassland types, delimited on the bases of topography and substratum. The dominant species and herbage (shoot) production (mean of 2 yr) of stands located on what were classified as silty range sites included, on rolling upland, (1) Stipa viridula (128 g m—2), Stipa comata (128 g m—2), and (3) Bouteloua gracilis (110 g m—2); on ravine slopes, (4) Stipa spartea—forbs (135 g m—2) and (5) Andropogon spp. (126 g m—2). On sandy range sites were found, in shallower ravine bottoms and at bases of slopes, (6) Sporobolus heterolepis—Stipa spartea (160 g m—2) and (7) Sporobolus heterolepis (185 g m—2); on slopes with very sandy soils, (8) A. scoparius—Stipa spartea (132 g m—2), (9) A. scoparius—forbs (107 g m—2), and (10) A. scoparius (119 g m—2). The highest annual herbage production was measured in Stand 7 (200 g m—2) and the lowest in Stand 3 (77 g m—2). Root mass ranged from 6230 g m—2 (Stand 7) to 3030 g m—2 (Stand 9). Annual root production could not be measured but was estimated to be equivalent to herbage production. Variation in herbage production between sites and between years was attributed to differences in soil moisture and fertility (especially the latter in sites with sandy soils). Fresh mulch was highest in the Stipa—dominated sites (288—320 g m—2) and lowest in Andropogon— and Bouteloua—dominated stands (212—234 g m—2). The highest fresh mulch decay constant and shortest equilibrium time were determined to occur in sites dominated by Sporobolus heterolepis, and the converse for Stipa—dominated sites. Chlorophyll values ranged from 78 mg m—2 (Stand 3) to 542 mg m—2 (Stand 7); variation depended on plant morphology and phenology as well as biomass. Maximum efficiency in herbage production occurred later in the year for stands with a large warm—season component (Sporobolus heterolepis an Andropogon spp.) than in sites dominated by cool—season grasses (Stipa spp.). On an annual basis, Stand 7 was most efficient (0.12%) and Stand 3 least (0.06%). The interrelationships between drought tolerance, phenology (cool— vs. warm—season development), physiology, and production are discussed.
The Chaetognatha Community of the Agulhas Current: Its Structure and Related Properties
Authors: Stone James H.
Published: April 2006

An objective technique was used to identify and study related groups of chaetognath species from the Southwest Indian Ocean off the coast of the Republic of South Africa. Groups or community—type data agreed with, corroborated, and extended the scope of data on single species, multiple species and hydrographic conditions. The most representative groups were from the data of the N70V—nets which formed 3 groups. The 1st (or main) group was composed of 9 species and was distributed as a unit almost solely in the upper 200 meters of oceanic waters, and occurred in neritic waters only when > 1 to 2 species were absent. This agrees with and corroborates individual species data as each was, according to its distribution and abundance, a resident of the general ocean and each showed, except 3 species found predominantly in deeper waters, a positive correlation with the flow—pattern of the Agulhas current. The 2nd (or associate) group of 3 species was found 61% of the time at mixed—water stations, the best of these being located along a line where mixing between neritic waters and the Agulhas current was most likely to occur. Hydrographic data gave no indication of the latter. The 3rd group was made up of 1 oceanic— species which was distinctive by virtue of its vertical distribution; it was a mesoplanktonic beast, inhabiting water >200 m in depth. Species fecundity correlated quite well with each group and its distribution, in that neritic individuals of only one species of the 1st (main) group contained significantly more eggs than corresponding oceanic individuals whereas neritic individuals of all 3 species of the 2nd (associate) group contained significantly more eggs than corresponding oceanic individuals. This evidence suggests that the species of each group were reacting similarly to their environment and that fecundity is a suitable index to hydrographic conditions. Despite the close similarity of chaetognath species in regard to their habitats and niches considerable overlap existed among the species as a result of differences in their horizontal and vertical distributions, in their abundances, in their size relationships, in food diversity which apparently allows for some specialization in feeding but not in a rigid manner, and finally in structural aspects of the food chain itself as 2 of the larger species appeared to function as 2nd—order carnivores.
Patterns of Use of a Flood Plain by Zambian Mammals
Authors: Sheppe Walter , Osborne Timothy
Published: April 2006

