bluuebunny/crossref_metadata_2025 huggingface dataset:
Changes in the Chemical Composition of Leaves During Processing in a Woodland Stream
Authors: Suberkropp K. , Godshalk G. L. , Klug M. J.
Published: May 2006

The weight loss and changes in the concentrations of major classes of macro—molecules were assessed in white oak (Quercus alba) and pignut hickory (Carya glabra) during processing in a third order hardwater stream in southwestern Michigan. Oak was processed more slowly than hickory. An initial rapid weight loss in hickory was followed by a linear rate of loss, while oak lost weight at a linear rate over the entire processing period. Losses of soluble reducing sugars and polyphenols were rapid, occurring mainly during the initial 2—wk exposure to the stream. Hemicellulose and cellulose losses were similar in both species and nearly paralleled weight loss. An absolute increase in the lignin fraction was observed in both species during the first 6—12 wk of processing. We consider this to be due to an increase of complexed N containing compounds in the lignin fraction. Lipid loss was intermediate between soluble sugars and polyphenols and the structural components. Of the constituents analyzed, only the concentration of Kjeldahl nitrogen and lipid was significantly higher in hickory than in oak leaves during the entire processing period.
Spatial Heterogeneity and Bird Species Diversity
Authors: Roth Roland R.
Published: May 2006

A heterogeneity index, D, derived from the point—quarter technique was significantly correlated with bird species diversity (BSD) for several shrub and forest areas. It predicted BSD for a series of similar brushlands where other indices had failed. Species richness increased faster than species overlap in a series of increasing complex habitats up to the forest stage. Species overlap was negatively correlated with patchiness. Additional species may be accommodated in preforest habitats primarily by horizontal spatial segregation facilitate by the presence of additional patches. New patches result from the addition of layers of vegetation. In late shrub or forest stages other kinds of segregation such as vertical segregation become important to species packing. Patchiness, as measured here, has a proximate effect on avian diversity. The extent or existence of latitudinal gradients in habitat patchiness and the effect on bird species diversity is unknown. The need remains for a universal simple, yet meaningful, heterogeneity index which incorporates both horizontal and vertical variability of vegetation.
Foraging Ecology and Community Organization in Desert Seed‐Eating Ants
Authors: Davidson Diane W.
Published: May 2006

Granivorous ants in the southwestern deserts of the United States are characterized by species—specific colony foraging behaviors that determine their efficiencies at utilizing seeds from different density distributions. Workers search for food either in groups or as individuals, and these feeding strategies represent specialization for high and low density resources respectively. While gathering experimental seeds, simultaneously supplied in clumped and dispersed distributions, feeding by group foragers focused on the high density resource patches, while individual foragers harvested seeds predominantly from the dispersed distribution, which required that prey be independently discovered. Collecting native seeds from unmanipulated environments, individual foragers tended to spend proportionately more time searching and experience lower foraging success than did group foragers in the same local habitat. During a period of high seed abundance, group foragers collected a much narrower range of prey types than did individual foragers. Colonies of Pogonomyrmex rugosus exhibited a mixed foraging strategy, with the most distinct feeding columns occuring during a period of peak seed abundance. Among group foragers, greatest activity coincided with periods of relatively high seed densities and low climatic stress, and these species utilized tactis such as seed storage, hibernation, and estivation to weather less favorable periods in a resting state. Individual foragers were active at intermediate levels during less favorable periods. Although high and low density seed resources are not renewed independently of one another, density specialization appears to promote coexistence between group and individual foragers. Their capacity to stably partition resources in this way should depend on (1) the degree of difference in their efficiencies at exploiting different density distributions of seeds and (2) the availability of seeds in the habitat as a function of seed density. In more mesic habitats, where seeds are renewed more frequently and probably also in large pulses, group foraging species are proportionately more abundant.
The Stability and the Intrinsic Growth Rates of Prey and Predator Populations
Authors: Tanner James T.
Published: May 2006

Models of prey—predator systems incorporating the intrinsic population growth rates of each species were analyzed mathematically and were also simulated. The behavior of the two basic models led to a hypothesis: either a stable prey population possesses strong self—limitation, e.g., territoriality, or the growth rate of the prey species is less than that of its predator. Intrinsic population growth rates calculated for eight species of prey and of their important predators showed that the prey had higher rates than their predators in two cases, in both of which the prey species are known to be self—limited. The prey and predator have approximately equal rates in the snowshoe hare and lynx, known to be cyclic. Five species of ungulates, lacking intraspecific population controls, had lower growth rates than their predators. Incorporation of two habitats, one with good and a second with poor cover for the prey, changed a limit cycle to a stable focus; this fits some observations of snowshoe hares. The properties with the greatest effect on stability shown by these models are prey self—limitation and predator searching time.
Natural Decline of a Pine Needle Scale (Chionaspis pinifoliae [Fitch]), Outbreak at South Lake Tahoe, California following Cessation of Adult Mosquito Control with Malathion
Authors: Luck Robert F. , Dahlstein Donald L.
Published: May 2006

