bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Refuges and Recruitment on Gonodactylid Stomatopods, a Guild of Mobile Prey
Authors: Steger Rick
Published: May 2006

Postlarval recruitment and features of the habitat, especially the availability of spatial refuges, were examined for their effect on the abundance of gonodactylid stomatopods along the Caribbean coast of Panama. Gonodactylus bredini, Gonodactylus oerstedii and Gonodactylus austrinus occupied and aggressively defended cavities in coral rubble as refuges from predators throughout an intertidal bed of turtlegrass, Thalassia testudinum. Although water height, Thalassia density, and rubble density all varied significantly within the habitat, variation in the local density of gonodactylids was related only to the density of coral rubble. Manipulation of rubble showed that any change in the abundance of rubble, either positive or negative, altered local densities of gonodactylids in the same manner. Cavities within the rubble, however, provided the true, potentially limiting microenvironment. Comparisons between gonodactylid densities and cavity densities showed that the number of cavities limited the local densities of individuals. However, noncontinuous and variable postlarval recruitment created temporal variation in the abundances of smaller gonodactylids and sometimes limited their densities below the capacity of their cavities. The impact of cavities, therefore, depended on the size of the individual and the recent history of postlarval recruitment. Nonuniform resource utilization and nonuniform resource abundance allowed both the shortage of necessary resource and the restricted time when the population's rate of increase was positive to limit the number of individuals simultaneously. Although the spatial distribution of cavities affected the availability of cavities and lowered the effective supply of refuges, its effect on gonodactylid densities was unclear. The inaccessibility of cavities most likely affects gonoacytlids qualitatively by contributing to the maintenance of aggression in these populations.
Insect Herbivory and Photosynthetic Pathways in Old‐Field Ecosystems
Authors: Pinder John E. , Kroh Glenn C.
Published: May 2006

Three old—field plant communities of varying composition near Aiken, South Carolina, were used to test the hypothesis that phytophagous insects avoid consuming plants possessing the C4 photosynthetic pathway and consume plants that possess only the C3 pathway. The relative abundances of stable carbon isotopes in insect tissues, which indicate consumption of C3 or C4 plants, were used to determine if insects were consuming C3 and C4 plants in proportion to their abundance in the plant community. In one community, the carbon isotope ratio for insects was significantly less than that expected for proportional consumption and indicated avoidance of C4 species. Insect consumption of C4 plants was °50% of that expected if insects were consuming C3 and C4 plants in proportion to their abundance. In the other two communities, the differences between observed and expected isotopic ratios were not significant. Levels of insect consumption of C4 plants in these two communities were, respectively, °82% and °126% of those expected for proportional consumption. The results suggest that the degree of avoidance varies among plant communities.
Exploitation Competition Influences the Use of Foraging Sites by Tits: Experimental Evidence
Authors: Alatalo Rauno V. , Eriksson Dag , Gustafsson Lars , Larsson Kjell
Published: May 2006

In coniferous forests of central Sweden, tits (Paridae) and the Goldcrest, Regulus regulus, exploit nonrenewable resources in their group territories during winter. The smaller and socially subordinate species, the Goldcrest and the Coal Tit, Parus ater, forage on the outermost tree parts, while the larger and dominant Willow Tit, Parus montanus, and Crested Tit, Parus cristatus, forage on the inner tree parts. We removed Coal Tits and Goldcrests in three flocks in early winter to see if their absence would cause changes in the foraging patterns of the two dominant species. In late winter, Crested Tits foraged farther outward on branches of spruce in experimental flocks than they did in the control flocks. In spruce, Willow Tits foraged nearer the trunk than Crested Tits, and they did not respond to the experiment. In pine, Willow Tits, however, did move from branches to twigs in the absence of Coal Tits and Goldcrests. The experiment indicates that exploitation competition directly based on food depletion, without any interference, may influence the use of foraging sites by tits in coniferous forests.
Interactions Between Birds and Fruits in a Northern Florida Hammock Community
Authors: Skeate Stewart T.
Published: May 2006

This paper examines interactions between 22 bird species and 45 species of bird—dispersed plants in a northern Florida hammock community from April 1982 to May 1984. Seasonal patterns of frugivore availability and fruiting phenology were determined and compared. Frugivore species diversity and abundance were highest during the fall and winter months and lowest in the spring and summer. Increases in frugivore numbers and diversity were attributed to the presence of migrant thrushes in the early fall and of overwintering frugivores in the late fall and winter. The timing of fruiting for the plant species studied over an annual period showed a significant correlation with frugivore diversity and abundance. Spring and summer months showed the lowest number of fruiting species and fall and winter the highest, with a peak of 28 species in fruit in December. Fruiting patterns fell into four seasonal groups: summer, fall, fall—winter, and winter fruiting. Nine species showed summer fruiting, bearing ripe fruit between early spring and late summer. Summer—fruiting species were uncommon in the community and typically occurred in disturbed areas. These species appeared to use both mammals and birds for seed dispersal. The 12 fall—fruiting species ripened fruit at the peak of frugivore migration through northern Florida. Fall—fruiting species were common or abundant in the community and dependent primarily on migrant thrushes for seed dispersal. The 20 fall—winter fruiting species produced highly persistent fruits that were available to migrant frugivores through fall and winter. The four winter—fruiting species matured fruit in December and depended on overwintering birds for seed dispersal. No clear seasonal trend were evident for ripening synchrony patterns. The bird—fruit dispersal system of the lower temperate latitude hammock community shows similarities in fruiting patterns to middle latitudes of North America. Mild winters, the presence of evergreen species, and large populations of overwintering frugivores, however, have made winter fruiting more prevalent at lower latitudes than middle latitudes.
Palatability of Woody Plants to Browsing Ruminants in a South African Savanna
Authors: Owen-Smith Norman , Cooper Susan M.
Published: May 2006

