bluuebunny/crossref_metadata_2025 huggingface dataset:
Ecological Segregation of Adelie, Gentoo, and Chinstrap Penguins at King George Island, Antarctica
Authors: Trivelpiece Wayne Z. , Trivelpiece Susan G. , Volkman Nicholas J.
Published: May 2006

Ecological segregation among Adelie, Gentoo, and Chinstrap penguins during summer results from differences in breeding chronology, foraging behaviors, and life history tactics. To determine the importance of these factors in segregating the niches of the three species, we collected data on their population size, breeding success, breeding chronology, feeding frequency, and foraging range. Gentoo Penguins feed inshore and are deep divers. Their small populations probably reflect the comparatively large amount of food they need to rear chicks and the small foraging range that is dictated by short nest relief schedules and nonfasting behaviors. Their deep diving ability enables them to exploit a niche that is unavailable to their more abundant congeners. Adelie and Chinstrap penguins are shallow—diving, offshore foragers that avoid competition by differences in breeding chronology, prebreeder behaviors, and molting strategies. Adelie chicks fledge just as Chinstrap chicks reach creche age. Different migration times and molting locations further reduce niche overlap. The ultimate factors responsible for this tropic segregation may be unrelated to these proximate factors, however. Nonmigratory behavior, short nest reliefs, nonfasting, and slow growth of chicks may be adaptations to the mild conditions experienced by Gentoo Penguins in their sub—Antarctic range. The differences in Adelie and Chinstrap breeding time may reflect adaptation of Adelies to early breeding in the harsh, short summer of the continental Antarctic, and adaptation of Chinstraps to later breeding in the milder Maritime Antarctic. We found no evidence to suggest that the niches of the Pygoscelis penguins were influenced by competition for food, and suggest that adaptation to conditions in the center of their respective distributions is the primary cause affecting ecological segregation in areas of sympatry.
Litter Placement Effects on Microbial and Organic Matter Dynamics in an Agroecosystem
Authors: Holland E. A. , Coleman D. C.
Published: May 2006

Two different agricultural tillage practices were used to study how changes in the structure of the soil—litter system affected litter decomposition rates, microbial community composition, and soil organic matter dynamics. Surface straw placement results in spatial separation of carbon—rich litter (C:N ratio 80:1) and mineralized soil nitrogen. In contrast, when the litter is plowed into the soil, straw carbon and soil nitrogen are in intimate contact. Our field studies in Colorado showed that fungal biomass in surface—straw treatments was 144% of that in the incorporated—straw treatments, probably because fungi, with their extensive hyphal networks, are able to utilize both the surface straw carbon and the available soil nitrogen. Field studies using 14C—labeled wheat straw showed that a greater proportion of added 14C was retained in the surface—straw treatment than in the incorporated—straw treatment. Maximum net N immobilization was higher and litter decomposition was slower in the surface straw than in the incorporated straw placements both with and without experimental nitrogen addition. Slower litter decomposition of the surface litter may contribute to reduced soil organic matter losses. Soil organic matter losses may also be reduced in no—till systems as a result of the increase in the ratio of fungal to bacterial activity because of the greater growth efficiency of fungi and the accumulation of carbon in the less decomposable fungal biomass. The surface placement of straw in no—till agriculture allowed management of microclimate and microbial populations so that losses of soil organic matter and nutrients were minimized.
Sex and the Single Forager: Gender‐Specific Energy Maximization Strategies in Fiddler Crabs
Authors: Weissburg Marc
Published: May 2006

The energy costs and benefits of foraging were quantified for male and female fiddler crabs. Benefits accrued during foraging were calculated using patch—specific intake rates, combine with an analysis of the energy value of food. Foraging costs per unit time were quantified by measuring O2 depletion of foraging animals. An energetics model constructed with these cost—benefit functions predicted the leaving rule, defined as the food concentration at which a crab should cease feeding in a patch, that maximized energy intake. A second model was designed for males feeding on fine (sieved) sediments, which reduced intake rate, and therefore changed foraging costs. Lastly, by building on a previous analysis of foraging mechanics, the model examined whether the structure of the foraging system constrains the ability of fiddler crabs to maximize energy intake. Analysis of sex specific intake rates and foraging costs indicates that females, although experiencing greater costs, also had higher intake rates, than males. The model predicted that females should acquire more energy from foraging in a patch, and should display a lower leaving threshold than males. The leaving rule prediction was tested and confirmed; observed leaving thresholds for all three treatments (females, males, and males on fine sediments) were statistically indistinguishable from model predictions. As predicted by the model, males feeding on fine sediments responded to increased costs by displaying a higher leaving threshold, relative to males on unsieved sediments. A sensitivity analysis, performed by using the 95% confidence interval of O2 consumption to bracket foraging costs, indicated the model to be robust; leaving threshold were extremely stable, with all the three predicted threshold remaining quite distinct, despite a 20% change in the cost function. Finally, the model showed that the architecture of the feeding system constrains the ability to implement energy maximizing strategies. Constraints are gender specific, and the associated energy penalty is frequently small.
The Influence of Thermal Conditions on the Surface Activity of Thirteen‐Lined Ground Squirrels
Authors: Vispo Conrad R. , Bakken George S.
Published: May 2006

