bluuebunny/crossref_metadata_2025 huggingface dataset:
Learning, Memory, and Foraging Efficiency in Two Species of Desert Seed‐Harvester Ants
Authors: Johnson Robert A.
Published: May 2006

I examined seed learning and memory in the desert seed—harvester ants Messor pergandei and Pogonomyrmex rugosus to determine (1) which aspects of handling and search efficiency are learned, (2) how fast this information is learned and then forgotten, and (3) how learning and memory affect foraging efficiency. Individuals of both species learned a suite of behaviors that enhanced foraging efficiency on a novel seed species under natural conditions. Handling time per seed and return time (travel time to and from the nest and time inside the nest) decreased for both species. Each species also used specific harvest techniques that enhanced foraging efficiency. Individuals of the larger P. rugosus increased efficiency by increasing the number of seeds harvested per trip. As a result, the percentage of individuals that carried > 1 seed per trip increased over time. The smaller M. pergandei decreased the number of seeds handled before harvesting one, and still harvested heavier than average seeds because they handled but did not harvest lightweight seeds. Messor pergandei also varied the number of seeds handled before harvesting one, such that the fewest number of seeds was handled at low natural seed densities. Overall, foraging efficiency of M. pergandei and P. rugosus individuals, measured as seed mass harvested per unit time, increased 1.9— and 3.85—fold, respectively, within one observation period, even excluding the effect of harvesting heavier than average seeds. The difference between species arose because individuals of P. rugosus could carry twice as many of the test seeds as M. pergandei. Memory was clearly an important component of foraging efficiency for both species, as all foraging efficiency indices fell much more slowly than they rose. Tactile—associated behaviors, such as handling time per seed and number of seeds handled, were lost quickly compared to olfactory—associated behaviors such as seed recognition and acceptance.
Larval Adaptation to Lauraceous Hosts: Geographic Divergence in the Spicebush Swallowtail Butterfly
Authors: Nitao James K. , Ayres Matthew P. , Lederhouse Robert C. , Scriber J. Mark
Published: May 2006

Geographic differences in host plant availability are thought to result in regional adaptation of herbivores to their sympatric host species. Larvae of Papilio troilus, the spicebush swallowtail butterfly, feed only on plants of the Lauraceae. Although this insect occurs throughout the eastern United States, each host species has a distribution smaller than the range of P. troilus. In peninsular Florida, the major host is Persea borbonia; Sassafras albidum and Lindera benzoin are the predominant hosts elsewhere. The hypothesis that P. troilus are better adapted to their sympatric host plants was tested using a split—brood design comparing the larval performance of Florida, Midwest, and hybrid Florida ° Midwest full—sib families on Persea, and Lindera. Florida larvae had higher first—instar growth rates and survival on Persea compared to Midwest larvae. In contrast, Midwest larvae had higher total larval growth rates on Sassafras and Lindera relative to Florida larvae. A significant host ° geographic strain interaction was also found for pupal mass. Performance of hybrid larvae was intermediate to the parental strains, or equalled the best performing parental strain, depending on the host and performance measure. Regional differentiation in genetically based traits for host use indicates the existence of geographic races in P. troilus.
Crown Asymmetry, Treefalls, and Repeat Disturbance of Broad‐Leaved Forest Gaps
Authors: Young Truman P. , Hubbell Stephen P.
Published: May 2006

Gap—edge trees were significantly more likely to fall into pre—existing gaps than in other directions in a moist tropical forest on Barro Colorado Island (BCI), Panama. Trees not at gap edges were more likely to fall away from near neighbors than toward them. We show that gross asymmetries of tree crowns were the rule rather than the exception for all trees. The crowns of most gap—edge trees were strongly asymmetrical into their adjoining gaps, and trees not at gap edges were strongly asymmetrical way from near neighbors. Trees had a strong and significant tendency to fall on these heavy sides. Such treefalls, and related limbfalls, can result in redistribution of gaps and a retardation of gap regeneration. An analysis of 5 yr of canopy height data from a mapped 50—ha plot on BCI revealed that sites within larger gaps were significantly more likely to be redisturbed by secondary treefalls than were sites in the smallest gaps or nongap quadrats. These results suggest that treefall gaps in tropical forest may be more persistent disturbances than previously thought.
The Effects of Stress on Reproductive Characters of Lobelia Cardinalis
Authors: Devlin B.
Published: May 2006

To determine the effect of environmentally induced stress on reproductive characters of perfect—flowered hermaphrodites, I grew 14 clonal pairs of Lobelia cardinalis plants under common—garden conditions of 27% available light, sufficient water, and equal amounts of soil from initiation of growth until flower bud formation. At that time, one member of each clonal pair was stressed by decreasing its available light to 10% of ambient. I then assessed nectar, pollen grain, and seed production from flowers on stressed and nonstressed ramet. Stress significantly decreased seed and nectar production but did not affect pollen grain number and mean seed mass. In addition, ganets produced significantly different amounts of nectar, pollen grains, and seed per flower. These data on the effects of stress complement the generalization that stressful environmental conditions induce greater male emphasis in monoecious plants and plants that change sex.
Roles of the Tent in Behavioral Thermoregulation of Eastern Tent Caterpillars
Authors: Joos B. , Casey T. M. , Fitzgerald T. D. , Buttemer W. A.
Published: May 2006

