bluuebunny/crossref_metadata_2025 huggingface dataset:
Temporal Changes in Insular Populations of the Pika (Ochotona Princeps)
Authors: Smith Andrew T.
Published: May 2006

Temporal change in size of populations of a small alpine mammal, the pika (Ochotona princeps), at Bodie, California, USA, was analyzed by comparing a 1972 census of the observed spacing of pikas on discrete islands of habitat to a similar 1977 census. The islands of habitat varied in size and distance from one another. Overall results from each census were strikingly similar. Not all of the islands were occupied and most seemed to contain fewer individuals than they could hold. The populations apparently represent dynamic equilibria between extinction (which was inversely related to island size), and recolonization (which was inversely related to inter—island distance). In spite of these similarities, the population size of individual islands changed dramatically. Eight previously unoccupied islands were colonized and 11 island populations became extinct. Fifty—three percent of 77 islands compared changed their degree of saturation within the 5—yr period. Changes in percent saturation of available habitat on islands varied among island sizes and degrees of isolation. All cases of turnover (extinction or recolonization) occurred on small or medium—sized islands. Within these island size categories, turnover increased with isolation. Extinction of populations on islands close to a source of potential colonists was apparently minimized by recurrent colonization, a verification of the rescue effect.
Foliar Nitrogen: A Basis for Host Suitability for Elongate Hemlock Scale, Fiorinia Externa (Homoptera: Diaspididae)
Authors: McClure Mark S.
Published: May 2006

The success of elongate hemlock scale, Fiorinia externa Ferris (Homoptera: Diaspididae) varied significantly on 14 of its coniferous host species and on Eastern hemlocks supporting different scale densities. An important factor determining host suitability was the concentration of nitrogen in the young foliage available to nymphs during their development. Nymphs residing on hosts whose foliage provided a relatively rich source of nitrogen suffered less mortality, developed at a faster rate, and ultimately produced more progeny than did nymphs on hosts with relatively low nitrogen foliage. Nymphs responded positively to increases in foliar nitrogen associated with fertilization, host phenology, and levels of herbivore—induced stress. Studies at two arboretums in southwestern Connecticut, USA revealed that the suitability of 14 host species to attack by F. externa was positively related to the nitrogen concentration and water content of the young foilage in mid—June, during peak colonization by nymphs. Interspecific differences in the concentrations of foliar nitrogen and water were attributable to differences in the degree to which the young foliage had matured by that time. Data from experiments on greenhouse, farm, and forest scale populations on hemlock indicated that feeding by nymphs significantly reduced the quantity (biomass) and quality (nitrogen concentration) of food available for subsequent consumption. Scales were more successful on trees which previously had supported lower densities. The availability of nitrogen to feeding nymphs underlies this mechanism of host suitability for F. externa.
On Foraging Time Allocation in a Stochastic Environment
Authors: Caraco Thomas
Published: May 2006

A model of foraging in a stochastic environment is presented. Drawing on statistical decision theory, this paper suggests that maximization of expected utility may be a useful optimization criterion for ecological problems. The common notion of feeding preferences is extended to preferences over the probability distributions of net energetic benefits associated with available resources. By incorporating a forager's response to environmental variation, the model can lead to predictions that contrast with those of most deterministic feeding models.
Hemlock Stands from Wisconsin to Nova Scotia: Transitions in Understory Composition Along a Floristic Gradient
Authors: Rogers Robert S.
Published: May 2006

The understories of 71 small stands strongly dominated by hemlock (Tsuga canadensis [L.] Carr.) were sampled in a transect from northern Wisconsin to Nova Scotia. Stands selected for sampling were at least 100 yr old and had homogeneous overstories composed of at least 75% hemlock. Stand area varied from 0.04 to 2.4 ha, with most stands between 0.2 and 0.8 ha. All stands were local areas of strong hemlock dominance within a much larger mixed forest. The hemlock understories, as a group, are not a distinctive community type with respect to composition (vascular plants). There are no species relatively abundant in the hemlock stands that are not also abundant in other types of mesophytic forest. Presence and coverage of species vary greatly from stand to stand, even on similar soils and topography, suggesting that community organization is haphazard. However, a few species are almost ubiquitous (Tsuga canadensis, Acer rubrum, Maianthemum canadense, Dryopteris austriaca), and others consistently attain high coverage when present (e.g., Taxus canadensis, Oxalis montana). Also, the mean association (Cole's index) between pairs of common species (50—69% presence) significantly exceeds the mean association in stands not strongly dominated by a single overstory species (i.e., mixed hardwood forests with varying proportions of sugar maple and basswood in the overstory). Half of the 66 understory species that are at least moderately common in non—hemlock forest throughout the study area are rare or absent under hemlock in some parts of the transect, yet common in other parts. Regional differences in composition, however, are not discrete. Regional differences in understory composition are poorly correlated with regional differences in the potential hemlock forest flora (i.e., plants observed growing in hemlock forests). Community transitions also correspond poorly to certain climatic differences between regions, such as the length of the freeze—free period.
The Experimental Analysis of Competition between Two Syntopic Species of Orb‐Web Spiders (Araneae: Araneidae)
Authors: Horton Charles C. , Wise David H.
Published: May 2006

