bluuebunny/crossref_metadata_2025 huggingface dataset:
Seed Predation and Seed Number in Scheelea Palm Fruits
Authors: Bradford David F. , Smith Christopher C.
Published: May 2006

Two Scheelea palm populations in Central America were studied: Scheelea rostrata in Costa Rica and Scheelea zonensis in the Panama Canal Zone. Most of the fruits in both populations contained one seed, but small proportions of the fruits on each tree contained two or three seeds. Compared to single—seeded fruits, seedlings from multiseeded fruits are apparently disadvantaged because of less endosperm and competition among seedlings emerging from the same indehiscent fruit. However, since seed predation of Scheelea is heavy, we hypothesized that differential seed predation may counteract this selection pressure against the production of multiseeded fruits and thus maintain multiseeded fruits in Scheelea populations. At least 61% of the fruits in the Costa Rica population and 81% of the fruits in the Canal Zone population were attacked by seed predators. Beetles (Family: Bruchidae) were the predominant predators in the Costa Rica population (84% of attacked fruits), whereas rodents predominated in the Canal Zone population (78%of attacked fruits). Attack by rodents was independent of seed number, but data from Costa Rica suggested that bruchids concentrated predation slightly on one—seeded fruits. Because seed predators were often restricted to one seed, seed survivorship in attacked fruits was directly related to seed number (0%, 22%, and 41% for attacked 1—, 2—, and 3—seeded fruits, respectively, in the Costa Rica population). As a result of an interaction of these aspects of predation, the proportion of seeds that escaped predation was also directly related to seed number (39%, 54%, and 60% for 1—, 2—, and 3—seeded fruits, respectively, in the Costa Rica population). The major difference in predation between the two populations appeared to be the predominance of bruchid predation in the Costa Rica population, which allowed a greater survivorship in attacked multiseeded fruits (29% survivorship of bruchid attack versus 12% survivorship of rodent attack). This apparent stronger selection pressure for producing multiseeded fruits coincided with a significantly greater proportion of multiseeded fruits in the Costa Rica population (27%) compared to the Canal Zone population (6%). These findings support the hypothesis that seed predation is a significant factor in maintaining multiseeded fruits in Scheelea populations.
The Seasonal Contribution of C"3 and C"4 Plant Species to Primary Production in a Mixed Prairie
Authors: Ode David J. , Tieszen Larry L. , Lerman Juan Carlos
Published: May 2006

A Mixed Prairie in the Northern Great Plains (South Dakota, USA) was studied to determine the floristic composition, according to C3 and C4 types, of the seasonal contribution to aboveground production. Twenty—seven of the 305 species present in the Ordway Prairia possessed Kranz anatomy and are assumed to be C4. Thirty—six percent (19) of the grasses were C4 whereas <2% (4) of dicots were C4. Dicots were often weedy and fugitive or introduced species. C3 plants possessed a mean °13C value of —26.7%, C4 plants a value of —12.9%. The biomass of lowland and upland communities possessed low °13C values in spring. These values increased significantly in the upland community in midsummer to —20.0% indicating a shift to C4 activity. In fall the biomass °13C values were again low in both communities indicating a return to C3 activity. These data show a clear seasonal displacement in production activity of C3 and C4 grasses in the Mixed Prairie. A general discussion of the evolution and significance of photosynthetic pathways is included.
Functional Complexity and Ecosystem Stability
Authors: Voris Peter Van , O'Neill Robert V. , Emanuel William R. , Shugart H. H.
Published: May 2006

The hypothesis that complexity and stability are positively correlated was experimentally tested at the ecosystem level of organization using intact terrestrial microcosms. Power spectral densities of hourly CO2 efflux, from 11 old—field microcosms, were analyzed for the number of low—frequency components. We postulate that the number of peaks is related to functional interactions among system components (i.e., population interactions, physical—chemical reactions, and biological turnover rates) influenced by nonlinearities, feedbacks, and time delays. Thus, the number of low—frequency peaks can be taken as an index of "functional complexity". Relative stability was based on the capacity of the system to retain essential nutrients and was measured by net loss of calcium after the system was stressed with a heavy metal, cadmium. Rank correlation supported the hypothesis that increasing ecosystem functional complexity leads to increasing ecosystem stability.
Seasonal Habitat Selection by Birds in the Lower Colorado River Valley
Authors: Rice Jake , Anderson Bertin W. , Ohmart Robert D.
Published: May 2006

