bluuebunny/crossref_metadata_2025 huggingface dataset:
Self‐Thinning Exponent Correlated with Allometric Measures of Plant Geometry
Authors: Weller Donald E.
Published: May 2006

The —3/2 power rule of self—thinning, which describes the course of growth and mortality in crowded, even—aged plant stands, predicts that average mass is related to plant density by a power equation with exponent —3/2. The rule is widely accepted as an empirical generalization and quantitative rule or law. Simple geometric models of space occupation by growing plants yield a power equation, but the exponent can differ from —3/2 when realistic assumptions about the allometric growth of plants are considered. Because such deviations conflict with the empirical evidence for the —3/2 value as a law—like constant, the geometric model have not produced an accepted explanation and the thinning rule remains poorly understood. Recent studies have concluded that thinning exponents can deviate more widely from —3/2 than previously thought, motivating the present re—evaluation of the geometric explanation. I extend the simple models to predict the relationships of the thinning exponent to allometric exponents derived from commonly measured stand dimensions, such as height, average mass, average bole diameter at breast height (DBH), and average bole basal area. If the form and exponent of the thinning equation arise from the geometry of space filling, then thinning exponents should be systematically related to the exponents of allometric equations relating average height to average mass, average height to average DBH, and average height to average basal area. I also predict some values for the slopes and intercepts of regression lines relating thinning exponents to the allometric exponents. The predictions are verified by statistically comparing the thinning exponents and allometric exponents of self—thinning populations. The expected negative correlations are present and statistically significant (P ≤ .05), and the slopes and intercepts of linear regressions relating thinning exponents to allometric exponents are near the predicted values. These results support the hypothesis that the thinning equation arises from the geometry of space filling, but recognition that thinning exponents differ from —3/2 as predicted by simple geometric considerations weakens the case for a quantitative rule or law.
Risk‐Sensitive Foraging Strategies of Two Spider Populations
Authors: Gillespie Rosemary G. , Caraco Thomas
Published: May 2006

We found that foraging mode differed between habitats in the long—jawed orb—weaving spider Tetragnatha elongata. In a creek habitat, where relatively few prey (on average) were captured over extended intervals, spiders remained at the same foraging site for long periods. In a lake habitat, where average prey availability was relatively high, spiders changed web sites frequently. We first compared models differing in assumptions about a spider's ability to rank sites by their long—term average prey densities when prey availability varied through time at each site. When temporal variation in prey availability within foraging sites constrains a spider's ability to respond to spatial variation among sites, choice of foraging mode can be analyzed as a simple problem involving risk—sensitive decision making. The corresponding model associates the fixed—site strategy with risk—proneness, and associates the mobile strategy with risk—aversion. When average prey availability is less than the spider's requirement for successful reproduction, the fixed—site strategy induces the lower probability of a reproductive failure. When average prey availability exceeds the requirement, the mobile strategy induces the lower probability of a reproductive failure. In the laboratory, we estimated the prey biomass a female must consume in order to produce an egg sac. Given this estimate, a subsequent field experiment indicated that the risk—sensitive model adequately predicted observed variation in foraging behavior. The other two models we examined did not assume that a spider's response to spatial heterogeneity was inhibited by temporal variation; these models did not predict observed behavior.
Variation in Capsule Size of Holopedium Gibberum (Zaddach): A Response to Invertebrate Predation
Authors: Stenson Jan A. E.
Published: May 2006

The gelatinous capsule of Holopedium gibberum is variable in size, and seasonal size development shows different courses in different lakes. Reasons for this variation were experimentally studied. Differences in density of invertebrate predators rather than abiotic differences were responsible. In experiments, adaptive change in capsule size took place without any physical contact between Holopedium and the predator Chaoborus obscuripes, suggesting a chemically triggered mechanism. The specific receptor in Holopedium and the key substance released by the predator are not yet known. The experiments showed that the same Holopedium individual that received the stimulus reacted by a direct phenotypical change. The reduction of capsule size in response to reduced predator density suggests a cost in maintaining this antipredator armor. A larger capsule leads to a higher cost for food gathering because of an increased drag in the water. The need for better armor may postpone the start of reproduction in Holopedium.
Recruitment and Postrecruitment Interactions in a Colonial Hydroid
Authors: Yund Philip O. , Cunningham Clifford W. , Buss Leo W.
Published: May 2006

The colonial athecate hydroid Hydractinia echinata encrusts gastropod shells inhabited by hermit crabs of the genus Pagurus. In field samples recruits were found clustered in specific locations on the undersurface of Urosalpinx cinerea shells. Assays of the behavior of larvae indicated that the bacteria that induce metamorphosis coat the entire shell. A series of laboratory experiments demonstrated that site—specific recruitment reflected the interplay between initial sites of contact with the shell, movement prior to metamorphosis, and differential mortality after metamorphosis. In situ experiments demonstrated that growth rates were high in the areas most frequently occupied by recruits. Recruitment sites determined the ontogenetic timing of subsequent intraspecific encounters between colonies, and thus influenced both the cost and outcome of competition. These results show that bacterial induction of metamorphosis is associated with extensive juvenile mortality, that field distribution of recruitment represents differential mortality rather than adaptive patterns of habitat choice, and suggest that environmental variation in recruitment position acts to maintain genetic variation in competitive ability.
Estimating the Location of the Focal Center in Red Squirrel Home Ranges
Authors: Lair Helene
Published: May 2006