Nonexperimental studies by several investigators have provided indirect evidence of interspecific competition between Argiope aurantia and A. trifasciata, two syntopic orb—weaving spiders common in old fields. We performed replicated field experiments in 1979 and 1980 to test directly for both inter— or intraspecific competition among these spiders. Densities of one or both species were altered within open 12 x 12 m plots in an old field in Maryland, USA. Evidence for competition was based upon the effects of the density manipulations upon web location, prey capture, growth, and survival. The experiments yielded no evidence of interspecific competition between A. aurantia and A. trifasciata. The manipulations did provide statistically significant evidence of intraspecific competition during 1979. However, no treatment effects were evident in 1980, though densities of both species were higher that year and prey appeared to have been less available than in 1979. Increasing the density of conspecifics caused an increase in the mean web height of both species during the early census period of 1979. The growth rate of A. aurantia was slightly less at the higher density in 1979, and survival (net effect of mortality, immigration, and emigration) of A. trifasciata was negatively density dependent that year. Responses to the density manipulations were variable between experimental plots and years. Some of the responses were correlated with seasonal changes in the weather and with variation in the composition and structure of the vegetation. Thus our observations provide indirect evidence of the importance of physical factors and possibly vegetation in the distribution and abundance of A. aurantia and A. trifasciata. Interspecific competition appears to have no, or only minor, influence on resource utilization, growth, and survival of these species.
Bird Species Distribution in the Galapagos and Other Archipelagoes: Competition or Chance?
Authors: Alatalo Rauno V.
Published: May 2006

Connor and Simberloff (1979) have argued that in many archipelagoes bird species are randomly distributed subject only to constraints of a minimum island size and a give number of occupied islands for each species and a given number of species per island. Thus, interspecific competition would not seem to be important in determining the distribution of birds. However, the Galapagos finches (Geospiza) are not distributed as predicted by a random model with contraints of minimum island size and each species inhabiting a given number of islands. Interspecific competition is a likely explanation for the dissimilarity of distributions in this genus, as availability of suitable habitat or isolation alone are not satisfactory as alternative explanations. For the birds and bats of the West Indies many cases of competitive exclusion may be included in the seemingly small difference that Connor and Simberloff found between the numbers of exclusive distribution and those expected under the constraints of their random model. However, factors other than interspecific competition may be responsible for the dissimilarity of these distributions. Random models are a weak method for studying the role of interspecific competition in animal distribution.
Responses in Abundance and Diversity of Cornfield Carabid Communities to Differences in Farm Practices
Authors: Dritschilo William , Erwin Terry L.
Published: May 2006

The response of carabid beetles to differences in tillage and chemical use was studied using four matched pairs of sites. Abundance and species richness were significantly different between treatments, but four commonly used measures of diversity were relatively insensitive indicators of change. It is suggested that the use of diversity indices in impact assessment is redundant or, when used with ground beetles, misleading.
Winter Water Relations of Red Spruce on Mount Monadnock, New Hampshire
Authors: Kincaid Dwight T. , Lyons Elizabeth E.
Published: May 2006

With the pressure chamber, the water potential of red spruce stems (Picea rubens) was measured in four altitudinal zones, including timberline, on Mount Monadnock, New Hampshire, during two winters. Average water potential over all zones and dates was —1.14 (sd @= 0.57, range —0.1 to —4.0) MPa for 545 stems from 91 trees. More variation in water potential occurred within single trees than between juvenile and adult trees, and than among altitudinal zones or among seasons. Temperature data from maximum—minimum thermometers and from thermistors implanted into trunks and soil show that bulk water flow periodically occurs in winter. The slow decline in water potential of branches excised in winter, and scanning electron micrographs of plugged stomates, indicate that high diffusion resistances maintain needles within the relatively high levels of water potential measured in this study. Our results disagree with traditional concepts, support the conclusions of Slatyer (1976) and Marchand and Chabot (1978), and indicate that the mechanisms behind the variability in water status of modular parts of single trees must be understood before the causes, timing, and consequences of winter death in timberline trees can be unraveled.
Loss of Mass in Breeding Wrens: Stress or Adaptation?
Authors: Freed Leonard A.
Published: May 2006

