bluuebunny/crossref_metadata_2025 huggingface dataset:
Parasitoids, Predators, and Group Size in the Paper Wasp, Polistes Exclamans
Authors: Strassmann Joan E.
Published: May 2006

The paper wasp Polistes exclamans loses part of its brood to the parasitoids Chalcoela iphitalis (Lepidoptera:Pyralidae) and Elasmus polistis (Hymenoptera:Chalcidoidea:Eulophidae) and loses entire nests to bird predation, Crematogaster laeviuscula (Hymenoptera:Formicidae) predation, and orphanage (loss of all adult wasps). More nests fail because of bird predation than from all other causes combined. Larger nests are neither better protected nor more vulnerable to nest predators, though larger nests are less likely to fail because of orphanage than are smaller nests. Larger nests are more heavily attacked by Elasmus polistis, which reinfests its natal nest. Polistes exclamans may subdivide larger groups of females by forming satellite nests because of the lack of an overall advantage to large nests. Satellite nests increase the probability that the brood in either the original nest or satellite nest will avoid bird predation.
Early Regeneration of a Clear‐Cut Southern Appalachian Forest
Authors: Boring Lindsay R. , Monk Carl D. , Swank Wayne T.
Published: May 2006

The components of hardwood forest regeneration on a southern Appalachian watershed were assessed during the 1st yr following clear—cutting. First—year net primary production (NPP) on the clear—cut was 1955 kg/ha, representing 22% of the NPP of a nearby undisturbed hardwood forest. First—year nutrient pools in NPP for N, P, K, Mg, and Ca were estimated at 29—44% of those in the NPP of the control. The greatest NPP and nutrient pools were represented in descending order by hardwood sprouts, herbs, vines, and seedlings. Woody successional species (Robinia pseudo—acacia, Liriodendron tulipifera, and Vitis aestivalis var. argentifolia) and herbs (Aster spp., Solidago spp., and Erechtites hieracifolia) were important in revegetation due to competitive advantages in growth rates, growth forms, and propagative capacities. The woody successional species had higher tissue concentrations of N and P than most other woody species. Herbs as a group had significantly higher foliar concentrations of K than woody species. Woody successional and herbaceous species collectively had higher biomass and elemental pools than other woody species. Following forest disturbance, these fast—growing species conserve substantial pools of nutrients in their biomass and initiate a rapid recovery of forest elemental cycling processes.
Insect Herbivore Abundance in Tropical Monocultures and Polycultures: An Experimental Test of Two Hypotheses
Authors: Risch Stephen J.
Published: May 2006

The population dynamics of six beetle pests (Chrysomelidae: Galerucinae) were studied for three seasons in monocultures and polycultures of the corn—bean—squash agroecosystem in Costa Rica. The study was designed to determine if beetle abundance differed among these treatments, and if so, to determine whether the differences were caused primarily by predators and parasites (Enemies Hypothesis) or by movement patterns of adult beetles (Resource Concentration Hypothesis). In polycultures containing at least one nonhost plant, the numbers of beetles per unit host plant (squash or bean) were significantly lower relative to the numbers of beetles on host plants in monocultures. This pattern was generally observed for each of the beetle species studied between 40 and 60 d after planting and continued until the end of the season. When a given beetle species fed on both crop species in a diculture (i.e., the squash—bean diculture), the general polyculture effect was reversed: beetle numbers were significantly greater on one or both of the host plants in the diculture relative to the respective monocultures. These effects were apparently caused by changes in species richness of the plots per se rather than changes in host density or total plant density in the plot. There were no differences in rates of parasitism or predation of beetles between monocultures and polycultures. Neither were there significant differences in beetle emergence between treatments that could account for differences in adult beetle numbers between monocultures and polycultures. Instead, several lines of evidence show that different movement patterns of adult beetles were primarily responsible for the observed pattern of beetle abundance and distribution. Direct measurements of beetle movements in the field showed that beetles tended to emigrate more from polycultures that included a nonhost plant than from host monocultures. Field observations and experiments showed that this was due to several factors: (1) beetles avoided host plants shaded by corn; (2) cornstalks interfered in some way with flight movements of beetles; (3) as beetles moved through polycultures they remained on nonhost plants significantly less time than on host plants and their overall rate of movement in polycultures was higher. These data suggest that the Resource Concentration Hypothesis rather than the Enemies Hypothesis can account for differences in beetle abundance between monocultures and polycultures. The implication of this finding for predicting herbivore abundance in yet untried agreocosystems are discussed. It is suggested that it is not sufficient merely to recognize that herbivores are less common in particular polycultures. Practical issues of agroecosystem design and more theoretical questions of plant—herbivore coevolution depend on knowing exactly what ecological processes create reduced herbivore loads in polycultures.
Foraging Efficiency and Body Size: A Study of Optimal Diet and Habitat Use by Bluegills
Authors: Mittelbach Gary G.
Published: May 2006

