bluuebunny/crossref_metadata_2025 huggingface dataset:
Pruning of Host Plant Neighbors by Ants: An Experimental Approach
Authors: Davidson Diane W. , Longino John T. , Snelling Roy R.
Published: May 2006

Some ants of myrmecophytic plants either obligately or facultatively prune vegetation surrounding their host trees. Pruning behavior occurs at higher frequency in ants with sting defenses than in those with chemical defenses, which may generally convey an advantage in aggressive encounters between ants. Experiments in tropical moist forest of Peru tested the hypothesis that pruning may reduce the threat of invasions by potentially dangerous alien ants. When unseverable contracts were made between Triplaris americana 1—3 m in height and neighboring plants, Crematogaster ants invaded more frequently and in greater numbers than on control plants and inhibited activities of behaviorally subordinate resident Pseudomyrmex dendroicus. Invaders occasionally carried away brood and usurped hollow stem nests of resident ants. Pseudomyrmex cut petioles of leaves on plants contacting their hosts as well as leaves of their own host plants, when these leaves formed bridges for major invasions. Basal clearings maintained by Pseudomyrmex around their hosts appear to reduced the likelihood that Crematogaster workers will occur in the vicinity of the tree trunk. On Cordia nodosa inhabited by Allomerus demararae, numbers of invading ants did not differ on experimental and control trees, perhaps because larger ants cannot invade through dense trichomes. Allomerus workers attacked encroaching vines only when these vines bore large numbers of Crematogaster.
Chemical Ecology of the Tiger Swallowtail: Mediation of Host Use by Phenolic Glycosides
Authors: Lindroth Richard L. , Scriber J. Mark , Hsia M. T. Stephen
Published: May 2006

Subspecies of the eastern tiger swallowtail butterfly exhibit striking differences in their ability to use quaking aspen (Populus tremuloides) and other members of the Salicaceae as larval host plants. Papilio glaucus canadensis survives and grows well on aspen, whereas Papilio glaucus glaucus does not. In earlier studies we isolated a crude fraction of aspen compounds that exhibited activity against P. g. glaucus and identified the components as a suite of four phenolic glycosides (salicin, salicortin, tremuloidin, and tremulacin). This study was designed to identify the specific phenolic glycosides, or interactions among glycosides, responsible for the differential abilities of Papilio subspecies to utilize quaking aspen. We bioassayed the glycosides individually and in combination against both Papilio subspecies, using neonate survival trials and fourth—instar feeding trials. None of the compounds or combinations of compounds negatively affected the survival, growth, consumption rates, or digestibility/conversion efficiencies of P. g. canadensis, or 72—h survival rates of P. g glaucus. Salicortin and tremulacin significantly increased fourth—instar duration and decreased growth rates for P. g. glaucus, primarily by reducing consumption rates. Salicin and tremuloidin showed no negative effects. Combinations of glycosides containing salicortin and tremulacin decreased larval survival and a dramatically lowered growth rates by decreasing both consumption rates and food conversion efficiencies. Observations of treated larvae indicated that these glycosides may have caused gut lesions. The active component of salicortin and tremulacin is a cyclohexeone saligenin ester, and its activity is synergized in tremulacin by the presence of a benzoyl ester on the same molecule. We propose that differences in the susceptibilities of P. g. canadensis and P. g. glacus to the active phenolic glycosides are due to differences in activity of their carboxylesterase detoxication systems.
Resource Partitioning Among Some Eusocial Insects: Bumblebees
Authors: Heinrich Bernd
Published: May 2006

Four common species of bumblebees in central Maine each utilized the nectar from a great variety of both native and exotic flowers. The number of individuals that utilized a nectar resource was directly related to supplies available. Seasonally, ° 92% of the “standing crop” of food energy available per day was utilized in 1972. No interference competition was observed, and competition was primarily exploitative. Small wild bees generally foraged at high temperatures from the small food rewards left after Bombus exploitation. Overlap in resource utilization in the guild of bumblebees was avoided by differences in tongue length. Open (short—corolla) flowers were visited primarily by short— and secondarily by long—tongued bumblebees. Deep—corolla flowers were visited primarily by long—tongued bumblebees. Pollen flowers were utilized more by short—tongued than by long—tongued bumblebees. However, the bumblebee forager spectra on given flower species varied from one locality to the next as well as seasonally. Colony polylecty is probably a necessary correlate of social behavior, given short blooming times of the host plants relative to the long durations of the colony cycles. In turn, colony polylecty of eusocial bees may decrease species richness in the bee community due to competition.
Growth Characteristics of Weediness in Portulaca Oleracea L.
Authors: Zimmerman Craig A.
Published: May 2006