Losses of mass in breeding passerines are commonly observed and presumed to reflect physiological stress from activity while feeding nestlings. Analyses of changes of body mass in House Wrens (Troglodytes aedon) show that: (1) females lose ° 13% of their body mass between onset of incubation and fledging, but reattain their original mass for second broods; (2) 50% of mass loss is achieved before hatching is completed; (3) loss of mass is virtually complete before food demands of the nestlings are greatest; (4) change in body mass during the period of highest food demand is independent of brood size; (5) males foraging at rates similar to females show no change in mass. These patterns do not reflect stress. Rather, the timing of mass losses of females may be adaptive by permitting a 23% reduction in power required to remain aloft in flight before the most demanding period of feeding nestlings.
Age and Size Structure Analyses of Engelmann Spruce, Ponderosa Pine, Lodgepole Pine, and Limber Pine in Colorado
Authors: Knowles Peggy , Grant Michael C.
Published: May 2006

Ages and diameters were measured in mature stands for each of 507 ponderosa pine, 541 lodgepole pine, 141 limber pine, and 217 Engelmann spruce trees in the Colorado Front Range of the Rocky Mountains, USA. Cumulative age distributions were statistically different for each species. The spruce and ponderosa pine age distributions each exhibited a strong infection point at °210 and 125 yr, respectively, while neither lodgepole nor limber pine showed such a sharp inflection point. We suggest that the presence of this inflection point may be indicative of climax type in mature stands and interpret the age distributions of these species as reflective of their climax, colonizing and fugitive ecological patterns, respectively. Diameter distribution curves exhibited patterns markedly different from the age patterns. Ponderosa pine and Englemann spruce showed almost identical distribution despite widely disparate age structure. We interpret this result to simply that to the complex of forces which influence size distributions in forest trees is very similar in ponderosa pine and spruce although they form very different ecological communities. Lodgepole and limber pine diameter distributions were quite different. Our conjectures that the presence of a sharp inflection point in cumulative age distributions indicates a climax stand and that climax species will often have coincident diameter distributions, are offered as empirically testable hypotheses.
Growth, Fecundity, and Mortality of Conus Pennaceus in Hawaii
Authors: Perron Frank E.
Published: May 2006

The population biology of the tropical marine gastropod Conus pennaceus was studied off the island of Oahu in the Hawaiian Islands. Growth, fecundity, and survivorship rates were determined in the field through a combination of mark—recapture studies, regular population censuses, and direct count of ova produced by individual females. Individuals of C. pennaceus grow to a shell length of 15 mm in their 1st yr, and females may reproduce by the end of their 2nd yr at a shell length of between 20 and 25 mm. Females probably produce 1 egg mass/yr, and age—specific fecundity increases with increasing female size. By year 10, annual fecundity of females has increased to over 6000 ova, while shell growth has slowed to <1mm/yr. Although C. pennaceus has no planktonic larval stage, over 99.93% of all fertilized ova fail to survive their 1st yr. Subsequent mortality is constant at an annual rate of 0.419, and very few animals survive to the end of their 10th yr. The population examined remained relatively constant in size during the 22—mo study. This stability is attributed to the shape of the reproductive value curve of the population and to the ability of C. pennaceus to utilize a wide variety of prey species.
Leafminers on Oak: The Role of Immigration and In Situ Reproductive Recruitment
Authors: Connor Edward F. , Faeth Stanley H. , Simberloff Daniel
Published: May 2006

The roles of overwintering reproductive recruitment and intertree immigration are examined for leaf—mining insects on oak trees. Full—exclusion caged trees that exclude immigrants, litter exclusion caged trees that exclude overwintering recruits, and control trees were sampled to determine leafminer abundance and species richness. Full—caged trees had little overwintering recruitment, and populations went extinct during the season's first generation. Litter exclusion trees recruited as many species as did control trees, but had lower leafminer densities. However, further experiments indicate that the lowered densities were a caging artifact. This result suggest that most leafminer populations on oak trees are maintained by yearly reimmigration from safe overwintering sites and not from in situ reproductive recruitment. Therefore, the distribution and abundance of leafminers on oak should not be viewed as the turnover of reproductive populations on individual trees, but rather as the immigration and failed colonization of species whose movements encompass several trees.
The Effects of Adaptive Behavior on the Type‐2 Functional Response
Authors: Abrams Peter A.
Published: May 2006

Holling's (1959) disc equation is the most commonly used model of type—2 functional response. The constant—parameter version of this model makes three assumptions that are seldom likely to be satisfied: (i) attack rate per unit search time per unit prey density is independent of prey density; (ii) handling time is independent of prey density; and (iii) the relative rates of successful and unsuccessful attack are independent of prey density. Optimal foraging theory is used to suggest how each of these components of the predation process might depend on prey density. Explorations of simple models suggest that these types of adaptive variation are often consistent with type—2 responses, but generally result in shapes that differ significantly from the disc equation. The consequence of using a constant—parameter disc equation model when it is inappropriate is limited ability to predict functional responses in altered circumstances, for example, in the presence of a second prey. The potential ability to understand multispecies systems from their subcomponents cannot be assessed unless appropriate models of the functional response are used to describe the component systems; these models are likely to include adaptive behavior.
Modelling Individuals Employing an Integrated Energy Response: Application to Daphnia
Authors: Hallam Thomas G. , Lassiter Ray R. , Li Jia , Suarez Luis A.
Published: May 2006