Every year the Kafue River, one of the main tributaries of the Zambezi River in Zambia, floods a broad level area, the Kafue Flats, to a depth of up to 5 m for several months. The flats are 235 km long and up to 40 km wide. The life of the flats is conditioned primarily by the alternating rainy and dry seasons and by the floods. About 80 cm of rain falls from November to April. The Kafue rises slowly during the rains, is highest in May, and falls during the latter part of the dry season. Vegetation is composed primarily of grasses, especially Oryza barthii. Because of the abundant water the primary productivity of the flats is much greater than that of the surrounding woodlands. Secondary productivity also is high. Every year there is an alternation of aquatic and terrestrial faunas. During the floods fish move onto the flats from the Kafue, and most spawning takes place there. Terrestrial species are driven off, but as the floods recede they reoccupy the flood plain and use what is by far the best grazing in the region. Large mammals find shelter in tall stands of grass, small mammals in the thick mat of vegetation that covers much of the ground or in the deeply cracked soil. There is a gradient in use of the flood plain: some species (hippopotamus, otter) always stay near the river at low water, others (lechwe, zebra, wildebeest) go for varying distances onto it, and more than half of the mammals (kudu, squirrel, vervet, aardvark) go onto it little if at all. Failure to use the flood plain seems to be due to absence of suitable habitats or food, rather than exclusion by the floods. The most abundant large mammal on the flood plain is the lechwe, the only terrestrial ungulate that sometimes feeds in the floodwaters. Several shrews and mice, especially Mastomys natalensis, are common on the food plain and breed there during the rains. During the floods they leave the flood plain or take refuge on natural levees along the Kafue. Large populations of water birds are common. Crocodile and monitor live near the water's edge and move in and out with the floods like the hippopotamus. Some snakes are common on the flats, but turtles and frogs are not. Although ants and termites are abundant in the surrounding region, they are largely excluded from the flats by the floods.
Feeding Ecology of the Red‐eyed Vireo (Vireo olivaceus) and Associated Foliage‐Gleaning Birds
Authors: Williamson Penelope
Published: April 2006

The sexes of the Red—eyed Vireo, an abundant insectivorous bird in eastern deciduous woodlands, differed in their foraging heights, with an overlap of only about 35%. The males foraged higher than the females, and non—random sequences of movements were employed to maintain this separation. The foraging level and the nest height of female Red—eyes was similar, whereas the males foraged closer to their singing perches. Thus the foraging efficiency of both sexes was likely greater in accordance with their differing behavior roles. The "small" territory (1.3—1.7 acres) of this species actually consists of a cylinder extending from the forest canopy to the low understory. The associated White—eyed Vireo (Vireo griseus) was generally separated from Red— eyed Vireo by habitat, and the Yellow—throated Vireo (Vireo flavifrons) overlaps in habitat but possesses structural and behavioral differences which indicate different prey preferences. Foraging behavior of migrating Red—eyed Vireos differs markedly from that of breeding birds. Differences were also noted between summer and winter foraging behavior patterns in the White—eyed Vireo. All three vireo species exhibit species—specific patterns of foraging movements. Two other foliage—gleaning insectivorous birds occupying the same forests, the American Redstart (Setophaga ruticilla) and the Acadian Flycatcher (Empidonax virescens), also possess species—specific foraging behaviors (niche—exploitation patterns).
The Dynamics of Aquatic Systems: A Comparative Study of the Structure of Four Zooplankton Communities
Authors: Lane Patricia A.
Published: April 2006

This study compares the structure of the limnetic zooplankton communities of four very different lakes ranging from an oligotrophic Great Lake to a small dystrophic reservoir. Only filter feeding cladocerans and calanoid copepods are considered. Using Levins' Theory of the Niche analysis, I measured habitat overlap values for each species pair to determine to what extent habitat selection can explain the number of limnetic zooplankton species coexisting in these lakes and to what extent other factors, such as resource allocation or differential predation, must be invoked to explain the observed diversity. A substantial portion of the ecological overlap of zooplankton populations can be alleviated by habitat selection. Zooplankton communities are dominated by two to three species, in both numerical abundance and grazing intensity. In Lake Michigan (large, oligotrophic), habitat selection was the major mechanism of competitive coexistence and it was achieved by extensive vertical migrations (mean a = .47). In Gull Lake (small, mesotrophic), habitat selection via vertical migration also predominated (mean a = .37) as the major mechanism of competitive coexistence. Three species, Diaptomus sp., Daphnia retrocurva, and Daphnia galeata exhibited high habitat overlap coefficients. In Cranberry Lake, a dystrophic reservoir, most species possessed high overlap coefficients (mean a = .62) and minimal migration amplitudes. Diaptomus spp. and Daphnia catawba predominated. In Lake George, a medium—sized oligotrophic lake, mean habitat overlap was also high (mean 2 = .65). The three dominant species were Diaptomus sp., Diaphanosoma leuchtenbergianum and Daphina galeata. For the latter two communities, which possessed similar high and mean overlap values, the basic equation was weighted by the standing crop and turnover rate of phytoplankton. The spatial–temporal positioning of the Lake George populations were correlated with algal standing crop and turnover patterns while the Cranberry Lake populations were not.
Classification, Description, and Dynamics of Upland Plant Communities within a Minnesota Wilderness Area
Authors: Grigal D. F. , Ohmann Lewis F.
Published: April 2006

The major upland plant community types of the Boundary Waters Canoe Area (BWCA) of northeastern Minnesota, identified by multivariate analyses (clustering and canonical and discriminant analysis) of 68 stands disturbed by logging and 106 stands undisturbed by logging, include the following: lichen, jack pine—oak, red pine, jack pine—black spruce, jack pine—fir, black spruce—feather moss, maple—oak, aspen—birch, aspen—birch—white pine, maple—aspen—birch, maple—aspen—birch—fir, fir—birch, and white cedar. Each of these types is based on a complex of 53 common species, through the name may incorrectly imply that one or two dominant overstory species are indicative of the type. Other forest stands from the BWCA are quantitatively related to the regional vegetation through discriminant analysis. Succession on the uplands in the area, without disturbance, leads to fir—birch and ultimately to the white—cedar community type. Whitetail deer may have had an impact on restricting the occurrence and reproduction of the white cedar type.
Ecological Aspects of Some Mixed‐Species Foraging Flocks of Birds
Authors: Morse Douglass H.
Published: April 2006

