bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement of Long‐Wave Radiant Flux in Organismal Energy Budgets: A Comparison of three Methods
Authors: Campbell G. S. , Mugaas J. N. , King J. R.
Published: May 2006

Long—wave radian flux density was inferred as the difference between short—wave and total flux, the latter being measured by modifications of 2 commercially available instruments. One of these was a Kipp pyranometer in which the glass domes were placed by a polyethylene dome and a thermocouple was affixed to the bottom of the thermopile. The other was a net radiometer in which the lower polyethylene dome was painted, shaded, and supplied with a thermocouple. Long—wave flux density was measured separately with a custom—made prygeometer, using a commercially available KRS—5 dome. Construction details are given for all 3 instruments. Comparisons through a 4—day span under field conditions showed that the 3 readings of long—wave flux density never differed by >10%, and usually differed by <2—3%, although differences as great as 20 W/m2 sometimes occurred when short—wave flux was high. Short—wave heating of the dome had a negligible effect on the accuracy of the new pyrgeometer, unlike the commercial model.
Fire Regime in Red Pine Stands at the Northern Limit of the Species' Range
Authors: Bergeron Yves , Brisson Jacques
Published: May 2006

In order to define more precisely the fire regime prevailing in red pine stands, and to evaluate its effect on the maintenance of marginal populations in the boreal forest, we documented the frequency, extent, and intensity of fires that affected red pine populations in northwest Quebec. We focused on two islands in Lake Duparquet, divided the islands into 5 x 5 m plots, and recorded topography, soil moisture, and vegetation for each plot. We mapped stems and noted fire scars, took core samples and stem cross sections to determine the age of trees, and used fire scars, post—fire regeneration, surviving trees, and changes in growth rates as indications of fire passage and intensity. Fire frequency was also estimated using fire history data from 13 other stands. Before 1906 the fire regime was characterized by: (1) a short but very irregular fire interval averaging °30 yr, (2) a large variation in burned area, including patches that were generally left unburned, (3) a low fire intensity, although some fires were locally very intense, and (4) the occurrence, roughly every 68 yr, of fires of sufficient intensity to kill most trees on the entire island. Since 1906 the frequncy of fire has decreased dramatically. The fire regime appears to be controlled by abiotic conditions. The high—frequency fires of variable intensity primarily affect the xeric habitats. The less frequent, intense (possibly crown) fires that affect the entire island may depend on stand development from a less susceptible pine and hardwood forest towards a more typical spruce—fir boreal forest in more mesic habitats. The fire regime described for these insular stands at the limit of species range is similar to that described for other parts of the range of red pine. We hypothesize that red pine is restricted to insular habitats at this northern limit of its range because of the particular fire regime that prevails there; the larger and more intense fires of the mainland here led to the elimination of the species.
Demographic Costs of Antipredator Defenses in Daphnia Pulex
Authors: Riessen Howard P. , Sprules W. Gary
Published: May 2006

Juvenile Daphnai pulex develop neck spines in response to a chemical agent released by predatory Chaoborus larvae. While these defensive structures reduce the vulnerability of Daphnia to the insect predator, they also entail a demographic cost. We investigated the nature and degree of this cost through an analysis of cohort life tables involving both the typical morph (TM), which lacks neck spines, and spined morph (SM) at 22°C in two different food regimes: "natural" food conditions (53—@m filtered pond water) and "ideal" food conditions (1 x 105 cells/mL Chlamydomonas sp.). No consistent pattern of differences between TM and SM occurred with respect to survivorship, clutch sizes, body sizes, mean egg volume, or number of juvenile instars. Development rates of both juvenile and adult instars, however, were significantly slower in SM. The presence of neck spines increased the age maturity for D. pulex by 8.4—14.6%, and the duration of adult instars exposed to Chaoborus—factor, whose eggs will develop into SM, was 2.8% longer than for those not exposed. This caused delayed reproduction in SM and resulted in a population growth rate (°) that was °8—9% lower than in TM. This relatively large demographic cost of spine formation in D. pulex produces a strong selection pressure to forgo the formation of these spines when Chaoborus predators are absent.
Chemical Leaf Repellency to an Attine Ant: Seasonal Distribution Among Potential Host Plant Species
Authors: Hubbell Stephen P. , Howard Jerome J. , Wiemer David F.
Published: May 2006