We examined the relation between surface activity and the surface microclimate in thirteen—lined ground squirrels. We statistically isolated the effects of thermal conditions on squirrel activity and tested the suitability of two optimization models for predicting thermoregulatory behavior. Metabolic and microclimatic data were used to represent thermal conditions as standard operative temperature. The proportion of time spent on the surface was highest at standard operative temperatures between 30° and 40°C. This range approximately matched the squirrel's physiological thermal neutral zone. This pattern was not qualitatively altered by using a multiple—regression model to remove the effects of potential confounding variables (e.g., time of day or time of year). At low temperature, the observed reduction in surface activity resulted in lost foraging benefits °6 times as great as the energy savings in thermoregulatory and activity costs. Therefore, we believe that these squirrels cannot be considered daily energy maximizers. Rather, optimization must be considered in the context of longer time periods. At high temperatures, the observed reduction in surface activity was described by a constrained thermal model of shuttling thermoregulation, which assumed squirrels maximized surface feeding time. The model also explained a systematic decline in body temperature when exiting and entering the burrow as surface operative temperatures increased.
Field Metabolism, Water Requirements, and Foraging Behavior of Wild Ostriches in the Namib
Authors: Williams Joseph B. , Siegfried W. Roy , Milton Suzanne J. , Adams Nigel J. , Dean W. R. J. , du Plessis Morne A. , Jackson Sue
Published: May 2006

We measured the field metabolic rate (FMR) and water influx rate (WIR) of the largest species of bird, the Ostrich (Struthio camelus), which lives in the Namib desert, one of the driest regions on earth. Along with doubly—labeled water (DLW) measurements of FMR and WIR, we examined the availability of plants in various habitats, the plants selected by Ostriches, and the daily activity patterns of these birds. During 6—8 d periods, adult Ostriches (88.25 kg) had an FMR of 18 040 kJ/d, while subadult birds (50.75 kg) metabolized energy at a rate of 15 428 kJ/d. Adult energy expenditure was 26% lower than predicted, but subadults had a FMR nearly equivalent to expectation, suggesting that adults may be more efficient at acquiring resources. Conclusions remain tentative because data for the Ostrich exceed previous data used for allometric equations by almost an order of magnitude. Gravel and stoney plains together accounted for 84% of the study area, whereas washes occupied °1%. Vegetation cover was sparse in all habitats, varying between 7 and 19% in most areas, but comprising °15% along drainage lines. Ostriches foraged on gravel plains nearly 65% of their daytime hours and 25.5% of their day in washes. They consumed a narrow range of green plants with Monechma arenicola, Schmidita kalahariensis, Blepharis spp., Trianthema triquetra, and Dicoma capensis representing the principal items in the diet. From a time—activity budget, Ostriches spent 7.5 h of their 24—h day walking, and roosted at night for 11.5 h. Transport between food patches accounted for 62.2% of FMR while nighttime rest was 19.0%. Preening and other maintenance behaviors were responsible for <1% of the Ostriches' energy budget. Ostriches had a frugal water economy when compared to other nonpasserine birds, with both subadults and adults having lower WIRs than predicted. Values for WIR were higher than allometric predictions based on four other desert birds previously studied, but because Ostriches are two orders of magnitude larger in body mass, firm conclusions await further data. Calculations of the water economy index (WEI), the ration of water influx to FMR (in millilitres of water kilojoule), showed that Ostriches conserved water like smaller desert birds. Four species of desert birds had WEI values averaging 0.16 mL/kJ; values of the Ostrich averaged 0.17 mL/kJ. An itemized water budget suggested that adults did not drink during the periods of observation, while water intake by subadults averaged 729 ml/d. This suggests that adults may have lower minimum water requirements than subadults. Combining available data for FMR and WIR with data for the Ostrich, we constructed new allometric equations that nearly span the entire range of bird size and include 62 species of birds.
Some Determinants of Dispersal by Crab Spiderlings
Authors: Morse Douglass H.
Published: May 2006

Dispersal of young from their natal sites may be determined by their numbers or by availability of hunting sites, possible correlates of maternal hunting success and habitat selection. To test these alternatives, I observed the dispersal (ballooning on silken threads, moving on threads among vegetation, walking) of second—instar crab spiders Misumena vitia (Thomisidae) from nests and experimental releases that differed in number of spiderlings and proximity to hunting sites. The probability of ballooning by young crab spiders did not differ with size of clutch, individual size, or egg size; however, substrate had a strong effect. Spiderlings often ballooned within minutes from nests not near hunting sites, but usually remained much longer, up to several days, on excellent foraging substrates, typically goldenrod (Solidago spp.) flowers. Spiderlings released on both goldenrod in bud and asters (Aster sp.) in bud left quickly, often ballooning directly from them. Spiderlings from nests within 1 m of flowering goldenrods usually moved to these flowers, ones several metres away seldom moved to them. The choice of nest sites by the spiderlings' mothers thus indirectly affected the spiderlings' dispersal behavior. The lack of a significant correlation between maternal clutch size and spiderling dispersal means that dispersal does not change maternal fitness differences originally resulting from differential success in foraging (reported elsewhere).
Plant Competition and Resource Availability in Response to Disturbance and Fertilization
Authors: Wilson Scott D. , Tilman David
Published: May 2006