I attempted to evaluate the relative usefulness of three statistical estimates of focal center location in red squirrel (Tamiasciurus hudsonicus) activity fields (defined as the distribution of an animal's time as a function of location). The statistics compared were the bivariate arithmetic mean center, the bivariate median, and the harmonic mean center. The arithmetic mean center, the classical center of activity, was an unsatisfactory estimate of focal center location; the median center was closer to the point of greatest activity in 77 of 81 cases, but still did not coincide with it; the harmonic mean center was a good estimate of focal center by definition, and it coincided with the behavior focal center whenever the latter could be identified from direct observation. Because the harmonic mean center usually did not coincide with either the arithmetic mean or the median centers, I recommend the use of the harmonic mean center to field biologists wanting to estimate the location of an animal's point of greatest activity from the geographical distribution of its active time, and I discuss its possible biological interpretation.
Landscape Ecology and Nature Reserve Design in the Boundary Waters Canoe Area, Minnesota
Authors: Baker William L.
Published: May 2006

Temporal change in the biosphere occurs at different rates and in different ways, depending on spatial scale. One hypothesis is that in environments where disturbances produce patches and temporal instability in small areas, the aggregate mosaic of these patches on larger areas may be constant, so that a "shifting—mosaic steady state" occurs. If so, then the appropriate minimum size for a nature reserve might be the minimum land area on which the patch—mosaic is stable. To test for a stable patch—mosaic I subdivided the Boundary Waters Canoe Area (BWCA) into smaller units at five spatial scales and used the fire history data published by Heinselman to reconstruct temporal changes in the patch—mosaic on each unit. A temporally stable patch—mosaic was not evident at any scale, largely due to (1) spatial heterogeneity in the fire—regime and/or environment, and (2) a mismatch between the grain of fire—patches and the grain of the environment. The pattern of temporal instability present on the BWCA as a whole was not replicated on smaller areas. Nature reserve design and management practices that focus on the landscape level, community level, or species level may conflict. If the management goal is to perpetuate natural fluctuations in landscape structure, then certain species dependent on landscape structure may fluctuate as well. Maintaining stable populations of these species may entail landscape manipulations that lower the value of the reserve for perpetuating landscape processes and structures.
Dispersal of Misteltoes by Honeyeaters and Flowerpeckers: Components of Seed Dispersal Quality
Authors: Reid Nick
Published: May 2006

The quality of the seed dispersal performed by a dispersal agent has two components. Disperser efficiency is the probability that a seed dispersed by the vector will lodge in a safe site and germinate. Disperser effectiveness is the proportion of seedlings in a population that any one seed vector is responsible for disseminating. Two birds, the Spiny—cheeked Honeyeater (Acanthagenys rufogularis) and the Mistletoebird (Dicaeum hirundinaceum), dispersed the seeds of the host—specific mistletoe Amyema quandang in arid western myall (Acadia papyrocarpa) woodland in South Australia. The Mistletoebird dispersed the mistletoe's seeds by defecating and depositing seeds in a string on the perch, with the majority of seeds being dispersed to branches between 5 and 10 mm in diameter. Honeyeater—dispersed seeds usually fell onto substrates below the defacting bird or were wiped onto a convenient branch, generating a bimodal distribution in the branch sizes to which seeds were dispersed. Mistletoebirds and honeyeaters were similarly efficient in dispersing seeds to the thin (1—6 mm diameter) western myall stems most susceptible to infection (0.9—1.2% of all seeds dispersed, respectively), but Mistletoebirds dispersed four times as many seeds as honeyeaters to the whole range of stem diameters infected (1—16 mm). Germination of mistletoe seeds and seedling growth did not differ significantly in the seeds dispersed by the two birds species across the main fruiting season, but Mistletoebird—dispersed seeds had higher viability and grew more strongly at the start of the season when ripe fruit was scarce. Disperser effectiveness was measured by locating and classifying recently disseminated Amyema quandang seedlings in western myall canopies as Mistletoebird or honeyeater dispersed on the basis of species—specific differences in the stools of the two birds. Honeyeaters had dispersed most of the older (>2 mo) seedlings to live myall branches in three trees, suggesting that they were the more effective disperser of A. quandang. The system is a good model for resolving problems in the ecology of seed dispersal, which are intractable with most terrestrial plant species.
Predation by Birds on Spruce Budworm Choristoneura Fumiferana: Functional, Numerical, and Total Responses
Authors: Crawford Hewlette S. , Jennings Daniel T.
Published: May 2006

