bluuebunny/crossref_metadata_2025 huggingface dataset:
Variation in Interception of the Direct Solar Beam by Top Canopy Layers
Authors: Herbert Thomas J.
Published: May 2006

Leaves that show paraheliotropic movement are inclined at a shallow angle with respect to the horizontal near dawn and dusk, and at a steep angle near solar noon. However, the mean inclination angle is only a first approximation to the distribution of leaf angles, and, as such, is not a good measure of the complexity of the underlying distribution. In order to ascertain the accuracies of statistical summaries of angular data used to measure the orientation of plant leaves for intercepting light, I used cellulation plots to analyze the distribution of leaf angles or cosines of the angle of incidence. Examination of these plots shows that inclinations of leaves in the top canopy layer of individual heliotropic plants are distributed over a wide range of angles during a period of several hours before and after solar noon, but that there is a sharp lower limit to this distribution of angles. Experimental observations show that near midday leaves are inclined at least at some minimum angle with respect to the horizontal. In order to interpret these data, I used a simple model of plant canopy photosynthesis and optimized the distribution of leaf angles to maximize total canopy photosynthesis. The results of this model suggest that the existence of a limit angle may be a function primarily of stress factors, and that the distribution of leaf angles more vertical than the limit angle may be important primarily for maximization of whole—plant photosynthesis.
Zonation of Spartina Patens and Spartina Alterniflora in New England Salt Marsh
Authors: Bertness Mark D.
Published: May 2006

In New England salt marshes the boundary between frequently flooded low marsh habitats and less frequently flooded high marsh habitats is characterized by striking plant zonation. Spartina alterniflora monocultures dominate low marsh habitats while the seaward border of high marsh habitats is generally dominated by Spartina patens. In this paper I examine the role of interspecific competition in maintaining this zonation pattern. Spartina patens turfs and tillers transplanted into the low marsh were severely stunted with or without S. alterniflora neighbors, and low marsh bare patches bordering S. patens monocultures were not significantly colonized by S. patens in three growing seasons. The limited ability of S. patens to oxygenate its rhizosphere in anoxic soils appears to limit S. patens to high marsh habitats. In contrast, S. alterniflora transplants were vigorously in the high and low marsh when buffered from neighbors, but were excluded from the high marsh in 2—3 yr when S. patens was present. S. alterniflora also rapidly invaded the high marsh in the absence of S. patens. These results support the hypothesis that S. alterniflora is restricted to low marsh habitats by competitive displacement. S. alterniflora thrives in anoxic low marsh habitats due to its ability to oxygenate its roots and rhizosphere. Rhizosphere oxidation by S. alterniflora, however, is not evident in seedlings and small colonizing patches and both seedlings and small colonizing patches of S. alterniflora are stunted in anoxic low marsh substrates. This suggests that the success of S. alterniflora in anoxic habitats is size dependent and may be driven by group benefits of rhizosphere oxidation. These results suggest that the maintenance of intertidal zonation in rocky beach and marsh plant communities is very similar. In both assemblages, competitive dominants monopolize physically benign habitats and displace competitive subordinates to physical stressful habitats.
Fluxes of Dissolved Organic Nutrients and Humic Substances in a Deciduous Forest
Authors: Qualls Robert G. , Haines Bruce L. , Swank Wayne T.
Published: May 2006

We evaluated the importance of dissolved organic matter as a vehicle for the movement of N and P from the canopy and the forest floor into the mineral soil of a deciduous forest. We also examined the origin and nature of dissolved organic matter from the forest floor to see whether it was simply soluble plant material or highly humidified matter. The average annual output from the forest floor in the form of dissolved organic matter was 18, 28, and 14% of the input in solid litterfall for C, N, and P, respectively. In throughfall, about half of the dissolved N and P was organic. But, in solution percolating from the forest floor, 94% of the N and 64% of the P was organic. Leaching from the forest floor was not a source of inorganic N and P for the mineral soil. Instead, the forest floor was a sink for the removal of these inorganic nutrients delivered in throughfall. Microbial immobilization was the most likely explanation for much of the inorganic nutrient removal. In contrast, the forest floor was an abundant contributor of N and P to the mineral soil in the form of dissolved, and possibly particulate, organic matter. Much of the dissolved organic matter entering the A horizon originated from the upper (Oa and Oe horizon) forest floor, but it was modified in several respects compared to the original soluble material. The solution percolating from the forest floor over most of the year was much richer in nitrogen, contained a much larger proportion of hydrophilic acids, and contained a much smaller proportion of carbohydrate—rich hydrophilic neutrals, than did the original water—extractable material in autumn litter. However, the fresh autumn litter did contain a similar proportion of soluble hydrophobic acids that resembled dissolved humic substances in several respects. Most of the flux of nitrogen from the forest floor to the A horizon was carried by humic substances and highly colored hydrophilic acids.
Effect of Algal Epiphytes on the Mussel Mytilus Californianus
Authors: Dittman Dawn , Robles Carlos
Published: May 2006

