bluuebunny/crossref_metadata_2025 huggingface dataset:
Predictability, Constancy, and Contingency of Periodic Phenomena
Authors: Colwell Robert K.
Published: May 2006

Temporal patterns in fluctuating physical and biological phenomena are of great interest in several fields of biology, primarily because of their importance as evolutionary constraints. To clarify and simplify the wide variety of terms used to describe aspects of temporal pattern, simple measures of predictability, constancy, and contingency are proposed. These are sufficient to describe the general characteristics of periodic phenomena. The measures are based on the mathematics of information theory. Methods of testing the statistical significance of these measures are given. Predictability, constancy, and contingency can be determined for either qualitative (categorical) or quantitative (discrete or continuous) variables measured over a period of time. Alternative patterns of seasonal flowering and fruiting of tropical trees are given as an example of a qualitative variable; precipitation data are analyzed as an example of a quantitative variable.
Quantitative Studies of Roots of Perennial Plants in the Mojave Desert
Authors: Wallace A. , Bamberg S. A. , Cha J. W.
Published: May 2006

Root and stem weights were obtained for 113 field samples of 10 species of perennial plants in the northern Mojave Desert in order to develop methods for determining belowground biomass under the given conditions. Root and stem weights for all were highly correlated, and linear regression in most cases adequately expressed the relationship between root weight and stem weight. Root weight for the total of all plants considered was about 45% of the sum of stem and root weights. There were species differences. The proportion that was root was generally independent of plant size. Root biomass can be estimated from stem weights for a population, for some species at least, within a possible error of ± 10% to 20%. The data were combined with stem weights by dimension analysis to calculate the belowground biomass per ha in the Rock Valley area of the Mojave Desert.
Density Compensation in Umbra‐Perca Fish Assemblages of Northern Wisconsin Lakes
Authors: Tonn William M.
Published: May 2006

Small, shallow seepage lakes in northern Wisconsin that have both low winter oxygen concentrations and low pH possess depauperate fish assemblages containing primarily or exclusively central mudminnows, Umbra limi, and yellow perch, perca flavescens. Six times during three years I determined the total densities of fish in five lakes by mark—recapture and catch—per—effort methods. The richness of the fish assemblages in these lakes varied from one to three species in a nested pattern. While seasonal variations in density occurred within and among lakes due to variations of growth, mortality, and recruitment, total densities overall were independent of species richness. the pattern, called complete density compensation, suggests that total densities are expressions of the lakes' similar abilities to support fish and that the species interact rather than being independent. An intense negative correlation between perch and mudminnow densities was found, suggesting a strong interspecific population dominance of perch over mudminnows. There was also indirect evidence that both competition (exploitation and interference) and predation by perch regulate mudminnow populations when the species co—occur. Because these assemblages were unpredictably variable in time and space, conclusions from single censuses would have been unreliable. An exercise simulating single seasonal censuses from each richness class showed that less than one—half of such single censuses would have produced the pattern of complete compensation that was observed in the field study. In general, my results support the idea that the relative importance of the factors that determine community structure, or even the factors themselves, can differ not only from assemblage to assemblage at any one time, but even from season to season or year to year within a single assemblage.
Herbivory and Between‐Habitat Diversity: The Differential Effectiveness of Defenses in a Marine Plant
Authors: Gaines Steven D.
Published: May 2006

The effects of consumers on prey populations are traditionally expressed as a function of the intensity or frequency of consumption. Interactions among the intertidal alga Iridaea cordata and five common herbivores of the Oregon coast emphasize that the variety of consumers can be as important as the magnitude of feeding. The five herbivores in this system divide into two functional groups with respect to feeding on Iridaea. (1) Katharina tunicata (chiton), Strongylocentrotus purpuratus (sea urchin), and Collisella pelta (limpet) readily feed on Iridaea in all reproductive conditions. In contrast, (2) Lacuna marmorata (coiled snail) and Idotea wosnesenskii (isopod) feed on reproductively mature plants, but avoid nonfertile individuals. Idotea is deterred from eating immature plants by the alga's cuticle. Lacuna, while impartial to the presence of the cuticle, avoids nonfertile plants for an unidentified reason. At several locations along the Oregon coast these herbivores are abundant and spatially segregated. In these areas Iridaea has a restricted distribution (isolated on vertical walls) that correlates with the qualitative feeding differences among the five herbivores. Iridaea persists with Idotea and Lacuna (although reproductively mature blades are heavily damaged), but is rare or absent when either Katharina or Strongylocentrotus is common. When Katharina is removed from isolated rocky benches, Iridaea establishes in areas where it was previously absent. Qualitative variation in herbivory as a result of feeding by different species enhances between—habitat algal diversity by restricting the distribution of Iridaea. When the effects of herbivores are experimentally reduced, Iridacea shows a more uniform distribution.
Effects of Soil pH, Competition, and Seed Predation on the Distributions of Two Tree Species
Authors: Goldberg Deborah E.
Published: May 2006

