bluuebunny/crossref_metadata_2025 huggingface dataset:
The Density Dependence of Parasitism by Tephritid Fruit Flies
Authors: Roitberg Bernard D. , Mangel Marc , Tourigny G.
Published: May 2006

The density dependence of parasitism in host—parasitoid systems has received considerable theoretical and experimental treatment recently, but usually at the intergenerational or interseasonal level. Here we study the density dependence of parasitism within a season, using tephritid fruit flies as a model system for parasites. Our work is motivated by conflicting observations that individual Rhagoletis pomonella flies foraged with direct density dependence (Roitberg et al. 1982), but that a population of flies of the sibling species Rhagoletis cornivora showed either no relationship between parasitism and host density (density vagueness) or inverse density dependence (Borowicz and Juliano 1986). Thus, we want to understand how a direct response to density at the individual level can be translated to a lack of response or to inverse density dependence at the population level. To do this, we employ behavior—rich simulations as well as individual—based state variable models. Our results resolve the apparent conflict: individual behaviors may not translate to the same population level patterns due to (1) a lack of defined patch boundaries (from the parasite's perspective), (2) repeated visits by parasites to patches, (3) uneven rates of host acceptance, and (4) egg limitations. Finally, we show the importance of relating the experimental region to the rest of the world of the parasite and propose that future studies of host—parasitoid systems should monitor both host and parasite populations.
Competition, Fire, and Nutients in a Mixed‐Grass Prairie
Authors: Wilson Scott D. , Shay Jennifer M.
Published: May 2006

Variation in the intensity of competition has been proposed as a mechanism that may control the species diversity and composition of many kinds of communities, including North American prairie. Competition intensity is predicted to increase with soil fertility and decrease with disturbance, causing variation in the abundances of competitively subordinate species. Variation in competition intensity was examined in an experiment in which fire and fertilizer were applied to native mixed—grass prairie. Treatments consisted of annual burns, a single burn, and untreated control, and fertilization (N ≥ 15 replicates) applied to 7 x 5 m plots for two growing seasons. The experiment was repeated at a second site 15 km away. Fire reduced standing crop and litter, and increased the frequency of bare ground. Nutrient addition increased standing crop and decreased bare ground. Species composition and diversity were largely unaffected by fire. Two common species, Bouteloua gracilis and Carex obusata, were significantly more abundant in fertilized vegetation than in controls at both sites; species diversity decreased in fertilized plots. Vegetation responses to the treatments were similar at the two sites. To tests for variation in competition intensity with fire and nutrient availability, removal experiments were performed in each treatment at one site. Two tussocks each of Festuca ovina and Stipa spartea were chosen in each plot. One plant of each pair had all neighbors within 30 cm clipped and removed during two growing seasons. Each species grew larger and faster in the absence of neighbors, but the magnitude of this effect did not vary with fire, fertilizer, or standing crop. Fire and nutrient addition produced significant changes in the community structure of mixed—grass prairie but did not alter competition intensity in the manner predicted.
Helmets and Invisible Armor: Structures Reducing Predation from Tactile and Visual Planktivores
Authors: O'Brien W. John , Kettle Dean , Riessen Howard
Published: May 2006

Several arctic Alaskan lakes were found to have populations of both visual—feeding planktivorous fish and the tactile—feeding predaceous copepod Heterocope septentrionalis. Feeding experiments and reactive distance measurements were carried out to determine the relationship between zooplankton size and susceptibility to both types of predation. Heterocope was found to feed selectively on small prey forms and fish on large forms, thereby confirming that the zooplankton in these lakes are subject to conflicting selective pressures. Two of the species of zooplankton appear to have adapted to this situation by developing structures that increase their actual size, thus helping to protect them from invertebrate predation while not altering the visual body size they present to fish.
Lack of Vesicular‐Arbuscular Mycorrhizal Inoculum in a Ponderosa Pine Forest
Authors: Kovacic D. A. , St. John T. V. , Dyer M. I.
Published: May 2006