We recorded food selection by free—ranging kudus (Tragelaphus strepsiceros), impalas (Aepyceros melampus), and domestic goats in the Nylsvley Nature Reserve in South Africa. The predominant vegetation is a deciduous wooded savanna growing on infertile sandy soils; patches of Acacia savanna occur on the more fertile sites of former human settlements. Woody species fall into two basic categories of acceptability to the animals: (1) species favored year—round; and (2) species generally rejected, except during certain periods. Among the latter, some species increase in acceptability during the dry season; others are favored temporarily while new leaves predominate. Some species remain low in acceptability year—round. We propose a palatability classification of woody species, considering features of leaf retention (deciduous vs. evergreen) and spinescence, as well as seasonal variations in acceptability. Estimated annual foliage losses to browsing ungulates were 1—3% for unpalatable deciduous species, compared with 10—30% for most palatable unarmed deciduous and evergreen species. Unpalatable deciduous species dominate the woody foliage biomass on the infertile soils, while palatable but spinescent species make up most of the leaf biomass of trees and shrubs on the more fertile sites. These patterns seem widely typical of African savanna. Plants known to have chemical defenses against vertebrate herbivory are prominent on nutrient—deficient soils, while those with structural defenses are prominent on fertile soils.
Predation Risk and Use of Intertidal Habitats by Young Fishes and Shrimp
Authors: Kneib R. T.
Published: May 2006

Salt—marsh populations of the killifish Fundulus heteroclitus and the grass shrimp Palaemonetes pugio exhibit similar age—specific habitat—use patterns. At low tide, the young of both species are found in shallow aquatic microhabitats on the surface of the intertidal marsh, separated from the adults, which reside in adjacent subtidal creek habitats. As a test of the hypothesis that predation is responsible for the segregation of age classes by habitat, I used 5—m2 enclosures containing artificial subtidal habitats (water—filled pits) to maintain two size classes (45—60 mm and 70—90 mm total length) of adult killifish on the intertidal marsh surface both in the resence and absence of adult grass shrimp. During the 10—wk experiment, larval killifishes and postlarval grass shrimp recruited into the enclosures through the 1.2 mm square mesh walls. The numbers of young killifishes (F. heteroclitus and Fundulus luciae) in enclosures that contained adult fish of either size were <5% of those in plots that excluded both adult fish and shrimp. The overall effect of fish on the abundance of shrimp postlarvae was not as great because large adults had less effect on young shrimp than did small adults. However, adult shrimp mortality was higher in enclosures that contained large fish than in those that contained small fish. Enclosures that received only adult grass shrimp additions contained °50% fewer young killifishes than those from which both adult fish and shrimp were excluded. Adult shrimp had no effect on the abundance of shrimp postlarvae. Treatment effects were enhanced with increasing elevation in the intertidal zone. The experimental results suggest that young killifishes and shrimp remain in the intertidal zone at low tide as an adaptation to avoid a high risk of mortality from concentrations of predatory adults in subtidal marsh habitats.
Treefalls Revisited: Gap Dynamics in the Southern Appalachians
Authors: Runkle James R. , Yetter Todd C.
Published: May 2006

In 1976—1977, 284 gaps (canopy—opening sizes 1—1490 m2) were sampled (age, size, species composition) from old—growth mesic forests in Great Smoky Mountains National Park, Joyce Kilmer Wilderness Area and Walker Cove Research Natural Area. In 1983, the woody vegetation (stems @>1 cm dbh) of 273 of these gaps was resampled, rates of gap closure by canopy tree branch growth and sapling height growth were estimated, and incidences of disturbances occurring since 1976—1977 were noted. The average yearly crown extension growth rate was 18 cm/yr, with much variation among species and individuals. Some individual crowns grew into the canopy opening as much as 4 m in the 7 yr. Saplings grew an average of 30 cm/yr in height, again with much variation. Overall, taller saplings grew somewhat faster than smaller ones and saplings in large gaps grew faster than those in small gaps. These two rates of gap closure together suggest that most saplings will require two or more gap episodes to reach the forest canopy. For woody vegetation, basal area per unit gap area was originally highest in small gaps, though it increased between sampling dates most in large gaps. Stem density had been highest in small old gaps, but decreased the most in old gaps. Tsuga canadensis, Fagus grandifolia, Acer saccharum, and Halesia carolina were the most important species in the gaps studied. Most species did not change in relative density or dominance between the two sampling dates and showed no significant correlations between those parameters and gap size and age. Overall, Tsuga and Fagus decreased and Acer saccharum increased in importance. High rates of repeat disturbance favor species able to grow in intermediate light levels and to survive several periods of suppression before reaching the canopy.
The Rise and Fall of a Dominant Planktivore: Direct and Indirect Effects on Zooplankton
Authors: Rudstam Lars G. , Lathrop Richard C. , Carpenter S. R.
Published: May 2006

