bluuebunny/crossref_metadata_2025 huggingface dataset:
Life History and Underwater Studies of a Heterodont Shark
Authors: McLaughlin R. H. , O'Gower A. K.
Published: April 2006

The biology of Australian reef populations of the shark Heterodontus portusjacksoni has been studied underwater with SCUBA. Additional life—history data have been obtained from catch records of adults and juveniles and from observations on captive animals. The mature breeding female H. portusjacksoni lays 10—16 eggs which are deposited in rock fissures on shallow reefs; the young emerge approximately 1 year later. The breeding season on the central coast of New South Wales occurs in late winter and spring. This species appears to segregate by sex into separate juvenile, adolescent, and adult populations. A slow growth rate and protracted immaturity is characteristic. Sexual maturity is reached at 8—10 years for males and 11—14 years for females. Most feeding occurs during darkness over both reefs and soft substrates, gut contents reveal that they feed on a variety of benthic invertebrates. Populations of H. portusjacksoni observed on sublittoral reefs consisted of adult sharks, the majority of which were females. A few specific sites, usually caves in which they shelter during the day, are preferred. Resightings of sharks tagged on a reef at Bondi, Sydney, revealed that animals which formed the local population moved freely to and from the reef and also between the several sites on the reef. Reef densities were highest during the breeding season and were correlated with low water temperatures. Emigration from inshore reefs occurs at the end of the breeding season in late September and October. A number of tagged sharks returned to the same reef in the following year and others after periods of up to 2 years. Recaptures made outside the Sydney area indicate that some sharks may travel several hundred kilometers during their migration.
Seasonal Variation in Composition, Plant Biomass, and Net Primary Productivity of a Tropical Grassland at Kurukshetra, India
Authors: Singh J. S. , Yadava P. S.
Published: April 2006

The variation in composition, plant biomass, and net primary productivity was analyzed in a tropical grassland situated within the campus of the Kurukshetra University, India, at 29° 58' N latitude and 76° 51' E longitude. A study of life forms indicated a therocryptophytic flora. Detailed phytosociological values of constituent species of the vegetation were studied at monthly intervals (May 1970 to May 1971) through tiller analysis. Most of the species were found to be contagiously distributed. The changes throughout the year in the aboveground plant biomass, standing dead, litter, and belowground biomass showed a maximum aboveground biomass in September (1,974 g/m2) and maximum belowground biomass in November (1,167 g/m2). Examination of vertical distribution of the aboveground biomass of total vegetation as well as of the individual species indicated that different layers of vegetation are dominated by different species in different months. The aboveground net primary production was maximum during the rainy season (1,706 g/m2), and the belowground maximum occurred during the winter season (785 g/m2). Total annual net primary production is estimated to be 3,538 g/m2. The system transfer functions revealed that productivity was more aboveground—directed during the wet period and more belowground—directed during the dry period. Annual efficiency of energy capture by the primary producers was calculated to be 1.66% on the basis of half total incident solar radiation.
A Simulation Model of Lagoon Algae Based on Nitrogen Competition and Internal Storage
Authors: Fong Peggy , Foin Theodore C. , Zedler Joy B.
Published: April 2006

Simulation modeling was used to explore the importance of resource competition to algal community structure of shallow coastal lagoons. We used modified Michaelis—Menten equations to describe the kinetics of nitrogen uptake for three co—occurring algal functional forms: phytoplankton, foliose algae, and cyanobacterial mats. Uptake rates and subsequent growth were modified by differential internal storage and recycling of nitrogen from decaying tissues. Analysis of model behavior indicated that a two—step model uncoupling nutrient uptake and growth was capable of producing a wide variety of dynamic behavior not possible with simple uptake kinetics. A nutrient storage term allowed for time lags between nutrient input and growth, and resulted in periodic changes in dominance between foliose algae and mats. Analyses of the most important parameters of the model, nutrient uptake and growth rates, indicate that our model can be adapted for algal communities composed of different dominant algal forms. The model demonstrates a dynamic response in both magnitude and timing of algal growth when the parameter values for nutrient uptake are manipulated. From this we hypothesize that the dynamic relationships between the algal groups are similar in different communities, but that the parameter values in the Michaelis—Menten equations for each algal form must be tailored to each system. We validated the model with three field experiments from the published literature. First, we compared model predictions to results of a replicate microcosm experiment modeling lagoonal communities treated with 20 nitrogen loading rates spanning three orders of magnitude. The model adequately predicted the dynamics of the simplified microcosm community within the range of loading rates where nitrogen was limiting (0—0.270 mg°L—1°d—1). As predicted by the model and confirmed with the experiments, microalgal mats dominated when nitrogen addition was low, but were replace by foliose algae at higher N loading levels (0.068—0.090 mg°L—1°d—1). Phytoplankton never dominated the community. Second, we compared the observed and predicted communities for a competition experiment excluding two of the algal forms. The magnitude of the phytoplankton biomass in each treatment is accurately predicted but predictions of timing are less accurate. The final biomass of foliose algae and mats is predicted accurately. Finally, we applied the model to an experiment using algal communities of San Francisco Bay. Results matched predictions when the kinetic parameters for foliose algae were changed to those for Ulva, the green macroalga from San Francisco Bay. These results support the hypothesis that nitrogen storage is important in understanding growth and nutrient utilization patterns of foliose algae and benthic cyanobacterial mats and the subsequent impacts on community structure.
Phylogeny, Ecology, and the Richness of Parasite Communities in Vertebrates
Authors: Poulin Robert
Published: April 2006

