bluuebunny/crossref_metadata_2025 huggingface dataset:
A Comparative Demographic Analysis of Clonal Reproduction in a Temperate Soft Coral
Authors: McFadden Catherine S.
Published: May 2006

In order to evaluate the relative importance of asexual and sexual reproduction to the fitness of a clonal organism, matrix projection models were used to quantify the contributions of each of these reproductive modes to population growth of the soft coral, Alcyonium sp., a species that undergoes frequent colony fission. The demographic and fitness consequences of eliminating either asexual or sexual reproduction from the life cycle of this species were examined by sensitivity analysis and by altering selected entries in the transition matrices to simulate changes in reproductive allocation. Four populations of Alcyonium sp. were monitored photographically for two years to record colony growth, mortality, fission, sexual reproduction, and larval recruitment, Despite high turnover, population densities remained reasonably constant. The 24—52% mortality was matched approximately by recruitment of daughter colonies produced by fission. Sexual reproduction was infrequent, and no larval recruitment was observed. The frequency of both fission and sexual reproduction increased with increasing colony size, while mortality decreased with increasing size. Size—class transition matrices constructed from the demographic data were analyzed by the methods of Caswell (1986, 1989). In both years of the study the size distributions of colonies observed in the field did not differ from the stable size distributions predicted from the projection models. Eliminating sexual reproduction from the life cycle did not alter the predicted stable size distributions; eliminating fission shifted the size distributions of all four populations towards the larger size classes. Reproductive value increased with increasing colony size. Damping ratio, °, a measure of the rate of convergence to the stable size distribution, increased with increases in both fission and sexual reproduction, suggesting that rapidly growing populations are more stable than slowly growing populations. Projected growth rates (=relative fitness, ‐l) of the four populations ranged from 0.66 to 1.15. Sensitivity analyses indicated that sexual reproduction contributes < 1% of the value of ‐l, while > 40% of l is accounted for by transitions between the upper size classes in the population, either by fission or growth. Eliminating sexual reproduction from the life cycle had a negligible effect on fitness (‐l); eliminating fission greatly reduced ‐l and suggested rapid extinction for all populations. This result held even when sexual reproductive output was increased by an order of magnitude. The results of these simulations suggest that selection for increased @l will increase fission at the expense of sexual reproduction in this species.
Estimating Gap Origin Probabilities for Canopy Trees
Authors: Lorimer Craig G. , Frelich Lee E. , Nordheim Erik V.
Published: May 2006

A method is presented for estimating the probability that existing canopy trees were growing in gaps during the sapling stage, given observed mean 5—yr radial growth rates starting at the point where the trees were 4 cm diameter at breast height (dbh). Required calibration data are frequency distributions of radial growth rate for existing saplings that are currently suppressed in the forest understory vs. those of saplings growing in gaps. Threshold growth rates for high confidence of gap origin are calculated in an iterative fashion using growth rate data and interpretation of radial increment patterns. The method is illustrated with data from sugar maple (Acer saccharum) and eastern hemlock (Tsuga canadensis) in mature and old northern hardwood forests of western Upper Michigan. The proportion of canopy trees that had been growing in gaps during the sapling stage ranged from 35—67% in different stands. This wide range suggests the adaptability of these two late—successional, shade—tolerant species to a variety of stand microenvironments.
On the Statistical Significance of the Hutchinsonian Size‐Ratio Parameter
Authors: MacNally Ralph Charles
Published: May 2006

A technique is presented for the analysis of sets of morphological data to test whether sympatric, closely related species form geometric series (or Hutchinsonian series, where the constant of proportionality, h, is 1.30 for linear characteristics). A basic two—parameter model is proposed. The logarithmically transformed version of the geometric model is used because of its simplicity, and the greater probability that variances will be homogeneous. The analysis provides methods for estimating the goodness of fit of a set of measurements to the model, a maximum likelihood estimate for h, h, and a 95% confidence interval for h. The mean, standard deviation, sample size, or, if possible, raw data for each species in the array are required. The present paper demonstrates the importance of using a method that intrinsically depends upon intraspecific variation. The assumptions of many existing analyses that are based only upon means or their ratios, such as homoscedasticity and negligible variances, generally are invalid. This result is damaging in more general terms because large variances often mean wide morphological overlap, even if means do differ. The method demonstrated that only a small fraction of collections of syntopic series conformed to the geometric model, and of these, few had confidence intervals for h in which 1.30 was included.
Impact of Shoot Galler Attack on Sexual Reproduction in the Arroyo Willow
Authors: Sacchi Christopher F. , Price Peter W. , Craig Timothy P. , Itami Joanne K.
Published: May 2006