We analyzed a 14—yr time series (1976—1989) of planktivorous fish and zooplankton from Lake Mendota, Wisconsin. Planktivory rates changed by an order of magnitude during this time period, primarily due to the rise and fall of the 1977 year class of cisco (Coregonus artedi) that dominated planktivory rates for a period of 10 yr. Planktivory increased between 1977 and 1978 due to an increase in biomass of that year class and decreased in August 1987 after a summer kill of cisco in the lake. Yellow perch (Perca flavescens) and other cisco year classes contributed °25% of the total planktivory during 1978 to 1987. Time series analysis revealed that this 10—yr pulse in planktivory rates was associated with changes in the Daphnia species and biomass. In years with low planktivory rates, higher biomass of daphnids dominated by Daphnia pulicaria developed earlier in the spring and lasted longer into the summer. This was also associated with an earlier and longer spring clear—water phase. In years with high planktivory rates, daphnid biomass was generally lower and dominated by the smaller Daphnia galeata mendotae. However, there was no significant effect of planktivory on the early summer peak in Daphnia biomass that is associated with a clear—water phase. The dynamics of this Daphnia peak are apparently regulated by Daphnia—algae interactions and not by planktivory rates. The seasonal and interyear changes in Daphnia species composition can be explained by the combined effects of planktivory, dynamics of food resources, and the physiological ecology of the two Daphnia species. There was no significant effect of increased planktivory on total zoo—plankton biomass due to a compensatory increase in cyclopoid copepods and no response by calanoid copepods. The recent history of Lake Mendota illustrates a 10—yr period of predation imposed by a single year class of a large, long—lived, obligate planktivore. It is an example of variability in a lake ecosystem scaled to the life—span of a dominant fish species.
Predicting the Consequence of Plant Spacing and Biased Movement for Pollen Dispersal by Honey Bees
Authors: Morris William F.
Published: May 2006

The distance that pollen is dispersed has important ecological and evolutionary implications. Nevertheless our understanding of the mechanism that generates spatial patterns of pollen dispersal in animal—pollinated plants, namely the combine processes of pollinator movement and pollen deposition, is at present qualitative. I show how easily measured characteristics of pollinator movement and pollen deposition can be used to build models that generate quantitative predictions about pollen dispersal distance. A field experiment in which I followed honey bees (Apis mellifera) foraging on one—dimensional arrays of mustard plants (Brassica campestris) indicated that individual honeybees have a biased direction of movement and alter their movement patterns in response to plant spacing. I used the model to explore the potential consequences of biased movement and plant spacing for pollen dispersal. Biased movement results in an increase in the expected pollen dispersal distance. In addition, the predicted dispersal distance increased with increasing interplant distance. These results demonstrate that models of pollen spatial dynamics based on individual pollinator behavior can be used to explore the consequences of plant spatial arrangement for gene flow in plant populations.
Consistent Interplant Variation in Nectar Characteristics of Mirabilis Multiflora
Authors: Hodges Scott A.
Published: May 2006

The volume and sugar content of nectar produced by flowers can affect plant reproductive success by influencing pollinator foraging behavior. Because temporally varying environmental factors can affect nectar characters, the likelihood of pollinator—mediated selection for nectar characters may be reduced. To determine the constancy of interplant variation in floral nectar, I measured the volume and sugar concentration of nectar produced by plants in a population of Mirabilis multiflora within and between seasons. The means and relative ranking of nectar characters varied widely among plants. The means were remarkably constant within a season and across three successive flowering seasons. Variation among plants in sugar produced per flower was significantly correlated with the number of flowers produced (° = 0.29) and sugar concentration of nectar was significantly correlated with the date of peak flowering (° = —0.33). Ramets of clones had similar nectar volumes and total sugar produced, while genetically distinct near neighbors did not, suggesting a genetic component to nectar production. I conclude that plants of M. multiflora vary widely, and consistently through time, in their nectar characters and that this variation could affect pollinator behavior.
Rapid N^2 Fixation in Pines, Alder, and Locust: Evidence From the Sandbox Ecosystems Study
Authors: Bormann Bernard T. , Bormann F. Herbert , Bowden William B. , Piece Robert S. , Hamburg Steve P. , Wang Deane , Snyder Michael C. , Li C. Y. , Ingersoll Rick C.
Published: May 2006