The growth responses of Portulaca oleracea plants from a central Michigan population are compared with representative populations of Portulaca grandiflora, a cultivar, and Portulaca pilosa, a plant of southern and relatively narrow distribution in this country. In comparison tests, and species show a significant interaction with photoperiod, light intensity, temperature, and soil types. Portulaca oleracea is a weed, in part because its process and pattern of growth give the plant quick response capability. In contrast to its companion species, P. oleracea uses a wide variety of photoperiods, and capsule numbers are positively correlated with amounts of light received. This weed is widely tolerant of light intensities, temperature regimes, and soil types, and the plants produce adequate levels of capsules over a wide range of these factors. In addition, individuals have the ability to produce large numbers of capsules rapidly when high levels of these factors become available.
Weather and Reproductive Variation in Shrubsteppe Sparrows: A Hierarchical Analysis
Authors: Rotenberry John T. , Wiens John A.
Published: May 2006

We used a hierarchial approach to investigate the relationships among within— and between—year variation in precipitation and temperature in the northern Great Basin and components of reproductive success in two species of common shrubsteppe sparrows. During a 5—yr period we monitored clutch sizes, brood sizes, and fledging success in Sage (Amphispiza belli) and Brewer's (Spizella breweri) Sparrows in shrubsteppe habitat in central Oregon. During the 5 yr of our study, the Pacific Northwest experienced precipitation extremes, with the driest bioyear (October—April) in 65 yr of records (1976—1977) followed by one of the wettest. Within each breeding season daily temperature and precipitation fluctuated widely and in most cases unpredictably over a scale of a few days. Both species achieved significantly greater final reproductive success (as measured by number of fledglings produced) in wetter, presumably more biologically productive years. In Brewer's Sparrows this was achieved through higher clutch sizes in wetter years, whereas Sage Sparrows had higher hatching rates. For Sage Sparrows, within—year weather variables accounted for a significant amount of variation in reproductive parameters beyond that statistically explained by bioyear precipitation alone. However, in Brewer's Sparrows addition of within—year weather variables was not significant, and thus this species appeared buffered from the effects of short—term variation in abiotic conditions. The observed patterns suggested that clutch—size variation in Brewer's Sparrows may represent adaptive flexibility, the ability to assess potentially favorable environmental conditions and adjust reproductive investment accordingly. In contrast, in Sage Sparrows the association between bioyear precipitation and final reproductive success seemed most likely to be a proximate consequence of resource availability at the time of oogenesis, and to be further modified by the effects of weather that each nest experienced.
The Effects of a Successional Habitat Mosaic On a Small Mammal Community
Authors: Foster Johanna , Gaines Michael S.
Published: May 2006

Small herbivorous mammals living in successional patchy environments may be affected by both patch size variations and successional changes. If patch size affects densities of residents then larger patches should have higher densities compared to smaller patches because larger patches often have greater habitat diversity and lesser chances of stochastic events destroying the population. If patch size affects length of time animals spend on patches (persistence rates) then smaller patches should have lower persistence rates compared to larger patches because smaller patches contain fewer resources. Larger animals may be constrained by body size, as well as population size, to reside only on the larger patches because smaller patches do not contain enough resources. If successional changes affect resident animals that are host specific on colonizing plant species, then the effects may be noticeable immediately following field abandonment because plant species replacement rates are most rapid then. Four small mammal species were included: Sigmodon hispidus, Reithrodontomys megalotis, Microtus ochrogaster, and Peromyscus maniculatus. The field was divided into patches that varied in area by two orders of magnitude with patches of each size treatment placed in each section of the field. Patch size effects on densities and persistence rates of the four mammal species were tested with analyses of variance. Plant species were grouped according to seven life history traits so as to include relative percent cover of most plant species. Multiple linear regression analyses were used to identify which variables (patch size, field sections, and percent relative cover) significantly explained variations in small mammal densities. Effects of patch size on animal densities and persistence rates varied among animal species. The largest species, Sigmodon hispidus, was never found on the smallest patches, but densities of Reithrodontomys magalotis, Microtus ochrogaster, and Peromyscus maniculatus were often highest on the smallest patches. Animal density variations were often significantly explained by patch size using multiple regression analyses (P ≤ .001). However, lack of consistent trends with respect to the other independent variables included in the equations and variable R2 values suggested that other, unmeasured, variables may have influenced the results. Persistence rates of the three species were often highest on the largest patches. Results suggested that animals visited small patches more often than large patches without staying, and used large patches to create territories. Small and medium patches may have been used as archipelagos of preferred habitat interspersed with less desirable habitat. The first stage of old—field succession was completed by the end of the 3rd yr: perennials replaced annuals as the dominant plant species. Neither patch size nor field sections significantly affected percent cover.
Source‐Sink Carbon Relations in Two Panicum Coloratum Ecotypes in Response to Herbivory
Authors: Dyer M. I. , Acra M. A. , Wang G. M. , Coleman D. C. , Freckman D. W. , McNaughton S. J. , Strain B. R.
Published: May 2006