The population dynamics of a pine needle scale infestation Chionaspis (Phenacaspis) pinifoliae (Fitch) were studied at South Lake Tahoe, California from 1969 to 1971. A uniparental population on lodgepole pine, Pinus contorta Dougl., and a biparental population on Jeffrey pine, P. jeffreyi Grev. and Balf., declined dramatically during the study period. The dominant mortality factors for the lodgepole pine population were crawlers which failed to settle, parasitization by Prospaltella bella Gahan, predation by Chilocorus orbus Casey var. monticolus Drea and Cryptoweisia atronitens (Casey), and an unknown mortality of adult female scale. In the Jeffrey pine population, the dominant factors were crawlers which failed to settle, unknown mortality of immature scale, and predation by the same cocinellid species. An intrageneration, intertip analysis of scale populations on lodgepole pine revealed that predation destroyed a constant proportion of the scale in all study areas. Prospaltella bella responded similarly in one lodgepole pine site. Futhermore, parasitization by P. bella showed a within—tip response that increased (proportionately) with decreasing needle age. In the Jeffrey pine site, intertip analysis indicated predation upon ;a constant proportion of the scale population. Achrysocharis phenacapsia Yoshimoto, like P. bella, increased its percent parasitism with decreasing needle age. Laboratory bioassays in which adult diaspid scale parasitoids were exposed to foliage collected from areas fogged for mosquito control indicated insecticide residues were present. The results of the bioassys, the location of the infested area, the field observations on arthropod associates as well as the decline of the C. pinifoliae population following cessation of malathion fogging (for adult mosquito control), all implicated the mosquito control program as causal to the scale outbreak at South Lake Tahoe.
Measuring Preference in Selective Predation
Authors: Chesson Jean
Published: May 2006

Selective predation occurs when the relative frequencies of prey types in a predator's diet differ from the relative frequencies in the environment. A measure of preference is proposed which is derived from a simple stochastic model involving probability of encounter and probability of capture upon encounter. The measure is applicable to any number of prey types and methods of estimation are given for both constant and changing prey numbers. Because the measure is based on a biological model, it can be manipulated and interpreted in a meaningful way.
The Dynamics of a Population in a Markovian Environment
Authors: Slatkin Montgomery
Published: May 2006

A model of population growth in a partially predictable environment is developed and analyzed. The essential features of the model are as follows: (1) There are only 2 possible environmental states, and the transitions between the 2 states are the same as for a Poisson process, so that time spent in a state before a transition follows an exponential distribution. (2) Arbitrary rules for population growth in each of the states are assumed. (3) The model is formulated in terms of the joint distribution function of environmental states and population sizes. An equilibrium solution is found, from which it can be determined if there is a stationary joint distribution by whether or not the equilibrium solution exists and can be normalized. Several specific growth models are analyzed, including the effect of variations in the intrinsic rate of increase and in the carrying capacity for the logistic and related models. The main result is that it is possible to have a stationary distribution of populations sizes indicating persistent fluctuations driven by environmental changes only when the correlation time of environmental changes is roughly comparable to the average response time of the population to those changes. Also, in many models of growth the dominant parameter that determines the tendency of the population to go extinct is the mean of the intrinsic rates of increase in different environments. There is no support for earlier conjecture that the relative values of the mean and variance of a parameter determine the extinction tendencies.
Responses of Annual Vegetation to Temperature and Rainfall Patterns in Northern California
Authors: Pitt M. D. , Heady H. F.
Published: May 2006

Weather patterns significantly influence annual vegetation, both within and between years. The impact of these weather patterns on standing crop, cover, and botanical composition were investigated during the years 1955—1973 inclusive. Standing crop increased as the growing season progressed from March to June, while cover declined. Erodium spp. Carduus pycnocephalus, Geranium spp. and Hypochoeris glabra all declined between these 2 calendar dates. Other annual species increased in botanical composition as the growing season progressed. Weather patterns primarily influenced total standing crop during the initial period of plant growth. Following germination, temperatures were typically warm and conducive to plant growth; drought at this time suppressed total standing crop. Consistent precipitation following fall germination, and again during the period of rapid plant growth in spring, contributes to a "grass year" in the annual type. The sequence of freezing temperatures in relation to phenology of developing grass seedlings determines the relative proportion of annual grass species from one year to the next. Botanical composition of Trifolium spp. and Medicago hispida depends upon mild autumn weather, particularly when such fall weather favors none of the annual grass species. Botanical composition of Erodium spp. positively correlated with weather patterns associated with dry fall conditions. Multiple regressions of standing crop on weather patterns produced useful guidelines for manipulating animal units to achieve complete utilization of available forage, particularly when these equations were implemented in seasonal stages.
The Influence of Seasonality and Stability on the Species Equilibrium
Authors: Osman Richard W.
Published: May 2006