Not all nitrogen (N) inputs have been accounted for in forested ecosystems. We sought to account for N2 fixation and dry deposition using a lysimeter mass—balance approach. Large sand—filled, field lysimeters were used to construct 5—yr nitrogen budgets for two N2—fixing trees, two pines, and a nonvegetated control soil. This approach is a promising and straightforward technique for quantifying otherwise difficult—to—measure fluxes. Accurate assessment of changes in N storage combined with direct measurement of N inputs in precipitation and losses from leaching allowed as to estimate fluxes. Gains of N in pine systems were greatest in vegetation and litter, overshadowing combined losses from mineral soil and leaching by about threefold. Rapid acetylene reduction in pine rhizospheres and in cultures from washed roots suggests that unexplained gains are due to associative N2 fixation. These results provide strong evidence for N2 fixation in pine systems of °50 kg°ha—1°yr—1 N. The symbiotic N2—fixing trees black locust and black alder fixed 2 and 5 times more N2, respectively, than did pines. In all systems, input in precipitation and dry deposition were relatively unimportant to the N budget. Unexplained losses of N from the nonvegetated control suggest that denitrification is an important flux. Mineral soil organic matter declined sharply and significantly in pines (20%) and even more so in the nonvegetated control (40%). Symbiotic N2—fixing trees caused a small, nonsignificant increase in mineral soil organic matter and large, significant increases in litter layer organic matter and large, significant increases in litter layer organic matter. Bulk density (0—20 cm) declined by 5% under symbiotic N2—fixing trees and increased by 5% in one pine sandbox. Correction for soil expansion or collapse did not greatly alter estimates of unexplained N or N2 fixation. Pines with rhizospheres that fix N2 at the rates we observed might be used to restore degraded land and to create silvicultural systems that are N self sufficient. We first need to better understand the microbiology, tree genetics, and soil conditions that lead to rapid N2 fixation in pine ecosystems.
Invasibility of Experimental Habitat Islands in a California Winter Annual Grassland
Authors: Robinson George R. , Quinn James F. , Stanton Maureen L.
Published: May 2006

In an experimental test of plant community invasibility, we introduced seeds of a native ruderal, California poppy (Eschscholzia californica), at fixed density into experimental plots in a California winter annual grassland. Each of the 42 plots, which ranged in size from 2 m2 to 32 m2, had been studied for 4 yr previous to the introduction, with the common observation that a subset of plots of each size consistently held more species than others. It was primarily in these more species—rich plots that establishment and reproduction by the experimental invader occurred. Success of the invader per plot, measured as the total number of plants germinating, producing seeds, or perennating, varied with plot size, but the statistical contribution of plot size was secondary to that of local species number. Contributing variables were the extent of small mammal disturbance (positive) and the degree to which a single resident plant species (in particular, Bromus diandrus) dominated a plot (negative). In contrast to theories of competitive exclusion via niche partitioning, species—rich plots were more invasible.
Maternal Traits and Reproduction in Richardson's Ground Squirrels
Authors: Dobson F. Stephen , Michener Gail R.
Published: May 2006

Differences among conspecifics in body mass result from underlying differences in structural size and physiological condition. To determine whether the structural or physiological component of body mass has a stronger influence on reproductive traits at parturition, we studied the body composition (lean dry mass and fat content), structural size (1st principal component scores computed from 10 skeletal measurements), and body condition (residuals from regression of body mass on structural size) of yearling and older female Richardson's ground squirrels (Spermophilus richardsonii). At parturition, differences among yearlings in body mass primarily reflected differences in structural size associated with continuing structural growth. Older females appeared to reach a deterministic adult structural size, and body mass of older females was most strongly associated with body condition. Structural size of yearling females and body condition of older females had significant positive effects on litter mass. For older females, date of parturition had a significant negative influence on litter size. Both yearling and older females exhibited significant positive effects of parturition date and body condition on neonate mass. In older females, trade—off resulted in smaller litters of heavier neonates as the breeding season progressed, perhaps reflecting the need for rapid growth of offspring before hibernation. Maternal size, maternal condition, and seasonal timing were important aspects of reproduction in Richardson's ground squirrels, but these characteristics are seldom considered concurrently in studies of life histories.
Carbon Dioxide Exchange between an Undisturbed Old‐Growth Temperate Forest and the Atmosphere
Authors: Hollinger D. Y. , Kelliher F. M. , Byers J. N. , Hunt J. E. , McSeveny T. M. , Weir P. L.
Published: May 2006