Field experiments were conducted to evaluate the effects of soil pH, competition, and seed predation on seedling establishment of two tree species in the Sierra Madre of northwestern Mexico. Quercus albocincta and Lysiloma divaricata occur on the same mountain slopes but have almost no overlap in distribution on a more local scale: Quercus dominates in distinct patches of open woodland found only on acid, nutrient—poor soil derived from hydrothermally altered volcanic rock while Lysiloma dominates in adjacent patches of drought—deciduous forest found only on less acid and more fertile soil derived from unaltered volcanic rock. The experiments were designed to test the hypotheses that Lysiloma is excluded from the oak woodland by the low soil pH and that Quercus is excluded from the deciduous forest by competition and/or seed predation. In the oak woodland, Lysiloma seedlings survived on the acid soil only if soil pH was increased, while Quercus seedlings showed no significant response to increase in soil pH. In the deciduous forest, Quercus seedlings required the exclusion of seed predators as well as removal of surrounding vegetation to survive. Lysiloma seedlings also required removal of some of the surrounding vegetation to establish in the deciduous forest, but less reduction in competition intensity was required than for Quercus seedlings.
Presettlement Regeneration Patterns in a Southwestern Ponderosa Pine Stand
Authors: White Alan S.
Published: May 2006

Tree stems @>106 yr old (i.e., established before significant European influence in this area) in a 7.3—ha old—growth ponderosa pine (Pinus ponderosa) forest in northern Arizona were aged and mapped. Age structure analysis showed that successful establishment of ponderosa pine was infrequent. The periods without successful establishment could be quite long, as suggested by four consecutive decades in which only two surviving trees were established. The stems were strongly aggregated, as measured with nearest neighbor analysis, and groups were visually distinct in the field. Most of the stems occurred in groups of three or more, with group size ranging from 3 to 44 stems and area occupied by a group ranging from 0.02 to 0.29 ha. Ages of stems within groups were variable, the most homogeneous group having a range of 33 yr and the least having a range of 268 yr. The data are not consistent with the commonly held view that southwestern ponderosa pine occurs in even—aged groups and that each group became established following the demise of the group previously occupying the site. Instead, it seems more likely that seedlings became established when one or two trees within the group died, the additional fuel surrounding the dead trees causing an intensely burned spot in the otherwise low—intensity fires that were frequent in the area. The hot spot would create a potential seedbed for pine by eliminating, at least temporarily, the competing grasses on that small area. This decreased competition, in conjunction with adequate seed production and favorable moisture conditions in the spring and early summer, may well have been critical for ponderosa pine establishment. The relative infrequency of all these events occurring in the necessary sequence could explain the erratic age structure data from this area.
Relating Body Size to the Rate of Home Range Use in Mammals
Authors: Swihart Robert K. , Slade Norman A. , Bergstrom Bradley J.
Published: May 2006

The area occupied or traversed by an aimal is a function of the time period considered, but few empirical estimates of the temporal component of home range use are available. We used a statistic called the "time to independence" to make an ecollogically meaningful estimate of the amount of time required for an individual to traverse its home range. Data from 23 species of terrestrial mammals indicated the existence of a size—dependent time scale governing the rate of home range use. Foraging mode influenced the rate of home range use; central place foragers traversed their home ranges approximately five times as rapidly as comparably sized noncentral place foragers. Numerous physiological measures of time are related to body mass raised to the 1/4 power. Our results suggest that the time scale governing the rate of space use by mammals is related similarly to body mass. This relationship permits a more critical examination of factors throught to influence home range size, including habitat prodictivity and social organization.
Biochemical Ecology of Canavanine‐Eating Seed Predators
Authors: Bleiler John A. , Rosenthal Gerald A. , Janzen Daniel H.
Published: May 2006

L—canavanine, a nonprotien amino acid structurally similar to L—arginine, is potentially toxic to many insects. In northwestern Guanacaste Province in Costa Rica, most insect seed predators do not feed on seeds that contain canavanine. However, several larval Coleoptera have become specialists on canavanine—containing seeds. Three of these beetles were compared with a number of other insects with regard to their biochemical ability to deal with and utilize L—canavanine. Our analysis revealed that certain biochemical capacities required by canavanine—feeding insects may have existed prior to their exposure to dietary canavanine.
The Postfire Environment and Earthworm Populations in Tallgrass Prairie
Authors: James Samuel W.
Published: May 2006

