bluuebunny/crossref_metadata_2025 huggingface dataset:
Body Size and Coexistnce in Desert Rodents: Chance or Community Structure?
Authors: Bowers Michael A. , Brown James H.
Published: May 2006

We use a simple test to evaluate the null hypotheses that desert rodent communities are composed of species assembled at random with respect to body size. This hypothesis is unequivocally rejected for species in the granivore guild: species of similar size (body mass ratios <1.5) coexist less frequently in local communities and overlap less in their geographic distributions than expected on the basis of chance, suggesting that their co—occurrence is precluded by interspecific competition. When granivore species and members of other guilds are combined in the same analysis, the patterns apparent in granivores diminish or disappear. These results indicate that ability to detect community structure depends to a large extent on thorough knowledge of the organisms and choice of appropriate statistical tests. We suggest that recent claims that many communities are indistinguishable from random associations of species are premature.
Reconstruction of a Mixed‐Species Forest in Central New England
Authors: Oliver Chadwick Dearing , Stephens Earl P.
Published: May 2006

A 0.36—ha area in the Harvard Forest, Petersham, Massachusetts, was intensively analyzed to determine its history. Natural and man—caused disturbances of varying magnitudes occurred periodically in the central New England mixed—species stand. Evidence of two hurricanes and a fire prior to 1803 were found. Between 1803 and 1952, 14 natural or man—caused disturbances of various magnitudes occurred in the area. Large disturbances created new age classes, but small disturbances did not. Species arising together after large disturbances formed a distinct vertical stratification, with northern red oak (Quercus rubra L.) arising after several decades to the dominant canopy. Smaller disturbances to the overstory allowed understory trees such as black birch (Betula lenta L.), red maple (Acer rebrum L.), and eastern hemlock (Tsuga canadensis L. Carr.) to emerge to the dominant canopy. The composition of this forest was more the result of allogenic influences rather than autogenic development.
Bruchid Bettles and Seed Packaging by Palo Verde
Authors: Mitchell Rodger
Published: May 2006

Cercidium floridum pods usually contain one or two seeds. If pods contained three or more seeds, pod biomass would be reduced by 19% and that energy could be diverted into seeds. Intrinsic factors do not explain the wasteful packaging; interactions at a node divert energy into several one— or two—seeded ponds rather than one large pod. The patterns of mortality from three seed predators are: (1) Rodents attack all pods equally; (2) All seeds are equally likely to be attacked by Mimosestes amicus (Bruchidae); (3) Stator limbatus (Bruchidae) attacks through exit holes cut by M. amicus and the entrances rise almost exponentially with seed count. Stator limbatus generally attacks all the seeds of a pod, and selects the larger pods, but pod size varies so much that pods with two seeds cannot be discriminated from each other. Stator limbatus destroys twice as many seeds in two— or more seeded pods which confers an advantage on seeds from one—seeded pods. An egg parasite, Uscana semifumipennis kills °40% of the eggs of both beetles. This mortality, plus a nearly 40°% loss to heat and desiccation, makes it advantageous for M. amicus to put one egg on top of another, thus sacrificing a second egg to double the survivorship of the first egg.
Spatial Patterns of Trees in a Caribbean Semievergreen Forest
Authors: Forman Richard T. T. , Hahn D. Caldwell
Published: May 2006

Spatial patterns of trees >10 cm dbh were studied in a 4—ha forest plot on St. John, United States Virgin Islands, by measuring nearest—neighbor distance for 28 tree species and mapping 5 uncommon species. Tree species diversity was high, with an even distribution of dominance (28 species out of 132 individuals, and H' = 4.37). A clumped distribution pattern with short intertree distances was prevalent. All but five species had average nearest neighbor distances of <25 m and most were <10 m. Indices of dispersion based on variance to mean rations for the 16 most abundant species suggested clumped distributions for 12 of the species, and only one regularly distributed species. Uncommon mapped species were located in distinct clumps of approximately 700—1300 m2, also with small average nearest—neighbor distances. The combination of high species diversity with many clumped distribution patterns and low interindividual distances for both common and uncommon species, suggests that mechanisms other than species—specific predators are important in understanding tree diversity differences between temperate and tropical forests. The importance of microhabitats is suggested. Available published evidences indicates a prevalence of clumped and a paucity of regular distribution patterns for tree species in tropical forests.
Mangabey (Cerocebus Albigena) Movement Patterns in Relation to Food Availability and Fecal Contamination
Authors: Freeland W. J.
Published: May 2006