The makeup of parasite communities is the result, among other factors, of interactions between the evolutionary history and ecological characteristics of hosts. This study evaluates the relative importance of some ecological factors (host body size, diet, habitat, latitude, and the mean number of parasite individuals per host) as determinants or correlates of parasite community richness in vertebrates, before and after controlling for potential effects of host phylogenetic relationships. Data were obtained from the literature on 596 parasite communities belonging to one of four distinct types: gastrointestinal parasite communities of fish, birds, or mammals, and ectoparasite communities of fish. There were positive correlations between the number of hosts sampled and mean species richness of the parasite community of each genus. In analyses treating host genera as independent statistical observations and using estimates of parasite species richness corrected for host sample size, positive correlations were observed between richness and host body size in gastrointestinal communities of all three groups of vertebrates. The mean number of parasite individuals per host also was correlated positively with species richness. In fish, richness increased with increases in the proportion of animal food in the host diet. Aquatic birds had richer parasite communities than their terrestrial counterparts, whereas marine fish had richer gastrointestinal parasite communities than freshwater fish. The richness of ectoparasite communities on fish showed no association with any of the ecological variables investigated. Using host genera as independent points in the analyses may lead to biased results since some host lineages are descended from recent common ancestors, and are therefore not truly independent. The comparative analysis was repeated using phylogenetically independent contrasts derived from the phylogeny of hosts. Once the effects of host phylogeny were removed, somewhat different results were obtained: host body size showed no relationship with parasite species richness in birds, and there was no evidence that habitat transitions resulted in significant changes in parasite species richness in any of the types of communities studied. Of the ecological factors studied, the comparative analyses suggest that only host body size can be an important determinant of parasite community richness in certain host groups. This study illustrates clearly the need to control for phylogeny in investigations of host—parasite interactions.
Mechanisms Associated With Decline of Woody Species in Riparian Ecosystems of the Southwestern U.S.
Authors: Busch David E. , Smith Stanley D.
Published: April 2006

Throughout western North America, riparian ecosystem function has been transformed by anthropogenic influences on riverine environments. Modified flood frequency, duration, or intensity; depressed floodplain water tables; and increased alluvium salinity have contributed to change in riparian forest communities formerly dominated by Populus fremontii and Salix gooddingii. The invasion of the naturalized arborescent shrub, Tamarix ramosissima, potentially alters competitive hierarchies and disturbance regimes in these riparian ecosystems. We evaluated the structure and function of two southwestern riparian communities that differed in the degree of streamflow perturbation to which they had been subjected: the highly regulated lower Colorado River and the less tightly regulated Bill Williams River. Ordination analyses provided evidence that these riparian communities are structured along gradients relating to moisture, salinity, disturbance from fire, and community maturity, with Colorado River sites being more xeric and saline than those on the Bill Williams River. Foliar elemental analyses revealed high sodium concentrations in Tamarix (Na:K ratio = 1.87) and in the native shrub Tessaria sericea (Na:K = 1.56). Evaluation of tissue water relations parameters showed that Tamarix had lower osmotic potentials than sympatric woody taxa, helping to confirm that Tamarix is halophytic and probably capable of greater osmotic adjustment than native species. Carbon isotopic discrimination (△) provided evidence for higher water use efficiency in Tamaris than in Populus, Salix, and Tessaria. Tamarix △ averaged over 1% less than that of the other riparian taxa. Experimental removal of Tamarix from stands where Salix was codominant resulted in growth augmentation, less negative water potentials, and higher leaf conductance in Salix, all providing evidence of interspecific competition. The persistence of Salix, but not Populus, on the Colorado River appears to be due to greater water— and salinity stress tolerance in Salix than in Populus. A preponderance of senescent Populus along the Colorado River is an indication that this formerly dominant species is effectively approaching local extinction in parts of this ecosystem.
The Role of Herbivorous Fishes in the Organization of a Caribbean Reef Community
Authors: Lewis Sara M.
Published: April 2006