Population samples of an African C4 grass, Panicum coloratum L., were collected from two locations in the Serengeti Grasslands varying in grazing intensity, one a high—grazing location (GA = grazing—adapted), the other a low—grazing location (NGA = nongrazing—adapted). Plants were cloned, put in controlled environments mimicking the natural photo—thermoperiod, and subjected to light grazing pressure by a generalist feeding North American grasshopper, Melanoplus sanguinipes. Carbon assimilation and redistribution were measured in the short term with an infrared gas analyzer and 11C—labelled CO2, coupled with a three—compartment analytical model, and by harvesting whole plants at the end of a 12—wk regrowth experiment. Results documented several significant differences between the GA and NGA samples, suggesting the evolution of physiological traits related to C assimilation, translocation, and storage in response to previous grazing history. Pregrazing net C—fixation rates, translocation rates, and C—storage pools were identical for the two ecotypes. After grazing, overall C—fixation rates were 39% higher for the GA plants, and the regrowth data suggest they remained higher than NGA rates throughout the experiment. Removal of < 10% of initial green—leaf biomass by grazing at each grazing period produced major differences in carbon flux between the two samples. Throughout the experiment GA plants produced and stored more C in leaves, stored less C in stem sinks, had higher phloem activity, and translocated more of the labile C to roots where it was stored in higher quantities. This suggests that storage of labile C reserves in sinks or pools readily accessible to the plant, which allows rapid mobilization after grazing, is an important element of adaptation to grazing. Major storage of labile reserves in stems, characteristic of the NGA plants, may be advantageous in ungrazed habitats where there is vertical growth due to canopy closure and competition for light, but such storage makes those reserves accessible to grazers. GA ecotype plants in the regrowth experiment compensated completely for the eight weekly defoliation events by the time of the 12—wk harvest; yield of the NGA ecotype was reduced 21% by the moderate level of grasshopper grazing. Increased total yield (biomass harvested plus compensation) by the GA ecotype was expressed in both above— and belowground biomass, suggesting that suppression of the latter does not contribute to compensation by the former.
Interactions Between Seedlings of Agave Deserti and the Nurse Plant Hilaria Rigida
Authors: Franco Augusto C. , Nobel Park S.
Published: May 2006

Seedlings of the succulent crassulacean acid metabolism (CAM plant Agave deserti in the northwestern Sonoran Desert were found only in sheltered microhabitats, nearly all occurring under the canopy of a desert bunchgrass, Hilaria rigida. Apparently because soil surface temperatures can reach 71°C in exposed areas, seedlings were generally located near the center or on the northern side of this nurse plant. Both species have shallow root systems, about half of the roots of H. rigida and all those for seedlings of A. deserti occurring above soil depths of 0.08 m. To examine competition for water between the nurse plant and an associated seedling, a three—dimensional model for root water uptake was developed. The model divided the soil into 17 conentric sheaths each subdivided radially into eight wedges and vertically into nine layers. Predicted pre—dawn soil water potentials at the mean root depth and total shoot transpiration agreed well with field measurements. Simulated annual water uptake by a seedling of A. deserti was reduced °50% when the seedling was moved from an exposed location to the center of the nurse plant. Shading by the nurse plant reduced total daily photosynthetically active radiation (PAR) by up to 74% compared with an exposed seedling. On the other hand, soil nitrogen under the canopy of H. rigida was 60%higher than in exposed locations. Assuming that the effects of nitrogen, temperature, PAR, and soil water on net CO2 uptake are multiplicative, the predicted net CO2 uptake by a seedling of A. deserti under the nurse plant was only °45% of that for an exposed seedling. Thus, although the nurse plant facilitates seedling establishment by reducing maximum soil surface temperatures and provides a microhabitat with higher soil nitrogen levels, its shading and competition for water reduce seedling growth.
Rapid Estimation of Coniferous Forest Leaf Area Index Using a Portable Integrating Radiometer
Authors: Pierce Lars L. , Running Steven W.
Published: May 2006

Canopy transmittance was measured at 1200 and 1400 local solar time using an integrating radiometer on seven coniferous forest stands in western Montana, ranging in projected leaf area index (LAI) from 1.7—5.3 m2/m2. Transmittance of each 1—ha stand was measured at 96,000 points, yet measurement required <1 h because the instrument instantaneously integrates 80 radiometer measurements at once. The Beer—Lambert Law was inverted to estimate LAI using measured transmittance and an extinction coefficient of 0.52. LAI estimated by transmittance was highly correlated with LAI measured by sapwood—based allometric equations at both the 1200 (R2 = 0.97) and 1400 (R2 = 0.94) measurement times. The results suggest that the technique has a wide applicability given the range of LAIs, stand densities (450—4140 trees/ha) and illumination angles (32°—57°) under which it was tested.
Daphnia‐Phytoplankton Interactions: Density‐Dependent Shifts in Resource Quality
Authors: Kerfoot W. Charles , Levitan C. , DeMott W. R.
Published: May 2006