Due to the structure of the vegetation in Kibale Forest, Uganda, mangabeys (Cercocebus albigena) cannot avoid, and do not try to avoid, defecating on vegetation they may subsequently use in normal feeding and movement activities. Mangabeys defecate throughout the day. Sleeping areas are major sites of fecal contamination. Wet weather removes fecal contamination from leaves within 24 h. Dry weather results in live intestinal protozoa persisting in feces for more than 24 h. Mangabeys were found to travel further, use more area and exhibit less day—to—day overlap in use of area during dry weather than on days of rain. The data suggest that differences in movement and activity on days of rain and no rain are not due to observational error, or to mangabeys' movement being inhibited by rain. I suggest that the difference is due to rain removing fecal contamination from the environment. This allows concentrated use of areas during periods of rain. During dry weather, mangabeys move long distances and use more area, apparently to avoid environmental contamination, and thus frequently encounter rare food items such as fig tress in fruit. Food density could not be shown to influence mangabey use of areas within the home range.
The Relationship Between Cellular Slime Molds and Bacteria in Forest Soil
Authors: Kuserk Frank T.
Published: May 2006

Observations of natural and manipulated populations were used to investigate the regulation of population sizes and community structure in a group of soil amoebae, the cellular slime molds (Dictyosteliida). Correlations of slime mold abundance and distributional patterns with soil bacteria in the field suggest that food supply is a potential factor in the regulation of these species' numbers. One species, Dictyostelium mucoroides, responds most visibly to seasonal changes, spring and fall being peak seasons. When total slime mold number are partitioned into active and encysted forms, amoebae account for as much as 51% and 24% of the population in the fall and spring, respectively, vs. only 10—12% of the population during the summer and winter months. Large additions of various bacteria to field plots caused significant increases in D. mucoroides numbers. Moreover, the ability of this species to respond to a second addition of bacteria, made several days later, depended on its density. High—density populations failed to respond to additional food, whereas those which had already returned to base levels showed increases. These findings support the hypotheses that cellular slime molds are food limited in nature, and that community diversity is due, at least in part, to differential resource utilization by the species in nature.
The Role of Understory Vegetation in the Nutrient Cycle of Forested Ecosystems in the Mountain Hemlock Biogeoclimatic Zone
Authors: Yarie John
Published: May 2006

The biogeochemical role of understory vegetation was investigated in three sites representing a typical topographic sequence of xeric, mesic, and hygric site types characteristics of the Mountain Hemlock Biogeoclimatic Zone. Overstory biomass on the three sites was estimated to be 60.9, 55.7, and 34.5 kg/m2 for the hygric, mesic, and xeric site types, respectively. Average values for understory biomass were: 44.1, 66.1, and 309 g/m2 for the hygric, mesic, and xeric site types. Understory aboveground production (UAP) values of 26.0, 14.2, and 63.1 g°m—2°yr—1 for the hygric, mesic, and xeric site types were equivalent to 11.3, 6.1, and 48.6% of the estimated overstory aboveground production. The understory was found to cycle a much greater proportion of its total nutrient (N, P, Ca, Mg, K, Zn, Cu) standing crop annually compared to overstory. Approximately 80% of the macronutrients present in the understory standing crop are found in the understory annual production on the hygric site type. Estimates of 17.6, 8.3, and 20.6 g°m—2°yr—2 of total understory aboveground litterfall (exclusive of the moss layer) were obtained for the hygric, mesic, and xeric sites, respectively. Understory was shown to return a significant proportion of the litterfall nutrients on a yearly basis, the bulk of which was retuned as a single pulse during the first autumn snowfall. Understory vegetation above the moss layer was shoen to have a significant effect on the quantity of nutrients present in throughfall precipitation. The effect was seasonal with PO4—P, NO3—N, and NH4—N being removed in the spring and Ca, Mg, and K being added to overstory throughfall in the autumn. Modifications of water chemistry previously attributed to the forest floor may in some cases reflect unmeasured influences of understory vegetation.
A Structural Approach to the Function of Buttresses of Quararibea Asterolepis
Authors: Richter Wayne
Published: May 2006

Buttresses of lowland tropical forest trees have long been thought to serve as structural supports. This morphometrical study of the buttresses and other characteristics of Quararibea asterolepis demonstrates and describes the nature of the support role played by buttresses. Buttresses tend to grow on the side of the tree that faces the prevailing winds, thereby functioning as tension members that help prevent the tree from being blown over. The buttresses become proportionately larger as the tree grows, providing the tree with greater support as its crown becomes exposed to the stronger winds of the upper forest canopy. In particular, buttress height increases much faster than tree height. This disproportionate increase reduces by °15% the potential vulnerability of the trunk to snapping or twisting under stress due to winds. On the other hand, no specific relationship independent of tree size can be demonstrated between the length of a buttress and the radius of the crown above or opposite that buttress. There is thus no evidence that buttresses counteract stresses due to crown asymmetries of a particular trees.
Field Water Relations of Sonoran Desert Annuals
Authors: Forseth I. N. , Ehleringer J. R. , Werk K. S. , Cook C. S.
Published: May 2006