Eastern tent caterpillars (Malacosoma americanuum) are behavioral thermoregulations and achieve higher body temperatures than expected for ectotherms of similar body size. Construction of a communal silken tent and aggregation behavior are also conspicuous features of their life history. Operative temperature (Te) models were used to evaluate the roles of the tent and of aggregation behaviors in the thermoregulation of tent caterpillars. When solar radiation was present, Te of models on the tent surface were higher than those of models away from the tent. Temperatures of a model aggregation on the tent surface were higher than those of a single caterpillar model. In the presence of solar radiation, the interior of the tent was always at least 4°C above air temperature. These data are used to evaluate the quantitative significance of the tent, aggregation behavior, and orientation to incident radiation in the thermoregulation of tent caterpillars.
Logistic Theory of Food Web Dynamics
Authors: Berryman A. A. , Michalski J. , Gutierrez A. P. , Arditi R.
Published: May 2006

Classical food—web theory arises from Lotka—Volterra models. As an alternative, we develop a model from the logistic concept of demand and supply. We first extend the logistic to an arbitrary species in a trophic chain or stack by developing a simple equation for any population Xi: 1/Xi dXi/dt = ai — biXi — Xi/ciXi—1 — diXi+1/Xi, which includes the effects of intra—specific competition for fixed resources (the term biXi), intra—specific competition for renewable resources in the lower trophic level (the term Xi/ciXi—1), and consumers in the upper trophic level (the term diXi+1/Xi). This equation emerges from the basic logistic concept of demand and supply, as captured by the consumer/resource ratios, and fulfills all the requirements for a plausible food—chain equation. We then generalize the equation to any population in a food web of arbitrary complexity 1/Xi dXi/dt = ai biXi — Xi/sj cijXjFrj(i) — sk dikXkFkc(i)/Xi, where Frj(i) is the fraction of population Xj that is a resource for i, and Fkc(i) is the fraction of population Xk that consumes i. This equation meets all the requirements for a general food web model. Some properties of the model are discussed.
Transmission Dynamics of a Virus in a Stage‐Structured Insect Population
Authors: Goulson D. , Hails R. S. , Williams T. , Hirst M. L. , Vasconcelos S. D. , Green B. M. , Carty T. M. , Cory J. S.
Published: May 2006

Despite the blossoming interest in host—microparasite epidemiology, and in use of viruses in the biological control of insect pests, few empirical studies have attempted to quantify transmission and mortality rates under field conditions. We report a laboratory and field study in which the transmission parameter (u) and mortality rate (a) due to nuclear polyhedrosis virus (NPV) are measured in different larval instars of the cabbage moth, Mamestra brassicae (Lepidoptera: Noctuidae). Laboratory studies of food consumption and virus susceptibility were used to produce crude estimates of relative transmission rates in successive instars. Increased in the rate of feeding outstrip increases in virus resistance with instar, so we predict that transmission rates should increase in older larvae (assuming rate of intake of virus to be proportional to rate of feeding). This prediction was tested in a field experiment in which a constant initial density of susceptible and infected (moribund) larvae were reared together on cabbage plants for 2—8 d. Estimates of the linear transmission parameter (u) did not differ between instars and gave a mean value of 2.16 x 10—12 for all instars and time points. Causes for the discrepancy between predictions based on laboratory data and field measurements are discussed. Differences were found between instars in the time from infection to death (?) (equivalent to 1/a, where @a is the rate of mortality due to viral infection). Second—instar larvae died more rapidly once infected than third instars, which, in turn, died more rapidly than fourth instars. The effect of host population stage structure on patterns of viral infection can be pronounced and if recognized, may significantly increase the accuracy and predictive value of models of host pathogen systems.
Water Losses in the Patagonian Steppe: A Modelling Approach
Authors: Paruelo Jose M. , Sala Osvaldo E.
Published: May 2006

In this paper we sought to answer questions related to the long—term soil water dynamics of the Patagonian steppe: What are the magnitude and seasonal dynamics of transpiration, evaporation, and deep percolation? How do these fluxes respond to fluctuations in annual precipitation? What is the pattern of soil water availability? We developed a soil water model for the steppe with a daily time step. The model gives weekly cumulative values of transpiration, evaporation, and drainage, maximum and minimum water content for the different soil layers in each week, and the weekly frequency of days with soil water potential higher than — 1 MPa for each layer. The model was tested against three sets of experimental data. Simulated data of total water losses were significantly correlated with observed data, and the slope did not differ significantly from 1 nor the y—intercept from O. On a long term basis, evaporation accounted for 56% of total water loss, transpiration 34%, and deep percolation the remaining 10%. Transpiration and evaporation had asynchronic dynamics. Evaporation was high during the coldest and wettest months of the year (mainly winter months). Transpiration, on the contrary, reached maximum values when energy and water availability were simultaneously high in late spring—early summer. Drainage took place during the coldest months, when most of precipitation occurred, and the soil remained near field capacity. Both evaporation and transpiration had a positive response to an increase in precipitation. However, the proportion of total water loss following these pathways decreased with increasing precipitation. Drainage had a positive exponential relationship with winter precipitation. Probabilities of soil water potential higher than – 1 MPa in the upper soil layer were very low during most of the warm season (P<0.15). At the beginning of the growing season the wettest layer was located at an intermediate depth (10—20 and 20—40 cm), and moved downward so at the end, only deep roots had high soil water availability.
Density‐Dependent Processes in Meadow Voles: An Experimental Approach
Authors: Ostfeld Richard S. , Canham Charles D.
Published: May 2006