Comparison of live beetle—killed ponderosa pine stands revealed that understory biomass was higher under beetle—killed pines. Carex heliophila, a nonmycorrhizal plant, was virtually the only species present beneath live ponderosa pines. We examined the extent to which vesicular—arbuscular mycorrhizae (VAM) colonized understory plants on both live and beetle—killed pine sites; the availability of live— and killed—pine soil VAM inoculum in situ and in the laboratory; the abundance of VAM spores in field soils; and the possible inhibition of VAM formation by forest floor decomposition products in live—pine soils. Roots of field species from live and beetle—killed pine soils, as well as bioassay plants (species not native to the study site) grown in situ and in the laboratory were cleared, stained, and examined for presence or absence of VAM colonization. Sixty percent of the understory of live pine was composed of nonmycorrhizal species, 26% of mycorrhizal species, and 14% of species not examined for VAM. Twenty—nine percent of the understory of killed pine was composed of nonmycorrhizal species, 51% of mycorrhizal species, and 20% of species not examined for VAM. All laboratory bioassy plants were nonmycorrhizal in live—pine soils and mycorrhizal in killed—pine soils. No in situ bioassay plants became mycorrhizal beneath live pines. Counts of 50—250 μm VAM spores in May 1981 and August 1982 were 0.013 and 0.098 spores/g, respectively, in live—pine soils and 0.36 and 4.97 spores/g, respectively, in killed—pine soils. We found no evidence that VAM colonization in corn plants was inhibited by forest floor decomposition products. Mycorrhizal colonization of field species was greater in the killed—pine understory than in the live—pine understory. Laboratory and field studies showed that VAM inoculum was absent in the understory of live ponderosa pines. Allelopathic—type suppression, either of host plants or of the mycorrhizal colonization process, might explain the lack of inoculum. Direct inhibition of the colonization process, however, was not observed in greenhouse experiments.
Bias and Random Errors in Estimators of Net Root Production: A Simulation Approach
Authors: Singh J. S. , Lauenroth W. K. , Hunt H. W. , Swift D. M.
Published: May 2006

Simulation of net primary production in a grassland and subsequent calculations of belowground net production revealed substantial sources of error that may be common to all estimates of belowground net production from field data. Inherent bias in the most frequently used estimators and a large counterintuitive effect of sample variability require that techniques be carefully considered before conclusions are drawn based upon estimates of belowground net production.
A Diet Model for Planktivores that Follow Density‐Independent Rules for Prey Selection
Authors: Butler Steven M. , Bence James R.
Published: May 2006

Whether prey are encountered simultaneously or one at a time is an important distinction. We have applied the concept of simultaneous encounters to models of prey selection by planktivorous fish. When the maximum distance from which a prey item can be detected is determined by the subtended angle a prey item makes the fish's eye (apparent size), selection for the apparently largest among simultaneously encountered prey results in prey preferences that are independent of the prey density or relative density. This occurs regardless of how many prey are likely to be encountered simultaneously, and the resulting diet is the same as that predicted by the simpler reactive field volume model. Furthermore, the assumptions of this apparent—size model can be greatly modified, to enhance realism, without changing our conclusion. In this context we allow prey selection and detection to become stochastic, and to depend upon aspects of prey consciousness other than prey length. The similar model proposed by O'Brien et al. (1976) as an alternative to Werner and Hall's (1974) optimal—diet explanation of size selection by bluegill sunfish yields a different conclusion. We find that there are, in all, five reasons for our different conclusions. In light of our results, we interpret the results of Werner and Hall (1974) and two more recent attempts to differentiate between apparent—size models and optimal—diet theory (Gibson 1980, Gardner 1981). We conclude that these studies do not provide strong support for the predictions of optimal—diet theory; field evidence does, however, suggest that fish can alter their diet in an adaptive fashion.
The Effect of Temperature Preconditioning on the Temperature Sensitivity of Net CO2 Flux in Geographically Diverse Populations of the Moss Polytrichum Commune
Authors: Sveinbjornsson Bjartmar , Oechel Walter C.
Published: May 2006

Gametophytes of five geographically diverse populations of the moss Polytrichum commune ranging from Barrow, Alaska (71°N), to Gainesville, Florida (29°N), were grown under constant temperatures of 5° and 20°C with 16 h of light each day. The net CO2 exchange of the shoot tissue produced under these conditions was subsequently studied in relation to temperature. The following patterns were noted. (1) The maximum net photosynthetic rate decreased in P. commune populations from sites along a gradient of increasing latitude, except that highest rates were found in the temperature St. Hilaire population; the difference in photosynthetic rates between the extreme values was about sevenfold, (2) Cool growth conditions resulted in low maximum net CO2 uptake rates. (3) The growth temperature affected the maximum net photosynthetic rate of low—latitude populations more than high—latitude populations. (4) The optimum temperatures for photosynthesis were equal to or higher than the growth temperatures. (5) The temperature at the center of the estimated optimal range for net CO2 uptake was correlated with both preconditioning temperature and the latitude of origin. (6) The upper temperature compensation points, which were unusually high for bryophytes, were correlated positively with the preconditioning temperature and negatively with the latitude of origin. (7) Northern populations had dark—respiration rates which were equal to or lower than those of southern populations. (8) After low—temperature acclimation, the dark respiration increased only in the southernmost population.
Predator‐Mediated Microhabitat Partitioning by Two Species of Visually Cryptic, Intertidal Limpets
Authors: Mercurio Kimberly S. , Palmer A. Richard , Lowell Richard B.
Published: May 2006

