bluuebunny/crossref_metadata_2025 huggingface dataset:
Measures of Niche Overlap
Authors: Slobodchikoff C. N. , Schulz William C.
Published: May 2006

This paper examines the mathematical bases of three commonly used measures of niche overlap along a single resource dimension: curve averaging, asymmetrical α and symmetrical α. Also, we examine the common methods (product α, summation α) for calculating overlap along two or more resource dimensions. The advantage of curve averaging is that it has a simple transition from discrete to continuous data and a simple geometric interpretation, but the method is relatively difficult to apply to a combined overlap along several resource dimensions. The symmetrical α method of computing overlap along a single resource dimension is useful biologically in that it: (1) has a convenient geometric interpretation; (2) is applicable for both discrete and continuous data; (3) provide a mathematical basis for computing overlap along several resource dimensions; and (4) provides a mathematical basis for testing whether patterns of resource use along different resource dimensions are independent. A test for resource independence is developed in this paper. The symmetrical α method, however, measures the patterns of resource uses, and not the actual competitive pressure resulting from niche overlap. A better estimator of competitive pressure is the asymmetrical α method.
Production of Coastal Louisiana Marsh Plants Calculated from Phenometric Techniques
Authors: Hopkinson Charles S. , Gosselink J. G. , Parrondo R. T.
Published: May 2006

Production and mortality of six marsh macrophytes were measured in Louisiana, USA using phenometric methods. The Williams—Murdoch technique estimates annual production from measurements of individual culm height, mass, growth, and longevity. Annual mortality was estimated from bimonthly field measurements of culm height, mass, and death rate as a function of height, and density. Annual production estimates averaged 1.8 times higher than annual mortality estimates although in a steady—state system the rates should be about equal. A probable and correctable reason for the discrepancy is discussed. These estimates are higher than those calculated from Smalley's method but lower than those obtained from the Wiegert—Evans paired—plot technique. The validity of various techniques are applied to wetlands is discussed.
Hatching Success and Nest Predation in the Green Sea Turtle, Chelonia Mydas, at Tortuguero, Costa Rica
Authors: Fowler Lynn E.
Published: May 2006

Green turtle hatching success and nest predation were investigated at Tortuguero, Costa Rica, during July—November 1977. Forty—two percent of 350 study area nests and 57% of 237 beach survey nests produced emerging young; 38% and 24%, respectively, were destroyed by dogs, coatis, and vultures. The mean emergence percentage for the successful study area nests was 83%. About 13% of all eggs deposited did not hatch. A mean incubation period of 62 d and a mean clutch size of 104 eggs were recorded. Emergence success was not influenced by other recorded parameters (nest position on beach, rainfall, turtle's tag year, time of season, incubation period, and clutch size). Incubation period was related to nest position and clutch size. Dogs, coatis, and black and turkey vultures were the chief predators at Tortuguero; dogs did the most damage. Dogs and coatis found nests at all stages of development, but destroyed more nests containing hatchlings than nests containing unhatched eggs. Predation was related to nest position, but not to nest density. Nests were destroyed in equal proportion on the entire 35.4 km of beach. Predator activity was not consistent throughout the season; proportionally more nests were destroyed near the end of the nesting season than during the beginning.
Influences of Seasonal Changes in Leaf Morphology on Water‐Use Efficiency For Three Desert Broadleaf Shrubs
Authors: Smith William K. , Nobel Park S.
Published: May 2006