The chemical repellency of leaves to the leaf—cutting, fungus—growing ant Atta cephalotes was surveyed for 42 plant species randomly selected from the dry forest of Santa Rosa national Park, Guanacaste, Costa Rica. The sample represents about one—sixth of the potential host plant species available to the ants in the secondary, semideciduous forest at Santa Rosa. Repellencies of leaf extracts were measured by bioassay. A laboratory colony of A. cephalotes collected from Santa Rosa was used. Three—quarters of the species exhibited significantly repellent nonpolar (lipid—soluble) extractables (e.g., terpenoids, steroids, and waxes), and half of the species exhibited repellent polar extractables (e.g., phenols, flavonoids, and glycosides). The occurrence of significant extractable repellents, particularly in the lipid—soluble fraction, correlated closely with species that were avoided in leaf—preference tests and with seasonal patterns of host—plant selection by ant colonies in the Santa Rosa forest. There were no clear correlations of repellency with plant growth form or with evergreen vs. Deciduous habit. Further chemical isolation of ant—repellent substances from a number of avoided species revealed that most of the repellent compounds are terpenoids, some of which have already been shown to be highly toxic to the ant's fungus and to many other fungi as well. A dramatic decline in the amount of extractable repellent occurred in almost all plants species in the latter half of the wet season, 1—2 mo before the dry season began. Because this decline was prevalent in evergreen as well as deciduous species, we tentatively concluded that the ant—repellent substances in the leaves are mainly the result of selection in plants for fungal resistance, which incidentally confers resistance to attack by leaf—cutting ants. We suggest that the decline in the amount of repellent substance is due to a reduction in the synthesis of antifungal secondary compounds in the dry season, when the risk of fungal attack is low.
Long‐Term Yield from Harvester Ant Colonies: Implications for Horned lizard Foraging Strategy
Authors: Munger James C.
Published: May 2006

For a long—term, prudent foraging strategy to evolve via natural selection at the level of the individual, three conditions must be met: (1) the forager must use resources from a discrete subpopulation, (2) use of that subpopulation must be virtually exclusive, and (3) the resource population must respond in such a way that a long—term strategy actually provides a greater economic benefit to the forager than does a short—term strategy. In this study, the third condition is tested for a predator (the horned lizard: Phrynosoma cornutum) that has been hypothesized to employ a long—term strategy when harvesting worker ants from colonies of the ant Pogonomyrmex desertorum. Five artificial harvesting experiments were undertaken to determine if, at some point, an increased harvesting intensity will lead to a decreased long—term yield. Such a result would indicate that it is economically advantageous for horned lizards to employ a strategy in which harvest intensity is reduced in order to promote the long—term availability of ants. However, in none of the five experiments was this result obtained. It is therefore concluded that evolution of a long—term strategy in horned lizards is unlikely. Failure to obtain the required result is attributed to three characteristics of P. Desertorum colonies. (1) Because colonies cease surface activity in response to depredation of surface—active workers, and increase in foraging effort (above some threshold) by horned lizards will not lead to an increase in losses from preyed—upon colonies. (2) Because colonies have an unharvestable reserve consisting of the queen, immature workers, and colony stores, they can, for a limited time, continue to produce surface—active workers (the resource used by horned lizards), even if all surface—active workers have been previously harvested. (3) In the short term, colonies are a finite resource; it should matter little whether the resource is harvested in one intense bout or in a series of less intense bouts.
On the Importance of Male Fitness in Plants: Patterns of Fruit‐Set
Authors: Sutherland Steve , Delph Lynda F.
Published: May 2006

A hypothesis is presented which states that flower production in hermaphroditic flowering plants is primarily controlled by male function. The male function hypothesis predicts a lower fruit—to—flower ratio for hermaphrodites as compared to monoecious or dioecious plants. The hypothesis also predicts that self—compatible hermophrodites should exhibit a higher percent fruit—set than self—incompatible hermaphrodites. These predictions are supported by fruit—set data complied from the literature. An alternative hypothesis relating fruit—set to the probability of self—fertilization also predicts low fruit—set for hermaphrodites as compared to monoecius or dioecious plants. The self—incompatibility hypothesis is tested and rejected on the basis of fruit—set patterns in self—incompatible andromonoecious, self—incompatible monoecious, and self—compatible monoecious species. The effect of the male function hypothesis on current ideas concerning low fruit—set in hermaphrodites is then examined.
Competitive Abilities of Sparse Grass Species: Means of Persistence or Cause of Abundance
Authors: Rabinowitz Deborah , Rapp Jody K. , Dixon Philip M.
Published: May 2006

Sparse species have chronically small local population sizes, even though they occur in several habitats over a wide geographic range. Greenhouse de Wit replacement series with seven species of sparse and common perennial grasses of tallgrass prairie were performed with seedlings and tiller fragments for 5, 10, and 15, mo. As younger and older seedlings, sparse grasses overyielded and were advantaged by the interaction with common grasses. The common grasses underyielded and were disadvantaged in mixture with sparse grasses. As tillers, the interaction was less antagonistic, and both common and sparse grasses either overyielded or were unaffected by the interaction. Seedlings of sparse species were largest when planted in low proportion, surrounded by individuals of a common grass. Because the sparse species are not disadvantaged by interactions with their common neighbors, their competitive abilities are not implicated as a cause of their local rarity. Rather, the good competitive abilities of these sparse grasses are best viewed as a mechanism that offsets the hazards of low density and makes local persistence more likely.
Factors Structuring Fish Assemblages Along a Bog Lake Successional Gradient
Authors: Rahel Frank J.
Published: May 2006

