bluuebunny/crossref_metadata_2025 huggingface dataset:
Interactions among Colonizing Annuals: Is There an Effect of Gap Size?
Authors: McConnaughay K. D. M. , Bazzaz F. A.
Published: May 2006

Small—scale gaps are often colonized by more than one plant, yet few individuals. We examined interactions among co—invading plants within small gaps in an old—field community as a function of the size of the gap being invaded. Several colonizing annual species were grown, as single individuals or in conspecific or heterospecific pairs, in artificially created gaps of 10, 20, and 40 cm diameter in a Poa pratensis L. background in two successive years. The identity of the neighbor influenced plant survivorship for one of the two years studied, but had no effect on the probability of becoming reproductively mature, or on levels of seed production. In contrast, vegetative growth was influenced by neighbor identity in both years. Both positive and negative plant—plant interactions were found; the presence of a neighbor did not always result in competitive reduction in the target individual. Conspecific neighbors were found to reduce vegetative biomass to a greater extent than heterospecific neighbors for one of the two years studied, primarily due to the sensitivity of one species, Setaria faberii, to conspecific neighbors. We found no evidence of a consistent competitive hierarchy present in this system. Finally, although increasing gap size had significant, positive effects on survivorship, growth, and reproduction, the size had being colonized had no effect on plant—plant interactions between co—invading individuals. In this system the number and identity of invading individuals, as well as the characteristics of the gap itself, interact to dictate the composition of the invading community and the relative performance of each individual colonist. This complexity may contribute to increased diversity in plant communities in which small—scale disturbances provide opportunities for earlier successional species to persist in localized patches within later successional vegetation.
Patterns and Controls of Nitrogen in Tallgrass Prairie Streams
Authors: Tate Cathy M.
Published: May 2006

A descriptive and experimental approach was used to study patterns of nitrogen concentrations in tallgrass prairie streams in Kansas. Nitrate concentrations varied depending on the year, season, time of day, stream flow conditions, source of water, and location where the sample was taken. Mean nitrate concentrations during the dormant season decreased from 1982 to 1984, corresponding to above—normal precipitation during this same period. Nitrate concentrations decreased from third—order (grass/shrub riparian vegetation) to fourth—order channels (gallery forests). Nitrate, organic N, and total N were higher in a third—order intermittent stream than in a third—order perennial stream. Water from seeps and tributaries had higher nitrate concentrations than stream water; however, streams and tributaries had higher organic N than seeps. Maximum nitrate concentrations during storm flows were directly related to the frequency of storm events. The activity of terrestrial vegetation influenced nitrate concentrations within streams, seeps, and tributaries; nitrate concentrations were always higher during periods of vegetation dormancy. Aquatic biota influenced both spatial patterns (downstream decrease in nitrate concentrations and increase in organic N) and diel patterns of nitrate concentration (night > day). Uptake rates of nitrate by biota were greater during the day than during the night as determined by experimental addition of nitrate. Nitrogen (N) and/or phosphorus (P) limitations of algal growth were tested experimentally using nutrient—diffusing substrata (clay saucers filled with enriched agar) and enrichment of artificial channels. Addition of N and P in combination increased algal biomass (chlorophyll a and ash—free dry mass) over controls or single addition of N or P. Thus, algal biomass was N and P limited; however, individual taxa responded to specific N and/or P concentrations or atomic N:P ratio.
Foraging Efficiency and Habitat Switching in Competiting Sunfishes
Authors: Werner Earl E. , Hall Donald J.
Published: May 2006

A series of experiments are presented which elucidate competitive interactions among 3 congeneric sunfishes (Centrarchidae). In the absence of competitors all species rank 2 habitats (vegetation, sediments) in the same order according to profitability but differ in their relative efficiencies at utilizing these habitats. The ordinal ranking of foraging efficiencies of these species in the 2 habitats permits qualitative predictions of the occurrence and order of habitat shifts as resources decline (competitive pressure increases). We present an experiment demonstrating such habitat shifts which corroborate these predictions. The patterns of habitat switching indicate that the addition of a competitor which is more effective in a given habitat can change the profitability ranking for a species to the extent that the preferred habitats are abandoned. Thus these results provide experimental evidence for the compression hypothesis as a mechanism for the development and maintenance of resource partitioning by habitat. Further, the ecological flexibility exhibited by the sunfish indicates that overlap or co—utilization of habitats can be a very dynamic process, determined by the relation between resource levels and differences in foraging efficiencies among species. The implications of these results to competition, optimal use of a patchy environment, and species abundance relations in natural lakes are discussed.
A Test of the Principle of Allocation Using Two Sympatric Species of Cockroaches
Authors: Rollo C. David
Published: May 2006