Variation in the intensities of above— and belowground competition is proposed to underlie changes in community structure associated with variable fertility and disturbance. Here we report measurements of light, soil nitrogen, and above— and belowground competition in vegetation that had received four combinations of fertilization and disturbance for 3 yr. Two levels of nitrogen addition (none and 17 g°m—2°yr—1) and two of soil disturbance (non and annual tilling to remove all vegetation) were applied in a factorial design to 5 ° 5 m plots (10 replicated) in a 31—yr—old field in southeastern Minnesota. During the 3rd yr of the experiment, community standing crop, light penetration, and soil ammonium and nitrate concentrations were measured every 3—4 wk. Fertilization significantly increased productivity and tilling significantly decreased community biomass at the start of the growing season, indicating that the experimental treatments created variation in stress and disturbance. Transplants of Schizachyrium scoparium, a native perennial grass, were grown with no neighbors, or with the roots of neighbors, or with all neighbors in each combination of fertility and disturbance in order to examine competition intensity. In both undisturbed and tilled plots, belowground competition was the dominant interaction in unfertilized plots, whereas both below— and aboveground competition controlled growth in fertilized plots. The intensity of aboveground competition was greatest in plots with the lowest light penetration and declined significantly with increasing light supply. Similarly, belowground competition was most intense in plots with the lowest nitrogen availability and decreased significantly with increasing availability. The intensities of above— and belowground competition were significantly negatively correlated. The total (above— and belowground) intensity of competition did not vary with fertility, but was significantly reduced by disturbance. Similarly, the effect of neighbor mass on transplant growth was significantly reduced by disturbance.
Foliar Nutrients During Long‐Term Soil Development in Hawaiian Montane Rain Forest
Authors: Vitousek Peter M. , Turner Douglas R. , Kitayama Kanehiro
Published: May 2006

We determined the consequences of systematic changes in nutrient availability during long—term soil development by measuring foliar nutrient concentrations. Sun leaves of the dominant tree Metrosideros polymorpha and of eight other species were sampled in Hawaiian rain forests developed on substrates that were 0.3 ° 103, 2.1 ° 103, 5 ° 103, 20 ° 103, 150 ° 103, 1400 ° 103, and 4100 x 103 yr old. Elevation, annual precipitation, parent material, and dominant species were nearly constant along this gradient. Foliar N and P concentrations in Metrosideros were lowest in the youngest site (0.72% and 0.052% for N and P, respectively), increased to a maximum on 20 ° 103 and 150 ° 103—yr—old substrates (1.45% and 0.108%), and then declined close to the initial concentrations in the oldest site (0.86% and 0.061%); N:P ratios in foliage varied relatively little across the sites. Most other species followed a similar pattern of variation. On a per unit leaf area basis, foliar N and P contents in Metrosideros also peaked on intermediate—aged substrates. Foliar nutrient concentrations in Metrosideros sun leaves were determined across a parallel but wetter substrate age gradient. The pattern of variation was similar on both gradients, but the magnitude of variation was smaller on the wetter sequence of sites. Overall, the pattern of variation in foliar nutrients with substrate age is consistent with conceptual models for the dynamics of soil nutrient availability during long—term soil development, and with measurements of soil properties along this sequence.
Forest Reorganization: A Case Study in an Old‐Growth Forest Catastrophic Blowdown
Authors: Peterson Chris J. , Pickett Steward T. A.
Published: May 2006

We studied the patterns and mechanisms of regeneration of a 400—ha wind—throw in an old—growth beech—hemlock forest caused by a tornado on 31 May 1985. Starting in 1986, and over a period of six growing seasons, we recorded percent cover and density of woody stems, and monitored seedling demography of nearly 5000 seedlings in the windthrow and adjacent forest. Plant community response to the disturbance was dramatic: by August of 1986, species richness, tree seedling density and total percent cover were significantly greater in the windthrow than in the adjacent forest. Shade—intolerant herbs (e.g., Erechtites hieracifolia) and shrubs (e.g., Rubus allegheniensis) established and rapidly increased in abundance during the first 3 yr, but began declining by the 5th yr of the study. Tree seedlings established in decreasing amounts through the 6 yr of the study, and the young tree canopy was dominated in 1991 by seedlings and sprouts that established prior to 1987. Fagus grandifolia, a shade—tolerant species that established via advanced regeneration, was dominant the first 3 yr, but was surpassed in the 5th yr by Betula alleghaniensis, a species of intermediate tolerance that established from seed germination just before or shortly after the disturbance. Tsuga canadensis seedling densities were initially high, but deer browsing prevented substantial growth and a drought in 1988 caused heavy mortality of browsed seedlings. Regeneration thus differed from the predictions of the gap and Hubbard Brook models of forest regeneration (which predicts dominance by shade—intolerant species), and the severity model (which predicts dominance by shade—tolerant species). The differences point out important influences of availability of propagules and the impact of herbivory; and the need for more attention to models that incorporate multiple contingencies.
Sexual Dimorphism Masks Life History Trade‐Offs in the Dioecious Plant Silene Latifolia
Authors: Delph Lynda F. , Meagher Thomas R.
Published: May 2006

