bluuebunny/crossref_metadata_2025 huggingface dataset:
Applications of Discrete and Continuous Network Theory to Linear Population Models
Authors: Lewis E. R.
Published: May 2006

The well—established methods of network construction and analysis are adapted to the problem of modeling single populations. A major advantage of the resulting approach is that it allows explicit incorporation of key processes in the life cycle of the organism being modeled, with feedback loops providing economy of representation where they are allowed. Thus, network structures provide heuristic vehicles by which population models can be developed and modified. When a model is linear and has parameters that do not vary with time, a characteristic dynamic function can be derived by inspection from a simple transform of the network representation. The zeros of the function can be found (analytically or by commonly available numerical methods) and used directly to deduce the modeled population's dominant growth pattern and its propensity to sustain oscillations. In addition, under certain conditions (i.e., that the network model not contain both time delays and integrators), a straightforward mathod (partial fraction expansion) is available for deduction of the modeled population's specific responses to a variety of perturbations.
Energetics of a Suburban Lawn Ecosystem
Authors: Falk John Howard
Published: May 2006

A study of the energetics of a sururban lawn was conducted in 1972—73 in Walnut Creek, California USA. Several major components of the annual primary and secondary production were measured, including man's role as manager and experimenter in the system. The system was extremely productive with net productivity of 1,020 g/m2 per yr compared to cornfields with productivity of 1,066 g/m2 per yr and exceeding tall grass prairie values of around 1,000 g/m2 per yr. Homopterans, with maximal values of 19 mg/m2 were plentiful; other typical grassland species, like Araneida, were scarce, representing only 1% by weight of the total invertebrate population. Food utilization per unit area by suburban birds considerably exceeded natural grassland bird utilization (46 kcal/m2 per yr vs. 1.01—2.33 kcal/m2 per yr); lawns are ideal foraging sites for open area adapted, flock—feeding species. Man was the dominant consumer in the community, accounting for 10% of the herbivory and nearly 100% of the scavenging. Energy inputs (labor, gasoline, fertilizer, etc.) amounted to 578 kcal/m2 per yr, equalling or exceeding corn production for a comparable net productivity, but not necessarily utilitarian return.
Divergence of Two Co‐Occurring Successional Annuals on a Soil Moisture Gradient
Authors: Pickett S. T. A. , Bazzaz F. A.
Published: May 2006

Polygonum pensylvanicum and Abutilon theophrasti, components of old—field communities in central Illinois were grown on a controlled soil moisture gradient to determine whether they exhibit different response centers. Germination response of a constant number of seeds per six moisture resource states, height, biomass, and reproductive response of initially constant numbers of plants per state were monitored. States, ranging from saturation down to 13.61 ± 5.53% moisture, are numbered in order of increasing drought. Maximum height of Polygonum occurred in states 4 and 5, and of Abutilon in 4, 5, 6. Vegetative and reproductive biomass maxima of Polygonum and Abutilon occurred in states 5 and 6, respectively. Abutilon reproduced only in states 5 and 6 where it overtopped the Polygonum canopy, while Polygonum reproduced in all states. Polygonum performance declined sharply in state 6. Thus, Polygonum and Abutilon response centers are displaced toward the wetter and drier ranges of the gradient, respectively. On a within—community level, Abutilon reproduces in less mesic resource states where Polygonum canopy height is reduced. In general, Abutilon reproduction may occur throughout the gradient when the Polygonum canopy opens. The performance of these species is strongly influenced by both physical and biotic conditions. Previously investigated adaptations, including drought resistance and rooting patterns in space and time, appear to underlie divergence along the moisture gradient. The response patterns found here are broad and exhibit considerable overlap as expected in fugitive strategies. The patterns of divergence shown on point and gradient scales in this and our previous work reflect a related complex of adaptations and strongly suggest that competitive divergence occurs in early successional communities.
Empirical Evaluation of the Hypothesis of Food Competition in Salamanders of the Genus Plethodon
Authors: Fraser Douglas F.
Published: May 2006

Two species of salamander, Plethodon cinereus, and Plethodon hoffmani were used in experiments designed to test the hypothesis that food is the object of competition. Plethodon hoffmani occurs in the Appalachian Ridge and Valley Province where it appears to exclude the closely related and contiguously allopatric P. cinereus. Detailed field observations and a laboratory experiment confirm Jaeger's (1972) hypothesis that the food of salamanders could be limited in availability during dry weather and suggest that the cause of the low availability is the reduced mobility of the salamanders rather than a reduced food supply. However, because stomach content data and behavioral observations indicate that salamanders do not forage when the surface is dry, it was reasoned that food competition could occur under only two restricted conditions: (1) when more than one animal is isolated under damp refugia, such as rocks and logs, during dry weather, and (2) when many animals emerge onto the surface to forage when wet weather follows a prolonged dry period. Empirical evidence suggests that salamanders do not clump in refugia as the habitat dries out and that they do not surface simultaneously to forage after a dry period. Thus, even though food is periodically unavailable, the staggered feeding schedule prevents the density of salamanders from ever reaching competitive levels. Interference mechanisms are not ruled out as mechanisms of the observed spacing patterns. Competition for space is suggested as an alternative to the food competition hypothesis.
Successional Relationships of Five Florida Plant Communities
Authors: Veno Patricia Ann
Published: May 2006