Two species of limpets appear to partition microhabitats according to substratum color within the mixed—barnacle zone of the Pacific Northwest rocky intertidal. A predominantly light—shelled species, Collisella digitalis, occurs most commonly upon the light—colored skeletons of barnacles (Pollicipes polymerus and Semibalanus cariosus), while a predominantly dark—shelled species, Collisella pelta, occurs most commonly on the dark valves of mussels (Mytilus californianus). Field experiments revealed significantly higher mortality of limpets mismtched to their background, due to predation by both fish and birds. When given a choice, in the absence of predation, both species moved actively onto the substratum where they were most cryptic. Strong selection by visual predators appears to maintain the partitioning of space by these limpet species. Competitive interactions are not required to explain this microhabitat partitioning, although they cannot be ruled out as one of the original selection pressures.
Assessing Probability of Interaction in Size‐structured Populations: Depressaria Attack on Lomatium
Authors: Thompson John N. , Moody Michael E.
Published: May 2006

Hypotheses on the direction of evolution in interspecific interactions generally rely upon the probability of interaction between the species. Attack by Depresssaria multifidae (Lepidoptera, Oecophoridae) on tagged individuals of the perennial herb lomatium grayi ( Umbelliferae) was studied for five (Smoot Hill population) or three (Little Tucannon population) years to analyze how plant demographic characteristics affect the probability that a plant is attacked at least once in its lifetime. The plant populations and the pattern of attack by the moths were analyzed based upon a model of size—specific demographic characteristics. Both plant populations were in steppe in eastern Washington, but plants at Smoot Hill had a higher probability of flowering eacy year within the larger size classes, spent fewer years in the largest size class, and had a lower probability of reaching the larger size classes than plants at Little Tucannon. The populations were similar, however, in the probability of attack each year within each size class. The probability that an established plant was attacked at least once in its lifetime was 0.12—0.17 at Smooth Hill and 0.30—0.55 at Little Tucannon. The results suggest that small to moderate differences in plant demographic characteristics can have large effects on the probability that a plant will be attacked at least once during its lifetime by a particular herbivore species; that is, small shifts in demographic characteristics can have large effects on “apparency” (sensu Feeny). The evolution of interactions between species may occur through slight shifts in life histories as often as it occurs through shifts in chemsity and morphology. The model developed here is applicable to other interactions involving size—structured populations and is a measure of probability of interaction that is useful in assessing how shifts in life histories and demographic variables can affect interactions between species.
Juveniles vs. Adult Competitive Abilities in Plants: Size‐dependence in Cattails (Typha)
Authors: Grace James B.
Published: May 2006

This paper tests the hypothesis that plant size can affect a species' relative competitive ability; i.e., species that are good competitors as adults may not also be good competitors as juveniles. Seeds of two Typha species were sown along a miniature gradient in water depth, both in monocultures and in mixtures, to ascertain the relative competitive abilities of juveniles. In monoculture, T. latifolia was found to germinate and grow better at slightly higher elevations than T. domingensis. In mixture, T. latifolia was the better competitor above the water table while T. domingensis dominated below the water table. Summed over the entire gradient, the two species did not differ significantly in their overall competitive abilities despite the fact that T. latifolia has larger seeds. Adult competitive abilities were studied experimentally in a large artificial pond by using transplants planted at low initial densities. When each species was grown in monoculture, T. latifolia grew best in shallow water with little growth in water ≥1.0 m deep, and T. domingensis had peak shoot growth in deep water (0.8—0.9 m) with comparatively less growth in shallow water. When grown in mixture, T. latifolia was the dominant competitor over virtually the entire gradient. Therefore, I conclude that relative competitive ability in Typha is size dependent. The factors most likely to cause the observed shift in relative competitive abilities are (1) the onset of flowering, (2) changes in morphology and physiology associated with growth and development, and (3) changes in the factors limiting growth as plants increase in size. Regardless of the cause, the observed relationship between size and competitive ability suggests that the type of initial propagules used in competition experiments can affect the outcome, and that in some cases, the outcome of competition may be dependent upon the initial density of propagules.
Variance Tests for Species Association
Authors: McCulloch Charles E.
Published: May 2006

Schulter recently discussed species association tests for handling situations where the data consist of a two—way table (species by samples) of either presence/absence or density data. In this paper, connections between those association tests and standard statistical tests are established and used as a guide to proper interpretation of the association tests. Association tests are sensitive to purely sample—to—sample differences and in some cases will not reflect species interactions at all. However, in cases where the association tests are appropriate, the connections furnish information on the choice of accurate critical values.
Subalpine Tree Reestablishment After Fire in the Olympic Mountains, Washington
Authors: Agee James K. , Smith Larry
Published: May 2006