The assumption that herbivores cause reduction of plant fitness is central to plant herbivore theory, but empirical support from field studies is scant. We tested the hypothesis that the shoot—galling sawfly Euura lasiolepis significantly influences the fitness of its obligate host, the arroyo willow (Salix lasiolepis). Surveys of reproductive attributes on field willow clones at different elevations indicated that the presence of shoot galls led to a significant reduction in reproductive buds, and thereby inflorescences, on individual shoots on both male and female plants. Shoots with galls suffered a loss of 43% of their reproductive buds relative to shoots without galls. Inflorescence size, measured as flowers per inflorescence, seed mass, germination success, and pollen germination success were not significantly affected by gall presence. Two sawfly exclosure—enclosure experiments on plants that had been established from cuttings from clones showed reduced reproductive bud production on plants with galls at both the whole—plant and individual—shoot levels; whole—plant losses of 34—56% and per—shoot losses of 17—55% of reproductive buds on different clones were coupled with densities that varied from 0.56 to 2.70 galls per shoot. Most plants suffered relatively small reproductive losses because of low sawfly densities. However, 20% of all plants suffered losses of 10%, while 7.5% experienced losses of 20% or more of their reproductive potential. A small willow clone attacked over the range of 0.5—2.0 galls per shoot would lose 381 000—1 500 000 seeds of a potential 3 760 000 seeds in the year of attack. Genetic variation among plants in susceptibility to sawfly attack and subsequent herbivore—related reproductive loss was suggested by a common garden study; some clones had 12 times as many galls as others.
The Response of Annuals in Competitive Neighborhoods: Effects of Elevated CO2
Authors: Bazzaz F. A. , Garbutt K.
Published: May 2006

Four members of an annual community were used to investigate the effects of changing neighborhood complexity and increased CO2 concentration on competitive outcome. Plants were grown in monoculture and in all possible combinations of two, three, and four species in CO2—controlled growth chambers at CO2 concentrations of 350, 500, and 700 μL/L with ample moisture and high light. Species responded differently to enhanced CO2 level. Some species (e.g., Abutilon theophrasti) had increased biomass with increasing CO2, while others (e.g., Amaranthus retroflexus) had decreased biomass with increasing CO2 concentration. In mixtures, species tended to interact strongly, and, in some cases, the interaction canceled out the effects of CO2. Furthermore, there were cleared differences in species behavior in different competitive mixtures as assessed by total biomass and seed biomass, and by an index of response to neighbors. In general, competitive arrays that had C3 species depressed the response of C4 species, especially Amaranthus. Ambrosia artemisiifolia was the strongest competitor in this assemblage. Strong statistical interactions between CO2 and the identity of the competing species in mixtures were found to be primarily due to the as yet unexplained response of plants with CO2 at 500 μL/L. The potential effects of CO2 on community structure could be profound, particularly at the intermediate levels of CO2 that are predicted to be reached during the first half of the next century.
Zooplankton Grazing in a Eutrophic Lake: Implications of Diel Vertical Migration
Authors: Lampert Winfried , Taylor Barbara E.
Published: May 2006

During summer and fall, depth profiles of zooplankton community grazing were determined in situ during day and night in the Schohsee, a small eutrophic lake. Labeled algae of two different sizes were mixed with the natural suspension of phytoplankton in a grazing chamber. A small blue—green alga (Synechococcus, I μm) was labeled with 32P; a larger green alga (Scenedesmus, 4—15 μm)was labeled with 14C. Grazing and vertical migration of zooplankton were strongly coupled. During summer, grazing in the upper 5 m was negligible during day but strong at night. Hence, algae grow relatively unimpeded by grazing during daytime but are harvested at night. Vertical and diel differences in grazing rates disappeared when the vertical migration ceased in fall. Selectivity of grazing was controlled by the zooplankton species composition. Eudiaptomus showed a strong preference for Scenedesmus (electivity index D = 0.9). Daphnia showed a slight preference for Scenedesmus, but Ceriodaphnia preferred Synechococcus. Cyclopoid copepodites did not ingest the small blue—green. Because Daphnia and Eudiaptomus were dominant, grazing rates on larger cells were usually higher than grazing rates on the small cells. Negative electivity indices for Scenedesmus occurred only when the biomass of large crustaceans was extremely low (near the surface, during day). Zooplankton biomass was the main factor controlling both vertical and seasonal variations in grazing. Highest grazing rates (65%/d) were measured during fall when zooplankton abundance was high. Differences between day and night in the grazing activity per unit biomass were not statistically significant. Laboratory experiments showed considerable losses of tracer from preserved animals, and loss rates differed for the two radioisotopes. Because such differential losses can produce substantial errors in the results, it was necessary to process the samples on the boat immediately after collection, without preservation.
Antiherbivore Chemistry of Larrea Tridentata: Effects on Woodrat (Neotoma Lepida) Feeding and Nutrition
Authors: Meyer Michael W. , Karasov William H.
Published: May 2006