In mesotrophic Lake Mitchell, density fluctuations of Daphnia cause rapid shifts within algal assemblages, while total phytoplankton cell densities persist at comparable levels before and after grazer fluctuations. The rapid shifting balance between naked flagellates and several digestion—resistant species is evident in both seasonal patterns and in enclosure experiments. The balance between flagellates and resistant species helps explain asymmetrical demographic responses of competing smaller bodied cladocerans to changes in Daphnia density. Shifts between refractory components and accessible, edible algae constitute a prime reason why general measures of phytoplankton (e.g., chlorophyll α, biomass, biovolume, primary productivity) may fail to reflect impacts of Daphnia fluctuations. Rapid changes in resource quality may also provide a simple explanation for why Daphnia's tendency to show unstable oscillatory dynamics in the laboratory is dampened in natural populations.
Antipredator Defenses and the Persistence of Amphibian Larvae With Fishes
Authors: Kats Lee B. , Petranka James W. , Sih Andrew
Published: May 2006

Communities of larval amphibians in fishless habitats often differ strikingly from those in habitats with fish. We surveyed larvae of 15 amphibian species to determine if presence or absence of specific defenses against fish was correlated with breeding habitat. Each species was tested for two key defenses: unpalatability and chemically mediated predator avoidance. In eight of nine cases, larvae of species that often encounter fish had at least one of these defenses. In contrast, larvae of seven species that breed in fishless pools consistently lacked defenses against fish. Lack of appropriate defenses appeared to be a primary reason why temporary pool species cannot successfully coexist with predatory fishes in permanent habitats. Palatability and responses to chemical cues from fish often differed among closely related taxa and were correlated strongly with frequency of encounter with fish. Thus, natural selection rather than phylogeny best explains interspecific variation in antipredator defenses. Our data show that members of at least two orders and four families of amphibians use chemical cues to reduce predation risk from predatory fish.
Parasite‐Induced Mortality in Mycophagous Drosophila
Authors: Jaenike John , Benway Heather , Stevens Gwen
Published: May 2006

Reductions in host survival due to parasitism have immediate effects on the population dynamics of both parasites and hosts. Despite its potential importance, parasite—induced host mortality has rarely been studied experimentally under natural conditions. We conducted a series of field releases of Drosophila putrida and Drosophila neotestacea that had been parasitized in the laboratory by the nematode Howardula aoronymphium. In 11 out of 12 independent tests, the proportion of flies that were parasitized declined significantly through time, suggesting that parasites cause host mortality. Independent tests showed that parasites have no detectable effect on dispersal of flies and that motherworms do not disappear from surviving hosts, thus supporting the conclusion that parasites cause significant host mortality in the field. Host mortality rate increased with worm burden, significantly so in four independent tests. This is a possible mechanism for density—dependent regulation of these parasite populations. Parasite—induced mortality was considerably greater among flies that had been released in the field than among those kept concurrently in the laboratory. Thus, laboratory or clinical traits may not provide an accurate assessment of parasite virulence in natural populations. These results support a central assumption of many recent ecological and evolutionary models, i.e., that parasites are harmful to their hosts under natural conditions.
Life History Variation and the Coexistence of a Daphnia Hybrid With Its Parental Species
Authors: Spaak Piet , Hoekstra J. Rob
Published: May 2006

Life history variation and genotype composition were studied in two species of the Daphnia longispina group: the relatively large D. galeata (1.4 mm) and the smaller D. cucullata (0.9 mm). Several multi—locus genotypes of these species were compared with genotypes of their interspecific hybrid, D. cucullata X D. galeata. Both species and the hybrid co—occur in the shallow eutrophic Tjeukemeer. For two successive years, laboratory life table experiments were conducted at three temperatures (12.5°, 17.5°, and 22.5°C) and two food levels (carbon concentrations 0.62 and 1.64 mg/mL) to study the mechanisms that permit coexistence of taxa in this Daphnia hybrid species complex. Three—way analyses of variance showed significant main effects (e.g., temperature, food level, taxon) for most reproductive traits (e.g., number of newborns, age at first reproduction, number of juvenile instars, intrinsic rate of increase) among the Daphnia taxa. For size—related traits (e.g., size at maturity and offspring size) only one main effect, taxon, was found. Hybrids were intermediate to the parental species with respect to size and maturity, offspring size, and number of newborns. For developmental traits (number of juvenile instars and age at maturity) the hybrids tended to have the lowest values for all conditions. The intrinsic rate of increase (r) of the hybrid did not differ significantly from that of D. galeata, but was significantly higher than that of D. cucullata. We argue that essentially r will reach higher values for the hybrids than for D. galeata, because of the strong influence of size—selective fish predation. Our results are discussed in reference to different theories about the maintenance of hybrid species complexes in nature.
Effects of Associated Fauna on Growth and Productivity in the Red Mangrove
Authors: Perry Diane M.
Published: May 2006