Rates of subalpine tree establishment were measured on the 1978 Hoh burn (3 yr old), the 1924 Mount Wilder burn (55 yr old), and the 1891 High Divide burn (88 yr old) in Olympic National Park, Washington, USA. All three sites were Abies lasiocarpa/Tsuga mertensiana forest at the time of burning: Vaccinium spp. were dominant after the fire. Tree establishment rates were higher on burned sites during periods with generally above—average to average spring/summer precipitation than during below—average periods. Highest rate of tree establishment occurred close to fire edges. Current species composition on the older burns closely reflects the composition of residual trees. Drought resistance of the residual tree species may interact with climate to affect establishment rates. Tsuga mertensiana establishes best during wet periods; Abies lasiocarpa, Pseudotsuga menziesii, and Pinus monticola establish well during normal periods. These patterns are quite different from tree invasion into heather (Phyllodoce/Cassiope) meadows, which occurred during a fairly discrete 1920—1940 regional drought when extended snow—free periods apparently existed in these meadows.
Local Food Depletion and the Foraging Behavior of a Specialist Grasshopper, Hesperotettix Viridis
Authors: Parker Matthew A.
Published: May 2006

Foraging behavior of the grasshopper Hesperotettix viridis was studied in a New Mexico grassland where it fed almost exclusively on the shrub Gutierrezia microcephala (Compositae). During its annual 4—mo feeding period, H. viridis progressively reduced the quality and quantity of edible host plant tissues. By midsummer, when most grasshoppers reached maturity, plants attacked by grasshoppers had significantly lower shoot water content, nitrogen content, and edible biomass per stem than plants experimentally protected from feeding damage. The timing and intensity of defoliation often varied strongly among nearby plants, but by the end of the summer all plants were seriously damaged. Using mark—recapture experiments, I examined how foraging of H. viridis was affected by plant spacing, grasshopper density, and the intensity of prior feeding damage to individual plants. Grasshoppers fed in short bouts and sampled different plants continuously; most animals found on a plant abandoned it within a day. The tenure of grasshoppers on plants tended to be shorter for plants growing in dense stands. Tenure was not affected by current grasshopper density on a plant, but did vary with a plant's history of feeding damage. Grasshoppers remained longer on plants that had been protected from herbivory than on plants that had suffered average or artificially enhanced levels of feeding damage. H. virdis nymphs grew more rapidly when reared on the protected plants. Animals confined for 7 d on high— vs. low—quality plants differed in their subsequent foraging behavior, suggesting that grasshoppers use information related to their recent feeding history to assess the quality of particular plants. Tenure of H. viridis was shorter on plants sprayed with a chemical deterrent (nicotine sulfate). However, grasshoppers preferred protected plants, even when sprayed with nicotine, relative to depleted plants exposed to natural herbivory. Gradual, spatially variable food depletion thus appears to be of key importance in the feeding biology of H. viridis. Selective foraging of grasshoppers for undamaged plants in turn tends to equalize the cumulative amount of feeding damage per plant. Extensive food depletion may reduce the amount of discrimination by herbivores against plants containing chemical deterrents.
Plant Quality and Grasshopper Feeding: Effects of Sunflower Condition on Preference and Performance in Melanoplus Differentialis
Authors: Lewis A. C.
Published: May 2006

The feeding of the grasshopper Melanoplus differentials was observed in fields of wild and cultivated sunflower, Helianthus annuus, in central Texas. Observation of 227 feeding grasshoppers revealed a strong bias toward wilted, damaged, diseased, or senescent leaf tissue or inflorescences over turgid, undamaged leaf tissue. Only 1% of the grasshoppers fed on undamaged leaf tissue, even though it was more abundant than other tissue types. The preference for girdled, wilted leaves was studied in laboratory experiments using excised leaves to simulate the leaves preferred in the field. In binary choice tests, grasshoppers of all instars and both sexes of adults preferred wilted leaves to turgid ones. In long—term growth experiments, grasshoppers offered a mixed diet of wilted and turgid leaves from hatching through oviposition had increased survival, growth rate, fecundity, and egg mass as compared with their turgid—leaf diet sibs. The growth difference appears in the first instar and is due to both increased consumption and increased efficiency of conversion of ingested food by mixed—diet grasshoppers. Excess water, while not responsible for the preference, may in part be responsible for the growth effect. Other chemical differences between the leaves may also have a role. These results and similar observations on other plants and grasshoppers suggest a strong selectivity by these generalists and may have implications for their population dynamics. Increased performance on stressed plant tissue is not found under all conditions nor in all herbivores, and possible reasons for this are discussed.
The Use of Removal Sampling in Comparative Censuses
Authors: Skalski John R. , Simmons Mary Ann , Robson Douglas S.
Published: May 2006

Estimators of proportional abundance (ratio of animal abundance of one population N2, to that of another, N1, defined as K = N2/N1) are derived using constant—effort removal sampling techniques. Based on assumptions about catchability, alternative estimators of K using absolute abundance and catch indices (counts of individual animals captured during a study) are presented. Power curves for comparative censuses using two or three removal samples are presented and the relative efficiency to tag—recapture studies is discussed. Removal sampling is found to be only slightly less efficient than tag—recapture methods if the use of catch indices is permissible; otherwise the efficiency is appreciably less if absolute abundance methods are employed. Analytic procedures are presented which can be used to design comparative census studies. The effects of heterogeneity in the trap response of animals on abundance estimation are discussed.
The Niche Concept Revisited: Mechanistic Models and Community Context
Authors: Leibold Matthew A.
Published: May 2006