The "principle of allocation" underlies much of current life history theory, but has never been critically tested. Although the cockroaches Periplaneta americana and P. brunnea are closely related and morphologically very similar, P. brunnea produces an ootheca nearly twice the mass of P. americana's. The principle of allocation predicts that P. brunnea should take longer to produce an ootheca, or that some other factor must be traded off. The principle was tested by comparing the mass budgets of Periplaneta americana and P. brunnea fed artificial diets under controlled conditions. Although dissections confirmed their morphological similarity, P. brunnea had a rate of dry biomass production of 11.2 mg/d, whereas that of P. americana was only 4.4 mg/d. The gross production efficiency of P. brunnea (38%) was also higher than that of P. americana (29%). These results challenge the generality of the principle of allocation. P. brunnea was much less resistant to starvation than P. americana. Although growth of the two species was similar on an adequate diet, P. brunnea grew faster but suffered higher mortality than P. americana on low—protein diets. The results suggest a view of organism design based on safety factors and adjustment to levels of key resources, as an alternative to a perspective of maximized production.
Identifying Biotic Boundaries Along Environmental Gradients
Authors: McCoy Earl D. , Bell Susan S. , Walters Keith
Published: May 2006

We outline a new procedure for placing boundaries of faunal and floral distributions along environmental gradients. Four data sets on species distributions from the marine literature, each of which had been scrutinized previously for boundaries, were analyzed with the procedure. Placement of boundaries proved straightforward in all cases. Our procedure possesses several attributes that make it superior to other methods of boundary location: (1) it is graphical and amenable to exploratory data analysis, (2) it is based upon taxonomic similarity between locations that is judged in a probabilistic manner, (3) it uses all locations simultaneously, (4) it provides a level of confidence in any boundary, and (5) it is precise: two workers employing it will arrive at the same conclusion.
The Longleaf Pine Islands of the Ocala National Forest, Florida: A Soil Study
Authors: Kalisz Paul J. , Stone Earl L.
Published: May 2006

Longleaf pine—wiregrass—turkey oak (Pinus palustris—Aristida stricta—Quercus laevis) occurs as isolated islands, 60—4000 ha in area, in a matrix of sand pine—scrub (Pinus clausa—Quercus spp.) on deep sands of the Ocala National Forest, Florida. Striking contrasts in physiognomy and species composition, and sharp, stable boundaries suggested that soil differences determined vegetation boundaries. Examination of soils to a 500 cm depth at 130 locations revealed no consistent differences in profile morphology, particle size distribution, or extractable nutrients. Thus the hypothesis that soil differences were responsible for the distribution of the two communities was not sustained. Organic matter and nitrogen contents of surface (0—60 cm) soils likewise did not differ between plant communities. Differences in surface horizon color were ascribed to differences in soil—forming processes: frequent additions of fine charcoal and active faunal mixing under longleaf pine vs eluviation under sand pine. Biogenic opal extracted from surface soils at 75 locations failed to provide evidence of long—term community stability. Isolines of total opal mass were concentrically patterned around longleaf islands, but opal mass and morphology did not consistently differ across vegetation boundaries.
Sources of Interpopulational Variation in Growth Respones of Larval Salamanders
Authors: Petranka James W.
Published: May 2006

In central Kentucky, larvae of small—mouth salamanders (Ambystoma texanum) in ponds grow faster, have shorter larval periods, and are larger at metamorphosis than those in streams. Comparisons of populations in the field with those in common environment in the laboratory indicated that variation in growth in nature is due largely to environmental rather than genetic differences between populations. In laboratory experiments, temperature and food level significantly affected larval growth. However, growth occurred independently of density when larvae were fed identical per—capita food allotments at low levels or were fed as libitum. In the field, larval growth rate and size at metamorphosis were positively correlated with mean environmental temperature. However, the relative size of metamorphs in warm (pond) vs. cool (stream) habitats was the reverse of that predicted from laboratory studies. In a field experiment for determining whether food level affects growth, supplementally fed larvae in enclosures in a stream grew faster (P < .0001) than larvae in the stream proper. Regression coefficients of larval density vs. mean size in isolated pools within the stream were uniformly negative, which suggests that larvae exploitatively competed for food. In streams, food limitation caused little direct mortality through starvation. Instead, food shortages lowered larval survivorship indirectly by lengthening the time to metamorphosis and thereby increasing their susceptibility to stream drying, flooding, and fish predation.
Application of a Primary Production Model to Epiphytic Algae in a Shallow, Eutrophic Lake
Authors: Jones R. Christian
Published: May 2006