We used the eddy—correlation technique to investigate the exchange of CO2 between an undisturbed old—growth forest and the atmosphere at a remote Southern Hemisphere site on 15 d between 1989 and 1990. Our goal was to determine how environmental factors regulate ecosystem CO2 exchange, and to test whether present knowledge of leaf—level processes was sufficient to understand ecosystem—level exchange. On clear summer days the maximum rate of net ecosystem CO2 uptake exceeded 15 μmol°m—2°s—1, about an order of magnitude greater than the maximum values observed on sunny days in the winter. Mean nighttime respiration rates varied between °—2 and —7 μmol°m—2°s—1. Nighttime CO2 efflux rate roughly doubled with a 10°C increase in temperature. Daytime variation in net ecosystem CO2 exchange rate was primarily associated with changes in total photosynthetically active photon flux density (PPFD). Air temperature, saturation deficit, and the diffuse PPFD were of lesser, but still significant, influence. These results are in broad agreement with expectations based on the biochemistry of leaf gas exchange and penetration of radiation through a canopy. However, at night, the short—term exchange of CO2 between the forest and the atmosphere appeared to be regulated principally by atmospheric transport processes. There was a positive linear relationship between nocturnal CO2 exchange rate and downward sensible heat flux density. This new result has implications for the development of models for diurnal ecosystem CO2 exchange. The daytime light—use efficiency of the ecosystem (CO2 uptake/incident PPFD) was between 1.6 and 7.1 mmol/mol on clear days in the summer but decreased to 0.8 mmol/mol after frosts on clear winter days. Ecosystem CO2 uptake was enhanced by diffuse PPFD, a result of potentially global significance given recent increases in Northern Hemisphere haze.
Bud Demography of the Mountain Birch Betula Pubescens Ssp. Tortuosa Near Tree Line
Authors: Lehtilä Kari , Tuomi Juha , Sulkinoja Matti
Published: May 2006

The aim of this study was (1) to evaluate the importance of dormant buds for the bud demography of the mountain birch Betula pubesecens ssp. tortuosa near the tree line and (2) to study whether sexual reproduction leads to costs for bud production rates. A bud population census was taken in two consecutive years for six branches of each of 90 mountain birch trees. The trees were growing in a common garden and belonged to 10 different progenies originating from different parts of Finnish Lapland. The data were analyzed with matrix population models. The most important transformations of the bud populations were between vegetative short and long shoots. However, if most apparently dead buds are actually latent dormants, they make an even more important contribution to the bud population growth rate than vegetative long and short shoots. Dormant buds may have considerable importance especially after events such as herbivore outbreaks, in which short and long shoots are damaged. Generative long shoots (with male catkins) and short shoots (with female catkins) had approximately the same bud production rate as the corresponding vegetative shoots, i.e., bud populations did not show any major costs due to sexual reproduction. Meristem costs, i.e., a decrease in the number of buds due to sexual reproduction, may be relatively low in mountain birch, because new axillary buds develop and compensate for lost shoot apices. This compensation capacity may be especially well developed under suboptimal conditions, where canopy expansion is limited by the harsh environment rather than by the availability of meristems. The resource cost of reproduction (e.g., in terms of carbon or mineral nutrients) may also be partly compensated especially when flowering intensity is sufficiently low.
Fruit Laxatives and Seed Passage Rates in Frugivores: Consequences for Plant Reproductive Success
Authors: Murray K. Greg , Russell Susan , Picone Christopher M. , Winnett-Murray Kathy , Sherwood Wendy , Kuhlmann Mark L.
Published: May 2006

To explore how plants may influence dispersal of their own seeds by manipulating the behavior and physiology of their dispersers, we studied the effect of a soluble chemical (or chemicals) in the fruits of Witheringia solanacea (Solanaceae), a Costa Rican cloud forest shrub, on passage of its seeds through the guts of one of its major dispersers, the Black—faced Solitaire, Myadestes melanops (Muscicapidae: Turdinae). Using artificial fruits containing natural seeds, we found that the presence of a crude pulp extract reduced the median seed retention time by nearly 50%. Estimation of seed dispersal distance as a function of retention time suggested that more rapid seed passage results in shorter average dispersal distances, especially for seeds retained <20 min. At the same time, germination trials revealed that seeds voided rapidly were far more likely to germinate than those remaining longer in Myadestes guts. We propose that “laxative” chemical(s) in Witheringia fruits balance these positive and negative consequences of ingestion by Myadestes.
Character Displacement, Sexual Dimprphism, and Morphological Variation among British and Irish Mustelids
Authors: Dayan Tamar , Simberloff Daniel
Published: May 2006

