bluuebunny/crossref_metadata_2025 huggingface dataset:
Microhabitat Use by Heteromyid Rodents: Effects of Artificial Seed Patches
Authors: Price Mary V. , Waser Nickolas M.
Published: May 2006

Coexisting heteromyid rodent species differ in their affinities for foraging microhabitats. Bipedal forms (Dipodoyms, Microdipodops) use open spaces preferentially, while quadrupedal forms (Perognathus) use spaces under or near vegetation. Despite consistent reports that these preferences occur in heteromyid assemblages throughout North American deserts, and despite frequent speculation about the importance of foraging economics and predation risk in producing them, few experimental analysis of microhabitat use exist. Here we describe studies showing that (1) microhabitats distinguished by heteromyids in nature differ in seed abundance, soil particle size, and soil density, all of which are features that have been shown to influence heteromyid foraging efficiency; (2) four coexisting species differ in their preferences for artificial seed patches in a large laboratory foraging arena, and differences in the properties of preferred and nonpreferred patches correspond qualitatively to differences in the attributes of preferred and nonpreferred microhabitats in nature; and (3) microhabitat use in nature, as measured by livetrapping, can undergo rapid shifts that track the location of preferred artificial seed patches placed either in open spaces or under vegetation. These results suggest that divergent microhabitat specializations of coexisting heteromyids are in part functions of divergent preferences for the particular combinations of seeds and soils found in various microhabitats. Further experimental studies are needed, however, to determine whether these preferences can be predicted accurately from knowledge of the economics of foraging, and to what extent other factors may also influence microhabitat choice.
Bipedal Hopping and Seed‐dispersion Selection by Heteromyid Rodents: The Role of Locomotion Energetics
Authors: Thompson Steven D.
Published: May 2006

Selection of seeds with different dispersion patterns, facilitated by the energetic differences between bipedal—hopping and quadrupedal heteromyid rodents, has been proposed as a mechanism of resource allocation for microsympatric species. To test several aspects of this hypothesis, field observations of foraging behavior and laboratory measurements of locomotion costs were made for several species of heteromyids. Neither the field observations nor the laboratory measurements supported predictions generated by the seed—dispersion hypothesis. Locomotion costs of the bipedal—hopping Dipodomys deserti and D. merriami were similar to those predicted for quadrupeds of equal mass. At low speeds, the cost of quadrupedal locomotion was not greater for a small, hopping biped than would be expected for a similar—sized quadruped. Free—living Dipodomys deserti, D. merriami, and Perognathus longimembris spent most of their foraging time either stopped or moving at slow speeds. High—speed locomotion, at which energetic "savings" by bipeds should be greatest, was rare. These results led to the conclusion that the energetic costs of different locomotory modes do not play a major role in resource allocation for Mojave Desert heteromyids.
Experiments on Disturbance in Old‐Field Plant Communities: Impact on Species Richness and Abundance
Authors: Armesto J. J. , Pickett S. T. A.
Published: May 2006

Changes in species richness and abundance following experimental disturbance were studied in a 7th—yr old field (the “older” field) dominated by Solidago canadensis, and 2nd—yr old field (the “younger” field) dominated by Ambrosia artemisiifolia. The two fields were disturbed in late spring 1981 by clipping and removing aboveground plant biomass in experimental plots, which were compared with undisturbed control plots. Treatments represented three intensities of disturbance (low, intermediate, and high), as defined by amount of biomass removed and extent of canopy disruption. Different patterns of change in richness were observed in each field during the 1981 growing season. In the older field the number of species decreased in control plots during the summer. In disturbed plots, richness increased with intensity of disturbance, resulting in maximum richness at the highest disturbance intensity during mid— and late summer. Several species increased significantly in cover and/or frequency in plots representing intermediate and high intensity of disturbance. In the young field, species richness fluctuated in control plots, with peaks in early and late summer. In disturbed plots, richness decreased with intensity of disturbance in early summer and increased in midsummer. By late summer no difference in richness or species composition was detected between control and clipped plots. These patterns of change in richness can be explained by the different effect of treatments on the vegetation structure and by the role of the dominant species in each field. In the bi—layered older field, disturbances greatly disrupted the closed canopy of S. canadensis, thus providing resource patches (i.e., increased light, moisture) for suppressed and invading species. In the younger field, the impact of disturbance was only detectable in early and midsummer, and changes in richness were mainly related to the seasonal cycle of the dominant annual A. artemisiifolia. Results from these experiments provide insights into the mechanisms of species coexistence under natural disturbance regimes comparable to our disturbance treatments. They also indicate that the impact of disturbance on species richness depends largely on the nature of the dominants and rates of successional or seasonal change of the community.
Forest Litter Decomposition in Relation to Soil Nitrogen Dynamics and Litter Quality
Authors: McClaugherty C. A. , Pastor John , Aber J. D. , Melillo J. M.
Published: May 2006