During winter, creosote bush (Larrea tridentata) dominates the diet of desert woodrats (Neotoma lepida) in the California Mojave Desert. The stems and leaves of creosote foliage are coated with large quantities of phenolic resins (10—25% of dry mass). We investigated whether woodrats feeding on creosote were influenced behaviorally by the resin and whether ingestion of creosote resin reduced the digestibility of protein as hypothesized by Rhoades and Cates (1976). Preference trials demonstrated that woodrats consistently selected creosote plant parts of low resin content. Removal of the resin with diethyl ether ameliorated this feeding preference and increased overall palatability of creosote foliage. Addition of the resin to laboratory chow ration caused this food to be avoided also. When woodrats were fed rations of creosote foliage or resin—treated laboratory chow no reduction in protein digestibility occurred. However, dry matter intake was reduced when woodrats were fed chow containing 12% resin. Our results indicate that creosote resin does deter feeding by desert woodrats. The antiherbivore mechanism of creosote resin to desert woodrats is likely one of toxicity and not digestibility reduction. This toxicity may limit the amount of creosote foliage woodrats can consume, which would explain the winter body mass decline and mortality that we observed in woodrats on our study site. We discuss the impact plant defense chemistry may have had on the evolution of Neotoma in the southwestern deserts of North America.
Community Organization of Erigeron Glaucus Folivores: Effects of Competition, Predation, and Host Plant
Authors: Karban Richard
Published: May 2006

Experiments were conducted to determine the relative importance of interspecific competition, predation, and host plant quality on the success of the common herbivores of Erigeron glaucus, the seaside daisy. Leaves of E. glaucus were attacked by spittlebugs (Philaenus spumarius), plume moth caterpillars (Platyptilia williamsii), and thrips (Apterothrips secticornis). Each species of herbivore was affected strongly by only one of the biotic factors examined. For spittlebugs, damage caused by caterpillars increased rates of desiccation and mortality of nymphs. This competitive effect probably caused the negative association between late—instar spittlebug nymphs and caterpillar damage wherever both species were encountered along the California coast. Plume moth caterpillars were most strongly affected by predator exclusion, at least during the late spring. Chicken—wire cages that excluded birds resulted in greater numbers of caterpillars on single rosettes and on whole clonal mats. Passerculus sandwichensis, Savannah Sparrows, were likely responsible for this effect which was only found during the season when adult birds were feeding insects to nestlings. On San Miguel Island, Savannah Sparrows were absent, densities of caterpillars were greater than on the mainland, and caging had no effect on caterpillar survival; these observations were all consistent with the hypothesis that Savannah Sparrows were important predators on the mainland. The factor that was most important for each herbivore species did not interact with other biotic factors. The relative ranking of the factors for each species was similar among the 3 yr of the experiments, with only one exception. Observations suggested that experiments conducted at Bodega Bay were representative of this community at other locations in California where all of these species were present. A scheme is proposed as a first step in predicting the importance of various biotic factors in herbivore community organization. If effects of predation, parasitism, or physical disturbance are very strong, host plant related competition and host quality are likely to be unimportant. If predation and disturbance are relatively less important, factors that reduce the choices available to an individual herbivore make host quality and interspecific competition more likely to play a significant role.
Host Genotype Choice by the Ball Gallmaker Eurosta Solidaginis (Diptera: Tephritidae)
Authors: Anderson Stephen S. , McCrea Kenneth D. , Abrahamson Warren G. , Hartzel Lori M.
Published: May 2006

We examined Eurosta solidaginis host choice among 38 naturally occurring clones of tall goldenrod, Solidago altissima. Significant variation was found in the percentages of (1) ramets ovipunctured, (2) punctured ramets with eggs, (3) ramets galled, and (4) punctured ramets that formed galls. These findings indicate that all genotypes of S. altissima are not equally suitable (capable of supporting larval development and/or acceptable (having the proper cues to stimulate oviposition). The two most resistant and two of the most susceptible of 38 clones were monitored for ovipuncturing the following season. A total of 244 apical buds on ramets were dissected at intervals of 1, 2, and 3 wk after oviposition. Significantly more larval mortality occurred in the meristems of resistant clones than in the meristems of susceptible clones. Further, 73% of the dead Eurosta larvae were surrounded by necrotic plant tissue, apparently due to a hypersensitive response by the plant to the gallmaker's stimulus. Finally, a common garden experiment using replicates of 30 clones of known resistance showed that susceptible clones were growing faster at the time of oviposition.
Influence of Growth from on Ecophysiological Responses to Variable Sunlight in Subalpine Plants
Authors: Knapp Alan K. , Smith William K.
Published: May 2006