A modification of Fee's phytoplankton model was used to calculate the production of algae epiphytic on the submersed macrophyte Myriophyllum spicatum in shallow, eutrophic Lake Wingra, Wisconsin. The model involved integration of four basic relationships using time— and depth—stepped difference equations: (1) depth distribution of algal biomass; (2) photosynthesis per unit biomass as a function of light intensity, (3) irradiance—depth relationship, and (4) diurnal variation in surface irradiance. Epiphyte production in the littoral zone of Lake Wingra, measured (as carbon) for two growing seasons, was 29—38 g°m—2°yr—1, placing the lake at the lower range of reported values. Since epiphyte production was 48—79% of macrophyte production, well within the reported range, the lower production values resulted from less dense weedbeds (lower substrate availability) rather than lower density of epiphytes. Algal biomass defined the upper bound for production, while irradiance levels on each day determined actual production by decreasing this potential by varying degrees. Other parameters of the production model, such as light extinction coefficient and photosynthetic parameters, explained little of the variation in production. These results emphasize the importance of accurately determining the quantity and vertical distribution of attached algal biomass when making production estimates.
Intertidal Mosaics: Patch Size, Propagule Availability, and Spatially Variable Patterns of Succession
Authors: Sousa Wayne P.
Published: May 2006

Localized disturbances transform most assemblages of sessile organisms into mosaics of patches differing in characteristics such as size and age (time since last disturbed). This mosaic nature of natural communities is especially evident on exposed intertidal shores along the northwest coast of North America, where the competitively dominant mussel, Mytilus californianus, occupies much of the space at mid—tidal levels. Nearly continuous beds of this species are interrupted by patches of open space generated mainly by the shearing forces of winter storm waves. These patches serve as foci for the recruitment, growth, and reproduction of many competitively inferior, fugitive species, including both algae and sessile invertebrates. These species are doomed to local extinction as the lateral encroachment of adult mussels closes the patch and excludes then from the area. This study examined the dynamics of algal succession within experimental patches cleared in mussel beds. In particular, two potentially important sources of variation in successional dynamics were investigated: (1) the size of the patch when first created, and (2) the location of the patch with respect to potential sources of propagules. The size of a cleared patch was found to influence strongly the course of algal succession. This effect was largely indirect, resulting from an interaction between patch size and grazing intensity. Small patches support higher densities of grazers, especially limpets, than do large patches. As a consequence, the assemblages of algae that develop within small and large patches differ markedly. The assemblage in small patches includes grazer—resistance but apparently competitively inferior species, whereas that in large patches is composed of grazer—vulnerable but competitively superior species. Small patches appear to serve as refuges from competition for grazer—resistant species. Recruitment was variable among the experimental patches. Percent cover of several species was found to be highly correlated with the cover of epozoic conspecific adults within 1 m of the edge of the patch. This result suggests that a number of the species inhabiting patches within mussel beds may disperse their propagules over relatively short distances. For such species, patch dispersion may influence the regional dynamics of their populations.
Experimental Community Studies: Time‐Series Tests of Competition between African and Neotropical Bees
Authors: Roubik David W.
Published: May 2006

Experimental studies of food competition were performed in lowland forest of French Guiana by controlled introduction and removal of a small number of African honey bee colonies, totaling 100 000—150 000 bees. Trends in brood production and food storage by observations colonies of native social bees, Melipona favosa and M. fulva, were described with linear stochastic statistical models. Data were taken after the introduction of honey bee colonies, and the models were used to construct falsifiable hypotheses that would suggest whether the honey bees disrupted colony growth and resource harvest of native species. During a 1—mo introduction of honey bees in the wet season, when floral resources were at low abundance, time—series intervention analyses showed that African Apis had no significant effect on native bee colonies. Repeated—measures ANOVA also showed no treatment effect on bee colonies, and removal of 5—10 honey bee colonies had no effect on brood dynamics and food storage of four conspecific colonies. All colonies foraged during experiments, yet no competition was recorded. Since the three species are diet generalists and share many of the same resources, competition at flowers was not disproved, although any cumulative effects were negligible. Linear, stochastic integrated autoregression/moving average (ARIMA) models were useful in removing statistical dependence in data series and allowing hypothesis tests, but intracolony variables showed seasonal decline that prevented introduction of deterministic components that would have improved some models. Perturbation of social tropical bee communities by colonizing African Apis is unlikely to become evident within a short time or at relatively low honey bee colony density.
Facilitation of Soft‐Bottom Benthic Succession By Tube Builders
Authors: Gallagher Eugene D. , Jumars Peter A. , Trueblood Dwight D.
Published: May 2006

Controlled field experiments were used to test the effects of surface—deposit feeders on succession at the Skagit flats, an intertidal sandflat in northern Puget Sound. The tube builders Hobsonia florida (Polychaeta, Ampharetidae), Pseudopolydora kempi japonica (Polychaeta, Spionidae), and Lanais sp. (Crustacea, Peracarida) facilitate the recruitment of other taxa on 10—cm2 azoic patches. Simulated animal tubes facilitated the immigration of Tanais sp. and oligochaetes. Macoma balthica, a tellinid bivalve, facilitated the immigration of H. florida, while inhibiting that of Tanais sp. These experiments clearly documented that facilitation rather than inhibition is the dominant process governing succession in the skatole community. The facilitation model of succession offers a viable alternate explanation for many soft—bottom benthic processes previously explained by the inhibition model.
Phosphorus Dynamics in a Woodland Stream Ecosystem: A Study of Nutrient Spiralling
Authors: Newbold J. D. , Elwood J. W. , O'Neill R. V. , Sheldon A. L.
Published: May 2006