Permanent 20—yr—old quadrats located in five plant communities in the University of Florida's Welaka Conservation Reserve in Putnam County, Florida, USA were inventoried to determine successional changes. The five communities included mesic hammock, xeric hammock, bluejack oak sandhill, turkey oak sandhill, and scrub. The reserve has been protected from fire and no fires had occurred on the study area for at least 40 yr. The quadrats were analyzed in terms of changes in species composition, number of stems, and growth. Increased similarity in species composition to the mesic hammock was found in the xeric hammock, bluejack oak sandhill, and turkey oak sandhill. Tree species composition changed most in the turkey oak sandhill, which gained six tree species and lost none, followed by the xeric hammock, the bluejack oak sandhill, the mesic hammock, and the scrub which exhibited no species change. A successional scheme for the xeric plant communities of north—central Florida was constructed. Rates of succession were also determined. It is proposed that the bluejack oak sandhill, turkey oak sandhill, and xeric hammock communities are changing toward a form of mesic hammock. Although the scrub was not changing in species composition, the individual trees were growing in number and size and the community may possibly become a xeric hammock. The successional scheme was derived for a group of ecosystems under man—induced fire suppression and thus is not necessarily true for undisturbed conditions in which natural fires play an important role. As the amount of land uninfluenced by man's management practices decreases, successional patterns as observed in this study will become the predominant forms.
Autumnal Processing of Dissolved Organic Matter in a Small Woodland Stream Ecosystem
Authors: McDowell William H. , Fisher Stuart G.
Published: May 2006

A mass—balance approach was used to construct a dissolved organic matter (DOM) budget for Roaring Brook, a small New England stream, in autumn. Total litter input to the stream ecosystem during the 77—day study period was 345 g/m2, 21% of which occurred as the lateral transport. Nearly 17% of litter input was released to the water as DOM within 3 days of entry. This leachate input represents 42% of total DOM input to the 1260—m study section. The remainder entered by way of inflowing surface and subsurface water. Autumn uptake of DOM was equivalent to 77% of leachate input and 33% of total DOM input. Approximately 88% of DOM removed from the water column is retained within the system and at least 67% is energetically degraded by consumer respiration in autumn. Based upon total annual energy budgets for similar streams, we estimate that DOM utilization in autumn accounts for from 7% to 11% of annual ecosystem respiration in Roaring Brook.
Prey Selection by a Setipalpian Stonefly Nymph, Acroneuria (Calineuria) Californica Banks (Plecoptera:Perildae)
Authors: Siegfried Clifford A. , Knight Allen W.
Published: May 2006

Predation by the stonefly nymph, Acroneuria (Calineuria) californica, was studied for 1 year in a Sierra Nevada foothill stream. Estimates of instantaneous consumption were obtained by analysis of the foregut contents of A. californica nymphs collected from the study stream at monthly intervals. The bulk of the prey items present in the guts consisted of chironomid larvae, baetid mayfly nymphs, and larvae of hydropsychid caddisflies. These three groups collectively made up > 78% of all animal prey (by numbers) in the stonefly guts in each month except November. The stonefly nymphs were not strictly carnivorous. Detritus was consumed in all months, and Ulothrix, a filamentous green alga, was an important food source in the spring. Quantitative benthic samples were obtained at each collection date to determine prey availability. Electivity coefficients were calculated each month for each prey item. The electivity values, though somewhat variable, were generally positive for baetid mayfly nymphs and chironomid larvae, and negative for other prey. The most important factor determining prey selection appears to be prey availability. No great preference for any particular prey was apparent. Size considerations operate to limit the size of prey a stonefly can ingest but make larger prey more suspectible to discovery by the foraging stonefly.
Precipitation and Streamwater Chemistry in an Undisturbed Forested Watershed in New Hampshire
Authors: Martin C. Wayne
Published: May 2006

Precipitation and streamwater from the Bowl, a watershed in central New Hampshire, were analyzed chemically during 1973 and 1974. The Bowl, covered by a northern hardwood forest with spruce and fir at higher elevations, has never been logged or otherwise disturbed by humans. Calcium, sulfate, and nitrogen ions dominated stream chemistry. The biogeochemistry of the cations seemed to be regulated more by precipitation, soil—water movement, and chemical weathering reactions than by forest succession. Nitrate concentrations remained nearly constant, at °2.5 mg/l throughout the study, with no apparent seasonal fluctuations; in contrast, nitrate concentrations in the stream draining a nearby 55—yr—old forest definitely declined during the growing season. Nitrate budgets indicated a net loss of this important plant nutrient from both watersheds. However, a net accumulation of ammonium was sufficient to give a net increase of total N in both watersheds. These data do not support the hypothesis of Vitousek and Reiners that old—growth forests may reach a point of no net growth and no net uptake of nutrients. Mixed deciduous—coniferous forests in New England, free from human disturbances, may reach an age where they become prone to natural disturbances that create a mosaic of similar—aged group of trees, each group having differing abilities to accumulate nutrients.
Seed Dispersal by Baboons in the Shai Hills, Ghana
Authors: Lieberman Diana , Hall J. B. , Swaine M. D. , Lieberman Milton
Published: May 2006