Field water relations were examined in winter and summer annuals of the Sonoran Desert. Both groups were characterized by low belowground biomass allocation. Winter annuals had large interspecific variation in water relations parameters, but their leaves generally had high conductances of water vapor and high water potentials during early growth periods. These high values were very short—lived and were reduced by peak flowering periods. Summer annuals were measured midway through the growing season; their leaves showed less interspecific variability in water vapor conductances and had values similar to the winter annuals for this stage of the growing season. Midday and diurnal courses of photosynthesis in the winter annuals exhibited lower rates than values previously reported for plants grown under unlimited water and nutrient regimes in glasshouses. This was probably due to the decreased leaf water potentials and increased vapor pressure deficits present in field situation. Both the winter and the summer annuals showed a dichotomy among species in leaf water potentials, associated with either variations in soil water availability or the ability of leaves to adjust osmotically to decreased soil water availability. Although winter and summer annuals are ephemeral, some species are capable of tolerating low leaf water potentials and therefore are not drought evading in the traditional sense.
Nutrient Dynamics in an Agricultural Watershed: Observations on the Role of A Riparian Forest
Authors: Peterjohn William T. , Correll David L.
Published: May 2006

Nutrient (C, N, and P) concentration changes were measured in surface runoff and shallow groundwater as they moved through a small agricultural (cropland) watershed located in Maryland. During the study period (March 1981 to March 1982), dramatic changes in water—borne nutrient loads occurred in the riparian forest of the watershed. From surface runoff waters that had transited °50 m of riparian forest, an estimated 4.1 Mg of particulates, 11 kg of particulate organic—N, 0.83 kg of ammonium—N, 2.7 kg of nitrate—N and 3.0 kg of total particulate—P per ha of riparian forest were removed during the study year. In addition, an estimated removal of 45 kg°ha—1°yr—1 of nitrate N occurred in subsurface flow as it moved through the riparian zone. Nutrient uptake rates for the cropland are riparian forest were estimated. These systems were then compared with respect to their pathways of nutrient flow and ability to retain nutrients. The cropland appeared to retain fewer nutrients than the riparian forest and is thought to incur the majority of its nutrient losses in harvested crop. The dominant pathway of total—N loss from the riparian forest seemed to be subsurface flux. Total phosphorus loss from the riparian forest appeared almost evenly divided between surface and subsurface losses. Nutrient removals in the riparian forest and thought to be of ecological significance to receiving waters and indicate that coupling natural systems and managed habitats within a watershed may reduce diffuse—source pollution.
Regulation of Freshwater Community Structure at Multiple Intensities of Dragonfly Predation
Authors: Thorp James H. , Cothran Marian L.
Published: May 2006

We examined the role played by predaceous dragonfly nymphs, Celithemis fasciata (Odonata: Libellulidae), in the regulation of the community structure of a benthic macroinvertebrate assemblage in Par Pond, an 1100—ha reservoir in South Carolina. Effects of predation intensity on species richness, evenness, and density wer evaluated by adding zero, two, four, and eight large dragonfly nymphs (antepenultimate and penultimate instars) to previously sieved (0.85—mm mesh) bottom sediment containing benthic macroinvertebrates. Predator and prey assemblages were then placed in individual field microcosms that consisted of polyethylene trays surrounded by underwater screens (mesh <2 mm diagonally) and suspended 15 cm below floating platforms. Twelve replicates of each treatment level were run during each of three 6—wk experimental periods: April—May 1980, August—October 1980, and January—February 1981. In addition, colonization of microcosms by invertebrates was quantified, and samples from natural, unenclosed benthic fauna were collected seasonally along a transect for comparison with experimental assemblages. We tested whether predator enhanced, depressed, both increased and decreased, or had no effect on the complexity of the community structure. The dual effect of predation on community structure is predicted by Connell's "intermediate disturbance hypothesis." Results showed that dragonfly nymphs can significantly influence the structure of the benthic community. However, the results did not show that invertebrate predation is the sole or even the primary regulator of community structure. Species richness was significantly greater at intermediate treatment levels (thus supporting Connell's general hypothesis), but the increase was not great (a range of °10%). The mechanisms by which species richness is maximized at intermediate intensities of predation are not entirely evident, but are probably a combination of prey refuges and nonselective predation with patch switching. In contrast, species evenness, as measured by equitability and by Simpson's index, was greatest at the highest predation level (which does not support the intermediate disturbance hypothesis). Dragonflies appeared to exert a greater influence on prey density than on community diversity.
Distribution, Abundance, and Reproductive Success in the Coral Reef Fish Acanthochromis polyacanthus
Authors: Thresher Ronald E.
Published: May 2006