The species equilibrium theory has proven to be a useful framework in which to analyze the distribution and abundance of species, especially for islands and habitats distributed as islands. In this study, the distribution of marine epifaunal invertebrates on rocks has been interpreted using this model. The relationship between area and the number of species present is sustained within this community. However, the increase in the number of species with increasing area appears to result from increasing immigration rather than decreasing extinction, as predicted by the model. Two factors have a major effect on the species equilibrium: seasonality and substrate stability. The seasonality of the climate causes the immigration rate to change seasonally. Colonization curves are dependent on the time of initial immersion of the substrate and vary in shape. However, with time, all converge. A simple model which allows immigration to fluctuate seasonally is developed. It predicts both the initial difference in the slope of the colonization curves and their eventual convergence. This same model also predicts that the colonization curve will cycle between a summer high and a winter low. The duration of the study was only long enough to test weakly this last prediction. The physical and temporal stability of a rock "island" is also a function of its size. Smaller rocks are disturbed more frequently than larger ones. This leads to an undersaturation in species on small rocks, causing an increase in the rate of change in the number of species with increasing area. Other changes in the slope of the area—species curve were found to be caused by changes in other environmental parameters. The species equilibrium theory is a fairly robust model in that changes in many parameters have a predictable effect on immigration and extinction which, in turn, determine species diversity. Although the utility of such an all—encompassing model may be questioned, its predictive framework makes it a very attractive tool for examining the diversity of communities.
Competition and Coexistence with A Guild of Herbivorous Insects
Authors: Rathcke Beverly J.
Published: May 2006

Resource exploitation and competition were measured for 13 herbivorous stem—boring insects inhabiting a climax prairie in Illinois. Nine species overlapped > 70% in their exploitation of plant species, stem sizes, and locations within stems, but competition was detected between only two species. In this case the competition resulted from aggression rather than exploitation for limited resources, and exclusion may be delayed or prevented by specific behaviors or refuges. To quantify how closely the guild members were packed upon the available resources, probabilities of coccurrences between species were calculated using resource abundances, population sizes, and overlaps in resource exploitation. The species packing averaged over the entire guild was 30%, but some species had several potential competitors and probabilities of cooccurrences were > 60%. This species structure offers the conditions for diffuse competition, but competition seldom occurred because stems could support several individuals and resources were generally nonlimiting. Although the potential for competition is high and species probably compete occasionally, the high resource overlap and species richness that competition has not been a major organizing force within this guild. When factors known to influence competition are considered, it seems possible that competition has been too infrequent, mild, or nondirectional to cause strong selection for competitive avoidance or displacement. This guild demonstrates that overlap values may not equal competition coefficients and that high overlap may exist because competition is rare. This guild is probably not unique in lacking competitive organization; herbivorous insects may be among the least likely groups to exhibit the patterns predicted by competition theory.
Vegetation Death and Its Importance in Primary Production Measurements
Authors: Bradbury Ian K. , Hofstra G.
Published: May 2006

Annual net aerial primary production (NAPP) was determined by two methods on two abandoned pastures of contrasting vegetation composition in southern Ontario, Canada. Summing the maximum biomass of each individual species gave NAPP values, 84% and 49% respectively, of those obtained when indirect determinations of vegetation death were made during the growing season as well as changes in standing crop of live material.
Ecology and Physiological Aspects of Reproductive Strategies in Two Lizards
Authors: Derickson W. Kenneth
Published: May 2006