The influences of seasonal variations in leaf length, pubescence, and the ratio of internal to external leaf area (Ames/A) on transpiration and photosynthesis were evaluated for 3 desert broad—leafs (Encelia farinosa [Grey], Hyptis emoryi [Tour.], and Mirabilis tenuiloba [Wats.]). Field data were incorporated into energy balance and gas exchange equations to determine water—use efficiencies (mass CO2 fixed/mass H2O transpired) at various times throughout a year. These results were compared with simulated values for leaves without the observed seasonal changes in morphology. For Encelia, leaf lengths were over 3—fold greater during the winter and spring than during the drier summer and fall (4.0 cm vs. 1.2cm). Increases in pubescence during dry periods led to substantial decreases in the leaf absorptance to solar irradiation (from 0.61 to 0.43). Similar alterations occurred for Hyptis, but with less month—to—month variation, while Mirabilis had comparatively small changes in these leaf parameters. For Mirabilis Ames/A was about 75% greater (maximum values of °70 vs. 40 for Encelia and Hyptis) and varied less (9% variation) during the year compared with Encelia and Hyptis (46% and 24% variation, respectively). Minimum leaf resistances to H2O vapor diffusion (Rwv) were similar for Encelia and Hyptis throughout most of the year, while Mirabilis had much higher values. The optimal temperature for photosynthesis was several degrees higher for Mirabilis (34°C vs. 30°C for Encelia and Hyptis). The calculated cellular resistance for CO2 diffusion (RcellCO2) was lowest for Mirabilis (147 s/cm) and almost identical for Encelia and Hyptis (282 s/cm). Encelia, and to a lesser degree Hyptis, had maximum photosynthetic rates at a lower water—use efficiency when water was available and greatest water—use efficiency at reduced photosynthetic levels during drier periods of the year. During a shorter growth period in the wet portion of the year, Mirabilis had a consistently high water—use efficiency (0.16) and a moderate photosynthetic rate, reflecting its much higher Ames/A and Rwv combined with a lower RcellCO2. Simulations for leaves without the observed changes in morphology that occurred during the drier months showed that the mean water—use efficiency for the year would have been °48% less for Encelia, 41% less for Hyptis, and only 3% less for Mirabilis. A decrease in leaf length, a decrease in absorptance, and an increase in Ames/A contributed about equally to the estimated seasonal increases in water—use efficiency for Encelia and Hyptis. Simulated transpiration and photosynthesis for leaves with morphology characteristic of drier periods resulted in water—use efficiencies similar to those for natural conditions, but annual photosynthesis was reduced °24% for Encelia, 17% for Hyptis, and <2% for Mirabilis.
Inferring the Properties of Predation and Other Injury‐Producing Agents from Injury Frequencies
Authors: Schoener Thomas W.
Published: May 2006

This paper analyzes how cumulative injury data in age—structured populations can be used to determine some properties of the injury—producing process, such as predation. If the injury—producing process is the only source of mortality or injury, raw frequencies of animals free from injury (e.g., tail breaks) are shown in a theoretical analysis to equal the efficiency of the injury—producing process (e.g., the fraction of predation attempts that result in death of the prey); they are not affected by the intensity of the injury—producing process. If alternate sources of mortality exist, this conclusion may still be approximately true unless those sources are large. If there exists alternative sources of injury that never result in death, raw frequencies of uninjured animals actually increase with increasing intensity of the process that produces both injury and death (e.g., predation). To estimate the intensity of the injury—producing and the per—unit—time rate of accumulating injuries in the simplest case, one needs both injury and survival frequencies. Formulae for these estimations are provided.
Competitive Interactions among Tadpoles: Responses to Resource Level
Authors: Steinwascher Kurt
Published: May 2006

Using experimental populations of Rana clamitans tadpoles, I examined the hypothesis that the nature of competitive interactions among individuals changes in response to the amount and distribution of the food resource. Competitive interactions within populations were dominated by the largest individuals. Large tadpoles outcompeted small tadpoles for the solid food resource. Small tadpoles ordinarily engaged in filter feeding and were thus exposed to relatively high concentrations to parastic yeast, Candida humicola. At lower food concentrations, large tadpoles responded by increasing the amount of filter feeding relative to substrate feeding, which increase the parasite population growth rate. Small tadpoles subsequently experienced a disproportionate increase in their parasite burden which decreased their competitive abilities relative to the large tadpoles. The aspect of the food resource which alters these competitive interactions appears to be the bivarate distribution of food items in time and space. The results of this investigation imply that deterministic models may be inadequate to predict outcomes even in laboratory situations because organism seem to be maximizing their probability of success conditional on their current situation.
Factors Controlling Transpiration and Photosynthesis in Tamarax Chinensis Lour.
Authors: Anderson Jay E.
Published: May 2006

Photosynthetic and stomatal responses of Tamarix chinensis to temperature, light, and humidity were investigated in the field in New Mexico and in the laboratory. Transpiration rates for T. chinensis were similar to those of several herbaceous plants and co—occurring phreatophytes. Net photosynthetic rates and water use efficiency of T. chinesis were lower than for other species. Photosynthesis was light saturated at a photon flux density equal to 44% of full sunlight. Carbon dioxide assimiliation was tightly coupled to irradiance below light saturation. Leaf resistances remained low at photon flux densities above one—third of full sunlight, but increased linearly with decreasing photon flux density below that level. Shading for 5 min resulted in a doubling of leaf resistance. The rapid response of stomata to changing light conditions is probably an adaptation to conserve moisture when light is limiting to photosynthesis. Optimal leaf temperatures for photosynthesis were 23°—28° C, which correspond roughly to ambient temperatures during the early part of the day when evaporative demand was relatively low. T. chinensis stomata appeared to respond directly to changes in the leaf—to—air absolute humidity gradient. At constant temperature, leaf resistance increased linearly with increases in the leaf—air humidity gradient. Midday depressions of gas exchange invariably occurred in the field, despite the fact that the plants had an abundant water supply. These depressions resulted from increases in leaf resistance in response to increasing evaporative demand of the air. This response results in improved water use efficiency during the hottest portion of the day. Plant water potential decreased from pre—dawn values of about —0.9 MPa to minimal values of about —2.6 MPa by midmorning. Improvements in bulk water status were often observed during the afternoon when leaf resistances were higher. Diurnal patterns suggested that leaf resistance was largely a function of temperature, light, and humidity, rather than plant water status.
Shoreline Vegetation in Axe Lake, Ontario: Effects of Exposure on Zonation Patterns
Authors: Keddy Paul A.
Published: May 2006