Periods of variable sunlight due to intermittent cloudcover are common in the subalpine zone of the central Rocky Mountains. To evaluate the ecophysiological significance of daily variations in sunlight, responses in net photosynthesis, stomatal conductance to water vapor, and water use efficiency (CO2 uptake per unit H2O lost) were measured in 12 subalpine species including trees, shrubs, and herbs. Measurements were made during intervals of alternating full sun and shade throughout a growth season that included daily and seasonal periods of water stress. In general, photosynthesis responded rapidly in all growth forms to changes in sunlight due to clouds, while stomatal conductance responded more slowly or not at all. Large reductions in photosynthesis were measured during clouds periods for all species, while stomatal conductance was considerably less responsive to changes in sunlight in woody species. The greatest and most variable reductions in stomatal conductance during shade (<10 to a 68% reduction of full sun levels) occurred in herbaceous species. Carbon gain in the woody species was enhanced because stomatal conductance remained relatively high during both sun and shade periods. Water use efficiency in most species was generally lower during shade periods compared to full sun periods. However, woody plants maintained higher xylem pressure potentials that were much nearer shade values compared to herbs. The greater decrease in stomatal conductance and photosynthesis in herbs during shade resulted in improved water use efficiency, although carbon gain was reduced during both shade and the subsequent full—sun period. These apparent growth strategies during non—steady—state response periods appear opposite to generalizations made previously for woody vs. herbaceous species under steady—state conditions.
Variation in Herbivore Response to Chemical Defenses: Zooplankton Foraging on Toxic Cyanobacteria
Authors: Kirk Kevin L. , Gilbert John J.
Published: May 2006

Some planktonic cyanobacteria possess chemical defenses (toxins) or morphological defenses (filamentous morphology) against herbivory by zooplankton; Anabaena affinis is both toxic and filamentous. The presence of A. affinis has a differential effect on various zooplankton taxa, causing reduction in the population growth rate of large cladocerans or rotifers. Here, we investigate the importance of differences in zooplankton foraging patterns in determining the differential inhibitory effect of A. affinis. Radiotracer feeding experiments were conducted on four cladoceran species (5 clones, and 15 body sizes), two rotifer species, and one calanoid copepod species. With one exception (the cladoceran Ceriodaphnia dubia), the ingestion rates of zooplankton on unicellular phytoplankton (Cryptomonas) were not reduced by the presence of A. affinis. Behavioral observations showed that cladocerans sometimes rejected A. affinis filaments but did not simultaneously reject Cryptomonas cells. Thus, the differential inhibitory effect of A. affinis is not caused by mechanical interference with feeding. When offered Cryptomonas and A. affinis simultaneously, small cladocerans, rotifers, and the copepod fed more selectivity than large cladocerans and avoided ingesting toxic filaments. As a result, large cladocerans received higher mass—specific doses of the toxin. Different tendencies to ingest toxic filament, perhaps combined with different physiological sensitivities to the toxin, cause the differential effect of A. affinis on various zooplankton taxa. This variation in herbivore response to cyanobacterial defenses may change the species composition and size structure of zooplankton communities.
Plant Protection as a Consequence of an Ant‐Membracid Mutualism: Interactions on Goldenrod (Solidago Sp.)
Authors: Messina Frank J.
Published: May 2006

In central New York, two chrysomelid beetles, Trirhabda virgata and T. borealis, frequently cause severe defoliation of tall goldenrod, Solidago altissima. This plant is also the primary host of Publilia concava (Membracidae), a sap—feeding treehopper that is tended by ants, especially Formica spp. Staged encounters indicate that Formica ants attack adult Trirhabda beetles on goldenrod stems bearing membracids. Such stems escape defoliation by Trirhabda, and show greater mean height increment and seed production than their nearest neighbors without ants. The degree of plant protection depends on the duration of Formica presence. During Trirhabda outbreaks, only stems bearing Formica ants for most of the season are likely to produce flowers and seeds. While Formica ants do not exclude Trirhabda larvae from goldenrod stems, they do deter feeding; plants with Formica ants experience significantly less defoliation by larvae than neighboring stems without ants. Two smaller, less aggressive an species (Prenolepis imparis and Myrmica sp.) do not affect either larval or adult beetle densities.
Postfire Succession of the Herbaceous Flora in Southern California Chaparral
Authors: Keeley Sterling C. , Keeley Jon E. , Hutchinson Steve M. , Johnson Albert W.
Published: May 2006

