bluuebunny/crossref_metadata_2025 huggingface dataset:
Rainfall and Grazing Effects on Net Primary Productivity in a Tropical Savanna, India
Authors: Pandey C. B. , Singh J. S.
Published: May 2006

The effects of rainfall amount, rainfall distribution, and grazing intensity on net primary production and herbivory were examined for a dry tropical savanna. Aboveground net primary production in permanently protected savanna ranged from 329 to 841 g.m—2.yr—1 and was positively related to annual rainfall. It was not related to early rainy season rainfall, but was related to rainfall of the later part of the rainy season. Grazing simulated 4—45% greater net primary production, the simulation being higher at lightly and moderately grazed sites. Consumption, as estimated by comparing net primary production inside temporarily fenced and outside the fenced plots, was 476— 34 g.m—2.yr—1. Consumption in the present savanna was positively related to net primary production, and to annual rainfall. Green biomass, being in function of soil moisture, was greater in the high rainfall year. Belowground net primary production in permanently protected savanna (BnpP) ranged from 404 to 838 g.m—2.yr—1 and was significantly related to annual rainfall. BnpP was not related to early rainy season rainfall but was related to late rainy season rainfall. Total net primary production ranged from 836 to 1579 g.m—2.yr—1 and was also related to annual rainfall than early rainy season rainfall. Grazing reduced belowground net primary production by 25—65% in grazed savanna (BnpG). Maximum reduction occurred in heavily grazed savanna. BnpG was inversely related with rainfall. Total net primary production (herbaceous + woody vegetation) of the savanna ranged from 11.9 to 19.1 Mg.ha—1.yr—1. The total net primary production of the savanna was similar to that of the native dry deciduous forest; however, the production of herbaceous species was 61% lower in the forest. The total net primary production of savanna was higher than that of the dry tropical forest when the belowground net primary production of the 0—50 cm layer was considered.
Plant and Soil Controls on Mycorrhizal Fungal Communities
Authors: Johnson Nancy Collins , Tilman David , Wedin David
Published: May 2006

A field experiment was conducted to examine the relative importance of soil factors and plant species on communities of vesicular—arbuscular mycorrhizal (VAM) fungi. Populations of VAM fungal spores were studies in 4—yr—old monocultures of five successional grass species grown in gradient of soil mixtures ranging from pure subsurface sand to pure sandy loam topsoil. A total of 19 species of VAM fungi were found across all treatments. Of the 12 most abundant VAM fungal species, 6 species had a significant dependence on both soil mixture and host species, while 2 were dependent only on soil and 2 only on host. To our knowledge, these are the first results indicating that even closely related hosts (five grasses) may cause divergence in VAM fungal communities on initially identical soils. Cluster analysis of the similarity of fungal communities by host plant species showed that fungal communities in the two late successional grasses to be most similar to one another and least similar to the fungal communities in the early successional grass species. Cluster analysis of the similarity of fungal communities by soil mixture showed the fungal communities in the sandy end of the soil gradient diverged predictably from the fungal communities in the black soil end of the gradient. These results support the hypothesis that soil factors and plant species may be of equal importance in regulating the species composition of VAM fungal communities.
Effects of Habitat and Body Size on Mortality Rates of Poecilia Latipinna
Authors: Trexler Joel C. , Travis Joseph , McManus Michael
Published: May 2006

We examined the effect of body size and habitat on mortality rates of juvenile and adult sailfin mollies (Poecilia latipinna) maintained in field cages that excluded predators. Mortality rates and the extent of size dependence varied among years and among seasons within years for juvenile and adult fish. When mortality rates were size dependent, large individuals had lower mortality rates than small ones. Differences between habitats in mortality rates were present in some experiments but not others. When there were habitat effects under winter conditions, both juveniles and adults had higher survival rates in a saltwater habitat than in a freshwater one. Some adults survived through the winter in all of our experiments. Adult mortality during the growing season exhibited a similar pattern of habitat effects although at lower mortality rates. Juvenile survival to sexual maturity in the growing season showed no consistent effects of habitat. Fish born during the autumn had a much lower probability of survival of sexual maturity than those born in spring or summer, and there was a threshold time beyond which newborn fish were virtually assured of perishing over the winter, presumably due to the inability to achieve a minimum body size capable of withstanding winter conditions. These experiments illustrate that the net direction of selection on molly body size in any generation depends on events that are stochastic on a yearly scale, such as the climatic severity of the winter and the intensity of predation pressure. Our results illustrate the complex interplay of different sources of selective mortality that operate in natural populations, and highlight the importance of ecological investigators of the different agents of mortality for a predictive understanding of the operation of natural selection.
Plankton Metabolism and Carbon Processes in the Amazon River, Its Tributaries, and Floodplain Waters, Peru‐Brazil, May‐June 1977
Authors: Wissmar R. C. , Richey J. E. , Stallard R. F. , Edmond J. M.
Published: May 2006