Females of dioecious plant species are expected to show greater trade—offs with other life history traits than males because of their high investment in reproduction. We investigated life history traits and resource allocation patterns for both males and females of the dioecious plant Silene latifolia, to document allocation both prior to and after reproductive investment. In addition, the impact of fruit production on subsequent allocation to reproduction and growth was investigated by varying the number of flowers pollinated on females. The mean date of first reproduction was the same for males and females. Early—flowering plants of both sexes invested less in leaf biomass than plants that delayed flowering. The percentage of flowers pollinated had a significant effect on how resources were allocated, and on the amount and timing of further flower production. Sexual dimorphism was observed in resource allocation patterns after, but not prior to, investment in reproduction. Overall, pollinated females were found to have more leaf biomass and invest nearly twice as much in reproduction as males, in spite of an apparent trade—off between leaf production and reproduction within both females and males. We conclude that sexual dimorphism in life history traits that affects resource acquisition causes the resource pool that is allocated to be variable, rather than static, thereby masking the underlying trade—offs.
Effects of Supplemental Food on Population Dynamics of Cotton Rats, Sigmodon Hispidus
Authors: Doonan Terry J. , Slade Norman A.
Published: May 2006

Variation in resource abundance affects population dynamics by altering demographic processes and interactions among individuals in the population. For small mammals, food is likely to be a critical resource. Population densities should vary directly with food abundance, but the underlying demographic changes are more difficult to predict. We experimentally increased food available to populations of hispid cotton rats, Sigmodon hispidus, to examine how rates of recruitment and disappearance varied with food abundance. We expected supplemental food to increase winter survival and to increase recruitment, perhaps advancing the date of first reproduction in the spring. We also thought larger, behaviorally dominant animals might dominate the point sources of supplemental food, altering age or size ratios and producing an excess of transient animals. Supplemental food increased population densities but did not dampen pronounced seasonal fluctuations. Supplemented populations contained proportionately more juveniles and small adults than did controls; social behavior may have limited increases in numbers of large adults. Survival rates did not change; density increases were due to increased reproduction and immigration. In supplemented populations, reproductive effort by females increased, but the proportion of reproductive females decreased. Reproductive females and nonreproductive animals of both sexes were less likely to be transients than were reproductive males. Transients constituted higher proportions of control populations because of successful settlement, primarily by juveniles and small adults, into supplemented areas. The strongly seasonal climatic conditions under which northern S. hispidus populations exist produce contrasting selective pressures which might favor season—specific foraging strategies. Our data support a scenario of territorial females and wandering males in the reproductive season, with females maximizing resource acquisition for production of offspring. During winter, both sexes may restrict foraging time (and exposure to predators and weather) to the minimum required for survival.
Habitat Fragmentation and Movements of Three Small Mammals (Sigmodon, Microtus, and Peromyscus)
Authors: Diffendorfer James E. , Gaines Michael S. , Holt Robert D.
Published: May 2006

We studied the effects of habitat fragmentation on the movements of cotton rats (Sigmodon hispidus), deer mice (Peromyscus maniculatus), and prairie voles (Microtus ochrogaster) living in a fragmented landscape using 7.7 yr (1984—1992) of mark—recapture data. The study area included three kinds of 0.5—ha blocks: single large patches (5000 m2), clusters of medium patches (27 88 m2), and clusters of small patches (32 m2). We tested three predictions regarding animal movements. First, animals should move longer distances as fragmentation increases. Second, the proportion of animals moving will decrease as fragmentation increases. Third, species will show more movement from putative sources to putative sinks. In support of our first two predictions, all species (except male cotton rats) moved farther, and lower proportions of animals moved, as fragmentation increased. In testing our third prediction, we found no trends, for all species, between patch size and the net number of animals a block either imported or exported, indicating source—sink dynamics were probably not occurring on our study site. Furthermore, animals of all species (except female deer mice) switched more frequently to blocks of larger patches. For prairie voles in the spring and deer mice in the summer, relative abundances among blocks predicted from a Markov matrix model of switching probabilities showed high congruence with the actual abundances, indicating movement and abundances were related. In both cotton rats and prairie voles but not in deer mice, more juveniles and nonreproductive animals switched between blocks than did adults or reproductive animals. Deer mice switched more frequently than did either cotton rats or prairie voles; the latter species switched in similar frequencies. We discuss the implications of our data for modeling and conservation.
Life History Consequences of Variation in Age of Primiparity in Bighorn Ewes
Authors: Festa‐Bianchet Marco , Jorgenson Jon T. , Lucherini Mauro , Wishart William D.
Published: May 2006