This study tests several hypotheses relating community structure to environmental disturbance, by testing for changes in the zonation patterns of lakeshore vegetation. Twenty—five transects were examined at different positions along an exposure gradient. The range of water depths tolerated by individual species (realized niche width) changes with exposure. Some (Lobelia dortmanna, Utricularia cornuta) reached their maximum on exposed shores, others (Drosera intermedia, Cladium mariscoides) at intermediate exposure, and others (Pontederia cordata, Triadenum fraseri) on sheltered shores. Species richness peaked significantly at intermediate levels of exposure (P < .01). In spite of changes in both species composition and richness, mean niche width did not change with exposure. The distribution of the upper and lower limits of species along the gradient was examined using measures of boundary clustering. The lower and upper boundaries of species were both significantly clustered (P < .001), suggesting discrete communities exist on the shoreline gradient. As exposure increased, upper boundaries became more cluster (P < .002); lower boundaries were unaffected. The distribution of boundaries shifted landward with increasing exposure. This fact is possibly related to an identical landward shift in the lower boundaries of shoreline shrubs, which appear to have a major influence on the distribution of herbaceous shoreline vegetation.
Some Comments on Measuring Niche Overlap
Authors: Abrams Peter
Published: May 2006

This article discusses the usefulness of niche overlap and niche overlap measures in studying competition. It is argued that (1) neither niche overlap nor niche overlap measures can be used to estimate the intensity of competition, but (2) niche overlap can be used to determine the relative amounts of inter— and intraspecific competition, while niche overlap measures are not useful in this regard. Before using overlap to estimate the relative intensities of inter— and intraspecific competition, it is necessary to have a model of the consumer—resource interaction, and some knowledge of the parameters in that model. The niche overlap and competition coefficient formulae proposed by Hurlbert (1978) are considered in some detail. Previously proposed niche overlap measures appear to be more useful than Hurlberts if one is interested in using the niche overlap measure for purely descriptive purposes.
Ecological Character Displacement
Authors: Slatkin Montogemery
Published: May 2006

A theoretical study is made of the conditions under which competition mediated by a quantitative character in each of two species will result in the convergence or divergence of the mean values of the character. Both within— and between—species competition are considered and a general model of the genetical basis of the character is used. Both the genetical and ecological parts of the model are based on the biological assumptions that are made in discussions of ecological character displacement, which include the assumptions that there are no constraints on the evolution of the character in either species and that individuals in each species with the same value of the character use the limiting resources in the same way. One of the main results is that the usual biological assumptions do not necessarily lead to permanent and significant displacement (i.e., d/w > 1). While some previous models have led to different conclusions, it is shown that those results are due to other, implicit, assumptions in those models. In the present model, two types of conditions lead to significant character displacement under biologically reasonable assumptions. One type of condition, which would apply to species that are roughly equally abundant at equilibrium, is that there be some differences in the set of limiting resources of the two species. In that case, the equilibrium displacement would be much larger than the differences between the resource spectra. Small and possibly unmeasurable differences between species could lead to significant displacements. The other type of resource distribution that leads to significant displacements is a skewed distribution. This requires no differences between the resource spectra for the two species but it leads to an equilibrium in which one of the species is much more abundant than the other. One consequence of these results is that the relative abundances of two species thought to have undergone character displacement are important to understand the possible mechanism leading to that displacement.
Changes in Chemical Composition of Food Parts in the Diet of Colobus Monkeys
Authors: Baranga Deborah
Published: May 2006

Foliage of Celtis durandii and Markhamia platycalyx are important foods of the red colobus (Colobus badius tephrosceles) and the black—and—white colobus (Colobus guereza occidentalis) at Kanyawara, Kibale Forest, Uganda. Seasonal variations in the chemical composition of this foliage were attributed to phenological changes. Moisture, crude protein, energy value, potassium, and phosphorus decreased, while acid detergent fiber, lignin, calcium, and sodium increased with advancing maturity. Intermonthly changes in plant chemistry of M. platycalyx were not so pronounced as in C. durandii; the latter also gave strong correlation among its chemical components, while similar correlations for M. platycalyx wee generally weak. The nutrient quality of leaves from C. durandii was highest during the brief period when buds (leaf and flower), flowers, and young leaves were most abundant. With increasing maturity, these leaves lost the characteristics that made them good forage. M. platycalyx, on the contrary, had foliage with varying nutrient quality. Intermonthly dietary variation of the colobus monkeys is governed by the interactions between food consumption rate and changing food quality, together with the availability of alternative food items.
Density Estimation of Small‐Mammal Populations Using a Trapping Web and Distance Sampling Methods
Authors: Anderson David R. , Burnham Kenneth P. , White Gary C. , Otis David L.
Published: May 2006