Patterns of population dynamics are determined by the interaction between density—dependent and density—independent processes. To examine the responsiveness of various demographic and behavioral processes to population density, we maintained replicate meadow vole (Microtus pennsylvanicus) population at low, medium, and high densities inside field exclosures in southern New York. Density was manipulated by removing subadults during biweekly live—trapping sessions. Under higher density conditions voles exhibited: (1) a truncated breeding season; (2) reduction in growth rates of individuals; (3) increase in age (but not in size) at sexual maturity; and 4) reduction in movement distances of individuals. Extremely large—bodied voles were most frequent in low—density enclosures and least common in high—density ones. Survival rates of adults were delayed density—dependent. Reproductive rates during the middle of the breeding season and survival rates of juveniles and subadults were not affected by vole density. We conclude that vole populations are stabilized by density—dependent reductions in reproductive rate and recruitment, but destabilized by density—independent survival rates of young, and by density—dependent compression on individual movements, which accommodates population growth. The time delay in the effects of density on survival of adults may provide a mechanism for vole cycles in which lagged density—dependent mortality plays a central role.
Community Changes Associated with the Spread of an Introduced Seagrass, Zostera Japonica
Authors: Posey Martin H.
Published: May 2006

Species introductions have provided a valuable source of information for understanding the factors that regulate community composition. However, the effect of such introductions has often been obscured by a lack of information on distribution and abundance patterns before or during an invasion event. I examined the changes in a benthic community associated with the ongoing spread of an introduced seagrass, Zostera japonica, by sampling transplanted seagrass plots and established Z. japonica patches of known ages. The sedimentary and faunal changes associated with Z. japonica were similar to those observed with native seagrass species. Mean sediment grain size declined and sediment volatile organics increased within Z. japonica patches. Faunal richness was higher within Z. japonica patches compared with adjacent unvegetated areas, and many numerically dominant species were positively associated with this introduced seagrass. However, the effects of Z. japonica on faunal abundance varied with both the age of a seagrass patch and site location. The introduction of this seagrass has thus changed the physical habitat as well as the richness and densities of resident fauna. Many studies of introduced species have concentrated on direct interactions between introduced and native organisms. In contrast, the community changes associated with the introduction of Z. japonica emphasize the potential importance of indirect and system—level effects of introduced species on community composition.
Analysis of the Functional Response in Foraging in the Sitka Black‐Tailed Deer
Authors: Spalinger D. E. , Hanley T. A. , Robbins C. T.
Published: May 2006

The hypothesis that forage intake rate in large (i.e., vertebrate) herbivores is a function of plant availability (biomass), bite size, and forage fibrousness was tested with tame Sitka black—tailed deer (Odocoileus hemionus sitkensis). Deer were offered single—species diets of eight commonly consumed plants or plant parts that provided a variety of bite sizes and chemical or structural characteristics. The hypothesis that intake rate is a function of plant availability was tested by varying the density of plants offered on an artificial "pasture" and measuring consumption rates. Intake rate was independent of plant availability, except for plants of small bite size at densities <0.5 g/m2. Intake rate was highly correlated with bite size and plant fibrousness, measured as neutral detergent fiber concentration. We conclude that intake rate is a direct function of processing constraints (chewing) imposed by plant tissues on the herbivore. Bite size indirectly affects intake rate because as bite size declines, the animal attempts to compensate by increasing bite rate. Since cropping directly competes with chewing, intake rate declines. Because bite size and fibrousness of plant tissues affect foraging time, foraging selection by large herbivores may be significantly affected.
Persistence and Structure of the Fish Assemblage in a Small California Stream
Authors: Moyle Peter B. , Vondracek Bruce
Published: May 2006

