bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth, Density, and Interspecific Influences on Pumpkinseed Sunfish Life Histories
Authors: Fox Michael G.
Published: May 2006

I collected data on 27 pumpkinseed (Lepomis gibbosus) populations in eastern and central Ontario and conducted a transplant experiment with one of these populations to test growth and competition—related predictions of several life history models. The predictions are that early maturity and high gonadal investment will occur: (1) in large and fast—growing juveniles; (2) in populations with low adult: juvenile growth ratios; (3) in populations with low density; and (4) in the absence of bluegill sunfish (Lepomis macrochirus), a competing congener. Predictions 1 and 4 were fully supported and Prediction 2 was partly supported, but Prediction 3 was not. The earliest maturing juveniles within populations were significantly larger than conspecifics of the same age that did not mature. Females from a lake population that were transplanted into a fishless pond exhibited both significantly faster growth and a significantly higher gonad to body mass ratio (gonado—stomatic index) that females living in the lake. At the population level, juvenile growth was significantly correlated with age at maturity (r = 0.52), but not with gonadostomatic index, whereas the Adult : Juvenile Growth Ratio showed a significant, negative correlation wih gonadosomatic index (r = 0.66), but not with mean age at maturity. Mean age at maturity showed a stronger correlation with the indicator of life—span and adult survival rate (r = 0.74), suggesting that mortality is more important than growth in the shaping of reproductive life histories of pumkinseed. High population density was associated with early maturity and high gonadal investment, the opposite of what was predicted. However, pumpkinseed populations that co—occurred with bluegills did mature significantly later and at a larger size and tended to have a lower gonadal investment than populations living in waterbodies without bluegills. This difference in reproductive patterns in the presence of bluegill is consistent with two—stage life history theory and may be the result of a direct effect of bluegills on pumpkinseed growth and survivorship, as well as differences in environmental conditions were the two species do or not co—occur.
The Impact of Fish Exclusion on the Abundance and Species Composition of Larval Odonates: Results of Short‐Term Experiments in a North Carolina Farm Pond
Authors: Morin Peter J.
Published: May 2006

Under natural conditions in a man—made North Carolina farm pond, a single small species, Perithemis tenera, numerically dominates a littoral assemblage of larval dragonflies. Censuses of exuvia of larvae metamorphosing from the pond in three successive years demonstrated a consistent negative correlation between numerical dominance in the assemblage and species—specific size in the final larval instar. This pattern suggested that size—dependent processes, such as vertebrate predation, might structure the odonate assemblage. Exclusion of vertebrate predators from patches of emergent vegetation by screen exclosures for 3 mo increased total abundances of larval odonates by an order of magnitude. Dominance shifted from small species to species of intermediate size, especially Pachydiplax longipennis, where fish were excluded. The largest species remained rare and were unaffected by fish exclusion. Four odonate species were facultatively multivoltine, completing larval development within 1—2 mo after colonization of the fish exclosures. Fish exclusion also increased the abundance of large microcrustacea, especially the cladoceran Simocephalus serrulatus. large microcrustacea increased in abundance despite a corresponding increase in the abundance of their macroinvertebrate predators, where fish were excluded. The usual dominance of small Perithemis in this assemblage was a probable consequence of differentially heavy predation by fish on moderate—size species, and the unexplained failure of large species to recruit well under experimental or natural conditions. These results support the general importance of predation in structuring freshwater communities and document an important mode of population regulation operating during the aquatic larval phase of the odonate complex life cycle.
Tests of Compensatory vs. Additive Hypotheses of Mortality in Mallards
Authors: Burham Kenneth P. , Anderson David R.
Published: May 2006

Band recovery data from over 410 000 adult Mallards (Anas platyrhynchos) banded in North America between 1950 and 1979 were analyzed to examine the effect of exploitation on annual survival rate. Two extreme hypotheses were defined; completely compensatory, and totally additive, and an explicit mathematical model was presented to represent each hypothesis. Comparison of the values of the log—likelihood functions at their maxima allowed discrimination between the two models (hypotheses). Extensive Monte Carlo studies were made on the performance of the discriminant test and the power of the test under the two extreme hypotheses and a range of intermediate hypotheses. The results of the discriminant tests on the data agreed closely with the expected performance of the test if the true underlying process was compensatory. We rejected the hypothesis of total additivity (P = .001). Instead, it appears that hunting mortalities are largely compensated for by other forms of mortality.
Effects of Short‐Term Selection Along Environmental Gradients on Variation in Populations of Amaranthus Retroflexus and Abutilon Theophrasti
Authors: Zangerl A. R. , Bazzaz F. A.
Published: May 2006