The relationships between population density, adult size, and various reproductive parameters (including summed reproductive effort and reproductive success) were examined for the planktivorous coral reef fish Acanthochromis polyacanthus. Data were based on multiple censuses conducted over a 2—yr period on One Tree Reef (southern Great Barrier Reef), at 10 sites that cover the range of habitats occupied by the species. Although several reproductive parameters varied significantly over this habitat range, between—site differences in mean reproductive effort per pair were not significant, and mean reproductive success per pair were significantly reduced at only 2 of the 10 sites. Multivariate analysis suggested that heterogeneity among sites in the mean number of broods produced per pair per spawning season is the major determinant of between—site differences in reproductive success, alone accounting for 69% of the among—site variance in reproductive success in 1979—1980, and 67% in 1980—1981. Principal components analysis of nine apparently independent population and fitness parameters over the 10 sites indicates 82.9% of the communal variance can be accounted for by three orthogonal source variables. These appear to relate to food availability, rates of predation, and, most likely, seasonal variation in water temperature. Results are consistent, overall, with the hypothesis that local populations of A. polyacanthus are equilibrial and resource limited. If this hypothesis is correct, adult density is a valid index of habitat quality. Theoretical considerations suggest that this is not likely to be true for most other small, site—attached species of reef fishes.
Effect of Fire and Drought on the Ecophysiology of Andropogon gerardii and Panicum virgatum in a Tallgrass Prairie
Authors: Knapp Alan K.
Published: May 2006

The effect of fire and drought on net photosynthesis, stomatal conductance to water vapor diffusion, leaf water potential, leaf osmotic potential, leaf nitrogen content, and growth were evaluated for the dominant tallgrass prairie species, Andropogon gerardii, and a less common (more mesic) grass, Panicum virgatum, at the Konza Prairie Research Natural Area in northeast Kansas. Early in the growing season in a burned plot, leaves of A. gerardii exhibited greater photosynthesis (maximum CO2 uptake = 41.6 @mmol°m—2@?s—1), leaf conductance, leaf nitrogen content, nitrogen use efficiency, leaf thickness, and shoot biomass compared to shoots from an unburned plot, Postburn differences in P. virgatum were less pronounced, and CO2 uptake rates (maximum = 46.0 @mmol°m—2@?s—1) were not significantly different between burned and unburned sites for this grass. Both grasses had broad temperature optima for photosynthesis, and neither was light—saturated at 2.2 mmol°m—2@?s—1 quantum flux. The response of A. gerardii to burning reflects the documented positive vigor of this grass, and is indicative of a substantial shift in the water, energy, and nutrient dynamics of tallgrass prairie following fire. Due to a midseason drought in 1983, leaf water potentials at midday decreased to < —6.6 and —5.74 MPa in A. gerardii, and —4.53 and —3.51 MPa in P. virgatum in burned and unburned plots, respectively. Lower leaf water potential in the burned site was a result of greater leaf area and transpiration per unit ground area in burned than in unburned prairie. As water stress increased, the osmotic potential at full turgor and at zero turgor decreased more rapidly in the burned plot for both grasses, but minimum values of each parameter (—2.04 and —2.68 MPa in A. gerardii; —2.04 and —2.56 MPa in P. virgatum) were recorded at the end of the season in the unburned plot. When water stress was most severe, photosynthesis decreased to near zero for both grasses at both sites, yet following substantial late season precipitation, photosynthesis increased from 28 to 48% of the early season rates. Attributes of A. gerardii that may contribute to its dominance in tallgrass prairie were the ability of this grass to maintain high rates of carbon gain over a greater range of leaf temperatures and at lower leaf water potential than P. virgatum.
Biogeochemistry of Aquatic Humic Substances in Thoreau's Bog, Concord, Massachusetts
Authors: McKnight Diane , Thurman E. Michael , Wershaw Robert L. , Hemond Harold
Published: May 2006

Thoreau's Bog is an ombrotrophic floating—mat Sphagnum bog developed in a glacial kettlehole and surrounded by a red maple swamp. Concentrations of dissolved organic carbon in the porewater of the bog average 36 mg/L and are greatest near the surface, especially during late summer. This distribution suggest that the upper layer of living and dead Sphagnum and moderately humified peat is the major site of dissolved organic material production in the bog. The dissolved organic material consists mainly of aquatic fulvic acid (67%) and hydrophilic acids (20%); these organic acids control the pH (typically 4 or somewhat lower) of the bogwater. The elemental, amino acid, carbohydrate, and carboxylic acid contents of fulvic acid from the bog are similar to those of aquatic fulvic acid from the nearby Shawsheen River, although the phenolic hydroxyl content of fulvic acid from Thoreau's Bog is higher. The hydrophilic acids have greater amino acid, carbohydrate, and carboxylic acid contents than the fulvic acid, consistent with the hypothesis that hydrophilic acids are more labile intermediate compounds in the formation of fulvic acid.
Regulation of Seed Yield Within and Among Populations of Prunella Vulgaris
Authors: Winn Alice A. , Werner Patricia A.
Published: May 2006