Patterns of seasonal variation in use of specific habitats by birds were investigated using monthly census data from 110 transects in riparian vegetation along the lower Colorado River from Arizona to California. Specific hypotheses were tested about changes in habitat breadth, timing of such changes, and the use of habitats which were preferred in seasons of maximum density. We generally found support for the hypotheses that habitat breadth narrows in seasons of supposed resource restriction, and that the narrowing occurs before the winter season. However, species with maximum winter populations tended to show the same kinds of trends in habitat use as were shown by species with summer maximum populations. Resource depletion does not provide a full explanation for habitat breadth restriction. Alternative factors, especially changes in population structure, are suggested, although at this point explanations must be undertaken on a piecemeal basis. Habitats preferred in the breeding season were found not to be consistently preferred in other seasons, implying that conclusions on habitat optimality may not be appropriate from studies during only part of a year.
Effects of Plant Density and Diversity on the Population Dynamics of a Specialist Herbivore, the Striped Cucumber Beetle, Acalymma Vittata (Fab)
Authors: Bach Catherine E.
Published: May 2006

To determine the effects of plant density and diversity on the population dynamics of a specialist herbivore, the striped cucumber beetle (Acalymma vittata [Fab.]), cucumbers (Cucumis sativus L.) were planted in monocultures and in polycultures with corn (Zea mays L.) and broccoli (Brassica oleracea L.).Population densities of A. vittata and its insect predators, as well as cucumber growth, survivorship, and reproduction were monitored in these experimental plots. By controlling total plant density, host plant density, and plant diversity, it was possible to distinguish the effects of these three confounding variables. Densities of A. vittata were 10–30 times greater in monocultures than in polycultures, both per plot and per plant, even when total plant density and host plant density were held constant. The lack of any effect of plant density on per—plot beetle abundances emphasized the importance of diversity per se in influencing beetle populations. These differences in beetle abundance caused by diversity resulted from differences in tenure time and movement patterns among plots, rather than from differences in colonization, reproduction, or predation. In mark—recapture studies, a greater proportion of beetles marked in monocultures than in polycultures was later found, and of those marked in monocultures, a greater number were later found in monocultures than in polycultures. All measures of growth (leaf area, growth rate, and vine length) and reproduction (fruit production and number of flowers) of Cucumis sativus were most strongly affected by diversity, but also were affected by plant density. Both per—plot and per—plant values were greater in monocultures than in polycultures. The number of beetles was strongly correlated with the total amount of plant growth and reproduction in monocultures but not in polycultures. For plots with equal amounts of leaf area, monocultures had an order of magnitude greater number of beetles; thus, differences in host plant quantity did not explain the differences in beetle abundances between monocultures and polycultures. Numbers of beetles were much more strongly related to total plot characteristics than individual plant characteristics. Although defoliation was negligible, beetles significantly decreased cucumber survivorship and longevity by disseminating bacterial wilt disease (Erwinia tracheiphila [E. F. Sm.]).
Influence of Amphibian Larvae on Primary Production, Nutrient Flux, and Competition in a Pond Ecosystem
Authors: Seale Dianne B.
Published: May 2006

In a pond ecosystem near St. Louis, Missouri, natural variations in tadpole biomass during 1971—1972 were accompanied by shifts in patterns of nutrient cycling and primary production, particularly when metamorphoses caused abrupt removal of these transient consumers. In the field, increased tadpole biomass was associated with: (1) reduced standing crop of suspended particles, including phytoplankton, the tadpoles' major food source; (2) a shift in the state of nitrogen from largely particulate to largely dissolved; (3) reduced rates of primary production, from both H14CO3 uptake and diurnal oxygen methods; (4) a nonliner effect on phytoplankton specific growth rates; (5) a shift in phytoplankton community structure away from filamentous blue—green algae; and (6) a reduced proportion of active chlorophyll @a in the photosynthetic pigments of phytoplankton. From laboratory experiments, the potential impact of tadpoles on nitrogen flux, through feeding and nutrient release, was estimated. Several conclusions were made: (1) Suspensions feeding by tadpoles reduced concentrations of suspended particles. Under conditions of low particles: high tadpoles, the specific growth rates of tadpoles reduced. Recruitment was absent except under conditions of high particles: low tadpoles; the most diverse community of new tadpole recruits (four species) was observed under such conditions. All four species (three genera) had similarly sized particles in their guts. These field observations are consistent with an hypothesis of competition among the tadpoles. (2) Tadpoles apparently were regulatory consumers; they became a large component with respect to phytoplankton. Nitrogen flux through tadpoles was within the same order of magnitude, and sometimes exceeded the estimated N uptake by phytoplankton. (3) Tadpoles probably regulated primary production by both reducing standing crop and altering specific growth rates of algae. At maximum tadpole biomass, suspended particle concentrations were stabilized near the laboratory—determined threshold concentration for feeding by these Rana tadpoles. When metamorphosis removed these transient consumers, rates of primary production increased dramatically. (4) Interactions within the pond ecosystem apparently determined aquatic—terrestrial nutrient balances for the amphibian communities. Some species deposited more nutrient in their eggs than was assimilated by larvae, but the community as a whole extracted nutrient from the ecosystem. Nutrient input in eggs was much less than that assimilated by autotrophs.
Interference Competition and the Coexistence of Two Competitors on a Single Limiting Resource
Authors: Vance Richard R.
Published: May 2006