Distance sampling methodology is adapted to enable animal density (number per unit of area) to be estimated from capture—recapture and removal data. A trapping web design provides the link between capture data and distance sampling theory. The estimator of density is D = Mt+1f(0), where Mt+1 is the number of individuals captured and f(0) is computed from the Mt+1 distances from the web center to the traps in which those individuals were first captured. It is possible to check qualitatively the critical assumption on which the web design and the estimator are based. This is a conceptual paper outlining a new methodology, not a definitive investigation of the best specific way to implement this method. Several alternative sampling and analysis methods are possible within the general framework of distance sampling theory; a few alternatives are discussed and an example is given.
Photosynthesis, Respiration, and Phosphate Absorption by Carex Aquatilis Ecotypes along Latitudinal and Local Environmental Gradients
Authors: Chapin F. Stuart , Oechel Walter C.
Published: May 2006

The genetic and environmental determinants of photosynthesis, respiration, and phosphate absorption were studied in Carex aquatilis populations from diverse thermal and nutritional environments, using reciprocal transplants to subarctic hot spring and muskeg sites and temperate alpine and subalpine sites. Ecotypic differences in growth rate (300% variation) reported by Chapin and Chapin (1981) were more important than ecotypic differences in maximum photosynthetic rate (<20% variation) or temperature optimum (5°C variation) in explaining population differences in tiller size. Acclimatization of photosynthesis to different gardens was most pronounced in populations from thermally variable environments. Differences in photorespiration among populations and among gardens generally paralleled differences in photosynthesis. The temperature optimum for photosynthesis of each population in all gardens was higher than average air temperature in any site and may have represented a compromise between (1) effective photosynthesis and (2) high nitrogen requirement associated with a low temperature optimum in a cold environment. All processes associated with plant carbon balance (respiration rate of shoots, rhizomes, and roots, and photosynthesis) lacked marked ecotypic variation. In contrast, phosphate absorption rate differed substantially among populations grown in a common garden, being highest in populations evolving in cold, phosphate—deficient soils. Soil phosphate status was more important than soil temperature in eliciting this evolutionary response. In contrast, acclimatization of phosphate absorption by each population was more sensitive to temperature than to phosphate availability. Rates of both phosphate absorption and photosynthesis were highest in those populations that had the highest genetic potential for growth. We conclude that rates of physiological processes associated with resource gain or loss are closely correlated with growth rate and can be best understood by considering the total selective regime impinging upon the plant.
Behavioral Response of a Sit‐and‐Wait Predator to Spatial Variation in Foraging Gain
Authors: Olive Cader W.
Published: May 2006

The orb—weaving spiders Argiope trifasciata and Araneus trifolium were tested for specific behavioral responses to spatial variation in the net rate of energy return. In laboratory feeding experiments with both species, no changes in attack time, capture success, or web design were caused by increased experience with particular prey types. Separate field experiments in enclosures of old—field vegetation were conducted with Argiope trifasciata. Spiders were free to move between four contiguous quadrats which differed only in the ration fed to spiders. Length of stay at a site was positively correlated but probability of moving and distance moved were negatively correlated with ration level. Spiders moved more often than predicted by random expectation when current rations were less than previous rations. These responses produced an aggregation of spiders in quadrats with high ration levels over an 11—d—period. This increase in search intensity which increasing return rate is qualitatively similar to that observed in searching predators in previous studies.
Early Spring Herb Communities in Mesophytic Forests of the Great Lakes Region
Authors: Rogers Robert S.
Published: May 2006