This study examines the effects of short—term selection along resource gradients on variation within plant populations. Populations of Amaranthus retroflexus and Abutilon theophrasti were subjected to four generations of selection in the following treatments: soil moisture (low, moderate, or high), diversity of competitor species (low, moderate, or high), and density (low or high). In several cases such selection resulted in significant divergence of the population from its original configuration. The changes occurred in such characters as photosynthetic rate, reproductive allocation, flowering time, root/shoot allocation, and shoot biomass. In all cases where significant changes in overall level of variation occurred, these were in the direction of reduced variation. The rapidity and nature of these changes differed according to the level and type of treatment. Reductions in variation occurred almost uniformly in the density treatments. In contrast, changes in variation were most unpredictable in the diversity treatments, owing to a high level of variability in the competitive background that was apparently associated with generation—to—generation variability in light and temperature. In some generations Abutilon theophrasti was greatly suppressed by heterosopecific competitors. This resulted in a significant reduction in variation in spite of high levels of environmental variability. Thus environmental variation in itself may not be sufficient to maintain variation in the population of extreme fluctuation in population size results in genetic bottlenecks. Levels of morphological and physiological variation were maintained in all but a low— and a moderate—moisture line of Amaranthus retroflexus, but the diversity of electrophoretically marked genotypes declined in all moisture treatments. The loss of variation at moderate soil moistures may be due to an overlying gradient of conspecific competition, a situation which cannot be avoided in even the simplest of gradients. Despite significant changes in several characters and in levels of overall variation, technological and analytical tools were found to be inadequate to determine if these changes led to significant and consistent differences in the response patterns of the species along these gradients.
Effects of Shading and Fertilization on Resistance of Winter‐Dormant Birch (Betula Pendula) to Voles and Hares
Authors: Rousi Matti , Tahvanainen Jorma , Henttonen Heikki , Uotila Ilari
Published: May 2006

Seedling representing four families of European white birch (Betula pendula) were grown through their first growing season in two fertilization and three shading treatments. The growth responses of seedlings were measured, and after complete winter hardening the basal parts of seedlings were offered to voles (Microtus agrestis) and the top parts of the same seedlings were fed to hares (Lepus timidus). The growth response of 1—yr—old birch seedlings to fertilization and shading was family specific. Nitrogen fertilization generally increased the biomass accretion and the palatability of the seedlings to voles. Fertilization significantly decreased resistance of seedlings to hares in only one out of the four birch families. Shading decreased the biomass accretion of seedlings and increased their palatability to hares but tended to decrease the palatability to voles. The differing preferences of the two mammalian herbivores for seedlings grown under different environmental conditions may be due to vertical differences in the defense of birch seedlings to herbivore feeding. No trade—off was found between seedling defense and growth.
Constant Predator‐Prey Ratios: An Arithmetical Artifact?
Authors: Closs G. , Watterson G. A. , Donnelly P. J.
Published: May 2006

Analysis of several large collections of food webs has shown that predator—prey ratios tend to be roughly constant at values close to one. We suggest that the constancy of the ratio may simply be an arithmetical artifact, a consequence of the way the ratio is defined. Taxa can be recorded as both predator and prey and, hence, be double counted. In many webs the proportion of species that are double counted is large; consequently, the ratio of predator species to prey species will inevitably by roughly equal to one. We present a simple random food—web model in which predator—prey ratios are approximately constant with values close to one.
Negative Effects of Scale Insect Herbivory on the Ectomycorrhizae of Juvenile Pinyon Pine
Authors: Del Vecchio Teresa A. , Gehring Catherine A. , Cobb Neil S. , Whitham Thomas G.
Published: May 2006

An important mycorrhizal mutualism in plants is known to be influenced by herbivory. In contrast to most previous studies that have focused on grass and crop species, here we examine this interaction with trees of known resistance and susceptibility to insect attack under field conditions. To examine the potential interactions between ectomycorrhizae and herbivory, we quantified the levels of ectomycorrhizal colonization of °30—yr—old pinyon pines (Pinus edulis) that were either resistant or susceptible to the sap—feeding scale insect Matsucoccus acalyptus. Resistant trees had lower scale survivorship and retained more of their foliage over a 7—yr period than susceptible trees. Resistant trees also had 28% more ectomycorrhizae than susceptible trees. To examine the cause of these ectomycorrhizal differences we removed scales from susceptible trees and compared their levels of ectomycorrhizal colonization with those of susceptible trees and compared their levels of ectomycorrhizal colonization with those of susceptible and resistant control trees. Following the removal of scales, the levels of ectomycorrhizal colonization of susceptible trees rebounded and did not differ from those of trees resistant to scale attack. These results demonstrate that herbivory by scale insects reduces the ectomycorrhizae of susceptible trees.
Pairing Probability of Schistosomes Related to Their Distribution Among the Host Population
Authors: Morand Serge , Pointier Jean-Pierre , Borel Gaetan , Theron Andre
Published: May 2006