Baboon dung was collected in the Shai Hills, Ghana at 4—wk intervals from November 1975 to January 1977, and was planted in sterilized soil in order to estimate the viable seed content of the dung. A total of 6,465 seedlings germinated, belonging to 59 species, although 70% of the seedlings belonged to 2 fleshy—fruited species. The species composition varied from month to month as did the relative importance of dry fruits. Germination of ingested seeds (removed from dung) was significantly improved over that of fresh seeds in 3 of 4 species tested (Securinega virosa, Azadirachta indica, and Nauclea latifolia). The 4th species, Diospyros mespiliformis, showed no significant improvement.
Fire History of a Sequoia‐Mixed Conifer Forest
Authors: Kilgore Bruce M. , Taylor Dan
Published: May 2006

Data on the years in which fires burned, on fire frequency, and on intensity and areal extent of fires were gathered from 935 scars on 220 stumps of mixed conifer forest species in an 1800—ha study area in the Sierra Nevada, California, USA. Before 1875, fires scarred clusters of living trees every 9 yr on west—facing slopes at Redwood Mountain and every 16 yr on east—facing slopes. Mean fire—free intervals between 1700 and 1875 varied by habitat phase from 5 yr in ponderosa pine on a dry ridge to 15—18 yr in most moist sites with white fir. For most 1—ha sites, the maximum time without fire was 14—28 yr. From 1700 to 1875, fires on various sizes were found every 2—3 yr somewhere in a given drainage (not necessarily the same site) and every 5—9 yr in 3— to 16—ha sites. This compares with fires every 8—18 yr in 1—ha clusters and 11—39 yr on individual trees. Scar records of pre—1700 fires suggest intervals fairly comparable to those from 1700 to 1875. Evidence of fires diminished greatly after Indian burning was eliminated in the early 1870's, and such fire records became almost nonexistent after 1900, when fire suppression became more effective. Most of the pre—1875 fires were small and of low intensity. Even the larger fires were usually confined to 1 slope or 1 drainage area. The short mean intervals between fires suggest that pre—1875 mixed conifer forests did not usually have heavy accumulations of litter or dense thickets of understory trees. Instead, small—acreage, low—intensity surface first must have consumed accumulated litter at frequent intervals and at the same time killed most of the conifer regeneration which had become established since previous fires. Such frequent fires would have led to an intricate mosaic of age classes and vegetation subtypes which, in turn, insured that a subsequent fire would not burn large areas with great intensity. Intense fires which moved from crown to crown were absent in the study area for the past 400 to 2000 yr. If frequency of lightning ignition of fires over the past 50 yr is typical, ignitions by Indians must have augmented lightning—caused fires to yield the pre—1865 frequency of fires in the Sierra mixed conifer forest. Since 1900, the lack of frequent, low—intensity fires has resulted in a major increase in understory forest and fuels.
Habitat Utilization, Competitive Interactions, and Coexistence of three Species of Cricetine Rodents in East‐Central Arizona
Authors: Holbrook Sally J.
Published: May 2006

The ecological relationships of three cricetine species–Peromyscus maniculatus, Peromyscus boylii, and Neotoma stephensi–and their utilization of habitat were revealed by species removals from unfenced plots and vegetation tailoring experiments. When N. stephensi, a woodland and shrubland species of woodrat, was removed from a manzanita—oak shrubland and pinyon—juniper woodland, P. boylii showed few significant changes in its use of the vegetational microhabitats on the grid or in the nature of its arboreal activity. However, when P. boylii, a woodland and shrubland mouse, was removed from a grid in juniper—oak shrubland and juniper grassland, both P. maniculatus, an inhabitant of open habitats such as grassland, and N. stephensi expanded their microhabitat utilization and changed their patterns of aboveground activity. Subsequent removal of both N. stephensi and P. boylii from this plot resulted in further expansion of arboreal activity by P. maniculatus. After crown removal of 0.75 ha of manzanita—oak shrubland, P. boylii, but not N. stephensi, avoided the newly opened area. Habitat selection, differential resource utilization, and interspecific competition all contribute to the coexistence of these three species in patchy habitats.
Least‐Squares Mapping Using Interpoint Distances
Authors: Rohlf F. James , Archie James W.
Published: May 2006

A new method for the mapping of the relative locations of organisms as points in a 2—dimensional space is presented. The proposed method is based on a least—squares estimate of the coordinates using observed distances between the objects being mapped (rather than triangulation based on horizontal angles). Only a small subset of the possible interpoint distances need be measured in order to obtain a very accurate map. The method is applied to 3 problems in mapping of trees in a forest.
Protozoan Density and the Coexistence of Protozoan Predators and Bacterial Prey
Authors: Habte M. , Alexander M.
Published: May 2006