Mixed—species flocks of birds composed of chickadees, titmice, woodpeckers, nuthatches, creepers, kinglets, and wood warblers were studied in several habitats in Louisiana, Maryland, and Maine. Chickadees and titmice usually led these groups. Both the size of the flock and density of birds varied with the habitat. An inverse relation existed between the density of birds in a habitat and the percentage of them that were species participating in flocks. Individuals of species regularly participating in flocks sometimes were found away from them. The tendency for such an individual to be in a flock was inversely related to the density of birds in the habitat. Hostile interactions were infrequent in flocks, being mostly of an intraspecific nature. A dominance hierarchy existed between the different species in flocks. A definite distance was maintained between individuals. As flock size increased the area occupied per individual decreased. The larger flocks were, the faster they usually moved. Each species favored certain parts of the habitat when foraging, though overlap existed between almost all pairs of species. The commonest species (Chickadees, titmice, kinglets) overlapped heavily. Subordinate species foraging in the presence of dominant species changed their areas of foraging, but little or no tendency was seen for subordinates to affect the foraging of dominants. Subordinates might either spread out over alternative foraging areas or concentrate their activities more strongly in a few areas. Most flock members responded markedly to predators; however, few predators were observed during the study. Many of the responses seen were delivered to inappropriate stimuli. Several factors suggest a direct relationship between population density and food supply. Flocking appears to be an effective adaptation to difficult conditions. If a species adjusts its foraging in the presence of another species, possibilities of hostilities should be minimized. Subordinate species usually moved to parts of the habitat to which they were well—adapted. If dominant species are absent, however, opportunism by the subordinate may allow it to take advantage of otherwise unused resources. Dominants may obtain a more predictable portion of the food supply by participating in flocks that if solitary. Constant movement may insure that birds obtain the most readily available food. Large flocks may move rapidly because their individuals are more crowded than those in small flocks. With this restriction either increased speed or more concentrated foraging would be predicted. The low frequency of predators suggests that evasive responses of flock members may have been more advantageous than at present.
Analysis of a North Dakota Gallery Forest: Vegetation in Relation to Topographic and Soil Gradients
Authors: Wikum D. A. , Wali M. K.
Published: April 2006

Vegetation—environment relationships were studied in forest stands located on slopes and a floodplain in northeastern North Dakota. Coverage values of all vascular plants and basal area, frequency, density, and mensuration data for tree species were obtained. A total of 109 species of vascular plants, 10 of mosses, and 15 of lichens are reported for the 40 plots studied. The three most important (based on importance value) overstory species are Quercus macrocarpa, Tilia americana, and Fraxinus pennsylvanica var. subintegerrima. Less important overstory species include Ulmus americana, Acer negundo, Populus tremuloides, and Betula papyrifera. Tree—ring analysis indicates that the present forest is less than 100 yr old. Correlation analyses were made between community characteristics of vegetation and the topographic parameters, slope inclination, aspect, and elevation. Principal component analysis and Swan—Dix—Wehrhahn ordination were used to establish five abstract community types. Environmental soil parameters measured for each soil horizon within the rooting zone include available water capacity; pH; electrical conductivity; cation exchange capacity; percentage organic matter; replaceable Ca, Mg, K, Na, Sr, and Mn; available P; and EDTA—extractable Al, Cu, Fe, Mn, Ni, Pb, Si, Sr, and Zn. Topographic and soil parameters were used for stepwise elimination multiple regressions to develop predictive equations for the distribution of plant species. Correlation analysis reveals that the basal area of trees and number and coverage values of herbaceous species is highest on lower elevations and south exposures. Whereas pronounced vegetation differences occur between slopes and the floodplain, the vegetation patterns on the slopes are continuous and overlapping. Based on the results from multiple regression analysis, gradients for potential solar beam irradiation, available water capacity, elevation, linear aspect, slope inclination, and replaceable K were established. By use of coverage values, ecological modalities of selected species are demonstrated along quantified gradients. Relationships of plots in several bivariate combinations show patterns of community type resembling those obtained by stand ordination.
Nitrogen Budget for a Small Coniferous Forest Stream
Authors: Triska Frank J. , Sedell James R. , Cromack Kermit , Gregory Stan V. , McCorison F. Michael
Published: April 2006