The niche concept is a central organizing aspect of modern ecology. Although its history has often been reviewed, the structure of the concept and its connection to advances in ecological theory has received less recent attention. I review the niche concept using “mechanistic” models of community theory to identify two distinct components. One describes the environmental requirements of organisms and the other describes the per capita impact of organisms on the environment. I argue that these correspond to significant differences between Grinnell's and Elton's concepts distinct from the previously discussed “habitat” vs. “functional” dichotomy. I illustrate the distinction between the requirement and impact components of the niche using models of resource competition and of keystone predators, and I discuss “Gause's axiom” and conventional “niche theory” in the context of these two distinct niche components. I suggest that the niche concept be elucidated by explicit reference to these two distinct components; the “impact” niche (corresponding to Elton's concept) describing instantaneous per—capita effects of species on the environment, and the “requirement” niche describing the responses of species to the environment (corresponding to Hutchinson's definition). This approach connects conventional niche theory with “mechanistic” individual—based ecological models and can help provide a more modern context for the niche concept.
Tasty Generalists and Nasty Specialists? Antipredator Mechanisms in Tropical Lepidopteran Larvae
Authors: Dyer Lee A.
Published: May 2006

To examine the effectiveness of larval antipredator mechanisms and to test the assumption that diet breadth and chemistry are important predictors of predation responses. I offered 70 species of lepidopteran larvae to the predatory ant Paraponera clavata (Hymenoptera: Formicidae). Prey chemistry and diet breadth were significant predictors of rejection, as was plant chemistry but to a lesser degree. Specialist caterpillars were better protected than generalists, and prey with unpalatable extracts were frequently rejected by P. clavata, while prey with palatable extracts were rarely rejected. Specialists with unpalatable host plant extracts were more likely to be rejected than specialists that had host plants with more palatable extracts. Other significant predictors included morphology, behavior, and interactions between developmental stage and prey chemistry and between developmental stage and diet breadth. I concluded that predation could be a substantial selective force in the evolution of narrow diet breadth and that plant chemistry could be the mechanism whereby specialists are better defended.
An Experimental Study of the Costs of Reproduction in the Kittiwake Rissa Tridactyla
Authors: Jacobsen Karl-Otto , Erikstad Kjell Einar , Saether Bernt-Erik
Published: May 2006

The cost of reproduction in Kittiwakes Rissa tridactyla were studied from 1990 to 1993 on Hornoy in northern Norway by enlarging and decreasing brood sizes of two chicks to three and one during the first week after hatching. No parents were able to raise the enlarged broods of three to fledging. Most chick mortality occurred in the 19—22 d age group. Chick body mass and fledging success were lower among enlarged broods than among reduced and control broods. Body mass of females, but not males, was lower at the end of the chick—rearing period in enlarged broods. Only those females rearing enlarged broods experienced a higher mortality during the next breeding season. Our results support Lack's hypothesis of clutch size regulation: Kittiwakes laid the maximal number of eggs they were able to raise successfully to fledging. Furthermore, as female Kittiwake mortality increased when clutches were enlarged, females have a trade—off between investment in current reproduction and investment in future reproduction. Because males and females responded differently to brood enlargement, fitness estimates showed that Kittiwakes may have a sex—specific optimal clutch size.
Storage Allocation in Seasonal Races of an Annual Plant: Optimal Versus Actual Allocation
Authors: Chiariello N. , Roughgarden J.
Published: May 2006

This study uses optimal control theory to predict the conditions under which an annual plant's optimal schedule of carbon allocation should include allocation of storage at some point during the growing season. The optimal allocation schedule is defined as the schedule that maximizes final seed set. Storage of carbon, and later remobilization for allocation to reproduction, is an option in the model, and by providing the option, we ask whether storage of carbon is ever optimal for annuals. Predictions from the model are tested by comparing the observed differences in allocation between two seasonal ecotypes of a grassland forb with differences in the optimal behavior we predict for each. The model predicts that the autumnal ecotype should store carbon, while the vernal ecotype should not. These predictions qualitatively agree with the patterns, in the two ecotypes, of total nonstructural carbohydrate, which is our measure of storage. Analysis of the model indicates that the parameter responsible for the different predictions for the two ecotypes is the reproductive (seed mass) loss rate.
Phenology and Water Relations of Three Plant Life Forms in a Dry Tree‐Line Meadow
Authors: Jackson Louise E. , Bliss L. C.
Published: May 2006