This paper proposes a mechanistic model of competition between two species for a single, nonaccumulating food resource. Actual organisms to which it may apply are sessile species like terrestrial plants and suspension—feeding benthic marine invertebrates whose energy source rains down upon them from the surrounding environment. The model's assumptions described general functions involving population growth, resource consumption, resource availability, and interference. The population consequences of competition thereby arise in a specified way from the properties of individual organisms. The generality of thevarious component functions allows them to accomodate a wide range of species' morphological, physiological, and behavioral properties as wells as relevant features of the physical environment. Accordingly, the overall model potentially approximates a correspondingly broad array of natural cases of competition. This model demonstrates the theoretical possibility that two species jointly limited by the same energy resource can coexist at a globally stable equilibriumpoint. The proof of this assertion involves standard isocline analysis. It reveals that, in this case, coexistence depends upon species' interference as well as resource exploitation properties. In particular, the two can coexist if each species interferes less with resource acquistion by the other than with resource acquisition by itself and if this difference is great enough to overwhelm the advantage of the more efficient exploitation competitor. The model includes the effects of space competition and abiotic disturbances. Field testing is discussed.
Effects of Ants and An Ant‐Tended Herbivore on Seed Production Of a Neotropical Herb
Authors: Horvitz C. C. , Schemske Douglas W.
Published: May 2006

This study examined a complex interaction among species, focused on (1) a neotropical perennial plant, Calathea ovandensis (Marantaceae), that produces extrafloral nectar, (2) ants that visit the nectaries, (3) an ant—tended specialist herbivore, Eurybia elvina (Lepidoptera: Riodiniadae), that feeds on buds, flowers, and fruits, and (4) other herbivores of reproductive tissues. We experimentally investigated: (1) to what extent seed production was influenced by ants and herbivores, and (2) whether spatial variation in seed production was influenced by spatial variation in the ant community. The lowest seed production (x = 6) was for inflorescences with Eurybia but without ants. Highest seed production (x = 21) was for inflorescences with ants, but without Eurybia. In the presence of ants, Eurybia lowered mean seed production by 33%. In the absence of ants, Eurybia lowered mean seed production by 66%. The eight ant species differed greatly in the magnitude of their beneficial effects on seed production. Ants were not equally distributed over sites, and spatial hetrogeneity in seed production by Calathea reflected differences in ant communities among sites. We concluded that variation and interdependence of various components of plant—animal interactions are likely to result in variation in the magnitude of their beneficial or detrimental impact on plants. Such variation should be incorporated into plant demograpic models as spatially varying alternate life history paths to yield insight into the evolutionary impact of plant—animal interactions.
Natural Disturbance and Nitrogen Mineralization: Wave‐Form Dieback of Mountain Hemlock in the Oregon Cascades
Authors: Matson Pamela A. , Boone Richard D.
Published: May 2006

Wave—form dieback of relatively pure stands of mountain hemlock provided an opportunity to study changes in nutrient availability following natural disturbance. Nitrogen mineralization rates of forest floor and mineral soil were estimated using laboratory and in situ incubations. Nitrogen mineralization rates in both the mineral soil and O2 horizon were at least doubled following the pathogen—induced disturbance. As the regenerating stands developed, rates declined again to the very low predisturbance levels. These changes in nitrogen availability may in turn influence tree resistance to the pathogen, suggesting that in this system the pattern of nitrogen availability is both a consequence and a case of natural disturbance.
Local Dynamics of Bird Assemblages in Small Forest Habitat Islands in Australia and North America
Authors: Howe Robert W.
Published: May 2006

Small isolated forest patches (0.1—7 ha) in eastern New South Wales and southern Wisconsin were surveyed regularly for at least 1 yr between 1977 and 1981. Forests and woodlands in both regions have been cleared extensively during the past century. This analysis examines how fragmentation of forest habitat has affected composition and dynamics of local bird assemblages. Species in forests islands are compared with those in equivalent "control plots" near the edge of a large, continuous forest. Disruption of continuous tracts apparently affects not only birds of the forest interior, but also those occurring along or near the edge. Birds in New South Wales and Wisconsin show common patterns of distribution among forest islands, despite considerable differences in taxonomic relationships, seasonality, and habitat. (1) Area was the best predictor of species richness in islands and control plots. Other habitat variables (except isolation) were relatively uniform among sites and did not contribute consistently to variation in numbers of species. (2) Species common in forest islands also tended to be common in large nearby forests, but bird assemblages in forest islands were statistically different from those in forest control plots. (3) Species assemblages in forest islands generally were more predictable over space and time than were local assemblages in larger forests. Although transients or regular visitors commonly were observed in forest islands, nonresident species occurred even more frequently in control plots. Apparently habitat fragmentation alters the spatial and temporal dynamics of native forest bird communities. This, in turn, may favor some species and leave others at a disadvantage. Differences between species assemblages of forest islands and control plots were most pronounced in Wisconsin. Numbers of forest or forest edge species were much lower in Wisconsin forest islands than in corresponding control plots. Isolation of forest islands was associated negatively with species richness only in Wisconsin. Long—term effects of forest fragmentation in New South Wales might not yet be realized, because relatively large areas of forest remain near existing habitat islands.
Sex Ratio and Nest Temperature in Turtles: Comparing Field and Laboratory Data
Authors: Bull J. J.
Published: May 2006