Schistosomes are dioecious blood helminth parasites of mammals, including humans, and are responsible for schistosomiasis in tropical countries. The distribution patterns of worms among their host populations and the pairing probabilities of female schistosomes are of considerable importance in understanding transmission dynamics of this parasite. Data were obtained from field studies carried out over 8 yr on natural populations of Schistosoma mansoni, a human parasite but heavily infecting rats (Rattus rattus) in Guadeloupe. The distribution of male and female worms within the whole host population was fitted to a negative binomial, and the aggregation coefficient was estimated. The observed prevalence of male—female pairs, the observed prevalence of worms, and the observed pairing probability were calculated. Results demonstrate a strongly clumped worm distribution. Observed pairing probabilities of female worms in cases where male and female schistosomes are aggregated together within the host population are of the same order (@Aobs ranging from 0.83 to 0.97) as the expected values obtained with the May (1977) model but significantly higher than in cases where the sexes are aggregated separately. A disequilibrated sex ratio biased towards male schistosomes increases the pairing probabilities of female worms.
Energetics of Foraging in Breeding Adelie Penguins
Authors: Chappell Mark A. , Shoemaker Vaughan H. , Janes Donald N. , Maloney Shane K. , Bucher Theresa L.
Published: May 2006

We studied foraging energetics in Adelie Penguins (Pygoscelis adeliae) using doubly labeled water (DLW) and time—depth recorders (TDR). Measurements were made during three nesting stages: late incubation, the guard stage (when small chicks were continuously guarded by at least one parent) and the subsequent creche stage (when large chicks were left unattended). Nest relief cycle times decreased from 229 h during incubation to 33.3 h during the creche stage, and the fraction of time birds spent swimming increased from 20.8% during incubation to 31.6% during the creche stage. The fraction of swim time spent in hunting dives and bottom time did not change significantly at different nesting stages. Field metabolic rates (FMR) wee 2.73 ° basal metabolic rate (BMR) during incubation, 3.03 ° BMR during the guard stage, and 3.29 ° BMR during the creche stage; due to high variance these values did not differ significantly. Estimated metabolic rate during swimming was 8.2 ° BMR. Rates of prey capture (grams of krill per hour of swimming, per hour of hunting dives, and per hour of bottom time) did not change at different stages. Food provided to chicks was 20.9% (guard stage) to 23.3% (creche stage) of the food metabolized by adults. Our data suggest that (1) prey capture by Adelies is limited primarily by their ability to find drill swarms and not by limitations in harvest rates or energy efficiency after prey have been located, and (2) reproductive effort in Adelies does not require a large increase in either energy expenditures or foraging time.
Beetle Folivory Increases Resource Availability and Alters Plant Invasion in Monocultures of Goldenrod
Authors: Brown D. Gordon
Published: May 2006

One way that insect herbivores can influence plant community structure is by altering the ambient availability of resources required by plants. To examine the importance of this mechanism, I tested the following three hypotheses in a field experiment in 1990: (H1) Folivory by the leaf beetle Trirhabda canadensis (Coleoptera: Chrysomelidae) increases availability of light, water, and N in monocultures of goldenrod (Solidago missouriensis). This hypothesis was partially supported. Folivory by Trirhabda reduced leaf, root, and total biomass of goldenrod and increased light penetration, soil water content, and soil nitrate concentration. However, Trirhabda grazing did not affect overall soil N availability. (H2) The invasion of goldenrod monocultures by other plant species and the response of goldenrod to folivory reflects an increased availability of resources in monocultures grazed by Trirhabda. This hypothesis was supported. Folivory by Trirhabda raised the probability that a species would invade the experimental monocultures and increased the species richness of the invading plant assemblage. Relative growth rate of goldenrod and production of aboveground biomass by invaders were higher in monocultures grazed by Trirhabda than in ungrazed ones. Prostrate and creeping forbs increased in relative abundance following folivory by Trirhabda. (H3) Reduced N uptake by goldenrod or increased N mineralization accompanies increased N availability in grazed monocultures. This hypothesis was also supported. Net N mineralization and nutrification were higher in monocultures grazed by Trirhabda than in ungrazed monocultures. Folivory by Trirhabda larvae reduced N uptake by goldenrod, but that of adults did not. The impact of herbivores upon plant communities may be effected through increased availability of important plant resources.
Indirect Interactions Between Prey: Apparent Competition, Predator Aggregation, and Habitat Segregation
Authors: Schmitt Russell J.
Published: May 2006