The relationship between physiological and phenological adaptations to drought was examined in three species of different life form in a tree—line meadow in the Sierra Nevada. Soil moisture in this meadow is predictably high following snow melt, but decreases through the summer because of infrequent precipitation. There were large differences among the life forms in allocation patterns and in vegetative and reproductive phenologies, but there were no comparable differences in their water relations. The annual Polygonum minimum and the hemicryptophyte Penstemon heterodoxus respond to decreasing soil moisture by reducing conductance; they consequently maintain a longer activity period. The geophyte Saxifraga aprica has little stomatal control of water loss and becomes dormant as soils dry. Penstemon heterodozus is a long—lived perennial with an intermediate root:shoot ratio (1.9), evergreen leaves, and high allocation to vegetative structures. Leaf growth occurs during the first part of the season, while the soil is moist. Flowering beings at the end of shoot growth, and fruits dehisce 3 mo after snow melt. During a dry summer, plants were observed to lose many leaves; 25% of the plants died, and none of the remaining plants flowered the next year. Although Penstemon decreases conductances under a limited range of leaf water potential, it is still not well adapted to long dry periods. Polygonum minimum is a very small annual with a low root:shoot ratio (0.15) and an indeterminate growth pattern. After a period of solely vegetative growth, leaves and floral structures are produced continually until °soil reaches —5 MPa. Seeds dehisce °2 mo after snow melt. Total seed production is a function of the length of the growth period. Regulation of water balance by stomatal control assures adequate time for fruit production. Other annuals in the community had a very similar phenological pattern. Saxifraga aprica has a high root:shoot ratio (5.0) and a small basal rosette of leaves. Leaf growth occurs for only 2 wk and is followed immediately by flowering. Only 5—10% of the plants flower each year. Fruits mature 5 wk after snow melt, and plants become dormant at —0.5 Mpa or within 6 wk of snow melt. This species and the other ephemeral geophytes complete their growth during the period of predictable high soil moisture, and therefore require little ability to regulate water balance. This study provides evidence that high diversity in life forms is maintained in environments with high yearly variability in the lengths of the drought and snow—free periods. No single set of phenological and physiological characteristics is optimally adapted to the variability in the subalpine environment that was studied, and therefore several adaptive strategies co—occur.
Phenetic Structure and Species Richness in North and Central American Bat Faunas
Authors: Schum Michael
Published: May 2006

Thirty—one North and Central American bat faunas were examined with multivariate morphometric techniques to determine if faunas of bats found in the tropics are more tightly packed in morphological attribute space than those found in temperate latitudes. Eighteen morphological features for each of 103 species of insectivorous bats wee measured. The density of morphological packing was calculated from nearest neighbor Euclidean distances between species within each fauna. Nearest neighbor distances were also computed for species' projections on principal component axes for each fauna. For these different character spaces, substantially different patterns emerged, resulting from the correlation of many, morphological characters. Morphological packing in these faunas was also compared to levels of packing predicted from two different neutral models in which species composition was generated by Monte Carlo sampling methods. In one neutral model, species were drawn from the total pool of insectivorous bats, including 11 species which were statistically significant morphological outliers. Frequency distributions of nearest neighbor distances of real and randomly assembled faunas with equal species richness were compared by Kolmogorov—Smirnov tests. Real faunas which did not contain the most significant outliers did not differ from random faunas. Seven faunas, which include outlier species with geographic ranges restricted to the tropics, differed from random communities in the majority of large numbers of Monte Carlo samples. In an alternate neutral model, outliers were excluded in the real and random faunas. Morphological packing was compared between real and random faunas by comparing regressions of average nearest neighbor distance on faunal species richness. In this analysis, levels of packing in real faunas of varying species richness were not significantly different from that predicted under a null hypothesis. Mean nearest neighbor distance decreased with increasing species richness in both real and randomly constructed faunas. Indicating that measures of species packing based on nearest neighbor distance values are sample size dependent. Both types of neutral models suggest that bat faunas are not more tightly packed in a morphological attribute space than would be predicted from chance alone. Tropical faunas, though, may accommodate more species than predicted from a neutral model when species can be added to the extreme peripheral limits of morphological variation.
Variation in Herbivore Infestation: Historical vs. Genetic Factors
Authors: McCrea Kenneth D. , Abrahamson Warren G.
Published: May 2006

Variation in insect infestation levels among plants is a result of both genetic variation in susceptibility and variation in previous insect attack. A plant's history of insect attack can either increase or decrease susceptibility and can also influence the number of potential colonizers. Experiments were conducted to determine whether infestation levels of Eurosta solidaginis, the goldenrod ball gallmaker, on Solidago altissima were a result of genetic variation among plant clones (i.e., resistance) or previous attack levels. Results indicated that current infestation levels are primarily due to genetic variation in the plant's resistance. Plant clones were found to differ in the percentage of ramets in which oviposition was attempted by Eurosta, the percentage of ramets galled, and in the percentage of ramets with Eurosta oviposition punctures that were galled. Field resistance or susceptibility was maintained in plant clones grown in a common garden.
Variation in Host‐Plant Quality: Implications for Territoriality in a Desert Grasshopper
Authors: Greenfield Michael D. , Shelly Todd E. , Downum Kelsey R.
Published: May 2006