The ability of herbivores to influence growth in marine plant communities is well documented. However, the role of nonherbivorous fauna in directly affecting growth and productivity in marine plants has only recently begun to emerge. The boring isopod Sphaeroma peruvianum and encrusting barnacle Balanus spp. are closely associated with prop roots of the red mangrove Rhizophora mangle on the Pacific coast of Costa Rica. This study first examines the impact, both spatially and temporally, of these two invertebrates on the growth and productivity of mangrove prop roots. Second, the ability of benthic or water column predators to mediate the extent of the faunal impacts on growth and production of prop roots is assessed. Roots were tagged and monitored seasonally for isopod and barnacle inhabitation and settlement. Experimental caging techniques established that the presence of burrowing isopods on prop roots can cause a 50%, and of encrusting barnacles, a 30%, decrease in root growth rate and a 62% (with isopods) and 52% (with barnacles) decrease in net root production. The magnitude and spatial extent of these effects are mediated indirectly by predation. Aerial roots not yet in contact with the substrate are susceptible to isopod and barnacle recruitment. Aerial roots in contact with the substrate (ground roots) are free from encrusting barnacles primarily because of the foraging activity of a benthic predator, the hermit crab Clibanarius panamensis, which climbs these roots during high tide to forage indiscriminately on newly settled barnacles. Isopod absence from lateral roots that originate from ground roots (lateral—G roots) is explained best by predation rather than dispersal barriers. Isopods burrowed and survived for extended periods in lower lateral—G roots when predators were excluded. Isopods are probably excluded from older roots that have entered the substrate because they cannot burrow into the tough, woody tissues of these roots.
Effects of Host Plant Patch Size on Herbivore Density: Patterns
Authors: Bach Catherine E.
Published: May 2006

The influence of host plant patch size on the population densities of three herbivorous insect species was investigated in seven sizes of experimental patches of squash plants (from 1 to 144 plants). Responses to patch size differed significantly for the three herbivore species, which are closely related but differ in the degree to which they require cucurbits as hosts. The number of individuals of Acalymma vittatum (the striped cucumber beetle) per plant was significantly greater in small patches (1—4 plants) than in larger patches (16—144 plants). The spotted cucumber beetle or southern corn rootworm (Diabrotica undecimpunctata howardi) was most abundant in intermediate—sized patches (64 or 100 plants). Population densities of Diabrotica virgifera (the western corn rootworm) were more variable, but tended to increase with increasing patch size. Relationships between herbivore density and patch size were nonlinear for all three species. The effects of patch size on herbivore population densities also differed between generations of the same species. Second generation Acalymma vittatum were most abundant in large patches, which was opposite to the pattern shown by first generation individuals. Greater numbers of second generation adults emerged in large patches than in small patches, indicating an effect of patch size on larval survivorship and/or oviposition rates. Densities of the spotted cucumber beetle were also differentially affected by patch size at different times throughout the season. Patch size significantly influenced plant growth, as measured by number of leaves and biomass, but did not influence parameters of plant reproduction (numbers of flowers of yield). Plant longevity was significantly greater in larger patches (16 plants or greater) than in small patches (1 or 4 plants). To examine the role of differences in plant size associated with patch size in influencing beetle densities, analyses of the number of beetles per leaf were carried out. Even when differences in plant size were taken into account, patch size still significantly affected densities of all three herbivore species. Thus, beetles responded to patch size per se, not just to associated changes in plant size. Relationships between plant parameters and herbivore parameters were analyzed in detail for the most specialized of the herbivore species, Acalymma vittatum. Beetle densities were positively correlated with numbers of flowers early in the season, but more strongly correlated with numbers of leaves late in the season. Plant longevity was negatively correlated with numbers of early colonizing beetles, indicating a potential effect of herbivory on plant survivorship. Variable herbivore responses to patch size are discussed with emphasis on mechanisms, including plant size/quality and insect movement.
On Productivity as a Predictor of Rodent and Carnivore Diversity
Authors: Owen James G.
Published: May 2006

Tilman's model predicts an asymmetric relationship, with ascending, peak, and descending parts, between plant species diversity and a resource gradient. I evaluated the applicability of Tilman's model to mammals by regressing rodent and carnivore species diversity on estimates of net aboveground primary productivity. Rodent diversity was highest at low productivity levels and declined as productivity increased. Although my data did not confirm the upslope and peak phases of Tilman's model, this may be because I did not sample from areas of extremely low productivity. Published studies from very dry desert habitats indicate that rodent diversity increases rapidly across a narrow gradient of very low productivity and peaks at moderately low levels. This suggests that rodents, which are largely herbivorous, exhibit an asymmetric productivity—diversity curve similar to that predicted by Tilman for plants. The rodent data are also consistent with the intermediate disturbance hypothesis, which predicts diversity as a peaked function of a disturbance gradient. Rainfall predictability, which may function as a disturbance, is inversely related to my plant productivity gradient. The regression curve for carnivores helps to distinguish between these two hypotheses. At a given locality carnivores and rodents should be exposed to about the same disturbance levels and according to the intermediate disturbance model should peak at about the same place. Since carnivores as a whole are about one trophic level above rodents, if plant productivity controls diversity the peak for carnivores should occur at about an order of magnitude of productivity higher than the peak for rodents. My carnivore diversity data exhibit upslope, peak, and downslope phases and are shifted to higher levels of productivity by about one order of magnitude. This provides evidence against the intermediate disturbance hypotheses and corroborates predictions of Tilman's model as applied to mammals.
Agonistic Asymmetries and the Foraging Ecology of Bald Eagles
Authors: Knight Richard L. , Skagen Susan Knight
Published: May 2006