Burning of tallgrass prairie is hypothesized to have positive effects on the resource base for belowground—feeding earthworms, but negative effects on the suitability of the soil climate for earthworms. To a certain extent these are inseparable due to the physiology of the grasses that dominate the ecosystem. A four—by—two factorial design using spring burning, autumn burning, addition of natural mulch, and control, with irrigation and without, was used to partially separate the different effects of burning on earthworm populations and to analyze the mechanisms of the responses. The biomass of native prairie earthworms. Diplocardia smithii and D. verrucosa increased 135 and 67%, respectively, with burning; the former species was unaffected by irrigation, and the latter was positively affected. Biomass of an introduced European species Aporrectodea turgida declined form 1.3 g/m2 on burned plots and increased with irrigation. These responses reflect increases in the resource base on burned plots, as indicated by an 11% increase in soil organic matter, and different physiological requirements of the three species.
Comparative Life History Characteristics of Two High Arctic Grasses, Northwest Territories
Authors: Grulke N. E. , Bliss L. C.
Published: May 2006

Two grasses wiith differing adaptations were studies in an extreme, but variable, high arctic environment. Phippsia algida is a ruderal species that colonizes bare soil in mesic sites, but also occurs across a broad range of habitats. Puccinellia vaginata occurs in xeric microsites with little soil disturbance. The phenology, carbon allocation, physiological adaptations to drought, and reproductive effort were compared between these two closely related species. Leaf water potential was variable on a daily basis in both species despite high soil water potential. Of the six physical factors studied (vapor pressure deficit, soil moisture, leaf and root temperature, wind, PAR), only wind and PAR (photosynthetically active radiation) were not limiting to leaf conductance at any level measured in the field. Phippsia experienced more frequent and severe drought stress at less extreme physical conditions than Puccinellia. In general, Phippsia had higher leaf turgor and growth rates. Puccinellia generally had higher leaf conductance, lower turgor and growth rates. Drought stress and low temperatures reduced whole plant biomass of Phippsia at all ages but affected only seedlings on Puccinellia. Phippsia root biomass was lower with drought and low soil temperatures, but was high and constant throughout the study for Puccinellia. Phippsia was relatively short lived (23 yr mean, maximum 34 yr) and Puccinellia long lived (35 yr mean, maximum 74 yr). The mean age of initial reproduction was relatively late when compared to temperate species, but it occurred sooner in PHippsia (17 ± 6 yr) than Puccinellia (26 ± 9 yr). Phippsia had high, constant seed set under all field conditions monitored. Production of viable seeds in Puccinellia was restricted by short growing seasons, low solar radiation, and low temperatures. During stressful years, Phippsia continued high carbon allocation to reproduction, but also had high rates of plant mortality. In contrast, Puccinellia generally had low carbon allocation to reproduction, and low rates of mortality during periods of stress. Although both species tolerate stress in the sense that they persist in an extreme environment, Phippsia algida has many characteristics of a ruderal and fills this role in a high arctic ecosystem. Puccinella vaginata has most characteristics of a stress—tolerant species.
Allometric Theory Explains Self‐Thinning Relationships of Mountain Beech and Red Pine
Authors: Osawa Akira , Allen Robert B.
Published: May 2006

Allometric theory on mechanisms of the self—thinning rule was tested for Nothofagus solandri populations from the Craigieburn Range, New Zealand and for Pinus densiflora stands from northern Japan. The self—thinning rule describes a consistent relationship of mean plant mass to the approximately —3/2 power of plant density in evenaged monocultures. Although his rule has been described for various species, mechanisms that produce certain relationships have not been well understood. We tested an allometric theory of Long and Smith of the self—thinning rule that assumes constant foliage mass density and allometry for mean dimensions of populations that represent dense conditions for given mean plant sizes. Only stands at maximum crowding were selected for analysis. The self—thinning boundary of N. solandri showed an exponent —1.13 with a 95% ci of —1.25 to —1.02 for mean stem mass. This was significantly shallower than the conventional value of the exponent —3/2, but was identical to the predicted exponent from the allometric theory. The thinning coefficient was also explained numerically by this hypothesis. In contrast, analysis of published data for P. densiflora indicated that the thinning exponent did not differ from the proposed —3/2. Empirical thinning lines varied substantially depending on species and plant parts considered; however, the allometric theory consistently provided predictions that agreed with the observed thinning relationships. Implications for the geometry of self—thinning populations and generality of the allometric theory are discussed.
Resource and Pollen Limitations to Lifetime Seed Production in a Natural Plant Population
Authors: Campbell Diane R. , Halama Kenneth J.
Published: May 2006

Using experimental manipulations, we examined pollen and resource limitations to seed production and their interaction in a natural population of a monocarpic plant, Ipomopsis aggregata. Our design cross two factors, pollen level (hand—pollinated or control) and resource level (water added, water and fertilizer added, or control). Both hand—pollination and fertilizing during the blooming season increased total seed production, while watering alone has no effect on any component of reproductive success. Hand—pollination boosted number of seeds per flower, with no effect on flower number. In contrasts, fertilizing had its primary effect on the number of flowers produced, while also increasing the number of seeds per mature fruit in hand—pollinated plants. Fertilizing increased nectar volume, but path analysis detected no indirect effect of this increased reward rate on seed production, suggesting instead that fertilizing has a slight, but direct, effect on seeds per flower. These results argue against a strict dichotomy between pollen limitation and resource limitation of female reproductive success in plants.
Physiological Benefits of Stem Curling for Resurrection Plants in the Field
Authors: Lebkuecher Jefferson G. , Eickmeier William G.
Published: May 2006