Two lizard species, the northern prairie lizard (Sceloporus undulatus garmani) from Reno County, Kansas and the northern sagebrush lizard (Sceloporus graciosus graciosus) from Washington County, Utah, were used to test four hypotheses and one assumption related to the theory of r— and K—selection. The northern prairie lizard is short—lived, matures early, and has a high reproductive effort (r—strategist) while the northern sagebrush lizard is long—lived, has delayed maturity, and a low reproductive effort (K—strategist). One of the assumptions of the r— and K—selection theory is that competition for food is more intense for K—strategists than for r—strategists. Given this assumption, greater food availability for the prairie lizard was hypothesized to result (1) a higher level of body lipids, (2) a higher rate of lipid utilization, (3) a lower percentage of ingested energy available for metabolism, and (4) and expenditure of less energy per offspring than in the sagebrush lizard. Total lipid levels in the two species collected before and after hibernation indicated that prairie lizards had significantly higher lipid levels than sagebrush lizards during both collection periods. A comparison of the amount of body lipids lost and the amount of egg lipids gained during vitellogensis of the first clutch suggested that more lipids are being utilized for egg production in prairie lizards than in sagebrush lizards. Sagebrush lizards apparently are better adapted physiologically to lower food levels since they had lower rates of lipid utilization during starvation studies and they extracted more energy usable for metabolism from ingested food than prairie lizards. Although both species expended about the same amount of energy on a given clutch of eggs, prairie lizards produced more offspring per clutch and therefore expended less energy per offspring. Prairie lizards produced as many as three clutches per season as compared to two clutches per season for sagebrush lizards, and changes in length—lean weight relationships for the two species over a given season suggested that the additional clutch of eggs produced by prairie lizards may be contributing to the higher mortality observed in this species. Indirect evidence, based on precipitation levels and insect biomass at the two study sites and mouthgapes of the two species, supported the assumption that more food was available to the prairie lizards than the sagebrush lizard. Higher precipitation levels (indicating greater insect biomass) occurred at the prairie lizard collection site and this species had a smaller mouthgape index, indicating greater specification in utilization of prey sites.
Causes of Juniper Invasion in Southwestern Idaho
Authors: Burkhardt J. Wayne , Tisdale E. W.
Published: May 2006

Invasion of western juniper into vegetation dominated by mountain big sagebrush and perennial bunchgrass on the Owyhee Plateau of southwest Idaho appears to be directly related to cessation of periodic fires. Evidence from adjacent climax juniper stands indicates that fires were frequent for at least several hundred years preceding white settlement. Fires have been much less frequent during the past century due to active fire control, development of roads and other fire barriers, and reduced fuel because of heavy grazing and a shift towards decreased precipitation. Physical and biotic factors affecting the establishment of juniper, seed dispersal mechanisms, and the fire history of the study area were investigated. Results indicated that range condition as such had a negligible effect on juniper establishment. Juniper seedlings became established most readily on areas supporting well—developed herbaceous and shrubby vegetation. Seed dispersal was primarily localized, and accomplished by gravity and disturbance by animal trampling. Abundant evidence of fire in the form of charred stumps and fire scars on living trees was found throughout the study area. Old juniper stands are confined to rocky ridges where understory vegetation is sparse and fires less intense. Juniper was apparently kept out of the denser vegetation of deeper soils by more intense fires. Most herbaceous and shrubby species survived this treatment due to greater tolerance to fire, or rapid reproduction from seed.
Energetics, Temperature and Water Relations in Winter Aggregated Sceloporus Jarrovi (Sauria: Iguanidae)
Authors: Congdon Justin D. , Ballinger Royce E. , Nagy Kenneth A.
Published: May 2006

Metabolic rates and water fluxes in winter—aggregated Yarrow's spiny lizards (Sceloporus jarrovi) were measured in the field using doubly—labeled water. Resting metabolic rates were estimated from measurements of microhabitat and lizard body temperatures. During the month (November) following aggregation at the hibernaculum and the month (April) before dispersal, lizards were relatively active and their metabolic rates averaged 0.071 ml CO2°g—1°h—1, °40% of their warm season rates. From December through March, lizards basked but were relatively inactive with metabolic rates averaging 0.031 ml CO2°g—1°h—1, °15% of warm season rates. Water influx rates averaged 10.3 ml°kg—1°day—1 during the 120—day inactive period. Water loss was slightly higher than gain and lizards lost weight slowly while hibernating, having ended the 6—mo aggregation period with a 22% decline in body mass. Total energy expenditures during the aggregation period averaged °7.53 kJ, <2 of which could have been supplied by stored body fat. This, together with other direct and indirect evidence, suggests that lizards fed during the cold season. As a result of basking and displaying some activity, S. jarrovi spent >3 times as much energy during the aggregation period as would a lizard remaining inactive in a crevice at 10°C for the entire 6 mo. Possible benefits of winter basking are discussed.
Food Sources of Estuarine Invertebrates Analyzed Using 13C/12C Ratios
Authors: Haines Evelyn B. , Montague Clay L.
Published: May 2006