Five native terrestrial mustelids are found in Great Britain. Only three of these occur in Ireland. Farmed American mink have recently established feral populations on both islands. We studied inter— and intraspecific size relationships, sexual size dimorphism, and morphological variation among these mustelids. We viewed each size as a separate morphospecies, skull length as a measure of body size, and the upper canine tooth as the organ used to kill prey. Geographic variation was low in both islands, so we considered the mustelid population of each island a single unit. Community—wide character displacement (evidence by equal size ratios) was found among British mustelids for canine diameter. For skull length it was seen only when the largely vermivorous badger was excluded. When we added feral mink the regular pattern disappeared, but when we substituted the mink for the polecat, which is now restricted to parts of Wales and adjacent England, community—wide character displacement was manifest. For Irish mustelids size ratios were not equal, but the pattern for canines was more regular than for skull lengths. Adding the local feral mink did not result in a regular pattern, but addition of the mink and exclusion of the badger yielded equal ratios for skull length but not for canines. These patterns plus published empirical data support a hypothesis of prey size partitioning. The significant differences in size between some of the British and Irish populations of the same morphospecies suggest the possibility of ecological release among Irish mustelids, whose populations originally derived from British ones. In particular, canine sexual size dimorphism is greater for Irish pine martens, stoats, and mink, as would be expected if there were fewer competitors. For the marten and the stoat, Irish females have evolved to be strikingly smaller than their British counterparts, in each case approximating the size of the male of a missing species (polecat for the marten, weasel for the stoat). For skull length there is no consistent patterns. Finally, morphological variation is greater in Ireland for five or six morphospecies, as predicted by the niche—variation hypothesis.
Detection of Density‐Dependent Effects in Annual Duck Censuses
Authors: Vickery William L. , Nudds Thomas D.
Published: May 2006

Annual censuses of duck populations from two localities differing in environmental variability were used to test the effectiveness of six tests of density dependence. The magnitude of Type I error for each test was estimated from simulated density—independent data having the same mean and variance as the observed censuses. The actual census data were used to evaluate the ability to detect density dependence. Only autoregression had the desired level of type I error, but this test was ineffective when populations grew or declined. The standard major axis was effective in the latter cases but was ineffective when populations did not grow or decline. The only effective test applicable to all cases was a regression test with a rejection region based oin simulated data. Results of these latter analyses agree with earlier descriptions of interspecific variation in life—history tactics among duck species and the importance of competition as a factor affecting guild structure in prairie—nesting ducks. Diving ducks, which occupy the most temporally stable wetlands, show a greater degree of density dependence than do dabbling ducks, and competition influences the guild structure of diving ducks more than that of dabbling ducks.
Seedling‐Scale Environmental Heterogeneity Influences Individual Fitness and Population Structure
Authors: Hartgerink A. P. , Bazzaz F. A.
Published: May 2006

This study examines the potential of small—scale environmental heterogeneity to influence population structure and fitness of individuals within a population. Two populations of Abutilon theophrasti were established at each of three densities in the greenhouse. At each density, one population was grown on substrate made heterogeneous before planting, with the addition of stone, localized nutrients, or sand, and soil compactions on a scale corresponding to individual seedlings. These were termed "patch types" and wee chosen to represent factors that would be unpredictable in both time and space to seedlings emerging in nature. Another population was established on homogeneous substrate at each density. Seedling height, final height, biomass, and seed number were measured. The variation in biomass of individuals within a population was significantly greater on heterogeneous than on homogeneous substrate at the lowest density. When the data where analyzed with ANOVA, there was a significant interaction of patch type with presence or absence of heterogeneity. Patch type explained 47—62% of the variance in 2—d seedling height and 27—33% or 30—37% of the variance in seed output or final biomass, respectively. A multiple regression model including patch type, seedling size, and random block together explained 58—76% of the total variance in final biomass, depending on density. Size of immediate neighbors showed no correlation with individual size. The data support our hypothesis that small—scale stochastic events can influence the position of an individual in a population hierarchy. This conclusion implies that genetic determinants of fitness may be overridden by commonly occurring stochastic events. The resulting lack of predictable selection against certain genotypes should contribute to the maintenance of genetic diversity in populations.
Induced Resistance and Plant Density of a Native Shrub, Gossypium Thurberi, Affect Its Herbivores
Authors: Karban Richard
Published: May 2006

This field experiment examined whether early—season damage to a wild, native shrub affected the survival and cumulative numbers over several generations of its most abundant herbivore, Bucculatrix thurberiella. The effects of reduced plant density on these herbivores and possible interactions between density and induced resistance were also considered. Half of the plants were damaged experimentally at the start of the season by feeding caterpillars; the other half served as controls. Half of the plants experienced experimentally reduced densities of neighbors, while the other half were surrounded by natural densities. Early—season damage did not influence the initiation of new mines, although plants experiencing reduced density received significantly more mines than controls. Miners were less likely to survive on plants that had been damaged in 1989. This was also true in 1990 but only for those plants at reduced density. Effects on one stage in the life cycle of an insect need not translate into similar effects on the size of its population on a host plant. In this study, I was able to measure effects of resistance induction on the cumulative numbers of B. thurberiella because it completes many generations in each season and because a record of each individual is preserved at the end of the season. Damaged plants and those that had natural densities of neighbors had fewer cumulative numbers of mines than plants that had not been previously damaged and those with reduced densities of neighbors. Plant density had a greater effect than induced resistance on the cumulative number of mines produced over an entire season.
Multiple Predator Effects: Predicting Combined Functional Response of Stream Fish and Invertebrate Predators
Authors: Soluk Daniel A.
Published: May 2006