The stems of the resurrection plant Selaginella lepidophylla, a piokilohydrous desert pteridophyte, curl dramatically as the plants dry and uncurl when rewetted. We tested the hypothesis that stem curling is a morphological feature that may serve to limit bright—light and/or thermal damage in resurrection plants with a field experiment at a Chihuahuan desert site in west Texas. Experimental plants were irrigated for 3 d and then permitted to dry. The plants wee either allowed to curl normally or mechanically restrained to prevent curling and either shaded (10% of ambient irradiance) or left fully exposed during desiccation. Nine days after complete desiccation, the plants were harvested and returned to the laboratory. When rehydrated, plants that had been unshaded and restrained from curling during field desiccation had significantly lower photosystem II electron transport rates, higher intrinsic chlorophyll fluorescence (Fo) yields, lower variable to maximum chlorophyll fluorescence (Fv/Fm) yield ratios, lower chlorophyll contents, and both lower whole—plant photosynthetic quantum efficiencies and light—saturated CO2 assimilation rates relative to plants from the other desiccation treatments. The results of both low—irradiance and high—irradiance hydrations in the laboratory suggest that the curling during desiccation and the uncurling during rehydration of S. lepidophylla in its natural habitat serves to protect the plant from damage due to high irradiance, high temperature, or perhaps a synergistic combination of the two. This protection permits the rapid recovery of photosynthetic competence when next the plants are wetted, thereby helping to maximize net carbon balance over time.
Natural Enemies and Plant Water Relations Influence the Distribution of an Armored Scale Insect
Authors: Hanks Lawrence M. , Denno Robert F.
Published: May 2006

In urban settings, the armored scale insect Pseudaulacaspis pentagona infests mulberry trees in disturbed landscape habitats (roadsides, parking lots) but not trees in forested habitats (woodlots). Populations of P. pentagona are contagiously distributed among mulberry trees in landscape habitats. We investigated how two factors, natural enemies and plant water potential, interact to affect the spatial distribution and abundance of P. pentagona. Survivorship of P. pentagona (caged with enemies excluded) was significantly lower on scale—free landscape trees than on scale—infested landscape trees or forest trees. Differences in scale survivorship disappeared, however, when scales were raised on greenhouse—grown clones of these same field trees, suggesting that variation in survivorship was due to the habitat in which trees grew. The survivorship of caged scales (enemies excluded) on field trees was positively related to midday water potential, an index of plant moisture stress. Scale—free landscape trees had significantly lower midday water potentials than did either scale—infested landscape trees or forest trees. Patterns of water potential and scale survivorship across trees suggested that the absence of P. pentagona on landscape trees with a history of scale absence was attributable to plant water relations. Moreover, highly significant differences in water potential occurred between infested and scale—free mulberry trees located <10 m apart, suggesting that moisture deficit influences the performance and abundance of scale insects over small distances. When cohorts of P. pentagona were raised on field trees and exposed to natural enemies for one generation, mortality was significantly higher on forest trees than on either scale—free or scale—infested landscape trees. High scale mortality in the forest habitat was attributable to abundant generalist predators. (e.g., phalangids, earwigs, and tree crickets). Scale mortality in the landscape habitat, due to specialized coccinellid predators and aphenlinid parasitoids, was much lower than in the forest habitat. The patchy distribution of P. pentagona in Maryland is best explained by the combined influences of natural enemies and plant water relations. P. pentagona infests a small subset of mulberry trees in the urban landscape that are without water deficits and free of generalist invertebrate predators.
Seedlings from Large Seeds Tolerated Defoliation Better: A Test Using Phylogeneticaly Independent Contrasts
Authors: Armstrong Doug P. , Westoby Mark
Published: May 2006

The characteristic seed size of a plant species may reflect evolution toward an optimal compromise between size and number of seeds produced. The extent to which large seed size conveys an advantage to a seedling probably varies according to circumstances, and this may explain at least some of the great variation in seed size among species. Here we test whether large—seeded species are better able than small—seeded species to withstand defoliation at the seedling state. We compared growth and survival of seedlings of 40 species of Australian angiosperms after removing 95% of their photosynthetic tissue shortly after emergence. We accounted for effects of phylogeny by selecting pairs of species that were "phylogenetically independent contrasts." That is, the phylogenetic path connecting the two species of any pair was independent of the path connecting the two species of any other pair we used. We also selected the species so that the two members of each pair differed at least 10—fold in seed mass. Therefore, each pair provided an independent case for examining the consequences of evolutionary divergence in seed size. We found that the large seeded species survived loss of photosynthetic tissue better than the smaller seeded species in 14 of the 16 pairs for which there was a significant difference in survival. The extent to which growth of survivors was reduced by loss of photosynthetic tissue also differed in 8 pairs. With the large—seeded species being less affected in 6 of these cases. We conclude that large seed size is generally associated with greater ability of seedlings to cope with loss of photosynthetic tissue. Consequently, large seed size may be favored in any circumstances in which seedlings are likely to experience carbon deficit early in development.
Emergence of 13‐Yr Periodical Cicadas (Cicadidae: Magicicada): Phenology, Mortality, and Predators Satiation
Authors: Williams Kathy S. , Smith Kimberly G. , Stephen Frederick M.
Published: May 2006