Postfire succesion of the temporary herbaceous and suffrutescent cover was studied after chaparral fires in San Diego County, California, USA. Four categories of species make up the temporary cover. (1) "Generalized herbaceous perennials" are present before and after fire. Populations of these herbs are sparse under the shrub canopy. They resprout after fire from bulbs or other underground parts and postfire populations are sparse. (2) "Generalized annuals" are present in openings before fire but produce their peak population size in the first few years after fire. (3) Specialized "fire—annuals" are more or less restricted to the 1st postfire yr. (4) Specialized "fire—perennials" (subshrubs) are uncommon before fire, establish from seed in the 1st postfire yr and reach maximum cover in the 3rd and 4th yr. Community—level changes in cover and diversity are interpreted in light of differences in population dynamics of the four groups. Specis richness was highest in the 1st yr after fire because this was the only time all four groups were present together. Throughout succession herbaceous species richness was positively related to herb cover, negatively related to elevation and unrelated to slope aspect. The number of annual species fluctuated greatly through succession at all sites, but the number of herbaceous perennials did not. Herb cover fluctuated markedly from year to year and was positively related to amount of annual precipitation and negatively related to subshrub or "fire—perennial" cover. Artificial seeding with annual rye grass Lolium multiflorum had not apparent effect on total herb cover since sites with poor Lolium establishment had as high or higher herb cover as sites with high Lolium establishment. Lolium success was at the expense of the native cover and this negative effect was greatest on the "fire annuals."
Conditions for a Species to Gain Advantage from the Presence of Competitors
Authors: Stone Lewi , Roberts Alan
Published: May 2006

The interaction between two species is usually assigned as though they were in isolation from all other species. Here we use a (known) method that determines species interactions more realistically, within the framework of the community to which they belong. This "inverse" method evaluates all the effects that one species experiences from another, both direct and indirect. We use this method to study the classical (though highly controversial) "competition community," where each species is considered (in the "isolated pair" approach) to suffer from the presence of every other. The model we use takes account of the fluctuations in interaction coefficients that one must expect in the real world, both from one species pair to another, and as the effect of ambient environmental variations. Remarkably, the "inverse" method finds that generally a high proportion (20—40%) of the interactions must be beneficial, or "advantageous," when not lifted out of the community context in which they actually occur. The contrary case, called here "hypercompetitive," in which each species suffers from every other species, can occur only if the environment is nearly constant, and the species closely akin to each other, with both of these conditions holding and persisting to a degree that must be considered implausible. The results of the model remain valid, even after incorporating a number of major structural modifications, thus indicating robustness in the predictions. We survey the available field data and show that they are in good general agreement with the conclusions reached, on the high proportion of interactions which must be "Advantageous in a Community Context" (ACC).
Ontogenetic Habitat Shifts in Bluegill: The Foraging Rate‐Predation Risk Trade‐off
Authors: Werner Earl E. , Hall Donald J.
Published: May 2006

The bluegill sunfish (Lepomis macrochirus) undergoes several habitat shifts between the littoral and pelagic zones of small lakes during its life history. After hatching in the littoral zone, bluegill fry migrate to the pelagic zone to feed on zooplankton. In this study the fry then returned to the littoral zone in four different lakes at a relatively constant size of °12.5 mm standard length. Several years are spent feeding in the littoral zone vegetation before the bluegill again shifts to feeding on zooplankton, first in the water column above littoral vegetation and subsequently in the true pelagic zone. This shift to feeding on zooplankton occurred at a specific body size within a lake, but the size ranged from 50 to 83 mm among five different lakes. The size at which the shift occurred was directly correlated with the density of the major predator of the bluegill in these lakes, the largemouth bass (Micropterus salmoides). We demonstrate that the bluegill is faced with a growth (or feeding) rate—predation risk trade—off in making these habitat shifts. Analysis of stomach contents, the growth of small fish caged in the pelagic zone, and predictions of foraging rates in both habitats all indicate that the pelagic zone is the more profitable habitat for all size classes of the bluegill. Through a series of pond experiments we further show that risk of predation by largemouth bass was 40—80 times as great for small bluegills in the open water as for those in the vegetation. We interpret the patterns of habitat use by the bluegill in terms of a model that explicitly weights the costs (predation) and benefits (growth) of making a size—specific habitat shift to the pelagic. Finally, we discussed evidence that the bluegill can facultatively respond to changes in feeding rates and predation risk, and the consequences of such ontogenetic habitat shifts to community dynamics.
Density‐Dependent Demography in Two Grasses: A Five‐Year Study
Authors: Fowler Norma L.
Published: May 2006

To determine the strength, frequency, and nature of density—dependent processes in natural plant populations, the densities of two perennial grass species, Bouteloua rigidiseta and Aristida longiseta, were perturbed in an otherwise undisturbed community. Both seed additions and adult removals, of each species separately, were used. Individual plant survival, growth, and reproduction were monitored, as well as recruitment to the population and the numbers in each size class of each species, for 5 yr. The responses to the experimental perturbations were small relative to the magnitude of the perturbations, indicating that density dependence in demographic parameters in both populations was weak throughout the period. The results therefore do not support the suggestion of Fowler (1986) that the weak density dependence in the Bouteloua rigidiseta population in the 1st yr of the experiment was a temporary result of drought. Significant responses to the density manipulations were scattered among annual intervals, characters, and replicates. This may be simply a result of testing responses at the limits of detectability, or it may reflect intermittent and scattered density dependence. Density effects were generally much smaller than other differences among quadrats and differences among plants within quadrats, indicating that other factors had much more effect on a plant's fate.
Effects of Gap Size on Litter Decomposition and Microbial Activity in a Subtropical Forest
Authors: Zhang Qishui , Zak John C.
Published: May 2006