A synoptic study of photosynthetic and respiratory activity of plankton communities in different Amazon surface waters indicates that large—scale events such as flooding can have a major impact upon the cycling of carbon and nutrients in these aquatic ecosystems. During high water, the major factors influencing primary production appeared to be nutrient concentrations in the mouthbays and varzea (floodplain) lakes and high levels of suspended matter in the Amazon mainstem. In riverine systems, plankton primary production (PPP) averaged 4.04 mg C°m—3°h—1, and measures of respiration (Re) averaged 0.67 mg C°m—3°h—1. In the more productive varzea lakes and mouthbays, PPP averaged 26.37 mg C°m—3°h—1 and Re averaged 2.30 mg C°n—3°h—1. Bacterial densities, 14 C—acetate rate constants for uptake, and plankton carbon: ATP ratios implied that heterotrophic microbiota were important components of the plankton communities in riverine waters. The importance of terrestrial organics to metabolic activity in all waters was implied by high particulate carbon: nitrogen ratios (20:1). These features were especially evident in riverine surface waters where integrated respiration rates exceeded those of plankton primary production. Riverine respiratory levels may be attributed to several factors: adequate supplies of terrestrial organic carbon, sufficient dissolved nutrient concentrations, increased surface area of suspended matter for microbial attachment and growth, and shading of phytoplankton by suspended matter which reduces photosynthetic activity. Observed supersaturation of Amazon waters by carbon dioxide was similar to observations for other rivers of the world. Shifts of CO2 solute components to CO2 in surface waters of varzea lakes and mouthbays and of some tributaries implied high partial pressures of carbon dioxide (°500—1500 Pa). The primary source of CO2 is most likely decomposing organic matter in planktonic and benthic environments of the rivers, lakes, and flooded terrestrial lowlands. The hypothesis that respiratory input of CO2 balanced by evasion (gas lost to atmosphere) is sufficient to explain high CO2 vapor pressures in the Amazon River appeared true from our calculations but needs further examination. Particular attention should be given to periods of rising water, when planktonic respiration appears to be two orders of magnitude greater than at periods of high water. Subsequent seasonal studies of the Amazon and other large rivers are needed to determine how the plankton community, the chemistry of terrestrially derived organics and their biological oxidation in water, and inorganic factors control CO2 supersaturation and exchange with the atmosphere.
Nitrogen and Phosphorus Effects on Secondary Succession Dynamics on a Semi‐Arid Sagebrush Site
Authors: McLendon Terry , Redente Edward F.
Published: May 2006

A sagebrush steppe community in northwestern Colorado was disturbed in 1984 and subjected to annual applications of nitrogen and phosphorus, and successional responses were studied over a 5—yr period. Phosphorus was not found to be significant but nitrogen did significantly affect succession for all years except the first. Three seral groups developed on the non—fertilized plots, the first two dominated by annuals and lasting 3 yr, the third transitional and dominated by perennials. The addition of N altered this successional pattern by allowing annuals to remain as site dominants through the 5th yr. Results of this study suggest that dominance of a site by annuals in early stages of secondary succession is related to high nutrient availability.
The Influence of Starvation and Predators on the Mating Behavior of a Semiaquatic Insect
Authors: Travers Steven E. , Sih Andrew
Published: May 2006

Few experimental studies of mating behavior have examined mating behavior shifts in response to ecological factors (e.g., predators, starvation). We conducted experiments designed to: (1) examine multiple trade—doffs between predator avoidance, feeding behavior, and copulating duration in a semiaquatic insect (Microvelia austrina); and (2) address the influence of altered copulation duration on male reproductive success. To examine multiple trade—offs, we used a 2 x 2 factorial design where we measured the amount of time that mating microveliids remained in tandem, for starved and fed microvellids in the presence and absence of predaceous water striders (Gerris remigis). We also looked at the effects of presence vs. absence of green sunfish (Lepomis cyanellus) on microveliid tandem duration. To address the impact of premature pair separation on male mating success, we separated paired microvellids in field populations and measured the time required for each sex to find new mates. To examine the effects of local sex ratio on remating times of males and females, we quantified the sex ratio of field populations via periodic censuses and correlated local sex ratio with remating times. Finally, to address oviposition rates while pairs are in tandem, we measured female oviposition patterns relative to tandem duration for poorly fed and well—fed females. Both Starvation and gerrid presence significantly reduced tandem duration. As a result of male—biased sex ratios, males that prematurely separated from females often faced long search times before securing a new mate. Females oviposited at a constant rate while in tandem. A simple computer simulation incorporating the above observations suggests that reductions in tandem duration can decrease fertilization rates.
Factors Affecting Gerbil Foraging Behavior and Rates of Owl Predation
Authors: Kotler Burt P. , Brown Joel S. , Hasson Oren
Published: May 2006