Herb composition in early spring (before leafout of hardwood overstories) was sampled quantitatively in 60 little—disturbed mesophytic hardwood stands in the Great Lakes region, USA. Disturbed hardwood stands and stands containing evergreens (hemlock) in the overstory were described by releve. Importance of strictly vernal growth forms, early spring annuals and ephemeroid perennials, was correlated with total cover of herbs. In places with nutrient—poor mineral soil, slow rates of litter decomposition, compressed growing season, or evergreen shade, populations of vernal herbs were generally sprase, with summergreen and semi—evergreen taxa relatively important. In mesophytic woods occupying physiographically similar sites, the most significant regional differences in species importance are associated with differences in soil fertility rather than climate. Superimposed on the soil—related, specific compositional differences were gradual, more general, southeast to northwest decreases in species richness, total cover of herbs, and importance of strictly vernal growth forms (annuals and ephemeroids) relative to herbs with summergreen shoots. Within stand, the most significant influences on vernal herb composition were soil drainage and microtopography. Plants with a large proportion of shoots of recent seed origin (annuals and "weedy" perennials) were often strongly dominant in areas with recently exposed mineral soil, as at the edges of receding vernal pools. Plants with large subterranean organs were rare in such sites. Cover of vernal herbs tended to be high on soil mounds and very low in pits, but differences in species composition were not as well defined as they are in large—area uplands vs. broad poorly drained areas. Species abundant on large patches of bare soil (as at the edges of receding vernal pools) also tended to be common in disturbed stands. Long—lived perennials with large storage organs tended to be uncommon in disturbed stands.
Dynamics of Single‐Species Population Growth: Stability or Chaos?
Authors: Mueller Laurence D. , Ayala Francisco J.
Published: May 2006

We have examined stability at the carrying capacity for 25 genetically different populations of Drosophila melanogaster. In spite of their genetic heterogeneity, 20 of the populations yield stable equilibria and none have eigenvalues significantly greater than one. Computer simulations demonstrate how selection at the individual level may account for population stability (and, hence, that group selection is not necessary for the evolution of stability). Recent theoretical studies on density—dependent selection in random environments provide predictions consistent with our empirical findings.
Patterns of Furanocoumarin Distribution and Insect Herbivory in the Umbelliferae: Plant Chemistry and Community Structure
Authors: Berenbaum M.
Published: May 2006

A chemical survey of 12 species of Umbelliferae in Tompkins County, New York, USA, revealed that species possessing furanocoumarins are most frequently found in open habitats: either swamps or thickets (Sium, Cicuta, Angelica spp.) or disturbed areas such as roadsides or old fields (Conium, Pastinaca, Heracleum spp.). Woodland species (Osmorhiza, Ziza spp.) characteristically lack furanocoumarins, though they may possess dihydrofuranocoumarin glycosides. Restriction of furanocoumarin production to plants in high—light environments is in keeping with the presumed mechanism of furanocoumarin phototoxicity towards insects, that is, ultraviolet—activated cross linkage of insect DNA. Cluster analysis of the insects on these umbellifers demonstrates that insect herbivores distribute themselves according to host furanocoumarin chemistry. By using a weighting factor that takes into consideration the differential discriminatory powers of specialists and generalists, it was possible to show that the insect faunae of plants containing angular furanocoumarins are more similar to each other than they are to the faunae of plants lacking angular furanocoumarins. The same case holds for the faunae of plants containing linear furanocoumarins, and for plants lacking furanocoumarins altogether. The chemical complexity of a plant species is reflected in the degree of dietary specialization of its insect associates; the more complex the furanocoumarin chemistry, as measured by the number and type of structures, the more specialized the feeding habits of the insects on the plant. Thus, plants lacking furanocoumarins have faunae dominated by generalists while plants with both linear and angular furanocoumarins have faunae dominated by extreme specialists. On the community level, then, it appears that furanocoumarin chemistry can account for patterns in distribution and abundance of insect herbivores on Umbelliferae.
Winter Hardiness of Tree Species at High Altitudes in the East Himalaya, Nepal
Authors: Sakai Akira , Malla S. B.
Published: May 2006

Maximal winter hardiness of trees in high—altitude forest zones, especially near timberlines in the East Himalaya, was assessed. A timberline coniferous species of the East Himalaya, Abies spectabilis, was marginally hardy to —20° to —23°C. Larix potanini, a high—altitude species which is the least hardy species of the genus Larix, also showed the same range of hardiness. The hardiness of these species is comparable to the least hardy Japanese Abietoideae that are native to the warm—temperature forest zone. Subalpine rhododendrons such as Rododendron campanulatum, R. campylocarpum, etc., were the hardiest of the Himalayan rhododendrons, resisting —20° to —23° in their flower buds. These were less hardy than those occurring in the subalpine forests of Japan and North America. The flower buds of Himalayan rhododendrons ranging as high as 4500 m in the alpine zone, such as R. anthopogon, R. setosum, and R. lepidotum, were slightly hardier. Thus in the Himalayas, rhododendrons and coniferous species, except Pinus and shrubby Juniperus, seem not to have evolved species capable of resisting below —30°, unlike boreal and subalpine forests in central Japan, probably due to high humidity and moderate temperatures in the Himalayan winter, and floristic history.
The Nitrogen Budget of Thoreau's Bog
Authors: Hemond Harold F.
Published: May 2006