A foraging model was developed to predict the optimal diet breadth and maximum energetic intake of a given—sized fish foraging in each of three aquatic habitats: the open water, vegetation, and bare sediments. Model parameters of prey encounter rates and prey handling times were quantified as functions of fish size, prey density, and prey size through a series of laboratory feeding experiments using the bluegill sunfish (Lepomis macrochirus). Results of these experiments show both searching ability and prey handling efficiency to increase with increasing fish size. Predictions of prey size selection and optimal habitat use based upon maximizing energetic gain were then examined in a small Michigan lake for three size classes of bluegills. Bluegills > 100 mm standard length were highly size selective in their feeding and their diets closely matched predictions of an optimal diet model. From two estimates of relative prey visibilities I show that these fish selected larger prey items than would be predicted if prey were consumed “as encountered.” Habitat use of large bluegills was also shown to maximize foraging return as fish switched from utilizing vegetation—living prey to utilizing open—water zooplankton as relative foraging profitabilities in the two habitats changed across the summer. Bluegills <100 mm standard length were restricted in their habitat use, remaining in or near the vegetation despite demonstrated increases in foraging return available in the open—water habitat. Size—related predation risk apparently accounts for differences in habitat use between bluegill size classes, and its consequences for intra— and interspecific competition between size classes are discussed.
The Effects of Watershed Disturbance on Dissolved Organic Carbon Dynamics of a Stream
Authors: Meyer Judy L. , Tate Cathy M.
Published: May 2006

The response of a stream ecosystem to disturbance in its watershed was investigated by comparing mass balances of dissolved organic carbon (DOC) for a stream draining an undisturbed watershed with a stream draining a watershed that was clear—cut 2 yr before the study began. These second—order streams are in Coweeta Hydrologic Laboratory, North Carolina. Both streams had similar, elevated DOC concentrations (from <1 to 5 mg/L) during storms. Rising and falling limbs of the hydrograph also had similar DOC concentrations. During the growing season DOC concentration increased from headwater seep to the weir in the undisturbed stream under baseflow conditions. No significant longitudinal change was observed in the stream draining the clear—cut watershed. Hence concentration was consistently lower in baseflow samples during the growing season in the stream draining the clear—cut watershed. As a result annual DOC export from the clear—cut watershed was less (9.8—11.5 kg/ha) than from the reference watershed (14.6—15.1 kg/ha). The lower DOC export was partly due to reduced DOC inputs from throughfall and leaching of fresh litter, but most importantly to lower DOC inputs in subsurface water and probably also less in—stream generation of DOC. The rate of recovery of this stream form disturbance is therefore dependent on the rate at which the terrestrial system recovers.
Primary Production and Abiotic Controls in Forests, Grasslands, and Desert Ecosystems in the United States
Authors: Webb Warren L. , Lauenroth William K. , Szarek Stan R. , Kinerson Russell S.
Published: May 2006

This paper, a synthesis based on data generated by the International Biological Program, deals with the relationships among biotic factors at the ecosystem level. Emphasis is placed on aboveground net primary production (ANPP), a major component of energy that drives ecosystem processes, and on potential evapotranspiration (PET), the abiotic variable most often used to explain variation in ANPP. The question addressed is: can ANPP be related to combinations of biotic and abiotic factors such that the relationships are independent of ecosystem type, whether it be forest, grassland, or desert? ANPP as a function of peak foliar standing crop (FSC) was best explained by models which showed a reduction in ANPP/FSC as FSC increased. Thus, deserts had a higher ANPP per unit of FSC than did other systems. As expected, photosynthetic efficiency (PE) was highest for forests, °100 times greater than for deserts. However, when PE was evaluated per unit of foliage, the differences in PE of ecosystems were much less. In fact, a hot—desert site had the highest PE/FSC. In terms of a theoretical maximum, the PE of forests was only 6—25% of the maximum value. Systems with nearly steady—state aboveground standing crop (ASC) showed an exponential decrease with decreased water availability (potential evapotranspiration minus precipitation). For these same systems, the ratio of ANPP to ASC increased with decreased water availability, suggesting that water—stressed systems need more energy from ANPP to drive internal processes. A model predicting ANPP of desert—shortgrass steppes was structured in terms of FSC, water availability, and temperature. The predictive power was found to be very high, and the model was successfully validated in two of three cases with an independent data set. A model predicting ANPP of forests was structured in terms of FSC, radiation, ASC, and temperature. The deviation of the observed ANPP relative to that calculated was 17%. Deviations from predicted values were highest for deciduous stands with high ANPP and low FSC. Most relationships exhibited good correlations between ANPP and the various independent variables including both biotic, abiotic, and combinations of the two. However, in many instances the data tended to be grouped by ecosystem type, suggesting that variation in ANPP can be reduced if ecosystem type is an added independent variable. It was surprising to find that, with the limits of our data, differences in ANPP at the ecosystem level are not glaring, especially considering that soil factors were not included in our analyses. When considering the broad range of genotypes in each ecosystem, and the much broader genotypic range representing all ecosystems, the control that native ecosystems have over abiotic factors in producing ANPP is evident but not large.
Trophic Cascades and Phytoplankton Community Structure
Authors: Vanni Michael J. , Findlay David L.
Published: May 2006