Experimental manipulations of grazing intensity were used to examine the role of herbivorous fishes in the families Acanthuridae (surgeonfishes) and Scaridae (parrotfishes) in determining distributions and abundances of benthic species within and among shallow tropical reef habitats. A back reef habitat along the Belizean barrier reef was characterized by a diverse benthic assemblage of algal turfs, coralline algae, and the coral Porites astreoides, but by extremely low macroalgal abundance. In contrast, several nearby shallow habitats were dominated by dense stands of several macroalgal species. Experimental reduction of herbivorous fish grazing in the back reef (achieved by constructing exclosures) rapidly and dramatically altered existing patterns of benthic species composition and species abundances. After 10 wk of reduced herbivory, total macroalgal abundance increased significantly in herbivore exclusion areas relative to unmanipulated controls, and was correlated with decreased percent cover of available space, several algal turf species, crustose coralline algae, and Porites. Some macroalgal species were able to directly overgrow and kill portions of Porites colonies within herbivore exclusion treatments. Successful recruitment and growth of several algal species under experimentally reduced herbivory indicated that macroalgal species distributions may be limited by herbivory rather than by lack of spore availability or unsuitable physical conditions. Algal turfs characteristic of many reef habitats appear to represent herbivore—tolerant assemblages, persisting under high grazing intensity but responding rapidly to reduced herbivory with increased abundances, morphological changes, and altered reproductive status. These results suggest that herbivorous fish grazing profoundly influences benthic species distributions and abundances within some tropical reef habitats. Spatial variation in herbivory appears to be of fundamental importance in determining regional patterns of benthic community structure on tropical reefs. The spatial mosaic of benthic community composition among shallow reef habitats was associated with patterns of grazing intensity by herbivorous fishes. Several reef habitats supporting dense macroalgal stands represented spatial refuges from herbivory, with low herbivorous fish densities and reduced grazing intensities. Transplant experiments revealed that algal species characteristic of these low—herbivory habitats were highly susceptible to grazing by herbivorous fishes. Spatial heterogeneity in grazing intensity may contribute to high regional diversity among tropical reef habitats by maintaining different benthic species assemblages under fundamentally distinct selective regimes.
Tests of Density‐Dependent Habitat Selection in a Patchy Environment
Authors: Morris Douglas W.
Published: April 2006

A simple regression analysis can be used to assess the response of animal density to differences in habitat quality. The same test can evaluate general predictions of habitat selection theory as well as search for differences in the shapes of habitat suitability density functions, something previous tests have been unable to do. Combined with demographic or other estimates of fitness, regression tests can provide new insights into the evolution of habitat selection. Regression and fitness tests were used to explore the pattern of density—dependent habitat use in two temperate—zone rodents. The intensity of population regulation appeared to be inversely related to a habitat's carrying capacity. Variation in density—dependent habitat choice suggests new and unexpected dispersal strategies that vary with habitat heterogeneity. The predictions of the theory are complicated when habitat quality varies independently of population density. Sweepstake fitness rewards may be reaped by animals that would previously have been assumed to have made a suboptimal habitat choice. At the level of microhabitat, habitat selection models appeared to be much less capable of predicting variation in population density. Such a relation could be due to complex and inverse relationships between microhabitat quality and carrying capacity, or it could simply reflect scaling patterns in habitat selection.
A 14 300‐Yr Paleoecological Profile of a Lowland Tropical Lake in Panama
Authors: Bush Mark B. , Piperno Dolores R. , Colinvaux Paul A. , De Oliveira Paulo E. , Krissek Lawrence A. , Miller Michael C. , Rowe William E.
Published: April 2006

The first paleoecological analysis of a complete sedimentary record spanning the period from the late Pleistocene to the present from lowland Panama, documents changes in lowland vegetation communities through major climatic change and the onset of human disturbance. Past sympatry is found among presently allopatric species, suggesting that tropical forest communities are not species—stable through time. Late Pleistocene floras at Lake La Yeguada (elevation 650 m), Panama, had high relative abundance of montane forest elements, e.g., Quercus and Magnolia, existing some 900 m below their present range, suggesting a climatic cooling of ≈5°C below present. This descent of montane forest taxa onto lowland hilltops denied the ground to postulated lowland rain forest refugia. The late Pleistocene (14 350—11 050 yr BP) was not uniformly cool and was interrupted by brief phases of near present—day warming. The onset of the Holocene was abrupt, taking <100 yr, and was almost coincidental with the start of human forest disturbance. Changes in climate at La Yeguada were found to be largely synchronous with those documented at Lake Valencia, Venezuela, but no fine—scale climatic synchrony was apparent with South American or European sites, and significant departures from the predictions of published climatic circulation models are found.
Geostatistical Tools for Modeling and Interpreting Ecological Spatial Dependence
Authors: Rossi Richard E. , Mulla David J. , Journel Andre G. , Franz Eldon H.
Published: April 2006