Sex is determined by incubation temperature in many reptiles, but precise quantifications of these effects have been obtained only in the laboratory. Here, temperatures and sex ratios were studied from 75 clutches of map turtles incubated in their natural nests. Clutch sex ratio showed a clear association with each of the two measures of nest temperatures studied: (1) hours per day above 30° or 32°C, and (2) the mean and variance of nest temperature. The best associations with sex ratio were obtained from temperatures experienced during the 4th through 7th wk of development and from temperatures during July. Results from this study were similar in several respects to results from previous laboratory studies on sex determination, despite the fact that natural incubation entails fluctuating temperatures and other environmental effects not incorporated in most laboratory studies.
Body Size, Food Concentration, and Population Growth in Planktonic Rotifers
Authors: Stemberger Richard S. , Gilbert John J.
Published: May 2006

Population growth rates of eight species of planktonic rotifers were assessed for five to seven food concentrations using daily renewed batch cultures. The food concentration for which population growth rate was zero (the threshold food level) varied by a factor of 17 among species. The log of threshold food concentration was positively and significantly related to the log of body mass. Similarly, a strong positive log—log relationship was found between rotifer body mass and the food concentration supporting one—half the maximum population growth rate (°max/2); this food concentration varied by a factor of 35 among species. There was a positive relationship between rotifer body mass and maximum population growth rate. Because the smallest species have the lowest threshold food levels and the food levels necessary for them to attain °max/2 are lowest, they appear well adapted to living in food—poor environments. Large species appear to be restricted to food—rich environments but may thrive there because of their high reproductive potential. These conclusions are consistent with observations on species distribution and community structure of rotifers in nature.
Does Competition Drive Dispersal?
Authors: Waser Peter M.
Published: May 2006

What distances would animals be expected to disperse if the only cost to staying on the natal site resulted from competition for resources limiting to reproduction? In such a situation the best" dispersal distance would be zero, but the constraint of competition would prevent many individuals from achieving this ideal; the actual dispersal distances would be shaped by population demography. The models I describe predict the distribution of dispersal distances as a function of home range turnover probability for animals that move to the first uncontested home range they encounter and no farther. The models provide a good match to data for some, but not all, avian and mammalian data sets, suggesting (1) that competition for resources limiting to reproduction may be the primary factor driving dispersal, and (2) that the action of inbreeding avoidance, habitat selection, and other processes suggested to influence dispersal distances can be detected as deviations from these model predictions.
Testing For Independence of Observations in Animal Movements
Authors: Swihart Robert K. , Slade Norman A.
Published: May 2006

Many analyses of animals movements assume that an animals's position at time t + 1 is independent of its position at time t, but no statistical procedure exists to test this assumption with bivariate data. Using empirically derived critical values for the ratio of mean squared distance between successive observations to mean squared distance from the center of activity, we demonstrate a bivariate test of the independence assumption first proposed by Schoener. For cases in which the null hypothesis of independence is rejected, we present a procedure for determining the time interval at which autocorrelation becomes negligible. To illustrate implementation of the test, locational data obtained from a ratio—tagged adult female cotton rat (Sigmodon hispidus) were used. The test can be used to design an efficient sampling schedule for movement studies, and it is also useful in revealing behavioral phenomena such as home range shifting and any tendency of animals to follow prescribed routes in their daily activities. Further, the test may provide a means of examining how an animal's use of space is affected by its internal clock.
The Role of Ecotones in the Distribution of Andean Birds
Authors: Terborgh John
Published: May 2006