We experimented on how illumination, habitat structure, and three different species of owls affected the foraging behavior of Gerbillus allenbyi and G. pyramidum, two gerbil species that coexist on sand dune habitats in the Negev Desert, Israel. We also tested how illumination and habitat structure affected rates of predation by owls on the two gerbil species. In a large aviary, we manipulated presence and absence of owls, owl species, presence and absence of illumination, and shrub cover. In response to the presence of owls or to increased illumination, gerbils foraged less, shifted foraging activity to the bush microhabitat, and quit patches at a higher giving—up density of resources. In accord with moonlight avoidance, both gerbil species suffered higher predation rates under illumination than in the absence of illumination. In addition, G. pyramidum distinguished among owl species, as indicated by changes in patch use and habitat selection. Habitat structure also affected foraging behavior and rates of predation. Gerbils foraged less in the open than in the bush microhabitat, foraged less when there was no cover present, and foraged less in the bush microhabitat when patches were encumbered by entangling branches. In accord with avoidance of open areas, both gerbil species suffered higher rates of predation when shrub cover was 0% than when shrub cover was 10%. With 0% cover, G. allenbyi suffered higher predation rates than G. pyramidum, but with 10% cover, rates of owl predation did not differ between gerbil species. Rates of owl predation on the two species corresponded to their natural patterns of macro— and microhabitat partitioning; relative to G. allenbyi, G. pyramidum predominates on open sand dunes and biases its behavior toward the open microhabitat. The results suggest that predation interacts with resource competition to determine the distribution and habitat separation of G. allenbyi and G. pyramidum.
Temperature and The Northern Distributions of Wintering Birds
Authors: Repasky Richard R.
Published: May 2006

Root (1988b) proposed that a physiological limit on metabolic rate constrains the northern distributions of wintering birds. Thermoregulation requires that metabolic rate increase as temperature declines, and hence a ceiling on metabolism that is reached before food or other factors become limiting would set the northern extent of a species distribution. I compared data with two predictions stemming from the proposal. First, there should be close correspondence between the northern range boundaries of species and temperature isotherms. To test this, I re—evaluated the data and found that only 8—42% of the species that Root classified as limited by temperature had northern range boundaries that corresponded to particular temperature isotherms. The second prediction examined was new: Species' northern limits should scale to body mass, because the ceiling on metabolism is stated as a multiple of basal metabolic rate, which itself scales to body mass. Large species should extend farther north than small species. A test of the prediction using Root's data showed little evidence that a constraint strongly affects the distributions of birds. In a second test, I analyzed additional recent Christmas Bird Counts that include areas farther north than were available to Root. I found that species of all sizes occur at the lowest temperatures, indicating that a ceiling on metabolism does not generally constrain the distributions of birds wintering in North America. My analysis turned up one other unexpected result: Few large passerines in North America have northern distribution boundaries that occur in warm, southerly areas, whereas small species have northern distribution boundaries in southern as well as in northern areas. The pattern might reflect the relationship between body size and the size of a species' geographic range coupled with the geography of North America. Large species tend to have only large geographic ranges, whereas small species are not so restricted. The avifauna of North America north of Mexico is bounded by a barrier formed by the Gulf of Mexico and a turnover in avifauna between the United States and Mexico. To the north of such a barrier, the northern distribution boundaries of large species should be well away from the barrier because of their large size.
Ecological and Evolutionary Consequences of Seed Size in Prunella Vulgaris
Authors: Winn Alice A.
Published: May 2006