Two predators of zooplankton, fish (yellow perch) and larval insects (Chaoborus), were manipulated within in situ mesocosms to assess their indirect effects on phytoplankton. The two predators caused similar reductions in zooplankton size and biomass, and as a consequence, zooplankton grazing rates on phytoplankton. However, phytoplankton increased relative to predator—free controls only in enclosures with fish and not in enclosures with Chaoborus, suggesting that fish have effects on phytoplankton independent of their effects on zooplankton grazing rates. Estimation of phosphorus excretion rates of zooplankton and fish suggests that fish render P more available to phytoplankton in two ways: (1) by directly excreting and egesting P, and (2) by increasing rates of P excretion by the zooplankton community. The latter effect resulted because small zooplankton have higher biomass—specific excretion rates than large zooplankton. The importance of nutrient regeneration is further supported by the response of phytoplankton community structure. Phytoplankton taxa exhibiting enhanced biomass in the presence of fish were those with high P requirements, including green and blue—green algae and dinoflagellates; other phytoplankton groups did not respond to manipulations. These results suggest that the mechanisms underlying the tropic cascade are complex and that predators affect phytoplankton by altering nutrient recycling rates as well as zooplankton grazing rates.
Population Dynamics of an Insect Herbivore in Simple and Diverse Habitats
Authors: Andow D. A.
Published: May 2006

Much of the recent experimental work on the effect of vegetational diversity on arthropod herbivores has tested Root's (1973) resource concentration hypothesis and natural enemies hypothesis. Two limitations of this work are that only a part of any arthropod life cycle has been examined, and that there is no common currency by which to compare the influence of resource concentration and natural enemies. In this paper, I describe and analyze the demographic response of Epilachna varivestis, a beetle that feeds on Phaseolus vulgaris (common bean), to four experimental habitats at Ithaca, New York: bean monocultures, bean—mustard mixtures, and two bean—weed mixtures. Beetle population densities were significantly lower in the weed mixtures, and only slightly lower in the mustard mixture compared to the bean monoculture. Colonization and egg survival were lower in the weed mixtures, confirming both hypotheses. Demographic analysis provided a common currency to compare the relative influence of colonization and egg survival, and showed that variation in colonization accounted for 58% of the variation in first generation offspring, while variation in egg survival accounted for only 1.7% of this variation. Larval/pupal survival was higher in the weed mixtures, which contradicted the predictions of the natural enemies hypothesis, and obliterated the potential effect of egg survival on first generation offspring. Independent experiments and observations suggested that non—host plants inhibited immigration and hastened emigration, that predaceous coccinellids ate more eggs in vegetationally diverse habitats, and that variation in larval/pupal survival could be related to greater larval movement at high densities. In this study, non—host plant interference influenced beetle populations more than mortality from natural enemies. Demographic analysis was essential for determining the causes of beetle response to vegetational diversity.
Pollen Removal by Bumble Bees and Its Implications for Pollen Dispersal
Authors: Harder Lawrence D.
Published: May 2006

Realized paternity should decelerate with increased allocation of resources to male function for hermaphroditic plants. If these diminishing returns result from the pollination process, total pollen dispersal would be maximized by restricting removal by individual pollinators and using the services of all available pollinators. The optimal restriction of removal by individual pollinators should depend on pollinator availability, the deceleration in dispersal, and the pattern of pollen removal during succeeding visits to a flower. In this study I measured pollen removal from six species (Aconitum delphinifolium, Aralia hispida, Lupinus sericeus, Mertensia paniculata, Pedicularis bracteosa, and P. contorta) to determine the extent to which removal is restricted and to quantify the removal pattern during repeated visits. For all species except Lupinus, I collected the pollen left in a flower after 1—4 visits by freely foraging bumble bees (Bombus). Pollen removal was estimated by substracting the pollen remaining in a visited flower from total pollen production by an adjacent unvisited anther or flower on the same plant. The pollen—dispensing mechanism of Lupinus allowed me to measure pollen removal directly. Much interspecific variation in pollen removal depends on the likelihood of contact between pollinator and anthers. Bumble bees removed a median of 50—80% of the available pollen during first visits to species with exposed anthers. In contrast, only 19° was removed during the initial manipulation of lupine flowers, which present pollen indirectly. Within species, removal increased with visit duration; it did not additionally depend on the number of visits involved. Removal typically changed in direct proportion to differences in pollen availability. Pollen size also affected removal, but it is unclear that this effect is direct. In most cases, removal was unaffected by the species or caste of bee involved. Succeeding visits to a flower removed either a fixed or, more commonly, a declining proportion of the pollen remaining in a flower. A model of pollen dispersal indicates that a constant removal proportion would maximize dispersal when the number of visits a flower receives varies little. In contrast, diminishing proportional removal would allow plants to maintain pollen dispersal when the frequency of pollinator visits is uncertain.
Kelp Beds and Sea Otters: An Experimental Approach
Authors: Duggins David O.
Published: May 2006