Males of Ligurotettix coquilletti, a univoltine grasshopper associated with Larrea tridentata shrubs in the Sonoran and Mohave Deserts, defend individual host plants as their mating territories. Field studies conducted between 1982 and 1985 demonstrated that certain Larrea shrubs were occupied by territorial males each year while others remained consistently vacant. Males that matured earliest in the season usually defended shrubs that had been most heavily occupied during previous years. Data on the movements of these males indicated that they located preferred territories, rather than simply maturing on and occupying sites that the parental generation had utilized during the preceding year. Occupancy, as measured by the number of different males arriving at a Larrea shrub throughout the season, was highest in large shrubs that were surrounded by small neighbors. The number of long—term resident males and the duration of their residence periods, however, were highest in shrubs that contained low amounts of methanol— or hexane—extractable compounds. Feeding trials showed that L coquilletti preferred foliage selected from bushes that harbored territorial males and contained low amounts of nordihydroguaiaretic acid (NDGA), a powerful anti—oxidant constituting a large percentage of the methanol—extractable compounds coating Larrea foliage. We propose that Larrea shrubs differ in quality as assessed by female L. coquilletti, that these differences reflect variation in foliage chemistry, and that males establish their territories on preferred shrubs to maximize their encounter rate with females. Furthermore, because the number of high—quality shrubs is limited, males in a local population often congregate on only 50—60% of the available shrubs, leaving the remaining ones vacant. This leads to a complex mating system in which males may alternatively defend a territory solitarily, defend a high—quality territory in the company of other males while at the same time behaving aggressively toward them, or adopt subordinate behavior on an occupied, high—quality territory.
Diet Variation in a Population of Darwin's Finches
Authors: Price Trevor
Published: May 2006

I studied the diets of a banded population of Darwin's Medium Ground Finches (Geospiza fortis) on Isla Daphne Major, Galapagos, between 1979 and 1981. I was able to show how an individual's diet was constrained to a subset of the foods that the population as a whole exploits, and considered how an individual selected from among the foods available to it. I examined both the evolutionary and short—term constraints on diet in the population. During times of food shortage there are essentially three food types on the island. These are small seeds, derived from several plant species, which are soft and easy for finches to handle, and two large seeds, derived from the cactus Opuntia echios and a semiannual plant Tribulus cistoides. The smallest finches are unable to crack the large seeds, while among those that crack large seeds, there are positive associations of seed handling ability and beak size. Generalization on all three seed types at the population level and some specialization of individuals on large or small seeds help to explain the high level of morphological variability observed in the population. Within the large—seed category some individuals specialized on Opuntia or Tribulus seeds, whereas others consumed both. Energy rewards from foraging on either seed appeared to be similar. Individuals did not behave in a manner suggesting that they were attempting to maximize energy returns when foraging on these seeds. Specialization on Opuntia or Tribulus is attributable to the patchy distribution of seeds on the island, and to a host of constraints on foraging, including microclimate and territorial behavior. If feeding abilities on each of the three seed types are considered as separate characters, then the joint evolution of the three feeding behaviors is tightly constrained through beak morphology, known to be highly heritable. This in turn implies high genetic correlations between the three feeding behaviors.
Tree Seedlings on Logs in Picea‐Tsuga Forests of Oregon and Washington
Authors: Harmon Mark E. , Franklin Jerry F.
Published: May 2006

Logs are the major seedbed for trees in coastal Picea sitchensis–Tsuga heterophylla forests. Field experiments were conducted at Cascade Head, Oregon, and Hoh River, Washington, to examine pathogens, predation, competition, and standing water as causes for this close seedling–log association. More seedlings survived on log blocks than on soil blocks, regardless of whether the blocks were raised or placed flush with the soil surface. Standing water was therefore an unlikely cause of the seedling–log association. Comparisons of plots protected from and exposed to predation revealed that predation was minor and of equal intensity on soils and logs. Sterilizing soils did not consistently increase seedling survival above controls. Clearing ground–layer vegetation from soil plots significantly increased the survival of conifer seedlings compared with that on uncleared soils. The seed penetration rates through moss mats indicated that <1% of the seedlings germinated within moss mats. Competition with herbs and mosses on the forest floor therefore appears to be responsible for the disproportionate number of tree seedlings found on logs. Recently fallen logs represent sites where competition is low enough for tree seedling recruitment within many Picea–Tsuga forests.
Importance of Energy Costs in Central Place Foraging by Hummingbirds
Authors: Tamm Staffan
Published: May 2006