We examined the effects of gap size on rates of mass loss of leaf, branch and bark litter, and microbial activity in a Castanopsis kawakamii forest over a 360—d period. Fiver forest gap size classes that compromise a gap size gradient were chosen: (1) closed canopy of pure Castanopsis kawakamii vegetation; (2) small gaps with a diameter of < 5 m; (3) small—to—intermediate gaps with a diameter of 5—15 m; (4) intermediate—to—large gaps with a diameter of 15—30 m; (5) large gaps with a diameter of > 30 m. After 360 d, plant litter decomposing under closed canopy or within the small gaps lost mass more rapidly than those in large gaps. Among the litter quality parameters measured, initial N concentrations were most strongly positively correlated with the annual decay rates, and among the microclimatic factors, soil moisture content was the best predictor. Total substrate—induced respiration (SIR) was highest under closed canopy, intermediate in small gaps, and lowest in large gap size classes. Within gap size classes, total microbial respiration was highest for leaves, intermediate for bark, and lowest for branches. Across all sample dates, soil moisture content was the best predictor of microbial activity among the measured microclimatic factors. Plant litter C/N ratios were best predictors of microbial activity among the measured litter quality indices. Decomposition rate constants were linearly correlated with overall mean values of microbial activities for all three types of decomposing litter within five different gap size classes (R2 = 0.984). The SIR rates of decomposing plant litter as a measure of potential active microbial biomass reflected the microclimate and litter quality. Our results suggest that large gaps significantly reduce microbial activity and decomposition rates by changing environmental conditions that consequently should reduce nutrient cycling rates in this system.
The Trade‐Offs Between Digestion Rate and Efficiency in Warblers and Their Ecological Implications
Authors: Afik Daniel , Karasov William H.
Published: May 2006

Frugivory in birds is associated with rapid gut passage whereas insectivory is associated with slower gut passage. This is interpreted by some avian ecologists to reflect an inherent digestive constraint on diet selection, but it could also result from dietary acclimation. We predicted that Yellow—rumped Warblers (Dendroica cornata) acclimated to fruit—, insect—, and seed—based diets would exhibit retention time that increase in that rank order, because this is the rank order of retention time across species that eat these types of food. We also predicted a trade—off between rate of processing (the inverse of retention time) and extraction efficiency. This is based on the assumption that digestive enzymes or absorptive capacity occur in the gastrointestinal tract at levels that are not in great excess, and so less contact time between enzymes and digesta reduces the extraction efficiency. To test these predictions, we measured retention time (using aqueous and lipid—phase inert markers) and extraction efficiency of glucose, sucrose, leucine, starch, and the lipid glycerol trioleate (using radio—labeled nutrients and inert markers). Our results were in accordance with predictions. Mouth—to—anus total mean retention time (TMRT) of Yellow—rumped Warblers acclimated to fruit—, insect—, and seed—based diets were, respectively, 46 + 9, 62 + 6, and 114 + 9 min for polyethylene glycol (PEG, an aqueous marker): results were similar for the lipid phase marker in most cases. But Yellow—rumped Warblers that were switched suddenly to an alternate diet did not readjust TMRT when tested 2 h later or did so incompletely. We found no diet—related morphological changes in the digestive tract, and thus attribute these results to changes in gut motility. Extraction efficiency was uniformly high across all diets for glucose (88 + 1%), but varied among diet groups according to out prediction for leucine (range 82—94%), sucrose (58—85%), starch (9—48%), and lipid (18—82%). We review how features of the Yellow—rumped Warbler's system permit relatively high energy uptake across a wide variety of food types. The most notable constraining feature was a low starch hydrolysis rate, probably restricting them from relying on starchy foods. Thus, digestive strategy (i.e., a combination of retention time of food in the gut and digestive efficiency) somewhat determines diet, but in more respects diet determines strategy.
Food Supply and Hatching Spans of Birds: Energy Constraints or Facultative Manipulation
Authors: Wiebe Karen L. , Bortolotti Gary R.
Published: May 2006