The effects of facultative epiphytes on a bivalve host were studied on an islet near Santa Catalina Island, California. The primary cover in the mid—intertidal zone was a mosaic of red algal turf (Corallina officinalis, Gigartina canaliculata, and Gelidium coulteri) and clumps of the mussel Mytilus californianus. In certain circumstances the algae attached to and overgrew the mussels. In other marine habitats, facultative epibionts benefit bivalve hosts by masking them from predators. At Catalina, spiny lobsters (Panuliris interruptus) and oystercatchers (Haematopus bachmani) preyed on intertidal mussels. However, in tagging studies, overgrown mussels displayed a nonsignificant trend towards lower survivorship. A factorial experiment manipulating exposure to predators and the cover of epiphytes demonstrated that overgrowth significantly reduced survivorship. There was no significant statistical interactions between treatment effects, indicating that the presence of algae did not protect the mussels from predators. Overgrown mussels also had significantly lower rates of growth and reproduction than naturally clear mussels of similar size and location. Experimental removal of the epiphytes caused significant increases in mussel growth and reproduction relative to overgrown controls.
Diet Choice in the Herring Gull: Constraints Imposed by Reproductive and Ecological Factors
Authors: Pierotti Raymond , Annett Cynthia A.
Published: May 2006

Studies of diet choice in carnivorous animals typically concentrate on maximizing rates of energy intake, and rarely emphasize either constraints on choice or alternate currencies. We examined diet choice in a generalist carnivore, the Herring Gull, Larus argentatus, in relation to both nesting habitat and reproductive performance over a 2—yr period. During prelaying and incubation periods 75—80% of Herring Gulls specialized on either intertidal organisms, human refuse, or other seabirds, while only 20—25% had generalized diets. Specific types of specialists tended to nest in particular habitats. Foraging tactics associated with each of three diets were related to time budgets and ecological constraints, i.e., levels of predation or intraspecific competition in specific habitats. Variation in diet choice was also strongly related to individual breeding performance. Intertidal specialists laid eggs earlier, produced larger and heavier clutches, and had higher rates of hatching than generalists and other specialists. This pattern held true over both years of the study even though weather conditions, and food accessibility, differed between years. Despite differences in performance related to laying and hatching, there were no significant differences in fledging rates, either between years or among diets. These results suggest that (a) choice of nesting habitat may constrain diet choice, (b) diet choice may have major effects on breeding performance, and (c) energy may not be the proper currency for examining diet choice in carnivores subject to a variety of constraints.
The Shape of a Gerbil Isocline Measured Using Principles of Optimal Habitat Selection
Authors: Abramsky Z. , Rosenzweig M. L. , Pinshow B.
Published: May 2006

The isoclines of Gerbillus allenbyi competing with G. pyramidum are negatively sloped and at least approximately parallel. They are not linear, however; they are steeper over both low and high G. pyramidum densities and only very slightly negative at intermediate densities. This nonlinearity agrees with the predictions of the theory of optimal density—dependent habitat selection in a two—species, shared—preference system, the system that obtains here. The isocline field was measured by a new method based on the single—species habitat selection theory of Fretwell. The species whose isoclines are being measured (the target species) is given a chance to equilibrate its per capita reproductive rate across a fence that separates two densities of its competitors. Equilibration requires that the target species distribute its density unequally on the two side of the fence. The general features of the method may be useful in other animal studies where densities can be experimentally manipulated.
Experimental Evaluation of Predator‐Prey Relationships in a Patchy Environment: Consequences For Habitat Use Patterns in Minnows
Authors: Fraser Douglas F. , Cerri Richard D.
Published: May 2006

An experimental study, using the minnows Semotilus atromaculatus and Rhinichthys atratulus, was conducted in a seminatural artificial stream to evaluate the interaction of predator presence, habitat structural complexity, and time of day in determining habitat use patterns of prey. Predators were adult S. atromaculatus, while juveniles of both species were used as prey. Prey of both species actively avoid locations in the artificial stream that contain predators. However, structural complexity and time of day each have a mediating effect on the response of the prey fish to the presence of predators. While others have shown differential survivorship of prey, dependent upon structural complexity and time of day, this study shows how prey respond behaviorally to the presence of predators in habitat patches. They are most likely to enter those patches during daylight or when structure is present. In natural headwater streams prey fish are often found in patches of high predator density where prey density is often correlated with the degree of structural complexity. Our findings offer an explanation for this variability in relative abundances of predators and prey, by relating patch use by prey habitat structural complexity and diel activity. Resulting implications for optimal foraging and guild organization are suggested.
Competitive Interactions in Plant Communities of Different Successional Ages
Authors: Parrish J. A. D. , Bazzaz F. A.
Published: May 2006