The complex nature of, and importance of detritus consumption in, estuarine food webs make analysis of specific pathways of carbon in estuaries very difficult. We have evaluated the use of the natural abundances of the stable carbon isotopes in estuarine flora and fauna as a qualitative indicator of food sources in a Georgia salt marsh estuary. The salt marsh ecosystem is particularly amenable to this technique because of the wide span of distinctive 13C/12C ratios of the dominant plants °13C of ‐12 to ‐26%). Laboratory experiments showed negligible further fractionation of the characteristic °13C of marsh plants during microbial degradation of dead plant material, and during assimilation of ingested detritus by estuarine invertebrates. Invertebrates having a variety of feeding habits were collected in monospecific and mixed species stands of marsh plants. Marsh snails and insects had °13C values close to those of the associated vascular plants. Deposit—feeding crabs had isotope ratios biased away from the marsh plant °13C values, likely a result of feeding on benthic diatoms and out—of—stand foraging. Mud snails and filter—feeding bivalves showed 13C/12C ratios similar to those of benthic and pelagic algae. The °13C values of marsh fiddler crabs varied from ‐12 to ‐24% in various habitats having predominately C4 or C3 food source plants. Individuals collected from populations of 3 crab species in a Spartina alterniflora marsh had 13C/12C ratios with a standard deviation of only about 1% from the mean°13C. Stable carbon isotope ratio analysis appears to be a powerful tool in the evaluation of carbon pathways in estuarine food webs.
Elevation Differentiation in Espeletia Schultzii (Compositae), A Giant Rosette Plant of the Venezuelan Paramos
Authors: Baruch Zdravko
Published: May 2006

Carbon dioxide exchanges, plant—water relations, and some morpho—anatomical characters were measured and compared between 3 elevational populations of Espeletia schultzii in the Venezuelan Andes. The paramo climate is characterized by large oscillations in temperature on a daily basis. Precipitation is strongly seasonal with rainy summers and dry winters. The elevational gradient chosen in the Sierra Nevada de Merida extends from 3450 to 4200 m and is characterized by an increase of solar radiation, wind velocity, evaporative forces, and a decrease of temperature and water availability from low to high elevation. Contrasting with temperature alpine areas, the growing season in the paramos extends throughout the entire year. Thus, the maintenance of adequate leaf temperatures on a daily basis and an adequate moisture balance on a seasonal basis are more critical than accumulating resources during a shortened growing season. Morpho—anatomical characters of E. schultzii such as thick leaves, dense pubescence, and water—storing parenchyma play important roles in regulating water loss by increasing water vapor diffusion resistance and buffering the periodic water stress during the dry season. The caulescent life form of the plant elevates the rosette of leaves above the soil surface avoiding extreme ground temperatures. Pubescence and thickness of the leaves may help to control leaf temperature by increasing albedo and heat capacity. The importance of such characteristics is evidenced by their increased prominence in plants of higher elevations. Photosynthetic rates, obtained in the field, using 14CO2 on mature leaves, were relatively low (2—4 mg Co2°dm—2°h—1) but correlate well with the slow growth rates and generation times reported. These low rates are not so disadvantageous in the paramos due to the extended growing season. Pubescence and density of the leaves probably have negative effects on photosynthesis by decreasing the photosynthetic active radiation (PhAR) reaching the chloroplasts and increasing resistance to CO2 diffusion. Controlled conditions in the laboratory produced different responses of the populations regarding photosynthesis. Under cold acclimation, high elevation populations showed highest rates. Under warm acclimation, low elevation populations showed highest rates. Also, different degrees of homeostasis were found among the populations which were correlated with their elevation of origin. The populations of E. schultzii are further differentiated by the degree of drought resistance; the higher elevation population being the most resistant. Among the populations, differences in density—thickness and leaf pubescence are maintained even under uniform laboratory conditions. The maintenance of differences among the populations of E. schultzi even under uniform conditions indicates that they represent genetically based ecotypes.
Foraging Behavior and Home Range Utilization of the Bannertial Kangaroo Rat (Dipodomys Spectabilis)
Authors: Schroder Gene D.
Published: May 2006

Radiotracking and live—trapping techniques were used to determine the movement patterns of Dipodomys spectabilis in central New Mexico, USA. In contrast with the live—trapping results which suggest extensive home range overlap, 133 h of radiotracking over a 2—yr period revealed that most individuals exclude other bannertails from a circular home range of approximately 0.05 ha with a centrally located burrow. Home range size remained relatively constant during the warmer months. The extent and intensity of activity varied greatly among rats, and although the contents of food caches changed qualitatively during the year, no seasonal foraging pattern was evident. The average bannertail spent 58% of the night hours inside or on the mound and less than 22% of the time more than 6 m from the mound. There was no evidence that moonlight affected activity. Foraging is an efficient process, perhaps accomplished concurrently with other activities. The tendency of this species to store large quantities of seed heads collected at an optimal stage of development from a feeding territory suggests that the bannertial may manage its food resources to an extent seen in few other rodent species.
Nearest Neighbor Assessments of Spatial Confirgurations of Circles rather Than Points
Authors: Simberloff Daniel
Published: May 2006

Classical nearest neighbor analysis, assuming points, underestimates expected mean nearest neighbor distance for circles and overestimates its standard error. The extent of distortion may be sufficient to lead to incorrect conclusions, such as regularity for an arrangement which is, in fact, clumped. Analytic expressions are derived which are accurate for small circles, and the results of simulations are plotted for use when circles are large.
A Competing‐Risk Model for Animal Mortality
Authors: Siler William
Published: May 2006