Seed size has significant demographic consequences in Prunella vulgaris. Measurements of the effects of seed size were obtained by sowing seeds of known size at four field sites and recording seedling emergence, survival, and size. The intensity of selection on seed size was calculated from these data. Large seeds had a significantly greater probability of emergence at most study sites in each of 2 yr. The effects of seed size were expressed most strongly during the early part of the life cycle, between sowing and emergence, and much less so in later phases of the life cycle. In contrast to a prediction based on the greater relative frequency of large—seeded species in later successional habitats, the effect of seed size on percent seedling emergence did not differ significantly between an old—field and a woodland habitat. However, it is likely that the intensity of natural selection on seed size is greater in an old—field than in a woodland population because the natural distributions of seed size in old—field populations include more small seeds than do those in woodland populations. An analysis of the costs and benefits of producing large and small seeds revealed that in addition to selection favoring large seeds, there was selection favoring individuals that produced large seedlings at three of the four study sites. At the fourth site, there was no selection favoring larger seeds or parents that produced larger seeds. Substantial capacity for phenotypic plasticity in seed size suggests that there may be little opportunity for an evolutionary response despite strong selection favoring large seeds.
Effects of Population Density on Long‐Distance Dispersal in the Goldenrod Beetle Trirhabda Virgata
Authors: Herzig Ann L.
Published: May 2006

Recent theoretical and empirical studies suggest that an animals's long—distance dispersal behavior can have a major influence on its population dynamics. In this study I examined the role of dispersal in the population dynamics of a goldenrod leaf beetle, Trirhabda virgata. I measured the long—distance emigration rates of this beetle in the field under different population densities. I manipulated densities of Trirhabda larvae early in the season to create crowded, heavily defoliated host patches and uncrowded, lightly defoliated patches. Newly emerged, teneral beetles were added to these two density treatments and their long—distance emigration rates were measured. Beetles flew from host patches that had high densities of adults and heavy damage caused by larvae. No flight was detected from low—density patches with little defoliation. Beetles that had developed in these crowded and uncrowded patches were placed on lush, undefoliated plants as teneral adults. Their emigration rates did not differ. In a final experiment, female beetles were placed on plants with or without mates. The plants were one of two types, either heavily defoliated or lush. On lush plants, females without mates showed a stronger tendency to leave than females that had access to mates. On defoliated plants, females left at high rates whether or not mates were present. These experiments indicate that long—distance emigration is triggered by conditions experienced during the teneral phase. Heavy host defoliation inflicted by high population densities appears to induce flight. This density—dependent emigration should reduce local population pressure and result in the spread of local outbreaks across a larger area. The influence of dispersal on population dynamics will vary depending on the pattern of beetle densities across the region.
Estimating the Mean Annual Seed Production of Trees
Authors: Greene D. F. , Johnson E. A.
Published: May 2006

Intraspecifically, plant seed production is a function of both seed size (mean mass per seed) and plant size. In this paper we examined the interspecific relationship between the size of seeds and plants and the mean long—term annual seed production per tree. For canopy trees, we show that seed production is highly (inversely) correlated with the mean seed mass as a power law argument. Tree size (basal area or leaf mass) is directly proportional to seed production over a limited range. Analysis of seed production for herbaceous plants indicates a relationship similar to that for trees. As the exponent relating seed size to seed production is >°1.0, it follows that large—seeded plants produce more total annual crop mass than do small—seeded species. However, this is balanced by the greater investment in ancillary reproductive tissue by smaller seeded species. The results obtained here ought to be of theoretical and applied interest in, for example, stand—level simulations of population dynamics or for planning the optimal size and shape of clearcuts intended to be regenerated naturally.
The Adaptiveness of Worker Demography in the Attine Ant Trachymyrmex Septentrionalis
Authors: Bershers Samuel N. , Traniello James F. A.
Published: May 2006

Castle theory states that worker size distributions in ant colonies have evolved for efficient division of labor and predicts that they should vary with habitat and have an effect on fitness. We tested these predictions in a comparative study of worker size variation and colony fitness in Florida and Long Island populations of Trachymyrmex septentrionalis. Colonies from the two populations were excavated shortly before the time of the mating flights, so that workers and reproductive forms could be censused. We measured worker head widths in each colony and analyzed the resulting size frequency distributions using the mean, standard deviation, skewness, and kurtoisis, and also principal components derived from these parameters. Size frequency distributions were unimodal and close to normal, with a tendency to negative skewness. Colonies in the Long Island population had both larger workers and greater size variation than those in Florida. Florida colonies followed a growth trajectory in which the mean and standard deviation of worker head width both increased with colony size, whereas in Long Island there was an inverse relationship between the mean and standard deviation that was independent of colony size. Analysis based on the principal components showed that worker size variation significantly affected fitness in both populations, and this was not a secondary effect of colony size. Population differences in colony life history strategies may account for the observed patterns: the Long Island population appears to be adapted for surviving the winter diapause, while the Florida population has experienced different selective pressures, possibly for rapid colony growth. Size variation that has evolved in the context of life history strategies may serve as a preadaptation for division of labor.
The Biogeochemistry of Phosphorus Cycling and Phosphorus Availability Along a Desert Soil Chronosequence
Authors: Lajtha Kate , Schlesinger William H.
Published: May 2006