Field experiments were performed to explore the nature of indirect interactions between two groups of ecologically distinct prey that occur on subtidal rocky reefs at Santa Catalina Island, California. Mobile gastropods (Tegula aureotincta, Tegula eiseni, and Astraea undosa) and sessile bivalves (mostly Chama arcana) share a common set of invertebrate predators (lobster Panulirus interrupts, cephalopod Octopus bimaculatus, and whelk Kelletia kelletii). The gastropods, which are secondarily preferred prey, principally occur on cobble reefs and less commonly on high—relief boulder reefs. Sessile bivalves and other species of favored prey are common on high—relief reefs and are rare or absent in cobble areas. The density of each predator species is greatest in high—relief areas containing abundant favored prey. Addition of bivalves to replicate cobble plots resulted in marked increases in predator density, relative to controls, because predators aggregated to areas containing favored prey. Greater densities of predators in the presence of experimentally added bivalves resulted in greater mortality and lower population densities of gastropods relative to controls. Conversely, mortality of Chama was higher where gastropods were common compared with cobble areas containing fewer gastropods. This occurred because the density of predators within a cobble reef, although low overall, was positively correlated with density of gastropods. Thus each group of prey was negatively affected by the presence of the other because each alternative prey increased the local density of predators. Such a doubly negative indirect interaction between prey, mediated by a shared predator, is known as apparent competition; this is the first experimental demonstration of its existence. Because gastropods and bivalves are superior at withstanding predation in different habitats, shared predation and apparent competition may be sufficient mechanisms to maintain the pattern of habitat segregation displayed by these prey groups.
A Comparison of Index‐Based and Pixel‐Based Neighborhood Simulations of Forest Growth
Authors: Armstrong Robert A.
Published: May 2006

In simulations, the computational method chosen to implement a model can critically affect quantitative results. Here I show that two reasonable implementations of a neighborhood model of plant growth produce predictions that differ quantitatively in several important respects. I conclude that for modelling forest canopies, pixel—based implementations should prove superior to implementations that are based on indices of neighborhood crowding.
Costs of Reproduction in an Intertidal Kelp: Patterns of Allocation and Life History Consequences
Authors: Pfister Catherine A.
Published: May 2006

Reproductive investment of the intertidal kelp Alaria nana was manipulated experimentally to investigate costs of reproduction. Allocation patterns were examined using treatments that employed vegetative and reproductive tissue removal. Growth, survivorship, size, reproductive investment, carbon, nitrogen, and phenolic secondary metabolities were monitored in tagged experimental plants. Further experiments investigated the contribution of reproductive tissue to drag forces on the plant in intertidal environments. Carbon, nitrogen, and phenolics were all higher in reproductive tissue (sporophylls) compared to vegetative tissue (the frond) in control plants. When some vegetative tissue was removed, carbon and nitrogen in the sporophylls decreased while percent phenolics remained unchanged. Reproductive tissue removal had no effect on plant survirorship, growth, or size compared with control plants. When the entire frond was kept clipped, sporophyll number decreased, often to the point of complete attrition. Although allocation patterns in Alaria nana are consistent with reproductive costs (i.e., the sporophylls are not self—supporting), there is no evidence that reproduction imposes costs on the plant that are demographically important such as growth, size, and survivorship. Instead, both reproductive investment and percent phenolics were positively correlated with growth and size in control plants.
Selective Herbivory by Abert's Squirrel Mediated by Chemical Variability in Ponderosa Pine
Authors: Snyder Marc A.
Published: May 2006

Abert's squirrels (Sciurus aberti) are restricted to stands of ponderosa pine (pinus ponderosa) and feed heavily on the inner bark tissues of these trees. Although preferences for inner bark of particular trees (target trees) have been well documented, the basis of the squirrels' selectivity has not been clearly demonstrated. Compared with matched nontarget (uneaten) trees, and with whole populations of ponderosa pine, target trees had significantly lower xylem oleoresin flow rates, and significantly lower levels of β—pinene and β—phellandrene in the xylem oleoresin. In feeding trials with free—ranging animals, food laced with ecologically realistic concentrations of either β—phellandrene were eaten significantly less than control food. The phloem of target trees had significantly higher concentrations of nonstructural carbohydrates and sodium, and significantly lower concentrations of iron and mercury, than the phloem of matched nontarget trees. Oleoresin characteristics associated with selective herbivory wee unaffected by simulated herbivory over 2 yr, supporting existing evidence that these characteristics are under strong genetic control. Because herbivory is associated with these host traits and results in dramatic reductions in host fitness, Abert's squirrels are probably important agents of natural selection in host ponderosa pine populations.
Multivariate Analysis of Golden Marmot Maximum Running Speed: A New Method to Study MRS In the Field
Authors: Blumstein Daniel T.
Published: May 2006