Decomposition and changes in nitrogen and organic—chemical content of six types of forest litter were studied for 2 yr in five adjacent Wisconsin forests. The five forests were floristically dissimilar, being dominated respectively by sugar maple (Acer saccharum), white oak (Quercus alba), bigtooth aspen (Populus grandidentata), white pine (Pinus strobus), and hemlock (Tsuga canadensis), Nitrogen mineralization rates in the five stands ranged from 29 to 125 kg°ha—1°yr—1. Decomposition rates of transplanted sugar maple leaves and red maple (Acer rubrum) wood were not correlated with nitrogen mineralization rates in all five stands, indicating that nitrogen mineralization rates do not affect initial decomposition rates. However, mineralization rates were correlated with decomposition rates of the native dominant foliage litter. Nitrogen first accumulated in all litters, but by the end of the 2—yr incubation period nitrogen release had begun in all foliage litters. Nitrogen concentrations increased approximately linearly with cumulative mass loss but eventually declined in some foliage litters. Neither maximum amount of nitrogen accumulated nor amount accumulated per gram of litter mass loss was related to the rate of soil nitrogen mineralization. Chemical composition of litter affected decomposition rates and patterns. Soluble substances and litters relatively rich in solubles disappeared rapidly during early stages of decomposition. Eventually, slowly disappearing acid—soluble and acid—insoluble substances dominated the pattern of mass loss in all litters.
Autumnal Resorption and Accretion of Trace Metals in Gallery Forest Trees
Authors: Killingbeck Keith T.
Published: May 2006

Autumnal resorption and accretion of copper (Cu), iron (Fe), zinc (Zn), and manganese (Mn) were measured in the foliage of five gallery forest tree species on the Konza Prairie Research Natural Area. Presenescence and postabscission leaves from five trees each of Quercus macrocarpa, Q. muehlenbergii, Fraxinus pennsylvanica, Celtis occidentalis, and Ulmus rubra were sampled. Three species resorbed 19, 25, and 26% respectively, of their presenescence foliar Zn, and one species resorbed 35% of its presenescence foliar Fe. This validates the prediction made by Whittaker et at. in 1979 that Zn and Fe are withdrawn from the scenescing foliage of at least some deciduous species. Net accretions of Cu (43, 44, 69%), Fe (36, 40%), and Mn (19.57%) occurred during the same period. The two oak species were responsible for most of the resorption, while the three non—oak species accounted for all of the significant accretion. Such well—defined differences in element conservation may influence interspecific competition by accentuating, or compensating for, species differences in element uptake ability and element use efficiency. Demand: availability ratios proved useful in predicting the likelihood that a given element would be conserved through resorption.
Recruitment and Natal Philopatry of Wood Ducks
Authors: Hepp Gary R. , Kennamer Robert A. , Harvey William F.
Published: May 2006

We tested whether hatch date and body mass at hatching affected recruitment of female Wood Ducks (Aix sponsa) to a breeding population in South Carolina. During six breeding seasons, day—old ducklings (n = 2945) were individually marked before leaving the nest. Females were recaptured later while nesting, and recruitment averaged 5.2% with no yearly differences. Most (82%) of the returning females nested as yearlings, but drought conditions in one year postponed the breeding of some females until they were 2 and 3 yr old. Most (60%) females did not return to nest on natal areas, but dispersed to sites a short distance away (mean = 1.6 km). Probability of dispersal was not related to competition for nesting space, and dispersing females did not begin nesting later than females that remained on natal areas. There was no evidence that early—hatched females had greater probabilities of being recruited than late—hatched females. In fact, late—hatched females had greater recruitment in two of six years than females that hatched early in the season. There also was no relationship between hatch date and the time that female Wood Ducks initiated their first nests. Heavy ducklings at hatching had higher probabilities of being recruited in one of six years, but composite statistics showed no overall relationship. We suggest that hatch date affects recruitment more at northern latitudes where time for producing and rearing offspring is short. Individuals hatching late at high latitudes may have insufficient time to mature before they must begin autumn migration.
Modeling Vertebrate Dispersal Distances: Alternatives to the Geometric Distribution
Authors: Miller Gary L. , Carroll Bradley W.
Published: May 2006

The geometric probability distribution is a common way by which to model frequency distributions of vertebrate dispersal distances. This model has two weaknesses: (1) it requires that the probability of stopping, p, is constant across the range of habitat units, and (2) it provides no mechanism for modeling the maximum dispersal distance. The weaknesses of this model are discussed and a family of alternative models, which allows the stopping probability to change and which takes into account the maximum dispersal distance (dispersal scale) is presented. The new models in their simplest form reduce to the goemetric probability distribution model. The new models are applied to five sets of published field dispersal data to determine the general properties of their fit as compared to the geometric and to assess their potential as better models of dispersal patterns.
Mapping Regional Forest Evapotranspiration and Photosynthesis by Coupling Satellite Data with Ecosystem Simulation
Authors: Running Steven W. , Nemani Ramakrishna R. , Peterson David L. , Band Larry E. , Potts Donald F. , Pierce Lars L. , Spanner Michael A.
Published: May 2006