The persistence and structure of a small but morphologically diverse fish assemblage of a California stream was examined to see if it had the characteristics of a deterministically regulated or stochastically regulated community. We evaluated persistence by monitoring the fish populations for 5 yr and examined resource use by measuring summer microhabitats and diets of each species. We also compared the microhabitats and diets of the juveniles and adults of each species to investigate the hypothesis that in stream fish assemblages at low diversity, juvenile fishes essentially function ecologically as separate species. Species composition in each of four stream sections was persistent over the 5—yr period, which included extreme flood conditions. Differences in species composition among sections reflected differences in their physical characteristics. The most abundant species were segregated by habitat, microhabitat, and/or diet. In addition, in all but one species, young—of—year were ecologically segregated from the adults; this increased the functional complexity of the assemblage. The observed fish assemblage thus appears to have the characteristics of a highly structured community, as expected under Connell's “niche diversification hypothesis.” The structure observed is probably characteristic of fish assemblages in cold—water streams throughout North America that contain the same or similar species.
Habitat Use and Habitat Overlap of Riparian Birds in Three Elevational Zones
Authors: Finch Deborah M.
Published: May 2006

I examined patterns of variance in habitat use and habitat overlap in 20 breeding bird species found along a riparian vegetational gradient in southeastern Wyoming to test whether habitat use in species differed (1) from availability of random habitat resources, (2) among elevation zones, and (3) between species that inhabited only one zone and species that occupied multiple zones. I sampled habitat features in bird territories and at random stations on 10 8.1—ha study grids distributed over an elevational gradient of 940 m. Principal components analysis was performed on both randomly sampled and territory—centered habitat data to examine the habitat use of each bird species in relation to the random centroid in n—dimensional habitat space. Using this transformed data set, I computed habitat size of each species, defined as degree of specialization in habitat use; species—habitat position, defined as use of common or scarce habitat; habitat overlap among species; and sum of variation in structure of the available habitat. Habitat size was significantly smaller for bird species found in subalpine riparian shrublands than for those found in foothill woodlands, but this result was partly a reflection of variation among elevational zones in structural diversity of the available habitat. The null hypothesis that species used habitat as it occurred randomly was rejected for all three subalpine bird species examined, 6 of 10 dominant mid—elevation species, and 1 of 10 lowland species. Habitat overlap among species did not vary among zones, but habitats used by lowland species were closer to the random expectation than habitats used by mid—elevation or subalpine species. In general, species restricted in distribution to one zone had smaller habitat sizes than species occupying multiple zones, but some zone—dependent species were generalists within their zones. Vegetation of lowland riparian habitats was structurally more complex than that of riparian habitats at higher elevations, and bird species richness and bird abundance were greatest in riparian lowlands. Because most lowland species were generalized in habitat use and occupied habitats that were similar among species, I concluded that bird species diversity was greater in lowlands than in mid—elevation and subalpine shrublands merely because woodlands were more heterogeneous. The large proportion of generalists in deciduous woodlands of the central Rocky Mountains may be explained by geographic dispersal patterns. Woodland species may have been historically prevented from dispersal by the ecological barrier of the Great Plains grasslands. Recent establishment of widespread riparian forest permitted colonization of woodland bird species, with generalists colonizing first.
Arboreal Perch Properties and the Operative Temperature Experienced by Small Animals
Authors: Bakken George S.
Published: May 2006

The effects of perch properties on the thermal environment of small animals were explored by placing hollow electroformed copper lizard models on simulated tree trunks of varying properties. The operative temperatures (Te) of models on perches were compared to reference models suspended away from the perch. Operative temperature increased with increasing perch diameter, decreasing heat storage capacity, and increasing absorption of solar radiation by the perch. Models on log perches measured Te values up to 12°C higher than the reference models. These results indicated that perch surface temperature and the heated thermal boundary layer of the perch were the primary factors increasing Te. Simulated thermoregulatory posture adjustments could affect Te by up to 6.5°. Movement from the sun to the shade side of 16+ cm diameter perches could change average Te by up to 18°. The thermal effects of sunlit aboveground perches should be explicitly considered in thermal energy budget models or thermal maps.
Patterns of Fish and Urchin Grazing on Caribbean Coral Reefs: Are Previous Results Typical?
Authors: Hay Mark E.
Published: May 2006

Strips of the seagrass Thalassia testudinum were used in a field bioassay to assess herbivory on 11 coral reefs scattered throughout the Caribbean. Patterns of herbivory on overfished reefs in Haiti and the United States Virgin Islands were compared to patterns on seven less fished reefs (United States Virgin Islands, Panama, Honduras, Belize, and the Bahamas). On the overfished reefs, the rate of Thalassia removal increased significantly with depth, urchin densities were high, and urchin grazing was equal to, or greater than, fish grazing in shallow (<10 m deep) habitats. On reefs subject to little fishing pressure, the rate of Thalassia removal decreased with depth, urchin densities were low, and herbivorous fishes were responsible for almost all Thalassia removal. Previous studies assessing the importance of urchin grazing in the Caribbean have been conducted on overfished reefs where urchin densities were unusually high and the density of grazing fish unusually low. It is doubtful that the intensity of urchin—algal and urchin—coral interactions observed on these heavily fished reefs occurs on reefs unaffected by humans.
Variable Ingestion Rate and Its Role in Optimal Foraging Behavior of Marine Deposit Feeders
Authors: Taghon Gary L. , Jumars Peter A.
Published: May 2006