This is the final paper of a five—part series on the distribution of birds in the Apurimac Valley of Peru. In one of the previous papers, I classified the upper and lower limits of avian distribution in the Cordillera Vilcabamba, Peru, as follows: (1) those that abutted on the range of a congener (presumably evidence of competitive exclusion), (2) those that approximately coincided with an ecotone, and (3) all others. A test of the limits presumptively enforced by competitive exclusion has previously been published. The present study involved limits in the second of these categories. For a set of 47 bird species whose limits (16 upper, 31 lower) coincided with the montane rain forest—cloud forests ecotone on the Vilcabamaba control transect, I determined whether, as predicted, the species expanded or contracted their distributions in localities in which the homologous ecotones were displaced upward or downward relative to the control elevation. Where the ecotone was displaced away from species' center of distribution (e.g., downward for species whose lower limits coincided with the ecotone on the control transect), 36 of 41 species (88%) were found to have expanded their distributions. This result upholds the provisional assessment of the ecotone as a distributional barrier to these species in the control locality. Where the ecotone was shifted toward species' centers of distribution, 43 out of 44 species (98%) had failed to contract fully in distribution; they were found on the other side of the ecotone, in what had been predicted to be alien habitat. This result was not anticipated. It is illustrative of a tendency of species to occupy a greater range of habitats near to vs. far from their centers of distribution. I also found that widespread Andean bird species had expanded distributions, in both upward and downward directions, in the biogeographically isolated Cordillera de la Costa of Venezuela. Reduced avian species diversity in this range relative to the main Andean chain appears to have led to a general relaxation of distributional restraints. Combining the results reported here with those presented in previous publications, I conclude that direct and diffuse competitive exclusion account for about two—thirds of the distributional limits of Andean birds, ecotones for about one—sixth, and unspecified factors that vary in parallel with the environmental gradient for the remaining one—sixth. Competitive exclusion emerges as the factor of overriding importance in the exceedingly diverse Andean fauna. In contrast, in temperature mountains, ecotones play a predominant role in limiting distributions, and competitive interactions are far less important.
Dynamics of Nitrogen Mineralization and Nitrification in Hemlock and Hardwood Treefall Gaps
Authors: Mladenoff David J.
Published: May 2006

In this study I have attempted to integrate two major topics: (1) the role of disturbance regimes in structuring communities, and (2) the effects of various disturbances on ecosystem—level processes such as nitrogen cycling. In an old—growth, northern hard—wood—hemlock forest in upper Michigan, USA, I measured net N (nitrogen) mineralization rates in small (90—250 m2), recent (0—3 yr) treefall gaps. These gaps were paired with plots in adjacent, undisturbed forest; half of the plots were dominated by eastern hemlock and half by sugar maple. Net N mineralization was measured using monthly on—site soil incubations. I made corresponding measures of other soil characteristics, moisture, and canopy opening (square metres) in each plot. The nitrogen mineralization rate varied with physical differences that existed between forest plots and gap plots, as well as with canopy species dominance. In gaps, N mineralization was higher under hemlock (forest, X = 95.3; gap, X = 107.3 kg°ha—1°yr—1) and lower under maple (forest, X = 102.0; gap, X = 88.5 kg°ha—1°yr—1). Nitrification of N was more than double in gaps under hemlock (forest, X = 27.4; gap, X = 69.8 kg/ha). Nitrification and soil pH were positively correlated in forest plots. I discuss the implications of these results for community compositional change and relevance to current hypotheses. These include hypotheses of mineralization rate change with disturbance, the role of nitrification in climax communities, and the influence of conifer canopy and litter on forest floor processes. Observed differences in these factors at this small—scale, single treefall level are compared with other large—scale whole—ecosystem studies. Implications for species adapted to this disturbance pattern are suggested. I hypothesize an interrelationship for the various observed changes in gaps and a mechanism for the exploitation of these increased resources by plants
Neighborhood Competition in an Old‐Field Plant Community
Authors: Goldberg Deborah E.
Published: May 2006

A series of field neighborhood experiments was performed to compare the competitive effects of seven plant species on the performance of transplanted individuals of Solidago canadensis. The neighbor species included conspecifics and species of a variety of growth forms that co—occur with S. canadensis in old fields. There were strong competitive effects on the transplants; average S. canadensis growth was reduced 17—62% by the presence of neighbors growing at natural densities. The responses to gradients of increasing neighbor density or biomass appeared to be nonlinear, with decreasing effects of adding more neighbors at higher neighbor density or biomass. Competitive effects were compared among neighbor species at three levels: effects at natural densities, per—individual (population density) effects, and per—gram (biomass density) effects. The magnitude of total effects at natural densities varied with the species of neighbor plants and was stronger for neighbor species with greater total biomass of plants per unit area. The magnitude of effects of each individual also varied with neighbor species and was stronger for neighbor species with larger mean biomass per individual. Consequently, per—gram competitive effects did not differ among the neighbor species. Thus, it appears that the differences in competitive effects among species were primarily due to differences in size or abundance, rather than to more subtle differences in aspects of resource use. The responses of target Solidago canadensis individuals to neighborhood competition were very variable. A maximum of 40% of the variance in individual performance was explained by any one of the measures of abundance of neighbors, and, in half of the neighbor species, <10% of the variance in target performance was explained by abundance of neighbors. Competition intensity seems to limit the maximum potential growth of individuals, but actual growth was often below the boundary determined by competition.
Evaluation of Character Displacement Among Plants in Two Tropical Pollination Guilds
Authors: Murray K. Greg , Feinsinger Peter , Busby William H. , Linhart Yan B. , Beach James H. , Kinsman Sharon
Published: May 2006