Knowledge of how seed production is regulated by components of seed yield can provide insight into what determines fitness in natural populations. This study examines patterns and sources of variation and interaction among components of seed yield within and among 10 populations of a perennial weed. Much of the variation in total yield among individuals within populations could be explained by differences in plant size. In general, relationships between components of yield were weak, indicating that total yield was not tightly buffered by trade—offs among yield components. The pattern of relationships between yield components differed among populations, demonstrating that these relationships are not a constant characteristic of a species. The patterns of relationship between each component and an estimate of the number of successful seedlings produced also differed among populations, but seed number was generally more important than biomass per seed in determining plant fitness. Yield component means and total yield differed between populations in old—field and woodland habitats. Populations in woodland habitats tended to produce fewer flowers per inflorescence and larger seeds than populations in old—field habitats. Reciprocal transplant and common garden studies showed that these differences were due to phenotypic plasticity. In contrast to the pattern observed within populations, variation among populations in mean yield was buffered by a trade—off between the numbers of flowers per inflorescence and seed mass. The results of this study indicate that the environment plays an important role in determining patterns of seed production and therefore fitness in natural populations. The high degree of phenotypic plasticity in components of seed yield also suggests that response to selection on these traits would be slow.
A Power Analysis for Detecting Trends
Authors: Gerrodette Tim
Published: May 2006

A power analysis allows estimation of the probability of detecting upward or downward trends in abundance using linear regression, given number of samples and estimates of sample variability and rate of change. Alternatively, the minimum number or precision of samples required to detect trends with a given degree of confidence can be computed. The results are applicable to an experimental situation in which samples are taken at regular intervals in time or space. The effects of linear and exponential change and of having sample variability be a function of abundance are investigated. Results are summarized graphically and, as an example, applied to the monitoring of the California sea otter population with aerial surveys.
Prey Depletion by Odonate Larvae: Combining Evidence from Multiple Field Experiments
Authors: Johnson Dan M. , Pierce Clay L. , Martin Thomas H. , Watson Charles N. , Bohanan Robert E. , Crowley Philip H.
Published: May 2006

In this paper we re—analyze previously published data regarding the response of several prey populations to manipulation of predaceous larval dragonfly (Insecta: Odonata) densities in four separate field enclosure experiments. Using a computer—intensive "rerandomization" approach to testing hypotheses, we show that the individual experiments were not sufficiently powerful to consistently reject false null hypothesis. Combining the data from three comparable experiments, we can enhance the power associated with such tests. Three prey categories (Trichoptera., Oligochaeta, and large Cladocera), constituting less than one—third of the typical odonate diet, were found to be consistently depleted in enclosures with odonate larvae; but the extent of their depletion was not increased at high (ambient) compared with low (half—ambient) odonate densities. These results support our previously published conclusions that exploitation competition was not an important phenomenon for odonate larvae in these experiments.
Life History Consequences of Sexual Dimorphism in a Chernetid Pseudoscorpion
Authors: Zeh David W.
Published: May 2006

Sexual dimorphism, in the form of sexual difference in pedipalp size, is highly variable among pseudoscorpions in the family Chernetidae. Male pedipalpal chela size ranges from 60 to 150% of that of the female. This study investigated potential life history costs associated with sexual dimorphism, and assessed genetic variation for male chela size in the chernetid Dinocheirus arizonensis. Results demonstrated that nymphal development is prolonged in males. A causal relationship between pedipalpal size and development rate was suggested by (1) a positive correlation between sexual difference in developmental period and sexual dimorphism in chela size, and (2) the absence of sexual differences in adult body mass after removal of the pedipalps. Parent—offspring regression provided some evidence for additive genetic variation underlying male chela size, but also suggested a high genetic correlation between the sexes for the trait. Life history compromises and ecological parameters (population density, patterns of female emergence/receptivity) appear to be important factors influencing sexual dimorphism. However, the degree to which sexual dimorphism actually evolves may depend critically on both the within—sex genetic variance and the between—sex genetic covariance existing in particular chernetid populations.
Growth of Sagebrush and Bitterbrush Following Simulated Winter Browsing: Mechanisms of Tolerance
Authors: Bilbrough C. J. , Richards J. H.
Published: May 2006

We investigated the herbivory tolerance characteristics of shrubs in a comparative study of two Great Basin shrub species with different abilities to tolerate browsing. Simulated winter browsing treatments were applied to the less tolerant species, mountain big sagebrush (Artemisia tridentata ssp. vaseyana (Rydb.) Beetle), and to the more tolerant species, bitterbrush (Purshia tridentata (Pursh) D.C.), growing in natural stands. These treatments ranged in severity from terminal bud removal to removal of 100% of current annual growth, and were applied to (1) individual branches on shrubs and (2) to entire shrubs. We measured growth responses on entire treated branches or on typical branches when treatment was to the entire shrub. We recorded the fate (long shoot, short shoot, mortality, and in Artemisia only, flowering stem) of all nodes on each measured branch and the length, node production, and estimated biomass of all new long shoots. We also estimated the proportion of resources removed by each simulated browsing treatment. Purshia compensated for the loss of tissue to a greater extent than Artemisia. In comparison to unclipped branches, Artemisia and Purshia both increased the frequency of new long shoots produced in response to simulated browsing. However, Artemisia tended toward reduced long shoot biomass with increased browsing, while Purshia was either insensitive to the severity of browsing treatment or, in one case, increased long shoots biomass after treatment. The contrasting responses were particularly evident in the most severe treatment, which resulted in death of all Artemisia branches but in biomass production equal to controls in Purshia. Additionally, node production declined in Artemisia but in Purshia was equal to or exceeded node production in control unclipped branches. In this study, neither aboveground resource availability nor bud availability fully explained differences in the abilities of Artemisia and Purshia to tolerate browsing. In addition, the results from this and related studies suggest that inherent relative growth rate was not correlated with herbivory tolerance in these species. Developmental plasticity and the resultant altered resource allocation patterns following winter browsing appear to distinguish Artemisia and Purshia in their ability to tolerate browsing.
How Tree Cover Influences the Water Balance of Mediterranean Rangelands
Authors: Joffre Richard , Rambal Serge
Published: May 2006