This research was conducted to test two predictions about niche and competitive relationships in the structure of plant communities. It has been demonstrated that early successional species of plants have broad, overlapping niche occupation on many gradients whereas later successional species show more niche differentiation. We predicted that, if the differences in resource use were for contested resources in limited supply, the more niche—differentiated species should: (1) experience less biomass reduction in pairwise interspecific competition than should broadly overlapping species of early succession, and (2) be able to convert relatively more of the available resources into plant matter as a heterospecific assemblage than can the conspecific stands from that assemblage. Six species from each of early (annual), mid— (early perennial), and late— (prairie) successional plant communities were grown in containers singly, in con— and heterospecific pairs, and in pure and mixed stands. Competitive interactions were assessed by comparing biomass production of species under the various treatment conditions. The late—successional community, in which we have previously shown more niche separation on several axes, showed a significantly higher mean ratio for the mass of individuals grown in heterospecific pairs divided by the mass of individuals of the same species grown in conspecific pairs than did the early successional community. All heterospecific pairs in the prairie and early perennial communities had a winner (ratio of mass in heterospecific pair to mass in conspecific pair >l.0) and a loser (ratio <1.0), while for most pairs in the annual community both competitors had ratios of <—l.0. Annuals showed the most reduction in total mass and prairie species the least when the mass of an individual grown in a heterospecific pair was compared to the mass of an individual of the same species grown by itself. Therefore, there was less competitive reduction in the more niche—differentiated community, the prairie. There was some evidence that mixtures of the more niche—differentiated late—successional species may be able to convert relatively more of the available resources into plant matter. The prairie assemblage had a slightly greater ratio of mixed stand to pure stand biomass with more variation among species than did the annual assemblage. There was also a tendency for the pairs of prairie species to have higher relative and absolute yields in mixture than in conspecific pairs. Thus members of the late—successional community experienced less competitive reduction in the presence of heterospecific neighbors than did species of early succession. This is strong evidence that niche reduction and separation are effective in reducing competition within a community and that selection to reduce competition could have been more important in the evolution of late—successional species than for early successional species.
Increased Reproductive Success at High Densities and Predator Satiation For Periodical Cicadas
Authors: Karban Richard
Published: May 2006

The reproductive success of periodical cicadas, measured as the number of offspring produced per adult, is found to increase as adult density increases. The inverse density—dependent pattern at the adult stage occurred over the entire range of adult densities encountered. This result could have been caused by (1) predator satiation, (2) increased fecundity at higher densities, (3) more efficient mating at higher densities, and (4) movement of adults from sites of low initial densities to sites of high densities. The hypothesis of predator satiation was explored in depth. Predation at each site was crudely estimated by collecting cicada wings, which are discarded by avian predators. If predator satiation accounts for the inverse density—dependent mortality, then each cicada's risk of capture will decrease as cicada density increases. The data suggest that this prediction is true; predation did not increase as cicada density increased so that the probability of escaping predation is greater at high—density sites. Estimated predator densities are independent of cicada densities, which indicates that predators cannot respond numerically to the cicada emergence. As cicada density increases, the number of cicada wings recovered per bird does not significantly increase, suggesting that birds are satiated. Significantly more cicada wings were recovered at sites with more birds. Total cicada predation was independent of cicada density but dependent on the density of predators. Some evolutionary consequences of these results are discussed.
Induced Settlement and Metamorphosis of Sand Dollar (Dendraster Ecentricus) Larvae in Predator‐Free sites: Adult Sand Dollar Beds
Authors: Highsmith Raymond C.
Published: May 2006

Dendraster excentricus larvae capable of metamorphosis presented with various substrates show a significant preference for adult—associated sand. Adult D. excentricus produce a chemical cue, possibly a small peptide (molecular mass <10 ,000), that is sequestered by some component in the sand and that is stable for at least 7 wk. Thus, larval settlement occurs within or adjacent to existing sand dollar beds which often contain several hundred adults per square metre. Experimental evidence indicates survival of newly metamorphosed D. excentricus is significantly reduced by an extremely abundant, tube—building predator. Leptochelia dubia (Crustacea: Tanaidacea). Tenaidacean and possibly other micropredators, however, are excluded from sand dollar beds by the reworking activities of adult sand dollars. Thus, preferential settlement of D. excentricus larvae near adults of the species should result in increased larval and juvenile survival.
The Ecological Impact of Nectar‐Robbing Bees and Pollinating Hummingbirds on a Tropical Shrub
Authors: Roubik David W.
Published: May 2006