In cloud forest at Monteverde, Costa Rica, two guilds of bird—pollinated plants exist; on guild pollinated by long—billed hummingbirds, primarily the Green Hermit (Phaethornis guy), and one guild pollinated by short—billed hummingbirds, primarily the Purple—throated Mountain—gem (Lampornis calolaema). Plants were assigned to guilds based on hummingbird visit patterns documented during >4000 plant—hours of field observations, and on identities of pollen grains collected from 600 mist—netted hummingbirds. Other studies indicated that pollination in these plants is often insufficient for maximum seed set. Each guild was examined for character displacement expected within a stable assemblage of plants structured by competition for pollination. (1) By comparing observed flowering phenologies with those obtained through a randomization procedure, we determined whether each species' phenology minimized overlap with the remainder of its guild. (2) We also examined complementarity between phenological displacement and morphological displacement in reproductive structures. Neither guild exhibited pronounced character displacement. (1) In most cases, flowering phenologies were indistinguishable from those generated at random; the few statistically significant departures mostly indicated aggregation, rather than displacement, of flowering seasons. (2) In most cases, morphological similarity was independent of phenological similarity. The only statistically significant result among the studied species was a positive correlation, among long—flowered species only, between rarity and uniqueness of flowering season. We do not conclude that this absence of expected pattern indicates that competition never occurs or that competition is an inconsequential ecological event. Rather, we attribute absence of pattern to the following aspects of biological variability, two of which we have demonstrated in other studies. (1) Within any one year, density—dependent competition for pollination is sporadic, and is not clearly related to flowering season or morphological similarity. (2) The nature of interspecific interactions varies among years, as neither the relative intensities of flowering nor the flowering seasons themselves are consistent from year to year. (3) The nature of interspecific interactions varies with changes in species composition, which occur over short distances. (4) The assemblage of species is probably not stable over long time spans; the species have Gleasonian ecologies that change distribution and abundance faster than natural selection or diffuse competition can screen out improper phenotypes or species, respectively.
Greater Land Bird Densities on Island vs. Mainland: Relation to Nest Predation Level
Authors: George T. Luke
Published: May 2006

Densities of Black—throated Sparrows (Amphispiza bilineata) are greater on Coronados Island in the Gulf of California than on the adjacent Baja peninsula, while those of Verdins (Auriparus flaviceps) and Costa's Hummingbirds (Calypte costae) are not. I tested the hypothesis that the higher sparrow densities on the island were influenced by greater nesting success there. Nesting success for 1984 and 1985 combined was significantly higher on the island than the adjacent Baja California peninsula for the sparrows, but not for Verdins or hummingbirds. Predation on artificial nests placed in sites similar to those used by Black—throated Sparrows was significantly lower on the island. Damage to eggs in artificial nests indicated that birds were responsible were responsible for at least some of the nest failures; correspondingly, densities of avian nest predators were significantly higher on the mainland. These data are consistent with the hypothesis that higher densities of Black—throated Sparrows are due to reduced predation on the island, and suggest that processes affecting density differences between island and mainland locations may differ among species.
Seed Predation and Cross‐Pollination in Mast‐Seeding Beech (Fagus Sylvatica) Patches
Authors: Nilsson Sven G. , Wastljung Urban
Published: May 2006

In this study we examined cross—pollination and levels of insect and vertebrate predation on the seeds of beech as a function of stand size and tree size. Cross—pollination in this wind—pollinated species is positively correlated with stand size and flower density. In two mast years, 6—14% of seeds contained no kernels, and in a nonmast year the figure was 24%. Predispersal destruction of seeds by a moth (Cydia fagiglandana) was 3.1% in a mast year and 38% in a nonmast year. The variation in seed destruction by vertebrates was much less between mast (5.7%) and nonmast (12%) years. Destruction by the moth was negatively correlated with tree size, while that by vertebrates was positively correlated with tree size. Vertebrate seed predation was negatively correlated with stand size. In a large stand, <10% of the seeds were consumed by vertebrates by April in two mast years. Both cross—pollination and seed predation may have been important factors in the evolution of mast seeding in beech.
Seed Predation in Relation to Tree Dominance and Distribution in Mangrove Forests
Authors: Smith Thomas J.
Published: May 2006