This study examined the hypothesis that predation by forest birds restricts expansion of low—density populations of the spruce budworm Choristoneura fumiferana (Lepidoptera: Tortricidae). Bird populations were censused in spruce—fir stands of low to moderate spruce budworm density in northern New Hampshire and western Maine. Diets were determined from stomach contents of collected birds. Budworm population densities were estimated by sampling fourth—instar larvae and successfully emerged pupae (counted after moth emergence). Populations of spruce budworms ranged from 80 x 103 to >22 x 106 larvae/ha and from 0 to 1.2 x 106 pupae/ha. Birds are spruce budworms throughout the daylight hours. Blackburnian Warblers consumed the most Choristoneura (X = 28 420 budworms/ha), followed by Cape May Warblers (X = 26 220 budworms/ha), and grading down to no consumption by some species. In plots where spruce budworm densities ranged from low to transitional, the entire bird community as well as specific groups of birds, i.e., overstory warblers and Golden—crowned Kinglets; sparrows, vireos, and juncos; and nuthatches, thrushes, and Purple Finches, showed significant functional responses to increasing budworm density. When transitional budworm densities were eliminated from the analysis, only the warbler—kinglet group exhibited a significant functional response. Only two species, Canada Warbler and Golden—crowned Kinglet, showed numerical responses to increasing budworm numbers. The percentage of the budworm population consumed by birds decreased as budworm numbers increased. Birds consumed 84% of the larvae and pupae where budworm populations were low, 22% where budworm populations were intermediate between low and high density. Once fourth—instar larvae exceeded 106 individual/hectare, bird predation was ineffectual. The hypothesis that birds can effectively limit budworm increase is compatible with three recent theories to explain budworm outbreaks. The forest stands we studied were dense, even—aged, with little understory. Numbers of birds potentially can be increased by modifying these stands through silvicultural means (Crawford and Titterington 1979). Birds are capable of dampening the seriousness of spruce budworm infestations when habitats are suitable for supporting adequate populations of these effective predators.
Development of Birch Leaves and the Growth Energetics of Epirrita Autumnata (Geometridae)
Authors: Ayres M. P. , MacLean S. F.
Published: May 2006

Larvae of Epirrita autumnata hatch at the time of budburst, and feed on the immature foliage of mountain birch, Betula pubescens ssp. tortuosa. The effects of leaf maturation, temperature, and larval instar on Epirrita growth performance were measured in two seasons. Leaf water, nitrogen, and phosphorus content declined with leaf maturation, while toughness and specific mass increased. There was a sharp decline in larval growth rate attributable to leaf maturation; for example the relative growth rate of third—instar larvae dropped from 0.52 to 0.11 mg°mg—1°d—1 in 3 d. Consumption rate, digestive efficiency, and nitrogen utilization efficiency declined on mature leaves; assimilation rate and Epirrita growth rate declined as a consequence. The rate of frass production remained very constant. Epirrita consumption rate appeared to be limited by postingestive handling of indigestible leaf tissue. Effects of leaf maturation were greatest at warm temperatures. Third—instar larvae grew faster than later instars on early season leaves, while fifth (last) instar larvae were least sensitive to seasonal effects; differences between instars make them well suited to the leaves that they normally encounter. The phenological constraint imposed by leaf maturation, and exacerbated by Epirrita adaptations that maximize growth rate on young foliage, seems to be an important selective force influencing the duration and timing of larval growth.
Population‐Level Variation in Photosynthetic Metabolism and Growth in Sedum Wrightii
Authors: Kalisz Susan , Teeri James A.
Published: May 2006

Four populations of Sedum wrightii were studied with respect to their photosynthetic carbon metabolism. This leaf—succulent species exhibits crassulacean acid metabolism (CAM). Plants that possess CAM have the potential to vary their rates of transpirational water loss greatly in response to drought. Leaf thickness and biomass °13C values were measured in mid growing season for the populations in their native habitats. The populations exhibited significant differences in both leaf thickness and °13C values ranged from —13.8% to 22.9%. In the field, populations differed in the proportion of day vs. night CO2 uptake during growth. Three of the populations were compared in a controlled—environment study. It was found that there are both environmental and genetic determinants of the differences in photosynthetic carbon metabolism. We observed a significant correlation between plant growth and biomass °13C value. The largest plants exhibited the greatest proportion of day vs. night CO2 uptake. These finding suggest that in this species there may be an inverse correlation between the ability to converse water and the ability to gain carbon. The unusually wide range of phostosynthetic phenotypes exhibited in this species may well explain its presence over a wide range of environments.
Evolution of Growth Rates in Eagles: Sibling Competition Vs. Energy Considerations
Authors: Bortolotti Gary R.
Published: May 2006