An Annual nitrogen budget is presented for a small steam draining Watershed 10, H. J. Andrews Experimental Forest, Oregon. The role of allochthonous debris in the input, flux, and export of nitrogen is emphasized in the material balance budget. All material entering the stream channel was presumed to enter the water sometime during the year. Material estimates are based on total channel area. The major annual nitrogen input (1974—1975) was subsurface flow (11.06 g/m2) as dissolved organic nitrogen (10.56 g/m2) and nitrate (0.50 g/m2).Biological inputs of nitrogen amounted to 4.19 g/m2 as direct terrestrial inputs of: litterfall (1.35 g/m2), lateral movement (1.78 g/m2), and throughfall (0.30 g/m2). Nitrogen fixation on fine wood debris contributed an additional 0.76 g/m2 based on rates from a nearby watershed. Total nitrogen input was 15.25 g/m2. The nitrogen pool was dominated by large amounts of particulate organic matter. Coarse wood constituted 32% of the nitrogen pool (3.80 g/m2) and fine wood fractions 18% (2.18 g/m2). The coarse wood fraction greatly influenced stream morphology. Fine organic particulates constituted an additional 40% of the nitrogen pool (4.77 g/m2). DON (dissolved organic nitrogen) export (8.38 g/m2) was less than input, presumably due to biological uptake associated with litter mineralization, sorption, and chemical flocculation. Due to effective retention of particulate inputs by debris dams, biological processing in the particulate nitrogen pool, and uptake and sorption of DON, most particulate organic inputs increased in nitrogen concentration prior to export. Particulate organic nitrogen input (3.13 g/m2) was greater than export (2.53 g/m2). Total annual nitrogen output was 11.36 g/m2, resulting in a gain of 3.89 g°m—2°yr—1 to the stream. Thus, the stream was not operating on an annual steady state, but on an input—output regime related to the processing of refractory wood debris and resetting by major storms. Although particulate and dissolved nitrogen loss per hectare was small for the 10—ha watershed, these losses passed through or were accumulated in a pool encompassing <1% of the watershed area. This concentration of N in the stream allowed establishment of a separate ecosystem whose processing efficiency and capabilities for nutrient cycling were related to the retention capacity of the channel and nutrient quality of inputs within the reach.
Thermal and Hydric Aspects of Environmental Heterogeneity in the Pitcher Plant Mosquito
Authors: Kingsolver Joel G.
Published: April 2006

In an attempt to define environmental heterogeneity and uncertainty in a meaningful manner, thermal and hydric aspects of the microenvironment of the pitcher plant mosquito (Wyeomyia smithii) were studied. Mechanistic mass and energy balance models were developed to predict surface temperatures in a Sphagnum bog and temperatures and water losses in Sarracenia purpurea pitchers. Field tests indicate that the models predicted surface and pitcher temperatures within 2°—3°C, and hourly and daily water losses from pitchers to within 30%, using only basic meteoroligical data as inputs. Experiments and observations in a natural population of W. smithii in northern Michigan, USA revealed significant differences in larval developmental rate, voltinism, and larval mortality due to microclimatic effects. The model predicts larval developmental rates and voltinism for each microclimate within 10%. Water loss simulations predict, and field observations confirm, that pitcher desiccation is a function of microclimate and pitcher size, and that rainfall patterns on the order of 5—30 d determine desiccation patterns. Identification of the spatial and temporal scale of both the environment and the organismic (population) phenomena in question is crucial to constructing a meaningful definition of environmental heterogenity. Thermal and hydric components of environmental variation may play an important role in the maintenance of fitness variation in W. smithii. These results support the hypothesis of Istock (1978) that environmental uncertainty favors mixed life strategies in Wyeomyia.
Autecology of Kobresia bellardii: Why Winter Snow Accumulation Limits Local Distribution
Authors: Bell Katherine L. , Bliss L. C.
Published: April 2006

In the Colorado Front Range Kobresia bellardii distribution is restricted to alpine meadows which are neither extremely wind exposed nor buried under snow in winter. This study examined reasons for such a limited distribution by comparing the behavior of undisturbed Kobresia with that of transplants into alpine habitats with less and more winter snow accumulation. Kobresia's success in snow—free meadows was related to rapid summer growth and to its use of an extended period for development, from about 1 April to 20 October. New leaves began elongation in the autumn and completed growth the following summer. Most carbohydrates were stored as oligosaccharides in the shoots. During periods of relatively warm weather (>—4°C) in winter, when soil water potential (ψs) rose above —2.0 MPa, leaves elongated slightly; they died back when cold, windy weather resumed. Kobresia did not survive in a fellfield primarily because of mechanical damage by windblown snow and sand. Low ψs led to leaf wilting in summer and winter. Shallow snow cover (°15 cm) that melted early permitted extensive April leaf elongation in saturated soils. The prematurely developed leaves apparently lost frost hardiness and were injured in late spring freezes. In sites of moderate (°75 cm) and deep (°140 cm) snow accumulations, autumn dieback was incomplete when drifts first formed in September and October. Winter freezing destroyed the apparently unhardened leaf tissues and meristems, resulting in loss of carbohydrate reserves. In the moderate snow accumulation sites, leaf elongation was delayed until snowmelt was complete in late June or early July. Interspecific competition appeared unimportant in determining Kobresia distribution. Flower and seed production were reduced up to 99% by habitat alteration. Kobresia may not be reproducing by seed in the Front Range at the present time.
Niche and Habitat Relations in Subalpine Bird Communities of the White Mountains of New Hampshire
Authors: Sabo Stephen R.
Published: April 2006