In Torch Bay, Alaska, USA, sea urchins structure shallow subtidal communities by consuming most macroscopic algae. Experimental removal of urchins leads to the development, within 1 yr, of a kelp association of high biomass and diversity. In the 2nd yr, however, these attributes decrease as a single kelp species, Laminaria groenlandica, becomes dominant. Additional field experiments confirm the competitive superiority of this kelp. These manipulations lead to predictions regarding the ecological role of sea otters, a once common (but now absent) sea urchin predator. The predictions are tested and verified by examining areas into which sea otters have recently been transplanted. As expected, a rapid and extensive modification of algal species composition and a dramatic increase in kelp biomass follow the return of sea otters.
Life History Characteristics of Two Introduced Populations of Mysis Relicta
Authors: Morgan Mark D.
Published: May 2006

Introduced populations of the opossum shrimp Mysis relicta take 4 yr to reach reproductive age in Lake Tahoe, California—Nevada and 1—2 yr in Emerald Bay, an isolated (for Mysis) embayment of Lake Tahoe. Reproduction in both populations occurs in the winter and spring. The duration of embryonic stages in the Emerald Bay population based on field population statistics ranged from 50 d for the egg stage to 6 d for the last embryonic stage. The number of eggs per egg—bearing female in Emerald Bay increased significantly with the total length of the female. Over the entire brooding period, the average number of embryos per female decreased from 13.89 eggs per gravid female to 10.33 stage 5 embryos per female. This decline in brood size with age of the brood was not due to size—related mortality, but probably resulted from embryos falling out of the marsupium. A similar decline in brood size with age of the brood was not observed in Lake Tahoe. The average number of eggs per brood in Lake Tahoe was 10.73 which was significantly less than that observed in Emerald Bay. However, there was no difference in the number of stage 5 embryos per brood in both systems. Differences in generation time and total reproductive output in the Lake Tahoe and Emerald Bay populations were most closely related to differences in the overall production and mean depth of the two systems. In addition, these traits and whether reproduction is seasonal or continuous among other mysid populations are strongly correlated with the total production of the environment. These results suggest that differences in M. relicta life history traits among populations may represent a phenotypic and not an evolutionary life history adaptation.
Skewed Flowering Distributions and Pollinator Attraction
Authors: Thomson James D.
Published: May 2006

The temporal distributions of flowering by animal—pollinated plant populations, flowering curves, can be viewed profitably as resource utilization functions. A conceptual model of plant competition for pollinators suggests that selection may favor asymmetrical, positively skewed curves, and that such skewness should be most evident in flowers which, at the initiation of flowering, are of a type unfamiliar to their pollinators. Both predictions are confirmed in an examination of 57 species of subalpine meadow plants from the Colorado Rocky Mountains.
Heteromorphic Life Histories of Certain Marine Algae as Adaptations to Variations in Herbivory
Authors: Lubchenco Jane , Cubit John
Published: May 2006

Many of the annual or ephemeral algae of the mid to high intertidal zones have heteromorphic life histories, existing as upright morphs during seasonal algal blooms and as crustose or boring morphs during other portions of the year. Experimental removal of herbivores on the coasts of New England and Oregon resulted in the occurrence of the upright morphs in the times of year when they were normally absent (summer in our areas), demonstrating that such uprights can survive the summertime physical regime (contrary to earlier speculation). We suggest that the upright and crustose or boring stages of these algae represent mutually exclusive adaptations to fluctuations in grazing pressure: the upright stages are adapted for high rates of growth and reproduction when grazing pressure is low, and the crustose and boring stages are adapted for surviving through times of high grazing pressure. We predict isomorphic species of algae would predominate in these sorts of habitats if grazing pressure were more constant.
Confounding Variables and Independent Observations in Comparative Analyses of Food Webs
Authors: Bengtsson Jan
Published: May 2006