We investigated the effects of both asymmetries and differing food levels on contest outcomes of wintering Bald Eagles (Haliaeetus leucocephalus) feeding on chum salmon (Oncorhynchus keta) carcasses. Large eagles, regardless of age, were more successful in pirating than smaller eagles. Small pirating eagles were usually unsuccessful unless they were adults attempting to supplant other small eagles. Feeding eagles were more successful in defeating pirating eagles according to (1) whether their heads were up to prior to a pirating attempt, (2) how long their heads had been up, and (3) whether they displayed. During periods of food scarcity pirating eagles were less successful, a fact attributed in a proximate sense to the increase incidence of retaliation by feeding birds. When food was scarce and eagles had a choice between scavenging the pirating, they chose to scavenge more often. Body size appears to be an important factor in determining social dominance and influencing differences in foraging modes of wintering Bald Eagles.
Analysis of Steady‐State Populations With the Gamma Abundance Model: Application to Tribolium
Authors: Dennis Brian , Costantino Robert F.
Published: May 2006

We develop a stochastic model for the abundance of adult populations of the four beetle Tribolium. The model is in the form of a stochastic differential equation containing adult recruitment and mortality rates perturbed by multiplicative noise. A deterministic version of the model (an ordinary differential equation) predicts a fixed, stable equilibrium; by contrast, the stochastic model predicts a stationary probability distribution for population size. The model can be approximated closely by a stochastic logistic equation having a gamma distribution as a stationary solution. We develop or clarify various practical aspects of making statistical inferences for the resulting gamma abundance model, including estimation of parameters, testing goodness of fit, obtaining confidence intervals for functions of parameters, and testing to compare two gamma distributions. Analyses of 11 data sets on Tribolium suggest that the gamma model deserves more widespread consideration as an equilibrium abundance model for other species.
Assessment of Sampling Stability in Ecological Applications of Discriminant Analysis
Authors: Williams Byron K. , Titus Kimberly
Published: May 2006

A simulation study was undertaken to assess the sampling stability of the variable loadings in linear discriminant function analysis. A factorial design was used for the factors of multivariate dimensionality, dispersion structure, configuration of group means, and sample size. A total of 32 400 discriminant analyses were conducted, based on data from simulated populations with appropriate underlying statistical distributions. Results from the simulations suggest that minimum sample sizes must exceed multivariate dimensionality by at least a factor of three to achieve reasonable levels of stability in discriminant function loadings. However, the requisite sample size would vary with respect to each of the design factors and, especially, with the overall amount of system variation. A review of 60 published studies and 142 individual analyses indicated that sample sizes in ecological studies often have met that requirement. However, individual group sample sizes frequently were very unequal, and checks of assumptions usually were not reported. We recommend that ecologists obtain group sample sizes that are at least three times as large as the number of variables measured.
Order and Chaos in Ecological Systems
Authors: Schaffer William M.
Published: May 2006

Systems of differential equations exhibiting complex periodic or chaotic behavior can sometimes be associated with one—dimensional mappings (difference equations) which encapsulate the properties of the attractor governing the full n—dimensional system. Recent advances make it possible to reconstruct the underlying attractor, and hence the one—dimensional map, from time—series data for a single state variable (species). The present paper illustrates this technique with reference to a hypothetical system consisting of a single predator species and two species of victims. Trapping records for Canadian lynx suggest that the method may have application to real—world populations.
Bumble Bee Foraging: The Threshold Departure Rule
Authors: Hodges Clayton M.
Published: May 2006

The response of queen bumble bees (Bombus appositus) to variations in the distribution and abundance of nectar in multiflowered Delphinium nelsonii inflorescences was used to identify a general rule for allocation of foraging time among plants. Bumble bees most often leave plants immediately after probing a flower that has a relatively small volume of nectar (short probe time) and most often stay to visit another unvisited flower on a plant after probing a flower that has a larger volume (longer probe time). The threshold departure rule was found to be resilient to changes in the number of open flowers on a plant, the arrangement of flowers on plants, the dispersion patterns of plants, and bee behavior of previously visited plants. The threshold departure rule was violated under two general circumstances: when the average energetic penalty for disobeying the rule was negligible, and when bees would have had to change the direction of flight, from up to down an inflorescence, to find an unvisited flower. This rule is distinct from a patch departure rule based on giving—up time that is prevalent in the behavioral ecology literature. When the threshold departure rule is property qualified it is similar to a “win—stay, lose—shift” departure rule prevalent in the psychology literature.
Biogeochemistry of C, N, and P in a Soil Catena of the Shortgrass Steppe
Authors: Schimel D. , Stillwell M. A. , Woodmansee R. G.
Published: May 2006