We investigated the lie history consequences of age at primiparity in two marked populations of bighorn sheep (Ovis canadensis), studied for 19 and 12 yr. Some ewes first lambed at 2 yr of age, others at 3 or 4 yr. Lambs of 2—yr—old ewes were smaller and less viable than lambs of older ewes. Lactation at 2 yr did not affect survival to 3 yr, but in one population it was associated with reduced survival from 3 to 4 yr. Two yr olds that weaned lambs gained less mass during summer than other 2 yr olds. Overwinter mass loss from 2 to 3 yr was correlated with autumn body mass, and was lower for parous than for nonparous ewes. Reproductive success at 3 yr was independent of reproductive status at 2 yr. At 4 yr, ewes that lactated as 2 yr olds were smaller than ewes that had not lactated as 2 yr olds. In one population, negative effects of early maturation upon mass gain and subsequent reproductive success became evident at high density, and very few 2 yr olds lambed when population density was high. The lifetime reproductive success of early maturing ewes was not lower than that of late—maturing ewes. To account for individual variation in initial reproductive potential, we controlled body mass as a yearling, a variable that affected reproductive success. Mass as a yearling, however, did not interact with age of primiparity to affect survival, growth, or reproductive success. Uncertainties about resource availability and possible effects of genotype and body fat may explain why many ewes postponed their first reproduction despite apparently low life history costs of early maturation.
Predicting the Response of Populations to Environmental Change
Authors: Ives Anthony R.
Published: May 2006

When subject to long—term directional environmental perturbations, changes in population densities depend on the positive and negative feedbacks operating through interactions within and among species in a community. This paper develops techniques to predict the long—term responses of population densities to environmental changes using data on short—term population fluctuations driven by short—term environmental variability. In addition to giving quantitative predictions, the techniques also reveal how different qualitative patterns of species interactions either buffer or accentuate population responses to environmental trends. All of the predictions are based on regression coefficients extracted from time series data, and they can therefore be applied with a minimum of mathematical and statistical gymnastics.
Vertical Hydrologic Exchange and Ecosystem Metabolism in a Sonoran Desert Stream
Authors: Jones Jeremy B. , Fisher Stuart G. , Grimm Nancy B.
Published: May 2006

Hyporheic metabolism in a Sonoran Desert stream was examined, focusing on the sources of detritus supporting hyporheic respiration. Two alternative hypotheses were specifically addressed: (1) organic matter derived from the surface stream supports hyporheic respiration, and (2) detritus buried during flash flood supports hyporheic respiration. As predicted for the surface—derived organic matter hypothesis, respiration was lowest immediately following flash floods and increased significantly with time after flood (P < 0.001). Hyporheic respiration ranged from 0.05 mgO2°L sediments—1°h—1 immediately following a flash flood to as high as 4.41 mgO2°L sediments01°h—1 late in algal succession. Respiration was significantly correlated with surface algal biomass during two spring/summer successional sequences (P < 0.05; partial correlation coefficients 0.58 and 0.88). Respiration was also consistently higher in downwelling than upwelling zones with overall mean rates of 1.12 and 0.46 mgO2°L sediments—1°h—1, respectively. Respiration exhibited a distinct diel pattern with highest rate coinciding with time of maximum photosynthesis and was also significantly correlated with dissolved organic carbon concentration (P < 0.05), further supporting the hypothesis of hyporheic dependence on algal production. Flash floods bury organic matter that is also respired in the hyporheic zone; however, based upon storage of organic carbon immediately following floods, an average of only 15% of the observed respiration could be supported. We conclude that hyporheic respiration in Sycamore Creek is tightly linked to surface production. It is spatially distributed in biotic "hot spots" where surface water enter hyporheic sediments and is most likely supported by organic matter that is supplied as dissolved organic carbon, perhaps from algal production.
Partial Differential Equations in Ecology: Spatial Interactions and Population Dynamics
Authors: Holmes E. E. , Lewis M. A. , Banks J. E. , Veit R. R.
Published: May 2006