Geostatistics brings to ecology novel tools for the interpretation of spatial patterns of organisms, of the numerous environmental components with which they interact, and of the joint spatial dependence between organisms and their environment. The purpose of this paper is to use data from the ecological literature as well as from original research to provide a comprehensive and easily understood analysis of geostatistics' manner of modeling and methods. The traditional geostatistical tool, the variogram, a tool that is beginning to be used in ecology, is shown to provide an incomplete and misleading summary of spatial pattern when local means and variances change. Use of the non—ergodic covariance and correlogram provides a more effective description of lag—to—lag spatial dependence because the changing local means and variances are accounted for. Indicator transformations capture the spatial patterns of nominal ecological variables like gene frequencies and the presence/absence of an organism and of subgroups of a population like large or small individuals. Robust variogram measures are shown to be useful in data sets that contain many data outliers. Appropriate removal of outliers reveals latent spatial dependence and patterns. Cross—variograms, cross—covariances, and cross—correlograms define the joint spatial dependence between co—occurring organisms. The results of all of these analyses bring new insights into the spatial relations of organisms in their environment.
Adaptation to Single Resources and the Evolution of Crossbill (Loxia) Diversity
Authors: Benkman Craig W.
Published: April 2006

I quantitatively test the hypothesis that four taxa or "types" (species or subspecies) of Red Crossbills (Loxia curvirostra) in the Pacific Northwest have diversified morphologically in bill characters in response to alternative adaptive peaks presented by their food: seeds in conifer cones. Hypothetically, each adaptive peak corresponds to one conifer species whose seeds are (1) produced regularly from year to year, (2) held in cones through late winter when seed is most limiting, and (3) protected from depletion by potential noncrossbill competitors. Four such conifers, termed "key conifers," are present (Tsuga heterophylla, Pseudotsuga menziesii, Pinus ponderosa, and Pinus contorta var. latifolia). I use data on foraging efficiency for 31 captive crossbills of four types to determine the optimal bill size and palate structure for foraging on the key conifers. As predicted, if each type is adapted for foraging on a key conifer, the observed morphology of a given type is often the predicted optimal size morphology of foraging on its respective key conifer. Two of the types have mean bill sizes (bill depth) equalling their predicted optimal size. For one of the remaining types, the observed differs from the optimum by 0.4 mm; I was unable to predict an optimal size for the remaining type. Optimal bill size varies with season. Bill sizes corresponded more closely to the optima for winter (lean period) than for summer. Observed mean width of the palate groove, in which crossbills hold conifer seeds while the seeds are being husked, was consistently close to the estimated optimal groove width. Optimal groove width was correlated (r2 = 1.00, n = 4) with seed size (cube root of mass), suggesting optimal groove width is determined by seed size. Overall, each crossbill type has either the optimal bill size or optimal husking groove width, or both, for foraging on their key conifers. Fitness set analyses indicate that there are substantial trade—offs in foraging efficiency. The best phenotype for foraging on one conifer is often only one—half as efficient on other conifers. All four fitness sets are concave, implying selection against intermediate phenotypes. I conclude, first, that reliability of seeds on key conifers during periods of food scarcity is a critical feature in the ecology and evolution of crossbills. Second, optimization of morphological traits occurs even in populations in highly variable environments. Third, disruptive selection against intermediate phenotypes is likely. This should maintain, if not reinforce, the distinctiveness of types. Fourth, the diversity of cone structure and seed size among key conifers is ultimately responsible for the diversification of crossbills.
Simple Methods for Calculating Age‐Based Life History Parameters for Stage‐Structured Populations
Authors: Cochran Margaret E. , Ellner Stephen
Published: April 2006

Stage—classified matrix models are important analytical and theoretical tools for the study of population dynamics; in particular, these models may be appropriate for populations in which survivorship and fecundity are dependent on size or developmental stage, populations in which the age of individuals is difficult to determine, and populations in which there are multiple types of newborns. Nevertheless, methods for analyzing the implications of a population's stage—transition matrix have been limited in comparison to methods available for age—structured models (life tables or Leslie matrices). In this paper we show that all of the standard age—based measures of life history traits can be derived from a stage—transition model. By decomposing the transition matrix into separate birth, survival, and fission matrices we derive simple, direct formulas for age—based life history traits such as the discrete survivorship function, lx, maternity function, fx, mean age at maturity, and net reproductive rate, Ro, and also population parameters, including the stable age distribution, age—specific reproductive value, and generation time. These provide a common set of parameters for comparing age—structured and stage—structured populations or comparing populations with differently structured life cycles. In addition, we define four measures of age and life—span that summarize the relationship between stage and age in a stage—structured population: age distribution and mean age of residence for each stage class, expected remaining life—span for individuals in each stage class, and total life—span conditional on reaching a given stage class. We illustrate the use of our methods to address specific ecological questions by applying them to several previously published demographic data sets. These questions include: (1) what are the demographic effects of crowding on the tropical palm Astrocaryum mexicanum?; (2) how important is the initial rosette size in determining life history of teasel, Dipsacus sylvestris?; and (3) how old are reproducing adults in a stage—classified population of pink lady's—slipper, Cypripedium acaule? Our results may also be useful for evaluating the adequacy of a given stage—transition model.
Woody Plant Regeneration in Four Floodplain Forests
Authors: Jones Robert H. , Sharitz Rebecca R. , Dixon Philip M. , Segal Debra S. , Schneider Rebecca L.
Published: April 2006