The populations, foraging behavior, and habitat selection of 20 insectivorous species of subalpine birds were studied during three breeding seasons in the white Mountains, New Hampshire, USA. The object was to clarify how an elevation gradient of environmental adversity affected diversity, dominance, niche, habitat, and species biology. Population densities and territory sizes were measured from census plots. Territory sizes correlated with species body mass, probably in response to size—related food requirements. Lower elevations had more even distributions of densities among species and less steep dominance—diversity curves than did higher elevations. To show habitat selection patterns in the subalpine vegetation, principal components analysis (PCA) was used to develop a model that emphasized elevation and broadleaf—to—coniferous foliage as the two major gradients. Bird species diversity (BSD) reached a maximum in well—developed preclimax red spruce—balsam fir forests with some broadleaf crowns present. Individual species responses were mapped using the model as a base chart, grouping together: (1) five species living at altitudes above 1100 m with small differences in their specific patterns, (2) five other species widely distributed through the mid—subalpine zone (800—1100 m), (3) six more mid—subalpine species segregating along the broadleaf/conifer foliage gradient, and (4) three species found near water. Niche relations were studied using detrended correspondence analysis, a modified version of reciprocal averaging. Resource use was divided by (1) terrestrial vs. arboreal position, (2) a gradient from fine distal (leaves) to coarse proximal (trunk) substrates, and (3) different specializations for food acquisition. BSD was greater in the resource categories requiring less energy and less specialized morphological adaptations to exploit, or having greater insect resource bases. To investigate the relations between habitat and niche, a matrix of the correlations between the niche and habitat variables, using species means for data, was analyzed by PCA. Three relations emerged: (1) the elevation/forest structure gradient was correlated with the terrestrial/arboreal foraging segregation, (2) the niche substrate gradient responded to vegetation structure along the broad—leaf/conifer gradient, and (3) in both cases the response of foraging behavior to habitat structure involved many variables acting together. Niche and habitat were thus closely linked as aspects of species ecotopes; species responses were illustrated for the Blackpoll Warbler and other wood warblers. Food was inferred to limit populations occasionally, so that interspecific competition plausibly underlies niche differentiation. Thermoregulation is hypothesized to affect habitat selection.
Larval Settlement and Juvenile Mortality in a Recruitment‐Limited Coral Reef Fish Population
Authors: Victor Benjamin C.
Published: April 2006

The temporal and spatial patterns of larval settlement of the bluehead wrasse, Thalasoma bifasciatum, were documented in the San Blas Islands of Panama. Daily censuses indicated that larvae settle onto coral reefs in brief episodes that tended to occur around the new moon and peak in intensity between August and December of each year. The magnitude of settlement each day was unrelated to changes in the local population size but was found to be significantly correlated with the nightly catch of planktonic reef fish larvae in the waters over the reef. The spatial pattern of settlement was examined on three scales. On a large geographic scale, 24 reefs within 1000 km2, there was tremendous variability in settlement intensity that was very consistent from one year to the next. The best correlate of recruit density was exposure to the onshore current. It is suggested that some large—scale oceanographic process determines the intensity of settlement of bluehead wrasses on this scale. The densities of adults on these reefs directly reflected the densities of recruits. Counts of daily otolith increments indicated that those larvae that settled onto low—density reefs subsequently grew significantly faster. On an intermediate scale, sites within 1 km2, there was no consistency among sites or months in settlement intensity, suggesting that variance in settlement intensity on this scale may be the product of random processes. On the smallest scale, habitat selection within a patch reef, it was found that recruits had distinct preferences in their choices of settlement sites. The daily mortality schedule of bluehead wrasses indicated that mortality was extremely high for the first 3 d on the reef. Juvenile mortality was, furthermore, found to be independent of density. As a result, the patterns of larval settlement of bluehead wrasses persisted into the adult population relatively unchanged. It is therefore proposed that the distribution and abundance of bluehead wrasses in this region are more a product of the external processes controlling larval settlement than of any biological interactions within the reef assemblage.
Interactions of Bacteria, Fungi, and their Nematode Grazers: Effects on Nutrient Cycling and Plant Growth
Authors: Ingham Russell E. , Trofymow J. A. , Ingham Elaine R. , Coleman David C.
Published: April 2006