Most of the fundamental elements of ecology, ranging from individual behavior to species abundance, diversity, and population dynamics, exhibit spatial variation. Partial differential equation models provide a means of melding organism movement with population processes and have been used extensively to elucidate the effects of spatial variation on populations. While there has been an explosion of theoretical advances in partial differential equation models in the past two decades, this work has been generally neglected in mathematical ecology textbooks. Our goal in this paper is to make this literature accessible to experimentally ecologists. Partial differential equations are used to model a variety of ecological phenomena; here we discuss dispersal, ecological invasions, critical patch size, dispersal—mediated coexistence, and diffusion—driven spatial patterning. These model emphasize that simple organism movement can produce striking large—scale patterns in homogeneous environments, and that in heterogeneous environments, movement of multiple species can change the outcome of competition or predation.
The Effects of Variability on Metapopulation Dynamics and Rates of Invasion
Authors: Goldwasser L. , Cook J. , Silverman E. D.
Published: May 2006

Many spatial models of ecological processes achieve analytical tractability by neglecting variability among individuals, in their patterns of movement, and among locations, in their favorability for colonization or persistence. We develop a series of simulations, based on standard metapopulation and partial differential equation models, that explicitly track variability among individuals or locations, and compare the results with what would be obtained by ignoring such variability. We find, first, that variability among patches can have a strong influence on the outcome of community dynamics, by promoting the coexistence of species that would otherwise exclude one or the other through competition. Second, variability among individuals in their rates of movement can markedly increase the rate of spread of a population. Construction and analysis of a modified PDE (partial differential equation) model for a population that consists of two classes of individuals, dispersed and nondispersers, confirms the second result, and also shows that the presence of even a few rapidly dispersing individuals can markedly increase the rate of spread of the population. Our simulations and analysis show that the incorporation of variability into ecological thinking is feasible and that it can strengthen our ability to investigate the effects of spatial processes.
Forging at Different Spatial Scales: Dorcas Gazelles Foraging for Lilies in the Negev Desert
Authors: Ward David , Saltz David
Published: May 2006

We examined the foraging strategies of dorcas gazelles (Gazella dorcas) at different spatial scales. When in sand dunes in the Negev desert of Israel, the gazelles fed on a single plant species, the madonna lily (Pancratium sickenbergeri). In summer, when all live plant material is subterranean, the gazelles dug holes in the sand to remove the stem and bulb. After the winter rains, only the tips of the leaves that had emerged above ground were eaten. We examined search patterns and patch choice on a large scale using both null hypotheses generated by a random walk model and “rules of thumb” that may approximate on optimal foraging strategy. At a smaller scale, we examined the selection of parts of individual lilies, again making predictions about "rules of thumb" that the gazelles should use. Gazelle search paths deviated considerably from a random walk. Gazelle made shorter move lengths in areas of high plant density. Bouts of concentrated feeding in small areas were interspersed with long moves to new forging areas, suggesting that these animals were repeatedly sampling their environment. Congruent with optimal foraging predictions, the gazelles selected plants with more and larger leaves than randomly available, and concentrated their foraging activity in areas of highest lily densities. In summer, gazelles altered the depth of digs in response to differences in sand compaction. There was a surprising negative correlation between the size of the plant and the amount eaten. This selection of small plants appears to be due to the increased probability of reaching the bulb, which contains most of the plant's volume. Summer foraging by gazelles had a negative impact on the lilies. There is strong selection on the lilies to grow to a sufficient depth that damage of the bulb by herbivory is minimized. Conversely, the gazelles must select plants with bulbs close to the surface to maximize energy intake and minimize the energy cost of digging.
Food Plant Choice of Pieris Butterflies as a Trade‐Off between Parasitoid Avoidance and Quality of Plants
Authors: Ohsaki Naota , Sato Yoshibumi
Published: May 2006

This report assesses the role of specialist parasitoids in providing a major selection pressure for food plant preference in herbivorous insects. Three Pieris butterflies, P. rapae crucivora, P. melete, and P. napi japonica, use different sets of cruciferous larval food plants. P. rapae is oligophagous and uses ephemeral plants. P. melete is polyphagous and uses persistent plants, as well as all of the ephemeral plants used by P. rapae. On the other hand, P. napi is locally monophagous, using persistent Arabis. We assessed the intrinsic suitability of these crucifers by measuring survival rates, development times, and pupal mass of larvae growing on them at a constant temperature. All of the food plants of P. rapae, and P. melete are suitable for larvae of the three Pieris species. On the other hand, food plants of P. napi are the least suitable for all three species. Pieris species. On the other hand, food plants of P. napi are the least suitable for all three species. Pieris larvae have two specialist parasitoids, the braconid wasp Cotesia glomerata (formerly Apanteles glomeratus) and the tachinid fly Epicampocera succincta. In newly established habitats, P. rapae can avoid both parasitoids. In long—lasting habitats, however, P. rapae is heavily parasitized by both parasitoids. P. melete and P. napi, by contrast, live only in long—lasting habitats, where the parasitic pressure is potentially high. However, P. melete can partially avoid parasitism by killing the eggs of C. glomerata by encapsulation, through parasitized by E. succincta. On the other hand, P. napi seems to have evolved behavioral avoidance of parasitoids by specializing on Arabis plants. The different food plant preferences of the three Pieris species can be interpreted as resulting from differences in the balance of a trade—off between parasitoid avoidance and the intrinsic quality of potential food plants of Pieris species.
Minimal Community Structure: How Parasitoids Divide Resources
Authors: Naeem Shahid , Hawkins Bradford A.
Published: May 2006