Annual evapotranspiration (ET) and net photosynthesis (PSN) were estimated for a mountainous 28 x 55 km region of predominantly coniferous forests in western Montana. A simple geographic information system integrated topographic, soils, vegetation, and climate data at a 1.1—km scale defined by the satellite sensor pixel size. Leaf area index (LAI) of the forest was estimated with data from the NOAA (National Oceanic and Atmospheric Administration) Advanced Very High Resolution Radiometer (AVHRR). Daily microclimate of each cell was estimated from ground and satellite data and interpolated using MT—CLIM, a mountain microclimate simulator. A forest ecosystem simulation model, Forest—BGC, was used to calculate ET and PSN daily for each cell. Ranges of estimated LAI (4—15), ET (25—60 cm/yr), and PSN (9—20 Mg°ha—1°yr—1) across the landscape follow the trends expected in both magnitude and spatial pattern. These estimates compared well with field measurements of related variables, although absolute validation of these predictions is not possible at large spatial scales.
Habitat and Community Modification by An Introduced Herbivorous Snail
Authors: Bertness Mark D.
Published: May 2006

Experimental removal of the introduced herbivorous snail Littorina littorea from a protected New England rocky beach resulted in rapid habitat and community changes. At normal snail densities, L. littorea grazing bulldozes sediments from hard substrate and precludes the presence of an algal canopy. Snail removal resulted in rapid sediment accumulation and the development of an algal canopy, which accelerated sedimentation and bound sediment to hard substrate. These changes led to the increased success of organisms characteristic of soft—sediment habitats, such as polychaetes, tubiculous amphipods, mud crabs, and mud snails, and decreased success of organisms characteristic of hard—substrate habitats, such as barnacles and encrusting algae. Snail removal also significantly influenced the success of the marsh grass Spartina alterniflora. L. littorea consumes the shoots and rhizomes of marsh grass, as well as mediating sediment accumulation, which is necessary for vegetative expansion of root mat. Removal of L. littorea resulted in expansion of the littoral area dominated by S. alterniflora, as well as increased productivity of the marsh grass. These results suggest that the North American invasion of the European periwinkle has altered habitats and communities in protected littoral waters. Prior to the introduction of L. littorea, soft—bottomed littoral habitats and fringing salt marsh environments may have been more common that they are presently. Herbivorous snails in general may have an important habitat—modifying effect in protected marine communities that has not been appreciated previously.
An Analysis of the Components of Chaoborus Predation on Zooplankton and the Calculation of Relative Prey Vulnerabilities
Authors: Riessen Howard P. , O'Brien W. John , Loveless Brad
Published: May 2006

We formulated a model for Chaoborus predation on zooplankton and analyzed its various components for fourth—instar Chaoborus americanus and three prey species (Daphnia pulex, Bosmina longirostris, and Diaptomus pallidus). This model is based upon the encounter rates between predator and prey, the probability of a first strike by Chaoborus, and the probability of ingestion given a first strike. This latter probability is a function of the strike efficiency, contact efficiency, and second—strike probability of the predator with each prey. The strike volume of Chaoborus was variable in both size and shape for the different prey species. In each case, however, it could be closely approximated by a cylinder. This strike volume was largest for Daphnia and smallest for Bosmina. The strike efficiency of Chaoborus (probability that a strike will result in a contact) was not significantly different for the three prey types, indicating that the different evasion behaviors of the prey do not influence this parameter. The contact efficiency (probability that a contact will result in a capture) on Daphnia was significantly lower than for the other ability that a contact will result in a capture) on Daphnia was significantly lower than for the other prey; this is a function of the differing sizes and shapes of the prey. Second—strike probability (probability that if a prey is initially missed by a Chaoborus strike, it will remain in the strike volume and be attacked again) was significantly different for each of the three prey species tested, with Bosmina > Daphnia > Diaptomus. This is mainly the result of the different evasion behaviors of the prey. Our model predicts that Chaoborus is equally efficient at capturing each prey once a first strike occurs. Differential vulnerabilities of these prey to Chaoborus predation are therefore the result of differing encounter rates, which are influenced by both the swimming speeds of the prey and the strike volume of the predator.
High‐Temperature Responses of North American Cacti
Authors: Smith Stanley D. , Didden-Zopfy Brigitte , Nobel Park S.
Published: May 2006