Motivated by problems where variation among individuals is necessary to explain properties of ecological systems, we develop a mathematical model of an individual organism. The model, based primarily upon energetics, is developed specifically for female daphnids, although with appropriate modifications it should be applicable to other aquatic animals such as fish. Mimicking the life history of an individual as it progresses from egg to juvenile to adult instars, the model consists of a coupled pair of nonlinear, nonautonomous ordinary differential equations. The growth of an individual is described through the dynamics of two compartments–lipid and structure–of the organism because of the importance of lipid dynamics in aquatic animals. Energy supply and demand are handled through an energy integrator compartment. Availability of energy is represented by potential flows from the lipid and structural compartments. Energy requirements of maintenance, activity, and reproduction are the sinks. An advantage of employing an individual—based technique is that ample information relating to the individual parameters–most of which are physiological in character–can usually be obtained, and reasonable estimates for model application found.
Dynamic Relations between the Antler Moth and Meadow Vegetation in Northern Sweden
Authors: Danell Kjell , Ericson Lars
Published: May 2006

The antler moth (Cerapteryx graminis L.) is a noctuid that occurs over wide areas of Europe and that has recently colonized North America. The larvae feed almost exclusively on grasses in spring and early summer. In northern latitudes and at high altitudes they can reach densities of 100—1500 individuals/m2. During the outbreak years this herbivore may have a great impact upon the vegetation. In a long—term investigation (11 yr) we recorded the larval densities, plant standing crop, and number of shoots of the abundant grass and herb species in northern Sweden. We also experimentally increased the density of larvae to study the effect of larvae on the individual size, mass, and nutrient concentration of grasses and herbs. During the study period the larval density was negatively correlated with the mean June temperature. We observed two outbreaks of Cerapteryx, which reduced grass biomass and led to increased population sizes of some herb species during the same and/or subsequent years. The annual herb Rhinanthus minor increased in abundance in the years of the Cerapteryx peaks. Increased numbers of flowering shoots were recorded in the following year for the perennial herbs Gnaphalium sylvaticum, Leontodon autumnalis, Trifolium repens, and Veronica serpyllifolia. The abundance of herbs decreased in the following 3—4 yr when the larval densities of Cerapteryx declined, while the grasses increased. There was a negative correlation between the biomass of grasses and the biomass of herbs harvested in early September for the 11 yr. suggesting that herbs also responded to grass consumption by larvae. The nitrogen concentrations in the aboveground parts of grasses and herbs were highest in the same year or in the year following the outbreak. The experiment with increased larval density (°560 larvae/m2) showed that at this level: (1) grass ramets of four species in late summer were lower in height and biomass while the individuals of four out of five species of herbs had greater dry mass, but not height; (2) the concentrations of nitrogen, phosphorus, and potassium in the aboveground parts of the plants were higher than found at the control level (°60 larvae/m2). Our data thus indicate that the grass—herb dynamics are influenced by Cerapteryx. Other important factors affecting the grass—herb dynamics are voles, summer air temperature, and precipitation.
Null‐Model Tests for Competitive Displacement: The Fallacy of Not Focusing on the Whole Community
Authors: Pleasants John M.
Published: May 2006

Null—model tests for competitive displacement involve comparisons of species' relationships in an observed community with those of randomly constructed communities. I examine the performance of several parameters that have been used to characterize community structure for the purpose of this comparison. These parameters have in common the fact that they focus on selected species pairs within the community rather than considering the community as a whole. Fleming and Partridge (1984) proposed a parameter for examining flowering phenologies that focuses on one species pair at a time. Fleming and Partridge compared the observed overlap between members of a particular species pair with the distribution of possible overlap values for this pair from randomly constructed communities. A second parameter, the overlap between a particular species and all other species combined, was employed in a similar way. Using hypothetical phenologies I show that using these parameters (a) will always result in acceptance of the null hypothesis when in fact a segregated flowering pattern exists, and (b) may lead to acceptance of the competition hypothesis when in fact the null hypothesis is true. A community—level parameter, mean pairwise overlap, is shown to make neither or these errors. Simberloff and Boecklen (1981) employed a family of parameters to examine the competition hypothesis that there is a constant difference in log body size (=a constant ratio of body sizes) between adjacent species ranked according to body size. Their parameters focus on the relative magnitude of the differences between two pairs of adjacent species, such as the two pairs with the smallest and largest log body size differences. Using hypothetical communities I show that these parameters may not always recognize a community with competitive displacement, and may mistake a clumped community for one with displacement. Several community—level parameters are shown to have greater power in discerning competitive displacement.
Effect of Floral Morphology and Temperature on Pollen Receipt and Removal in Ipomoea Trichocarpa
Authors: Murcia Carolina
Published: May 2006