Three alternate and distinct fish assemblages were identified in 43 northern Wisconsin lakes encompassing a gradient from bog ponds to small, oligotrophic lakes having little bog character. Multivariate analysis of physical/chemical variables identified three general environmental factors that influence community type and species richness. These factors are: (1) habitat size and heterogeneity; (2) lake productivity— pH; and (3) winter oxygen concentrations. Ordination of lake fish assemblages along these environmental gradients revealed that two factors, lake pH and winter anoxia, were especially crucial in determining assemblage type. Maintenance of these discrete assemblages is also the result of biotic interactions (exclusion of cyprinids due to predation or competition from centrarchids). A centrarchid assemblage dominated lakes across a wide range of habitat types and pH, providing winter anoxia does not occur. Lakes with low winter oxygen concentrations harbor a cyprinid assemblage if the pH is above 5.2—5.4. Lakes that are acidic and also have low winter oxygen contain only two physiologically hardy species comprising the Umbra—Perca assemblage: central mudminnow (Umbra limi) and yellow perch (Perca flavescens). Five fishless lakes had physical and chemical characteristics similar to lakes with an Umbra—Perca assemblage. Within the fish assemblages, no subgroups of closely associated species were evident. For Umbra—Perca lakes, few environmental differences were evident between lakes having (1) no fish, (2) central mudminnows only or, (3) yellow perch and central mudminnows. The sequential loss of species along a gradient from oligotrophic seepage lakes to bog ponds reflected the increased harshness of the chemical environment and the loss of near—shore habitat zones. Species that are specialized zooplanktivores, that utilize hard—bottom substrates for spawning for feeding activities, or that are intolerant of low pH were limited to lakes with little bog character. Lakes in intermediate stages of succession contain acid—tolerant species that are food and habitat generalists. Lakes near the final stages of senescence contained only yellow perch and central mudminnows. The location of bog lakes at the lower end of several environmental gradients makes them valuable sites for studying the influence of environmental factors and biotic interactions on the structure of aquatic communities.
Use of a Secondary Host by Non‐Outbreak Populations of the Gypsy Moth
Authors: Rossiter MaryCarol
Published: May 2006

Oak species are the favored host of gypsy moth populations in the northeastern United state, although the herbivore expands its host range dramatically during an outbreak. Pitch pine, a secondary host because of its unacceptability for early development, was found to be frequently used for oviposition in oak—pitch pine forests with non—outbreak populations. This observation led to the study of ecological and behavioral factors that can contribute to the use of a secondary host under low—density conditions by an irruptive herbivore species. A series of manipulative field and laboratory experiments plus a study of natural history provided data on the pattern of pitch pine use during the life cycle of the gypsy moth, the effect of pitch pine on larval growth, and the differential impact of natural enemies depending on host use. I found that (1) egg masses occurred far more frequently on pitch pine than was expected based on the frequency of pitch pine in forests with low—density gypsy moth populations; (2) in the laboratory, early—instar larvae could not survive on pitch pine while late—instar larvae grew well; (3) in the field, larvae began to use pitch pine to feed and rest after the onset of the fourth instar. Compared to oak, (4) egg masses on pitch pine experienced less parasitism; (5) the microhabitat of pitch pine held less nuclear polyhedrosis virus (NPV), a major mortality agent of the gypsy moth; (6) individuals hatching from eggs laid on pitch pine were less infected with NPV; and (7) larvae dosed with a known amount of NPV survived longer when feeding on pitch pine foliage. The use of pitch pine by individuals in low—density gypsy moth populations appeared to be beneficial and may have an important effect on population dynamics. The mobility associated with host switching by late—instar larvae and with dispersal by first—instar larvae oviposited on unacceptable food may represent an important mechanism for host—range extension.
Age‐Specificity of Black‐Capped Chickadee Survival Rates: Analysis of Capture‐Recapture Data
Authors: Loery Gordon , Pollock Kenneth H. , Nichols James D. , Hines James E.
Published: May 2006

The ornithological literature indicates a widespread belief in two generalizations about the age—specificity of avian survival rates: (1) survival rates of young birds for some period following fledging are lower than those of adults, and (2) after reaching adulthood survival rates are constant for birds of all ages. There is a growing body of evidence in support of the first generalization, although little is known about how long the survival difference between young and adults lasts. This latter question can be addressed with capture—recapture or band recovery studies based on birds marked in the winter, but the inability to determine age in many species during winter has prevented the use of standard methods. There is very little evidence supporting the second generalization, and we are in need of methods and actual analyses that address this question. In the present paper we restate the two generalizations as hypotheses and test them using data from a wintering Black—capped Chickadee (Parus atricapillus) population in Connecticut, which has been studied by Loery for 26 yr. We use a cohort—based Jolly—Seber approach, which should be useful in other investigations of this nature. We found strong evidence of lower survival rates in 1st—yr birds than in adults, but could not determine whether this was the result of higher mortality rates, higher emigration rates, or a combination of the two. We also found evidence that survival rates of adult birds were not constant with age but decreased at a rate of ° 3.5%/yr. As adult birds are very faithful to their wintering areas, we believe that almost all this decrease can be attributed to an increase in mortality with age. Simulation results suggest that heterogeneity of capture probabilities could not explain the magnitude of the decrease in survival with age. Age—dependent tag loss is also discussed as an alternative explanation, but is dismissed as very unlikely in this situation. This analysis thus provides some of the first evidence of declining adult survival probabilities with age for any bird species.
Predation Risk and Fasting Capacity: Do Wintering Birds Maintain Optimal Body Mass?
Authors: Rogers Christopher M.
Published: May 2006