Low numbers of Tetrahymena pyriformis provided with °2 x 109 Klebsiella pneumoniae or Rhizobium sp. cells/ml reduced the bacterial population only to 107/ml in solution. When 5—7 x 108 K. pneumoniae or 1 x 108 Rhizobium sp. were preyed upon, 106 bacteria/ml survived and coexisted with the ciliate. Low initial densities of the protozoan provided with 3 x 106 or 5 x 107 K. pneumoniae or 3—4 x 107 Rhizobium sp./ml reduced the prey population t 2—3 x 105/ml. However, when the initial number of T. pyriformis was high (2—34 x 104/ml) and the K. pneumonia or Rhizobium sp. density was °107/ml, the animal did not reduce the bacterial population size. The data suggest that the number of K. pneumoniae or Rhizobium sp. cells able to coexist in solution with T. pyriformis was governed by the number of protozoa initially present or developing as a result of predation. By contrast, 105 Xanthomonas citri cells/ml coexisted with the ciliate whether this bacterium was provided at 7 x 106 or 8 x 108 cells/ml. In soil treated with 3.5 x 108 and 3.1 x 109 Rhizobium sp./g, the density of indigenous protozoa rose to 1—2 x 104 and 4—5 x 105/g, respectively, yet the density of rhizobial survivors stabilized at 3—7 x 107/g, a range higher than that observed in solution.
An Experimental Test of a Guild: Salamander Competition
Authors: Hairston Nelson G.
Published: May 2006

The guild of plethodontid salamanders of the deciduous forest of the southern Appalachians shares the common resource of food, which consists of insects and other invertebrates inhabiting the forest floor and low vegetation. Since in practice, use of the guild concept implies competition for the shared resource, the validity of accepting a guild as delimited for the terrestrial Plethodontidae was tested by field experiments covering the 5 yr 1974—1978. The test consisted of regularly removing the most abundant species, Plethodon jordani, from one set of experimental plots, and the second most abundant species, P. glutinosus, from another set of plots. Each of these two species was favorably affected by the removal of the other P. glutinosus increased significantly in abundance where P. jordani was removed. Removal of P. glutinosus resulted in a significant increase in the proportion of young P. jordani. The remaining species, including the congeneric P. serratus, were not affected by the removal of either of the two most abundant species. The results call into question the common assumption that competition is the organizing force in all coexisting assemblages of species which share common resources. The data also imply that food is not the resource for which P. jordani and P. glutinosus compete. Nest sites may be the critical resource, but much information is needed before they can be implicated.
Seed Predation by Rodents on Three Ant‐Dispersed Plants
Authors: Heithaus E. Raymond
Published: May 2006

Seed predation by small mammals has been proposed as one selective force in the evolution of ant—dispersal (myrmecochory) in many herbaceous plants of mesic deciduous forests. Three myrmecochorous species, Asarum canadense, Jeffersonia diphylla, and Sanguinaria canadensis, were studied in the Monongahela National Forest, West Virginia, USA, to (a) assess the impact of seed predators on seed mortality, (b) compare seed removal rates when ants were present vs. absent, and (c) evaluate the effects of elaiosome removal, seed burial, and doubling seed number per depot on rates of seed predation. Seed predators, especially rodents, cause more mortality among these woodland herbs than previously thought. Predispersal seed predation by rodents destroyed 24% and 47% of the seed crops of A. canadense and J. diphylla, respectively. The proportions of A. canadense seeds lost to predators varied among sites. Sanguinaria canadensis also suffered seed predation before capsules dehisced. Peromyscus leucopus was the most common seed predator. In exclosure experiments, seed predators removed 70% of the A. canadense seeds when dispersal by ants was prevented, but only 24—39% of the seeds suffered predation when ants were allowed access. Seed predators removed 84% of the S. canadensis seeds when ants were excluded, but 13—43% were lost when also exposed to ants. These estimated minimal proportions of seeds lost to predators when ants were allowed access differed between two study sites, but myrmecochory clearly reduced seed predation at parent plants. In laboratory experiments, P. leucopus located buried A. canadense and S. canadensis seeds less frequently than seeds on the surface (67.5% vs. 100%). Consumption of buried seeds by rodents was reduced when elaisomes were removed, as would be done by ants in natural conditions. Increasing the detectability of seeds through adding olfactory cues or doubling seed number per depot did not increase the frequency of predation, because mice were able to locate depots without these additional cues.
On Measuring Fluctuating Environments: Predictability, Constancy, and Contingency
Authors: Stearns Stephen C.
Published: May 2006

By simulating 640,000 time—series either 30 or 300 observations long, I evaluated the precision and power of significance tests for four measures of nonmetric time—series: Colwell's C and M and Goodman and Kruskal's @lR°C and @tR@?C. Significance tests for all four measures are powerful but do not fit published g2 and t values. An Appendix gives critical levels for the test statistics of all four measures for tables from 2 x 2 to 5 x 5 and for time—series with lengths 30 and 300. To detect the significant periodicities in a time—series with C and M, one should calculate the measures for many different time classes (contingency will peak when the number of time classes matches a natural periodicity), and sample at least twice as frequently as the period of the highest frequency fluctuation that one wants to detect (more frequently if asymmetric fluctuations are suspected). Colwell's measures were not as sensitive as Fourier transforms in detecting significant periodicities in metric test data, but their overall performance on metric data was good enough to justify their use on nominal and ordinal data. A method is given for estimating from pilot data the sampling effort required to detect a significant fluctuation.
A Theory of Habitat Selection
Authors: Rosenzweig Michael L.
Published: May 2006