We examined emergence of male and female 13—yr periodical cicadas (Brood XIX) and mortality, due to avain predation, fungal infection (Massospora cicadina), environmental factors, and senescence. We compared relative contributions of the sources of mortality, and determined the temporal pattern of avian predation associated with predator satiation and mortality. Based on a mean density of 6.65 emergence holes/m2, we estimated that 1 063 300 cicadas emerged on our 16—ha study area in northwestern Arkansas during May 1985. Males appeared first in emergence traps in early May and emerged more synchronously than did females. About 50% of the population emerged during four consecutive nights, and peak abundance of adult cicadas occurred in late May. Based on samples from mortality traps, at least 40% of the population died in severe thunderstorms during the first week of June, demonstrating the stochastic factors can be major influences on periodical cicada populations. Fungal infection was not a major source of mortality. The first cicadas that emerged in early May were eaten by birds, but avian predators became satiated. Birds consumed 15—40% of the standing crop at low cicada densities, but very little of the standing crop was consumed when cicada densities were >24 000 individuals/ha on the study site. Avian predators appeared to be satiated for several weeks, and by the time their foraging activities increased due to demands imposed by feeding young, density of adult cicadas was relatively low. Mortality due to predation gradually increased to near 100% as the density of adult cicadas declined in June. Based on estimates of cicada emergence and deaths due to avian predation where adult cicada activity was greatest, birds consumed only °15% of the adult cicada population. Therefore, only a small proportion of the adult cicada population was actually consumed by avian predators. These results demonstrate that, indeed, the synchronized, explosive emergences of periodical cicadas may be classical examples of predator satiation.
Diving Behavior During Foraging in Breeding Adelie Penguins
Authors: Chappell Mark A. , Shoemaker Vaughan H. , Janes Donald N. , Bucher Theresa L. , Maloney Shane K.
Published: May 2006

We used electronic time depth recorders to examine diving patterns of Adelie Penguins (Pygoscelis adeliae) breeding near Palmer Station, Antarctica. Most hunting dives consisted of a rapid descent to depth, a period of bottom time at near—constant depth, and a rapid ascent to the surface. Most hunting activity occurred in bouts of consecutive dives to similar depths. Adelies foraged at depths between 3 and 98 m, with a mean of 26 m. descent and ascent rates averaged 1.2 and 1.1 m/s, respectively. Foraging was primarily diurnal, but there was relatively little circadian change in foraging depth. The birds' overall hunting effort (cumulative bottom time) was concentrated between 0500 and 2100 at depths between 10 and 40 m. Bottom time decreased slightly with increasing depth but the correlation was weak. Dive duration was positively correlated with dive depth. Maximum dive duration was 160 s; most hunting dives lasted 60—90 s with a mean of 73 s. Post—dive surface intervals averaged °50% of dive duration. Time use efficiency during dive bouts (bottom time/[dive duration+ surface interval]) decreased with increasing dive depth. Estimates of oxygen stores and diving metabolic rates indicate that the aerobic dive limit of Adelies is 46—68 s and that most hunting dives require some anaerobic metabolism. Use of anaerobiosis engenders and energy penalty and probably affects both the behavior and energetics of foraging.
Top‐Down and Bottom‐Up Control of Stream Periphyton: Effects of Nutrients and Herbivores
Authors: Rosemond Amy D. , Mulholland Patrick J. , Elwood Jerry W.
Published: May 2006