Measurements of carbon, nitrogen, and phosphorus content were carried out in the soils of a hillslope of shortgrass steppe. Plant biomass, soil morphology, and soil physical properties were also measured. Soil morphology indicated that the site had undergone several cycles of rapid erosion and deposition. Total mass of C, N, and P increased downslope, following a trend in soil depth, but the summit A horizon had higher C, N, and organic P concentrations than the backslope, reflecting a higher clay content. Laboratory and field incubations showed that N availability increased downslope, while relative N mineralization (N mineralized: total N) decreased. Organic matter content and mineralization rate were closely coupled to physical properties of the soil, which reflect the geomorphic history of the site.
Nutrient Budgets for Agricultural Watersheds in the Southeastern Coastal Plain
Authors: Lowrance R. Richard , Leonard Ralph A. , Asmussen Loris E. , Todd Robert L.
Published: May 2006

Watershed—level agroecosystem studies are essential to relate land management to the external environmental effects produced by agricultural nutrients and to enhance our understanding of agricultural nutrient cycles. Inputs and outputs of N, P, K. Ca, Mg, and Cl were determined for four subwatersheds of the Little River in the Georgia Coastal Plain from 1979 through 1981. The four watersheds had 40, 36, 54, and 50%, respectively, of their land in agricultural uses (row crop and pasture). Prescipitation inputs and streamflow outputs were determined by field sampling of water volumes and nutrient concentrations. Agronomic inputs (from fertilizer and symbiotic N—fixation) and outputs in harvested material were estimated from land use data; countrywide averages of fertilizer applications and crop yield, and plot studies on peanuts and soybeans. All elements except Cl had greater inputs than outputs on each watershed each year. The general order of streamflow loads was Cl > Ca > K > Mg> N > P. Fertilizer inputs exceeded precipitation inputs for all elements on all watersheds. Outputs on N, P, and K in harvest generally exceeded streamflow loads, but harvest outputs of Ca, Mg, and Cl were generally lower than streamflow loads. The two watersheds with more agricultural land had consistently higher loads of N, K, Ca, Mg, and Cl in streamflow and had No3—N loads 1.5 to 4.4 times higher than loads from the less agricultural watersheds. Streamflow loads on the Little River watersheds were similar to those on other Coastal Plain agricultural watersheds with comparable land use and discharge volumes. Budgets for the upland portion of one of the watersheds indicated that large amounts of N, P, K, Ca, and Mg were not accounted for. about 56 kg°ha—1@?yr—1 of N were retained or lost to gaseous emissions from the uplands. Apparently, a large percentage of the nutrients applied to these watersheds was being retained somewhere in the watershed or being lost in some unquantified way.
Conditional Mutualism in a Membracid‐Ant Association: Temporal, Age‐Specific, and Density‐Dependent Effects
Authors: Cushman J. Hall , Whitham Thomas G.
Published: May 2006

We examined the impact of a tending ant, Formica altipetens, on the population dynamics of a membracid, Publilia modesta. Controlled ant—exclusion experiments revealed three ways in which the strength and occurrence of this mutalism was conditional. First, we detected yearly variation in the impact of ants on membracids. Ants had a significant positive impact on membracid abundance in 1985 and 1987, but not in 1986. Second, we found age—specific effects of ants of membracids. In 1985 and 1987 our experiments revealed that only membracid nymphs benefit directly from and tending; we did not detect a direct positive impact of ants on the survival of membracid adults in any year. Third, we documented a density—dependent effect of ants on membracids. In 1985 and 1987, nymphs in large aggregations benefited from ant tending more than nymphs in small aggregations. Observations in 1985—1987 suggest that protection from a predatory salticid spider, Pellenes sp., may be at least one mechanism by which ants benefit membracid nymphs. In 1985 and 1987, when the association was mutualistic, spiders were significantly more abundant on membracid—infested plants without ants than on infested plants with ants, whereas this was not the case in 1986. We hypothesize that the age—specific benefits in this system may result because nymphs are especially susceptible to predation by spiders whereas the more agile and sclerotized adults are not.
Temporal Patterns of Nectar and Pollen Production in Aralia Hispida: Implications for Reproductive Success
Authors: Thomson James D. , McKenna Mary A. , Cruzan Mitchell B.
Published: May 2006