Nestlings of birds that raise only one young (B1 species) grow slower than those of birds that raise more than one young (B2 species). B2 species may be fast growing because sibling competition exerts a strong selective force for rapid growth. Other hypothesis have been proposed to explain the slow growth of B1's. To overcome the problem inherent in previous investigations of comparing species widely divergent in ecology and phylogeny, I analyzed patterns of variation in growth of one group only: eagles of the world. I fitted growth curves to data obtained from the literature for some species, but most eagles were compared using the length of the nestlings period as an indicator of growth rate. I also collected data on the growth of wild Bald Eagle (Haliaeetus leucocephalus) nestlings. I investigated how postnatal growth may be related to brood size, clutch size, nestlings mortality, body size, and prenatal growth. B1 eagles were found to grow slower than B2 eagles. Differences in foraging ecology, diet, and mode of development are unlikely to account for the observed variability in the rate and form of growth. Contrary to the sibling competition hypothesis, growth rate was not correlated with intensity of sibling competition. Eagles that regularly lay two eggs but always lose one chick because of sibling competition (obligate fratricide) grow slower than those species in which such mortality may or may not occur (facultative fratricide). Species characterized by facultative fratricide grow at a rate expected for their adult body size, whereas to B1 species do not. B1 eagles are characterized by growth curves with early inflection points compared to B2 eagles. The shape of the growth curves and slower growth rates of B1 species and contrary to predictions of the sibling competition hypothesis and may instead have resulted from selection to reduce peak energy requirements of the nestlings. I present a model of the effect of hatching interval, species—specific growth rate, and relative growth of siblings, on the probability of fratricide, which suggests that sibling competition is not an important factor in selecting for rapid growth.
How Body Plans Limit Acclimation: Responses of a Demosponge to Wave Force
Authors: Palumbi Stephen R.
Published: May 2006

The biomechanical basis of morphological acclimation to wave force was studied in intertidal demosponges. Colonies of Halichondria panicea were found to be stronger and stiffer in high wave force habitats than in low wave force habitats. These biomechanical changes are due to increase spicule number and size in sponges from areas of high wave action. The spicule changes follow the predictions of theories developed for particulate composite materials (e.g., those comprised of a flexible matrix with ridge imbedded stiffeners), suggesting that the habitat—dependent changes observed in H. panicea are engineering solutions to environmental stresses. An additional constraint imposed upon the basic Porifera body plan is the necessity of pumping water through the skeleton for feeding and respiration. In H. panicea, piping elements decrease in diameter in high wave force environments. This increases the resistance of oscular systems to water flow, thereby increasing the costs of water pumping. Environments with extremely high wave force are not inhabited by H. panicea, possibly because the high cost of pumping water through a skeleton dense enough to persist would limit or preclude growth. This limitation, however, is peculiar to the engineering trade—offs required by the body plan and feeding mode of H. panicea. Species in other taxa with different basic body plans may be immune to these trade—offs, and may be ecologically limited over identical environmental gradients in different ways.
A Model for Seed Scatterhoarding: Coevolution of Fox Squirrels and Black Walnuts
Authors: Stapanian Martin A. , Smith Christopher C.
Published: May 2006

Granivores are likely to store food in numerous, widely scattered, small caches if they are unable to defend concentrated large caches against interspecific competitors. This scatterhoarding of seeds makes it impossible for individuals to defend all their scattered caches against intraspecific competitors as well. Optimal spacing of scattered caches should result from a balance between decreasing loss of caches to naive competitors with decreasing density and increasing cost of storage with decreasing density. A mathematical model predicting optimal density is presented. One prediction of the model is that as the (θ) of habitat suitable for seed burial surrounding a seed source is decreased, the average distance (D) a cache is taken from the source by scatterhoarders should increase. Another prediction is that increasing the number of seeds (N) at a source, either by an increase in the size of a single seed crop or by the presence of 2 or more conspecific seed producers in close proximity to one another, should increase D. One trait of trees which increases single crop sizes and average seed dispersal distance (D) is the pattern of withholding energy from reproduction some years to allow unusually large crops during mast years. Three field tests of the model and its predictions were conducted: (1) Juglans nigra seeds were buried at 3 densities and their survival in time from predation by Sciurus niger was found to increase with decreasing density; (2) average distances that J. nigra saplings occurred from 16 parent walnut trees were found to have a statistically significant correlation with the D calculated from the model and the θ of suitable habitat surrounding each tree; (3) Sciurus niger individuals were observed to scatterhoard Juglans nigra seeds in a pattern that maintained a mean cache density that gives low rates of loss to naive competitors as indicated from field test (1). The results strongly suggest maintenance of optimum cache density by S. niger. The coevolution of trees which have mast years and the animals that scatterhoard their seeds is discussed.
Three ‐Speed Foraging During the Breeding Cycle of Yellow‐Rumped Caciques (Icterinae: Cacicus Cela)
Authors: Robinson Scott K.
Published: May 2006