A graphical theory of habitat selection is built in steps. The theory treats two species in an environment with two usable patch types in a matrix of unusable space. The first step assumes habitat selection is density independent and free of search costs. The second assumes density independence, and the third assumes neither. The first two steps produce results already known from earlier theories. The third, however, requires a new analytical device, the isoleg, which is a line in a two—dimensional—state space of the two species' densities. An isoleg is a set of points in such a density space, such that on one side of the set, individuals of a species optimize their foraging by being strict habitat selectors, whereas on the other side, they do so by using at least a bit of a poorer patch. The population dynamics of the competitors is analyzed using their isolegs. The isolegs allow us to deduce that the zero isoclines of the species are warped into nonlinear forms capable of producing competitive coexistence. It is shown that at the equilibrium point of this coexistence, there may be no overt competition remaining. The difficulties this presents to the field investigator are mentioned, and a modified definition of interspecific competition is suggested.
Longitudinal and Seasonal Changes in Functional Organization of Macroinvertebrate Communities in Four Oregon Streams
Authors: Hawkins Charles P. , Sedell James R.
Published: May 2006

Relative numerical dominance and densities of invertebrate functional feeding groups are compared with longitudinal and seasonal changes in food resources in a Cascade Range stream system in Oregon. We also compare our data with hypothetical predictions of the River Continuum model. We found that both relative abundances and densities of functional groups fit qualitative characterization of stream reaches and the River Continuum model: Shredders dominated upper shaded reaches; scrapers were most important in intermediate—sized sections; collectors increased in importance progressively downstream; predators were nearly constant in relative abundance at all sections; other functional groups were either rare or only locally abundant. Seasonal shifts also occurred but were not as dramatic as longitudinal differences. Correlation analyses between quantitative measures of food sources and functional group abundance were usually significant. Lack of significance for some correlations is likely due to inaccurate characterization of food availability, but small size and improper classification of species' feeding behavior also may be important. In addition, invertebrate densities and food standing crop are static parameters and only approximately reflect the dynamics of consumer production and food availability.
Frequency Distributions of the Concentrations of Essential and Nonessential Elements in Largemouth Bass, Micropterus Salmoides
Authors: Pinder John E. , Giesy John P.
Published: May 2006

From data on elemental concentrations in human tissues, Liebscher and Smith (1968) hypothesized that the frequency distributions of concentrations for essential elements were normal distributions, whereas the frequency distributions for nonessential elements were lognormal distributions. Although Liebscher and Smith's justifications for this hypothesis are flawed, other researchers have reported similar observations. We measured the concentrations of Cd, Ca, Cr, Co, Cu, Fe, Pb, Mn, Hg, K, Se, Na, and Zn in the muscle, liver, and egg tissues of female largemouth bass to determine (1) to what extent the frequency distributions of elemental concentrations are affected by errors in measuring concentrations and (2) whether the previously observed differences between essential and nonessential elements could be demonstrated if we restricted our comparisons to only those elements whose concentrations can be accurately measured. Variance component analyses of elemental concentrations in muscle tissue indicated that variations among replicate tissue samples due to measuring errors were large relative to the variations among individual fish for Cd, Ca, Mn, K, and Na. For elements where variation among individuals was not obscured by errors in measuring concentrations, there were no apparent differences between the frequency distributions for the essential elements, Cr, Cu,Fe, Se, and Zn, and the distributions for the nonessential elements, Pb and Hg. The hypothesis of Liebscher and Smith was not supported by our data.
Vegetation on Limestone and Granite in the Mule Mountains, Arizona
Authors: Wentworth Thomas R.
Published: May 2006

Gradient analysis was employed to determine vegetation pattern in relation to environment on limestone and granite in the Mule Mountains, Cochise County, Arizona, USA, using data obtained from 175 0.1—ha (20 x 50 m) quadrats located over ranges of elevation and topographic position. Sequences of major community types from mesic to xeric positions on the mountain slopes were pygmy conifer—oak scrub, open oak woodland, and desert grassland on granite and Cercocarpus breviflorus scrub, mesic phase Chihuahuan desertscrub, and xeric phase Chihuahuan desertscrub on limestone. Calcareous bajadas (alluvial plains) southwest of the Mule Mountains support a disturbed Chihuahuan desertscrub type. An environmental scalar was developed to quantify site position along complex—gradients of elevation and topographic position. The scalar was correlated with indirect ordination axes from reciprocal averaging, principal components analysis, and Wisconsin polar ordination. Over similar ranges of elevation and topographic position, granitic sites have higher species richness, greater importance of herbaceous and arborescent species, and lower importance of shrubby species than limestone sites. Both rock types support vegetation rich in widely distributed southwestern species but the vegetation on limestone has a distinctly Chihuahuan character, while that on granite is Madrean. Investigation of species distributions indicated a modal shift toward more mesic environments on limestone as compared to granite. This shift complemented other observations which suggested more xeric conditions on limestone. The low—elevation plant communities on limestone represent the northwesternmost occurrence of the Chihuahuan Desert formation. This may be due to three factors: (1) limestone habitats are arid (as judged from observations of vegetation); (2) increased aridity favors desert species, but the local climate (particularly cool winter temperatures) selects for Chihuahuan, rather than Sonoran, species; and (3) among desert species the Chihuahuan are better adapted to calcerous soils, which occur widely in the Chihuahuan Desert.
Community Differentiation Along a Dune Succession: An Experimental Approach With Corprophilous Fungi
Authors: Yocam D. H. , Wicklow D. T.
Published: May 2006