High—temperature tolerances of 14 species of North American cacti were investigated. A reduction in the proportion of chlorenchyma cells taking up a vital stain (neutral red) and reduced nocturnal acid accumulation were used as indicators of high—temperature damage. All species tolerated relatively high tissue temperatures, the mean maximum tolerance being 64°C, with an absolute maximum of 69° for two species of Ferocactus. Such tissue tolerances to high temperature may be unsurpassed in vascular plants. Morphological features can affect tissue temperatures. Specifically, thin—stemmed species such as the cylindropuntias attain lower maximum temperatures under identical microclimatic conditions than do more massive species; they also tend to be less tolerant of high—temperature stress. Stem diameter changes of three species of columnar ceroid cacti along a Sonoran Desert latitudinal transect were previously attributed to adaptation to progressively colder temperatures northward. Such changes can also be interpreted as a morphological adaptation to high temperatures, particularly in the southern Sonoran Desert. Interspecific differences in high—temperature tolerance may account for distributional differences among other species, e.g., the coastal—sage—inhabiting Ferocactus viridescens was less tolerant of high temperature than were the three Ferocactus species that inhabit desert regions. Acclimation of high—temperature tolerances in response to increasing day/night air temperatures was observed in all 14 species, especially at higher growth temperatures. From 40° day/30° night to 50°/40°, the tolerable tissue temperatures increased an average of 6°. Half—times for the acclimation shifts were 1—3 d. Although cacti attain extremely high tissue temperatures in desert habitats, tolerance of high temperatures and pronounced acclimation potential allow them to occur in some of the hottest habitats in North America.
Co‐Occurring Tree Species Change Rank in Seedling Performance with Resources Varied Experimentally
Authors: Latham Roger Earl
Published: May 2006

Parallel hypotheses are advanced: (1) the distribution of resources in mesic upland temperate forests is patchy at a scale that is fine for adult trees and coarse grained for seedlings and (2) co—occurring tree species differ in abilities to exploit resources, resulting in differences in species rank order, by absolute growth rates of seedlings, among different resource combinations. Spatial variation in growth rate ranking among species may translate, through competition, into patterns of relative species abundances of seedlings that could persist in the community composition of adult trees. Here I report the results of factorial greenhouse experiments that address the second hypothesis. Seedlings of six co—occurring tree species of temperate eastern North American upland forests showed strong interspecific differences in growth rates and patterns of biomass allocation in response to variation in light and mineral nutrients. The ranks of four species in growth measures associated with competitive ability, absolute rates of change in stem height, total leaf area, and root biomass, varied significantly among nutrients treatments. Seedlings of American chestnut, Castanea dentata, ranked highest in traits associated with competitive ability over the broadest range of combinations of resource levels.
Life Histories in an Epifaunal Community: Coupling of Adult and Larval Processes
Authors: Bingham Brian L.
Published: May 2006

Marine invertebrates growing epifaunally on red mangrove (Rhizophora mangle) prop roots in the Indian River, Florida, USA, were studied in a small mangrove island (Jim Island) through which a number of channels had been cut. Roots hanging down into the water supported diverse epifaunal communities including sponges, oysters, barnacles, bryozoans, and acidians. To determine what factors control species' population dynamics and contribute to the high degree of spatial heterogeneity characteristic of communities in the unique habitat, two hypotheses were tested: (1) Distributions of species on the roots are controlled by differential growth and mortality due to physical features; and (2) Recruitment, as influenced by larval supply, structures the community. Four channels of the island were chosen for comparison and experimentation. Distributions of abundances of epifauna in the channels were determined and physical parameters (i.e., temperature, pH, salinity, flow, turbidity) were measured over a 13—mo period. Adult and early juvenile organisms were transplanted among channels and growth and survival were monitored. Patterns of water flow in the island were studied, and plankton samples were taken to determine how larval supply varied among channels in different parts of the island. General patterns or recruitment were measured for 9 mo and patterns were compared to adult distributions. Epifaunal cover differed among the study channels, with dramatic differences in abundance and species diversity. Except for flow, physical factors did not differ significantly among channels. Flow rate, per se, was not responsible for disjunct distributions since neither adult nor juvenile survival (processes expected to be most affected by flow) differed among channels. Plankton samples and recruitment measurements revealed that the importance of larval supply depended on the life history of the individual species. Those producing short—lived lecithotrophic larvae showed patchy distributions that were strongly affected by the location of source populations and prevaling patterns of water flow. Species with long—lived planktotrophic larvae were more evenly distributed and post—settlement processes played a more important role in their populations dynamics. On large temporal or spatial scales, the effects of physical factors on juvenile and adult organisms are probably quite important in controlling epifaunal distributions. However, in this study, the distributions of organisms on Rhizophora mangle root within Jim Island were best explained by differential larval input, with larval life history determining the strength of coupling between adult populations, larval supply, and patterns of recruitment.
Nesting Frequency and Success: Implications for the Demography of Painted Turtles
Authors: Tinkle Donald W. , Congdon Justin D. , Rosen Philip C.
Published: May 2006