We conducted two experiments to determine the relative effects of herbivory and nutrients on an algal community in Walker Branch, a stream having effectively two trophic levels: primary producers and herbivorous snails. The first study (1989), performed in streamside channels, tested the effects of three factors: (1) stream water nitrogen (N), (2) phosphorus (P), and (3) snail grazing, on periphyton biomass, productivity, and community composition. The second study (1990), conducted in situ, tested the effects of snail grazing and nutrients (N + P). In the 1989 study, nutrients had positive effects, and herbivores had negative effects, on algal biomass (chlorophyll a, ash—free dry mass, total algal biovolume) and primary productivity (area— and chlorophyll—specific). Likewise, both nutrients and snail grazing exerted effects (+ and —< respectively)( on biomass measured in the 1990 study (chlorophyll a, algal biovolume). Grazed communities were dominated by chlorophytes and cyanophytes, which were overgrown by diatoms when herbivores were removed. Algal species that were reduced most by herbivores were increased most by nutrient addition, and vice versa, suggesting a trade—off between resistance to herbivory and nutrient—saturated growth rates. Increases in algal biomass and productivity were slight with the addition of either N or P compared to responses observed when both nutrients were added together, suggesting that both nutrients were at growth—limiting levels. The greatest changes in periphyton structure or function were observed when both nutrients were added and simultaneously, grazers were removed, in contrast to lesser effects when nutrients were added under grazed conditions or grazers wee removed at low nutrient levels, indicating dual control by both factors. Nutrient addition also positively affected snail growth in both experiments, indicating tight coupling between herbivore and algal growth (top—down effects) and that bottom—up factors that directly affected plant growth could also indirectly affect consumers belonging to higher trophic levels. Indices quantifying the direct effects of top—down factors relative to bottom—up factors (top—down index, TDI) and the importance of interactions between these factors (interaction coefficient, IC) wee computed. These indices showed that the relative strength of top—down and bottom—up factors varied among biomass and productivity parameters and that top—down and bottom—up effects, alone, were less important than their combined effects.
Early Consequences of Seed Dispersal for a Neotropical Tree (Virola surinamensis)
Authors: Howe H. F. , Schupp E. W. , Westley L. C.
Published: May 2006

We examined the effects of distance from a fruiting conspecific, light environment, and initial seed size on survivorship using Virola surinamensis seeds and seedlings placed in concentric rings around fruiting Virola trees. Insects and mammals killed 99.2% of seeds and seedlings between fruit drop and exhaustion of parental endosperm at 12 wk after germination. During this time, survival was not influenced by either seed size or light environment, but up to a 44—fold advantage accrued to seeds dropped 45 m from a fruiting tree as compared with those underneath Virola crowns. Seeds that germinated under and near fruiting Virola trees suffered disproportionate attack from a curculionid weevil (Conotrachelus sp. nov.). Both large initial seed size and favorable light environments (assessed by canopy photographs) enhanced seedling vigor at 18 wk of age. Among those seedlings that escaped predation beyond the exhaustion of parental seed stores, a difference of as little as 1% in the amount of canopy open overhead distinguished seedlings with three leaves (3.5% open) from those with two leaves (2.3% open). Our results demonstrate clear advantages to local seed dispersal for a tree the rainforest canopy. They further suggest that dispersal of seeds >20m by large birds, such as guans (Penelope purpurascens) and toucans (Ramphastos sulfuratus and R. swainsonii), is more favorable for seedling survival than that of the smaller trogons (Trogon massena) and motmots (Baryphthengus martii), which regurgitate seeds under or near the tree crown.
Leaf‐Cutting Ant Herbivory in Successional and Agricultural Tropical Ecosystems
Authors: Blanton Chantal M. , Ewel John J.
Published: May 2006

Herbivory by Atta cephalotes was measured in four plant communities of different complexity in Costa Rica. The four communities were a monoculture of cassava (Manihot esculenta) and three diverse assemblages, each 1.5 yr old: (1) successional vegetation, unmodified by the investigators; (2) imitation of succession, a community of investigator—introduced species designed to mimic the unmodified succession; and (3) enriched succession, a successional vegetation that the investigators had augmented by propagule inputs. Each and colony had access to all four community types simultaneously. The average herbivory rate (daily cutting of leaf per square metre of ground, all treatments combined) was °150 mg (or 38 cm2). In communities with greater leaf area index, structural complexity, and species richness. A. cephalotes cut lower portions of total leaf area. Before harvest of the cassava monoculture, the herbivory rate of A. cephalots was 87.9 cm2°m—2°d—1 in the monoculture, 21.4 in the imitation, 14.7 in the succession, and 6.8 in the enriched succession community. These amounts represented 0.3% of total leaf area in the monoculture and a mean of 0.03% of total leaf area in three complex ecosystems. Cassava, which occurred in three of the four communities, was attacked most heavily (per unit leaf area) in the imitation successional community, least heavily in the enriched succession, and at intermediate intensity in the monoculture. In response to loss of their preferred forage (cassava) through harvesting by humans, the ants cut more leaf tissue in the three species—rich communities, especially the imitation. As cassava resprouted in the monoculture, A. cephalotes' rate of attack on the three diverse treatments reurned to preharvest levels. Atta cephalotes cut only 17 of 332 available plant species. They cut proportionally more woody than herbaceous species, more introduced species than natural colonizers, and species with below—average water contents. Plant relative abundance alone did not determine host plant selection, but most of the attacked species were cut in proportion to their total leaf area.
Host Leaf Selection by Leaf Miners: Interactions among Three Trophic Levels
Authors: Faeth Stanley H.
Published: May 2006