Exposure of laboratory—collected rabbit feces in different habitats associated with a dune succession in Presque Isle State Park, Erie, Pennsylvania, USA, affected the structure and composition of the coprophilous fungal communities developing on these feces. Nylon mesh litter bags containing 4—g samples of rabbit feces, the latter incorporating a similar fungal inoculum, were exposed in the field for either 2— or 4—wk intervals throughout the summer of 1975. This was followed by a 40—d incubation in laboratory moist chambers, during which time the development (sporulation) of fungal populations was studied and the frequency of occurrence of fungal taxa recorded. Community analysis was accomplished by position vectors ordination, and analysis of species diversity values. Experimental evidence is offered for the first time that microclimatic amelioration and stabilization in later stages of ecosystem development can be important in determining microfungal diversity. Community diversity increased when dune habitat and/or exposure interval were associated with a less stressful environment (i.e., presence of vegetational cover or greater available moisture).
Coevolution of the Life History of a Tropical Seed‐Feeding Insect and its Food Plants
Authors: Derr Janice A.
Published: May 2006

Field and laboratory studies of Dysdercus bimaculatus (Pyrrhocoridae; Heteroptera) were conducted in Costa Rica and Panama. The purpose of this work was to relate the life history of this insect and its host plants to the broader evolutionary context of the Dysdercus—Malvales association. Nymphs and adults feed on seeds of four tree species: Sterculia apetala (Sterculiaceae), Ceiba pentandra, Pseudobombax septinatum, and Bombacopsis quinata (Bombacaceae). The distance between trees and the hystolysis of wing muscles upon oogenesis effectively limit all the offspring of a female to one seed crop. These crops mature at different times during the tropical dry season. Crops are usually short lived and seeds are highly oleganious. Thus, the host plants of D. bimaculatus create a course—grained patter of point concentrations of energy—rich food resources occurring unpredictably in time and space. Migration and diapause are important "options" in the life history of the adult. Evidence that adult D. bimaculatus do fly between seed crops is indirect. Adult insects first appeared in the ripening fruits of the crops of one species after fifth—instar nymphs first occurred in the samples at the earlier ripening species. In the laboratory, females tended to delay egg development when food was plentiful but water limited. In the field, a declining proportion of nymphs appeared to remain and reproduce as adults at an usually long—lived S. apetala crop as conditions became drier. I proposed that uncertainty in the location in time and space and the level of moisture stress at different crops would account for variability in the tendency of females to develop eggs when water was limited. Two evolutionary processes would account for the association of D. bimaculatus with its particular type of food plants, relative to the many types of plant forms, seeds, and fruiting schedules in the Malvales: (1) I proposed that energy—rich food resources are required for D. bimaculatus to grow to its large adult size. Large body size is, in turn, though to enhance survival during diapause, migratory capacity, and reproductive output of this species. At present, these relationships are inferred from ecologically similar species. (2) I assumed that the risk associated with diapause is a function of time. This leads to the conclusion that D. bimaculatus populations would be dominated by individuals which included in their range all Malvales that produced concentrations of energy—rich seeds.
Variable Developmental Period: Intraspecific Competition Models with Conditional Age‐Specific Maturity and Mortality Schedules
Authors: Frogner Karl J.
Published: May 2006

A simulation model is presented incorporating a variable time to maturity resulting from density—dependent scarcity of resources for the immature stages. This mechanism is shown to be capable of regulating a population and the model population persists under these conditions. The model is qualitatively robust to changes in the parameters: resource input rate, offspring input rate, and immature death rate. Implications drawn from the model behavior with respect to pest control, “r—selection” under “K” conditions and field experiments are discussed.
Sampling Data on the Bactivorous Ciliates of a Small Pond Compared to Neutral Models of Community Structure
Authors: Taylor William D.
Published: May 2006

Small—volume samples of bactivorous ciliates were collected from a small pond. Qualitative (presence—absence) data were obtained from a set of 230 samples collected over all seasons during 3 yr and quantitative data were obtained from 81 samples collected during one summer. Analyses of these data demonstrate that apart from a segregation of taxa between aerobic and anaerobic benthic habitats, taxa were essentially independent in their distribution among samples. There was some indication of a net association in the qualitative samples, possibly due to seasonal and annual variations in ciliate abundance. There was little tendency for faunistically similar samples to be collected from the same site, at the same time, or form visually similar microhabitats. The frequency of samples containing different numbers of individuals and the ranked species abundance lists for samples taken at the same time and site are compatible with the species undergoing a stochastic pure birth and death process with rates linearly dependent on population size. The relative commonness of species is compatible with the hypothesis that the total number of occurrences is randomly partitioned among the species. Although communities of protozoa are thought to possess a complex structure maintained by interspecific interactions, the bactivorous ciliates in the pond appear to be an assemblage of essentially independent species undergoing density—independent growth processes and generally conforming to simple, neutral models of community structure.
Energetics of the Solidago Canadensis‐stem Gall Insect‐parasitoid Guild Interaction
Authors: Stinner Benjamin R. , Abrahamson Warren G.
Published: May 2006