The biochemistry of the weathering, landscape movements, and chemical transformations of phosphorus and its availability of plants were examined in a chronosequence of soils developed from quartz monzonite alluvium in southern New Mexico. Total P in the soil profile decreased with increasing soil age and was removed from the ecosystem as readily as the most leachable base cations. Although Ca—bound forms of P decreased with increasing soil age, Ca—P remained the singlee largest fraction of total P in all soils. In contrast, Fe— and Al—bound P was a very small percent of total P in all soils. There was little evidence for the stabilization of P by soil organic matter within this ecosystem; both soil organic P and microbial P represented very small pools of total soil P. Phosphorus availability, measured by in situ resin bags, was not well correlated with soil age or total soil P, and P concentrations in shrub tissues did not reflect changes in forms or total amounts of soil P. The biogeochemical cycle of P in this system differs sharply from that in a more mesic, forested system, where fixation by iron and aluminium oxides and biologic activity play more dominant roles in the conservation of P within the ecosystem.
Permanence and the Assembly of Ecological Communities
Authors: Law Richard , Morton R. Daniel
Published: May 2006

A numerical technique for assembly of ecological communities of Lotka—Volterra form is described. The technique is based upon a global criterion for coexistence of species known as permanence. This provides a relatively fast and accurate method to determine the sequence of communities that develops when species are drawn sequentially and in an arbitrary order from a regional pool of species. Steps in the assembly sequence that cannot be resolved by this method are determined by numerical integration. The results are as follows. (1) At each step in an assembly sequence, a species that succeeds in invading when rare persists in the resulting community even if one or more of the resident species becomes extinct. (2) Assembly sequences are terminated with a community that is uninvadable by any of the remaining species from the pool. The number of these endpoints is small, even when the species pool is large. (3) In some cases, the final community cannot be reassembled from the species left in it; other species, which are absent at the end, are needed for the endpoint to be reached. (4) Invasion resistance builds up in three stages during an assembly sequence. Over much of the sequence, invasion resistance shows little if any increase; during this period, species composition continues to change until the sequence happens to land on an endpoint. (5) Communities assembled from large species pools are more resistant to invasion than those assembled from small species pools.
Interspecific Competition Controls Abundance and Habitat Use of Territorial Caribbean Damselfishes
Authors: Robertson D. Ross
Published: May 2006

The prevailing notion that interspecific competition has little impact on the abundances of tropical reef fishes has been tested by few experiments, all of which examined its effects on recruitment and juvenile demography rather than on adult abundances. This study, in Caribbean Panama, examined whether a common territorial damselfish, Stegastes planifrons, limits the abundances of adults of four ecologically similar congeners in S. planifrons' primary habitat: S. partitus and S. variabilis, which commonly occur in that habitat, and S. diencaeus and S. leucostictus, which rarely do. When S. planifrons was nearly eliminated from 16 natural patch reefs for 4—6 yr, adult populations of S. partitus, which is less aggressive and half the size of S. planifrons, doubled and expanded their range into a microhabitat previously used almost exclusively by adults of S. planifrons. Population increases by S. partitus occurred within 1 yr of the removal of S. planifrons, were persistent, but only reached their maximum ≈ 4 yr (and about four generations) after the initial removal. Adult populations of S. variabilis, which is less aggressive and 20% smaller than S. planifrons, also doubled following the removal of S. planifrons. Combined populations of S. partitus increased most on reefs where S. planifrons previously had been most dense. However, increases by those two species reached only ≈ 70% of the population density and ≈ 40% of the biomass density that S. planifrons had before the removals. Removal of S. planifrons had no effect on the abundances of S. diencaeus and S. leucostictus. Removal of S. partitus from eight reefs for 3 yr did not affect the abundances of S. planifrons or S. variabilis. Eleven years monitoring of adult populations and juvenile recruitment by S. partitus before and during the experiment show that its population increase after the removal of S. planifrons occurred while its overall abundance was gradually increasing, but was not associated with a period of unusually high juvenile recruitment. Thus asymmetric competition among closely related, ecologically similar, territorial coral reef fishes does sometimes control both abundances and microhabitat use in shared habitat. Interspecific territoriality and differences in body size and aggressiveness that lead to asymmetric competitive ability occur commonly among reef fishes in various trophic groups. Interspecific interactions within such groups of species often affect their patterns of habitat use. Hence the role that interspecific competition plays in limiting abundances of coral reef fishes requires reevaluation.
Ecological Explanation through Functional Morphology: The Feeding Biology of Sunfishes
Authors: Wainwright Peter C.
Published: May 2006