The term spiralling refers to the interdependent processes of cycling and downstream transport of nutrients in a stream ecosystem. To describe spiralling in Walker Branch, a first—order woodland stream in Tennessee, we released 32PO4 to the stream water and measured its uptake from the water and then followed its dynamics in coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), aufwuchs, grazers, shredders, collectors, net—spinning filter feeders, and predators over a 6—wk period. Rates of transfer among compartments and rates of downstream transport were estimated by fitting a partial differential equation model of the ecosystem to the data. With the resulting coefficients, the model was run to steady state to estimate standing stocks and fluxes of exchangeable phosphorus. Phosphorus moved downstream at an average velocity of 10.4 m/d, cycling once every 18.4 d. The average downstream distance associated with one cycle, defined as the spiralling length, was therefore 190 m (10.4 m/d ° d). Spiralling length, at steady state, is approximately the ratio of the total downstream flux of phosphorus per unit width of stream (720 mg°d—1°m—1) to the rate of P uptake from the water (3.90 mg°m—2.d—1). CPOM accounted for 60% of the uptake, FPOM for 35%, and aufwuchs for 5%. Turnover times of P in particulates ranged from 5.6 to 6.7, except for FPOM, which showed a second, slower turnover time of 99 d. Of the P uptake from water by particulates, 2.8% was transferred to consumers, while the remainder returned directly to the water. About 30% of the consumer uptake, in turn, was transferred to predators. The spiralling length was partitioned into: (1) an uptake length associated with transport in the water column (165 m), (2) a particulate turnover length associated with transport in FPOM and CPOM (25 m), and (3) a consumer turnover length associated with animal drift (0.05 m). FPOM transport accounted for 99% of the particulate turnover length. The small consumer turnover length reflected low consumer uptake of P from particulates and slow downstream drift velocity (0.013 m/d). In spite of the low rate of phosphorus uptake, the combined consumer—and—predator community accounted for 25% of the standing stock of exchangeable P in the stream. The retentiveness of this community is attributable both to the low drift rate and to a long turnover time (152 d) for P within the community.
Patterns of Prey Selection in the Cruising Calanoid Predator Euchaeta elongata
Authors: Greene Charles H. , Landry Michael R.
Published: May 2006

The invertebrate predators of marine and freshwater plankton assemblages typically employ encounter tactics that enable investigators to characterize them as either ambushers or cruisers. In this study, we examine the implications of cruising tactics on the prey selection patterns exhibited by the marine calanoid copepod Euchaeta elongata. Patterns and mechanisms of prey selection exhibited by Euchaeta were investigated in the laboratory with predation experiments and videotaped observations of predator and prey swimming behavior. Prey types offered to Euchaeta included each of the developmental stages of Calanus pacificus and the adults of Pseudocalanus sp. Calanus' vulnerability to Euchaeta predation steadily increased through the naupliar stages, reached a peak in the early copepodid stages, and declined rapidly after the molt to the fourth copepodite. Pseudocalanus adults were as vulnerable to Euchaeta predation as the early copepodites of Calanus. The behavioral mechanisms underlying the observed patterns of prey selection are discussed and compared with the results from other investigations of selective planktivory by marine calanoid copepods. It is concluded that prey selection by Euchaeta is largely determined by differences in prey vulnerability; some evidence for active behavioral choice was observed, however.
Common Tern Foraging: Seasonal Trends in Prey Fish Densities and Competition with Bluefish
Authors: Safina Carl , Burger Joanna
Published: May 2006

We present a method for making direct quantitative measurements of population dymamics of fish species preyed upon by seabirds. We studied foraging behavior of Common Terns (Sterna hirundo) in relation to density of fish prey of Long Island, New York. We used an echo sounder to record the abundance of small prey fish and large predatory fish near a tern colony. Prey fish abundance was highest when terns were feeding young chicks and declined sharply later in the season, coincident with the arrival of large numbers of predatory bluefish (Pomatomus saltarix). Mean depth of occurrence of prey fish was shallower after the arrival of bluefish. Terns fished in areas with high concentrations of prey, but there was no correlation between numbers of feeding terns and prey density. Number of feeding terns was inversely correlated with prey depth, however. Terns were strongly attracted to predatory fishes feeding near the surface. We suggest that in addition to concentrating prey fish near the surface, predatory fish may have been important as competitors.
Postdispersal Seed Predation in Lomatium Spp. (Umbelliferae): Variation among Individuals and Species
Authors: Thompson John N.
Published: May 2006