I develop a multivariate technique permitting the study of variation in the maximum velocity of animals. The method was applied to study maximum running speed (MRS) of adult golden marmots (Marmota caudata aurea). Subjects were encouraged to run to home burrows after being livetrapped, and were timed while running over a randomly selected distance between 1.7 and 36.0 m. Body mass, sex, distance run, substrate, and incline the individual ran across for 52 different subjects were entered into a linear model, which implicitly modeled MRS as a function of the running time. Body mass did not significantly explain variation in running time and was deleted. A new model fitted to the remaining variables significantly explained variability in time, and therefore MRS. The model did not violate any of the assumptions of linear models, and appeared to be robust. Intrayear repeatability of individual MRS was small, suggesting that environmental factors may account for much of the variation in MRS in golden marmots. The model predicts that the four significant variables (sex, distance run, substrate, incline) should influence space use and patch selection in golden marmots since they influence the speed at which an alarmed marmot can reach a refuge. The general method developed here can be applied to other species and should shed light on patch use and foraging behavior of refuging species.
Nonlinear and Logistic Growth In Experimental Populations of Guppies
Authors: Barlow Jay
Published: May 2006

Previous experiments have shown that for a specific experimental system of guppies (Poecilia reticulata), per capita growth rate is linearly related to population biomass. A length—specific population model is developed to examine how the various mechanisms of density dependence (i.e., changes in somatic growth rates, reproductive rates, and cannibalism) act in concert to shape the overall population response to intraspecific competition. Despite many nonlinear components, equilibrium per capita growth in the model is linearly related to population biomass, mimicking the logistic growth seen in the experiment. Using the model, the components of density dependence are varied to determine how each component shapes the overall density—dependent response. Density dependence in somatic growth rates is found to be the most important population—regulating mechanism in this system. In the past, emphasis has been placed on density dependence in birth and death functions. The importance of somatic growth in shaping per capita population growth curves may be underestimated.
Sex Allocation in the Simultaneously Hermaphroditic Polychaete Worm Ophryotrocha Diadema
Authors: Sella Gabriella
Published: May 2006

O. diadema is a simultaneously hermaphroditic polychaete worm with a brief protandrous phase. Pairs are formed preferentially between two simultaneous hermaphrodites, which reciprocally change sex roles and trade eggs. Allocation of reproductive effort during simultaneous hermaphroditic and protandrous phases was evaluated quantitatively, by microscopic examination of both fixed and living specimens. Ovarian tissue is four times as abundant as testicular tissue. The mean number of sperm counted in sectioned animals was taken as an approximation of the number of sperm offered per egg. As expected in species where the mating group always consists of two individuals, this number is low (50 sperm are offered per egg). Like eggs, sperm released at fertilization are replaced within 3 d. Such reduced allocation to testicular tissue allows saving in the physiological cost of sperm production. Efficiency of fertilization in the protandrous phase is low but positively correlated with body length (measured by the number of setigers). A fertilization ratio of 95% is obtained only at the simultaneous hermapohroditic stage. This explains why protandrous males are generally not selected as mates in the reproductive strategy of O. diadema. Self—fertilization is avoided by behavioral constraints. The observed female bias in reproductive effort in the mating system of O. diadema suggests that hermaphroditism should be evolutionarily stable, as the male investment in reproductive effort follows the law of diminishing returns.
Growth Responses of Tropical Shrubs to Treefall Gap Environments
Authors: Denslow Julie Sloan , Schultz Jack C. , Vitousek Peter M. , Strain Boyd R.
Published: May 2006

To investigate the effects of differences in light and nutrient availability on growth, we planted seven species of shrubs in two genera, Miconia (Melastomataceae) and Piper (Piperaceae), into the centers, edges, and adjacent forest understory of four natural treefall gaps (275—3355 m2) in the tropical premontane rain forest of Costa Rica. We used rooted cutting of species typical of forest understory environments on the one hand and large clearings or disturbed areas on the other. We also compared growth rates of three Miconia species grown in shade houses under 2, 20, and 40% full sunlight. Both light and nutrient availability in newly formed gaps of these sizes were strongly buffered by the canopy and root systems of the surrounding forest. Total incident radiation was higher in gap centers (9—23% full sunlight) than in gap—forest edges (3—11%) or under intact forest canopy (.04—2%), but varied among similar microhabitats from different sites. Relative stem growth rates (RGRs) of all field—grown plants were significantly greater in gap centers than at edges or beneath forest understories. Fertilization did not significantly affect growth rate in any light environment. Light appears to be the most critical resource limiting growth at these gap sizes. In general, shade—tolerant species were less plastic than light—demanding species, but at these gap sizes grew as fast or faster in the gap centers. In shade—houses, the shade—tolerant species grew faster at 20% full sunlight and light—demanding species grew faster at 40% full sunlight. We found no evident of a trade—off between growth and foliar phenolic concentration in these species.
Intense, Natural Pollution Affects Arctic Tundra Vegetation at the Smoking Hills, Canada
Authors: Freedman B. , Zobens V. , Hutchinson T. C. , Gizyn W. I.
Published: May 2006