In Gainesville, Florida, the coastal morning glory Ipomoea trichocarpa varies considerably in flower size, shape, and stigma—anther separation. I studied some of the causes and reproductive consequences of this variation by exploring its effect on pollen receipt and removal. Variation in flower size was caused by variation within genets and, more drastically, by temperature fluctuations. While several insect taxa visited I. trichocarpa flowers, I concentrated on one hawk moth species (Enyo lugubris) and the bumble bee Bombus pennsylvanicus. Hawk moths visited the flowers from the time of anthesis until sunrise, when bumble bees arrived. Stepwise regression analyses indicated that flower morphology affected pollen deposition during single hawk moth visits, single bumble bee visits, or unlimited hawk moth and bumble bee visits. The morphological variables with strongest effects differed in each case, and could be related to the foraging technique of each pollinator. Floral morphology affected pollen removal from flowers more than it influenced pollen receipt. The morphological variables that best explained variation in the amount of pollen removed from the anthers during a single bumble bee visit differed from those influencing pollen receipt. In the field, hawk moths could potentially fully pollinate I. trichocarpa before pollen from later rising bumble bees could send tubes to ovules. However, tube growth rate increased with rising morning temperatures. Thus, ovules could potentially have been fertilized by a mixture of pollen deposited by hawk moths and by bumble bees. Furthermore, low morning temperatures excluded hawk moths entirely from I. trichocarpa pollination, by delaying anthesis. On cool mornings, flowers opened in time to receive bumble bee visits only. The effect of temperature on flower size and pollinator type, coupled with the physical interaction between flower and pollinator morphologies, determined the amount of pollen removed from and received by I. trichocarpa.
The Role of Substratum in Benthic Macroinvertebrate Microdistribution and Litter Decomposition in a Woodland Stream
Authors: Reice Seth R.
Published: May 2006

This experiment tested the effect of substratum particle size, in the absence of velocity variation, on the determination of macroinvertebrate microdistribution and decomposition in woodland streams. Replicate baskets of three different substrata were placed in a single riffle of New Hope Creek, North Carolina, USA. The substrata were fine gravel (°1 cm diameter), pebbles (°2.5 cm diameter), and large cobbles (°8.5 cm diameter). Half the baskets had leaf packs attached to their upper surfaces. Replicate samples were removed on five dates at 2—wk intervals and analyzed for macroinvertebrates and leaf pack biomass. Colonization and distribution patterns of individual taxa were used to assess substratum preferences, which were correlated with physical properties of the sediments and the presence or absence of the leaf covering. Overall, litter decomposition did not vary among substrata. The fraction of animal populations in leaf packs was proportional to the biomass remaining. Animals showed substratum preferences even when velocity difference were eliminated. Preferences of common taxa were unaffected by the presence or absence of leaf packs on the substratum. Common taxa showed strong preferences for either leaves or substrata. Abundance data (the number of individuals per basket) strongly contradict the density data (number per square metre of substratum surface). The latter measure offers more biological insight. These results emphasize the importance of substratum size as a prime determinant of the structure of lotic macroinvertebrate communities.
Effects of Macroalgal Dynamics on Recruitment of a Temperate Reef Fish
Authors: Carr Mark H.
Published: May 2006

Spatial and temporal variation in the recruitment of dispersive life stages can strongly affect the distribution, dynamics, structure, and gene flow of local populations. I investigated the causes of variable larval recruitment to populations of a temperate reef fish, the kelp bass (Paralabrax clathratus). Spatial and temporal variation in the density of settlement habitat on a reef, the giant kelp (Macrocystis pyrifera), explained much of the variation in recruitment of kelp bass. Although recruitment was positively related to Macrocystis density, the relationship was asymptotic at high plant densities, indicating that kelp bass recruitment saturates at intermediate densities of Macrocystis (°100—130 stipes/30 m2). Field experiments demonstrated causation between Macrocystis density and kelp bass recruitment and indicated that kelp bass recruitment was linearly related to the local abundance of kelp structure (i.e., the number and biomass of overlapping Macrocystis blades). This linear (nonasymptotic) relationship between Macrocystis blade biomass and density of fish recruits indicated that both the quantity and quality of the recruitment habitat limited larval recruitment. Structural complexity per unit length of plant (e.g., blade biomass per unit plant length) was inversely related to plant density. This relationship underlies the asymptotic relationship between structural complexity per unit reef area (the product of per—plant structural complexity and plant density) and recruit density. Thus, density—dependent constraints on blade biomass in the macroalga, rather than larval supply, better explained the general asymptotic nature of the relationship between Macrocystis and kelp bass recruitment. These results indicate that density—dependent effects on plant biomass and architecture (e.g., plant height and structural complexity), common among both macrophytes and terrestrial plants, can greatly influence the relationship between plant density and recruitment of other species. For organisms with dispersive life stages whose local recruitment is influenced by the occurrence of living habitat structure, spatial and temporal variability characteristic of such biogenic habitats can be a major source of variation in their recruitment.
Predation Risk and Competition Affect Habitat Selection and Activity of Namib Desert Gerbils
Authors: Hughes Jessica J. , Ward David , Perrin Michael R.
Published: May 2006