Dehesa ecosystems of the southwestern Iberian Peninsula are characterized as a savanna—like rangeland dominated by scattered mediterranean evergreen oak trees. We investigated whether isolated trees modify the water balance of this ecosystem and if so, what implications this finding might have on models that assume homogeneity of soil water resources. The water balance of the two ecological components of the dehesas–(1) the tree—grass component, and (2) the open areas between the tree canopies with unshaded grass vegetation–was studied for three consecutive years in three locations in the Sierra Norte de Sevilla region of Andalusia in southern Spain. In this region, annual rainfall was generally between 600 and 800 mm, and the summer drought lasted °130 d. Soil water storage was measured with a neutron moisture gauge outside and under the tree canopy. Deep drainage between two consecutive census dates was calculated using field—measured drainage characteristics. Evapotranspiration (Ea) and surface runoff were computed from the water balance equation assuming that Ea is limited by Penman potential evapotranspiration. Monthly Ea by annual species in open areas was poorly correlated with rainfall levels in the autumn and was limited during the spring by availability of water in the top 40 cm of soil. During summer, monthly Ea by trees ranged from 30 to 50 mm. Mean annual Ea was 400 mm outside and 590 mm under the tree cover. In open areas, water yield (WY), defined as the sum of deep drainage and surface runoff, ranged from 65 to 100% of total Ea, whereas under the tree canopy WY was only 20 to 40% of the Ea. Under the tree canopy, when annual precipitation was <570 mm, WY was negligible and all precipitation was lost by evapotranspiration. Outside the tree canopy, WY occurred as soon as annual precipitation exceed 250 mm. Models of competition between trees and grass generally assume a spatial homogeneity of soil hydrodynamic properties. Our results, however, show that both soil water storage and evapotranspiration are greater for the tree—grass component. Consequently, these models must account for this spatial variability in water resources according to species.
Predicting Methane Emission from Bryophyte Distribution in Northern Canadian Peatlands
Authors: Bubier Jill L. , Moore Tim R. , Juggins Steve
Published: May 2006

A predictive model for bryophyte distribution, water table position, and seasonal mean methane (CH4) emission was developed for two areas of northern peatland: the Clay Belt of Ontario and the Labrador Trough of Quebec. Water table position and CH4 flux were the most important environmental variables in canonical correspondence analyses (CCA) of bryophyte data. Water chemistry constituted a second environmental gradient, independent of hydrology and CH4 flux. Weighted averaging regression and calibration were used to develop a model for predicting log CH4 flux from bryophyte distribution. The model showed an increase in log CH4 flux from hummock to carpet and pool species, corresponding with a decrease in height above the mean water table position. The exceptions were rich—fen pool species, which had low CH4 flux optima in spite of their moisture status. Tolerances were greatest for mid—hummock species and least for carpet and pool species. No overlap in tolerances occurred between hummock and pool species, suggesting that species at either end of the height gradient are the best predictors of CH4 flux. Error analyses showed that bryophytes are equally as effective as water table position for predicting mean CH4 flux even though bryophytes are only surrogates for the degree of anaerobism/aerobism in the peat profile. Bryophytes are distributed in well—defined zones along microtopgraphic gradients; they integrate long—term changes in the water table, which fluctuates on a daily and season basis along with CH4 flux, and may be more easily mapped with remote—sensing techniques. Bryophytes, however, are only useful for predicting CH4 flux within a region; similar species values cannot be extrapolated to other northern peatlands where different climatic and biogeochemical factors may result in different ranges of CH4 emission. The model may be used in paleoreconstructions of methane emission and for biological monitoring of climate change.
Effects of Late Holocene Forest Disturbance and Vegetation Change on Acidic Mud Pond, Maine, USA
Authors: Rhodes Thomas E. , Davis Ronald B.
Published: May 2006