Dinitrogen fixation and atmospheric deposition are the dominant sources of nitrogen to Thoreau's Bog, contributing °1.0 and 0.7 g·m—2°yr—1, respectively. Denitrification must compete with nitrate uptake and the reduction of NO3— to NH4+; the latter process accounts for the fate of °25% of nitrate input to the bog. Gaseous ammonia is not an input, but appears to be lost in small amounts from the bog surface. Organic N is deposited into the peat by plants at an annual rate of almost mg/m2. About a third of this amount is "permanently" deposited, and the balance is recycled through mineralization and uptake. Compared to other ecosystems, the bog processes nitrogen in a parsimonious fashion. Annual fluxes are small compared to total N stored in the ecosystem, and nearly 80% of annual N input is retained with the system.
Differential Costs of Reproductive Effort for Male and Female Richardson's Ground Squirrels
Authors: Michener Gail R. , Locklear L.
Published: May 2006

The costs of reproductive effort for adult male and female Richardson's ground squirrels were compared to determine whether these costs differed in timing or magnitude in a manner related to sexual differences in mating and parental effort. Reproductive effort was assessed from 1982 to 1986 for a population of Richardson's ground squirrels in southern Alberta, Canada, by monitoring seasonal mortality schedules, fecundity, and changes in body mass and fat content. Although the adult sex ratio was female—biased, the operational sex ratio was male—biased. Males lost mass and sustained injuries as a result of male—male conflict during the mating season. From 50 to 79% of males disappeared annually during reproduction (between spring emergence and the end of the mating season), whereas <20% of females disappeared during reproduction (between spring emergence and litter emergence). Overwinter mortality rates were similar for adult males and females. Active seasons of adult males and females were of similar durations, but because reproductive effort finished earlier for males, they had a longer period in which to prepare for hibernation, and they entered hibernation (immerged) with larger fat stores. Juvenile males had a longer active season and attained a greater proportion of adult structural size than juvenile females (90 vs. 75%). Although males emerged from hibernation with more residual fat than females, fat stores were rapidly depleted during mating 2—4 wk postemergence, whereas females did not deplete residual fat until late lactation 7—8 wk postemergence. Survival rates of females that did and did not wean litters did not differ significantly, and survival rates of mothers were not significantly or negatively correlated with litter size or litter mass. Although litter size tended to regress toward the mean in consecutive years of reproduction, neither size nor mass of litter weaned negatively influenced litter size or mass in the subsequent year. Depletion of larger stored fat reserves and lower survival rate during reproduction indicate that, from both energetic and survival perspectives, mating effort is more costly for male Richardson's ground squirrels than parental effort is for females.
Food Limitation of Avian Reproduction: An Experiment with the Cactus Wren
Authors: Simons Laurie Stuart , Martin Thomas E.
Published: May 2006

Food limitation of reproduction during the nestling stage was tested by supplementing food only during this stage in Cactus Wrens. Parents offered supplemental food fed their young more (volume per unit time) than unsupplemented parents in 2 yr of experimentation. In 1986, food—supplemented nestlings had greater mass and linear measures, whereas in 1987, most food—supplemented nestlings had greater mass, but not linear dimensions, than their controls. In 1986, survivorship of young both in the nest and during 4—6 wk after fledging, was greater with food supplementation than without. In 1987, survivorship differences were not significant. Food supplementation enhanced annual reproduction of parents because more food—supplemented Cactus Wrens laid second clutches than controls in 1987, the only year when this was examined. We conclude that food availability during the nestlings stage can limit present reproduction, enhance the likelihood of additional reproduction, or both, in Cactus Wrens.
Variation in Food Quality and Temperature Constrain Foraging of Gregarious Caterpillars
Authors: Stamp Nancy E. , Bowers M. Deane
Published: May 2006

We examined the separate and combined effects of changing food quality, thermal environment, and larval behavior on the foraging pattern of a gregarious caterpillar. Hemileuca lucina (Saturniidae), which specializes on the shrub Spiraea latifolia (Rosaceae). First—instar larvae, which encounter only new leaves under field conditions, showed no preference for new leaves over mature ones. In contrast, third—instar larvae, which normally encounter both leaf types, exhibited a preference for new leaves, in both 12—h and instar length choice tests. In laboratory tests, larvae in groups were more likely to survive and exhibited less variation in biomass gained than solitary larvae. An experiment was conducted to determine the interactions of temperature (with daytime temperatures of 20°, 25°, or 30°C), group size (solitary or in groups of 10), and diet (new or mature leaves) on third—instar larvae. The effect of diet and group size on relative growth rate depended on the daytime temperature. At a daytime temperature of 20°, representing overcast and cool spring conditions, growth rates were similar for all group size—diet combinations. Thus, at such a cool temperature, there was no advantage in feeding on new leaves or in larval groups. In contrast, solitary larvae fed mature leaves gained less mass and developed more slowly than the other group size—diet combinations at the mean maximal day temperature (25°) for field conditions. At 30° representing the average temperature reached through basking, larvae eating new leaves grew significantly faster than those on mature leaves, regardless of larval group size. These results indicate that in the absence of predators, in which case larvae eat new leaves, bask, and remain aggregated, the larvae can grow twice as fast and attain more biomass per instar than solitary larvae eating mature leaves in the shade, which is often the case when they avoid predators.
Plant Response to Herbivory and Belowground Nitrogen Cycling
Authors: Holland Elizabeth A. , Detling James K.
Published: May 2006