This paper tests the optimal body mass (OBM) model developed by Lima (1986) for small birds overwintering in a temperate climate. The OBM model views winter body mass as reflecting a trade—off between predation risk and the risk of starvation during periods of inclement weather. Specifically, stochastic computer simulations of winter seasons showed that costs (e.g., risk of being preyed upon) and benefits (e.g., fasting capacity) of fat associated with an intermediate mass combine to maximize the probability of overwinter survivorship (Lima 1986). The OBM model predicts that when benefits but not costs of extensive fattening are eliminated by predictable resources, the optimal solution will be for bird species to avoid costs by remaining leaner than species utilizing unpredictable resources. In the latter environment, benefits of fattening are important to surviving suddenly imposed fasting spells. Thus if mass and therefore winter survivorship are optimized among environments, size of the fat reserve should vary inversely with resource predictability. As a test of this prediction, this study compares visible subcutaneous fat among three avian foraging guilds that occupy a gradient of winter resource predictability. Strictly tree—foraging species utilizing tree—borne resources were considered to occupy a predictable environment relative to strictly ground—foraging species. In the latter foraging guild, resources may be unexpectedly and temporarily eliminated by moderate to heavy snowfall. Species with a partial dependence on each of these resource environments were assumed to occupy a resource environment of intermediate predictability. In four separate data sets involving small birds from southern Michigan, central Tennessee, northern Michigan, and southern Indiana, visible fat class varied inversely with resource predictability, as predicted. This trend was also observed in a comparison of species assumed alike in physiological factors that may independently influence size of fat reserve (body size, hypothermia, thermal conductance, basal metabolic rate, nocturnal roost microenvironment). In further concordance with theory, a comparison of dawn fat class between the Dark—eyed Junco and Tree Sparrow (from an unpredictable resource environment) and the Downy Woodpecker and Tufted Titmouse (from a predictable resource environment), which factored out all but a slight effect of basal metabolic rate, indicated significantly greater reserves in the two ground—foraging species. I conclude that aside from possible influences of physiological factors, resource predictability likely affects winter body mass in small birds. This influence takes the form of guild—specific trade—offs between predation risk and fasting capacity (see also Stuebe and Ketterson 1982, Nolan and Ketterson 1983). These results support the OBM model (and similar models), and along with previous studies suggest the widespread importance of predation risk as a factor structuring biological systems.
Mechanisms for Coexistence Among Three Species of Dipodomys: Habitat Selection and an Alternative
Authors: Schroder Gene D.
Published: May 2006

High—resolution radio—tracking was used to study the habitat affinities of Dipodomys merriami, D. ordii, and D. spectabilis in an area of mixed shrubs and grasses in central New Mexico. Detailed habitat maps were examined to place the locations of stops in one of seven habitat classes, and preferences were identified by comparing the number and duration of stops with that expected if habitat use were proportional to habitat abundance. Neither Dipodomys merriami nor Dipodomys ordii used habitats in the available proportions. Both stopped more frequently at burrow locations beneath Larrea, the dominant shrub, and also used sand dunes more than expected. Dipodomys merriami avoided grassy areas entirely. These two species also used significantly different mixes of habitats. Analyses limited to stops either in the open or beneath shrubs showed that D. ordii preferred the shrubs and that D. merriami had no relative preference for either habitat. Limited observations of D. spectabilis showed that they avoided shrubby habitats. These results support the hypothesis that habitat selection facilitates coexistence between Ord's and Merriam's Kangaroo rats. However, the adequacy of this mechanism to account fully for the coexistence of these species is questioned. An alternative, nonequilibrium hypothesis is proposed, which is based on the assumptions that neither species has a clear competitive advantage in the mixture of habitats where they coexist, and that the course of the interaction is frequently disturbed by migration from nearby, allopatric populations.
Vertical Stratification of Activity of Two Peromyscus Species: An Experimental Analysis
Authors: Harney Barbara A. , Dueser Raymond D.
Published: May 2006