Tests of optimal foraging theory have focused generally on food item selection by mobile, high—trophic—level predators. Deposit—feeding invertebrates are aquatic organisms with limited mobility and hence limited ability to forage actively for food—rich patches. In addition, there is little evidence for a major role of behaviorally mediated food item choice in these animals, and growing evidence of mechanical limitations in food particle choice. Given such limited food—selection ability, varying ingestion rate in response to changes in food value is likely to be an important animal response affecting feeding energetics. A previously developed optimal foraging model predicted that ingestion rate and food value should covary positively in order to maximize net time rate of energy gain. To test this general prediction, we fed three species of deposit—feeding polychaetes artificial sediments which varied only in protein content (food value); other physical and chemical properties which might affect ingestion rate were kept constant. In support of the model, ingestion rates increased as protein levels increased.
Some Adaptations of Gastropods to Physical Stress on a Tropical Rocky Shore
Authors: Garrity Stephen D.
Published: May 2006

Activity patterns, microhabitat occurrences, and behaviors during low tides of 10 common gastropods inhabiting rocky intertidal shores of the Pacific coast of Panama were examined. All species were active when wet or washed by rising or falling tides, but were inactive during periods of emergence and subemergence. During daytime low tides, most snails occurred in cervices, on vertical surfaces, or in tidepools; few were found on horizontal or sloping rock. Measurements of substrate temperature, water evaporation, and snail tissue temperature indicated that daytime physical stress at a given tidal level increased along the microhabitat gradient; cervices < tidepools < vertical surfaces < slopes < Horizontal surfaces. In addition to the above mechanisms, six gastropods showed specific behaviors during sunny low tides that apparently further ameliorated stress. The importance of these observed patterns was tested in the field. Experiments included (1) transfers of gastropods from "refuge" microhabitat to adjacent, horizontal or sloping surfaces during low tides; and (2) manipulations of possible thermoregulatory or antidesiccatory behavior. Differences between snails in experimental and control treatments were measured by (1) amount of mass lost, (2) changes in tissue temperature, and (3) relative survival. For the predatory gastropods, one herbivorous snail, and two limpets, microhabitat was vital importance in reducing stress. However, several snails and a limpet had additional behavioral or morphological adaptations that further lessened stress and allowed them to persist over a wider range of microhabitats. A neritid formed nultilayered aggregation, loss extravisceral water at a controlled rate, and reduced tissue temperature by evaporative cooling; two littorinids used mucus to attach or orient shells to lessen heat gain as well as to reduce water loss; and at least one pulmonate limpet maintained a raised posture on its home scar to lessen contact with substrate and allow evaporative cooling. Gastropods were ecologically constrained by physical stress. They were constrained also in time and space by predaceous fish during high tides. Thus intense, complementary, and cyclically occurring biotic and abiotic factors severely limited foraging for the gastropods of this region.
Field Energetics and Foraging Mode of Kalahari Lacertid Lizards
Authors: Nagy Kenneth A. , Huey Raymond B. , Bennett Albert F.
Published: May 2006

We examined the energetic costs associated with foraging mode in the widely foraging lizard Eremias lugubris (mean mass 3.83 g) and the sit—and—wait lizard Eremias lineoocellata (3.27 g). These lizards are broadly sympatric in the Kalahari desert. Individuals of both species were probably abroad every day, but the wide forager was abroad for much shorter periods (2.75 h/d vs. 10.25 h/d). Nevertheless, the widely foraging species had significantly higher field metabolic rates (800 vs. 544 J/d, as measured with doubly labeled water), feeding rates (metabolizable energy of 1165 vs. 739 J/d), production rates (365 vs. 195 J/d) and water influx rates (0.285 vs 0.156 mL/d). Measurements were made before the productive season began; there were no significant differences in these measures between sexes within either species. Resting metabolic rates (measured as O2 consumed) were similar at 37°C (0.240 vs 0.252 mL°g—1°h—1) and at 26° (0.094 vs. 0.103 mL°g—2°h—1), the field active and nocturnal burrow temperatures, respectively, of both species. Field metabolic rates, on a 24—h basis, were 3.1 x resting in E. lugubris and 2.2 x resting in E. lineoocellata. Energy expenditures during the activity period were 12.0 x resting in the wide forager and 2.8 x resting in the sit—and—wait predator. Foraging efficiency (metabolizable energy gained while foraging/total energy spent while foraging) was higher in the wide forager (2.0) than in the sit—and—wait predation (1.6). However, when foraging efficiency is expressed as metabolizable energy gained/activity cost (above resting cost), the wide forager (2.2) was less effective than the ambush predator (2.6), apparently because the cost of searching is substantial for E. lugubris but negligible for E. lineoocellata. The hourly cost of foraging was much higher for E. lugubris (almost 5 x that of E. lineoocellata) but the hourly benefit of foraging for E. lugubris was even higher (nearly 6 x that of E. lineoocellata). Thus E. lugubris made a greater profit on its investment (46 vs. 36% each day), and the wide forager grew nearly twice as fast as did the sit—and—wait predator during this study. On an annual basis, variation in food availability of differences in predation rate may alter the relative fitness of these foraging modes.
Adaptation to Frugivory of Mediterranean Avian Seed Dispersers
Authors: Herrera Carlos M.
Published: May 2006