A series of experiments in the steppe of eastern Washington and the laboratory was designed to test the hypothesis that postdispersal seed predators can distinguish among conspecific seeds derived from different plants within a population as well as between seeds of congeneric species within a community. Lomatium (Umbelliferae) seeds are eaten by ground—foraging beetles and mammals after the seeds are dispersed by wind from the plants. Field experiments showed that removal of L. dissectum seeds from seed piles was faster than removal of L. triternatum seeds in both single—species and mixed—species piles. Removal of seeds from 5—seed piles did not differ from removal from 10—seed piles. In piles containing seeds from different L. dssectum plants in the same population, seeds from certain plants were removed significantly faster than those from others. Laboratory trials using adult Eleodes nigrina beetles confirmed that plants in the L. dissectum population differed consistently in the palatability of their seeds. Beetles ate the entire seed when chewing on seeds of some plants but consistently ate only the wing surrounding the seeds of other plants, leaving the embryo and endosperm intact and viable. These results indicate that (1) postdispersal seed predators can distinguish among congeneric and even conspecific Lomatium seeds, (2) the shape of survivorship curves for the seed stage can depend upon the proximity of congeneric seeds and the mix of conspecific phenotypes (potentially genotypes) in seed piles, and (3) outcomes of interactions with E. nigrina beetles make it inaccurate to give this interaction a single typological label, since the interaction ranges from predation to commensalism among Lomatium plants within a population.
Physiological and Ecological Consequences of Sleeping‐Site Selection by the Galapagos Land Iguana (Conolophus Pallidus)
Authors: Christian Keith A. , Tracy C. Richard , Porter W. P.
Published: May 2006

Field observations and biophysical models were combined to analyze sleeping—site selection by Galapagos land iguanas (Conolophus pallidus). Iguanas slept in different kinds of sleeping sites during different seasons. In the coolest season (garua), adults land iguanas were found in sleeping sites that were warmer than the coolest sites a available. This may be because the garua season (cool, overcast, and foggy) is a time when environmental conditions mitigate against rapid warm—up in the mornings, so lizards may regulate nighttime body temperatures so that it is easier to warm up to preferred daytime body temperatures. In the warmest season, adult iguanas were found in the coolest sleeping sites available. This observation is consistent with hypotheses of voluntary hypothermia, which can be advantageous in energy conservation and in avoiding detrimental effects associated with maintenance of constant body temperatures throughout the day and night. Juvenile iguanas were found sleeping in rock crevices regardless of the ambient thermal environments. Such sites are likely to be important as refugia for this life stage, which, unlike the adult stage, is vulnerable to predation. We conclude that selection of sleeping sites is a process that may help in avoidance of predation, optimization of body temperature at the end of the sleeping period, and reduction of metabolic costs during sleeping. The importance of some of these factors may change with the thermal milieu (e.g., season).
Graphical Display and Interpretation of Antelope Census Data in African Wildlife Areas, Using Correspondence Analysis
Authors: Greenacre M. J. , Vrba E. S.
Published: May 2006

Census data are represented on 17 antelope genera, constituting 9 tribes, in 16 sub—Saharan African wildlife areas. A multidimensional graphical technique, correspondence analysis, is described and then used to represent the genera, or tribes, and the wildlife areas in a joint planar display. The interpretation of these displays is discussed in the light of supplementary ecological information available for each wildlife area. In the displays of the present analysis gross vegetational physiogonomy, that is height and spacing of the cover, emerges as the most important influence on the grouping of ecosystems, antelope tribes, and ecosystems with tribes.
Carbon Isotope Dynamics During Grass Decomposition and Soil Organic Matter Formation
Authors: Wedin David A. , Tieszen Larry L. , Dewey Bradley , Pastor John
Published: May 2006

We analyzed changes in the stable C isotope composition (°13C) of bulk tissues and lignin fractions during a 2—yr decomposition study in east—central Minnesota (USA) of aboveground and belowground litter from four perennial grass species: Schizachyrium scoparium (C4), Agropyron repens (C3), Poa Pratensis (C3), and Agrostis scabra (C3). Although lignin concentrations increased for all litter types during decomposition and lignin fractions were consistently depleted in 13C compared to bulk tissues (3.6% more negative on average), we found neither convergence of bulk tissue °13C values towards lignin °13C values, nor greater stability of °13C values for lignin fractions. Furthermore, °13C values of C3 and C4 species shifted in opposite directions during decomposition. Thus, our data do not support the hypothesis that °13C values decrease during decomposition because of the selective preservation of lignin and we instead suggest the isotopic shifts are caused by the incorporation of new C from soil organic matter into litter by microbial decomposers. We estimate that this new C comprised 12—19% of the total litter C, depending on species, at the point of 70% mass loss. In monocultures of these four species plus another C4 grass (Andropogon gerardi) growing on initially homogeneous soils with a predominantly C3 isotopic signature, soil °13C values increased 1.6—2.2 for the C4 species and remained relatively unchanged for the C3 species after 4 yr. Averaging across the C4 species and the experimental soil organic matter gradient, 14% of the total soil C in these plots must be new C4 C to account for this isotopic shift. We estimate that this amount of new soil C equals 30% of NPP summed over 4 yr in these plots.
The Spatial Distribution of Nematode Trophic Groups Across a Cultivated Ecosystem
Authors: Robertson G. Philip , Freckman Diana W.
Published: May 2006