Long—term, natural emissions of sulfur dioxide and acidic aerosols have had an impact on remote tundra at the Smoking Hills. The emissions have caused plant damage by SO2 toxicity, and have severely acidified soil and freshwater. At the most intensively fumigated locations closest to the sources of emission, pollution stresses have devegetated the terrestrial ecosystem. The first plants that are encountered along a spatial gradient of decreasing pollution stress are Artemisia tilesii and Arctagrostis latifolia, which dominate a characteristic, pollution—tolerant community. Farther away at moderately polluted sites there are mixed communities with floristic elements of both fumigated and reference, unfumigated tundra. This pattern of ecosystem response to a concatenation of stresses caused by natural air and soil pollution is qualitatively similar to the damage that occurs in the vicinity of anthropogenic point sources of air pollution, such as smelters.
Annual Variation in Seedfall, Postdispersal Predation, and Recruitment of a Neotropical Tree
Authors: Schupp Eugene W.
Published: May 2006

Knowledge of the dynamics of seed production and seedling recruitment of individual tree species is crucial for a complete understanding of tropical forest dynamics, yet multiyear studies on the seed and young seedling stages of tropical trees are virtually nonexistent. In a 4—yr study of the understory tree Faramea occidentalis on Barro Colorado Island, Panama, I quantified natural levels of viable seedfall, seedling emergence, and seedling establishment, and experimentally estimated postdispersal seed predation. The levels of viable seedfall, seed predation, seedling emergence, early seedling survival, and seedling recruitment all differed significantly among years. The proportion of fallen seeds destroyed by predators before germination was not related to the quantity of F. occidentalis seedfall. Within a year, however, F. occidentalis seed predation appeared to be influenced by community—wide seedfall, with high predation rates during times of low seed abundance and very low predation during the late dry season peak in seedfall by the community. Most of the annual variation in recruitment can be explained by the combination of seedfall and seed predation; in 3 of the 4 yr seedling emergence could be predicted from a knowledge of viable seedfall an the probability of a seed surviving until the peak of germination. The 4th yr, however, demonstrated that environmental conditions provide a sporadic, though important, limitation to recruitment. In comparison to many tree species, early seedling survival was relatively high, as was the ratio of seedlings recruited per seed falling. The highly successful recruitment of F. occidentalis is associated with a high population density of both saplings and adults in the study area.
Biological Control of the Cottony‐Cushion Scale: Experimental Tests of the Spatial Density‐Dependence Hypothesis
Authors: Thorarinsson Kristjan
Published: May 2006

According to the spatial density—dependence hypothesis, successful biological control is stabilized by parasitoid foraging behavior, such that increasing rates of parasitism accompany increasing local host (pest) density. Three field experiments were conducted to try to detect spatially density—dependent parasitism in the biological control of the cottony—cushion scale, Icerya purchasi, by the parasitic fly Cryptochaetum iceryae. Experimental colonies of the scale were established on terminal shoots of one of its host plants, mock orange (Pittosporum tobira). These colonies, consisting mostly of second— and third—stage nymphs, were exposed to parasitism for 11—14 d, depending on the experiment, and the resulting spatial patterns of parasitism rates recorded. In the first experiment the developmental stage of each scale was recorded; in the remaining two experiments, the length of each individual scale was also recorded. Weighted regression analysis was used to study the relationship between spatial density and parasitism rate, using mean length of scales (when available) as a covariate. The relationship was studied on two spatial scales, first by terminal shoots and then by individual leaves nested within shoots. No spatial density dependence was found in any of the three experiments on either spatial scale. Rates of parasitism increased with scale body length, indicating parasite preference for larger hosts. The results suggest that spatial density dependence may not be necessary to the success of biological control. The results should, however, be interpreted with caution because of the limits imposed by finite statistial power; also, it is possible that spatial density dependence may express itself on other host plants or on another spatial scale. Alternative stabilizing mechanisms, such as partial invulnerability of adults and individual variation in risk, are discussed with reference to the cottony—cushion scale system.
The Vernal Dam: Plant‐Microbe Competition for Nitrogen in Northern Hardwood Forests
Authors: Zak Donald R. , Groffman Peter M. , Pregitzer Kurt S. , Christensen Soren , Tiedje James M.
Published: May 2006

Nitrogen (N) uptake by spring ephemeral communities has been proposed as a mechanism that retains N within northern hardwood forests during the season of maximum loss. To understand better the importance of these plants in retaining N, we followed the movement of 15NH4+ and 15NO3— into plant and microbial biomass. Two days following isotope addition, microbial biomass represented the largest labile pool of N and contained 8.5 times as much N as Allium tricoccum L. biomass. Microbial immobilization of 15N was 10—20 times greater than uptake by A. tricoccum. Nitrification of 15NH4+ was five times lower in cores containing A. tricoccum compared to those without the spring ephemeral. Spring N retention within northern hardwood forests cannot be fully explained by plant uptake because microbial immobilization represented a significantly larger sink for N. Results suggest that plant and microbial uptake of NH4+ may reduce the quantity of substrate available for nitrification and thereby lessen the potential for NO3— loss via denitrification and leaching.
Single‐Species Spatial Dynamics May Contribute to Long‐Term Rarity and Commonness
Authors: Hanski Ilkka
Published: May 2006