The consequences of nectar robbing by highly social bees were studied at 71 patches of the forest—edge shrub, Pavonia dasypetala (Malvaceae) at two tropical moist forest habitats in central Panama. Groups of Trigona (Trigona) ferricauda (Apidae: Meliponinae) arrived at flowers in early morning; each bee dominated a single flower that it perforated to rob nectar (Fig. 1). Robbers in each patch aggressively defended their feeding sites from the sole pollinator of the plant, the Hermit Hummingbird Phaethornis superciliosus (Trochilidae: Phaethorninae). Less visitation by hummingbirds and reduced seed production resulted from nectar robbing. Flower number in patches was not associated with visitation per flower by pollinators, but patches having the most flowers were entered more often by P. superciliosus. In contrast, thieving P. longuemareus and robbing T. ferricauda showed no patch preference related to total flower number. Seed production per flower was not related to the total number of flowers presented in patches at one time. The average proportion of robbed flowers through the entire flowering episode of P. dasypetala was 36% among 66 patches at one site and 35% among 5 patches at a second site. Reduced seed production was not caused directly by robber damage to flowers, but indirectly through successful deterrence of birds via joint attacks by bees on and near the flower visited by a hummingbird. Further, although fewer seeds were produced by robbed flowers overall, robbed and unrobbed flowers in patches that were heavily robbed did not differ in seed production. Thus robbing primarily influenced reproductive success at the level of the flower patch. P. dasypetala was self—pollinating, yet unrobbed flowers that were not bagged produced 67% more seeds than those not visited by P. superciliosus. Persistent robbing by Trigona may result in selective advantage for autogamous or apomictic P. dasypetala.
A Limpet‐Coralline Alga Association: Adaptations and Defenses Between a Selective Herbivore and its Prey
Authors: Steneck Robert S.
Published: May 2006

The population density of the limpet Acmaea testudinalis is highest on the crustose coralline alga Clathromorphum circumscriptum in both tide pool and subtidal environments in the Gulf of Maine. Juvenile limpets recruit to C. circumscriptum and both juveniles and adults preferentially feed on this species (not its epibionts) over a choice of other corallines, foliose algae, microalgae (diatoms), and detritus. Abundances of the predator (limpet) and the prey (coralline) are positively correlated, and the fitness of both organisms may be increased by the association. Acmaea testudinalis possesses a radula apparently adapted to eat efficiently C. circumscriptum, a food of low caloric value. The coralline is not only an abundant and dependable food throughout the geographic range of the limpet but also provides a superior holding surface for it. Clathromorphum circumscriptum requires grazing to remove potentially lethal epiphytes, but is not harmed by limpet grazing since it possesses a uniquely thick protective tissue over the region of growth. Reproductive structures of C. circumscriptum develop in winter and are buried below the surface so the effects of grazing on them are minimal. Over a wide range of depths, the rate of cell removal by limpets matches the rate of cell production by Clathromorphum. The number of apparently specific adaptations of both organisms, with mutual advantages, suggests evolution toward a coevolved interdependency. Trophic specialization of small organisms with low mobility (limpets) on larger, longer lived prey (coralline algae and kelps) is more likely than in other herbivore/prey interactions. By specializing on a long—lived and predictable food source, A. testudinalis may have evolved a strategy of economy by minimizing its cost of foraging (risk to predation and energy expenditure) relative to its absolute intake of calories. This foraging strategy may have been reinforced over time since it also improves the survivorship of the preferred prey.
Bird Activity and Seed Dispersal of a Tropical Wet Forest Tree
Authors: Howe Henry F.
Published: May 2006

Feeding assemblages of birds were observed throughout a fruiting season at a Costa Rican population of the rain forest tree Casearia corymbosa in order to distinguish effective dispersal agents from visitors which used arils for food without dispersing seeds. This system is of special interest because the tree fruits during an annual period of fruit scarcity in a forest characterized by high proportions of animal—dispersed plants and obligate frugivores. Results show that the Masked Tityra (Tityra semifasciata) is an effective dispersal agent of seeds of this tree because it: (1) regurgitates viable seeds, (2) is a common and regular visitor throughout the season, (3) has high feeding rates, (4) removes seeds from the vicinity of the parent tree before processing them, and (5) depends on this rather than other plants fruiting at the same time. Twenty—one other visitors were deficient dispersers, although some depended heavily on fruit of this tree for food. Two parrots (Amazona autumnalis and Amazona farinosa) preclude dispersal by stripping arils with their bills and dropping all seeds in situ. Fourteen species are occasional visitors. Five visitors (Ramphastos sulfuratus, Ramphastos swainsonii, Pteroglossus torquatus, Myiozetetes similis, and Myiozetetes grandensis) process seeds undamaged and sometimes numerically abundant, but are absent part of the season and tend to regurgitate seeds in situ. Because fruiting activity of this tree occurs during annual scarcity, it represents a pivotal species in the community. Three obligate frugivores, including one effective disperser and two fruit thieves, depend almost entirely on it for food periods of 2—6 wk. I hypothesize that extinction of this plant at this site would lead to disappearance of these birds, and would almost certainly depress recruitment of other species of trees for which they serve as dispersal agents at other times of the year.
Insect Species Richness: Hispine Bettles of Heliconia Latispatha
Authors: Strong Donald R.
Published: May 2006