A three—component competing—rick model for animal mortality is presented, in which the additive hazards include a new model, dominant during the prematurity period; a constant hazard, dominant during the period of maturity; and the conventional Gompertz hazard, dominant during senescence. A good fit of the model is obtained to survival data for a variety of species, with both laboratory and field data being represented. Interpretation of the model parameters in terms of animal adjustment to hazards is offered.
Growth of Herbivorous Caterpillars in Relation to Feeding Specialization and to the Growth Form of Their Food Plants
Authors: Scriber J. Mark , Feeny Paul
Published: May 2006

Larvae of 9 species of swallowtail butterflies (Papilionidae), 10 species of bombycoid moths (Saturniidae and Bombycidae), and of the southern armyworm (Noctuidae: Spodoptera eridania) were reared under standardized conditions on mature leaves of many of their typical food plants. Growth rates, feeding rates and efficiencies of food and nitrogen utilization of larvae in their penultimate and final instars were measured by standard techniques. The study revealed a clear relationship between larval growth and growth form of the food plants quantified as leaf water content. Larvae grew faster and more efficiently on herbaceous plants than on the foliage of shrubs and trees. These differences were greater than could be accounted for by variation in the degree of feeding specialization among the insect species tested. The particular leaf characteristics responsible for the relationship between plant growth form and larval growth are not known, but they probably include leaf water content, nitrogen content, toughness, and fiber content. Since trees are likely to be more apparent to enemies than are most herbaceous plants, our results are consistent with recent suggestions that the mature foliage of apparent plants is generally poorer food for herbivores than is the foliage of unapparent plants. However, the extent to which the low food value of mature tree leaves is actually a consequence of the selective action of herbivores remains an open question.
A Simulation Model of Primary Production and Carbon Allocation in the Creosotebush (Larrea Tridentata [DC] COV.)
Authors: Cunningham Gary L. , Reynodls James F.
Published: May 2006

A preliminary simulation model of primary productivity and carbon allocation in creosotebush (Larrea tridentata [DC] Cov.) is described. The model utilizes a systems approach in which movement of assimilate within the plant is in response to changes in source—sink strengths of leaves, stems, roots, early reproductive buds, maturing reproductive buds, flowers, and fruits. Two distinct compartments are defined per organ or developmental stage to separate assimilate into a pool fraction (labile or translocatable) and a structural fraction (nonlabile). The changes in magnitude (within upper and lower limits) of a pool compartment during the course of a simulation (i.e., growth and development of the plant) are a function of the rates of maintenance respiration and growth as well as a priority scheme governing allocation of assimilates; the increases and decreases in dry weight of a structural compartment are a function of aging and the magnitude of its associated pool (which determines structural growth and physiological death). A 1—yr simulation of a hypothetical Larrea plant shows that the model exhibits a reasonable behavior, although no validation is attempted at this stage in its development. The heuristic value of the model is illustrated in the sensitivity analysis, which shows the need for detailed knowledge of priority carbon movement during both vegetative and reproductive growth periods, the importance of substrate controlled respiration rates, and the need for further studies of the dynamics of labile pools in the plant. The model has been an excellent tool in our initial attempt to integrate the voluminous information on Larrea into a complete functional description of the autecology of the species. Further refinement of this model as data from our current research and that of other investigators become available should lead us to be better understanding of the ecological role of Larrea in desert ecosystems.
Canopy Structure and Potential Light Competition in Two Adjacent Annual Plant Communities
Authors: Turitzin Stephen N.
Published: May 2006

Two annual grassland communities, 1 of which occurred on serpentine soil, were compared as light—competitive habitats for Bromus mollis L. Gramineae, a common component of each. Potential light competition was assessed as the dependence of simulated photosynthetic rate on position within the canopies. The 2 communities were found to differ markedly in foilage canopy structure and light penetration. Incident sunlight was less attenuated by the serpentine canopy. The vertical gradients of light intensity within the 2 communities were such that a plant in the nonserpentine grassland would have to display its leaves as a greater elevation than it would in the serpentine canopy in order to achieve the same rate of photosynthesis. The nonserpentine canopy was thus judged to be potentially more light competitive.
A Simple Measure of Niche Breadth
Authors: Feinsinger Peter , Spears E. Eugene , Poole Robert W.
Published: May 2006

Niche breadths in animal and plant populations are often measured without regard to the relative frequencies of the various resources available to the organisms. Recent papers have provided more accurate indices for use when available resources can be quantified. These indices may lack wide applicability or simple biological interpretations. We suggest that niche breadth be defined as the degree of similarity between the frequency distribution of resources used by members of a population and the frequency distribution of resources available to them. Similarity can be quantified with the familiar Proportional Similarity (PS) Index. This index, which measures objectively the similarity between two frequency distributions, reflects in satisfactory fashion the breadth of a population's niche.
Water Relations of Populus Clones
Authors: Pallardy S. G. , Kozlowski T. T.
Published: May 2006