Energy budgets are developed for two goldenrod stem gall insects: Eurosta solidaginis (Diptera: Tephritidae) and Gnorimoshema gallaesolidaginis (Lepidoptera: Gelechiidae). Energy budgets are also constructed for the parasitoid guilds associated with each of the gall insects. Over 25 kJ or °7% of mean ramet production were used in formation of these falls and support of the insects. If the gall insects were parasitized, only 15—20 kJ were used in gall and insect production. The ecological efficiencies of energy transfer between the gall insects and their parasites on a square—metre basis were 10 and 8% for Eurosta and Gnorimoshema, respectively. These insect energetics are then related to Harnett and Abrahamson's (1979) results for the effects of the two stem gall insects on resource allocation patterns in goldenrod, where gall—bearing ramets compared to non—gall—bearing ramets showed decreases of 45% (Eurosta) and 28% (Gnorimoshema) in ramet porgagule production. The energy lost form the ramet by having a ball gall is greater than is accounted for by energy flow into the gall, and the presence of either gall can have an appreciable impact on plant fitness.
Competition, Foraging Energetics, and the Cost of Sociality in Three Species of Bees
Authors: Schaffer William M. , Jensen Deborah B. , Hobbs Donna E. , Gurevitch Jessica , Todd James R. , Schaffer M. Valentine
Published: May 2006

Competition among three species of bees (Apis mellifera, Bombus sonorus, and Xylocopa arizonensis) visiting Agave schottii was studied. Honeybees were found to predominate in the most productive habitats, Xylocopa in the least; Bombus as most abundant in patches of intermediate quality. It is suggested that these observations are consistent with a simple graphical model relating the standing crop of available nectar to foraging energetics and the additional costs of colonial life (cost of sociality). Temporal variations in foraging behavior were also observed, with Apis again preferring to forage at those times of day (early morning and late afernoon) when resources were most abundant. Bombus was less crepuscular than Apis, and carpenter bees foraged without obvious regard to variation in standing crop. Several factors which may account for this pattern are discussed.
The Influence of Body Size on the Energetics and Distribution of Fossorial and Burrowing Mammals
Authors: McNab Brian K.
Published: May 2006

The energetics of four fossorial mammals, Spalacopus cyanus, Cannomys badius, Aplodontia rufa, and Scalopus aquanticus, are examined and compared to data available in the literature. Basal rates of metabolism are lower than expected from mass if fossorial mammals weigh >80 g but they are higher than expected if they weigh <60 g. This conclusion also applies to mammals that spend only part of the time in burrows. The season that most hibernating rodents have low basal rates of metabolism is that they burrow and weigh >100 g. Small burrowers may have low basal rates unrelated to burrowing habits. Minimal thermal conductances in fossorial and burrowing mammals are usually near the expected values at masses <1 kg, unless these species live in warm burrows, in which case minimal conductances are high. Burrowing mammals weighing > 1 kg have high minimal conductances. The temperature differential maintained at the lower limit of thermoneutrality and at temperatures above thermoneutrality is proportional to the ratio of basal rate to minimal conductance. Adaptations to a burrowing habit include (1) maintaining a small temperature differential independent of mass, which is accomplished by matching the mass sensitivity of the basal rate to that of the minimal conductance, (2) reducing basal rate, (3) increasing minimal conductance, and (4) reducing mass. The limits that exist to a reduction in mass and a reduction in basal rate of metabolism and to an increase in minimal conductance result in geographical limits to the distribution of fossoial mammals, but not necessarily to the distribution of other burrowing mammals.
Patterns of Primary Succession on Granite Outcrop Surfaces
Authors: Shure Donald J. , Ragsdale Harvey L.
Published: May 2006

The patterns of primary succession were studied in soil—island communities on a granite outcrop in Georgia. The island communities increase in area and depth over time and the soil becomes more organic. The strong moisture and temperature fluctuations that occur in shallow pioneer soils are significantly reduced in the later stages. Plant biomass and vertical stratification increase throughout succession as larger plant species invade the deeper communities. A small winter annual is the dominant pioneer species. Lichens, annuals, and eventually perennial species invade as succession progresses. Interspecific competition for moisture and nutrients regulates plant species composition at successive stages. Macroarthropod populations increase in density, biomass, and diversity throughout succession. The few soil microarthropod species that occur in the pioneer stages often exhibit rapid density oscillations in the shallow substrates. The deeper and more environmentally constant substrate of later stages contains a greater variety of microarthropods. Biotic diversity generally increases during primary succession on the outcrops. Plant diversity peaks at intermediate stages while microarthropod and macroarthropod diversity increase from pioneer through later stages. The strong physical factors on the outcrops determine the rate and extent of community development in particular soil—islands. However, as the many soil—islands undergo succession they converge in community characteristics such as total density, biomass, and diversity.
Disperal in Island and Mainland Voles
Authors: Tamarin Robert H.
Published: May 2006