Parasitoid communities show patterns in the distribution and abundance of species that fit predictions of niche partitioning models. These models treat community resources as unit volumes and conceptually treat niche partitioning, or resource division, as breakage of this unit into fractions that correlate with proportional densities of the species controlling that fraction of the resource. Unlike many other communities, parasitoid communities are particularly appropriate for applying these models. We examine four types of niche partitioning models employing three types of breakage algorithms. Of these, the overall best—fit model was unidimensional—niche model using a breakage algorithm in which each subsequent break occurred on the smaller or larger fraction at random. Successful fit of these empirically derived, abundance distributions of parasitoid communities to this theoretically derived, expected distribution supports the hypothesis that these communities are minimally structured by a hierarchical process of random niche partitioning along a single resource axis.
Consequences and Plasticity of the Specialized Predatory Behavior of Stream‐Dwelling Stonefly Larvae
Authors: Peckarsky Barbara L. , Cowan Cathy A. , Anderson Chester R.
Published: May 2006

The purpose of this study was to determine experimentally the potential mechanism(s) favoring specialized foraging behavior of stream—dwelling predatory stonefly larvae (Megarcys signata), and whether this specialized behavior was fixed or flexible. We measured stonefly growth rates after 10 d of conditioning on Baetis bicaudatus (Ephemeroptera) or one of two alternative mayfly prey species in replicated circular plexiglass flow—through chambers powered by water from a stream in western Colorado. We then compared attacks per encounter (as an index of predator choice) of conditioned stoneflies to those of unconditioned controls given equal densities of all three prey types. We conducted additional behavioral observations to determine whether predators differentially encountered the alternative prey types on natural substrates, and to determine whether predator—prey microhabitat overlap was mechanism explaining differential prey encounter rates among prey species. Megarcys exhibited a stereotyped attack bias toward swimming mayflies, such as Baetis, following the simple rule, "attack any prey that swim," which results in Megarcys' selection of prey types that swim in response to stonefly encounters. This behavior remained unmodified after extended experience with alternate but palatable nonswimming mayfly species. A potential fitness advantage (measured as relative growth rate) of this behavioral specialization occurred for female Megarcys, but not for males. Female Megarcys grew faster on Baetis diets because they ate more Baetis, not because Baetis was a more intrinsically profitable prey item (in terms of promoting predator growth). Further, high feeding rates on Baetis were a result of disproportionately high encounter rates and attacks per encounter with Baetis compared to the other mayfly species. Our data are consistent with the hypothesis that this predatory behavior arose and is maintained in female stoneflies due to a fitness advantage to individuals specializing on a relatively abundant, easily recognized prey resource. We speculate that this specialized behavior has not been lost in male stoneflies, for which we did not measure a fitness advantage, because there is no strong selection pressure against it. Effective pre—and—post—contact prey defenses may exert enough selection pressure to prevent stoneflies from evolving on alternative mayfly types.
Movements, Survival, and Settlement of Red Squirrel (Tamiasciurus Hudsonicus) Offspring
Authors: Larsen Karl W. , Boutin Stan
Published: May 2006

Movements and settlement patterns of animal offspring, along with the costs of occupying familiar and unfamiliar habitats, have been inferred frequently, but rarely have they been documented directly. To obtain such information, we monitored the individual fates of 205 (94%) of the 219 offspring born over 3 yr in a population of the North American red squirrel (Tamiasciurus hudsonicus), at Fort Assiniboine, Alberta, Canada, (54.20°N, 114.45°W). We located neonates by radio—tracking mother squirrels, and thereafter we documented the movements, survival, and settlement patterns of the offspring, using a combination of telemetry, live—trapping, and visual observations. Prior to settlement, offspring made forays of up to 900 m (° = 126 m) off the natal territory, but they did not abandon the natal territory until they had settled on their own territory. Foray distance was not related to the age or size of the offspring. We used the locations of offspring kills to show that the risk of predation significantly increased when the offspring were travelling off of their natal territories. Just under half of the 73 offspring that acquired territories did so no or immediately adjacent to their mother's; the farthest settlement distance was only 323 m from the natal territory, or about the distance of three territory widths. Movement data from adults in the population showed that all offspring settled within potential contact of their mother (and possibly their father). Offspring that settled relatively farther away from their natal territory were more likely to obtain larger territories, with traditional hoarding and overwintering sites (middens). These offspring also had higher overwinter survival, suggesting that the costs of making forays off the natal territory may be balanced by the advantages of locating a superior territory.
Beaver Influences on the Long‐Term Biogeochemical Characteristics of Boreal Forest Drainage Networks
Authors: Naiman Robert J. , Pinay Gilles , Johnston Carol A. , Pastor John
Published: May 2006