Lowland Costa Rican populations of Heliconia latispatha (Zingiberales) are attacked by six species of rolled leaf hispine beetles (Chrysomelidae). Each species feeds upon a specific plant part in both larval and adult stages: either the inflorescence, the edge of the rolled leaf, or the blade of the rolled leaf away from the edge. Lowlands in the Pacific North have a long dry season, and H. latispatha grows only ° 6 mo of the year and supports from zero to three hispine species. The Pacific Southern dry season is shorter, and H. latispatha grows ° 10 mo and supports four hispine species. The Caribbean lowlands have sufficient rain for H. latispatha to grow year round, and four rolled leaf hispine species are supported. Heliconia latispatha may be saturated with four species of rolled leaf Hispinae in the Caribbean lowlands and Pacific South. Between these two regions, the species associated with H. latispatha change, the pool of species with the potential to attack H. latispatha increases to five, and the total number of species attacking congeners of H. latispatha increases from six to eight.
Nutrient Recycling and Stability: A Reevaluation
Authors: Harwell Mark A. , Cropper Wendell P. , Ragsdale Harvey L.
Published: May 2006

A general linear, donor controlled model of nutrient cycling in ecosystems has been previously developed. Stability analysis of this model was equated to analysis of the parameters of the second—order damped oscillation equation, and rankings among eight aquatic and terrestrial ecosystems were ascertained by those authors. In this paper, the results of direct experimental perturbations of the general model and the second—order approximation are presented. These digital and analog computer analyses show that the terms said by those authors to measure the resistance and resilience components of asymptotic stability do not reflect the actual response of either stability component for either model. Ecosystem resilience and resistance rankings, based upon the linear models, are reassigned according to observed response.
Energy Expenditure in Free‐Ranging Lizards
Authors: Bennett Albert F. , Nagy Kenneth A.
Published: May 2006

Metabolic rates of the western fence lizard Sceloporus occidentalis (Sauria: Iguanidae) were measured in the laboratory by gas exchange, and in the field with doubly labeled H2O. Field metabolic rates of adults during the spring reproductive season (0.22 ml CO2 g—1 h—1 or 141.4j g—1 day—1) were not different from those during the fall nonreproductive season (0.20 ml CO2 g—1 h—1 or 127.2j g—1 day—1). Field and laboratory metabolic rates of @V @V were not different from those of ♀ ♀ in spring or fall. Average daily field metabolic rates were 2.0 to 2.5 x resting laboratory rates measured on a simulated normal thermal cycle; estimated rates during the field activity period were 2.5 to 3.1 x resting levels measured at 35°C. Only 11 to 17% of total daily energy expenditure was used while lizards were inactive in their burrows. Daily field metabolism of this lizard is only 3 to 4% that of a bird or mammal of equal size.
Chemical Defenses of Ascidians Against Predators
Authors: Stoecker Diane
Published: May 2006

Many benthic ascidians lack strong mechanical defenses but are relatively free from predation; chemical defenses against predators are important in certain species. A number of ascidians have highly acidic tunic fluids (pH ≤ 2) which deter predators. The high vanadium content (≥100 mg/g wet mass) of some ascidians reduces their palatability to predators.
The Flax Pond Ecosystem Study: Exchanges of CO"2 Between a Salt Marsh and the Atmosphere
Authors: Houghton R. A. , Woodwell G. M.
Published: May 2006

Profiles of CO2 concentration, windspeed, and temperature were used in the aerodynamic flux technique to calculate the CO2 exchange between a Long Island salt marsh and the atmosphere. Uptake of CO2 by the marsh during hours of sunlight and release during the night occurred during all times of the year. The rates of CO2 exchange were highest during midsummer, 2.3 g CO2°m—2°h—2 averaged over the daylight hours of July, and 1.3 g CO2°m—2°h—2 averaged over the dark hours. Winter hourly rates averaged °0.2 g CO2°m—2°h—1 for both uptake and release. The net 24—h exchange rates followed Spartina growth and senescence during the summer and fall and photosynthesis of benthic algae during the late winter and spring. There was a net uptake of CO2 over 24 h by the marsh during all seasons except autumn. The net annual flow of carbon was from the atmosphere to Flax Pond (°300 g C°m—2°yr—2 averaged over the entire marsh ecosystem). This flux was larger than the net exchange of carbon between the marsh and either uplands, sediments, or coastal waters. The net uptake of CO2 during summer was less than the net productivity of the vascular plants, indicating that some of the CO2 assimilated by the plants came from heterotrophic respiration within the marsh. Nevertheless, respiration by the plants was by far the largest source of CO2 from the marsh surface. Nighttime respiration of the ecosystem released a total of °510 g C°m—2°yr—1 to the atmosphere.
Density‐Dependent Survival of Scatterhoarded Nuts: An Experimental Appoach
Authors: Stapanian Martin A. , Smith Christopher C.
Published: May 2006