The foraging behavior of a color—marked population of the polgynous, colonial Yellow—rumped Cacique, Cacicus cela (Icterinae), was studied throughout the breeding cycle in southeastern Peru. Females changed their foraging rates and microhabitat use at each stage of the nest cycle, apparently in response to changes in protein needs, travel costs, and time constraints. Nonnesting females, with few such constraints, searched for insects slowly and inconspicuously high in the forest canopy where they caught prey (mostly small larval lepidopterans) at a low rate. When protein needs were high (i.e., during egg—laying and fledgling—feeding) or foraging time was restricted (i.e., during incubation), females accelerated their searching rates in all microhabitats. During the nestling—feeding period, when both protein needs and travel costs to and from the nest were high, females searched at the fastest rate and foraged more in the forest understory where they caught relatively more large orthopterans. These data suggest that females with few constraints minimize their exposure to predators, while energetically constrained females maximize their rate of energy intake.
Antipredator Responses and the Perception of Danger by Mosquito Larvae
Authors: Sih Andrew
Published: May 2006

I conducted a series of laboratory experiments to contrast the responses of two species of mosquito larvae to a common predator, the freshwater bug Notonecta undulata. One prey species, Culex pipiens, commonly co—occurs with Notonecta, whereas the other, Aedes aegypti, shares no evolutionary history with this predator. I tested the evolution—based prediction that Culex should be killed at a lower rate due to stronger and more precise antipredator responses. I then studied the types of cues used by these prey to gauge predation risk by examining prey responses to simple disturbances and to potential chemical cues associated with predation. The results were that (1) both mosquito species responded to Notonecta by moving less and by shifting their microhabitat use; (2) Culex was indeed killed at a lower rate; (3) Culex showed far stronger responses to notonectids; (4) the response of Culex was correlated with actual predation risk even after encounter rate with predators was accounted for; and (5) Culex, but not Aedes, responded to water in which conspecifics had been preyed upon by Notonecta. The differences in the responses of the two species can be explained by a fundamental difference in the way in which they estimate predation risk. Whereas the behavior of Aedes can be understood as a response to disturbance per se, Culex behavior is also mediated by chemical cues associated with the actual predation act. Since more predation on Culex should produce more alarm chemical, this cue is probably responsible for the observed "precise" (i.e., correlated with risk) responses.
Ant Constancy at Passiflora Extrafloral Nectaries: Effects on Caterpillar Survival
Authors: Smiley John
Published: May 2006

I examined the role of ants in controlling the magnitude and distribution of caterpillar survival at Corcovado National Park, Costa Rica. Ants of 29 species were observed visiting the extrafloral nectary glands of Passiflora vitifolia and P. quadrangularis. Plants attended by ants had ants present 60% of the time. Approximately equal numbers of plants attended by ants and plants without ants were treated with Tanglefoot to exclude walking predators from a single branch on each plant. Heliconius ismenius caterpillars were placed on the treated plants and an equivalent set of unmanipulated controls, and their growth and survivorship were measured. Forty—eight percent of caterpillars died over a 2—d period on unmanipulated plants with many ants, while only 20, 22, and 23% died on unmanipulated plants with few ants, Tanglefoot—treated branches with many ants outside the Tanglefoot barrier, and Tanglefoot—treated branches with few ants outside the Tanglefoot barrier, respectively. This suggests that ants were the only significant walking predator of Heliconius ismenius caterpillars in this habitat, and that °75% of the deaths observed on control plants were on plants with ants. After 4 d all caterpillars were replaced by a second set of caterpillars. Measurements of these latter individuals revealed that mortality was 62, 46, and 24% on control plants on which caterpillars previously survived for 0, 2, and 4 d, respectively. No trend was observed on branches with ants excluded. This indicates that the effects of ant predation are locally stable, i.e., that ant constancy causes plants to be heterogeneous in their suitability as host plants. Caterpillars thus are much more likely to survive on some plants than on others. Analysis of caterpillar growth rates did not reveal any effects on survival; i.e., slow—growing caterpillars survived as well as fast—growing ones.
Reproductive Interactions Between Sympatric Dalechampia Species: Are Natural Assemblages "Random" or Organized?
Authors: Armbruster W. Scott
Published: May 2006

By considering patterns of species co—occurrence and pollinator use, I attempted to answer the question: are natural assemblages of Dalechampia species (Euphorbiaceae) organized in ways that reduce the amount of interspecific pollination and/or competition for pollinator service? The observed structure was tested against three null models, one operating by species assemblage processes, one operating by species evolution processes, and a third operating by both processes. Two alternative operations definitions of sympatry were used with each model. The outcome (rejection or acceptance of the null hypothesis) was sensitive to the null model employed and the definition of sympatry used. This underscores the importance of using biologically realistic null models, assumptions, and definitions. Using the biologically more reasonable definition of sympatry, tests against all three null models suggest that assemblages of Dalechampia are structured with respect to pollinator use; indirect evidence suggests that some of this structuring has occurred through evolution of component taxa.
Structure of the Encounter between Goldenrod (Solidago Altissima) and Its Diverse Insect Fauna
Authors: Maddox G. David , Root Richard B.
Published: May 2006