A model of spatial population dynamics was developed in which small and large local populations are distinguished. In the model, emigration from small populations is assumed to be inconsequential, but the dynamics of currently small populations are affected by immigration from large ones. If immigration substantially increases the likelihood that a small population will grow large, two alternative stable states are possible in the model corresponding to regional commonness and regional extinction (or regional rarity, if dispersal from safe sites or from outside the region is allowed for). Analysis of the model showed that even a small number of large local populations may play a crucial role in the regional dynamics of some species. The process of regional extinction, when it occurs, may show as an unexpected crash in the time trajectory.
Causes and Consequences of Territoriality in a Grazing Stream Insect
Authors: Hart David D.
Published: May 2006

The use and defense of food and space in the territorial larval caddisfly Leucotrichia pictipes (Trichoptera: Hydroptilidae) were evaluated through field observations and experiments in southwestern Michigan. These grazing stream insects live in silk cases attached to hard substrates and aggressively expel intruders (both conspecifics and other species) that enter their algal—covered foraging areas. Defended areas contained significantly greater algal standing crops, which were a benefit resulting from resource defense. This territorial behavior often led to patterns of uniform spacing between larvae in high—density patches. Exceptions to these patterns tended to prove the rule; when Leucotrichia cases were much closer than normal, at least one of the individuals had entered a nonterritorial life history stage. Larval territory size was determined by food requirements and food availability. For example, territory size is related to the resident's body mass according to the equation: Area = 2.39 Mass0.84. The exponent for this power function is similar to the exponent relating metabolic rate to mass in many insects, further supporting the view that territoriality in Leucotrichia is related to the defense of a limiting food resource. Testing the effect of food availability on territory size provided support for several optimal feeding—territory—size models; when availability was experimentally reduced, residents expanded the size of their territories relative to control larvae. Ordinarily, other competing grazer species (e.g., the mayfly nymph Baetis flavistriga) spent less time per unit area foraging within Leucotrichia's territory than they spent foraging elsewhere. However, when resident caddisflies were experimentally removed from their territories, these mayfly nymphs spent more time inside than outside the territory. The territory of Leucotrichia often provided a resource oasis containing significantly higher algal standing crops than unprotected areas, which were heavily utilized by a diverse guild of grazers. The mayfly nymphs seem to have adopted a territory—parasite strategy similar to that of some nectar—feeding birds, in that they regularly robbed resources from Leucotrichia territories by a combination of stealth and speed. Other members of the grazer guild were less able to gain access to these limited resources, so that Leucotrichia acted as a dominant competitor in this community.
Heterogeneity of Tropical Floral Microclimates and the Response of Hummingbird Flower Mites
Authors: Dobkin David S.
Published: May 2006

Analyses of microclimatic patterns on Heliconia wagneriana inflorescences inhabited by nectarivorous hummingbird flower mites (Gamasida: Ascidae) in secondary lower montane rainforest reveal the existence of substantial microclimatic variation both among and within individual inflorescences. Field experiments demonstrate that the mites respond to this variation by exploiting thermally favorable locations and avoiding thermally adverse conditions within inflorescences. Field introductions of mite population inocula to uninhabited inflorescences show that mite reproduction (and probably survival) is affected strongly by inflorescence microclimate. Floral microclimate, in conjunction with hummingbird visitation patterns, controls mite dispersal among inflorencences. These experiments illustrate the impact of microclimatological heterogeneity on the population biology of a tropical phytophagous arthropod.
Late‐Quaternary Climatic and Vegetational Change in the Lower Mackenzie Basin, Northwest Canada
Authors: Ritchie J. C.
Published: May 2006