Nesting ecology and reproduction of painted turtles (Chrysemys picta) in southeast Michigan were intensively studied from 1975 to 1978. The average clutch size of Michigan painted turtles was 7.55, with body size accounting for only 9—13% of the variance. Data on nesting frequency indicate that from 30 to 50% of the females possibly do not reproduce every year and that °6% reproduce twice in a given year. Predation within 48 h of egg—laying is responsible for the failure of 20% of the nests. An additional 12% nest failure is due to various other causes. These data substantially alter the life table previously reported for this population of painted turtles.
Small‐Scale Variability in the Demography of Transplants of Two Herbaceous Species
Authors: Fowler Norma L. , Antonovics Janis
Published: May 2006

Individuals of Plantago lanceolata and Salvia lyrata, both herbaceous perennial species, were transplanted into plots of otherwise unmanipulted vegetation. Individuals came from one of 12 half—sibships (common mother). Individuals were marked and leaf and inflorescence numbers counted at intervals for 14 mo. Consistent differences among plots, in survivorship, size, and reproduction, were found in both species, although the plots were only metres apart. Causes of this spatial variation are discussed. Evidence for genetic and genetic x environmental variation in Plantago lanceolata is suggestive but not conclusive.
Experiments on Competition and Predation Among Shrimps of Seagrass Meadows
Authors: Coen Loren D. , Heck Kenneth L. , Abele Lawrence G.
Published: May 2006

A series of experiments was conducted to evaluate the distribution and abundance patterns of two species of macrophyte—inhabiting caridean shrimps found in Apalachee Bay, Florida, USA. Experiments using Palaemonetes vulgaris and Palaemon floridanus included microhabitat association and behavioral interactions. Predator—prey relationships, as they apply to habitat complexity and interspecifc interactions, were also examined using the pinfish, Lagodon rhomboides, a natural predator. Pt. vulgaris individuals were excluded from the experimental habitats by P. floridanus over a range of densities. We conclude that their nonoverlapping microgeographical distribution is primarily a consequence of interspecific competition rather than just microhabitat selection. Predation success on single species was inversely related to the physical complexity of the experimental habitat. However, when placed together, a significantly greater proportion of Pt. vulgaris is consumed than P. floridanus. We posit that the displacement of Pt. vulgaris from cover by P. floridanus significantly increases the former's risk of detection and capture. The importance of refuges in regard to: (1) seagrass predator strategies; (2) prey tactics, and (3) species abundance and richness patterns in macrophyte—dominated communities are discussed.
Species Interactions and Community Structure in Alaskan Seabird Colonies
Authors: Whittam Thomas S. , Siegel-Causey Douglas
Published: May 2006

Previous studies of colonial seabirds suggest that similarities among geographically distinct seabird communities, in terms of ecologically matched species, are strong evidence for structure in these bird assemblages. We quantify species associations and community structure within five nesting guilds of Alaskan seabirds using multiway contingency tables and log—linear models. The observed species combinations differ in frequency from those expected using an independent assortment model. Simple models that adequately describe the data structure contain mostly positive two—way interactions between species. Higher order interactions, although uncommon, may suggest a type of diffuse competition. A ranked measure of resource overlap is uncorrelated with the magnitude of species interactions within guilds, but is positively correlated with deviations from independence among all species pairs.
Patchiness, Density, and Aggregative Behavior in Sympatric Allochronic Populations of 17‐Year Cicadas
Authors: Simon Chris , Karban Richard , Lloyd Monte
Published: May 2006

Densities of two allochronic, geographically overlapping populations of periodical cicadas were censused in the eggnest stage in order to test the hypothesis that larger populations can be supported when the root—feeding nymphs are of different ages. In a frequently burned chest—high stand of scrub oak (Quercus ilicifolia) on Long Island, Magicicada septendecim emerged in 1974 (Brood XIV), then again in 1978 (Brood I) from under the same individual plants over an area of about 1.2 x 3.5 km. Adjacent to this, in the same vegetation, periodical cicadas emerged in 1974 but not in 1978. A nested scheme of sampling revealed significantly higher total eggnest densities when Broods I and XIV occupied the same ground than when Brood XIV existed by itself. The major component of variation in density was among quadrats within plots. That is, cicadas exhibited patchiness on a small scale (metres) which was not reflected in patchiness on a larger scale (tens of metres). Microspatial distributions were thus patchy in both Brood I and XIV, yet dense patches of one brood were usually not correlated with dense patches of the other. Apparently the vegetation appears about as uniform to the cicadas as it does to the human observer, the primary cause of the patchy oviposition pattern being either mutual attraction among oviposition females or lack of dispersal from the chorusing and mating centers. The degree of patchiness is inversely density—dependent, in that the most patchy distributions were invariably those with lowest density. However, several plots with low density were not very patchy. This pattern makes biological sense, remembering that mutual attraction cannot operate unless the animals perceive one another, and at low densities they may sometimes do so and sometimes not. Further analysis, plotting mean crowding against mean density (following Iwao 1968) suggests that mutual attraction among females is weakened at high densities. As modes of data analysis, both kinds of plots–mean crowding vs. mean density and patchiness vs. mean density–bring out patterns that would not otherwise be apparent. In this connection, we offer an empirical definition for the concept, density—independent aggregation."
An Empirical Test of Community Stability: Resistance of a Fouling Community to a Biological Patch‐Forming Disturbance
Authors: Smedes G. W. , Hurd L. E.
Published: May 2006