Large plants of Aralia hispida present their pollen and nectar in hundreds of small flowers than open sequentially over 2—3 wk in a pattern of synchronized protandry that alternates male and female phases. The primary pollinators, bumble bees, are able to discover individual plants with elevated levels of either nectar or pollen, and to return to them more often than to less rewarding plants. Both pollen and nectar are presented gradually over time in such a way as to favor traplining behavior by the bees, with many, frequent visits. In controlled environments, lifetime sugar production per flower varies among plants, among umbel orders within plants and between male and female phases of bloom within umbel orders; there are further interactions among these sources of variation, which complicate any consideration of differential male—phase vs. female—phase nectar production. After draining, floral nectar is rapidly replenished, rendering umbels attractive to bees again within 15 min. Pollen production per flower and pollen grain size vary among umbel orders, increasing toward the end of bloom: grain size (but not production) varies among plants as well. Given the high visitation rate by bees, the gradual presentation of pollen should result in more efficient dispersal than simultaneous presentation would.
Control of Nitrogen Mineralization a Sagebrush Steppe Landscape
Authors: Burke Ingrid C.
Published: May 2006

Factors controlling N turnover in sagebrush ecocystems are separable into two groups. The first group consists of properties that exhibit strong spatial patterning at a landscape scale but are temporally static at time scales of years or tens of years. These static properties include plant species assemblages, total soil nutrient pools, and soil texture. A second group includes properties that vary across the landscape over shorter time scales, i.e., annually, seasonally, and diurnally. These dynamic properties include soil moisture, temperature, and amount of available nutrients. This paper evaluates the landscape variability of properties in both of these groups, and examines the extent to which these factors control N turnover. Static ecosystem properties were entered into a principle components analysis resulting in four axes of landscape variability. A statistical analysis of the relationship of net N mineralization with the principal components and with soil temperature and moisture suggested that soil microclimate and organic matter quality both control in situ N turnover. Soil microclimate limited N mineralization to a short season in early spring and summer; only during this time did soil organic matter exert control. In landscape positions where soil organic matter pools were low, improved soil microclimate conditions did not increase N mineralization rates. A similar approach may be useful in evaluating control over ecosystem processes in other systems that are characterized by strong seasonal and spatial variability.
Randomized Intervention Analysis and the Interpretation of Whole‐Ecosystem Experiments
Authors: Carpenter Stephen R. , Frost Thomas M. , Heisey Dennis , Kratz Timothy K.
Published: May 2006

Randomized intervention analysis (RIA) is used to detect changes in a manipulated ecosystem relative to an undisturbed reference system. It requires paired time series of data from both ecosystems before and after manipulation. RIA is not affected by non—normal errors in data. Monte Carlo simulation indicated that, even when serial autocorrelation was substantial, the true P value (i.e., from nonoautocorrelated data) was <.05 when the P value from autocorrelated data was <.01. We applied RIA to data from 12 lakes (3 manipulated and 9 reference ecosystems) over 3 yr. RIA consistently indicated changes after major manipulations and only rarely indicated changes in ecosystems that were not manipulated. Less than 3% of the data sets we analyzed had equivocal results because of serial autocorrelation. RIA appears to be a reliable method for determining whether a nonrandom change has occurred in a manipulated ecosystem. Ecological arguments must be combined with statistical evidence to determine whether the changes demonstrated by RIA can be attributed to a specific ecosystem manipulation.
Distribution and Cycling of Nutrients in an Aspen‐Mixed‐Hardwood‐Spodosol Ecosystem in Northern Wisconsin
Authors: Pastor John , Bockheim J. G.
Published: May 2006

The distribution and cycling of N, P, K, Ca, Mg, S, Fe, and Zn were examined over a 2—yr period in an aspen—mixed—hardwood—Spodosol ecosystem in order to determine the roles of overstory aspen and understory sugar maple in nutrient retention. Stand biomass and net primary production (NPP) were 198 Mg/ha and 12.5 Mg°ha—1°yr—1, respectively. Overstory quaking aspen (Populus tremuloides) comprised 60% of the biomass and 50% of the NPP, and understory sugar maple (Acer saccharum) comprised 24 and 25% of these values, respectively. The total soil N pool and extractable soil P and Mg pools were larger than the respective vegetation nutrient pools. Nutrient cycling within this ecosystem is characterized by large uptake rates, retention of between 40 and 60% of nutrient uptake in perennial tissues, and leaching losses of <3kg°ha—1°yr—1 for each nutrient. Between 22 and 40% of total stand nutrient uptake is retained in perennial tissues of quaking aspen and between 4 and 20% in perennial tissues of sugar maple. Aspen was more effective than maple in retaining nutrients because a larger proportion of its production was perennial tissues with higher nutrient concentrations. Because nutrient—rich leaves constituted a smaller proportion of aspen's production compared with sugar maple's, aspen used nutrients more efficiently in production of dry matter. Translocation of nutrients out of leaves back into perennial tissues was an important retention mechanism for N, P, and K; was less important for S, Mg, and Fe; and did not occur for Ca and Zn. Aspen translocated proportionally more of most nutrients out of green leaves than did sugar maple, but differences between the two species were often small. Uptake as rainwater passed through the canopy and immobilization with the A1 horizon of the soil were additional retention mechanisms for NH4—N and NO3—N. A fragipan at the base of the rooting zone also slowed leaching losses.