The effects of different types of predators may combine in complex ways to impact prey populations. In a series of microcosm experiments I empirically derived the functional response curves for two common types of stream predators, sculpins (Cottus bairdi) and stonefly larvae (Agnetina capitata), on two behaviorally and morphologically distinct types of mayfly prey (Baetis tricaudatus and Ephemerella subvaria). Data from these separate trials were combined using simple additive models to generate predicted combined functional responses for both types of predator. For both types of prey, actual combined functional response curves (both types of predators together in a treatment) differed significantly from predicted combined functional response for some ranges of prey densities. When Baetis was used as prey, significant departures from the predicted values occurred at moderate and high prey densities and were in a negative direction (interference between predators). When Ephemerella was the prey, significant departures occurred at low and moderate densities and were in a positive direction (facilitation between predators). Additive models for predicting combined predator effects cannot account for non—linearities in combined functional response introduced by non—trophic (behavioral) interactions that occur between fish and stoneflies, and between these predators and their prey. Other aquatic and terrestrial food webs contain similar linkages between vertebrate and invertebrate predators and their prey. Inclusion of behavioral interaction terms seems likely to be a necessary part of any general predicting combined effects of predators on prey populations in these systems.
The Effects of Pathogen‐Induced Pseudoflowers and Buttercups on Each Other's Insect Visitation
Authors: Roy B. A.
Published: May 2006

Pseudoflowers induced by the rust fungus Puccinia monoica on Arabis spp. are flower—like in color, shape, size, nectar production, and scent. Pseudoflowers attract insects that aid the rust's reproduction in a way that is analogous to pollination in flowering plants. I explored the effects of pathogen—induced pseudoflowers and co—blooming buttercups (Ranunculus inamoenus) on each other's insect visitation by comparing visitation in single—species plots and mixtures. Visitation to pseudoflowers was greater in mixtures containing buttercups than in pseudoflowers plots of the same density, and visitation to buttercups was also greater in the mixtures than in pure buttercup plots. Because fungal pseudoflowers and buttercups both appeared to receive greater visitation when associated with each other than when alone, and both species produce food for insect visitors, this interaction could be described as Mullerian floral mimicry. However, several characteristics of this system do not fit standard definitions of mimicry: (1) pseudoflowers are not necessarily mimicking any particular species but instead may be generalized flower—mimics, (2) flower visitors may not always mistake pseudoflowers for flowers but may actually choose them, and (3) under some circumstances pseudoflowers and flowers may compete for visitors. Competition in favor of pseudoflowers is most likely when flies are the primary flower visitors or on the rare occasions when pseudoflowers are common relative to other species.
A Comparison of Rate of Succession Over 18 Yr in 10 Contrasting Old Fields
Authors: Myster Randall W. , Pickett S. T. A.
Published: May 2006

We explored local variation in rate of plant succession by computing three contrasting measures of change for 10 New Jersey old fields through the first 18 yr after abandonment. We found that all three measures, euclidean distance in ordination space, turnover analysis, and regression analysis of species loss from the initial cohort, showed the same trend of decrease in the rate of plant succession with time. However, we did not find a monotonic decrease, and the specific years of transient increase differed with the analytical measure used. Both turnover and regression analysis showed that those old fields that had previously been planted in orchard grass (Dactylis glomerata) had a much lower initial change than fields previously in soybeans, or soybeans and sorghum, but this difference dissolved after 10 yr of succession. The study fields compared favorably to other mesic successions and agreed with the general pattern of more rapid rate decrease in mesic compared to xeric successions. Our results echo the general decrease in the rate of plant succession with time reported in the literature and also show important relationships between this decrease and the land—use history of particular fields.
Effects of Interannual Climate Variation on Aboveground Phytomass in Alpine Vegetation
Authors: Walker Marilyn D. , Webber Patrick J. , Arnold Elizabeth H. , Ebert-May Diane
Published: May 2006

Relationships between peak annual vascular aboveground phytomass and annual climate variation in alpine plant communities located on Niwot Ridge, Colorado, were analyzed using path analysis. The five community types, fellfield, dry meadow, moist meadow, wet meadow, and snowbed, represent a snow depth—soil moisture gradient and broadly represent the most common vegetation types on east—facing slopes of the Front Range alpine zone. Using nine successive years of data, this is the first longer term analysis of alpine phytomass and climate and one of the longest nonagricultural production records available. Live phytomass ranged from 97 g/m2 (snowbed) to 237 g/m2 (fellfield). Among—community differences in phytomass were greater than differences among years, but there was also significant phytomass variation among years. Path analysis indicated that climate accounted for 15—40% of the variation in phytomass. The dry communities, fellfield (exposed rocky summit areas dominated by cushion and mat plants) and dry meadow, were most sensitive to previous year precipitation, the moist and wet meadow communities were most sensitive to current growth season soil moisture, and the snowbed community was most sensitive to date of snow release. Because of the relatively high amount of variation attributable to variables related to precipitation, changes in precipitation regimes that may occur in alpine ecosystems will likely result in changes in phytomass that are detectable with clip—harvest methods.
Long‐Term Experimental Study of a Chihuahuan Desert Rodent Community: 13 Years of Competition
Authors: Heske Edward J. , Brown James H. , Mistry Shahroukh
Published: May 2006