Stability, defined as resistance to external perturbation, was evaluated for two developmental stages of an estuarine subtidal fouling community subjected to a biological patch—forming disturbance. Three stability criteria: (1) magnitude of response, (2) rate of response, and (3) recovery, were assessed on the basis of differences between control and perturbed panel communities with respect to physical structure (percent bare substrate, and sessile species volume), species richness (S), evenness (VH'), interaction diversity (H'), and community similarity, over time. Both motile and sessile species assemblages were examined, and responses of individual species were also determined. The older fouling community was found to be less stable than the younger one in terms of both measures of physical structure, for all three stability criteria. This was also true for species richness. Differences in evenness and H' between control and perturbed communities showed no consistent trend in this regard, and motile and sessile species exhibited different degrees of response. Multiple discriminant analysis indicated that fewer species contributed to the discriminant function in the younger community than in the older one. Community similarity between control and perturbed panel communities was lower overall for the older community. Individual species responses that contributed to these effects were examined. The results of our study, as well as an examination of other empirical evidence from the literature, reveal a lack of consistent support for traditional hypotheses of community stability. We conclude that evolutionary pressures, and resulting ecological strategies, are more important in determining community resistance stability than statistical properties such as species diversity.
Ecological Segregation, Color Matching, and Speciation in Lizards of the Amphibolurus Decresii Species Complex (Lacertilia: Agamidae)
Authors: Gibbons John R. H. , Lillywhite Harvey B.
Published: May 2006

The Amphibolurus decresii complex comprises three species, A. decresii, A. fionni, and A. vadnappa, having restricted distribution in South Australia and extreme western New South Wales, Australia. Adults of all three species are similar in appearance, apart from color and pattern differences, particularly in males, and are restricted to rocky outcrops. Amphibolurus decresii and A. vadnappa are partially sympatric, but are ecologically segregated according to the color of rocks on which they perch. Amphibolurus decresii is found on rocks which are predominantly pinkish yellow, whereas A. vadnappa is found on rocks which are dark reddish brown. Each species appears camouflaged on its respective mineral type which it prefers in choice tests. Amphibolurus fionni is allopatric from the other species and exhibits variable coloration in its "substrate races." It has a distinctive color patter of its own, but resembles one or the other species in areas where rocky habitats are correspondingly similar. Studies of reflectivity of lizard skin demonstrate that the dorsal coloration on the back, tail, and legs is relatively labile. Maximum blanching occurs at high body temperatures. Lability is correlated with latitude, southern populations being darker and possessing less lability than northern populations. The dorsal coloration is thought to be important in thermoregulation. Reflectance from dorsolateral patterns of lizards shows little lability (<3% at all visible wavelengths) and matches the reflectance of respective mineral backgrounds. These patterns also resemble the colors and shapes of lichens that grow upon the rocks. Both dorsal and dorsolateral coloration are believed to have camouflage functions, but the latter appears more important because (1) it matches the substrate more closely, and (2) it does not change appreciably with body temperature. Evidence suggests that species comprising the A. decresii complex evolved from a common ancestor in South Australia. With respect to A. decresii and A. vadnappa, a model of ecological speciation is proposed involving divergent color adaptation and possibly sympatric or parapatric populations. Color divergence and substrate matching seen in populations of A. fionni conceivably reflect evolutionary processes similar to those that promoted speciation of A. decresii and A. vadnappa.
Mature Mesophytic Hardwood Forest: Community Transitions, by Layer, from East‐Central Minnesota to Southeastern Michigan
Authors: Rogers Robert S.
Published: May 2006