Researchers have been using simple morphological measures as indicators of ecological features for some time. The utility of morphological variables as ecological indicators depends upon our understanding of how the variable effects the ability of the organism to perform a particular task. Functional morphological analyses identify those features that can be directly related to behavioral performance and help to distinguish causal functional relationships from spurious correlations. The behavioral abilities of the individual, in turn, shape patterns of resource use and fitness by placing limits on the range of resources that can be utilized and by shaping the cost/benefit curve for resource choices. Examples from research on the feeding biology of North American sunfishes are discussed to illustrate how functional morphology can be used to provide explanations for differences between species in patterns of prey use, patterns of habitat use, ontogenetic diet switches, and population size. Trends from analyses of the evolution of fish feeding mechanisms suggest specific functional features that are most likely to vary among taxa and underlie differences in feeding performance and diet. Included in this group of predictive variables are the organization of lever arms in the jaw opening and closing systems, the size of the mouth, and the size of muscles used in prey—crushing behaviors. The link between morphology and ecology will be made most firmly when variables are chosen that clearly reflect the ability of the organism to perform relevant behaviors.
Response of Insect Herbivores to Multiple Allelochemicals Under Different Thermal Regimes
Authors: Stamp Nancy E. , Yang Yuelong
Published: May 2006

Temperature and food quality are well known to constrain the growth of insect herbivores, but less is known about how these factors may interact. We examined the effects of three tomato allelochemicals (chlorogenic acid, rutin, and tomatine) and two thermal regimes (21°:10°C and 26°:15°C, representing spring and summer respectively) on three insect herbivores: a Solanaceae specialist, Manduca sexta, and the polyphagous Helicoverpa zea and Spodoptera exigua. The alkaloid tomatine slowed development of the polyphagous H. zea, but the two phenolics had little effect. Development of the polyphagous S. exigua was faster with tomatine at the cool thermal regime, and slower with the two phenolics together at the warm thermal regime. All three of the tomato allelochemicals slowed development of the specialist M. sexta. Values of approximate digestibility (or assimilation efficiency) for the two polyphagous species spanned a range twice that of the Solanaceae specialist, increasing when allelochemicals were added to diet. Our techniques permitted us to separate pre— and post—ingestive effects of allelochemicals on mass gained, but we found complex interaction of allelochemicals and of temperature and allelochemicals. The effects of an allelochemical on larval performance often depended on the combination of other allelochemicals in the diet. But the allelochemicals did not always have a negative effect, and multiple allelochemicals did not always have a more negative effect than a single allelochemical. Interaction between thermal regime and the allelochemicals was also common. For example, whether rutin had a negative, neutral, or positive effect on relative growth rate of S. exigua depended on thermal regime and presence or absence of chlorogenic acid. Often the negative effect of an allelochemical was greater at the warm regime than at the cool regime, but not consistently so. Another experiment showed that thermal regime did not affect allelochemical concentration in the diets. Over time (7 d), only tomatine concentration declined. Therefore, the interactive effects of thermal regime and allelochemicals on the insect herbivores were not an artifact of the experimental procedures. Overall, the results indicated that insect performance is not a simple function of temperature and food quality because interactions occur between these factors. They also suggest that the array of chemical defenses exhibited by a particular plant species may be shaped in part by temperature altering the efficacy of allelochemicals.
Biodiversity: Population Versus Ecosystem Stability
Authors: Tilman David
Published: May 2006

The relationships between biodiversity and stability were determined for both population and ecosystem traits in a long—term study of 207 grassland plots. Results demonstrate that biodiversity stabilizes community and ecosystem processes, but not population processes. Specifically, year—to—year variability in total aboveground plant community biomass was significantly lower in plots with greater plant species richness both for the entire 11—yr period and for the nine non—drought years. The change in total plant community biomass from before the drought to the peak of the drought was also highly dependent on species richness. For all three measures of total community biomass stability, multiple regressions that controlled for covariates showed similar significant relationships between plant diversity and stability. In contrast, year—to—year variability in species abundances was not stabilized by plant species richness for either all years or non—drought years. This difference between species vs. community biomass likely results from interspecific competition. When climatic variations harm some species, unharmed competitors increase. Such compensatory increases stabilize total community biomass, but cause species abundances to be more variable. These results support both the predictions of Robert May concerning the effects of diversity on population stability and the diversity—stability hypothesis as applied to community and ecosystem processes, thus helping to reconcile a long—standing dispute.
Response of Stream Invertebrates to a Global‐Warming Thermal Regime: An Ecosystem‐Level Manipulation
Authors: Hogg Ian D. , Williams D. Dudley
Published: May 2006