The importance of competitive interactions in maintaining the vertical microhabitat segregation of two ecologically similar Peromyscus species was evaluated by reciprocal species—removal experiments during a year of peak density. The white—footed mouse, Peromyscus leucopus noveboracensis, and cloudland deer mouse, Peromyscus maniculatus nubiterrae, are syntopic in southwest Virginia and are commonly trapped aboveground in mixed deciduous forest. Patterns of arboreal habitat use and levels of arboreal activity were determined for both species before the removal experiments. During the summer and fall of 1981, six live—trapping grids, equipped with both ground and aboveground traps, were randomly allocated as control, leucopus removal, or maniculatus removal grids. On all grids, maniculatus used arboreal habitat more extensively than leucopus. Overall, 48% of 1357 maniculatus captures, but only 24% of 2270 leucopus captures, were in aboveground traps. Before removals, both species occupied structurally similar arboreal habitat, and following removals were detected no shifts in arboreal habitat use that would indicate competitive release by the unmanipulated species. Removals also had no effect on the degree of individual arboreal activity shown by the remaining species. Both before and after removals, maniculatus consistently showed more arboreal activity than leucopus for all treatments. While maniculatus significantly increased the number of arboreal sites occupied in the absence of leucopus, and encroached substantially on arboreal habitat formerly used by its congener, this expanded arboreal habitat use occurred in the absence of any numerical increase or change in activity levels. We attributed it to greater arboreal orientation of maniculatus and opportunistic use of arboreal habitat in areas of former range overlap with leucopus. The experiments suggest that differential use of arboreal habitat by these species results from differences in their behavioral responses to vertical habitat, and is independent of dynamic competitive interactions.
Effects of Mate Size and Mate Number on Male Reproductive Success in Plants
Authors: Nakamura Robert R. , Stanton Maureen L. , Mazer Susan J.
Published: May 2006

Hermaphroditic plants gain fitness through both female and male reproductive success (RS). We present and equation for estimat6ing male RS in panmicitic populations of self—incompatible plants with perfect flowers. The results of the simulations conducted using this model demonstrate that male RS and plant gender are sensitive to the frequency distribution of mate size and to the number of mates. When the frequency distribution of lower number per plant is changed from normal to L—shaped, the male fitness function decelerates with increasing flower number. Compared to large populations, in small populations small individuals will reproduce relatively more through pollen, while large individuals will be relatively more female.
Choice of Oviposition Site by Hyla Chrysoscelis: Role of Predators and Competitors
Authors: Resetarits William J. , Wilbur Henry M.
Published: May 2006

The role of predators and competitors in the choice of oviposition site by the treefrog Hyla chrysoscelis was examined in a randomized complete block experiment using 90 replicated experimental ponds. Control ponds containing neither predators nor competitors were contrasted with treatment ponds into which one of four species of predators (Ambystoma maculatum larvae, Enneacanthus chaetodon adults, Notophthalmus viridescens adults, Tramea carolina larvae) or one of two species of competitors (Rana catesbeiana, Hyla chrysoscelis) was added. Treatments had significant effects on the mean number of eggs deposited in ponds. Fewer eggs were laid in ponds with Ambystoma, Enneacanthus, or Hyla, as a result of fewer females laying eggs and fewer eggs laid per visit, compared with control ponds. Notophthalmus, Rana, and Tramea had no effect on the number of eggs laid. Ovipositing Hyla discriminated among potential oviposition sites based on the species present. Choice of oviposition site can determine the success of a female's reproductive investment, and it can be a mechanism affecting the structure of ecological communities as well. Our results emphasize the importance of oviposition site choice in the evolution of reproductive patterns and implicate species avoidance by ovipositing females as a mechanism important in generating variability in ecological communities.
Resource Competition During Seasonal Succession Toward Dominance by Cyanobacteria
Authors: Sterner Robert W.
Published: May 2006

An annual replacement of eucaryotic algae by nostocalean cyanobacteria was studied by monitoring the algal and zooplankton communities as well as various nutrient components during three growing seasons in Pleasant Pond, Minnesota. Short—term factorial experiments consisting of nitrogen and phosphorus enrichments and macrograzer removal were performed, and algal population growth was observed to determine whether nitrogen, phosphorus, or both were potentially limiting to different phytoplankton taxa. The algal taxa that were consumed by the dominant grazer, Daphnia pulex, were also determined. Competition for nitrogen together with grazing on the inferior competitors occurred. Several other potential mechanisms–a changing nitrogen—to—phosphorus supply ratio, increasing grazing pressure, and competition for silicon–did not appear to be important. Nitrogen was more limiting than phosphorus. At least some nonheterocystous species were nitrogen limited in every experiment. During the transition to dominance by Nostocales, Nostocales did not usually respond to nitrogen additions, whereas many other species did. This taxonomic difference in nutrient limitation suggests that some of the species unable to produce heterocysts were being outcompeted for nitrogen. Daphnia grazers consumed Anabaena flos—aquae, Fragilaria sp., gelatinous greens, pennate diatoms, and Scenedesmus spp. Algal responses to added nitrogen and to grazers were often interdependent. A behavioral indirect effect via the grazers' functional response could explain this dependence.
Ordination Analysis of Components of Resilience of Quercus Coccifera Garrigue
Authors: Malanson George P. , Trabaud Louis
Published: May 2006