Between 1987 and 1990, we estimated seedfall and recorded age, growth, and survival of 10 933 tree and vine seedlings growing in the understories of four bottom land hardwood forests in South Carolina. The forests differed in flood frequency, soils, and vegetation structure but had a number of woody plant species in common. Several demographic processes were consistent for all four forests as well as for floodplain forests described in other published studies. Smaller seeded species had larger numbers of seeds dispersed, germinants, and established seedlings. Seed size, however, was not clearly related to seedling survival. Published rankings of shade and flood tolerances were also unrelated to survival, at least during the first growing season after germination. Seedling survival rates were least during the first growing season and greater in subsequent years. Within a growing season, early germinants had greater survival. For some species, survival was negatively related to basal area of neighboring conspecific adult trees. Some aspects of the regeneration process were more site specific. Within species, seedfall densities relative to adult tree abundance differed across forests by an order of magnitude. First—year seedling mortality rates were significantly affected by site and site x species interactions. Mortality in subsequent years was also significantly affected by site. Although the role of flooding in site—specific mortality was not clear, small elevation changes within flooded sites were correlated with changes in germination and survival for some species.
Ecological Shift and Release in an Isolated Population: Conus miliaris at Easter Island
Authors: Kohn Alan J.
Published: April 2006

With the exception of a recently discovered rare species, the gastropod genus Conus is represented only by Conus miliaris at Easter Island, and isolated outpost of the Indo—West Pacific (IWP) marine biota probably <2.5 million yr old. Throughout most of the IWP region. C. miliaris co—occurs with 4—23 congeners on marine benches and coral reefs. Compared with these central populations, Easter Island C. miliaris (1) is more abundant in favorable habitats, (2) utilizes different habitat and food resources, and (3) is ecologically released with respect to food and probably depth range, but not microhabitat. Most striking, the major prey species (Onuphis sp.; 31% of the diet) belongs to a family of polychaetes not known to be exploited by any Conus species elsewhere. In all, 6 species of polychaetes eaten rarely or not at all by C. miliaris elsewhere comprise 77% of its diet at Easter Island. Sixteen prey species are eaten at Easter Island (vs. 2—8 in central IWP localities) and prey species diversity measured by the H' statistic is 2.0 vs. 0.2—1.3. Shifts to larger prey species with increasing size of C. miliaris account for some of the high prey diversity, but analysis of multiple feedings revealed that 68% of such individuals had eaten polychaetes of 2 or more species. Several polychaete species that C. miliaris preys on the Easter Island but not elsewhere are important components of the diets of other Conus species in the central IWP. Some of the ecological expansion of C. miliaris at Easter Island is thus directed toward resources that absent congeners would be expected to utilize were they present, or if they should colonize the island in the future. It is suggested that the virtual absence of congeneric species and increased intraspecific competition in dense populations at Easter Island have selected for the observed ecological expansion.
Ecology and Resource Partitioning among Nine Species of African Rain Forest Squirrels
Authors: Emmons Louise H.
Published: April 2006

Nine species of sympatric rain forest tree squirrels were studied in the field and in captivity at Makokou, Gabon. The findings on resource partitioning among them include: (1) two species are restricted to special habitat types. The other seven which commonly co—occur in the mature rain forest formation represent a maximum of taxonomic diversity. (2) Differential use of the vertical vegetation column divides these seven species into four essentially arboreal and three ground—foraging forms. (3) The species occupying a given foraging level and habitat differ in body size by approximately Hutchinson's ratio. Body size can act to partition fruit resources by size and hardness. (4) The squirrels are frugivores or omnivores; there are quantitative differences in diet between some species. These differences vary seasonally and may only be important during the most difficult season. (5) The four largest species have short active periods. I suggest that the greater species richness in African compared to Nearctic forests is permitted by a more constant productivity of fruit and arthropods, and a higher plant species diversity in the African forest.
The Natural Forests of Maryland: An Explanation of the Vegetation Map of Maryland
Authors: Brush Grace S. , Lenk Cecilia , Smith Joanne
Published: April 2006