A 355—cm sediment sequence from Twin Tamarack Lake near Inuvik, Northwest Territories, Canada, provides a 14 500—yr record of pollen percentages and accumulation rates, interpretable in terms of climatic change and plant community dynamics. Four distinct pollen assemblage zones are recognized: a Gramineae—Artemisia—Cyperaceae—Salix zone from 14 500 to 11 800 BP; a Betula glandulosa zone from 11 800 to 8400 BP, subdivided into four subzones (Gramineae—herb, Populus, Salix, and Juniperus); a Picea—Betula zone, from 8400 to 5900 BP; and a Piceae—Betula—Alnus zone from 5900 to 0 BP. The vegetation during the late—glacial at this site (14 500—11 800 BP) was relatively stable, consisting of sparse herb tundra on uplands in a slowly warming climate. Dwarf birch increased during this phase. The early Holocene period of maximum summer radiation produced thermal conditions °10% warmer than present, initiating a change from tundra to woodland at most sites, effected by the efficiently dispersed Populus (chiefly P. balsamifera). Relatively rapid changes in regional vegetation occurred during this period (11 800 to 6500 BP); Juniperus invaded the poplar woodlands, occupying well—drained, favorable sites, and later the slower—dispersed Picea (chiefly P. glauca) arrived and formed stands on alluvial sites, largely replacing poplar. At °7500 BP, Betula papyrifera spread on to the uplands; the accumulation of fuel, accelerated by the spread of conifer woodlands, likely caused an increase in fire frequency, which in turn maintained arboreal birch in the area. Fire opened up the woodlands locally and promoted the spread of Alnus crispa, expressed in the pollen record as a prominent rise in both percentage and PAR values. Picea mariana and other species adapted to mire habitats spread at this time (°7000 BP) as paludification, peat growth, and permafrost aggradation occurred in the extensive lowlands. The termination of summers warmer than at present (°6000 BP) initiated the development of a relatively stable vegetation that has persisted to the present.
Food Supplementation in a Tropical Mainland Anole, Norops Humilis: Demographic Effects
Authors: Guyer Craig
Published: May 2006

The concept of food limitation has played a key role in the theoretical framework of population and community ecology, especially for anoline lizards. A common hypothesis is that food is limited for island anoles but is relatively unlimited for mainland anoles. In this study I manipulated prey availability of investigate how food affects population parameters for Norops humilis at a site in the Atlantic lowland wet forest of Costa Rica. Mark—recapture data were collected on control and experimental plots over a 9—mo pretest and 7—mo test period during which 9910 observations were made on 1582 individuals. Manipulation (increase) of food resulted in an approximate doubling of population density. Significant increases in the numbers of hatchlings, immigrants, and survivors resulted in increased numbers of juveniles. However, I observed no increase in the numbers of juveniles recruited to adulthood. Instead, losses of juveniles due to death and emigration were significantly increased. The juveniles population represented a "doomed surplus" that had little direct effect on increasing the size of the reproductive population int he presence of extra food. The density of adult males and females increased survival and immigration. Population density increased proportionally more for adult females than for males and for juveniles than for adults. Increased female population density resulted in a significant increase in the number of eggs laid. Loss (mortality plus emigration) of adults was not affected by supplemental feeding. As the increased number of juveniles did not increase the number of adults recruited from these juveniles, must have increased in severity on experimental plots. These effects of food on N. humilis demography suggest a conceptual model for cycles of abundance of this species and other predators of arthropods within the leaf litter of primary wet forest. The cyclic increase in arthropod food resources later in the dry season appears to be triggered by leaf drop of forest trees, which in turn triggers demographic responses in predators of leaf—litter arthropods, such as N. humilis.
Temporal Coupling of Production and Recruitment of Larvae of a Caribbean Reef Fish
Authors: Robertson D. Ross , Green David G. , Victor Benjamin C.
Published: May 2006

Variation in larval recruitment is thought to have profound effects on the structure of coral reef fish communities, and planktonic processes often are cited as the major factor controlling the temporal and spatial patterns of such recruitment. We looked at the relationship between temporal patterns of larval production and settlement of planktonic larvae of the Caribbean damselfish Stegastes partitus at one site and attributed any differences to processes acting in the plankton. In doing so we assumed that the pattern of production we observed was representative of the regime that produced fish that settled in the study area. We monitored spawning and larval recruitment continuously for 3 yr. Both spawning and settlement followed (unimodal) lunar cycles, and both activities spanned °3 wk of the lunar month. Although the form of the average settlement cycle matched that of the average production cycle, monthly settlement episodes were shorter and (slightly) more variably timed than equivalent production episodes. Although monthly variability in the magnitude of settlement was fourfold greater than corresponding variability in the magnitude of larval production, monthly settlement success did not vary in an extreme manner. There was no significant correlation between the magnitude of larval production in a month and of settlement the following month. Daily growth increments in the otoliths of settlers indicated that (1) larvae were °5 wk old at settlement, (2) there was low overall variability in age at settlement, (3) there were no differences between the ages of settlers arriving early and late in the monthly settlement period, and (4) age variability among settlers collected on the same day was not different from that among settlers collected on different days. Hence, the basic lunar periodicity of settlement is determined by the periodicity of production of relatively fixed—age settlers. Planktonic processes enhance the temporal variability of settlement, principally by affecting the magnitude of settlement events, but also by influencing the duration and precise timing of monthly settlement episodes. Planktonic processes also determine that most of a month's successful settlers arrive (and are produced) over a few consecutive days and mix cohorts of larvae that are produced on different days. We conclude that the timing and magnitude of settlement are strongly influenced by both production and planktonic processes, and the latter only partly decouple settlement and production.