Fragments of old—growth forest in which sugar maple (Acer saacharum) is a principal dominant, i.e., late—successional mesophytic forests, were sampled layer by layer across a floristic gradient from east—central Minnesota to southeastern Michigan, USA. This includes the northern Maple—Basswood region ("Big Woods" section) and northwestern Beech—Maple region as described by E. L. Braun (1950). All sampled areas were relatively undisturbed, at least 0.5 ha (surrounded on all sides by >30 m of forest), and occupied similar sites: deep, gently sloping, well—drained drift with high water—holding capacity. The object was (1) to determine whether the community importance of species that are common throughout the study area differs significantly from region to region, perhaps as a result of within—layer competition with species that are restricted floristically, but become important as soon as their ranges are entered (e.g., beech, Fagus grandifolia), and (2) to determine whether regional differences in tree composition correlate with regional differences in the composition of inferior layers. Regional differences in composition of mature trees tended to support Braun's earlier findings that "climax" forests on mesic sites west of the beech border are characterized by the prevalence of basswood (Tilia americana) as a codominant, but as soon as the beech border is crossed, beech is the principal codominant. Basswood ranked at least second in density in 70% of the Big Woods stands (mean density = 53.4 boles/ha), but in only 20% of the stands within the range of beech (mean density = 8.3 boles/ha), where beech ranked at least second in 73% of the stands (mean density = 42.0 boles/ha). Mean basal area of basswood was 14.0 m2/ha west of the beech border and 1.6 to the east, compared with 8.7 for beech. Basswood, in fact, attained about the same relative density (0.20) and basal area (0.36) averaged over all stands in the Big Woods as beech did within its range (i.e., 0.23 and 0.29, respectively). (Mean total density per stand in the two regions was 267 ± 38 and 184 ± 37 boles/ha; mean total basal area was 39.0 ± 5.4 and 30.8 ± 4.6 m2ha.) Regional differences in composition were not readily apparent when inferior layers (vascular plants <2 m tall) were sampled quantitatively, excluding rare species and species with restricted floristic distributions. When understories so defined were clustered according to several measures of species importance, broad overlap of stand groups resulted in each case. Overlap was often similar to that obtained when understories were clustered on the basis of species weightings randomly allocated to stands, that is, when the distribution of scores for each species is not changed, but scores are assigned to stands randomly.
Do Tropical Bird‐Pollinated Plants Exhibit Density‐Dependent Interactions? Field Experiments
Authors: Feinsinger Peter , Tiebout Harry M. , Young Bruce E.
Published: May 2006

In a tropical cloud forest at Monteverde, Costa Rica, three understory shrub species overlap greatly in flowering seasons and share hummingbird pollinators (Lampornis calolaema). We conducted two field experiments to determine if a plant's pollination and subsequent reproductive output reflect local densities of conspecific and heterospecific flowers. We controlled floral composition in the neighborhoods surrounding 12 focal plants of self—compatible Besleria triflora (Gesneriaceae) and of self—incompatible Palicourea lasiorrachis (Rubiacaea), producing prescribed mixtures of flowers of the focal plants' conspecifics and of Cephaelis elata (Rubiaceae). Each experiment subjected each focal plant to four different treatments; a floral neighborhood of 10 conspecific flowers and 10 of Cephaelis within a radius of 20 m (Treatment A); 90:10 (B); 10:90 (C); and 50:50 (D). Focal plants themselves always had 10 open flowers. We assigned the sequence of treatments to each focal plant in a Latin Square crossover design using three balanced blocks of four plants each. Dependent variables were: frequency of hummingbird probes per flower; pollen tubes in styles; and seed output per flower. From standard models of competition for pollination, we predicted that pollination levels and seed production would be ranked among treatments: B(90:10) > D(50:50) > A(10:10) > C(10:90). These overall predictions were supported for both Palicourea and Besleria, based on tests for directionality showing that the rank order among adjusted treatment means did not differ from the order predicted. Separate a priori treatment contrasts indicated that in both species increasing flower density above base levels (Treatment A, 10:10) by adding conspecific flowers alone (B, 90:10) or along with Cephaelis (D, 50:50) increased the rate at which hummingbirds visited focal plants' flowers. In Palicourea, both the number of pollen tubes in styles and seed output per flower increased in the same fashion. In Besleria, the only effect was increased pollination (but not seed output) from A (10:10) to B (90:10). In contrast, enriching the basic neighborhood (A. 10:10) with Cephaelis flowers (C, 10:90) did not alter frequencies of hummingbird probes to either species of focal plant. Heterospecific enrichment did, however, decrease both pollination levels and seed output in Palicourea, and seed output in Besleria. In general, altering the floral neighborhood affected Palicourea more strongly than Besleria. Apparently the intensity of density dependence, at least in terms of female reproductive success, varies among plant species sharing pollinators. Seed outputs in self—incompatible plants such as Palicourea may closely reflect flower densities of neighbors, but interspecific effects on fertilization in self—compatible plants such as Besleria may be density vague.
Fire Intensity Effects on Germination of Shrubs and Herbs in Southern California Chaparral
Authors: Moreno Jose M. , Oechel Walter C.
Published: May 2006