Beaver (Castor canadensis) affect biogeochemical cycles and the accumulation and distribution of chemical elements over time and space by altering the hydrologic regime. Aerial photograph analyses of beaver activities on the 298—km2 Kabetogama Peninsula, Minnesota, were coupled with site—specific studies of soil and pore water concentrations of nutrients (nitrogen, phosphorus) and other ions (potassium, calcium, magnesium, iron, sulfate, chloride), nitrogen cycling processes (nitrogen fixation and denitrification), and biophysical environment variables (vegetation, temperature, organic matter, soil structure, pH, and oxidation—reduction potential). Our analysis demonstrate that beaver influence the distribution, standing stocks, and availability of chemical elements by hydrologically induced alteration of biogeochemical pathways and by shifting element storage from forest vegetation to sediments and soils. Over the 63 yr of aerial photo records (1927—1988), beaver converted 13% of the peninsula to meadows and ponds. Elemental concentrations in soils (in micrograms per cubic centimetre) and in pore water (in milligrams per litre) revealed complex patterns within and among the principal hydrologic zones (e.g., forest, moist meadow, wet meadow, pond, stream). Principal components analysis (PCA) suggested that anaerobic conditions caused by saturation of soil by water was the fundamental control over subsequent alterations of biogeochemical pathways. Although few clear statistical trends were detected for mass— or volume—specific elemental concentrations among habitats, organic horizon (O and A) depths were greatest in the wet meadows and ponds (@>15 cm), causing the standing stocks of chemical elements to be greatest there. We argue that the net effect of beaver activities has been to translocate chemical elements from the originally inundated upland forest vegetation to downstream communities and to pond sediments. As the upland vegetation dies and decays after dam construction, only a portion of the chemical elements are exported downstream (except for calcium and magnesium) or returned to the atmosphere (C and N only ). Consequently, the organic horizons of pond sediments accumulate substantial standing stocks of chemical elements that are available for vegetative growth when dams fail, the ponds drain, and meadows are formed. Since 1927 beaver activities have augmented the standing stock of chemical elements in the organic horizons by 20—295%, depending on the element. These influences are spatially extensive and long lasting, affecting fundamental environmental characteristics of boreal forest drainage networks for decades to centuries.
Top‐Down Effects in a Tritrophic System: Parasitoids Enhance Plant Fitness
Authors: GÓmez JÉse M. , Zamora Regino
Published: May 2006

We determined the direct and indirect top—down forces in a tritrophic system composed of a guild of three parasitoid species (Necremnus tidius, N. folia, and Aprostocestus sp.), a single weevil seed—predator species (Ceutorhynchus sp. nov.), and the host plant, Hormathophylla spinosa, a woody crucifer of the high—mountain elevations of the Sierra Nevada (southeast Spain). By Means of observational and experimental approaches, we focused on the possible regulatory effects of parasitoids on the weevil population as well as on the analysis of the sign and strength of the resulting indirect effects on plant fitness. Weevill attacks 21.7% of the fruit produced by H. spinosa during the study, selecting fruits having the most seeds. Seeds eaten by weevils were significantly bigger than uneaten ones. The number of seeds per fruit after being depredated by Ceutorhynchus was significantly smaller than for fruit not attacked. Thus, weevils affect the fitness of the host plant both by decreasing the number of seeds (quantitative effect) and by reducing the average size of surviving seeds per fruit (qualitative effect). When parasitoids were excluded experimentally, the percentage of attacked fruit (which corresponds to the number of weevil larvae) rose from 20 to 43%. We assume that the increase in the experimental weevil population may be because excluding parasitoids eliminates not only death by parasitoid oviposition for some Ceutorhynchus larvae, but also death by direct predation of parasitoids on weevil larvae. We have also experimentally verified that parasitoid activity benefits the entire plant, by way of parasitoids contributing to a higher average number of seeds dispersed per fruit in each plant, by influencing both the abundance of the weevis and the behavior of the weevil larvae.
Markov‐Recapture Population Estimate: A Tool for Improving Interpretation of Trapping Experiments
Authors: Wileyto E. Paul , Ewens Warren J. , Mullen Michael A.
Published: May 2006

This paper describes a method of population estimation in which a random, unknown number of individuals is marked using a self—marking bait station (a trap modified for mark and release); animals (both marked and unmarked) may then be captured in an otherwise identical trap, which is available simultaneously. The estimate of the unknown population size is based on the assumption of a closed population and a simple Markov model in which the rates of marking and capture are assumed to be equal. The population size estimator is based on the maximum likelihood technique, and is given by the nest integer less than N = (C + R)2/2(R + 1), where R and C are, respectively, the numbers of marked and unmarked individuals found in the trap. The estimator is almost unbiased for a wide range of true population sizes, and over a wide range of times over which the experiment is run, although it becomes negatively biased when the mean number of recaptures is <5. Confidence limits may be obtained using asymptotic maximum likelihood arguments, although relative likelihood methods perform better when the number of recaptures is small.