The most common system responses attributed to microfloral grazers (protozoa, nematodes, microarthropods) in the literature are increased plant growth, increased N uptake by plants, decreased or increased bacterial populations, increased CO2 evolution, increased N and P mineralization, and increased substrate utilization. Based on this evidence in the literature, a conceptual model was proposed in which microfloral grazers were considered as separate state variables. To help evaluate the model, the effects of microbivorous nematodes on microbial growth, nutrient cycling, plant growth, and nutrient uptake were examined with reference to activities within and outside of the rhizosphere. Blue grama grass (Bouteloua gracilis) was grown in gnotobiotic microcosms containing sandy loam soil low in inorganic N, with or without chitin amendments as a source of organic N. The soil was inoculated with bacteria (Pseudomonas paucimobilis or P. stutzeri) or fungus (Fusarium oxysporum), with half the bacterial microcosms inoculated with bacterial—feeding nematodes (Pelodera sp. or Acrobeloides sp.) and half the fungal microcosms inoculated with fungal—feeding nematodes (Aphelenchus avenae). Similar results were obtained from both the unamended and the chitin—amended experiments. Bacteria, fungi, and both trophic groups of nematodes were more abundant in the rhizosphere than in nonrhizosphere soil. All treatments containing nematodes and bacteria had higher bacterial densities than similar treatments without nematodes. Plants growing in soil with bacteria and bacterial—feeding nematodes grew faster and initially took up more N than plants in soil with only bacteria, because of increased N mineralization by bacteria, NH4+—N excretion by nematodes, and greater initial exploitation of soil by plant roots. Addition of fungal—feeding nematodes did not increase plant growth or N uptake because these nematodes excreted less NH4+—N than did bacterial—feeding nematode populations and because the N mineralized by the fungus alone was sufficient for plant growth. Total shoot P was significantly greater in treatments with fungus or Pelodera sp. than in the sterile plant control or treatments with plants plus Pseudomonas stutzeri until the end of the experiment. The additional mineralization that occurs due to the activities of microbial grazers may be significant for increasing plant growth only when mineralization by microflora alone is insufficient to meet the plants' requirements. However, while the advantage of increased N mineralization by microbial grazers may be short—term, it may occur in many ecosystems in those short periods of ideal conditions when plant growth can occur. Thus, these results support other claims in the literature that microbial grazers may perform important regulatory functions at critical times in the growth of plants.
Seasonal Succession in a Natural Daphnia Assemblage
Authors: DeMott William R.
Published: April 2006

This study uses descriptive data and field experiments to examine a seasonal succession between two planktonic grazers (Daphnia rosea and Daphnia pulicaria) in a small Vermont lake. Each year Daphnia populations hatched from resting eggs during May and both species produced numerous parthenogenetic generations before the appearance of males and ephippia in late September. During the 2 yr of study, D. rosea had an apparent advantage in spring and early summer, but was rapidly replaced by D. pulicaria during late summer and autumn. These shifts in the species balance were associated with reversals in reproductive success. Moreover, the species with the higher birth rate also tended to have a lower mortality rate. An analysis of population structure, reproduction, and mortality suggests that inter— and intraspecific competition, not fish predation, limited population growth. Field experiments in both small (3.8—L) and large (12 000—L) enclosures demonstrate that the demise of D. rosea during late summer was a direct consequence of competition with D. pulicaria. However, the outcome of field experiments was sensitive to seasonally changing conditions. During spring and early summer, when D. rosea had a reproductive advantage at low densities in the lake, it was also the stronger competitor at high densities within experimental enclosures. The shift in species advantage during summer seems associated with a changing resource base. In spring the food for grazers was composed mostly of highly edible flagellates, while "resistant" algae (mostly gelatinous greens) were predominant during late summer and autumn. Grazing experiments and observations on gut contents suggest how D. pulicaria was favored by this change in food conditions. Grazing experiments with a radioactively labeled flagellate (Chlamydomonas) show that the two Daphnia species had very similar feeding rates on this alga. In addition, direct observations on the composition of gut contents demonstrate that they ingested virtually identical proportions of common phytoplankton species. However, experiments with labeled natural algae and observations on the quantity of ingested algae indicate that D. pulicaria was more effective in utilizing "resistant"algae.
Variation in Algal Dispersal and Recruitment: The Importance of Episodic Events
Authors: Reed Daniel C. , Laur David R. , Ebeling Alfred W.
Published: April 2006

Spatial and temporal variation in dispersal and recruitment of the kelps Macrocystis pyrifera and Pterygophora californica and of filamentous brown algae (FBA) consisting primarily of Ectocarpus siliculosus were examined off southern California, USA. These three taxa of sublittoral brown algae show enormous differences in adult morphology and demography, but release propagules near the bottom that are similar in size, shape, and motility. Their weekly variation in recruitment was measured on replicate frosted glass slides placed at different distances from stands of fertile adults at three sites. For all taxa, significant temporal variation was observed in recruitment at the zoospore source as well as at distances out to 4000 m from the source. For the kelps, this variation was due to variable rates of both zoospore settlement and early postsettlement mortality. In general, recruitment density of both kelps rapidly declined with distance from the adult stand; significantly lower recruitment was observed at as little as 3 m away. In contrast, dispersal of FBA was much greater; no significant decrease in recruitment density was observed out to 500 m. The differences in dispersal between these two algal groups may result from behavioral differences of their zoospores. Unlike those of kelps, zoospores of FBA are positively phototatic. This enables FBA zoospores to remain in the water column longer, thus increasing the distances over which they disperse. The predominant pattern of limited dispersal in the two kelps, however, was not constant over time. Dispersal was greatly expanded during episodic periods of high recruitment that coincided with winter storms. Turbulent water flower associated with these storms may resuspend kelp zoospores normally occurring near the bottom and allow them to be transported greater distances by prevailing currents. Although species characteristics of the adult stand appeared to influence the distribution of recruits, the effect disappeared within 10 m of the stand. At a short—range study site, decreases in spatial variation in recruitment density between the zoospore source and 3 and 10 m away indicated that the distribution of zoospores became more uniform with distance. Likewise at a long—range site, recruitment out to 4000 m appeared to occur uniformly and coincided with recruitment at the zoospore source and at all intermediate distances. This uniformity and coincidence of recruitment indicated that dispersal over longer distances was probably via individual zoospores rather than via clumps of drifting plants or reproductive fragments as suggested in previous studies. Although episodic, such dispersal is probably essential in promoting the persistence of these plants in highly disturbed and unpredictable environments.
Larval Growth and Survivorship of the Black Swallowtail Butterfly in Central New York
Authors: Feeny Paul , Blau William S. , Kareiva Peter M.
Published: April 2006