The trajectories or vectors of sites through the statistical space of ordinations were used to quantify components of resilience for data on garrigue in southern France, from six experimental fire treatments and a control, through 13 yr of annual records. Four types of data were ordinated: foliar cover, point contacts, point contacts within horizontal strata, and presence—absence. An additional data set was created by adding the data on foliar cover to the cover data of 156 releves sampled by Braun—Blanquet and his associates earlier in the century, and which we took to represent the surrounding vegetation. Only the data of point contacts in strata consistently showed directional movement that could feasibly be used to quantify components of resilience, indicating the processes by which the vegetation, and especially the dominant Quercus coccifera, recovered. Postfire resprouting was so prevalent that measures of foliar cover or presence—absence could not distinguish well between prefire and postfire conditions, although they obviously were different. The ordinations showed that garrigue has a very high initial elasticity, which slows later; it is a function of growth. High elasticity was maintained in all treatments throughout the experiment and was the best indicator of potential resilience. The amplitude of the community was never exceeded, even by very frequent fires. Vectors of recovery toward the predisturbance condition were always evident; when the garrigue was examined in the context of the surrounding vegetation it never actually moved into the areas in ordination space that defined other communities. While the experiments disturbed the community too often to compare a steady postdisturbance location in ordination space with the predisturbance location, the garrigue was already recovering to the point where malleability was obviously low, and damping was high. The vegetation recovered in such a way that little opportunity was afforded for invasion from other communities. Inertia, seen indirectly as the distance from the predisturbance point to the point representing the sample the following spring, was also quite strong. The factors that did make a difference among the components of resilience are those previously identified as important factors in the recovery of garrigue: season of fire first, frequency second. The season of fire was important, because for autumn burns the plants had little time and poor conditions for regrowth before the census the following spring. This difference is seen in the initial vectors from the origin, and although the subsequent vectors show strong recovery, the initial deficit was never overcome. The fire intensity of controlled burns is shown to have little effect on resilience.
Second‐Order Neighborhood Analysis of Mapped Point Patterns
Authors: Getis Arthur , Franklin Janet
Published: May 2006

A technique based on second—order methods, called second—order neighborhood analysis, is used to quantify clustering at various spatial scales. The theoretical model represents the degree of clustering in a Poisson process from the perspective of each individual point. The method is applied to point location data for a sample of ponderosa pine (Pinus ponderosa) trees, and shows that heterogeneity within the forest is clearly a function of the scale of analysis.
Leafcutting Ant Diet Selection: The Role of Nutrients, Water, and Secondary Chemistry
Authors: Howard Jerome J.
Published: May 2006

The relationship of plant secondary chemistry, water content, and nutrient content to the palatability of leaves to the leafcutting ant Atta cephalotes was determined in a study of individuals from 50 woody plant species in tropical dry forest of Costa Rica. The study took place during the yearly period of maximum leaf harvest, in the early rainy season. The palatability of plants was determined by presenting leaf disks to ants on active foraging trails of three ant colonies. The distribution of several classes of polar secondary compounds in leaf samples was determined using chemical spot tests, and that of ant—deterrent nonpolar compounds using laboratory bioassays of plant nonpolar extracts. Nitrogen, energy, and water availability in leaves were assessed using quantitative analyses. Plants of high relative palatability generally lacked deterrent nonpolar extracts but did contain hydrolyzable tannins; the opposite was true for plants of low palatability. Little or no relationship was found to exist between plant palatability and measures of leaf nitrogen, energy content, and moisture content, or the presence of alkaloids and condensed tannins in leaves. No statistically significant interactions among leaf characteristics were found. The number of leaf disks harvested by ant colonies decreased during the study period. This decrease was unrelated to nutrient and moisture availability in leaves, but may be related to changes in secondary chemistry over the course of the season. Although plants were not equally palatable to all colonies tested, there was no evidence that colony—to—colony differences in plant preferences were related to the plant characteristic studied. Overall, the results suggest that plant secondary chemistry is the best predictor of plant species utilization by leafcutting ants.
Effects of Competition, Disturbance, and Herbivory on Salicornia Europaea
Authors: Ellison Aaron M.
Published: May 2006

The effects of interspecific competition, physical disturbance, and insect herbivory on the distribution and abundance of Salicornia europaea in a New England salt marsh were studied using manipulative field experiments. In New England salt marsh plant communities, S. europaea is restricted to disturbance generated patches. S. europaea was unable to colonize successfully low—marsh habitats that were flooded daily by tides, because seedlings germinating in these areas could not anchor in soft substrate and were easily uprooted. In high—marsh habitats, S. europaea seeds reliably colonized recently created patches because the seed's hairy coat bound to the plant debris that created the patches. S. europaea was successful in naturally occurring and artificially maintained patches. However, it was rapidly outcompeted for light by perennials in undisturbed areas and eventually was overgrown by perennials growing into the patches. When the perennial canopy was removed, S. europaea increased in biomass and fecundity relative to conspecifics that were beneath the perennial canopy. S. europaea, however, was attacked by insect herbivores more frequently in patches than beneath the perennial canopy and this herbivore pressure reduced survivorship in patches to approximately the same level as beneath the perennial canopy. Interactions among physical disturbance, seed dispersal patterns, interspecific competition, and herbivory appear to be major factors controlling patterns of distribution and abundance of Salicornia in New England salt marsh plant communities.
Do Predators Always Capture Substandard Individuals Disproportionately From Prey Populations?
Authors: Temple Stanley A.
Published: May 2006