We tested our earlier prediction that the food content of buried nuts will influence the density at which fox squirrels (Sciurus niger) scatterhoard each species of nut. We buried black walnuts (Juglans nigra), bur oak acorns (Quercus macrocarpa), and chinquapin oak acorns (Q. muehlenbergii) at varying densities in monospecific and mixed—species grids in three different years and three different seasons within a year, and analyzed their survival from predation over time. We also observed free ranging squirrels scatterhoard walnuts and bur oak acorns, which we made available in equal numbers. The survival of buried nuts was dependent on the nut species, nut density, and squirrel density. The lowest meaningful density of buried nuts for squirrels was related to the food content of the nut species. Analysis of nut survival at the same burial sites in successive seasons indicated that squirrels did not use memory to return to specific sites where they had previously found nuts. Rather, they foraged preferentially in certain microhabitats. Squirrels were observed burying black walnuts farther away from a source pile than they buried bur oak acorns. The difference is consistent with the difference in food content between the two species and also with the threshhold density below which naive squirrels have no density—dependent effect on buried nuts. Our data on nut survival suggest that squirrel foraging varies with nut density but not with nut spacing.
Influence of Microtopography and Canopy Species on Spatial Patterns of Forest Understory Plants
Authors: Beatty Susan W.
Published: May 2006

In a maple—beech forest in eastern New York, equal numbers of mounds, pits, and adjacent undisturbed soil microsites were caused for plant species density and cover over the growing seasons of 1978 and 1979. Several soil properties were also measured for each microsite. Detrended correspondence analysis (DCA) and chi—square goodness—of—fit tests were used to test whether the species were uniformly distributed over the microrelief positions. Comparisons of species composition, diversity, and vegetative phenologies were made between microsites and between hemlock— and hardwood—dominated forest areas. Differences in soil properties between microsites and between the two forest areas were assessed with one—tailed paried t test and Wilcoxon rank—sum tests, respectively. Microrelief created a mosaic of soil properties and a vegetation pattern in the understory, both of which were affected by the presence of hemlock in the canopy. Ordinations (DCA) indicated two factors that affected species compostion of the microsites: (1) presence of hemlock (Tsuga canadensis) as the nearest canopy tree, and (2) microtopography (whether the microsite was a mound, pit, or undisturbed—soil site). Species in non—hemlock areas had patchy distributions in the forest community: eight species grew mainly on mounds, six in pits, and five on undisturbed soil. The mounds and pits had characteristic assemblages of species. Species richness and total density and cover of plants in each microsite were constant over the growing season. In contrast, little spatial or temporal community pattern was apparent in forest areas influenced by hemlock. The effect of hemlock on species distributions was due in part to its effect on soil properties. For all microsites, forest areas with hemlock had greater soil organic matter content, available nitrogen, cation exchange capacity, and little depth, lower soil calcium content, moisture content, temperature, pH, and A1 horizon depth; and no frost heaving. Microrelief also affected soil properties significantly. In comparison with pits, mounds were drier and poorer in nutrient content, and had a lower cation exchange capacity, less organic matter, less little cover, a thinner A1 horizon, and less snow accumulation. In areas without hemlock, the mounds were also more acid, warmer in summer and colder in winter, and more subject to frost heaving than pits. Hemlock—influenced areas had no consistent microsite differences in pH or temperature. The different microenvirnments, created by the interaction of microrelief and hemlock, resulted in patchy distributions of most understory species. This pattern was likely a reuslt of species requirements for and tolerances of envirnomental conditions, tempered by competitive interactions.
Substrate‐Mediated Response of Stream Invertebrates to Disturbance
Authors: Gurtz Martin E. , Wallace J. Bruce
Published: May 2006

The response of aquatic invertebrates to a major watershed (catchment) disturbance, clear—cutting, was examined in a second—order stream in the southern Appalachian Mountains. For 21 mo after the start of logging, invertebrates were sampled in four substrate types: rock face (moss—covered boulders and outcrops), cobble riffles, pebble riffles, and sand. The type of substrate was an important factor in determining the direction and magnitude of the response of many taxa. In the stream that drained the clear—cut watershed, more taxa increased in density (compared with a nearby reference stream) in moss—covered rock face than in any other substrate; cobble riffles were next, followed by pebble riffles and sand. Conversely, the number of taxa with significant reductions in density was highest for sand substrates, followed by pebble, cobble, and rock face. Among functional groups of insects, collector—gatherers and scrapers increased, while the dominant shredder, Peltoperla, declined. The differential response of invertebrates among substrates suggests that biological stability is closely coupled with physical stability; however, moss associated with larger particles may be a factor in enhancing the biological stability of these substrates. Clear—cutting of a watershed is a largescale, low—frequency, anthropogenic disturbance to associated stream ecosystems. Certain successional characteristics of these headwater streams following a major anthropogenic disturbance differ from those observed in streams in which succession may be truncated by frequent disturbances such as flash flooding.
Energy Utilization by Free‐Ranging Jackass Penguins, Spheniscus Demersus
Authors: Nagy Kenneth A. , Siegfried W. Roy , Wilson Rory P.
Published: May 2006