The objectives of this study were to measure larval growth and mortality rates in a butterfly population and to determine how these rates vary locally in space and time. The survivorship and growth rates of 1353 Papilio polyxenes larvae, placed on natural food plants in typical habitats, were followed during both generations of 1973 and 1974. In each year and generation, cohorts of first instar larvae were set out on three commonly used host—plant species and at two elevations. Larvae reaching the fifth instar were returned to the laboratory, where the surviving pupae gave rise to 87 adults. Together with estimates of egg mortality in the field (50%) and the sex ratio of emerging adults (50/50), this result indicates that females must lay at least 60 eggs, on average, to replace themselves once in the next generation. Stage—specific mortality rates (qx) averaged 0.59, 0.33, 0.35, and 0.41 for the first four instars, respectively, approximating a Type II survivorship curve (constant mortality rate). Both the shapes of survivorship curves and the probability of reaching the fifth instar were highly variable from one treatment to another, however, providing a heterogeneous pattern of survivorship in space and time Survivorship was greater in 1973 than in 1974, probably due to different weather conditions; it was greater at high—elevation than at low—elevation sites; and it was greater on wild carrot, Daucus carota, and on parsnip, Pastinaca sativa, than on poison hemlock, Conium maculatum. Survivorship of larvae placed artificially did not differ from that of larvae hatching from wild—laid eggs on the same host species at the same time and place. Arthropod predators attacking larvae included spiders as well as hemipterans of the families Nabidae, Reduviidae, Coreidae, and Pentatomidae. Parasitoids emerging from larvae or pupae belonged to the families Phoridae, Tachinidae, Ichneumonidae, and Braconidae. Our results, supplemented by predator—exclusion experiments in 1977, indicate that predation is the major source of larval mortality, small (arthropod) predators having the greatest impact on younger larvae and large (vertebrate) predators the greatest impact on older stages. Larvae grew faster in 1973 than in 1974, probably because of more favorable weather in 1973. Growth was faster at low— than at high—elevation sites and markedly faster on poison hemlock than on wild carrot or parsnip. Unpredictability of larval survivorship in space and time is likely to favor widely spaced oviposition by P. polyxenes females. Such behavior may also be favored by variability in larval growth rates, since adult emergence times may be crucial to the reproductive success of surviving male progeny.
A Nutritional Constraint in Detritivory by Fishes: The Stunted Population of Sarotherodon mossambicus in Lake Sibaya, South Africa
Authors: Bowen Stephen H.
Published: April 2006

Juvenile Sarotherodon mossambicus in Lake Sibaya, South Africa, grow rapidly and maintain good body condition, but adults grow slowly, show extensive marasmas and appear to suffer from severe malnutrition. The cause of this malnutrition was sought by comparison of juvenile and adult diets. Juveniles feed on benthic detrital aggregate from nearshore sand terraces, whereas adults feed on benthic detrital aggregate in deeper offshore waters. Chemical analysis of 39 benthic detrital aggregate samples collected from juvenile and adult feeding areas showed that detrital aggregate from these 2 areas did not differ in caloric content (° =0.97 J/mg organic), organic content (° = 43.0%), soluble carbohydrate content (° = 16.0% of organic), total carbohydratecontent (° = 45.7% of organic). Protein content ranged from 1.8 to 14.2% of organic matter and decreased with increasing depth. Assimilation efficiencies determined experimentally for juveniles feeding on benthic detrital aggregate were: energy 42%, organic matter 37%, soluble carbohydrate 36%, total carbohydrate 35%, and protein 46%. A large proportion of the organic matter assimilated was nonliving amorphous detritus. Ratios of digestible protein to digestible energy calculated for the 39 samples show a highly significant inverse relationship to increasing depth. These ratios indicate that juvenile diets contain adequate protein to support good growth, but adult diets would be expected to result in malnutrition due to protein deficiency. Nutritional constraints imposed by low protein levels in detrital aggregate account for the characteristic nearshore distribution of detritivorous fishes and the paucity of exclusively detritivorous fish species.
Interbiome Comparison of Stream Ecosystem Dynamics
Authors: Minshall G. Wayne , Petersen Robert C. , Cummins Kenneth W. , Bott Thomas L. , Sedell James R. , Cushing Colbert E. , Vannote Robin L.
Published: April 2006