Morphological and digestive correlates of seed dispersal were investigated in 41 bird species of mediterranean scrublands of southern Spain. Seed dispersers (feeding on whole fruits and voiding seeds unharmed) are indistinguishable from nonfrugivores and fruit predators (feeding on pulp or seeds and not performing dispersal) in the ratio of gizzard mass, liver mass, and intestine length to body mass, but differ significantly in bill morphology and average gut passage time (GPT). Seed dispersers tend to be flatter and broader billed than other groups, and have a wider mouth relative to bill width. GPT of seed dispersers is significantly shorter than that of other groups. Morphological correlates of seed dispersers suggest that adaptations for insectivory serve as preadaptations for feeding on whole fruits. Shorter GPT's, in contrast, appear to be an adaptation indispensable to sustained, intense frugivory. Seasonal frugivory imposes limits on commitments to permanent structural modifications, and more subtle functional adaptations are to be expected. In the context of plant—disperser coevolution, unapparent adaptations of seed dispersers to plants are as relevant as the more conspicuous structural adaptations reported for year—round frugivores. In mediterranean scrublands they suffice to maintain a high—efficiency seed dispersal system.
The Ecology of Plants, Large Mammalian Herbivores, and Drought in Yellowstone National Park
Authors: Frank Douglas A. , McNaughton Samuel J.
Published: May 2006

The purpose of this study was to examine the effect of abundant native large herbivores on ecosystem function of a spatially and temporally heterogeneous temperate grassland. Net aboveground primary production (ANPP), large herbivore consumption (C), and dung deposit (D), an index of nutrient flow from herbivores to the soil, were measured in grassland ad shrub—grassland habitat on winter, transitional, and summer range used by herbs of elk (Cervus elaphus) and bison (Bison bison) in northern Yellowstone National Park. Temporary exclosures (5—7 per site) were moved ever 4 wk during the snow—free season to determine ANPP and C. Data were collected during 1988, a year of drought and unusually high elk and bison population levels, and 1989, a climatically near—average year, with dramatically fewer elk and bison. All three processes, ANPP, C, and D, varied widely among sites: ANPP range: 16—589 g/m2, C range: 0—306 g/m2, and D ranged: 0—68 g.m2. An average of 45% of ANPP was consumed by herbivores. Production and consumption, and consumption and dung deposition were positively correlated across all sites. In addition, sites were grazed when plants were growing. There was a 19% reduction in ANPP from 1988 to 1989, likely caused by death or injury to plants during the 1988 drought. Drought also appeared to be partially responsible for reductions in elk and bison from 1988 to 1989, which were coincident with declines in C and D. Results indicate direct effects and suggest indirect effects of a single—season drought on grassland function that will persist for several years after the event.
Density‐Dependent Mutualism in an Aphid‐Ant Interaction
Authors: Breton Lorraine M. , Addicott John F.
Published: May 2006

Our objectives were to determine if the mutualistic effect of ant tending on aphid population growth varied with aphid density, and to consider possible mechanisms explaining such a context—dependent effect. We studied the interaction between Aphis varians and Formica cinerea on fireweed (Epilobium angustifolium) in the Rocky Mountains of Colorado. In 1988 we used two experiments to examine the interaction between ant tending to aphid density on the per capita growth of aphid populations at naturally occurring low aphid densities, and naturally occurring high aphid densities. In 1989, we conducted a similar experiment, but we manipulated aphid densities to include the full range from low to high. The results of these experiments were consistent: ant tending significantly improved the growth of small aphid populations, but the benefit from ant tending decreased or disappeared at higher aphid densities. We considered possible mechanisms for the density—dependent mutualistic effect by examining the relationships among aphid density, the number of ants tending, and predator abundance. We show that the relative number of ants tending declined as aphid density increased, and ant tending did not reduce predator abundance at any aphid densities. Therefore, the cause of the density—dependent mutualism is probably a decline in the direct effect of ants on aphids resulting from a decline in the relative number of ants tending aphids at higher aphid densities. We discuss the implications of density—dependent mutualism for the dynamics and evolution of mutualistic systems.
Nesting Synchrony by Female Red‐Winged Blackbirds: Effects on Predation and Breeding Success
Authors: Westneat David F.
Published: May 2006