Rates of CO2 production by breeding Jackass Penguins (means mass, 3170 g) were measured by using doubly labeled water. Time budgets were estimated from behavioral observations, and energy budgets were calculated for a typical 2—d period of 24 h sitting on a nest and 24 h off the nest (which includes 9 h foraging at sea). Distances traveled at sea were determined by using speed/time meters harnessed to five birds containing doubly labeled water, and these measurements were used to calculated the energetic costs of swimming. Field metabolic rates (as CO2 production) averaged 0.991 mL°g—1°h—1, or (in energy units) 3890 kJ°penguin —1°2 d—1. Energy expenditures were 1.7 times the standard metabolic rate (SMR) while brooding, 6.6 ° SMR while foraging, 9.8°SMR while swimming underwater, and 2°6 ° SMR integrated over a 2—d period. Penguins spent °48% of their energy budget and 19% of their time budget to obtain food. Foraging efficiency (the metabolizable energy gained while foraging ° the energy used during foraging) was 2.1 for birds with small chicks. Fresh mass consumption rates calculated on the basis of the chemical composition of Cape anchovy, Engraulis capensis, were °758 g°penguin—1 2 d—1 for adults brooding small chicks, and °75 kg of anchovy per nest (2 adults, 2 chicks) over a 70—d nestling period. An adult penguin would need to capture °138 kg of fish per year. The 160,000 penguins using the South African fishing grounds would consume about 2.21 ° 107 kg of fish per year, of which °80% or 1.77 ° 107 kg would be Cape anchovies. This is °8% of the average commercial anchovy catch in that area.
Food Availability, Sea Urchin Grazing, and Kelp Forest Community Structure
Authors: Harrold Christopher , Reed Daniel C.
Published: May 2006

Unlike most previously reported kelp forest, which exhibit long—term persistence over large spatial scales, the kelp forest community on the west end of San Nicolas Island, California, occurs as a dynamic patchwork of barren areas characterized by grazing sea urchins and an algal assemblage consisting of upright and encrusting coralline algae, and kelp—dominated areas characterized by high densities of perennial brown algae, including the giant kelp Macrocysits pyrifera. In this study we examine the factor that regulate the grazing of sea urchins (Strongylocentroutus franciscanus), which in turn determines the structure of this community. In the barren area, drift algae sparse, sea urchins were poorly nourished, occupied open, unprotected microhabitats, and actively grazed the substratum. In the kelp—dominated are, drift algae were abundant, sea urchins were well nourished, moved little, occupied cracks and crevices, and probably fed on the drift algae. Early in the study substantial recruitment of brown macroalgae occurred in both sites, and the barren area gradually transformed into a kelp—dominated area. Concomitant with this change, the abundance of drift algae in the barren area increased. Urchins in this area abandoned open microhabitats for protected crevices and pockets, and sea urchin grazing intensity was reduced to levels characteristic of the kelp—dominated area. From these observations we present a qualitative model in which the transformation from one configuration to another is triggered by a behavioral switch in the mode of feeding of red sea urchins. This switch is controlled by the availability of drift algae, which in turn is dependent upon the abundance of attached macroalgae. The adult algal standing stocks appear to be regulated by prevailing hydrographic conditions, which are either favorable or unfavorable for kelp recruitment, survivorship, and growth. In this model grazing intensity is independent of urchin density and therefore provides a new view of how kelp forest communities function.
Disturbance, Nitrogen Availability, and Nitrogen Losses in an Intensively Managed Loblolly Pine Plantation
Authors: Vitousek Peter M. , Matson Pamela A.
Published: May 2006

The interactive effects of harvest intensity, site preparation, and herbicide treatment were evaluated in a clear—cut Piedmont site in North Carolina. Forest harvesting caused increased nitrogen mineralization and nitrification in all treatments, but harvesting without additional treatment had little effect on nitrate—nitrogen pool sizes and losses. The removal of most surface organic material during intensive site preparation led to greatly increased nitrate pool sizes and losses. Treatment with herbicide accentuated this effect, and in combination such removals and herbicide applications led to accumulations of nitrate—nitrogen in surface soil of 27 and 24 kg/ha in the first and second summers following site preparation. Nitrogen losses by leaching, denitrification, and erosion were also greatest in the plots where organic residues had been removed and herbicides applied. These results suggest that microbial immobilization controlled nitrogen pool sizes and losses, and this suggestion was confirmed using 15N. Soils from the plots without residue removal or herbicide treatment immobilized >90% of added 15N in 28 d, while those from the residue removal/herbicide treated plots immobilized <70%. Microbial activity was the predominant process regulating nitrogen availability and losses following disturbance in this site; site—preparation practices that removed important substrates for microbial metabolism caused elevated nitrogen losses.
Interaction of Disturbances in Tallgrass Prairie: A Field Experiment
Authors: Collins Scott L.
Published: May 2006