Microhabitat partitioning based on microhabitat—specific predation rates has been well documented as a mechanism of coexistence in desert rodents. In the Namib Desert, two species of rodent, a nocturnal dune hairy—footed gerbil, Gerbillurus tytonis, and a diurnal striped mouse, Rhabdomys pumilio, co—occur in a vegetated island in the dune sea. The species show a marked preference for the same microhabitat, although gerbils also inhabit a second microhabitat where predation risk is greater. We provide direct experimental evidence for the role of predation risk and both inter— and intraspecific competition in affecting foraging behavior and habitat selection of gerbils. Using two techniques (tracking and seed trays), we were able to show that gerbils decrease foraging activity in vegetated areas during periods of increased illumination (full—moon periods). Removal of a potential competitor, the striped mouse, as well as removal of conspecifics, resulted in a significant increase in gerbil foraging activity. We propose that high susceptibility to predation results in a shared preference for the safest habitat and leads to competition for limited resources.
Causes and Consequences of Female‐Biased Dispersal in a Flock‐Living Bird, The Pinyon Jay
Authors: Marzluff John M. , Balda Russell P.
Published: May 2006

Pinyon Jays (Gymnorhinus cyanocephalus) live in permanent flocks of 50—500 individuals on apparently undefended home ranges. We present data collected from 1973 to 1982 on the movement of individuals into and out of a color—banded flock. We banded 681 wanderers from other flocks, 46 of whom immigrated and bred in the study flock. Three hundred and eighty—three juveniles and yearlings born in the study flock disappeared from the natal flock; 22 of these bred in a neighboring flock. One was collected in 1973 as a yearling 600 km from its place of birth. Most dispersal occurred in the fall, but yearling females also dispersed during the breeding season. Fall dispersal was primarily by juveniles and included males and females in equal abundance. Females transferred between neighboring flocks more often than males. Many males returned to their natal flocks after months of wandering. As a result, 89% of male yearlings, but only 48% of female yearlings, in the study flock were born there. Annual variability in dispersal by yearlings was correlated with sex ratio in their natal and neighboring flocks, females joined flocks with strongly male—biased sex ratios and females left flocks with female—biased sex ratios. Low status of juveniles and yearlings, especially females, may explain why some dispersal occurred each year regardless of the sex ratio of the flock. Homing ability and the number of relatives in a flock were only weakly related to the percentage of yearlings and juveniles emigrating. Five consequences of dispersal were: (1) flock members may become familiar with resources outside their home range; (2) the sex ratio of yearlings approaches 1:1, despite extreme and variable sex ratio biases among yearlings born in the flock; (3) in some years the yearling cohort is predominantly composed of immigrants and in other years natives predominate; (4) immigrants have fewer relatives in the flock than natives have; and (5) the effective population size ranges from 222 to 370 and includes breeders of a flock and breeders in neighboring flocks.
Parental Effects on Spatial Patterns of Plants: A Contingency Table Approach
Authors: McCanny Stephen J. , Cavers Paul B.
Published: May 2006

We examined the effects of plant size and seed production on the seed shadows and mature offspring patterns of isolated parent plants of six biotypes of Panicum miliaceum. Spatial patterns of offspring were summarized in contingency tables that were classified by distance and orientation from the parent. These tables were analyzed with reciprocal averaging ordination and dispersions profile analysis. The first and second components of the ordination of a detailed data set reflected the distance and orientation, respectively, of the seed shadows from their parents. The mean infructescence height and the mean lateral spread of the parent plants were correlated with the first component of the ordination while the orientation of the parent plants was correlated with the second component. Little support was found for the hypothesis that biotypes with high numbers of seeds produced should have greater dispersal ability. However, when the individuals were grouped by seed production, irrespective of biotype, the more fecund groups had more distant cohorts of mature offspring. This latter trend was partly attributable to the increased density—dependent mortality near fecund parent plants and partly attributable to the larger sizes and hence larger seed shadows of these parents.
Analyzing Discrete Movement Data as a Correlated Random Walk
Authors: McCulloch C. E. , Cain M. L.
Published: May 2006