Plant responses to herbivory and links to belowground nitrogen cycling were investigated at Wind Cave National Park, South Dakota. Laboratory estimates of net nitrogen mineralization were highest in soils from the more altered areas of prairie dog colonies (Cynomys ludovicianus) and lowest in the adjacent, lightly grazed, uncolonized grassland. The ratio of CO2: net nitrogen mineralized, as index of immobilization, was highest in the uncolonized grassland and lowest in the altered core areas. Soil moisture was an important modifier of in situ field estimates of net nitrogen mineralization. Root biomass, an important carbon source for decomposers in perennial grasslands, was lowest in the altered core area and highest in the adjacent uncolonised grassland. Decreased nitrogen immobilization and increased net nitrogen mineralization in the laboratory incubations likely resulted from decreased root carbon inputs in grazed areas, which limited carbon availability to decomposers. Such increases in plant—available nitrogen may partially explain the frequently reported grazing—induced increases in shoot nitrogen concentrations. These studies suggest that carbon allocation to roots is a key link determining nitrogen—cycling responses to herbivory.
Allocation and the Transient Dynamics of Succession on Poor Soils
Authors: Gleeson Scott K. , Tilman David
Published: May 2006

Biomass and nitrogen allocation to leaf, root, stem, and reproduction was determined in a 35—field chronosequence that spans the first 60 yr of secondary succession on a Minnesota sand plain. Biomass (grams per square metre) in leaf and root increased during succession, but reproductive biomass declined, and that in stem remained constant. Because root biomass increased twice as rapidly as leaf biomass, the proportion of total biomass in root increased during succession, whereas that in leaf, reproduction, and stem declined. In an additional study, biomass allocation was determined on a species—by—species basis for 46 species common at different times during succession. This study showed a similar pattern of increasing proportional root allocation and declining proportional reproductive and stem allocation during succession. These changes were accompanied by an increase in total soil nitrogen and a decrease in light penetration to the soil surface during succession. Increasing root allocation and decreasing reproductive allocation suggest that succession on these nutrient—poor soils is the transient dynamics of colonization and competitive displacement, with later successional species being superior nitrogen competitors because of higher root allocation. Allocation trade—offs root, stem, leaf, and seed can lead to initial dominance by species with high seed and leaf allocation, presumably because of greater colonization and/or maximal growth rates. Thus, this succession differs markedly from successions on rich soils, for which stem allocation is increasingly important. These results contradict the resource ratio hypothesis as an explanation for the pattern of early succession on impoverished soils.
Relating Carbon Allocation Patterns to Tree Senescence in Metrosideros Forests
Authors: Gerrish Grant
Published: May 2006

The ratios of carbohydrate sink to source in the shoot systems of mature and juvenile Metrosideros polymorpha trees were compared, testing a prediction of the pipe model theory that this ratio is proportional to height, thus explaining the synchrony of forest decline in even—height stands. Volumes of sapwood (S) and inner bark (IB), and leaf mass (L) were measured by destructive sampling in 5 mature and 12 juvenile trees. S:L and IB:L of mature tree shoot systems were higher than of juvenile trees, strongly supporting the hypothesis that mature trees have a lower surplus of carbohydrate production and are less resistant to many forms of stress. The specific leaf burden (S:L and IB:L jointly) within the mature tree sample varied by a factor of about 2, compared with a variation in height by a factor of about 1.35, suggesting that a relatively strong environmental stress is needed to trigger the synchronous death of all or most trees in a stand. The pipe model, with its prediction of constant Huber value (stem cross—sectional area per unit supported leaf mass), does not accurately represent the distribution of either the sapwood or the inner bark in M. polymorpha. The sapwood Huber values varied significantly among the organ systems of individual trees and between life states. Within each organ system the sapwood cross—sectional area in larger stems was strongly correlated with supported leaf mass, while the inner—bark cross—sectional areas was better correlated with the cross—sectional area of the stem. Sapwood Huber values were similar in twigs and small branches of juvenile and mature trees, but were much lower in the larger stems of mature trees than at any point in the juvenile trees. The pattern of variation of the inner—bark Huber values among organ systems was similar in juvenile and mature trees. A reduction of the sapwood Huber value in the large stems of mature trees associated with heartwood development decreased the rate of increase of S:L with size. IB:L increased very little in the juvenile state, but it increased rapidly in the mature state when leaf mass failed to increase in linear proportion with total tree volume. The volume of inner bark was found to be about one—half as great as the volume of sapwood in mature trees, but is likely to be responsible for a disproportionate part of total aboveground respiration.
Flow Analysis of Models of the Hubbard Brook Ecosystem
Authors: Finn John T.
Published: May 2006