Predators are commonly thought to capture substandard individuals (those in poorer condition than the average individual) in higher than expected proportions, but evidence for this paradigm is scant, biased, and inconsistent. I describe the outcomes of 447 attacks by a trained Red—tailed Hawk (Buteo jamaicensis) on three species of typical mammalian prey (eastern chipmunks, cottontail rabbits, and gray squirrels). A strong correlation existed between the difficulty the hawk had capturing individuals of each species and the difference between the proportions of substandard individuals in the hawk's diet and the prey populations. The degree to which substandard individuals of a particular prey species are taken disproportionately by a predator seems to be a direct function of how difficult it normally is for the predator to capture and kill individuals of that species. A review of other studies shows a similar relationship between vertebrate predators and their prey.
The Selective Basis for Dispersal of the Prairie Vole, Microtus Ochrogaster
Authors: Johnson Michael L. , Gaines Michael S.
Published: May 2006

Three 0.8—ha fenced enclosures (X, Y, and Z) were used to determine the consequences of dispersal on the fitness of individual prairie voles, Microtus ochrogaster. Dispersers from enclosure X were marked and were either placed in a vacant enclosure (Z) or returned to their original enclosure (X) (frustrated dispersers). Enclosure Y contained a population of residents where all dispersers from that population were allowed to leave. The survival and reproduction of animals in all enclosures were monitored for 10 wk. Nine replicates of the experiment were performed over a 2—yr period. There was a greater proportion of subadults among dispersers than among residents. There were no differences in sex ratio or reproductive condition between dispersers and residents. Dispersers had the highest survival, followed by residents on enclosure X, and finally, frustrated dispersers. The reproductive activity of dispersers and frustrated dispersers was equal, and both were greater than that of residents on enclosure X. When survival and reproductive activity were combined into a general fitness index, dispersers had the highest relative fitness. Residents on enclosure X and frustrated dispersers had lower and approximately equal fitness. A comparison of the survival and reproductive activity of animals on enclosures X (where one—half of the dispersers were returned to the enclosure) and Y (where all dispersers were allowed to leave), indicated that only the survival of adult females on enclosure Y increased as a result of the reduced density on that enclosure. It was concluded that (1) dispersal into optimal habitat resulted in high survival and reproduction, (2) frustrated dispersal resulted in low survival, but once these animals became established, they showed high reproductive activity, and (3) females appeared to benefit most from living in a population when density was reduced by dispersal.
Resource Allocation and Demography of Sexual and Apomictic Antennaria Parlinii
Authors: Michaels H. J. , Bazzaz F. A.
Published: May 2006

Survivorship and allocation to seed and ramets of sexual and apomictic Antennaria parlinii were studied to determine whether these forms differed in colonizing and competitive ability. Apomictic plants produced more seeds per inflorescence, but their mean mass per seed was lower compared to sexual plants. However, because sexual seedling survivorship was higher than that of apomictic seedlings, the estimated numbers of offspring recruited, when male seedlings were included, were not different. Thus the primary difference in colonizing ability between sexuals and apomicts appears to be in the potential for dispersal provided by the greater number of lighter seeds produced by apomictic plants. The pattern of allocation to ramet production, one indication of potential competitive ability, was different among males, females, and apomicts. In general, both mean length of a stolon and total stolon length per plant were lower in apomicts than in males and females. Apomicts also had the highest frequency of ramet mortality, while males had the lowest. This pattern of ramet production in sexuals results in a growth form of a solid mat of long—lived, wandering stolons. Males consistently had the lowest mortality and also the highest total allocation to ramet production. Although all of an apomict's offspring bear the costs of maternity, the sons of a sexual female do not. Therefore, sexuality may be advantageous in competitive environments because of the higher adult survivorship and increased opportunity for future reproduction in those offspring that can reproduce as males.
Intraspecific Resource Partitioning in the Bunble Bees Bombus Ternarius and B. Pennsylvanicus
Authors: Johnson Robert A.
Published: May 2006