The forests of Maryland have been mapped at a scale of 1:250 000 on the basis of 15 regional associations: (1) tamarack, (2) bald cypress, (3) hemlock—yellow birch—black birch, (4) river birch—sycamore (5) sycamore—green ash—box elder—silver maple, (6) shingle oak, (7) chestnut oak—post oak—blackjack oak, (8) chestnut oak—bear oak, (9) chestnut oak, (10) loblolly pine, (11) basket oak—loblolly pine, (12) willow oak—loblolly pine, (13) basket oak, (14) sugar maple—basswood, and (15) tulip poplar. The associations differ from each other in species composition and in abundances of species common to many associations. Each was identified in the field by the presence of relatively few common discontinuous tree species referred to as characteristic species. Correlations between forest associations and geologic, topographic, and soils units mapped at a similar scale suggest that patterns of available water are important in controlling distribution of woody species throughout Maryland.
The Role of Habitat Patchiness in the Population Dynamics of Snowshoe Hares
Authors: Wolff Jerry O.
Published: April 2006

Population densities of snowshoe hares in Alaska fluctuate on a 10—yr cycle. A snowshoe hare population was studied near Fairbanks, Alaska USA from 1971 through 1977 during the peak, decline, low, and early increase phases of the cycle. The mosaic of habitat types was utilized by hares according to the amounts of available food and cover. Habitats utilized were from the most dense to the least dense during the increase phase of the cycle and subsided in reverse order during the population decline. Presaturation dispersal was the causal force for habitat expansion during the increase phase of the cycle, while predation was the primary force for habitat contraction during the population decline. Snowshoe hares tend to move from winter to summer ranges in response to availability of food and cover. A patchy environment which provides refuges like dense black spruce or willow—alder thickets in winter and open summer range allow hares to shift their use seasonally in response to changes in diet and to take advantage of changing environmental conditions. During summer, hares feed on herbaceous material and low shrubs in open areas; during winter, when herbs and shrubs are covered with snow, they move into the dense spruce thickets to feed on spruce, willow, and alder. During periods of high hare densities, browsing intensity reached 100% in all habitat types and diameter at point of browsing exceeded 10 mm. Both indices suggest that a hare—vegetation interaction can initiate cyclic decline. These conclusions are consistent with Keith's model for the 10—yr cycle of snowshoe hares. Dense spruce or willow—alder thickets play an important role in snowshoe hare cycles in that they provide protective cover from predators during a population crash. Snowshoe hares, which are thus able to avoid local extinction by seeking these refuges, provide the stock for subsequent population increase. In the northern part of their range snowshoe hare populations are cyclic but in the southern part they are not. The cycle in the north is regulated in part by the presence of obligate hare predators which exhibit a delayed density—dependent cycle with snowshoe hares and the existence of a vegetational mosaic which provides both suboptimal and optimal habitats. The discontinuous coniferous forests, the facultative resident hare predators, and the presence of at least three other lagomorphs are all proximate factors which help to maintain a stable hare population in the southern portions of its range.
Comparative Influences of Light and Temperature on the Growth and Metabolism of Selected Submersed Freshwater Macrophytes
Authors: Barko John W. , Smart R. Michael
Published: April 2006

Under controlled experimental conditions, the influences of light and temperature on the growth and metabolism of Egeria densa Planch, Hydrilla verticillata Royle, and Myriophyllum spicatum L. were comparatively examined. Light was controlled at six levels ranging between 5 and 75% of full sunlight at solar noon. Water temperature was controlled at five levels ranging between 16° and 32°C. Growth considerations included morphology, biomass, and nutrition. Photosynthesis, respiration, and CO2 compensation points were determined to evaluate physiological differences in plant growth as affected by the experimental ranges of temperature and light. External morphology in these species was significantly affected by the different experimental light and temperature conditions. Both low light and high temperature promoted extensive shoot elongation and associated canopy formation. Biomass production and carbon metabolism in all species were influenced more by temperature than by light. Each of the species demonstrated metabolic acclimation to light over a broad range. Conversely, the macrophyte species considered here were not strictly capable of acclimating to temperature. Growth rate and the seasonal progression of senescence were interrelated in these species. Higher temperatures stimulated growth and promoted a compression of the growth cycle. The relationship between photosynthesis and respiration (P:R) was appreciably reduced by senescence, but the CO2 compensation point did not reflect this condition. In the species examined, CO2 compensation points decreased with increasing temperature, suggesting adaptations to low free CO2 levels in the environment. Latitudinal differences in integral seasonal temperature, in relation to species—specific ranges of thermal tolerance, appear to be important in influencing the geographical distribution of the species considered here. Light may be the primary determinant of their depth distribution, but its importance in this regard could be somewhat diminished by their significant abilities to extend to the water surface under low light conditions.
Pollination Ecology of the Spring Wildflower Community of a Temperate Deciduous Forest
Authors: Motten Alexander F.
Published: April 2006