I analyzed some of the factors affecting female breeding success in an eastern population of Red—winged Blackbirds (Agelaius phoeniceus). Over three seasons, 37% of all nests produced at least one fledgling. Over 77% of nests that failed were destroyed by predators. Nests in all 3 yr were clumped in time, with 50% of the nests being started in just 16% of the season. Temporal proximity of other nests (number of other nests started within ±2 d) was positively associated with the number of chicks fledged. The probability of avoiding predation increased with number of temporal neighbors even with date of first egg and female age controlled in a multivariate analysis. Clutch size was associated only with settlement date and not number of temporal neighbors nor date of first egg. Breeding earlier by females resulted in higher total success for some measures in some seasons, but early nests failed sooner and more frequently. Returning females bred earlier than new females, and even though their first nests were more likely to fail, they typically renested within the peak of nesting or, if their first nest succeeded, sometimes started a second brood before the end of the season. New females to the study are started nests late, but tended to do so within the peak of nesting thereby improving their changes of success. These results highlight the importance of considering demography in studies of the ecological causes of synchronous breeding and the timing of avian breeding seasons.
Tropical Avian Phenology in Relation to Abundance and Exploitation of Food Resources
Authors: Poulin Brigitte , Lefebvre Gaetan , McNeil Raymond
Published: May 2006

We studied avian breeding and molting activity in relation to rainfall, temporal fluctuations in food resource abundance, and food exploitation by birds, in four arid and semiarid tropical habitats in Venezuela. Twice a month we used mist to monitor changes in breeding and molting conditions of captured birds and forced them to regurgitate to determine their diet and feeding guild membership. Food abundance was assessed by measuring the flowering and fruiting seasonality of marked plants and by evaluating arthropod abundance with four different trapping methods. Flowering activity was limited largely to the wet season. Fleshy fruits, although produced year—round, were also more abundant in the rainy period. Anthropod abundance followed the same general pattern with numbers highest in the wet season and lowest in the dry season. Birds of all feeding guilds predominantly bred and molted during the wet season, synchronously with the highest abundance of most food resources. However, the diet analysis revealed a higher occurrence of arthropods coupled with a sharp decrease in the intake of vegetable matter during birds' breeding season. Consequently, we suggest that arthropod abundance is a crucial factor governing the timing of breeding activities, even in species that normally include a high proportion of nectar and fruits in their diet. We also postulate that, in tropical habitats receiving >1500 mm of rain per year, breeding in nectarivores and frugivores in the dry season may be related to the lower reduction in arthropod numbers over the less severe drought period.
Sexual Dimorphism and the Annual Flowering Pattern in Jacaratia Dolichaula (D. Smith) Woodson (Caricaceae) in a Costa Rican Rain Forest
Authors: Bullock Stephen H. , Bawa Kamaljit S.
Published: May 2006

Flowering phenology and other aspects of reproductive biology were investigated to determine the extent of differences between male and female plants of Jacaratia dolichaula (Caricaceae), a small tree of neotropical lowland rain forests. The study revealed a number of differences between sexes. (1) The sex ratio was 1.08 males: 1 female, though the "opertional sex ratio" ranged from 10.0:1 to 1.4:1 during the blooming period. (2) The male trees tended to mature earlier than females, as indicated by the frequency distribution of size classes; however, there were no differences in growth rates between sexes over a 1—yr period. (3) The flowers were strongly dimorphic; male flowers had a long corolla tube and nectar, but female flowers had free petals and no nectar. Pollination was presumed to be effected by intersexual mimicry of nonhomologous flower parts. (4) The population flowered once per year for °9 mo from October to June, but with a distinct peak in February. Female trees tended to begin flowering later in the season than males, and had individual flowering periods less than half as long as males. (5) Females produced fewer flowers per year than males by a factor of °20. (6) Although the diameter at breast height and the size of the flower crop were correlated in both male and female trees, the correlation was stronger in males than in females. However, in females, the correlation between trunk size and fruit production was better than for flower production. (7) Female trees flowered less frequently than males, and there were marked fluctuations in fruit production between years. (8) There was no variation in sex expression of individual trees in 4 yr. The differences in the reproductive features of male and female plants are discussed in the context of sexual selection, dynamics of the pollination system, and energetics.
Parasitoids, Patches, and Phenology: Their Possible Role in the Local Extinction of a Cynipid Gall Wasp Population
Authors: Washburn Jan O. , Cornell Howard V.
Published: May 2006

The cynipid gall wasp Xanthoteras politum (Bassett) and its associated chalcid and inquiline parasitoid community were studied from 1976 to 1978 at a postfire site near Lake Oswego in the New Jersey Pine Barrens. X. politum and two other cynipid species selectively attacked new sucker shoots growing from larger fire—damaged trees. Chalcid and inquiline parasitoid attack was the major mortality factor in the population in all 3 yr. In each successive year a higher frequency of the host population was killed by parasitoid attack, and by 1978 on 76 host galls occurred at the site. Inquiline cynpids were minor parasitoid elements the 1st yr of the gall outbreak, but dominated the parasitoid guild in subsequent seasons. Their dominance is attributed to two major factors. First, they attack galls earlier than other parasitoids, and those galls attacked remained at a smaller size and were not chosen for oviposition by later arriving chalcids. Secondly, inquilines did not emerge in spring and infest other gall species as did the chalcid parasitoids. These inquilines remained in the leaf litter and reinfested the next asexual generation of X. politum which appeared on trees in mid—July. The parastioid community displayed neither strong stability nor consistent composition. The apparent overexploitation of the host population by parasitoids may be to their selective advantage as phenological changes in host plants eventually eliminate X. politum from postfire sites anyway.
On Determining the Statistical Significance of Discontinuities with Ordered Ecological Data
Authors: Cornelius Joe M. , Reynolds James F.
Published: May 2006