Habitat differences in variables describing food web structure were examined using data from two food web compilations, taking into account differences between webs in species richness and using independent data points only. Log—log regressions of connectance, linkage density, and food chain length on species number were calculated. In most cases these relationships were significant. The residuals from the regression lines were used to analyze differences between habitats in connectance, linkage density, and food chain length. In one compilation of insect—dominated webs, aquatic habitats had comparatively high connectance and linkage density when species number had been accounted for. In the other compilation, marine pelagic webs had higher relative connectance, relative linkage density, and mean chain length. In addition, desert webs had higher relative connectance. Several of the results when taking species richness into account do not agree with previous studies. Therefore it is concluded that it is important in examinations of food web patterns to account for web species number and to use independent observations as far as possible.
Sensitivity Analysis of Periodic Matrix Models
Authors: Caswell Hal , Trevisan Maria Cristina
Published: May 2006

Periodic matrix models are used to describe the effects of cyclic environmental variation, both seasonal and interannual, on population dynamics. If the environmental cycle is of length m, with matrices B(1), B(2),...., B(m) describing population growth during the m phases of the cycle, then population growth over the whole cycle is given by the product matrix A = B(m)B(m—1)...B(1). The sensitivity analysis of such models is complicated because the entries in A are complicated combinations of the entries in the matrices B(i), and thus do not correspond to easily interpreted life history parameters. In this paper we show how to calculate the sensitivity and elasticity of population growth rate to changes in the entries in the individual matrices B(i) making up a periodic matrix product. These calculations reveal seasonal patterns in sensitivity that are impossible to detect with sensitivity analysis based on the matrix A. We also show that the vital rates interact in important ways: the sensitivity to changes in a rate at one point in the cycle may depend strongly on changes in other rates at other points in the cycle.
Distribution and Extinction Patterns within a Northern Metapopulation of the Pool Frog, Rana Lessonae
Authors: Gulve Per Sjogren
Published: May 2006

The distribution and extinction patterns within a northern metapopulation of the pool frog (Rana lessonae) were analyzed with reference to a metapopulation theory. Occupied ponds were permanent and differed from unoccupied ones in terms of higher water temperature during May—June and a closer proximity to neighboring pool—frog localities, but local climate was not spatially autocorrelated. Two types of population extinctions occurred (average rate = 2% per population and year): (1) deterministic extinctions due to succession or draining pool—frog pond, and (2) extinctions of populations whose isolation had increased to a critical degree because of Type 1 extinctions of neighboring populations, increasing their susceptibility to predation and combined demographic/environmental stochasticity. The Type 2 extinctions were spatially correlated to a moderate degree, which may reflect the great impact of environmental stochasticity in the system. The results confirm and emphasize the importance of interpopulation proximity and connectivity of metapopulation persistence.
The Effect of Larval Density on Adult Demographic Traits in Ambystoma Opacum
Authors: Scott David E.
Published: May 2006

Factors that affect traits of aquatic larvae of amphibians may have long—lasting effects on terrestrial juveniles and adults. I manipulated larval densities of marbled salamanders, Ambystoma opacum, in large—scale field enclosures during 2 yr, released the juveniles that metamorphosed from these enclosures, and tested for effects on adults that returned to the pond during 6—7 subsequent breeding seasons. Individuals from low larval density treatments tended to have greater lipid stores at metamorphosis than those from high densities and survived longer in a laboratory inanition study. In the field, individuals that experienced low larval density returned for their first reproductive bout as larger adults than those from high—density treatments. For 5—yr—old females released in 1986, low larval density was linked to greater clutch size; clutch size in 4—yr—old animals from the 1987 cohort did not differ between larval treatment groups. Larval density also influenced age at first reproduction, as animals reared at low densities returned to breed at younger ages. Averaged across both cohorts, the proportion of animals that returned to breed at least once was 21% for low—density groups compared to 6% for the high—density groups. The larval environment exerted a strong influence on postmetamorphic traits, and thus larval density likely plays an important role in population regulation in both the aquatic and terrestrial phase of the life cycle.
Climatic Influences on the Growth of Subalpine Trees in the Colorado Front Range
Authors: Villalba Ricardo , Veblen Thomas T. , Ogden John
Published: May 2006

We examined variations in tree growth responses to climatic variations among different tree species and habitat types in the subalpine zone of the Colorado Front Range. We constructed 25 tree ring site chronologies (11 of Picea engelmannii, 9 of Abies lasiocarpa, 4 of Pinus contorta var. latifolia, and 1 of Pinus flexilis) from a series of subalpine habitats ranging from xeric to wet. To establish tree growth responses to climatic variation, we used correlation and response function analyses to compare variations in ring widths with monthly temperature and precipitation records. At the driest sites, growth of Picea and Abies tracked climatic variation similarly. At mesic and wet sites, however, these species differed in their responses to climatic variation. The responses of Pinus contorta, sampled over a narrower range of habitat types, differed from those of Picea and Abies but did not differ among sites. Steep environmental gradients in the subalpine zone of the Front Range accounted for most of the observed differences in growth responses to climatic variation. Even at adjacent sites that differ only slightly in topographic position, tree growth responses to climatic variation were distinct. Interspecific differences in response to climatic variations generally were less important than site differences. Intersite differences in tree growth responses to climatic variation can be used as indicators of environmental differences among subalpine habitats.
Bootstrapping Sparsely Sampled Spatial Point Patterns
Authors: Solow Andrew R.
Published: May 2006