The influence of leaf characteristics on host leaf selection and survivorship in an endophagous insect were studied in a leaf—mining species, Stilbosis juvantis. Dry mass measurements of mined and unmined leaves (after abscisson) showed that the leaf miner does not select leaves for oviposition and subsequent larval feeding based on leaf size, possibly because densities of S. juvantis are typically low and this miner consumes only a small part of a leaf. However, census of 18 host trees, on 6 of which the proportion of damaged leaves had been experimentally increased, revealed that these leaf miners do select significantly more intact leaves than damaged ones for ovisposition. S. juvantis on damaged leaves experienced significantly lower survivorship, owing to increased parasitism, than did miners on intact leaves. The higher rate of attack by parasitoids is probably attributable to physical or chemical alterations in damaged leaves. Thus, damaged to leaves by previous herbivores modifies leaf selection by miners. These results indicate a subtle interaction among species at three trophic levels in this plant—herbivore system. Indirect negative interactions occur between guilds (leaf chewers and leaf miners) via changes in rates of parasitism rather than through resource—based competition.
Evolution of Optimal Group Attack, with Particular Reference to Bark Beetles (Coleoptera: Scolytidae)
Authors: Berryman Alan A. , Dennis Brian , Raffa Kenneth F. , Stenseth Nils Christian
Published: May 2006

A general model for the benefits and costs of group attack by small predators that kill large prey was developed, and provided support for the hypothesis that bark beetles infesting pine trees have evolved group attack behavior that maximizes individual fitness. The model also provides a basis for evaluating the conditions under which social hunting behavior is likely to evolve, and provides insights on the population dynamics of animals that hunt in groups.
Dynamics of Cyanobacterial and Ostracod Interactions in an Oregon Hot Spring
Authors: Wickstrom Conrad E. , Castenholz Richard W.
Published: May 2006

Microbial mat communities are arrayed along a hot spring outflow as a result of biotic and abiotic requirements and/or antagonisms. It is generally assumed that temperature is the ultimate determined of species distributions and growth, hence of community composition and biomass. This assumption was tested through experimentation and detailed seasonal observations of mat communities in s southern Oregon hot spring. The microbial community most extensively studied was dominated by two species of cyanobacteria (blue—green algae), Calothrix thermalis and Pleurocapsa minor. These species comprised >70% of the community biomass and formed a distinctive crustose, nodular mat. This mat occupied the 35°—40° to 48°—51°C zone along the thermogradient of Fenceline Stream (Hunter's Hot Springs complex), as in many other thermal springs of Oregon, Nevada, and California. The geographical distribution of the nondular mat closely paralleled that of the thermophilic ostracod Potamocypris sp. Contrary to predictions, the lower border of the Fenceline nodular mat extended farther downstream in winter than in summer, and standing crop biomass was greater in winter. During all seasons the distribution of the nodular species was truncated with respect to their potential temperature ranges. Observations of ostracod behavior and population dispersion patterns, as well as results from temperature—effect and grazing experiments, indicated that the ostracods were responsible for the observed mat anomalies. The dynamics of the ostracod population and the grazing pressure exerted by this population were influenced by diel and cumulative (i.e., seasonal) thermogradient extensions and retractions. During diel cooling periods, ostracods controlled the upstream border of the Pleurocapsa Calothrix mat by grazing back the faster growing but less resistant Synechococcus—Chloroflexus mat. During autumnal cooling, the herbivore population dispersed upstream as food resources became available, thereby decreasing the grazing pressure over the lower portions of the nodular mat. This resulted in increased biomass and downstream growth of the nodular mat until it reached a winter maximum. Vernal thermogradient lengthening again changed the grazing range of the ostracod populations, which subsequently increased the grazing pressur over the Pleurocapsa—Calothrix mat. The coalesced nodular mat was reduced to patchy nodules and the lower border gradually shifted upstream, thus completing the atypical seasonal cycle. A grazer—induced dynamic equilibrium prevented exclusion of Pleurocapsa by the faster growing Calothrix, as the latter organism was easily grazed yet gained refuge within the partially refractory masses of Pleurocapsa cells. The persistence of the Pleurocapsa Calothrix mat community in hot springs of the area is grazer dependent, since without Potamocypris the temperature zone inhabited by the nodule species becomes overgrown by Synechococcus or other cyanobacteria
Calling Site Choice by Hyla Chrysoscelis: Effect of Predators, Competitors, and Oviposition Sites
Authors: Resetarits William J. , Wilbur Henry M.
Published: May 2006