In order to better understand the spatial distributions of soil trophic groups and the potential significance of these distributions to ecosystem functioning we initiated a study to describe the within—site variability of nematode feeding groups in a row—crop ecosystem. Soil cores were removed from a 48—ha corn (Zea mays) field in the U.S. Midwest prior to spring planting, and nematodes were identified by phenotypic criteria to four groups: bacterivores, fungivores, omnivores/predators, and plant parasites. Within—site variability was high for all groups; population counts spanned two orders of magnitude, with coefficients of variation ranging from 40—130% (n = 115—138 soil samples). Probability distributions were strongly lognormal. Geostatistical analysis showed that a major part of this variability was spatially dependent; variograms suggest that 70—99% of sample population variance was related to spatial autocorrelation over our geographic range of 6—80 m, except for the parasitic group, for which we detected no autocorrelation to 1200m. Maps of nonparasitic feeding groups across the field showed large multi—hectare areas of low to moderate population densities, with sub—hectare clusters of high—density populations towards one end of the site. Individual feeding groups were only weakly correlated with one another across the field (Kendall's t < 0.363, P < 0.001). Edaphic factors (bulk density, texture, pH, C availability, N availability) could collectively explain <30% of the variability in the nonparasitic groups across the area sampled. Results suggest that important soil food web components are strongly patterned at sub—hectare scales in this site. That this patterning is maintained in an ecosystem subjected to the homogenizing influences of annual soil tillage and a monoculture plant population is remarkable, and suggests that such patterning may be even more common in less—disturbed sites. Inclusion of these patterns in studies of ecosystem processes and soil community dynamics may significantly improve soil trophic models and our understanding of the relationship between soil populations and ecosystem function.
Water Limitations and Plant Community Development in a Polar Desert
Authors: Gold Warren G. , Bliss Lawrence C.
Published: May 2006

Polar deserts of the High Arctic contain vast areas of minimal plant cover and low primary productivity. Significant development of polar desert plant communities is largely restricted to areas with considerable cover of a cryptogamic soil crust, which develops in sites exposed to continued surface runoff from melting snow for some of the short growing season. Thus, soil drought and plant water stress have often been assumed to be major constraints to plant community development in polar deserts. To examine this issue, water availability and plant water relations of common herbaceous perennial plants were studied over three growing seasons in a typically barren ("noncrusted") site and a site with a well—developed cryptogamic crust and vascular plant community ("crusted"). Soil water content was consistently higher in the crusted site than the noncrusted site through all growing seasons. These differences had limited biological relevance because subsurface soils at both sites remained effectively saturated (soil water potential > —0.1 MPa) through each growing season, despite low amounts of precipitation that varied nearly twofold from year to year. However, the surface soils (0.5—2.0 cm in depth), especially in noncrusted sites, dry considerably in some years. There were no significant differences in plant water potential and midday values of whole—plant transpiration and water vapor conductance for adult plants growing in crusted vs. noncrusted sites. Water stress for established plants was minor in both sites. Greater plant community development in crusted areas of this polar desert does not result from a reduction in plant water stress by a greater supply of meltwater through the growing season. Instead, surface meltwaters probably benefit vascular plants indirectly by facilitating the growth, development, and nitrogen fixation activities of cryptogamic organisms in the soil crust. The presence and activity of these cryptogams favor vascular plant success through increased nutrient availability, soil organic matter, surface temperatures, reduced soil cryoturbation, and more favorable sites for germination and seedling establishment.
Gap Partitioning Among Maples (Acer) in Central New England: Survival and Growth
Authors: Sipe T. W. , Bazzaz F. A.
Published: May 2006