The bootstrap is a practical, nonparametric method that can be used to estimate the sampling distribution of a broad class of parameter estimates. We propose using the bootstrap to estimate the samplings distribution of a distance—based estimator of intensity in a sparsely sampled spatial point pattern. An illustration of the method is given.
Effects of Load‐Lifting Constraints on the Mating System of a Dance Fly
Authors: Marden James H.
Published: May 2006

Load lifting was studied in nature for the fly Hilara species novum (Diptera: Empididae). During mating, male Hilara sp. flies present insect prey to females and then fly in tandem while the female consumes the prey. During prey transfer and initiation of copulation, males lift their own mass, the mass of the female, and the prey. The maximum mass that males carried was consistent with limitations predicted from laboratory experiments (maximum lift force of 54—63 N/kg of flight muscle: Marden 1987). Maximum loads frequently approached, but rarely exceeded, the predicted limitation, whereas simulated random combinations of body masses of males, females, and prey frequently exceeded those limits. Thus, load—lifting limitations must frequently constrain the size of females and prey that a male can carry in flight. Observed consequences of this constraint were that males in copula had significantly greater body mass and flight muscle ratio [(flight muscle mass)/(body mass)] than randomly captured males; males and females and mated assortatively for body size (larger males have more flight muscle and consequently can carry larger loads); and larger females were presented with smaller prey (when male size was held constant).
Behavioral Differences between Enallagma Species (Odonata) Influencing Differential Vulnerability to Predators
Authors: McPeek Mark A.
Published: May 2006

In previous research I have shown that fish predation contributes to the exclusion of a group of Enallagma species (Odonata: Zygoptera) from lakes containing fish, whereas dragonfly predation contributes to the exclusion of another group of Enallagma species from fishless lakes. In the study reported in this paper I performed a series of laboratory experiments to identify behavioral differences between the two groups that could contribute to their differential vulnerabilities to the two predators. In one experiment I observed the behaviors of four Enallagma species, two from fishless lakes and two from lakes containing fish, in the presence of no predators, dragonfly larvae (Anax junius), and fish (Lepomis macrochirus) to identify behavioral differences that may cause the species to be differentially detected by the predators. In another experiment, I observed Aeshna dragonflies feeding on Enallagma larvae to evaluate whether species in the two groups differ in how they attempt to avoid attacking dragonflies. The Enallagma species from fishless lakes walked in rapid bursts of 2—3 s and frequently performed a conspicuous behavioral display, while the Enallagma species from lakes containing fish walked very slowly for durations of 20—30 s and almost never performed the conspicuous display. The species from lakes containing fish also swam for longer durations and remained motionless longer following swims than species from fishless lakes. The species from fishless lakes also greatly reduced their movement and responsiveness toward prey and increased the duration of motionless periods in the presence of dragonflies, but only reduced the number and duration of the conspicuous display in the presence of fish. In contrast, the species from lakes containing fish reduced their movement and responsiveness toward prey and increased the duration of motionless periods in the presence of both fish and dragonflies. In videotaped feeding trials, larvae of an Enallagma species from fishless lakes usually swam away from attacking Aeshna dragonflies. Larvae of an Enallagma species from lakes containing fish usually did not respond to attacking dragonflies, and were usually captured without attempting to evade the dragonfly. These behavioral differences are consistent with observed differences in their vulnerabilities to predators, and probably contribute to causing the pattern of habitat separation observed in the field.
Ecomorphology and Biogeography of a Tropical Stream Fish Assemblage: Evolution of Assemblage Structure
Authors: Wikramanayake Eric D.
Published: May 2006

The wet—forest stream fishes of Sri Lanka are segregated in ecological and morphological space. Suites of selected morphological attributes of the fishes were correlated with microhabitat and feeding ecology. Previous studies have shown that these fishes partition resources with pronounced complementarities along niche axes, a pattern consistent with assemblages structured by interspecific competition. Since such competitive interactions may have been significant historically, the speciations and extinctions that have occurred to produce the contemporary assemblages, since the early Pleistocene isolation of wet—forest fish fauna from the mainland fauna, most likely occurred in an arena of interspecific competition. Invasions of the Sri Lankan wet—forest streams by mainland fishes after the initial isolation of the island were also unlikely due to various biogeographic, climatic, and geologic barriers, Thus the data favor the hypothesis that the ecomorphological relationships exhibited by these fishes may be the results of evolutionary adjustments to facilitate resource partitioning, thereby reducing interspecific competition.
Mortality Risk vs. Food Quality Trade‐Offs in a Common Currency: Ant Patch Preferences
Authors: Nonacs Peter , Dill Lawrence M.
Published: May 2006