Several models of energy and nutrient flow on the Hubbard Brook Ecosystem, New Hampshire, USA, were analyzed for the pattern of flow through the models, cycling index CI, path length PL, and straight—though path length PLs. CI represents the proportion of flow that cycles through components of the system. PL is the average number of components that a unit of flow passes through on its way from inflow to outflow. PLs is that portion of PL attributable to flow passing straight through the system without cycling at all. The energy model had low values for PL, PLs, and CI, compared to nutrient models. Flow measures for nutrients varied greatly. The order of the elements in terms of CI and K > Na > N > Ca > P > Mg > S. This could be explained by the mobility and biological role of each element, although Na is somewhat anomalous. PL was very large for K (24.3) but small for S (5.2). PLs was near 4 for all elements but Na (2.6) and S (2.5). This was a reflection of different flow patterns for Na, which flows primarily between available nutrients and belowground biomass, and S, which flows primarily between available nutrients, and below— and aboveground biomass. Sulfur returns to available nutrients via stemflow and throughflow, bypassing the forest floor. The remaining elements cycle between above— and belowground biomass, forest floor, and available nutrients. Three different models of Ca flow were compared. When all three models were analyzed using the same nutrient flux data, flow measures become very close, despite structural differences in the models. For these models, flow values were more important than the architecture of the models in determining cycling and flow characteristics.
Leaf Ultraviolet Optical Properties Along a Latitudinal Gradient in the Arctic‐Alpine Life Zone
Authors: Robberecht Ronald , Caldwell Martyn M. , Billings W. D.
Published: May 2006

Leaf epidermal transmittance of terrestrial solar ultraviolet—B radiation (295—320 nm) was examined along a latitudinal gradient of solar UV—B radiation. In high UV—B radiation zones, e.g., equatorial and tropical regions, mean epidermal transmittance for the species examined was less than 2%. At higher latitudes, mean epidermal transmittance exceeded 5%. Although this latitudinal solar UV—B gradient represents more than a seven—fold difference in daily integrated UV—B irradiance, the calculated mean effective UV—B irradiance at the mesophyll of low—latitude species is not substantially different from that of species at higher latitudes. Species in high UV—B radiation environments appear to attenuate this radiation more effectively than those in lower irradiance environments. In most cases, absorption of UV—B in the epidermis is the major parameter effecting low transmittance. Reflectance from glabrous leaves is generally less than 10%. In some species, pubescent or glaucous leaf surfaces can reflect more than 40% of the UV—B radiation incident on a horizontal leaf, although such surface characteristics do not necessarily indicate high UV—B reflectance. Under controlled conditions, epidermal transmittance in Pisum sativum L. decreased in response to UV—B irradiation. The modification of epidermal transmittance, resulting in lower UV—B irradiance at the mesophyll, may represent a mechanism of plant acclimation to UV—B radiation. Such acclimation may have occurred in several wildland species of temperate—latitude origin that have invaded high UV—B irradiance equatorial and tropical regions.
Foraging in a Lethal Environment: Fish Predation in Hypoxic Waters of a Stratified Lake
Authors: Rahel Frank J. , Nutzman Julie Waymire
Published: May 2006

Development of low oxygen conditions (hypoxia) in bottom waters of stratified lakes has a profound effect on the distribution of aquatic organisms. Fish avoid hypoxic waters by moving into oxygenated water and suffer massive die—offs when migration is not possible. However, central mudminnows (Umbridae: Umbra limi) were captured regularly in severely hypoxic bottom waters of a lake in northern Wisconsin, USA. Caging experiments confirmed the lethality of this environment to mudminnows. The presence of freshly ingested phantom midge larvae (Diptera: Chaoborus) in the stomachs of mudminnows during the day coupled with the observation that Chaoborus are restricted to hypoxic waters during the day indicate that mudminnows routinely ventured into a lethal environment to forage. This conclusion was supported by caging experiments whereby mudminnows denied access to hypoxic water during the day did not contain Chaoborus in their stomachs. In laboratory experiments, ingestion of air bubbles enhanced the ability of mudminnows to forage in hypoxic water, but even in the absence of surface access, mudminnows readily foraged in hypoxic waters. These data help explain reports of other fish venturing into hypoxic waters and suggest that foraging in hypoxic waters may be common among some fishes when food abundance is low in surface waters.