We manipulated, in accord with global—warming predictions, the thermal regime of a permanent first—order stream near Toronto, Ontario, Canada. We examined the effects of a 2—3.5°C water—temperature increase on densities, biomass, species composition, and life histories of resident stream invertebrates. The stream was divided longitudinally at the source into two channels, one control and one experimental, and a before and after (BACI) design was employed such that one pre—manipulation year was followed by 2 yr of the temperature manipulation. Changes in the experimental channel following commencement of the manipulation included: (1) decreased total animal densities, particularly Chironomidae (Diptera); (2) earlier onset of adult insect emergence; (3) increased growth rates and precocious breeding in Hyalella azteca (Amphipoda); (4) smaller size at maturity for Nemoura trispinosa (Plecoptera) and H. azteca; and (5) altered sex ratios for Lepidostoma vernale (Trichoptera). These results partially corroborate previous laboratory and field studies. However, variation in the responses of individual target species to the manipulation was unexpected and may have been influenced by the genetic structure of local populations. We conclude that levels of gene flow among habitats may be critical to the degree of impact seen as a result of large—scale thermal perturbation (e.g., global warming).
An Experimental Demonstration of Exploitation Competition in an Ongoing Invasion
Authors: Petren Kenneth , Case Ted J.
Published: May 2006

A native asexual gecko, Lepidodactylus lugubris, declines numerically when the sexual gecko Hemidactylus frenatus invades urban/suburban habitats throughout the Pacific. Previous studies showed that the competitive displacement occurs rapidly and is facilitated by clumped insect resources. Five lines of evidence suggest that the mechanism of displacement is primarily due to differences in the ability of each species to exploit insect resources. (1) These species show nearly complete diet overlap. (2) Insects are a limiting resource for both geckos as evidenced by positive demographic effects with increased insect abundance. (3) Hemidactylus frenatus depletes insect resources to lower levels than L. lugubris, which results in reduced rates of resource acquisition in L. lugubris. (4) This reduced resource acquisition translates into significant reductions in the body condition, fecundity, and survivorship of L. lugubris individuals. (5) Evidence for interference (and other) mechanisms does not account for these negative demographic effects on L. lugubris. Interspecific competition is stronger than intraspecific competition for L. lugubris, with increasing L. lugubris density having negligible effect on H. frenatus, mirroring the asymmetry of the large—scale displacement. The superior harvesting ability of H. frenatus is most pronounced when insects are clumped spatially and temporally, and is attributable to a variety of species—specific traits such as their larger body size, faster running speed, and reduced intraspecific interference while foraging. We conclude that clumped resources can increase interspecific exploitation competition, and this mechanism may contribute to species turnover when human environmental alterations redistribute resources.
Virus Transmission in Gypsy Moths is not a Simple Mass Action Process
Authors: D'Amico Vincent , Elkinton Joseph S. , Dwyer Greg , Burand John P. , Buonaccorsi John P.
Published: May 2006

We used the nuclear polyhedrosis virus (LdNPV) of the gypsy moth, Lymantria dispar (Lepidoptera: Lymantriidae), to test one of the basic assumptions of most models of disease dynamics, that the rate of horizontal transmission is directly proportional to the product of the densities of healthy larvae and virus. To do this we made measurements of virus transmission, using small—scale experiments in bags on red oak (Quercus rubra) foliage and field data on naturally occurring populations from a previous study. We observed a decline in the transmission constant as the densities of both healthy larvae and pathogen increased, indicating that the rate of disease transmission is not directly proportional to the product of these variables.
Differential Responses of Growth and Two Soluble Phenolics of Tomato to Resource Availability
Authors: Wilkens Richard T. , Spoerke Jill M. , Stamp Nancy E.
Published: May 2006

We conducted two experiments to determine how resources influenced the intraspecific and within—plant allocation by tomato plants (Lycopersicon esculentum) to the soluble phenolics rutin and chlorogenic acid. We also measured the effect of resource availability on growth by measuring mass and other physical and cellular attributes of the plant. In the first experiment, we subjected plants to four levels of potassium nitrate fertilizer. In the second experiment, we subjected plants to high and low levels of potassium nitrate fertilizer and light. Both experiments yielded results consistent with the growth—differentiation balance hypothesis. Plants grown with low resources showed low levels of soluble phenolics and low plant mass. Plants grown with intermediate levels of resources showed high phenolic concentrations but inhibited growth. Plants grown with high resources had high growth but no increases in phenolic concentrations. The results were also consistent with the general prediction of the optimal defense hypothesis that there should be a negative relationship between growth and defense. We discuss possible adaptive explanations for the specific patterns observed. There were also consistent within—plant differences in phenolic concentrations. These differences in phenolic concentrations were large enough to have potential consequences for insect herbivores feeding on tomato plants.
How Risky is Biological Control?
Authors: Simberloff Daniel , Stiling Peter
Published: May 2006