Clutches of altricial birds generally hatch over a period of hours or days. Numerous hypotheses have been generated to explain hatching asynchrony, but most experiments have focused on the nestling stage to determine some post—hatch benefit such as increased reproductive success. We concentrated on proximate causes in the prelaying period, which have largely been ignored. In northern Saskatchewan, we censused prey species (small mammals) in early spring and measured the asynchrony of 273 clutches of the American Kestrel (Falco sparverius) over 4 yr. Hatching asynchrony was quantified in two ways: time for all the eggs within a clutch to hatch and mass hierarchies among nestlings. In good food years, the hatchling spans of clutches were more synchronous than in poor years. Similarly, parents on good territories or females in good physical condition had more synchronous broods than parents under food stress. In an experiment, we supplemented food to kestrels during the prelaying period. Parents that were supplemented laid larger eggs and hatched those eggs more synchronously, but control pairs were more successful at raising young to the fledging stage. Synchronous hatching when food is abundant is consistent with the hypothesis of facultative manipulation of hatching spans in relation to nestling food supply, but not with the energy constraint hypothesis.
Linking Local Species Interactions to Rates of Speciation in Communities
Authors: McPeek Mark A.
Published: May 2006

Regional species diversity limits the diversity of local communities by defining the pool of species that are available to colonize sites. Biogeographical processes that influence speciation and extinction rates determine the size and composition of this regional species pool. Community ecologists are beginning to recognize the importance of these biogeographical processes in influencing diversity patterns among local communities, but the potential for local interactions to influence biogeographical processes, especially speciation, has been little considered. In this paper I discuss one mechanism by which variation in the strengths of local interactions can influence the potential for population differentiation and thus for influencing speciation rates in the allopatric model of speciation. When more than one selective agent acts on the phenotype, the shape of the overall fitness surface depends critically on the relative strengths of the various selective agents. If one selective agent, which imparts strong selection, acts in all populations of a species, population differentiation via adaptation to local ecological conditions or via differentiation of sexual systems is retarded because the overall fitness surface in all populations strongly resembles the shape of the strongest selective agent. Consequently, the potential for speciation is reduced. Alternatively, if selective agents in various populations impart relatively equitable strengths of selection, the potential for differences in the overall fitness surfaces among populations is enhanced, which will enhance the potential for population differentiation and thus speciation. Analogous results obtain when multiple selective agents impact genetically correlated characters. Because the strength of selection generated by a species interaction should increase with the strength of the interaction, and because fewer species can usually coexist when the strengths of interactions are greater, the number of coexisting species and the potential for speciation in component taxa may covary among communities. This analysis indicates that the relative strengths of interactions can be as important to diversification in communities as the number of niche dimensions along which differentiation can occur.
Impact of a Parasitic Plant on the Structure and Dynamics of Salt Marsh Vegetation
Authors: Pennings Steven C. , Callaway Ragan M.
Published: May 2006

We investigated the effect of a native parasitic plant, Cuscuta salina, on the structure and dynamics of the plant community in a California salt marsh. Cuscuta was common in the middle marsh zones. The abundance of Cuscuta was positively correlated with the abundance of Limonium californicum at two sampling scales (0.25— and ≈\approx40—m2 quadrats). Sampling at the scale of individual plants indicated that the dominant plant in the marsh, Salicornia virginica, was preferred by Cuscuta as a host over Arthrocnemum subterminale, Limonium californicum, and Frankenia salina. This result was confirmed with host—choice experiments in the field. Based on spatial correlations and host—choice experiments, we hypothesized that Cuscuta indirectly facilitated Limonium and Frankenia, increasing plant diversity and initiating vegetation cycles. This hypothesis was supported by sampling patches with different histories of Cuscuta infection. Patches with recent heavy Cuscuta infection had reduced Salicornia biomass and increased Limonium and Frankenia biomass relative to controls. This effect was most pronounced at higher elevations and in larger, older patches. The hypothesis was also supported by sampling permanent quadrats repeatedly over time. The probability that Cuscuta would invade a quadrat increased with increasing cover of Salicornia. Quadrats containing Cuscuta increased in Limonium and Frankenia cover between censuses relative to quadrats lacking Cuscuta. Our results indicate that parasitic plants can have strong effects on the structure and dynamics of natural vegetation assemblages. However, these effects are mediated by physical and biological gradients across the landscape.
Density‐Dependent Dynamics of Soft Coral Aggregations: The Significance of Clonal Growth and Form
Authors: Karlson Ronald H. , Hughes Terence P. , Karlson Susan R.
Published: May 2006

In clonal plants and animals, stolons and runners often promote rapid directional growth and escape from crowded microhabitats. Here we evaluate the effects of density on clonal growth and dispersal by stolons, on colony mortality, and on recruitment in the soft coral Efflatounaria sp. This colonial organism forms dense aggregations on mid—shelf and outer reefs of the Great Barrier Reef, Australia, where it is subjected to frequent physical and biological disturbances. Stolonal growth and asexual recruitment of new colonies (by budding) were enhanced by experimentally reducing local density. Within unmanipulated aggregations of Efflatounaria, per—capita rates of asexual recruitment were higher at low density, but colony survivorship was lower. Furthermore, the effect of density on stolonal growth and dispersal of daughter colonies varied as a function of a colony's history of disturbance. Disturbance was simulated by detaching from the substrate a newly budded colony that was still connected by a stolon to the parent colony. At low density, these pairs of partially detached colonies moved apart, while at high density, stolonally connected colonies moved closer together. Our results suggest that Efflatounaria employs a plastic life—history strategy that promotes recovery from injuries and the formation of dense aggregations. At low density, stolons facilitate rapid directional growth, asexual recruitment, and aggregation. At high density, clonal growth is inhibited, and mortality rates are greatly reduced. Enhanced survival within aggregations provides the adaptive context for interpreting the influence of density and disturbance history on the population dynamics of this clonal organism.
The Population Dynamics of Brucellosis in the Yellowstone National Park
Authors: Dobson Andrew , Meagher Mary
Published: May 2006