Correlated random walk models provide a framework for making quantitative predictions about an organism's rate of spread. In this paper we present an algorithm that calculates the variance in squared displacement for a correlated random walk model. This can be used to make statistical comparisons of observed displacements to the model and to approximate the expected net displacement of the model. We illustrate the techniques with examples of insect movement and the vegetative spread of clonal plants.
Replication and Treatment Strength in Whole‐Lake Experiments
Authors: Carpenter Stephen R.
Published: May 2006

Replication of large—scale experiments is desirable, but the numbers of replicates needed are not known. Costs and feasibility of ecosystem experiments depend critically on the numbers of replicates needed because of the high cost per replicate and the scarcity of experimental ecosystems. This paper examines the numbers of replicates and magnitudes of manipulation needed to detect changes in lake primary productivity resulting from piscivore manipulations. Substantial (°10 x changes in the independent variable, piscivore biomass) and sustained (at least 3—5 yr) manipulations using five reference and five experimental ecosystems produced significant t test results in >80% of simulated experiments. The need for substantial and sustained manipulations is consistent with published results of whole—lake experiments on nutrient inputs, chemical contaminants, and the biota. In many cases, limited numbers of experimental systems or high costs will prevent adequate replication of ecosystem experiment. When large—scale experiments employ insufficient (e.g., 2 or 3) replicates and/or modest perturbations of the independent variate, there is great risk of erroneously accepting the hypothesis of no treatment effect. Therefore, unreplicated paired—system experiments (one reference and one experimental system) are often preferable even though classical statistics cannot be used to determine whether manipulation caused a change in the experimental system. A series of unreplicated paired—system experiments, staggered in time and performed in many locations, will provide more ecological insight than a replicated experiment in a single region. Few statistical methods pertain to large—scale ecological experiments; innovations could be very beneficial.
Population Responses of Surfperch Released from Competition
Authors: Schmitt Russell J. , Holbrook Sally J.
Published: May 2006

Previous short—term (behavioral) field experiments revealed that two temperate marine reef fish, black surfperch (Embiotoca jacksoni) and striped surfperch (E. lateralis), compete with one another along a depth gradient. Here we report abundance responses of these viviparous species following 4 yr of sustained release from competition with each other. We tested predictions derived from the short—term experiments concerning the symmetry of interspecific competitive effects on abundance. We also explored among—habitat variation in the occurrence and magnitude of density compensation. Populations of both surfperch species increased on the order of 40% at experimental (competitor—removal) sites; no change in density was detected for either species at unmanipulated control sites. These strong, reciprocal effects on density conformed with our expectations based on behavioral shifts in microhabitat use immediately following competitive release and on correlation analyses between population densities and feeding—associated resources, but not with predictions from habitat shifts (e.g., changes in depth distribution) that occurred shortly after manipulation. Density compensation was detected only in habitats (depths) where behavioral responses to competitive release had occurred (i.e., in shallow reef habitats). The correlation analyses accurately predicted variation in the relative magnitude of density compensation among habitats and among populations; predictions of the absolute changes in density based on the analyses were less accurate. Take together, these results suggest that behavioral experiments may be useful in identifying potential spatial variability in competitive effects on density. “Scaling—up” the results of behavioral experiments to project relative interspecific effects on density will be more problematic, in part because the relative demographic costs of the interaction may not be accurately portrayed.
Recruitment of a Tropical Colonial Ascidian: Relative Importance of Pre‐Settlement vs. Post‐Settlement Processes
Authors: Stoner Douglas S.
Published: May 2006

The relative importance of pre—settlement and post—settlement processes in limiting recruitment rates of the colonial ascidian Diplosoma similis was established through (1) direct observations of larval dispersal, (2) comparison of pre—settlement and post—settlement mortality rates, and (3) determination of whether larval settlement rates limit the areal coverage of adults. During a 1—5 min planktonic larval phase most larvae swam 0—4 m, usually in the direction of the reef, and settled at a depth less than or equal to that of the parent colony. Between larval release and settlement, benthic and planktonic mortality sources killed 41—47% of the larvae, while at least 90% of all newly settled colonies died within 1 mo post—settlement. The larval settlement rate was greater than 4600 larvae. m—2°yr—1, which in the absence of growth, was too low to account for the percent cover of adults. However, since the annual supply of recruits would occupy >40% of the space normally inhabited by potentially long—lived adults, it seems unlikely that larval settlement rates limit the percent cover of this ascidian. These results suggest that larval habitat selection determines the vertical zonation of recruits and post—settlement mortality determines the density of juvenile colonies.
Differential Effects of Anabaena Affinis on Cladocerans and Rotifers: Mechanisms and Implications
Authors: Gilbert John J.
Published: May 2006