Dispersal was studied in the beach vole, Microtus breweri (Baird) by monthly removal of animals from an experimental 0.8—ha grid. Microtus breweri is endemic to Muskeget Island, Massachusetts. A nearby live—trap grid provided information about residents. The same design was used on the mainland to trap the meadow vole. Microtus pennsylvanicus, to compare with the beach vole, which does not undergo the regular 3— to 4—yr cycles typical of the mainland. Both populations, however, had similar peak densities (160—180/ha). In M. breweri, dispersal is a simple process. Dispersers move short distances and are a random sample of the control population for age structure and sex ratio. In M. pennsylvanicus dispersal movements are longer. More mainland ♀ ♀ disperse in summers and more ♂ ♂ in winters. The age structure of mainland dispersers is not a random sample of the control population. Seventy percent of the variance in number of dispersers is explained by the density and rate of increase of the control population in M. breweri, but only 46% is thus explained in M. pennsylvanicus. Density is the more important of the 2 variables. Twenty—nine percent of the losses in the control population are caused by dispersal on Muskeget Island with a corresponding figure of 7% for the mainland. Virtually none of the mortality in the cyclic decline on the mainland can be accounted for by dispersal. In both species, dispersers are in reproductive condition to a greater extent than residents. Mice in the island population have larger home ranges than the mainland mice. Males have larger home ranges than ♀ ♀, and on the mainland, adults have larger home ranges that subadults. Both species have larger home ranges in summers. A model is constructed in which the differences in dispersal characteristics between the 2 species are attributable to differences in availability of a place into which the dispersers can move. I suggest that the mainland population has selective dispersal leading to cycles, whereas the island has a simple drainage of excess recruits, no selection by way of dispersal, and subsequently no cycles. The model suggest how this can evolve.
Desert Granivore Foraging and Its Impact on Seed Densities and Distributions
Authors: Reichman O. J.
Published: May 2006

Field experiments were conducted to determine the foraging behaviors of granivorous desert ants and rodents and to ascertain the impact of these foraging efforts on the density and distribution of seeds in the soil. Foraging experiments were performed which involved providing seeds within taxon—specific enclosures in scattered and clumped distribution on the soil surface and 1.5 cm below the surface. These experiments indicated that ants only foraged on the surface, whereas rodents were able to garner seeds from below the ground. Ants foraged °85% of the experimental seed distributions on the surface and were able to retrieve °45% of the seeds they detected during any 24—h experimental run. Rodents detected 100% of all distributions and got °96% of the seeds from all distributions except scattered/belowground, from which they gathered only 75% of the seeds. Three—year experiments using exclosures which excluded either ants, rodents, both taxa, or neither taxon, indicated that either taxon alone, or both taxon together, have a severe impact on the density of seeds in the soil. When neither is present, seed densities remain high. Furthermore, any combination of granivores greatly reduces the number of samples with either high numbers of seeds or high biomass of seeds, although the ants tend to leave a greater number of large clumps of seeds than the rodents. Clumping indices are also extremely high in the absence of granivores. In opposition to the apparently detrimental effect of seed use by the granivores, surface seed—caching by rodents appears to enhance seed germination.
Plutonium Inventories in Two Old‐Field Ecosystems in the Vicinity of a Nuclear‐Fuel Reprocessing Facility
Authors: Pinder J. E. , Smith M. H. , Boni A. L. , Corey J. C. , Horton J. H.
Published: May 2006

Plutonium inventories were determined in two old—field ecosystems, Fields 1 and 2, located near a nuclear—fuel reprocessing facility that had released approximately 640 mCi (24 GBq) 238Pu and 57o mCi (21 GBq) 239,240Pu to the atmosphere in the 20 yr preceding sampling. Field 1 was 230 m from the point of release (a 62—m stack) and contained 57 nCi (2.1 kBq) 238Pu/m2 and 225 nCi (9.4 kBq) 239,240Pu/m2. Field 2 was 420 m from the stack and contained 15 nCi (0.56 kBq) 238Pu/m2 and 79 nCi (2.9 kBq) 239,240Pu/2. The distribution of Pu between soil and vegetation components was similar in the two fields with only 1.3% of the 238Pu and 0.2% of the 239,240Pu occuring in pines, herbaceous vegetation and litter. More of the Pu could be suspended from the soil into the atmosphere by a 6 m/s wind on Field 1 than on Field 2 due to the lower biomass of herbaceous vegetation and litter on Field 1. Approximately 2% of the 238,240Pu and 0.5% of the 239,240Pu could be resuspended on Field 1, whereas only 0.5% of the 238Pu and 0.1% of the 239,240Pu could be resuspended on Field 2. Plutonium contamination of the vegetation occurred primarily by (1) direct deposition of recently released Pu onto vegetation, and (2) resuspension of Pu from the soil to vegetation surfaces. Root uptake apparently made negligible contributions to the Pu content of above—ground vegetation.