The role that pathogens play in structuring ecological communities needs to be examined from both a theoretical and empirical perspective. This paper provides a brief introduction to the ecology and epidemiology of brucellosis in Yellowstone National Park. Observed patterns of infection for brucellosis in bison in national parks in the United States suggest that Brucella has a relatively sharply defined threshold for establishment and that the proportion of the host population infected increases as a relatively simple function of population density. Unfortunately, the threshold population for Brucella establishment in bison is low, a herd of at least 200 individuals; this makes it very hard to eradicate Brucella from wild populations. Simple calculations based on observed prevalence and recruitment data for the Yellowstone bison herd suggest that unacceptably high levels of culling would be required to eradicate brucellosis in the park. A simple mathematical model is used to describe aspects of the population dynamics of Brucella. The analyses suggest that an understanding of infectious disease dynamics is crucial to the management and conservation of wild and domestic ungulate species in and around national parks and other nature reserves. The consequences of disease control for the maintenance of biodiversity in other natural and captive communities need to be better understood.
Effect of Temperature on the Response of Planktonic Rotifers to a Toxic Cyanobacterium
Authors: Gilbert John J.
Published: May 2006

Laboratory experiments show that temperature can play an important role in modifying the effect of toxic cyanobacteria on freshwater, planktonic herbivores. A reproductive—rate assay assessed the effect of temperature on the response of Brachionus calyciflorus and Asplanchna girodi to Anabaena flos—aquae and its endotoxin, anatoxin—a. Using a bootstrap technique, reproductive rates (λ from rm0 in experimental and control treatments were determined from complete life tables, and responses were expressed as mean ratios of these rates (experimental rate as a fraction of control rate) with asymmetric 95% confidence limits determined nonparametrically from distributions of bootstrapped ratios. Thus, at all temperatures, exposure to treatments was over a standard period of physiological or development time–one lifetime or generation. In rotifers acclimated for many generations to low (12° — 14°C), intermediate (19°C), and high (25°C — 26°C) temperatures, susceptibility to the cyanobacterium and its toxin increased significantly with temperature; ratios of λ values were 1.5—2 times greater at the low than at the high temperatures. The results indicate that seasonal increases in water temperature, and climate warming, may exacerbate the impact of toxic cyanobacteria on rotifers and perhaps other zooplankton taxa.
New Approaches to the Analysis of Population Trends in Land Birds
Authors: James Frances C. , McCulloch Charles E. , Wiedenfeld David A.
Published: May 2006

Although there is strong evidence that a few Neotropical migrant land birds have severely declining populations, we know of no evidence of general overall declines. One widely cited analysis of data from the Breeding Bird Survey (BBS), the only continental—scale monitoring program for land birds in the breeding season, indicates declining populations in eastern forests in the 1980s, but other analyses conclude that most species have had stable or increasing populations in the last 25 yr. These conflicting results call for a new perspective. Here, we propose two approaches to the analysis of BBS data that emphasize the detection of temporal and geographic variation in population trends. The first approach uses nonlinear regression to detect temporal changes in population trends, and permits the construction of graphs that compare nonlinear trends among regions. The second approach provides statistical tests of multispecies patterns of geographic variation in population trends. We report analyses of BBS data for 1966 to 1992 for the 26 species of wood warblers (Parulinae) for which the most data are available and the chance of obtaining reliable estimates is best. Using pooled data for all 26 species for eastern and central North America, nonlinear regression indicated declining populations in the Appalachian Mountains and the Eastern Foothills in the 1970s, and stability elsewhere. As examples of the use of this first approach in single—species analyses, we give maps, graphs, and statistical tests to illustrate variation in population trends by physiographic strata in the Blue—winged Warbler (Vermivora pinus) and the Ovenbird (Seiurus aurocapillus). As an example of the second approach, we applied a probit—normal model to a matrix of increases and decreases in the 26 species in 37 geographic strata. The analysis identified specific highland areas as having unusually high proportions of declining populations. These results suggest that correlates of elevation should be considered as factors possibly regulating populations of warblers and other land birds. Our perspective is that focus on studies of temporal and geographic variation in population trends of all birds, including Neotropical migrants, could help conservation biologists identify where species and groups of species are in the most trouble.