Many studies have shown that predation risk affects foraging behavior, but quantitative predictions are rare because of the lack of a common currency for energy intake and mortality. This problem is soluble in ants. We gave 12 Lasius pallitarsis colonies the choice between foraging in two patches that differed both in food quality and in associated mortality risk. We independently measured the growth that colonies could achieve on the diets offered in those patches. With no risk at either patch, colonies always preferred the higher food quality patch. When mortality risk (a large Formica subnuda ant) was associated with the trail to the higher food quality patch, the use of that patch depended on the magnitude of the growth differential between feeding in risky or safe patches: the greater the benefit of feeding in the risky patch, the greater was its relative usage. Colonies valued risky patches equally to safe patches at the point where forager mortality rates were approximately offset by colony growth gain. This ability to reduce mortality risk while foraging may be a factor that favors insect sociality, in general.
Plant Foraging: A Marginal Value Model and Coiling Response in Cuscuta Subinclusa
Authors: Kelly Colleen K.
Published: May 2006

Stems of the parasitic plant Cuscuta subinclusa (Convolvulaceae) discriminate among host species and invest in resource acquisition (coil around the stem of a host plant) relative to host quality in a way predicted by a marginal value model of resource use. C. subinclusa can be said to forage in that: (1) stem soiling, the necessary antecedent and determinant of resource uptake, precedes exploitation of host materials; and (2) mean coiling on a host species is proportional to mean reward per unit of investment (measured as biomass accumulation per unit length of stem in coil). Preliminary evidence indicates that coiling in C. subinclusa is induced by host bark flavonoids. Larger parasites are more likely to overwinter and set seed a second season than smaller ones, and parasites that start from overwintered tissue are significantly larger at flowering than are those that have started from seed. Thus, seed set is correlated with parasite size at the end of the season, linking the foraging response and fitness of the plant.
Decay Rates, Nitrogen Fluxes, and Decomposer Communiies of Single‐ and Mixed‐Species Foliar Litter
Authors: Blair John M. , Parmelee Robert W. , Beare Michael H.
Published: May 2006

Decomposition rates, N fluxes, and abundances of decomposer organisms were quantified in mixed—species litterbags (containing leaves of two or three of the following tree species: Acer rubrum, Cornus florida, and Quercus prinus) and in litterbags containing leaves of a single species. Data from single—species litterbags were used to generate predicted decay rates, N fluxes, and abundances of decomposer organisms for mixed—species litterbags, against which observed values could be compared to determine if significant interaction effects occurred when litter of different species, and different resource quality, was mixed. Decay rates of mixed—species litterbags during the 1—yr study were not significantly different than predicted from decay rates of individual component species. However, there were significant interaction effects on N fluxes and abundances of decomposer organisms. In the C. florida—A. rubrum and C. florida—A. rubrum—Q. prinus litter combinations there were significantly greater initial releases of N and lower subsequent N immobilization than predicted. In the A. rubrum—Q. prinus and C. florida—A. rubrum—Q. prinus litter combinations, lengths of fungal hyphae were significantly less than predicted on at least half the collection dates. Bacterial numbers in the mixed—litter combinations were also generally less than predicted. Nematode abundances, especially fungivores, were generally greater than predicted in mixed—species litterbags until the last sample date. Observed mean abundances of nematodes over all dates were 20—30% greater than predicted. Microarthropod abundances were more variable, but tended to be lower than predicted. Our results indicate the measurement of N flux in single—species litterbags may not reflect actual N flux in the field, where leaves of several tree species are mixed together. The differences in N flux between single— and mixed—species litterbags can affect ecosystem—level estimates of N release or accumulation in decomposing litter. For example, estimates of ecosystem—level N fluxes at our field site, based on data from single—species litterbags, resulted in a 64% underestimate of N released by day 75 and a 183% overestimate of N accumulated in the litter by day 375, relative to estimates based on data from mixed—species litterbags. We suggest that the deviation of observed N fluxes in mixed—species litterbags from those predicted using single—species litterbags are the result of differences in the decomposer community, such as lower microbial and microarthropod densities and higher nematode densities, resulting when litter of varied resource quality is mixed together. Longer term studies will be needed to determine if the differences between observed and predicted decomposer communities in mixed—species litter combinations influence the latter stages of decomposition where invertebrate—microbial interactions may have a greater effect on decay rates and nutrient release.
Elevated Atmospheric CO2 Effects on Belowground Processes in C3 and C4 Estuarine Marsh Communities
Authors: Curtis Peter S. , Balduman Lisa M. , Drake Bert G. , Whigham Dennis F.
Published: May 2006