Stomatal aperture and water balance in the field of eight Populus clones varying in growth rate were closely related to environmental factors and clonal differences were clearly expressed. Leaf water potential (ψl) was influenced by solar radiation, leaf conductance, evaporative demand, and soil moisture content. The effects of soil moisture on ψl were greatly modified by atmospheric conditions and stomatal conductance. Several slow—growing clones exhibited extended periods of ψl below that of rapidly growing clones, despite high evaporative demand and the much greater transpiring surfaces of the fast—growing clones. Stomata of all clones responded to changes in light intensity and vapor pressure gradient (VPG). Pronounced stomatal sensitivity to VPG of two rapidly growing clones of common parentage, and the resultant capacity of these clones to moderate water deficits under high evaporative demand, were associated with drought resistance in one of the parents. Seasonal maximum leaf conductance was positively related to growth in several clones, suggesting that rapidly growing clones possess the capacity to carry on higher rates of gas exchange under favorable conditions. Analysis of changes in ψl with changes in transpirational flux density (TFD) showed that for four clones, ψ l change per unit change in TFD decreased as TFD increased, indicating plant adaptation for prevention of damaging ψl even at high TFD. More rapidly growing clones exhibited a larger initial role of decline in ψl with TFD, but reduced the rate of decline more than slow—growing clones as TFD increased.
Long‐Term Replacement Cycles in Cladoceran Communities: A History of Predation
Authors: Kerfoot W. Charles
Published: May 2006

Populations of the waterflea Bosmina undergo temporal replacement cycles (seasonal successions of clones or closely—related sibling species) in many modern lakes, where long—featured winter clones are subsequently replaced by short—featured summer clones. Inspection of lake sediment reveals that populations have also undergone long—term replacements. Remarkably well—preserved remains, heavily biased towards summer morphology by production dynamics, document a progressive shift from late—glacial long—featured phenotypes to recent short—featured phenotypes. Several lines of evidence suggest that these long—term substitutions reflect a changing balance between planktivorous fishes and predatory copepods.
Field Experiments on Responses of a Freshwater, Benthic Macroinvertebrate Community to Vertebrate Predators
Authors: Thorp James H. , Bergey E. A.
Published: May 2006

We examined the seasonal importance of vertebrate predators in potentially regulating the abundance and diversity of the benthic macroinvertebrates in the littoral zone of a soft—bottom reservoir that receives thermal effluent from a nuclear production reactor. Thirty—six predator (fish and turtle) exclusion cages (4 m2) were placed in shallow water at six locations along a thermal gradient in Par Pond, a 1100—ha cooling reservoir on the Savannah River Plant near Aiken, South Carolina, USA. An additional 36 control plots (4 m2) were also set up. Cages were in place during three, 3—mo test periods beginning in September 1977. Estimates of benthic density, taxon richness, and distribution within functional groups (defined by feeding mechanism) were calculated for each test period. Effects of temperature on predator—prey relationships were also determined. Experimental results of this study suggest that vertebrate predation was not the fundamental parameter organizing the benthic macrionvertebrate community in the littoral zone of this reservoir. Neither taxon richness nor density of total macroinvertebrates was conclusively related to predator treatment. Relationships between predator treatment and community response (changes in density and taxon richness) were generally unaffected by either plot locality, temperature fluctuations from thermal effluent, or seasonal changes. When data from caged and control plots were pooled, however, both location and water temperature individually had direct impacts on the benthic community. From our results and other field studies we hypothesize that individual species of "keystone" benthic predators (Paine 1969b) do not occur in the littoral zone of freshwater lentic environments with soft bottoms. This hypothesis is based on the following four arguments. First, environmental heterogeneity should reduce predator efficiency relative to that found in rocky intertidal systems where keystone predation may be present. Second, relative food web complexity should reduce the community importance of an individual predator species. Third, the apparent relatively greater resource partitioning of space and food in the freshwater littoral zone hinders monopolization of rate—limiting environmental resources by a single dominant prey species. Fourth, the effects of predation may not be extensive enough in time or area to provide sufficient resource space (and thus food) for exploitation by fugitive species. Possible regulation by a guild of predators and cases where the general hypothesis might be falsified are discussed.
Nitrification Potentials in Primary and Secondary Succession
Authors: Robertson G. Philip , Vitousek Peter M.
Published: May 2006