I studied the spring wildflower community of mesic deciduous forests in piedmont North Carolina to determine (a) the extent to which fecundity is pollination—limited in the community, (b) the importance of competition for pollination in affecting seed—set, and (c) the characteristics of plants and their floral visitors that most contribute to full pollination. Although inadequate pollination seems likely in the community, supplemental hand—pollination significantly improved fecundity in just 3 of the 12 species I examined. Pollination—limited reproductive success was evident only in a distinctive subset of the community, species pollinated primarily by queen bumble bees. The majority of wildflower species are pollinated by flies and solitary bees. Measurements of visitation rates and pollinator effectiveness on these plants confirmed that they are usually adequately pollinated in spite of a short blooming season, considerable overlap in flowering times, extensive pollinator sharing by concurrently blooming species, and inclement weather that frequently interrupts insect activity. Many of the flies and solitary bees are inconstant foragers, yet competition for pollination among wildflower species through differential pollinator attraction or interspecific pollinator movements usually does not significantly decrease the seed—set of plants with shared visitors. Competition may act with other causes of insufficient pollination, however, as a selective force to maintain a characteristic set of floral biology traits within the community, including autogamy and self—compatibility, extended receptivity, and pollination by a variety of visitor types. That these floral traits contribute significantly to the successful pollination of vernal herbs was demonstrated by observations of visitor behavior, plant caging experiments that excluded visitors or restricted their access to selected flowers, and measurements of floral lifetimes and seed—set for individual plants. These traits are effective regardless of the source of pollination—limited fecundity, and it is the prevalence of such traits, rather than floral specialization or character displacement, that distinguishes the forest spring wildflower community from other communities with potentially inadequate pollinator service.
A Hierarchical Model of Lotic Ecosystems
Authors: McIntire C. David , Colby Jonathan A.
Published: April 2006

This paper presents the structure and properties of a total stream model that simulates the dynamics of small, flowing—water ecosystems in the northwestern United States. Conceptually, the model is hierarchically structured, consisting of 7 basic processes: periphyton dynamics, grazing, shredding, collecting, invertebrate predation, vertebrate predation, and detrital conditioning. These processes are subprocesses of 3 echelons of higher level processes: detritivory; herbivory; primary consumption; predation; and the total ecosystem. The model has 14 state variables in the 7 basic processes, and is conceptualized in discrete time with a basic time step corresponding to 1 day. Behavior of the stream model relative to different schedules of energy inputs and to the practice of clear—cut logging was investigated and related to contemporary theory of lotic ecosystems. In general, model behavior indicated that the regulation of biological processes in streams is complex, the mechanisms of which vary seasonally and from process to process. If a process is regulated by food supply, its annual production tends to increase as predation increases, while mean biomass may or may not be affected appreciably. In contrast, an increase in predation tends to decrease both mean biomass and annual production in processes regulated primarily by predation and such life history phenomena as insects emergence. The stream model provided the stimulus that led to a mathematical expression for the rate of production at the level of the entire ecosystem, and model behavior suggests that this rate tends to remain constant along a continuum from small, first—order streams with no tributaries to larger rivers which eventually drain into the sea.
Species Number and Compositional Similarity of the Galapagos Flora and Avifauna
Authors: Connor Edward F. , Simberloff Daniel
Published: April 2006

The flora and avifauna of the Galapagos Islands are used to reexamine quantitative analyses of species numbers and compositional similarities. Conflicts in the results of previous analyses of the Galapagos flora are reconciled and are shown to be the result of using different species numbers and physiographic parameter measurements. The idea that Galapagos plant species numbers largely reflect conditions in the archipelago during the Pleistocene glaciations (i.e., that relaxation times are very long) is criticized, and the more parsimonious alternative, that they reflect recent conditions, is proposed. Generalizations about factors determining species numbers that are based on multiple regression and correlation are precarious. The number of botanical collecting trips to each of the Galapagos Islands is a better predictor of species numbers than are area, elevation, or isolation. Two null hypotheses concerning the determination of floral and avifaunal compositional similarities among the Galapagos Islands are proposed and tested, through the application of a new method of similarity analysis employing the "expected number of taxa shared" (Ets) and its variance (VEts). These hypotheses view compositional similarities as resulting from the stochastic dispersal and persistence of a pool of species equally likely to colonize (Null Hypothesis I), and of a pool of species with different dispersal and persistence abilities (Null Hypothesis II). Although both hypotheses are found to be inadequate models of compositional similarity in the Galapagos, the results suggest that a substantial proportion of compositional similarity can be considered a consequence of stochastic processes of dispersal and persistence, and that compositional similarity arises indirectly as a result of similar species numbers rather than directly in a fashion determined by the physical environment or species interactions. Analyses of the distribution of congeneric bird species in the Galapagos, based on similarity analyses, are also criticized.
A Comparison of the Structure, Primary Productivity, and Transpiration of Cypress Ecosystems in Florida
Authors: Brown Sandra
Published: April 2006