Belowground carbon allocation is a major component of a plant's carbon budget, yet relatively little is known about the response of roots to elevated atmospheric CO2. We have exposed three brackish marsh communities dominated by perennial macrophytes to twice ambient CO2 concentrations for two full growing seasons using open top chambers. One community was dominated by the C3 sedge Scirpus olneyi, one was dominated by the C4 grass Spartina patens, and one was a mixture of S. olneyi, S. patens, and Distichlis spicata, a C4 grass. Root and rhizome growth were studied in the 2nd yr of exposure by measuring growth into peat cores previously excavated and refilled with sphagnum peat devoid of roots. Growth under elevated CO2 resulted in an 83% increase in root dry mass per core in the Scirpus community. Those roots were also significantly lower in percentage of nitrogen than roots from ambient—grown plants. There was no effect of elevated CO2 on root growth or nitrogen content in the Spartina community or in the C4 component of the Mixed community.
"Majoring" and "Minoring" by Foraging Bumblebees, Bombus Vagans: An Experimental Analysis
Authors: Heinrich Bernd
Published: May 2006

On their first 2 foraging trips out of the hive, young Bombus vagans workers visited, on the average, 4 or 5 different kinds of rewarding as well as unrewarding flowers, and few of each kind in succession. But, after 3—7 foraging trips, most of the bees specialized on jewelweed, which was the most numerous flower available with high nectar reward. When jewelweed specialists became numerous, and the food rewards in jewelweed declined, the bees resampled the reward spectrum. They again visited, and continued to visit, at least 3—4 different kinds of flowers on successive foraging trips in an enclosure where patch size was limited. Flowers in open inflorescences (aster, goldenrod) were handled appropriately from the start, but handling accuracy at zygomorphic flowers (jewelweed, turtlehead) was initially 40—50% at the first 10 flowers encountered, and increased to >90% in 60—100 flower visits. It is concluded that the most important problem faced by foraging bees attempting to enhance food intake is that of assessing the resources, which often change rapidly. Individual bees specialize on flowers yielding rewards that are "perceived" to exceed some minimum. However, the difference between perceived and actual rewards is, in part, determined by handling skills that are affected by foraging experience. Thus, optimal foraging in the bees must be explored from the perspective of long— rather than short—term energy balance.
Photosynthesis and Microclimate of Camissonia Claviformis, A Desert Winter Annual
Authors: Ehleringer J. , Mooney H. A. , Berry J. A.
Published: May 2006

The photosynthetic characteristics of the C3 desert winter annual Camissonia claviformis and the microclimate of the Camissonia habitat in Death Valley, California on 21 March 1976 are presented. This species has one of the highest photosynthetic capacities of any higher plant, with rates exceeding 6 nmoled CO2 cm—2 sec—1 at midday irradiances and optimum leaf temperatures. The photosynthetic temperature optimum is at 20°C, in concert with the prevailing cool winter temperatures of this desert habitat. The daily totals of primary production and water loss for Camissonia on 21 March are calculated using leaf energy budget and CO2 diffusion photosynthetic equations. Measured and predicted leaf temperatures were 2—4°C below air temperatures during the day. The rate of primary production is predicted to be 68.1 g CO2 m—2 day—1 under high stomatal conductances, while daily transpiration at these stomatal conductances, will be 5.54 kg m—1 day —1. The effects of decreases in stomatal conductance on daily primary production, transpiration, and water—use efficiency are calculated and show that the values of these 3 parameters diminish rapidly with decreasing stomatal conductance.
Plant Demographic Studies in Sagebrush‐Grass Communities of Southeastern Idaho
Authors: West Neil E. , Rea Kenneth H. , Harniss Roy O.
Published: May 2006

Generalizations on plant demography were examined in the light of >21 yr of pantograph records on perennial taxa in plots of sagebrush—grass vegetation at the United States Sheep Station in southeastern Idaho. Specific hypotheses tested were (1) grazing decreases plant longevity, (2) a semi—logarithmic model best describes plant population survivorship curves, and (3) recruitment to semidesert plant populations occurs in pulses. Mean longevities of the grazed plants, which varied between 2 and 4 yr, differed significantly from the ungrazed populations. For the pastures grazed by sheep in the fall, the average longevities of major grasses were enhanced, whereas the average longevity of three—tip sagebrush was reduced by grazing. The overall survivorship curves for most species closely fit a log—log regression for the 1st 11 yr of life, indicating that the chance of survival improved with age, up to the physiological maximum. No linear relationships of increasing size to age were found, indicating considerable plasticity in yearly growth with adjustments of volume to the fluctuating favorableness of the environment. Although exceptionally high numbers of seedlings were observed in certain years, overall age—class—density distributions do not usually deviate significantly from the log—normal model. Labeling these events as "pulse" phenomena becomes a matter of choice of definitions.