The study reported here investigates the effects of fire intensity and combustion products (ash and char) on shrub and herbaceous seedling emergence in the field in a mixed southern California chaparral, which was dominated by a facultative seeder, Adenostoma fasciculatum, and by an obligate seeder, Ceanothus greggii. Fire effects were studied in two winter burns, one of which as manipulated by adding brush to experimental plots to increase fire intensity. Adenostoma was very sensitive to increasing fire intensity, with decreasing seedling production as fire intensity increased. This was detected within the normal range of fire intensities of a winter burn, and to a greater extent when intensities were experimentally modified. Ceanothus germination was either stimulated by, or very resistant to, increasing fire intensity. Increasing fire intensity also promoted earlier germination of Ceanothus. Herb species were also differentially affected by increasing fire intensities. Among the abundant herbs, fire annuals like Phacelia brachyloba were more resistant to increasing fire intensity than opportunistic annuals like Cryptantha muricata. Seedling production of one opportunistic annual, Lotus strigosus, was however, stimulated by all levels of increased fire intensity. In most cases, ash and char did not affect field germination. Fire intensity can profoundly modify seedling production of shrubs and herbs after a fire. By substantially altering post—fire seedling populations, fire intensity may represent a major factor in controlling species composition and diversity in chaparral.
Complex Predator‐Prey Interactions within an Estuarine Benthic Community
Authors: Posey Martin H. , Hines Anson H.
Published: May 2006

Indirect predator—prey interactions have been suggested to be important in a variety of communities. However, we still understand little about the factors determining the relative importance of indirect and direct effects of predation or what forms indirect predator—prey interactions may take. We examined the direct effects of predation by an omnivorous grass shrimp. Palaemonetes pugio, and the indirect effects for prey species of interactions between this shrimp and other predators in an estuarine benthic community. Direct and indirect effects of predation were examined in the field using a combination of large— and small—scale enclosure/exclosure experiments. Specific interactions and mechanisms of effects were examined in laboratory predation studies, utilizing both two—species and multi—species systems. Grass shrimp predation reduced the densities of a variety of benthic fauna, but the effects varied seasonally and with prey size. Interactions between grass shrimp and other predators took two forms: reduction of an infaunal predator by shrimp predation and predation on grass shrimp by fish. Both types of interactions had a mixed effect on lower trophic levels, with enhancement of certain benthic prey and no effect on other species. Interactions between grass shrimp and their fish predators resulted in changes in shrimp distribution independent of changes in shrimp abundance, creating spatial refungia for shrimp prey. The dynamics of indirect interactions in this community are strongly affected by the omnivorous nature of the major predators. Our results emphasize the general importance of omnivory in regulating predator—prey interactions and in predicting the relative importance of indirect predator effects.
Age‐Specific Weaning Success of Northern Elephant Seals in Relation to Previous Breeding Experience
Authors: Sydeman William J. , Huber Harriet R. , Emslie Steven D. , Ribic Christine A. , Nur Nadav
Published: May 2006

We studied the effect of age and previous breeding experience on the probability of successful reproduction in female northern elephant seals (Mirounga angustirostris). Reproductive data, collected over a 12—yr period, were analyzed by multiple logistic regression to describe the functional relationships among the variables. We also examined the relationships among age, experience, date of breeding, and reproductive success to determine if differences in timing explain age—specific productivity. Finally, we investigated the relationship among age, experience, and maternal behavior to determine if correlates of reproductive effort could be related to age— and experience—specific patterns of reproductive success. Reproductive success increased between the maternal ages of 3 and 7 yr and then levelled off, demonstrating an asymptotic relationship. However, the relationship between reproductive success and age, after statistically controlling for previous experience, showed instead that success increased linearly throughout an animal's lifetime. In contrast, after statistically controlling for age, reproductive success was related to experience in a parabolic fashion; success increased with experience for the first few breeding attempts, but declined later in an animal's reproductive lifetime. The effects of experience depended on age: among young animals experience was apparently beneficial, but among old animals greater experience was deleterious. These results suggest experience—related senescence. Timing of breeding was related to experience, rather than age, in a curvilinear fashion; arrival and parturition dates were later for inexperienced and very experienced females, and were earliest for moderately experienced animals. Young animals that gave birth later in the season experienced reduced reproductive success; we observed no such decline in older animals. Thus, among young females, arriving and giving birth earlier each year was adaptive. Aggressiveness and dominance increased with age and experience, but we were unable to separate the effects of age and experience. We contend that the experience—related drop in reproductive success reflects the cost of reproduction. Breeding at a young age and/or continuously appears to result in lower reproductive success later in life. Furthermore, curvature in age—specific reproductive success can be explained by experience—related senescence. This result has general applicability, suggesting that age—related senescence instead may be due to reproductive "burn—out". These results indicate that previous breeding experience and differences in individual quality do not explain the pattern of increasing reproductive success with age in northern elephant seals, and provide indirect support for the hypothesis that animals increase reproductive effort as they age to offset a concomitant decrease in residual reproductive value.