Current ecological theory hypothesizes that boundaries between adjacent ecosystem units are important in determining ecosystem structure and function across heterogeneous landscapes, and that such boundaries are potentially important sites for early detection of global climate change effects. Hence, there is an increasing research effort to elucidate the structure and function of ecological boundaries. Yet traditional data analysis methods focus primarily on homogeneous units rather than on the boundaries between them; thus, new methods are being developed for detecting, characterizing and classifying boundaries, e.g., split moving—window boundary analysis (SMW). SMW is a simple yet sensitive method for locating discontinuities that may exist within multivariate, serial data (ordered in one dimension) at various scales relative to the length of the data series. However, SMW is subjective and relative, and therefore locates apparent discontinuities even within random, serial data. In this paper we present two nonparametric methods for determining the statistical significance of discontinuities detected by SMW. First, we describe a Monte Carlo method for determining the statistical significance of scale—dependent discontinuities (i.e., discontinuities that are significant relative to only one scale). Second, we propose a nonparametric, scale—independent method (it also is dependent upon scale size, but to a much lesser degree than the Monte Carlo method) that is more appropriate for locating statistically significant discontinuities that separate different, relatively homogeneous groups of varying size along a series. We examine the robustness of these two methods using computer—generated data having varying intensities of imposed discontinuities, and illustrate their application to locating boundaries between vegetation samples collected at systematic intervals across a desert landscape in southern New Mexico, USA.
Spatial Heterogeneity in Freshwater Zooplankton: Variation with Body Size, Depth, and Scale
Authors: Pinel-Alloul Bernadette , Downing John A. , Perusse Martin , Codin-Blumer Gabriella
Published: May 2006

The effects of body size, depth, and sampling scale on spatial heterogeneity were examined in the zooplankton community of a small lake. Analyses were performed by regression analysis of 27 sets (3 scales ° 3 depths ° 3 dates) of replicate (n = 4) samples of the natural zooplankton (cladocerans, copepods, rotifers) community of Lake Cromwell, Quebec, Canada. Spatial heterogeneity was measured as the variance among the four randomly arranged replicate samples of taxa taken at each scale—depth—date combination. The spatial distribution of populations of zooplankton in this community was found to be typical of the spatial heterogeneity encountered in other freshwater and marine ecosystems. The effect of population density on spatial heterogeneity was comparable to that found for other flora and fauna. Small animals were more heterogeneous than large ones, and populations sampled on large spatial scales or at greater depths showed greatest spatial variability. These effects were stable over the season. Few taxa or life stages diverged significantly from these trends.
Predation and the Evolution of Sociality in the Paper Wasp Polistes Bellicosus
Authors: Strassmann Joan E. , Queller David C. , Hughes Colin R.
Published: May 2006

We investigated the role of nest predation in selecting for group living in the primitively eusocial wasp Polistes bellicosus. Predation rates were so high that most colonies had to build new nests at least once during the season. To determine the responses of females to nest predation we marked all females on 49 nests and then removed the nests without harming the adults. Larger groups of females were more successful at building new nests. There was a per capita advantage in nest success for females in groups as measured 44 d after the simulated predation event, and the advantage did not diminish with increasing group size. Reproductive competition among females was dependent on number of females on the nest and age of females, and not on whether the nest was original or rebuilt. Ovaries developed as females grew older and were more developed in females on nests with greater numbers of females. Group living may be maintained in this population of Polistes bellicosus because of the advantages larger groups have in recovering from nest predation.
The Ecophysiological Significance of Leaf Movements in Rhododendron Maximum
Authors: Boa Yijia , Nilsen Erik T.
Published: May 2006

The patterns of leaf movements and their environmental stimuli have been documented for over a century. Although there have been several studies on the physiological significance of heliotropisms, thermotropic movements have been largely ignored. The objective of this study was to evaluate the ecophysiological significance of thermotropic leaf movements in a subcanopy plant through controlled experiments. Rhododendron maximum L. was selected because of its strong winter season, thermotropic leaf movements, and the ease of identifying even—aged leaf cohorts. A series of leaf manipulations was established to restrict curling, changes in leaf angle, or both. The effects of these treatments on leaf energy budget, chlorophyll content, photosynthesis, and leaf survival were evaluated. Leaves that were maintained in a horizontal position (whether they were allowed to curl or not) had the greatest decrease in chlorophyll content and the most inhibited photosynthetic rate. Reduced photosynthetic potential was due to both lower quantum yield and lower light—saturated nthetic rates. Optimal temperature for photosynthesis was several degrees above air temperature, corresponding to a horizontal leaf temperature in a sunfleck. Leaf angle influenced leaf temperature during daylight and night time; but the influence was small and most likely had no ecophysiological significance. Leaf curling had little to no influence on the ecophysiological parameters measured in this study.