An experimental study of competition between kangaroo rats (Dipodomys spp.) and other sympatric desert rodents using exclosures with "semipermeable" fences has been continuously maintained at a site in the northern Chihuahuan Desert since 1977. A new set of experimental manipulations begun in 1988 at the same site repeated this study. As reported previously for this community, exclusion of three species of Dipodomys from both original and new experimental plots resulted in greater abundances of five species of small granivorous rodents (Chaetodipus penicillatus, Perognathus flavus, Peromyscus eremicus, P. maniculatus, Reithrodontomys megalotis) on these plots relative to controls. In contrast, there were no significant treatment effects on the abundances of insectivorous grasshopper mice (Onychomys spp.). The long time lag before the response by small granivores to Dipodomys removal observed in the original experiment was not repeated in the experiment begun in 1988. Long—term (10 yr) exclusion of kangaroo rats from experimental plots has resulted in changes in vegetative cover, particularly increased grassiness, on these plots relative to controls. We used the repetition of the Dipodomys exclusion experiment in 1988 to evaluate the importance of this potential indirect effect of kangaroo rats on other rodents in this community. By examining differences in rodent capture numbers on the original and new sets of Dipodomys exclusion plots, we could identify four species (C. penicillatus, Perognathus flavus, Peromyscus eremicus, P. maniculatus) whose responses to Kangaroo rat removal reflected direct competition from kangaroo rats, one species (R. megalotis) whose response reflected both direct and indirect (via vegetation changes and habitat selection) effects, and two species (Sigmodon hispidus, S. fulviventer) whose responses reflected only vegetation—mediated effects. The continuous presence of competition between small granivores and kangaroo rats over the 13—yr study despite large, specie—specific fluctuations in abundances suggests that competition is pervasive within this community.
N and P Cycling Coupled by Grazers' Activities: Food Quality and Nutrient Release by Zooplankton
Authors: Urabe Jotaro
Published: May 2006

In order to clarify the role of zooplankton in the cycling of elements and its subsequent effect on algal populations, food elimination and nutrient release rates by zooplankton in a small pond dominated by daphnids were estimated for nitrogen and phosphorus from April to September. Comparison of the elemental contents of food and zooplankton showed that N was the element in shortest supply in the food sources utilized by zooplankton. The net clearance rate of the zooplankton was higher for N than for P and C throughout the study period, indicating that these organisms can eliminate the scarcest element from the food pool with a higher efficiency. Soluble N and P released by the zooplankton consisted mainly of ammonium nitrogen and soluble reactive phosphorus, and on a dry mass basis N release ranged from 0.27 to 1.05 mg°mg—1°h—1 and P release from 0.045 to 0.30 mg°mg—1°h—1. Statistical analysis revealed that the N release rate was most affected by food abundance, whereas the P release rate and N:P release ratio were affected by the N:P ratio of the food. The net production rate of the zooplankton, estimated from the difference between elimination and release rates, showed a quite similar N:P ratio to that of the body tissue, indicating that zooplankton can keep their elemental composition roughly constant even if the N:P ratio of their food supply varies. Thus, the N and P release rates of zooplankton are linked through processes maintaining the elemental composition of body tissues. The present results indicate that the cycling rate of an element in pelagic systems depends highly on the grazer's requirements as well as the relative abundances of other elements. Zooplankton can modify the nutritional environment for algae through accumulation and regeneration processes, as suggested by recent theoretical models.
Early Survival of Juvenile Snowshoe Hares
Authors: O'Donoghue Mark
Published: May 2006

Juvenile snowshoe hares (Lepus americanus) were radio—tagged at birth on one food addition grid and one control grid, to determine early juvenile survival, the effects of the food addition on survival, and proximate causes of mortality. Indices of survival were also estimated by live—trapping on these grids and on a replicate pair of grids. The overall 30—d survival rates of radio—tagged leverets were 0.46, 0.15, and 0.43 for the first, second, and third litters of the year, respectively. There were no differences between early juvenile survival on the food addition and control grids in any of the litter groups. The main proximate cause of juvenile mortality was predation by small mammalian predators, the most important being red squirrels and arctic ground squirrels. Seventy percent of early juvenile mortality occurred during the first 5 d after birth. Survival of littermates was not independent; litters tended to all live or die as a unit more often than expected by chance. Fifty—one percent of litters had no known survivors after 14 d of age. Individual survival rates were negatively related to litter size and positively related to body size at birth, and litter size was negatively correlated with body size. These correlations were most closely related to differences in life history traits among litters born at different times of the summer, rather than to trade—offs among traits within litter groups.