A field study was conducted to address the roles of fire, cattle grazing, and the interaction of these two disturbances on plant species abundance and community structure in a tallgrass prairie. In 1985 and 1986, species composition was sampled in three replicates of the following grassland treatments: ungrazed + unburned (undisturbed), grazed + unburned, ungrazed + burned, and grazed + burned. Cover of the matrix—forming species Andropogon gerardii was significantly greater, while cover of the nonmatrix species Bromus tectorum was significantly lower, on the burned treatments than on the unburned treatments. The number and cover of annuals were significantly higher on the grazed treatments than on the ungrazed treatments. Species richness increased with increasing disturbance intensity. Diversity, however, was lowest on the ungrazed + burned treatment and highest on the grazed + burned treatment. Burning significantly reduced species diversity on ungrazed treatments, and grazing significantly increased diversity on the burned treatment. This study documented the dissimilar effects of different natural disturbances on grassland species, growth form characteristics, and community structure. In addition, the results emphasize the important role of interaction among disturbances on plant community structure in grasslands.
Effects of Predation and Light on Seedling Establishment in Gustavia Superba
Authors: Sork Victoria L.
Published: May 2006

Large—seeded trees whose seeds are dispersed by mammals often have high levels of seed dispersal but high seed losses due to predation. Consequently, seedling establishment is influenced both by the impact of mammalian seed predators as well as by the ability of the seedling to survive where the seed is dispersed. I examined the relative roles of seed/seedling predation by mammals and of the light environment in seedling establishment in three populations of the tropical tree Gustavia superba. One population was located in semideciduous forest on Gigante Peninsula and two were on Barro Colorado Island (BCI), Panama. On Gigante Peninsula, mammals that feed on Gustavia fruit and seeds were much less abundant than on BCI. My objectives were: (1) to quantify seedling densities in each population, (2) to measure the intensity of seed predation, and (3) to determine the relative importance of seed/seedling predation and light in seedling survival and growth. Seedling density was significantly greater on the Gigante Peninsula than on BCI. This pattern was influenced by postdispersal seed predation that was much lower on the Gigante Peninsula than on BCI. Seedling transplant experiments, which measured the effects of site, light condition (gap, edge, understory), and protection from mammals (tall cage, short cage, no cage), showed that protection from mammals significantly increased all three measures of seedling performance: seedling survival, seedling height, and overall seedling success (survival times growth). In addition, as expected from differences in mammal abundance among sites, the three sites differed in the extent to which each measure of seedling performance was affected by seedling protection. Seedling survival, growth, and overall success were significantly greater on the Gigante Peninsula than on BCI. However, even on the Gigante Peninsula, seedlings benefitted significantly by protection from mammals. All three measures of seedling performance were greater in light gaps than in the understory, but seedlings survived well in the understory at all three sites. I conclude that seedling density was determined primarily by mammalian predation on both seeds and seedlings, although seedling establishment was influenced also by site and light conditions. Large seeds size, which probably increases vulnerability to mammalian see predators, facilitates the establishment of seedlings under low light conditions.
Life Histories of Mammals: Analyses Among and Within Spermophilus Columbianus Life Tables
Authors: Zammuto Richard M.
Published: May 2006

Eleven theoretical predictions (or assumptions) of life history evolution are considered for the montane Columbian ground squirrel, Spermophilus columbianus, using age—specific survival and fecundity from six life tables of natural populations. The following statements are supported among age classes among populations, among age classes within populations, and (or) within age classes among populations: (1) mortality rates are high after birth, drop to a minimum by age 1 yr, and then rise with age; (2) fecundity increases with age and seldom decreases at the last age reproduction; (3) reproductive value and residual reproductive value rise to a peak and then fall with age; (4) age—specific mortality rate and age—specific mortality covary inversely with reproductive value; (5) residual reproductive value, survival, and survival rates covary inversely with fecundity; (6) residual reproductive value is positively correlated with adult survival; (7) no relationships were found between fecundity and successive survival probabilities in the life table; (8) no relationship was found between age at maturity and life expectancy; (9) no relationship was found between litter size and generation length; (10) future fecundity is positively correlated with present fecundity; and (11) age—specific fecundity varies inversely with modified reproductive value. Life history patterns among populations, within populations, within age classes, and among species are not always similar, so that theoretical predictions should explicitly delineate the level of organization to which they pertain.
Interference and Exploitation in a Guild of Woodland Ants
Authors: Fellers Joan H.
Published: May 2006

This paper evaluates the effects of exploitation and interference on the foraging behavior of nine species of ants in a Maryland woodlot. These species overlapped substantially in both the size and type of prey taken. Three types of encounters occurred between species at baits: attacks, avoidance, and coexistence. Camponotus ferrugineus, Lasius alienus, Prenolepis imparis, and Formica subsericea fell into a dominant group, while the Myrmica spp. (M. punctiventris and M. emeryana), Aphaenogaster rudis, Tapinoma sessile, and Leptothorax curvispinosus were increasingly subordinate. Subordinate species significantly reduced feeding time when in the presence of other species. Exploitative abilities of the species were studied by observing ant activity at baits. Leptothorax curvispinosus, Aphaenogaster rudis, and the Myrmica spp. arrived first at a higher percentage of baits than did Camponotus ferrugineus, which was significantly slower at finding new food sources. An inverse correlation was found between exploitative and interference ability, which enabled the subordinate species to obtain resources. In most cases, the speed of bait location was no correlated with distance of the bait from the colony.