Most plant species are attacked by insects with widely different feeding styles and life histories. As a first step toward understanding how plants adapt to such diverse pressures, we investigated the population responses of 17 insect species to the genotypic variation exhibited by clones of goldenrods, Solidago altissima, that were drawn from the same population and grown in common gardens. A cluster analysis on the genetic correlations between the insects' responses produced four groups, termed "herbivore suites," that are attracted and repelled by similar plant genotypes (and their underlying sets of traits). There was no obvious correspondence of an insect's phylogenetic affinity, host range, or feeding style with its membership in a particular suite. Put another way, insects belonging to the same family or guild respond differently to genetically variable traits in populations of this native, perennial plant. We found no negative genetic correlations among suites and few among pairs of species, which suggests that trade—offs between the plant's resistance traits are relatively unimportant. The existence of suites of herbivores, whose members exhibit correlated patterns of abundance on different host genotypes, has several interesting consequences. First, the genetic variability of the plant population will be reflected in the diversity and composition of the insect community. Second, selection caused by one herbivore species can result in increased resistances to other, rarer species whose responses are correlated with those of the damaging species. Third, selection by suites of herbivores may be more intense and less variable than selection by individual insect species, especially when many species are rare. By recognizing that resistance can evolve as a function of selection by entire suites of species we can develop better estimates of selection and perhaps achieve a greater understanding of the process of diffuse coevolution. The linkage of interactions within suites suggests some unexplored possibilities for explaining the rarity of many insect species.
Indirect and Direct Effects in a Tropical Agroecosystem: The Maize‐Pest‐Ant System in Nicaragua
Authors: Perfecto Ivette
Published: May 2006

This study examines the direct and indirect effects of insecticides in maize—pest—ant system in Nicaragua. Population levels of two herbivores, the fall armyworm, Spodoptera frugiperda J. E. Smith, and the corn leafhopper, Dalbulus maidis De Long, were monitored and compared among four insecticide treatments: (1) carbofuran, a systemic insecticide; (2) chlorpyrifos, a broad—spectrum insecticide; (3) a combination of carbofuran and chlorpyrifos; and (4) a control, where no insecticide was applied. Ant foraging activity also was monitored throughout the season for all four insecticide treatments. Predation of S. frugiperda pupae was estimated with relation to insecticide treatment and ant foraging activity. Carbofuran reduced ant foraging activity and increased population levels of the fall armyworm. Ant predation was apparently responsible for a higher pupal removal rate in the plots with no insecticides. Chlorpyrifos significantly reduced ant foraging activity as well as densities of the fall armyworm, but it resulted in higher levels of the corn leafhopper. The results of this study are discussed in light of the possibility of developing integrated pest management programs in which ants may play an important role as agents of biological control. In such a case, ant susceptibility to chemical insecticides should be taken into consideration.
The Ecological Consequences of Flowering Asynchrony in Monoecious Figs: A Simulation Study
Authors: Bronstein Judith L. , Gouyon Pierre‐Henri , Gliddon Chris , Kjellberg Finn , Michaloud Georges
Published: May 2006

For plants with temporally separate sexual phases to outcross, population—level flowering asynchrony is necessary, but this can decrease the resource base available for pollinators. We developed a simulation model to examine the consequences of such asynchrony for individual reproductive success and long—term pollinator maintenance within monoecious fig populations. In figs, flowering is synchronous within a tree and the specialist pollinators/seed predators can only survive briefly away from trees. Consequently, population—level flowering asynchrony must extend year—round for pollinators to persist locally. In repeated stochastic simulations using phenological traits of one well—studied species (Ficus natalensis), a median of 95 trees was required to produce an asynchronous sequence that could maintain local pollinator populations for 4 yr. However, many trees in those simulated populations were either male—sterile (10%) or both male— and female—sterile (35%), because their sexual phases were not well timed with the opposite phases of other trees. Sterility within a population approached zero at 2—3 times the critical population size. Both the predicted critical population size and frequency of success of the trees within it depended strongly on the duration of reproductive episodes and the intervals between episodes. The level of within—tree reproductive synchrony was also critical: doubling the length of time over which individuals could donate pollen resulted in a 39% decrease in critical population size and a 27% increased likelihood that individuals would achieve at least some reproductive success. These results point to the need for precise phenological data for estimating plant fitness and population structure both in models and in the field.
A 5‐yr Record of Aerial Primary Production and Stand Characteristics of Spartina Alterniflora
Authors: Morris James T. , Haskin Betsy
Published: May 2006