I tested predictions from three models of central place foraging (CPF) in two field experiments with Calliope Hummingbirds (Stellula calliope). The models were based on different "currencies," or optimization critieria: (a) maximization of net rate of energy intake while foraging and traveling between a central place and feeding areas; (b) minimization of proportion of total time spent foraging and traveling; (c) maximization of energetic efficiency while foraging and traveling. All three models differ from most CPF models in that I assumed that gross rate of energy intake while foraging in a patch remained constant. Net rate of energy intake decreased with time spent foraging because the cost of hovering flight increased during foraging bouts due to the accumulating mass of ingested nectar. In a dual choice experiment repeated with the same male in two years, I offered a choice between a "fast—weak" feeder that provided a fast instantaneous energy intake rate but low sucrose concentration (low energy/mass ratio), and a "slow—rich" feeder with slow instantaneous intake rate and high concentration (high energy/mass ratio). As predicted by all three models, the male preferred the fast—weak feeder when the feeders were near his central place, and he increased his relative use of the slow—rich feeder when they were farther away. In another experiment in which I offered only one type of feeder at a time, males increased meal size with travel distance from their perches to feeders, and took larger, heavier meals from fast—weak than from slow—rich feeders. Both of these results were predicted by the time minimization and efficiency maximization models, but not the rate maximization model. Females who were incubating or feeding young did not change meal size significantly with travel distance from their nests to feeders. This qualitative result was predicted by the rate maximization model but not the time minimization and efficiency maximization models. Females took larger, heavier meals from slow—rich than from fast—weak feeders, and this was not predicted by any of the models. Mean meal sizes of both males and females were smaller than predicted by any of the models at all travel distances.
Environmental Productivity Indices for a Chihuahuan Desert Cam Plant, Agave Lechuguilla
Authors: Nobel Parks S. , Quero Edgar
Published: May 2006

Productivity of Agave lechuguilla, a commercially harvested plant that occurs over vast areas of the Chihuahuan Desert, was measured using conventional dry mass changes in the field and was predicted based on physiological responses to environmental variables in the laboratory. An environmental productivity index (EPI) was constructed as the product of indices for water status, leaf temperature, and photosynthetically active radiation (PAR). Each of these component indices was assigned a maximum value of unity when that variable was not limiting net CO2 uptake over a 24—h period. Soil water potential, daily air and leaf temperatures, and PAR in the planes of the leaves at the field site in Coahuila, Mexico, could thus be quantitatively described in terms of their effect on net CO2 uptake. Water status proved to be the most important of the three variables considered, nearly all leaf unfolding occurring during the relatively wet summer and early fall. The rosette habit and low leaf area index (1.23) for the plant groups monitored led to a relatively uniform and high level of PAR over the leaves. Seasonal changes in PAR proportionally affected both the modest daytime net CO2 uptake and the predominant nighttime net CO2 uptake for this crassulacean acid metabolism plant, while seasonal variations in temperature had relatively small effects on net CO2 uptake over a 24—h period. EPI was highly correlated with the number of new leaves unfolding each month in the field (°2 °0.83); counting unfolding leaves in a nondestructive method of estimating productivity. For the 1—yr study period EPI averaged 0.28, which led to a predicted annual dry mass gain per unit leaf area of 0.68 kg/m2. Field measurements indicated that the actual dry mass gain was about half this value, the difference representing photosynthate needed for constructing and maintaining folded leaves, stem, and roots. The productivity of A. lechuguilla per unit ground area explored by its roots was 0.38 kg.m—2.yr—1, which although much less than for agricultural crops, is still much greater than the average productivity for desert ecosystems.
Environment‐Dependent Intraspecific Competition in Phlox Drummondii
Authors: Clay Keith , Levin Donald A.
Published: May 2006

The relative success of six populations of the annual Phlox drummondii raised under conditions of intraspecific competition was studied in five field sites and an experimental garden located in central Texas. All pairwise combinations of seedlings from six Phlox populations and pure stands of each were planted at a density of eight plants per styrofoam cup. The cups were taken to the field sites, the bottoms were removed, and the cups sunk into the soil. Five replicates of each population pair were transplanted into each of the six sites from which the seeds had been collected. After the flowering season the pots were collected and the number of flowers (or fruits) and amount of biomass per plant were determined. Flower production was strongly related to biomass (° =0.96, P <.0001). In general, pure stands had higher mean flower production than mixed stands, and in mixtures there was a significant tendency for plants grown at their native site to outcomplete aliens in most sites. The production differential between natives and aliens was greater in mixed stands than in pure stands. No one population performed best in all sites and all populations performed best in all sites and all populations performed best in the experimental garden. The results indicate that Phlox drummondii is locally adapted to its microenvironment. The specific outcome of competition is dependent on the site in which competition occurs and the orgin of the competitors.