The ecology of seed predation was studied in mangrove forests. Controlled field experiments were used to assess the magnitude of predation on mangrove propagules, to test for differences in predation among five species of mangroves, and to examine how differential amounts of predation on seeds are correlated with conspecific dominance and may contribute to observed patterns of tree distribution across the intertidal region. Averaged over all species and forests, 75.7 ± 28.7% (° ± 1 sd) of the seeds used in the experiments were consumed by predators (small crabs belonging to the family Grapsidae). Significant differences existed in the amounts of predation among mangrove species. Avicennia marina was most heavily preyed upon, followed in decreasing order by Bruguiera exaristata, Ceriops tagal, Bruguiera gymnorrhiza, and Rhizophora stylosa. Differences in chemical composition of the propagules accounted fro ≥97% of the differences in amounts of predation among the five species. Nutritive quality of the propagules, expressed as a function of protein, simple sugar, fiber, and tannin content, was more important in explaining the differences than was propagule size. Significant differences also existed among three forest types, which differed in species dominance. Lowest amounts of predation for C. tagal were in the forest where adults of this species were absent, and highest in the forest where the species was most dominant. For A. marina, the two Bruguieras, and R. stylosa, however, amounts of predation were significantly higher in forests where conspecifics were rare or absent and lowest where conspecifics were dominant. For these latter four species, conspecific dominance in the canopy explained ≥63% of the variation in predation among forest types. The pattern of differences in seed predation among forests across the intertidal region indicates that seed predators may play a significant role in determining the intertidal distribution of the mangrove species studied.
Adult Recruitment in Chorus Frogs: Effects of Size and Date at Metamorphosis
Authors: Smith David C.
Published: May 2006

A cohort of tadpoles of the chorus frog, Pseudacris triseriata, on Isle Royale, Michigan, was marked to determine the effect of body size and date at metamorphosis on survivorship to maturity. The cohort was classified at metamorphosis into four categories based on size and date of metamorphosis: large—early, small—early, large—late, and small—late. The number of frogs in each category that returned to breed on the study area was monitored for the following 2 yr. Long larval period and small body size at metamorphosis influenced recruitment to the breeding population by delaying maturity. Frogs that metamorphosed at large size maintained their size advantage at maturity. Large body size and early date at metamorphosis increased survivorship to maturity by enhancing the chance that reproductive size was attained within 1 yr of metamorphosis. Frogs that were recaptured in the 2nd yr after metamorphosis, when all frogs had attained mature size, were from all four categories in the same proportions marked at metamorphosis, indicating that unless breeding during the 1st yr, led to higher mortality of large individuals, survival rate after metamorphosis was not related to size or date at metamorphosis.
Prey Size Influences Female Competitive Dominance in Nestling American Kestrels (Falco Sparverius)
Authors: Anderson David J. , Budde Christa , Apanius Victor , Gomez Juan E. Martinez , Bird David M. , Weathers Wesley W.
Published: May 2006

Body size differences typically influence sibling competition among nestling birds. In many species hatching order determine nestling size rank; however, sexually dimorphic growth may confound this correlation, giving one sex a consistent competitive advantage and leading to differential survival of the sexes. We found that captive female American Kestrel (Falco sparverius) nestlings were larger than male sibling nestmates from shortly after hatchling through fledging. When food delivered by parents was small enough to be seized and swallowed immediately by a nestling, females showed clear competitive superiority over male sibling nestmates in scramble competition. Females also took larger portions than males of small, yet unmonopolizeable, items. Competitive ability and gender were unrelated when parents delivered large, unmonopolizeable prey. Choice experiments demonstrated that parent feeding preferences did not account for the females' advantage. Based on the above, we expected fledging sex ratio to become increasingly female biased as nestling mortality increased, because nestling mortality increases and prey size decreases when preferred prey (unmonopolizeable rodents) are not available. Data from a wild kestrel population in eastern Pennsylvania supported this prediction, showing a significant positive relationship between fledging brood size and representation of males in the brood, although we note an alternative explanation for this result. Female competitive dominance and its interaction with prey size may influence the evolution of sexual size dimorphism and primary sex ratio in this species and other raptors.
Moose Browsing and Soil Fertility in the Boreal Forests of Isle Royale National Park
Authors: Pastor J. , Dewey B. , Naiman R. J. , McInnes P. F. , Cohen Y.
Published: May 2006

Selective foraging by moose on hardwoods and avoidance of conifers alters community composition and structure, which in turn can affect nutrient cycles and productivity. The effect of moose browsing on the nutrient cycles of boreal forests was studied using three 40—yr—old exclosures on Isle Royale, Michigan. Two alternative mechanisms by which moose affect ecosystems were tested: (1) moose depress both the quantity and quality of litter return to the soil, and hence N mineralization and net primary productivity, by browsing on hardwoods and avoiding conifers; (2) moose stimulate N mineralization, and hence net primary productivity, by opening the canopy and by dropping fecal pellets. Soil nutrient availability and microbial activity, including exchangeable cations, total carbon and nitrogen, nitrogen mineralization rates, and microbial respiration rates, were uniformly higher in exclosures than outside. These differences were more significant where browsing intensity was high and less often significant where browsing intensity was low. N mineralization in browsed plots declined with increasing moose consumption rates. Net primary production in exclosures and browsed plots was strongly correlated with N mineralization. N mineralization in turn was positively correlated with litter N return and negatively correlated with litter cellulose content. These differences in litter quantity and quality were caused by an increased abundance of unbrowsed spruce outside the exclosures. Moose pellets alone mineralized less N but more C than soil alone, but pellets combined with soil stimulated N and C mineralization more than the sum of the two separately. However, this did not appear to be sufficient to offset the depression in nitrogen and carbon mineralization in soil resulting from the increased abundance of unbrowsed spruce. We conclude that, in the long term, high rates of moose browsing depress N mineralization and net primary production through the indirect effects on recruitment into the tree stratum, and subsequent depression of litter N return and litter quality. These results suggest that the effects of herbivores on ecosystems may be amplified by positive feedbacks between plant litter and soil nutrient availability.
Post‐Fire Litter Microsites: Safe for Seeds, Unsafe for Seedlings
Authors: Lamont Byron B. , Witkowski E. T. F. , Enright N. J.
Published: May 2006