The potential harmful effects of non—indigenous species introduced for biological control remain an important unanswered question, which we addressed by undertaking a literature review. There are few documented instances of damage to non—target organisms or the environment from non—indigenous species released for biological pest control, relative to the number of such releases. However, this fact is not evidence that biological control is safe, because monitoring of non—target species is minimal, particularly in sites and habitats far from the point of release. In fact, the discovery of such impacts usually rests on a remarkable concatenation of events. In addition to trophic and competitive interactions between an individual introduced species and a native one, many effects of introduced species on ecosystems are possible, as are numerous types of indirect interactions. Predicting such impacts is no mean feat, and the difficulty is exacerbated by the fact that introduced species can disperse and evolve. Current regulation of introduced biological—control agents, particularly of entomophages, is insufficient. At the very least, strong consideration should be given to the likely impact of both the pest and its natural enemy on natural ecosystems and their species, and not only on potential costs to agriculture, silvi—culture, and species of immediate commercial value.
A Simplified Forest Model to Study Species Composition Along Climate Gradients
Authors: Bugmann Harald K. M.
Published: May 2006

Forest models based on the gap dynamics hypothesis (gap models) have gained an important role in forest ecology and have grown rather complex in the last 20 yr. They have been applied extensively to study the impacts of climatic change on ecosystems although they originally were not built for this purpose. The objectives of this study were (1) to develop a new forest gap model, ForClim, that includes only a minimum number of ecological assumptions but robust parameterizations of the effects of climate on plant population dynamics; (2) to test the realism of ForClim as compared to its predecessor model, FORECE; and (3) to examine the behavior of FORECE and ForClim systematically along climate gradients in Europe. ForClim is composed of three modular submodels: ForClim—P for plant population dynamics, ForClim—S for soil carbon/nitrogen turnover, and ForClim—E for providing reliable parameterizations of the abiotic environment. For the core model, ForClim—P, it was found that only four factors are sufficient to model tree growth, another four factors are required to model tree establishment, and only two factors are required to model tree mortality. The behavior of ForClim was tested at a large number of sites in the European Alps. The model yields tree species compositions that conform to field data and are very similar to those of the predecessor model. Based on this evaluation alone, it would not be possible to favor one of the models over the other. The behavior of both models then was examined systematically in a parameter space spanned by the annual mean temperature and the annual precipitation sum. From this exercise it became evident that both the pattern of aboveground biomass and the realized niches of the dominating tree species are simulated realistically by ForClim. Extremely steep gradients are characteristic of FORECE, and many ecotones are simulated to occur in the wrong places in FORECE. Thus, some of the current forest gap models can be simplified without reducing the realism of their behavior, and models other than FORECE should be scrutinized in this respect as well. The present study also suggests that the evaluation of model behavior at scattered sites is insufficient to show their validity for simulating forest dynamics along climate gradients. Further rigorous model comparisons and validation studies are required to increase the reliability of this promising class of models.
An Evaluation of the Accuracy of Kernel Density Estimators for Home Range Analysis
Authors: Seaman D. Erran , Powell Roger A.
Published: May 2006

Kernel density estimators are becoming more widely used, particularly as home range estimators. Despite extensive interest in their theoretical properties, little empirical research has been done to investigate their performance as home range estimators. We used computer simulations to compare the area and shape of kernel density estimates to the true area and shape of multimodal two—dimensional distributions. The fixed kernel gave area estimates with very little bias when least squares cross validation was used to select the smoothing parameter. The cross—validated fixed kernel also gave surface estimates with the lowest error. The adaptive kernel overestimated the area of the distribution and had higher error associated with its surface estimate.
Inference of Parasite‐Induced Host Mortality from Distributions of Parasit Loads
Authors: Rousset Francois , Thomas Frederic , De Meeus Thierry , Renaud Francois
Published: May 2006

Evidence for parasite—induced host mortality from field data remains difficult to obtain. We have developed analytical models to investigate the effects of parasite—induced host mortality on the distribution of macroparasites among individual hosts as a function of host age. Under the assumptions of these models, it is shown that a decrease of the mean parasite load or of its dispersion implies that there is parasite—induced host mortality and that infection rate is variable for different hosts in the population. Observed patterns will depend on the nature of such variation and of the relation between parasite load and host mortality.