Potential nitrogen mineralization and nitrification were measured in soils from a primary successional sequence developed on sand dunes and from a secondary successional sequence on old fields. Potential nitrogen mineralization in soils from the primary sere increased through the first five stages and then leveled off. Nitrogen mineralization was relatively constant in soils from the secondary sere, except that the highest rates were observed in the oldest site. Nitrification was very closely correlated with nitrogen mineralization in soils from both the primary and secondary seres. Only one site had both substantial nitrogen mineralization and low nitrification. The results of this study do not support the hypothesis that nitrification is progressively inhibited in the course of ecological succession.
Ecological Relationships Among Arboreal Desert Lizards
Authors: Vitt Laurie J. , van Loben Sels Richard C. , Ohmart Robert D.
Published: May 2006

Three arboreal desert lizards (Urosaurus graciosus, U. ornatus, and Sceloporus magister:Iguanidae) occur in close association in riparian habitats of central Arizona. The lizards were studied over 2 yr, to determine patterns of resource utilization. Urosaurus graciosus and U. ornatus are similar in body size, but Sceloporus magister is much larger. Juvenile S. magister are, however, similar in body size to adults of both Urosaurus. Ratios of head and body size (larger species:Smaller species) reveal similarities between the Urosaurus species and between both species of Urosaurus and juvenile S. magister, but large differences between Urosaurus species and adult S. magister. Active body temperatures of all three species were similar, and the only significant difference in time of activity was between S. magister and U. ornatus. The habitat of U. graciosus reached higher air and substrate temperatures than that of U. ornatus and received greater incident radiation (S. magister occurred in both habitats). Urosaurus ornatus and S. magister avoid extremes of temperatures by entering crevices in trees, retreating under loose bark, or entering mammal burrows or nests, whereas U. graciosus retreat to shaded portions of tree trunks and are thus less able to avoid extremes. Urosaurus graciosus occur in relatively small trees and forage in the canopy; U. ornatus inhabit large trees and forage on trunks and large limbs; S. magister occupy trees of all sizes but forage primarily on trunks and large limbs, and on the ground. Prey sizes were significantly different among species and with one exception (S. magister), significantly different within species among years. Diets of all three change seasonally and yearly, presumably in association with resource availability. Diet overlaps are often high but fluctuate seasonally and yearly. Data on resource availability suggest that all species exploit the temporally abundant insects. Data presented on habitat utilization, activity times, body temperatures, and food eaten reveal partitioning of the arboreal habitat in three dimensions between U. graciosus and U. ornatus and partitioning of prey size between the two Urosaurus as a group, and S. magister.
Gaussian Analysis: Identifying Environmental Factors Influencing Bell‐Shaped Species Distributions
Authors: Westman Walter E.
Published: May 2006

In seeking to identify environmental factors controlling the distribution of individual species, ecologists have most commonly used techniques which assume a linear relationship between an environmental factor value and species response (e.g., canonical correlation analysis, multiple linear regression analysis, discriminant analysis). Many species assume nonmonotonic, curvilinear response curves along a variety of environmental axes, however. A method is presented to help identify environmental factors controlling species distributions when the latter species response occurs. The technique builds on theoretical and empirical observations indicating that the distribution of species importance values is most often approximately bell—shaped when arrayed along an axis representing linear change in an environment factor strongly influencing the growth and survival of the species. The method consists of identifying those environmental factor axes along which a species distribution curve best fits a Gaussian form. The method is illustrated using coastal sage scrub vegetation and habitat data from southern California. The "Gaussian analysis" technique described here will not identify all of the important predictors of species response, for reasons arising largely from the multifactorial nature of influences on species performance. The technique does, however, permit identification of some important environmental predictors which are missed by traditional techniques that assume linear response curves.
The Effect of Environmental Disturbance on a Tropical Butterfly Population
Authors: Blau William S.
Published: May 2006

Near Turrialba, Costa Rica, the only common host plant of the black swallowtail butterfly Papilio polyxenes Fabr., Spananthe paniculata, is an early successional umbellifer that occurs in ephemeral patches that vary in age, size, and degree of isolation from one another, and support localized colonies of P. polyxenes. Within one patch, egg and larval survival was highest shortly after the plants began to flower. Eggs experimentally placed on the leaves of plants prior to flowering suffered rates of mortality nearly twice as great as those placed on flowers just 2 or 3 wk later. Beyond the point egg and larval survival decreased as rates of predation increased, local population densities increased, or infertility occurred. During this period, the numbers and kinds of insects from predominantly predaceous and parasitic families increased significantly, as did the numbers of spiders. The overall importance of three measured sources of mortality were: predation by large predators > predation by small predators > drowning due to rain. Year—round breeding and dispersal by black swallowtail adults allows them to colonize new patches as they appear and to track the spatial availability of the ephemeral host in time. New patches provide a continual supply of temporary refuges from natural enemies and areas of high intraspecific population density. The black swallowtail is only one of a variety of anthropods that appears to be adapted for colonizing asynchronous habitat patches produced by localized disturbances where the growing season is continuous.