We explore the effect of post—fire microsites on seed and seedling distribution and hence their potential role in community restoration. A summer wildfire and control burn in a sclerophyll shrubland in mediterranean Australia produced mosaics of physically and chemically contrasting microsites of litter and sand. Most seeds (>75%) of all species released from the burnt canopies fell, or were redispersed by wind, into the litter patches after both fires. Data on microsite characteristics and wind exposure (fire intensity), height of fruits, time of release, and seed properties were required to interpret relative distribution between the litter and sand patches. Seeds remained equally viable (up to 100%) over summer—autumn in the litter and sand and had equally high rates and levels (up to 100%) of subsequent winter germination. However, seedlings were 2—3 times less likely to survive in the litter and survivors were 35% smaller than those in the sand by the end of the first summer. Banksia hookeriana was particularly vulnerable to microsite properties, whereas the needle—leaved Hakea polyathema showed only minor responses. Pre—summer thinning of seedlings in the litter increased survival of the remainder by 2 times and size of the survivors by 31%. The fire—sensitive, small—seeded B. hookeriana had 17 times more seeds in the backburn litter than the resprouting, larger—seeded B. attenuata, which more than compensated for its 3 times greater seedling mortality levels over the dry summer. Recruitment of species prone to density—dependent mortality in the litter was enhanced by the retention of some seeds in the sand where competition for water was minimal, as indicated by the 2.2 times greater stomal conductance of their seedlings in early summer.
Patch Formation and Maintenance in an Old‐Growth Hemlock‐Hardwood Forest
Authors: Frelich Lee E. , Calcote Randy R. , Davis Margaret B. , Pastor John
Published: May 2006

Cause of patch formation was investigated on a 7.2 ha study area in Sylvania Wilderness Area, a primary forest remnant in Upper Michigan comprising a mosaic of hemlock, sugar maple, and mixed—forest patches. Spatial autocorrelation analysis of the stem map indicated that, although most species pairs have a neutral association between canopy trees and understory trees of other species, hemlock and sugar maple canopy trees both have strong positive self association and negative reciprocal association with each other. No species pairs have a positive reciprocal association on regeneration with each other. MOSAIC, a Markov simulation model in which transition probabilities depend on neighborhood species composition, shows that the negative reciprocal association between hemlock and sugar maple of the intensity observed in this study, could lead to spatial separation into monodominant patches over long time periods (3000 yr). The mixed—forest patches are along spatial continua of varying steepness between sugar maple and hemlock patches. Interactions sugar maple and hemlock overstory and understory trees, along with the pattern of invasion of hemlock, provide a reasonable explanation for the patch structure. Pedological, topographical, and disturbance history differences do not coincide with the location of patches within upland forests on the study area.
The Relative Importance of Fuels and Weather on Fire Behavior in Subalpine Forests
Authors: Bessie W. C. , Johnson E. A.
Published: May 2006

Surface fire intensity (kilowatts per metre) and crown fire initiation were predicted using Rothermel's 1972 and Van Wagner's 1977 fire models with fuel data from 47 upland subalpine conifer stands varying in age from 22—258 yr and 35 yr of daily weather data (fuel moisture and wind speeds). Rothermel's intensity model was divided into a fuel component variable and weather component variable, which were then used to examine the relative roles of fuel and weather on surface fire intensity (kilowatts per metre). Similar variables were defined in the crown fire initiation model of Van Wagner. Both surface fire intensity and crown fire initiation were strongly related to the weather components and weakly related to the fuel components, due to much greater variability in weather than fuel, and stronger relationship to the fire behavior mechanisms for weather than for fuel. Fire intensity was correlated to annual area burned; large area burned years had higher fire intensity predictions than smaller area burned years. The reason for this difference was attributed directly to the weather variable frequency distribution, which was shifted towards more extreme values in years in which large areas burned. During extreme weather conditions, the relative importance of fuels diminishes since all stands achieve the threshold required to permit crown fire development. This is important since most of the area burned in subalpine forests has historically occurred during very extreme weather (i.e., drought coupled to high winds). The fire behavior relationships predicted in the models support the concept that forest fire behavior is determined primarily by weather variation among years rather than fuel variation associated with stand age.