I tested for an association between intraspecific size differences and differences in resource utilization in worker bumble bees (Bombus ternarius and B. pennsylvanicus). Bees were observed foraging in two fields in Minnesota where flower species with short corollas and species with long corollas occurred in both single—species and mixed—species stands. In mixed—species stands, foragers on the species with short corollas were found to have shorter proboscides than conspecific foragers on the species with long corollas. However, proboscis lengths of foragers on the species with a long corolla in single—species stands did not differ from those on the species with a short corolla in single—species stands. Thus, where a choice exists, bumble bee foragers select the species having a corolla most compatible to their proboscis length. I then compared conspecific foragers on a single flower species in one—species versus mixed—species stands. For foragers on flowering species with a short corolla, proboscis length was significantly shorter in the mixed—species than in the single—species stand, but this difference was not found for the species with a long corolla. Thus, presence of other flowering species can influenced the pollinator population of a species, relative to a single—species stand. Resource choice by foragers was analyzed using a probability function, calculated as the ratio of foragers with a given proboscis length on the species with a long corolla to the number with that proboscis length collected on both species. The shape of the probability function was fitted to a sigmoidal curve using probit analysis Regression of the linearized probabilities was significant. This procedure is a better measure of intra— or interspecies association than are measures such as niche overlap, because it quantifies a continuous distribution and accounts for variance in the species distribution.
Statistical Analysis of Survival and Removal Rate Experiments
Authors: Pyke David A. , Thompson John N.
Published: May 2006

We consider both parametric and nonparametric statistical analyses of survivorship curves and of removal rates, including assumptions, tests, ecological applications, and the difference between censored and uncensored survival data. For censored data, the Gehan—Wilcoxon test, the longrank test, and the likelihood ratio test and appropriate and are readily available in mainframe computer statistical packages. In cohort analyses, these tests can determine if cohorts of different ages have different age—specific death rates or different death rates over the same time period.
Surface Temperatures of Cacti‐‐Influences of Environmental and Morphological Factors
Authors: Nobel Park S.
Published: May 2006

An energy budget model predicted the average stem surface temperatures of a Mammillaria dioica 9.4 cm tall and a Carnegiea gigantea 5.64 m tall within 1°C of the hourly measured temperatures over 24—h periods. For the ranges of various environmental factors considered in a sensitivity analysis, the average surface temperature of the stem was most influenced by air temperature. Greater diurnal extremes of stem temperature occurred at lower wind velocities. Asymmetrical heating of the stem by direct solar irradiation was clearly evident in the measurements and the simulations. The model was used to study the effect of changes in spine coverage, apical pubescence, stem absorptance, tissue thermal properties, plant height, and diameter on stem surface temperatures. In the absence of spines the average surface temperature of Mammillaria became 1 to 2°C cooler at night and 2 to 6°C warmer during the day. The effect of spines on the simulated diurnal temperature variation was even greater for the apical meristem, which had the most dense coverage of spines. A mat of pubescence additionally protected the apical meristem of Carnegiea and Ferocactus acanthodes, substantially reducing diurnal temperature extremes there. Simulated decreases in the diameter of Carnegiea increased the daily range of temperatures of the apical meristem, suggesting that some adaptive optimization of plant diameter may have occurred. Simulations for the 3 species under the same environmental conditions showed that the lowest stem and apical temperatures occurred for Carnegiea, consistent with its restriction to lower (presumably warmer) elevations than Ferocactus and Mammillaria.
Seasonal Water Balance in Orthoporus Ornatus, A Desert Millipede
Authors: Crawford Clifford S.
Published: May 2006

Seasonal H2O balance was studied in Orthoporus ornatus (Girard) (Spirostreptidae) inhabiting a volcanic escarpment in central New Mexico. Dormant, subterranean millipedes were collected periodically between November and June; feeding, surface—active millipedes mainly between July and October. For each specimen weights and total body H2O, cuticle—tissue H2O, gut—tissue and content H2O, and remaining H2O (assumed to be mostly hemolymph) were recorded. Fluctuation of these parameters for each collection was determined by regressing individual values against corresponding midsegment widths. Total body H2O was relatively high in spring and summer; lower in fall and winter. Cuticle—tissue H2O remained low from June (time of annual subterranean molt) to early winter, then increased to its maximal value during spring. Gut H2O was highest during summer feeding, then declined until after molt. Remaining H2O was highest in recent postmolt millipedes surfacing early or imbibing from a moist substrate, and lowest in early winter. Seasonal levels of cuticle—tissue H2O and gut H2O were similar between sexes at any midsegment width. Total body H2O loss was increased by molting and appeared relatively independent of O2 consumption. Less than half of total body H2O loss during the feeding season appeared due to excretion; other avenues were cuticular transpiration and secretions from repugnatorial glands. Total body H2O gain was little affected by metabolic—H2O production, but gain due to H2O ingestion during feeding and to inferred transcuticular uptake during dormancy was considerable. Such uptake should have been active because diffusion gradients of H2O activity (aw) between hemolymph and soil were generally unfavorable to dormant animals gaining cuticle—tissue H2O.
An Open Chamber Technique for the Measurement of Carbon Dioxide Evolution from Soils
Authors: Schwartzkopf Steven H.
Published: May 2006

This paper describes a new open chamber technique for measuring CO2 evolution from soil in situ, which minimizes measurement errors due to soil disturbance and chamber effects. An analysis of the data collected by this new technique, and a reanalysis of data already published, shows that there is clearly a relationship between the metabolic activity of a soil and the velocity of wind passing over that soil. The mechanism and implications of this relationship are discussed.