The purpose of this paper is to document and explain the interannual variability in aboveground primary productivity from salt marshes at North Inlet, South Carolina. A census method of measuring production was applied to salt marsh sites vegetated by the grass Spartina alterniflora, and a statistically significant relationship between stem age and cumulative leaf loss was used to estimate leaf turnover. Aboveground productivity was 2.3 times as large as the positive increment in standing biomass density due to the turnover of stems and leaves, with stem turnover accounting for 33% and leaf turnover 23% of total aboveground productivity. A numerical simulation demonstrated the sensitivity of destructive harvest methods to sampling errors that are propagated by spatial variability. Monthly measurements, made at one site for >5 yr, document a twofold variation in annual aboveground production, which has important implications for the biogeochemistry and trophic dynamics of estauries. Positive correlations indicate that mean sea level and/or rainfall are important determinants of annual production, and their effect on sediment salinity is postulated to be the proximate determinant of interannual variation in production. Monthly growth rates and standing biomass densities from two different marshes, both situated at the mean high tide elevation, have been equivalent for 3 yr despite a consistent twofold difference in stem density. Thus, equivalent aboveground productivity was maintained in populations differing in their allocation of photosynthate between stem growth and asexual reproduction.
Sex Determination in Daphnia Magna
Authors: Hobaek Anders , Larsson Peter
Published: May 2006

The factors inducing male offspring and the mechanism of sex—ratio adjustment were investigated in Daphnia magna, a cyclically parthenogenetic cladoceran (Crustacea). Laboratory experiments were conducted on individual animals living in flow—through chambers, to separate the effects of various density—dependent factors. Fluctuating food levels had no effects on offspring sex when starvation was not severe. Water from crowded populations of different Daphnia species contained substances that induced up to 43% male broods. The chemically mediated crowding effect was detectable under conditions representing a low population density, implying that animals responded to their own metabolites. Daphnia species differed in their inducing effects, and in one case (water from D. pulex) strong male offspring induction was associated with growth inhibition. A short daylength was also demonstrated to induce male offspring effectively. Evidence was found to support a mechanism of sex—ratio regulation that implies a regular alternation of brood sex, resulting in a sex ratio close to 1/2, the expected optimum ratio. Even individual mothers adjusted the sex ratio of their offspring within a sequence of seven broods. No resting eggs were formed in the experiments, indicating that formation of male offspring and resting eggs are independently controlled, possibly by distinct sets of environmental cues.
An Experimental Evaluation of Density Dependence in a Subtidal Algal Population
Authors: Reed Daniel C.
Published: May 2006

The effect of density on the survival, growth, fecundity, and recruitment of the subtidal kelp Pterygophora californica was experimentally evaluated by thinning a natural population of 1—yr—old plants to four different densities. Tagged individuals were followed through time, and changes in their estimated tissue volume were calculated from in situ morphometric measurements. Highly seasonal patterns of growth and reproduction were observed. During fall senescence, plants in stands thinned to the lowest density (i.e., 2 plants/m2) showed a significantly greater decline in volume than those in denser stands (i.e., 58 and 102 plants/m2). During periods of high growth, lower density plants showed significantly greater gains in volume and were the only plants that became reproductively mature. In the winter when volume was lowest, the proportion of individuals in the smallest size classes decreased as treatment density increased. In contrast, in the summer when volume was greatest the predominance of small individuals increased as treatment density increased. New recruits of Pterygophora as well as other macroalgae were observed only in lower density stands. Lower density plants also had higher levels of epiphytes. The distributions of the white sea urchin (Lytechinus anemesus) and two sea hares (Aplysia californica and A. vaccaria), the three most conspicuous grazers during this study, were correlated with Pterygophora density. Lytechinus were significantly more abundant in low—density plots, while Aplysia spp. were common only in high—density plots. Both grazers appeared to remove substantial amounts of biomass, and there was no obvious effect of stand density on the amount of plant tissue lost as a consequence of grazing. These findings suggest that density can strongly influence patterns of growth and reproduction in this alga. A likely mechanism is competition for light at high density. The relative importance of light competition at higher densities, however, probably varies with other factors such as nutrient dynamics, abrasion, and grazing, which may or may not operate in a density—dependent manner. The degree to which dominance and suppression are expressed in high density stands of marine algae most likely varies with species architecture and depth and may account for differences between the results of previous studies on marine algae and those reported here.
The Annual Energetics Cycle of Little Penguins (Eudyptula Minor)
Authors: Gales Rosemary , Green Brian
Published: May 2006

Isotope turnover techniques were used to measure the metabolic and food consumption rates of free—living Little Penguins (Eudyptula minor) throughout the annual cycle in Bass Strait, Australia. Water and sodium flux rates were also measured, and all parameters showed significant seasonal differences. There were no differences between male and female adult Little Penguins in any of the parameters measured. The estimates of energy turnover were used to construct time/energy budgets for comparison with time/activity budgets. Although chick—rearing occupies only 16% of the annual time budget, this period is the most energetically expensive phase and accounts for 31% of the annual energy budget. The high energy costs of chick—rearing are particularly extreme toward the end of chick growth. At this time the daily rates of food consumption by attending adults exceed 60% of adult body mass. During the nobreeding season (winter) adult energy expenditure exceeds that acquired from feeding, and the birds lose body mass. The metabolic costs of pursuit diving appear to be similar to those associated with flapping flight. The total annual food requirements of Little Penguins in Bass Strait were estimated to be 37 000 Mg of food (fish, squid and krill). The penguins are most vulnerable to fluctuations in food availability, either as a result of natural perturbations or from commercial fishing activities, during the winter and during the chick—rearing phase of breeding. _kw Albatross Island, Tasmania
Trade‐Offs in Activity Time and Physiological Performance for Thermoregulating Desert Lizards, Sceloporus Merriami
Authors: Grant Bruce W.
Published: May 2006