We examined the effect of predators, competitors, and conspecifics on the choice of calling sites by male gray treefrogs, Hyla chrysoscelis, and the correlations between choice of calling sites by males and choice of oviposition sites by females. The experiment was conducted in an array of wading pools and utilized a replicated block design for variance analysis. Pools were open to natural colonization by male and female treefrogs. The treatments consisted of controls containing no predators or competitors, four treatments containing one species of predator (either Ambystoma maculatum larvae, Enneacanthus chaetodon adults, Notophthalmus viridescens adults, or Tramea carolina larvae), and two treatments containing one species of competitor (either Rana catesbeiana or Hyla chrysoscelis). Male and female treefrogs avoided pools containing conspecific tadpoles and pools containing adult black—banded sunfish, Enneacanthus chaetodon. Females also avoided pools containing larval spotted salamanders, Ambystoma maculatum. Correspondence between calling sites and oviposition sites was examined in response to treatment, block (consisting of a location and a time), and location. Correspondence between male choices and female choices was weak. Number and location of calling males showed no significant correlation with the number and location of eggs along any of the axes of choice. Both males and females chose sites based on the species present, and both showed preferences in regard to location and time. The differences between male and female choices indicate that the potential choices have different values to each sex, or that different criteria are used to rank potential choices. Anuran reproductive behavior is not a simple consequence of the direct interactions between males and females, but is a result of complex interactions between the behavior of the two sexes and the environment in which their behavior is embedded.
Effects of Breeding Experience and Familiarity on Site Fidelity in Female Red‐Winged Blackbirds
Authors: Beletsky Les D. , Orians Gordon H.
Published: May 2006

We used data from a 12—yr study of dispersal and breeding success to investigate how female Red—winged Blackbirds (Agelaius phoeniceus) make decisions about movements between nests, both within and between years. Females were studied on eight separate marshes that were sufficiently close to permit inter—marsh movement. Our analyses showed that (1) a female's previous success affects her subsequent nesting decision, especially between years; (2) females exhibit strong marsh fidelity; and (3) the presence of familiar male neighbors affects female movement decisions. Little support was found for the ideas that females move in response to different types of nest predators, familiarity with current mates, or nesting success of other females on their marshes. Our results suggest that (1) there is more social constraint on changing territories and marshes, especially within breeding years, than previously suspected, and (2) although mate fidelity is relatively weak and unimportant, male "neighborhoods" or "breeding groups" affect female nesting success and, hence, influence movements.
A Test of Risk‐Sensitive Foraging in Wild Bumble Bees
Authors: Cartar Ralph V.
Published: May 2006

Risk—sensitive foraging decision (i.e., those based on both reward mean and variance) have often been observed in laboratory contexts, but there are few observations of risk—sensitive foraging in the wild. Indeed, wild foragers need not show risk sensitivity, since they can adopt alternative tactics for minimizing their probabilities of energy shortfall. I tested for risk—sensitive foraging in a natural context in coastal southwest British Columbia, Canada. Seablush (Plectritis congesta) and dwarf huckleberry (Vaccinium caespitosum) are co—occurring plants that offer foraging bumble bees equivalent expected rates of net energy intake, but that differ in variability. The equalization of mean profitabilities of floral rewards results from the actions of nectar—collecting foragers. Risk—sensitive diet choice can therefore be based solely on the variances of the foraging alternatives. I manipulated the energy requirements of bumble bee colonies (Bombus melanopygus, B. mixtus, B. sitkensis) by draining or enhancing their honey pots. I then censused foragers from these colonies on the two flower species. If bees were risk sensitive in their diet choice, they should have increased their relative use of the more variable flower type (dwarf huckleberry) when their colonies were nectar depleted compared to when their colonies were nectar enhanced. This was indeed the case. Wild—foraging bumble bees thus appear to be sensitive to both the mean and the variance of the energy rewards offered by alternative flower species, and this sensitivity is affected by colony energy requirements. These results suggest that wild foragers may change their risk sensitivity.
Plant Litter: Light Interception and Effects on an Old‐Field Plant Community
Authors: Facelli Jose M. , Pickett Steward T. A.
Published: May 2006

We studied the effects of litter of the annual grass Setaria faberii, the perennial herb Solidago spp. (mostly S. canadensis), and leaves of the hardwood tree Quercus alba on a successional plant community. We also assessed light interception by these litter types in the laboratory. Light extinction followed the Beer—Lambert exponential law. Solidago litter had the highest transmittance constant and Quercus litter the lowest. The three types of litter produced different light mosaics at the microsite (0.8 cm diameter) scale. In the field, all three litter types affected community structure, by the effect of Quercus was the strongest. Litter reduced the density of the two dominant grasses, Setaria faberii and Panicum dichotomiflorum. Quercus and Setaria litter resulted in biomass compensation (i.e., fewer but larger individuals) by S. faberii but not by P. dichotomiflorum, which probably was always outcompeted by S. faberii. Solanum carolinense, the main dicot in the community, was unaffected by litter addition. Setaria and Solidago litter enhanced the establishment of Erigeron annuus, but Quercus litter reduced it. Litter reduced the number of flowering individuals of S. faberii and the number of seeds per plot; Quercus litter increased the production of seeds per individual. We conclude that species—dependent effects of litter on plant populations may significantly alter interspecific interactions and change plant community structure through direct and indirect effects.