Studies were conducted in four distinct geographic areas (biomes/sites) in northern United States to examine changes in key ecosystem parameter: benthic organic matter (BOM), transported organic matter (TOM), community production and respiration, leaf pack decomposition, and functional feeding—group composition along gradients of increasing stream size. Four stations ranging from headwaters (1st or 2nd order) to midsized rivers (5th to 7th order) were examined at each site using comparable methods. The results for each parameter are presented and discussed in light of the River Continuum Concept of Vannote et al. (1980). The postulated gradual change in a stream ecosystem's structure and function is supported by this study. However, regional and local deviations occur as a result of variations in the influence of: (1) watershed climate and geology, (2) riparian conditions, (3) tributaries, and (4) location—specific lithology and geomorphology. In particular, the continuum framework must be visualized as a sliding scale which is shifted upstream or downstream depending on macroenvironmental forces (1 and 2) or reset following the application of more localized "micro"—environmental influences (3 and 4). Analysis of interactions between BOM and TOM permitted evaluation of stream retentiveness for organic matter. Headwaters generally were most retentive and downstream reaches the least. Estimates of organic matter turnover times ranged between 0.2 and 14 yr, and commonly were 1—4 yr. Both turnover times and distances were determined primarily by the interaction between current velocity and stream retention. Biological processes played a secondary role. However, the streams varied considerably in their spiraling of organic matter due to differences in the interplay between retentiveness and biological activity. Differences in the relative importance of retention mechanisms along the continuum suggest that headwater stream ecosystems may be functionally more stable, at least to physical disturbances, than are the r intermediate river counterparts.
Bottom‐Up and Top‐Down Impacts on Freshwater Pelagic Community Structure
Authors: McQueen Donald J. , Johannes Mark R. S. , Post John R. , Stewart Thomas J. , Lean David R. S.
Published: April 2006

For freshwater pelagic ecosystems, the biodiversity and cascading trophic interaction theories both predict that decreased piscivore populations will result in direct, short—term (a few years) increases in planktivore biomass, reductions in crustacean herbivore biomass, and increases in chlorophyll a concentration and phytoplankton biomass. An Alternate view is offered by the bottom—up:top—down theory, which predicts that in eutrophic lakes changes in piscivore biomasses will have strong impacts on planktivore numbers, weaker but observable impacts on zooplankton biomass, and little or no long—term effects on phytoplankton biomass. A partial winterkill at Lake St. George, Ontario, Canada allowed us to test these predictions. The data set comprised measures of: (1) piscivore and planktivore numbers, (2) zooplankton species composition, size structure, and biomass, (3) chlorophyll a concentration and Secchi depth, and (4) water chemistry from 1980 through 1986. Prior to the winterkill of 1981—1982, the piscivore population was high (1000—2000 piscivores/ha), the planktivore population was intermediate (8000—10 000 planktivores/ha), zooplankton biomass was intermediate (2400 µg/L), and chlorophyll a concentration was high (5—12 µg/L). In the year following the winterkill (1982), piscivore and planktivore numbers were low, and zooplankton biomass and chlorophyll a concentration were high. During the next 2 yr (1983—1984) the planktivore population increased rapidly to densities >20 000 individuals/ha, zooplankton biomass density decreased to <1600 µg/L and chlorophyll a concentration decreased. During the final 2 yr of the study, piscivores recruited to near prewinterkill levels, planktivores were reduced to <8000 individuals/ha, zooplankton biomass increased, and chlorophyll a concentration decreased. Over the 7 yr data set, we found a strong negative correlation between numbers of piscivores and planktivores, a weaker correlation between numbers of planktivores and zooplankton biomass, and no between—year correlation between zooplankton biomass and chlorophyll a concentration. There was, however, a positive correlation between total epilimnetic phosphorus and chlorophyll a concentration. These data are consistent with predictions made by the bottom—up:top—down model, and the implication is that at Lake St. George, the trophic cascade uncouples at the zooplankton → phytoplankton link. We speculate that this may be due to the combined effects of lake trophy and Daphnia species composition and size.
An Analysis of Avifauna‐Resource Relationships on the Serengeti Plains
Authors: Folse L. Joseph
Published: April 2006

The objectives of this study were: (1) to test the hypothesis that food was the principal proximate factor limiting the distribution of birds on the Serengeti Plains of Tanzania, and (2) to generate a satisfactory description of the relationships of the Serengeti avifauna to selected habitat factors. I analyzed natural changes in the avifauna among habitats through time and related these changes to variation in avian food and habitat resources. Food limitation was apparently not an important proximate factor affecting short—term variations in bird population densities, although many species were functionally capable of responding to fluctuations in food supply. Habitat partitioning was the principal means of "niche segregation" among the various species. Sites with greatest total avian biomass had low vegetation biomass, low vegetation height, but well—developed vertical vegetation structure. These conditions provided cover along with a great degree of ground—level mobility for most species. Individual bird species were associated with specific types of vegetation structure, while vegetation condition (primarily green biomass) was the main factor associated with increased densities of bird species within vegetation types. Neither conventional competition theory nor Wiens' (1977) "bottle—neck" theory can account for the characteristics of the Serengeti grassland avifauna.