The limnological effects of natural forest disturbance and succession were studied by analyzing the chemistry, charcoal, pollen, chrysophytes, and diatoms in sediments deposited from °3500 to °700 yr BP in Mud Pond, Maine, USA. Mud Pond was chosen for this study because its oligotrophy (mean chlorophyll a = 0.5 mg/L), large ratio of catchment area to lake area (°50:1), and steeply sloping catchment (°12%) are likely to make the lake unusually sensitive to the terrestrial environment. Fine—grained sediments rich in organic matter are interrupted every 100—500 yr (X = 280) by coarser and more minerogenic sediments, reflecting brief pulses of soil erosion. These disturbance horizons contain high concentrations of charcoal, indicating forest fires, low percentages of pollen of late—successional trees such as Tsuga, and high percentage of pollen and spores of early—successional plants such as shrubs and ferns. Within a few decades, pollen percentages of Alnus peak, followed by Betula. Tsuga pollen percentages increase for roughly 200—400 yr after disturbances, and generally stabilize or decrease several decades before the following disturbance horizon. Limnological responses to these terrestrial changes are indicated at disturbance horizons by high percentages of diatoms (e.g., Rhizosolenia sp., Fragilaria spp., and Synedra spp.) and chrysophytes (e.g., Synura spinosa and Mallomonas caudata) of waters of near neutral pH. Diatom—inferred pH typically increases abruptly from °5.0 to °6.1, and chrysophyte—inferred pH from °5.5 to °5.8 at disturbance horizons. These increases may be caused by increased inputs of wood ash and mineral soils rich in base cations after fires or other disturbances. This pulse is followed by a two—step recovery. First, within a few years or a few decades, the inferred pH decreases nearly to the predisturbance condition as PH—circumneutral diatoms are replaced by taxa of waters of pH <5.5 (e.g., Asterionella ralfsii var. americana, and Eunotia spp.) and pH—circumneutral chrysophytes are replaced by taxa of waters of pH <6.0 (largely Mallomonas allorgei and M. duerrschmidtiae). This response likely results from soil stabilization and uptake of base cations by successional vegetation. Second, a slower acidification of °0.1 pH unit per century occurs as early successional shrubs and hardwood forest are replaced by old—growth conifer forest. This process may involve further sequestration of base cations in vegetation, and greater availability of organic acids from the forest floor. Increasing percentages of pollen of Picea, Alnus, and Salix by 1350 yr BP, and decreasing percentages of hemlock and beech around 1350 yr BP imply cooler conditions. Large changes in diatom and chrysophyte assemblages in approximately this period may be related.
Convariation of Spider Egg and Clutch Size: The Influence of Foraging and Parental Care
Authors: Simpson Mark R.
Published: May 2006

The purpose of this study was to investigate the effects of body size, foraging mode, and parental care on the relationship between clutch size and egg size in spiders. Specifically, covariation of egg size and clutch size with body size was investigated, using data from the literature, in 156 species of North American spiders from 22 families. Variance in clutch size and egg size was found to be mainly attributed to differences among genera. I therefore conducted a comparative analysis at the generic level. Results indicated that clutch and egg size were influenced by body size. Spiders from genera with larger body sizes produced larger offspring and more offspring per reproductive event than spiders from genera with smaller sized species. Relative egg number was inversely related to size of eggs produced, indicating a reproductive trade—off. Web building spiders produced more and smaller eggs than cursorial hunting spiders of a similar body size. Furthermore, cursorial spiders that guard and carry their young produced more young than those that only guard the eggs before hatching or do not guard. However, residual clutch size or residual egg size (residuals from regressions of clutch size and egg size, respectively, against female body size) did not differ among parental care groups. This study suggests that the method of food acquisition and type of parental care contributed to the offspring are associated with clutch size and egg size in spiders.
Microtine Rodent Dynamics in Northern Europe: Parameterized Models for the Predator‐Prey Interaction
Authors: Hanski Ilkka , KorpimÄki Erkki
Published: May 2006

We added seasonality to a general predator—prey model to make it a more realistic description of the interaction between microtine rodents (Microtus) and their mustelid predators (Mustela nivalis) in northern Europe. The model parameters were estimated from field data, with the exception of three parameters, for which a range of plausible values was selected. Most parameter combinations generated high—amplitude, chaotic oscillations with a distinct periodic component; the median parameter values predicted dynamics closely resembling the observed 3—5 yr rodent oscillations in northern Fennoscandia. The results were not sensitive to a number of structural changes in the model, suggested by empirical observations: stochastic variation in the prey—carrying capacity, predator refuge at low density, predation on mustelids by large generalist predators in the years when rodents decline, and an alternative assumption about winter breeding in rodents. The model—predicted period and amplitude of oscillations agreed most closely with observations when the model included a low—density refuge for the mustelids. Another predator—prey model with ratio—dependent functional response did not predict dynamics resembling the observed dynamics. The present results provide further support to the hypothesis that the rodent oscillations are maintained by delayed density dependence imposed by specialist predators. Our results also suggest that the dynamics may be chaotic, with a periodic component, rather than a limit cycle with environmental stochasticity.
Developmental Plasticity in the Shell of the Queen Conch Strombus Gigas
Authors: Martin-Mora Elizabeth , James Frances C. , Stoner Allan W.
Published: May 2006