We measured survival and growth of three shade—tolerant species of maple (Acer pensylvanicum, A. rubrum, and A. saccharum) in response to understory and experimentally created small canopy gaps of two sizes (8 x 12m, 75 m2; 16 x 24 m, 300 m2) in central New England. Seedlings of the three species (2160 total, 720 per species) were transplanted into five plot locations (center plus northwest, northeast, southwest, and southeast gap edges) within all gap and understory sites. Measurements of microclimates, architecture, photosynthetic performance, survival, and growth were made over 1 yr before, and 2 yr following, gap release. Red maple (A. rubrum) survived better overall across the study due to greater persistence in the north and center plots of large gaps. The small gaps and understories showed no differences among the species. Survival rates exceeded 80% in most sites and plots, with low values (30—65%) only in the exposed plots of large gaps. There were no relationships between post—gap survival and previous age, height, or basal diameter. By the end of 2 yr of gap release, both gap sizes induced greater distinctions among the species in all growth variables than the understory. Striped maple (A. pensylvanicum) exhibited greater leader extension, absolute stem height, net height change, absolute basal diameter, and net basal diameter change than red maple and sugar maple (in that order) in nearly all sites and plots. The exception was large—gap center and north plots, where red maple equalled or exceeded striped maple in net basal diameter change, but not net height increase. Sugar maple (A. saccharum) was the least responsive of the species to the gap—understory gradient. As with survival, there were no predictable relationships between pregap age or size and post—gap growth. Photosynthetic performance paralleled growth by these species across the gradient, particularly for shoot assimilation. When growth variables were plotted against irradiance and temperature measured at seedling plot positions, there were consistent and clear distinctions among species across the gap—understory gradient, providing limited evidence for gap partitioning in our system. Striped maple appears to be a superior generalist, red maple is a weaker generalist, and sugar maple shows the poorest performance in a manner that is nearly insensitive to the gap—understory gradient, in our experimental system.
Is Mean Litter Size the Most Productive? A Test in Columbian Ground Squirrels
Authors: Risch Thomas S. , Dobson F. Stephen , Murie Jan O.
Published: May 2006

We studied the evolution of litter size in natural and experimentally manipulated populations of Columbian ground squirrels (Spermophilus columbianus) in eastern Washington state and southwest Alberta. Litter size at weaning in a large natural population (mean = 3.51 pups/litter, 248 litters) was significantly lower than the litter size (6 pups) that produced the most offspring surviving to yearling age class. This evidence contradicted Lack's "optimal litter size" hypothesis, which predicts that the most productive litter size should approximate the mean. Litter size had no significant effect on the subsequent survival or reproduction of mothers, contrary to the negative effects predicted by the "cost of reproduction" hypothesis. Litter success varied among years, and good and bad years for reproduction could be experimentally simulated with food supplementation. However, small samples of the largest litters rendered the "bad—years" hypothesis inappropriate for application to our data. Proportional survival of offspring was relatively constant among different litter sizes, contrary to the necessary condition of the "cliff edge" hypothesis of a dramatic decrease in survival of young from the largest litters. The data supported the "individual optimization" hypothesis most strongly. As predicted, the number of surviving offspring increased with litter size, although in one population the six largest litters (2.4% of 248 litters) suffered reduced success. In natural and food—supplemented populations, changes in maternal body mass were associated with changes in litter size (r = 0.205 to 0.926). Because survival of young remained relatively constant as litter size increased, these correlations resulted in greater offspring success for mothers in better body condition.
Bias in Estimating Forest Disturbance Rates and Tree Lifetimes
Authors: Fox John F.
Published: May 2006

Intervals between fires, tree blowdowns, and other large and small forest disturbances are often estimated by dating tree rings. Dates are taken from living trees, fallen logs or stumps, or other indicators such as peaks in tree age distributions and ages of understory plants damaged by treefalls. Such indicators disappear gradually over time; this will bias estimates of disturbance recurrence times or tree lifetimes. Models and simulations employing plausible rates of disturbance and indicator disappearance show that the bias can be substantial. Comparison of a realistic simulation with simpler models suggests that it may be difficult to correct the bias, given estimates for indicator survival rate. I recommend that investigators estimating forest—disturbance recurrence items also estimate survival rates for their indicators, and collaborate with statisticians (about estimating survival times with censored data) before designing field studies.
Seed Availability and Biotic Interactions in Granite Outcrop Plant Communities
Authors: Houle Gilles , Phillips Donald L.
Published: May 2006

Factors controlling the structure of plant communities occupying soil—filled depressions on granite outcrops in southeastern United States were investigated by adding and/or removing species. The effects of the treatment on vascular plant density and cover, and on moss and lichen cover, bare ground, and some soil characteristics (depth, bulk density, pH, and organic matter content) were followed over 2 yr. Addition experiments performed in shallower soil island communities revealed that shallower soil species often significantly increased the establishment of deeper soil species. This positive effect might have resulted from indirect interactions mediated by mosses and lichens. Extreme abiotic factors (i.e., drought) seemed to have overridden this initial facilitative effect, however. Species removals performed in the tension zones of island communities indicated that, although physiological tolerances, and not biotic interactions, appeared to determine the lower limit of some species (annuals) on the depth gradient, for others (perennials), biotic interactions with shallower soil species seemed to be significant in restricting the populations to the deeper section of the gradient. Few soil characteristics responded to our experimental manipulations over the 2—yr study period. Succession occurs in the soil—filled depressions on granite outcrops, and because the spatial patterns studied here appear to be good analogs of the temporal patterns, our results suggest that mechanisms of facilitation followed by tolerance and inhibition are operative in the sere.
Density‐Dependent Size Regulation in Diadema Antillarum: Effects on Fecundity and Survivorship
Authors: Levitan Don R.
Published: May 2006