To investigate how inputs of water and nutrients influence the structural and functional characteristics of cypress wetlands, primary productivity, respiration, transpiration, tree growth, biomass, basal area, stem density, standing stocks of phosphorus, and litterfall were measured in several kinds of Florida cypress ecosystems subjected to varying inflows of water and nutrients. A scrub cypress forest, natural and sewage—enriched cypress domes, and a cypress floodplain forest were studied. Floodwaters in these ecosystems ranged from stagnant and phosphorus poor (0.01—0.08 mg P/L) to flowing and phosphorus rich (0.66—5.86 mg P/L). Inputs of total phosphorus transported by water to these forests varied from 0.11 g P°m—2°yr—1 in the scrub cypress forest to 1625 g P°m—2°yr—1 in the floodplain forest. Phosphorus content of aboveground biomass (ranging from 0.25 to 4.78 g P/m2) appeared to be positively related to phosphorus inputs. Biomass and other structural characteristics of the study sites appeared to be related to stand history rather than to phosphorus inputs. For example, there was little difference in aboveground biomass between natural cypress domes (20.6—26.6 kg/m2), a sewage—enriched dome (21.7 kg/m2), and a floodplain forest (28.4 kg/m2). Aboveground biomass for the scrub cypress forest was low (3.6 kg/m2). Net daytime photosynthesis (2.1—13.7 g C°m—2 ground surface°d—1), plant respiration (0.9—10.9 g C°m—2) ground surface°d—1) and an estimate of gross primary productivity increased with increasing inputs of total phosphorus. Aboveground biomass production (sum of wood production, ranging from 44 to 1080 g°m—2°yr—1, and litterfall, ranging from 224 to 941 g°m—2°yr—1) increased with increasing total phosphorus inputs in the low range, but biomass production leveled in the high input range. Total water loss from cypress forests increased with increasing P inputs. Mean daily transpiration rates from the scrub cypress forest (1.0 mm) and cypress domes (3.1—3.8 mm) were lower while those from the floodplain forest (5.6 mm) were similar to evaporation rates from open water bodies. Ratios of transpiration to net daytime photosynthesis were also low (156—221 g H2O/g organic matter), showing that cypress is efficient in its water use. In cypress domes and scrub cypress forest, where water may be limiting, trees appear to adjust t potential water stress through leaf morphology adaptations and minimum canopy development (leaf area index, LAI = 0.53—3.4 m2/m2). When water is not potentially limiting, as in the floodplain forest, adaptations to conserve water were lacking (e.g., LAI = 8.5 m2/m2). A dome with added sewage effluent had higher leaf biomass, leaf area index, and chlorophyll ° content, and more phosphorus stored in leaves than untreated domes. Net primary productivity, litter production, and wood production increased approximately twofold over pre—effluent rates or untreated cypress domes.
Ecological History of Lake Valencia, Venezuela: Interpretation of Animal Microfossils and Some Chemical, Physical, and Geological Features
Authors: Binford Michael W.
Published: April 2006

The developmental history of Lake Valencia, Venezuela, over the past 12 500 yr was studied by means of animal remains and some chemical and physical variables in three sediment cores. Radiocarbon dates of abandoned beach terraces also provided information for the interpretation of former lake levels. Transects of surficial sediments in the lake served as modern analogues for both animal communities in the sediments and characteristics of the sediment itself. Interpretation of cluster and multiple discriminant analyses led to the recognition of six stratigraphic zones of animal microfossils. The zones were expressions of climatically controlled fluctuations of water levels that have dominated successional processes within Lake Valencia. The animal remains were mostly carapaces of Ostracoda and Cladocera, with fewer head capsules and mouthparts of immature insects and flatworm (neorhabdocoel) egg cases. Study of the variation of animal assemblages with various environmental requirements led to the following interpretation of the zones. The basin was dry from at least 13 000 to 11 000 yr before present (BP). It filled rapidly °10 500 yr BP, and had an outflow for 2500 yr. The level then gradually declined during the time from 8000 yr BP to 2500 yr BP, but was periodically stable during which intervals several terraces were cut. The next 2000 yr included several rises, some to the outflow. As late as 1727 AD the lake was again at the outflow level, after which it began to desiccate. Cultural influences, both indigenous and European, are expressed in the sediments of the past 2000 yr. In 1979 the level was °25 m below the outlet and had been dropping at °0.1—0.2 m/yr.
Abstract Spaces: A Workshop Approach
Authors: Cole David
Published: April 2006

I am going to describe a workshop technique that makes the abstractions of conceptual thought available as physical experiences, and in so doing, makes abstraction itself—usually thought of as a danger to theatre work—available to theatre, both as a subject matter and as an untapped source of performance energies.

There is a tendency, in most branches of contemporary thought, to use metaphors of space for all sorts of concepts and relationships that are not in themselves spatial. Literary critics locate the nuances of a word in “semantic space.” Psychiatrists refer to the “therapeutic space” in which they encounter their patients. Anthropologists imagine networks of kin-relationships spread out in “social space.” Musicologists speak of a “harmonic space” within which tones and timbres are distributed.