Average body temperatures (Tb) of summer—active Sceloporus merriami were lower in the morning (33.3°C) than in the evening 37.0°) at an arid locality in Big Bend National Park, Texas. During midafternoon, lizards were inactive and retreated to deep crevices and caves. According to a multivariate analysis, overall rates of lizard activities (movements, feeding, and social displays) were similar between morning and evening activity, indicating that lizards were behaving similarly during the different activity periods. However, at 37°, estimated rates of metabolism and water loss were greater and sprint speed was lower relative to these rates at 33°, suggesting that substantial resource and performance costs may have been imposed by the observed higher evening activity Tb. To investigate this unusual pattern of body temperature variation, I compared the morning and evening distributions of available operative environmental temperatures (Te) as estimated by an array of randomly deployed and oriented copper models of S. merriami placed on rock and ground surfaces within lizard home ranges. In the morning, lizards had access to a wide range of Te(30°—60°), and a large fraction of the Te distribution was below 36° at the time of peak morning activity. During the morning, a hypothetical increase in Tb to within the interval 36°—38° would have allowed lizards to occupy <10% more of the available thermal microclimates. But in the evening, most of the available microclimates were in the interval 36°—38° (>90% depending on the exact time), and microclimates below 36° were unavailable until just before dark. The available Te distributions in the evening forced lizards to accept a higher activity Tb, and concomitantly higher metabolic and water loss rates, and lower locomotor performance, or remain inactive in crevices and caves. Thus in the evening, and not in the morning, the gain in usable home range due to accepting a higher Tb was great, and the lizards exploited this fraction at an increased Tb only in the evening. These data suggest that the distribution of available Te can directly influence the cost/benefit economics of lizard body temperature selection.
Habitat Selection: An Experimental Field Test with Two Gerbil Species
Authors: Abramsky Z. , Rosenzweig M. L. , Pinshow B. , Brown J. S. , Kotler B. , Mitchell W. A.
Published: May 2006

We provide experimental evidence for the isoleg theory of habitat selection in a pair of psammophilic gerbil species. Gerbillus allenbyi (mean mass: 26 g) and G. pyramidum (mean mass: 40 g) coexist in Israel's Negev desert in areas that may contain three distinct sandy habitats: stabilized sand fields, semistabilized dunes, and drifting dunes. When all three habitat types are available, coexistence between the two species has been explained by a centrifugal model of community organization that has been untested until now. To begin testing it, we conducted a field experiment in six 1 ha enclosures, each containing similar proportions of two of the sandy—habitat types (stabilized sand and semi—stabilized dune). This experiment tested the following hypotheses concerning the coexistence of the two species: (1) both species prefer the same primary habitat type; (2) G. allenbyi and G. pyramidum exhibit intraspecific density—dependent habitat selection; (3) habitat preference of both G. allenbyi and G. pyramidum is affected by the interspecific density of the other species; and (4) in the presence of the two habits of our experiment, the theory predicts that the habitat preferences of the two gerbil species should collapse from a centrifugal to a shared—preference model of habitat selection. We tested these hypotheses during two summers by measuring activity of the two species after introducing predetermined and different densities of gerbils into the enclosures during eight 3—4 wk long temporal replicates. During each temporal replicate, we further manipulated the density of G. allenbyi. Activity and habitat utilization of the rodents was measured by tracks left in the sand. Results of the experiments supported all four of our hypotheses and allowed the construction of their isoleg graph.
Leaf and Simulated Whole‐Canopy Photosynthesis in Two Co‐Occurring Tree Species
Authors: Hollinger David Y.
Published: May 2006

The goal of this study was to explore how leaf and canopy level difference interact in determining total canopy productivity. Maximum rates of carbon gain and transpiration were found to be higher in leaves of the deciduous oak quercus lobata than in the co—occurring evergreen oak Quercus agrifolia. However, the efficiency of water and nitrogen use is greater in leaves of Q. agrifolia. A biochemically based model of annual leaf photosynthesis using environmental field data indicates that the evergreen habit in Q. agrifolia partially compensates for the lower photosynthetic capacity of leaves and increases the amount of photosynthate produced per unit water or nitrogen. Simulations indicate that with low N or water availability, the evergreen species can produce canopies with greater annual production than the deciduous species; the converse is true at higher levels of resource supply. Some of the patterns of distribution of these species in northern California may be explained by examining the relationships between source availability and estimated annual production.