Food limitation can often result in reduced fecundity or death. However, a reduction in body size can minimize these negative effects. This study demonstrates that the sea urchin Diadema antillarum has the ability to adjust body size quickly (by positive or negative growth) to a resource— or density—dependent level. In a field experiment, small (27.5 mm test diameter) and large (37.5 mm) urchins were placed in cages at low (12 individuals/m2) and high (24 individuals/m2) densities. After 26 wk, body size in the high density treatment had converged at 30 mm, and body size in the low density treatment had converged at 36 mm test diameter. Mortality was not size or density dependent. After 32 wk, the effects of initial size on mass, gonad volume, and gamete production could not be detected. Diadema adjusted body size to a density—dependent level predicted by the inverse relationship between mean size and density seen in field populations. In a laboratory experiment, urchins were placed in groups of one or three, and fed one of four rations of food. After 23 wk, the urchins had adjusted test diameter, mass, and gonad volume to a level determined by per capital food availability and not crowding. Gonad volume was not significantly different from that predicted by body size (compared to field populations), regardless of whether the urchins had grown or shrunk. Among the fed urchins, mortality was not resource dependent, even though the urchins in the low—food group exhibited significant negative growth. Size regulation thus increases survivorship and reproductive output under rapidly changing conditions of population density and resource availability. A consequence of this life history strategy is that populations tend to track carrying capacity closely.
Size and Shape of Sawfly Assemblages on Arroyo Willow
Authors: Boecklen William J. , Price Peter W.
Published: May 2006

We present new techniques for the analysis of abundance data from multispecies communities. We partitioned variation in abundance into two components: variation in absolute abundance and variation in relative abundance. We examined patterns of absolute and relative abundance for multispecies assemblages from a multivariate morphometrics perspective. Multivariate morphometric analyses of abundance patterns have both heuristic and quantitative advantages over information theoretic approaches. We extended the concepts of isometry and allometry to community structure and illustrated these principles with density data from a group of five closely related tenthredinid sawflies that infest clones of arroyo willow (Salix lasiolepis). There were significant differences among willow clones in the size and shape of their sawfly assemblages. In fact, willow clones are virtually unique in terms of absolute and relative densities of sawflies. We used size and shape analysis to compare community structure of sawflies on male and female willows. Male clones differed from female clones in absolute densities of sawflies but not in relative densities.
Intial Investment, Clutch Size, and Brood Reduction in the Common Grackle (Quiscalus Quiscula L.)
Authors: Howe Henry F.
Published: May 2006

Female Common Grackles (Quiscalus quiscula L.) laying clutches of 2, 3, and 4 eggs show different patterns of initial investment in offspring than those laying clutches of 5 and 6. Females laying small clutches do not increase egg size with laying sequence and hatch young synchronously by starting incubation after the last egg is laid. Such parents usually raise the entire brood. Females laying clutches of 5 and 6 increase provisioning of yolk and albumen with laying sequence, start incubation before the clutch is complete, and have variable success in space and time. Few raise the entire brood. Late—hatching young are favored by large egg size, but disadvantaged by hatching order because older siblings are fed first and grow faster. Youngest members of a brood are maintained as long as possible, but differentially starve if food is scarce. A review of the literature suggests that similar patterns may be widespread in passerine birds. Antagonistic forms of parental investment and brood reduction are interpreted as “bet—hedging” tactic for minimizing losses of parents attempting to raise large broods during periods of uncertain food availability. The most common clutch of 5 in Common Grackles is, on the average, the most productive of fledglings. But complete success of small broods and variability in the success of adults laying "the most productive clutch" suggest selection for individual optima related to demographic or ecological circumstances. Population averages emphasized in the literature appear as effects of such selection.
Seasonal Variation in Reproductive Output of House Sparrows: The Determination of Clutch Size
Authors: Murphy Edward C.
Published: May 2006

Seasonal changes in reproductive output of house sparrows, Passer domesticus, were studied at 2 localities: Lawrence, Kansas, USA and Calgary, Alberta, Canada. In both localities the most productive clutch (6 eggs) was larger than the modal clutch (5 eggs). Most variation in clutch size could be attributed to within—individual seasonal changes. Laying of 6—egg clutches was generally restricted to the middle of the breeding season. Clutch size changes were not consistently synchronized with trends in predictable environmental variables such as photoperiod and temperature. Because seasonal trends in clutch size and fledging success were poorly correlated, there is no evidence that seasonal changes in clutch size represent adaptations to maximize the number of adequately nourished offspring. Although clutch size is ultimately determined by contribution to future generations, it is probably proximately controlled by resources at the time of oogenesis.