Preliminary observations of plankton community structure in Star Lake, Vermont, over several years indicated that blooms of the filamentous cyanobacterium Anabaena affinis were associated with low densities of Daphnia pulex and high densities of rotifers. A series of laboratory experiments was conducted to analyze this pattern. A. affinis was isolated from Star Lake and cultured in a defined medium. Suspended filaments, averaging 322 μm in length, variably affected the population growth rates (rm) of seven cladocerans (six species, one with two strains) and five rotifers on their algal (Cryptomonas) or ciliate food. High filament concentrations (1—2 x 104 cells/mL, °155—310 filaments/mL) affected none of the rotifers, intermediate concentrations (5 x 103 cells/mL, ° 78 filaments/mL) inhibited four of the seven cladocerans, and low concentrations (103 cells/mL, °16 filaments/mL) inhibited the three most sensitive cladocerans. The inhibition of some moderately sensitive cladocerans increased greatly in the second generation of exposure. Cladoceran susceptibility was positively and significantly correlated with maximal adult body length. A competition experiment showed that the presence of A. affinis filament prevented the suppression of a rotifer by a large cladoceran and led to dominance by the rotifer. Such differential sensitivities of cladocerans and rotifers to cyanobacterial filaments should increase the relative importance of rotifers in zooplankton communities. Experiments indicated that a sensitive strain of Daphnia pulex was inhibited by ingesting an A. affinis endotoxin. Starved Daphnia lived longer than Daphnia fed only A. affinis filaments, and extracts of A. affinis were just as inhibitory as filament suspensions at comparable concentrations. Sonically disrupted filaments were no less inhibitory than whole filaments, indicating that the inhibition could not be attributed to mechanical interference with feeding. Filtrates of 1—d—old filament suspensions were inactive, suggesting that an exotoxin was not being excreted. The pattern of different susceptibilities of the cladocerans and rotifers to A. affinis filaments is at least partially due to different sensitivities to the endotoxin, possibly reinforced by a similar pattern of different tendencies to ingest the filaments. The Star Lake Strain of D. pulex was unaffected by the highest tested concentration of A. affinis filaments (5 x 103 cells/mL), indicating a co—evolved resistance to the toxin, possibly a counter to an antiherbivore defense.
Effects of Variation Body Size on Intraspecifc Competion among Larval Salamanders
Authors: Smith Charles K.
Published: May 2006

I examined the effects of body size on competition among larvae of the salamander Ambystoma opacum. Two opposing hypotheses were tested: (1) size differences among competing individuals reduce the negative effects of competition on larval growth, and (2) size differences reflect a hierarchy in competitive ability in which larger larvae are superior competitors. When larvae preyed on macrozooplankton in their laboratory, a twofold difference in larval size had no effect on prey capture rate. In two other laboratory experiments I reared larvae of three size classes in pairs, and showed that the growth of small larvae was independent of the size of co—occuring individuals when the larvae competed exploitatively for macrozooplankton. Larger individuals negatively affected the growth of smaller individuals only if the larvae physically interacted. The physical interactions included attempts at cannibalism. I also manipulated larval density and the sizes of co—occurring larvae in two field experiments. Although in both experiments I found adverse effects of high density on larval growth, larval growth was independent of the size of co—occuring conspecific larvae. These results suggest: (1) variation in body size does not lead to a reduction in the negative effects of competition; (2) under purely exploitative competition for small prey, larger larvae are not superior competitors; (3) larger larvae do not have a size advantage in physical interactions, but physical interactions may be minimized under field conditions even when density adversely affects larval growth.
Operationally Defining Home Range: Temporal Dependence Exhibited by Hispid Cotton Rats
Authors: Spencer Stephen R. , Cameron Guy N. , Swihart Robert K.
Published: May 2006

To determine whether cotton rats (Sigmodon hispidus) establish home ranges, and whether home—range size and shape vary temporally, we measured site fidelity at daily and multiday intervals. The concept of site fidelity can be used to provide a quantitative measure of the existence of a home range. Mean squared distance from the center of activity (MSD) and a linearity index (LI) were used to measure site fidelity of the hispid cotton rat. Significant differences for these metrics between actual and simulated, random movement showed that home ranges existed for both daily and multiday periods. Home—range characteristics varied with temporal scale. Daily home ranges were more concentrated (lower MSD), linear (higher LI), and elongate (higher eccentricity, ECC) than multiday home ranges. The effects of sex, age, and season on MSD, LI, and ECC differed for daily and multiday home ranges. The area of daily home range was not different between sexes or ages, or among seasons, but multiday home ranges was larger for males, for adults, and in winter and summer. Knowing that home ranges exist is a necessary prerequisite for ecology or behavioral interpretation of space—use patterns before comparisons of home—range characteristics are made, however, the temporal scale of measurement must be considered, because scale alone can yield differences in size and shape.