To evaluate the developmental plasticity of shell morphology in the queen conch Strombus gigas, we conducted transplant experiments at five sites in the Exuma Cays, Bahamas. Juvenile conchs from a sixth site were placed in cages at three sites, in an open cage at a fourth site, and in a naturally confined area at a fifth site. All animals spent 8 mo in the field. At the end of the experiments, the morphology of their shells was quantified by means of univariate and multivariate analyses of size and shape as advocated by Mosimann and James (1979). Comparisons were made within and among experimental groups at the first three sites, between groups at the fourth and fifth sites, and between experimental groups and naturally occurring animals at all six sites. Although the experiments extended over only 20% of the juvenile growth period, analyses showed that experimental animals at three of the sites differed significantly in the shapes of their shells. The shapes of these shells were also significantly different from those of the long—spined, bulky shells of naturally occuring animals of similar age at the source site. The greatest change in shape and growth rate occurred at the fifth site, where the rapidly growing uncaged experimental animals developed relatively slender, short—spined shells with large apertures and short spires. These changes were clearly in the direction of the size and shape of unmanipulated animals in the foster population. The results of these experiments, when combined with those of others, suggest that, although variation in the morphology of marine gastropods can have a genetic basis and can be influenced by natural selection, direct environmental induction can be the dominant process underlying patterns of local among—population differentiation. We recommend (1) that the morphometric methods used here to investigate the complex consequences of variation in the size and shape of organisms be applied in other taxa and (2) that the extent to which among—population patterns of geographic variation are directly induced by the environment be given more attention.
Nutrient Uptake from Enriched Soil Microsites by Three Great Basin Perennials
Authors: Black R. Alan , Richards James H. , Manwaring John H.
Published: May 2006

Relative nutrient uptakes efficiency was measured using a dual—radiotracer (32P and 33P) technique for Agropyron desertorum, Artemisia tridentata, and Pseudoroegneria spicata, species from the arid shrub—steppe of the Intermountain Region in the western U.S. These species differed in the timing and magnitude of their response to experimentally enriched soil microsites. The most rapid and significant increase in the uptake of phosphorus tracer from enriched microsites was exhibited by Agropyron, followed by Artemisia, while there was only a trend toward an increase in uptake for Pseudoroegneria. The volume of soil enriched was only 4% of the total pot volume yet Agropyron extracted more phosphorus from the enriched microsite than was removed from the remainder (96%) of the soil volume based on tracer uptake from enriched and unenriched microsites. For Agropyron and Pseudoroegneria, plant uptake of phosphorus from microsites was approximately proportional to the soil microsite enrichment and all three species exhibited increased root length density within the microsites in proportion to the concentration of available nutrients. Integrated physiological and morphological response of the plants to nutrient enrichment increased phosphorus uptake, normalized to the amount of phosphorus available in the soil microsites, by 19.7 to 97.5 x compared to uptake from a similar volume of unenriched soil. Preferential allocation of recently assimilated carbon to roots within enriched soil microsites was also demonstrated. The most important differences detected among species were associated with the temporal flexibility they exhibited in response to enrichment of soil microsites. Differences in flexibility and response among these species have been reported for other aspects of nutrient uptake, suggestive of species—specific differences contributing to demonstrated differences in productivity, persistence, and competitive ability.
Ontogenetic Scaling of Competitive Relations: Size‐Dependent Effects and Responses in Two Anuran Larvae
Authors: Werner Earl E.
Published: May 2006

This study examines size—dependent competitive interactions between larvae of two species of ranid frogs, the bullfrog (Rana catesbeiana) and the green frog (R. clamitans). Surveys of naturally breeding populations on an experimental pond site indicated that the two species often compete in the larval phase, and because both species may overwinter as larvae, interactions can occur between disparate size (year) classes. Using an additive design, I conducted field experiments manipulating densities of four species—class combinations (year I and II bullfrogs and green frogs) in the presence of low numbers of the other classes. The design permitted estimates of the effect of manipulated species/classes on different target classes and, because the latter were held constant, the effects of manipulated classes on themselves. Strong competitive effects were found within and between species/classes with one important exception. Neither class of green frogs affected small bullfrogs despite the fact that both had strong effects on large bullfrogs. Both classes of bullfrogs, however, effected both classes of green frogs. In general, smaller classes had a greater per unit biomass effect on target classes than larger classes and tolerated competitive effects better. Independent of size effects, bullfrogs appeared to be better competitors than green frogs because of other traits, presumably greater activity levels. I interpret these results in terms of a simple model of the ontogenetic scaling of foraging costs and gains. This model enables a mechanistic interpretation of the above results through the two components of competitive interactions, effect and response. Larger classes have greater per capita effects on resources, but because the gain curve is decelerating, smaller classes have greater per unit biomass effects. On the other hand, smaller classes respond less to competitors because net gain for these classes is higher or affected less by a decline in resources. I argue that to understand size—structured interactions in many taxa, we must first isolate the underlying ontogenetic or size—scaling component of competitive relations, and then add to these relations the effects of unique species traits.