bluuebunny/crossref_metadata_2025 huggingface dataset:
Demography and the Genetically Effective sizes of Two Populations of Darwin's Finches
Authors: Grant Peter R. , Grant B. Rosemary
Published: May 2006

The purpose of this study was to identify and quantify the demographic and genetic factors that favor long—term persistence of resident bird populations on small islands. Two species of Darwin's Ground Finches, Geospiza scandens (Cactus Finch) and G. fortis (Medium Ground Finch), were studied on Isla Daphne Major, Galapagos, from 1975 to 1991. Four cohorts born in the years 1975—1978 were followed to the point where almost every individual had died. Life tables were constructed from survival and reproductive data, and used to calculate genetically effective population sizes. Annual rainfall was highly variable and erratic. Extremes were 0 and 1359 mm. As a consequence the finch populations fluctuated in all demographic parameters. In years of little or no rainfall breeding did not occur, in years of abundant rainfall as many as eight breeding attempts were made by individual pairs. Maximum ages were 15 yr for G. scandens and 14 yr for G. fortis. Males of both species tended to live longer than females and to breed later; most females bred for the first time at ages 1—3 yr, whereas most males bred for the first time at ages 2—6 yr. An unusual feature of the survival and reproductive schedules is an increase in reproductive value sometimes occurring moderately late in life more than once, associated with occasional extremely favorable conditions for reproduction. Harmonic mean breeding population Sizes (N) were 94 G. scandens and 197 G. fortis. Effective population sizes (Ne) were much lower, principally as a result of a large variance in the production of recruits per parent, especially by G. fortis. Average effective sizes were 38 G. scandens and 60 G. fortis by one method of calculation, and slightly larger by another. The proportional rate of loss of selectivity neutral heterozygosity or additive genetic variance in quantitative traits potentially caused by random genetic drift (1/2Ne) in these populations is °0.003—0.005, or 0.3—0.5%/yr, and between 0.8 and 1.4% per generation. We suggest that a general estimate for the effective size of terrestrial bird populations is about one—quarter of average breeding numbers. These results are discussed in relation to the long—term viability of the populations and the maintenance of genetic variation. The demographic features that enable the finch species to persist in the face of extreme environmental stochasticity are a high maximum life—span, a generally high adult survival under the stressful conditions of drought, a flexible period of maturity, and a high reproductive rate. Despite their relatively small effective population sizes and the likelihood of genetic impoverishment through random drift they remain genetically variable through gene flow, principally hybridization. This study highlights fluctuating (unstable) age structures as a methodological constraint on some of the theoretical calculations. An expanded demographic and population—genetic theory is needed to overcome this constraint.
Phytoplankton Dynamics and the Role of Grazers in Castle Lake, California
Authors: Elser James J.
Published: May 2006

During summer 1987, a series of experiments was performed in conjunction with monitoring of in situ plankton dynamics in nutrient—deficient Castle Lake, California. These factorial manipulations of nutrients and zooplankton were designed to separate the direct (grazing mortality) and indirect (stimulation of algal growth by nutrient recycling) effects of grazers on both species—specific phytoplankton dynamics, as well as aggregate properties such as total biomass, productivity, and biomass turnover. Grazing loss rates differed significantly among algal species with the mean grazing loss rate suffered by an algal species a negative function of cell or colony size. Stimulation of division rates in the presence of zooplankton (compared to an unenriched treatment lacking zooplankton) was commonly observed. Nutrient and grazer treatments also had significant impacts on algal biomass (indicated by chlorophyll concentration), productivity (PPR), and PPR/chlorophyll, and significant nutrient—grazer interactions were frequently observed. Species—specific algal responses to grazers were aggregated across the phytoplankton by weighting species—specific parameters by the contribution of each species to total community biomass in order to provide community—wide estimates of the importance of direct and indirect effects on algal dynamics. Finally, linkages between population—level responses and aggregate properties were demonstrated by examining relationships between total phytoplankton biovolume concentration and chlorophyll concentration in various treatments, by comparing community—wide grazing rates estimated from aggregated species—specific grazing rates and from changes in chlorophyll growth rates, and by relating the aggregated degree of species—specific growth stimulation to stimulation of biomass turnover. These results indicate that phytoplankton responses to grazers in Castle Lake reflect a mix of direct and indirect effects that varies considerably both between species and through time during the stratified season.
Piscivory and Recruitment: Mechanisms Structuring Prey Populations in Small Lakes
Authors: Tonn William M. , Paszkowski Cynthia A. , Holopainen Ismo J.
Published: May 2006

Detecting mechanisms that structure ecological communities often requires investigations at appropriate spatial and temporal scales. A 15—mo, whole—lake experiment revealed direct and indirect mechanisms by which a fish predator structure its prey population, primarily by affecting recruitment of young—of—the —year (YOY) fish. Piscivorous Eurasian perch (Perch fluviatilis) were added to two of four sections of a divided lake in eastern Finland previously stocked with crucian carp (Carassius carassius). Although the stocked crucians quickly grew beyond vulnerable sizes, recruitment of their offspring through their 1st yr of life was reduced by 90% in sections with perch. Surviving YOY in predator sections were completely confined to vegetated inshore areas. This restriction to refuges lowered their growth rates in comparison with YOY in predator—free sections, which can further reduce recruitment by prolonging susceptibility to size—limited predation and decreasing energy reserves needed for anaerobic overwintering. In the presence of piscivores, surviving prey subsequently benefit from reduced intraspecific competition, increasing growth rates to achieve large, invulnerable body sizes rapidly. Resulting populations of crucian carp, consisting of relatively few but relatively large individuals, contrast with populations living in the absence of piscivores, which are characterized by high densities of small, stunted individuals. Such divergent population patterns are displayed by a number of prey species in Scandinavia and North America, suggesting that a combination of direct and indirect effects of predation on younger age classes, such as documented for crucian carp, may be common in fish populations of small northern lakes. Detecting these patterns and identifying their underlying mechanisms may often require a combination of extensive and intensive studies.
Differential Larval Predation by Ants Can Influence Willow Sawfly Community Structure
Authors: Woodman Robert L. , Price Peter W.
Published: May 2006

We experimentally tested the ability of generalist predatory ants to structure sawfly gall communities through differential predation on sawfly—galler larvae in relation to their life history patterns. Galler larvae of three species exit their galls in late summer and are exposed to predatory ants, while larvae of one species (stem—gallers) remain within their galls to overwinter. Many ant species (Formicidae) are patchily abundant in willows in northern Arizona. The abundances of four galler species and the activity levels of ant assemblages were assessed in census studies performed on variable numbers of the willow clones between 1986 and 1989. Two distinct types of sawfly gall communities were found on willow clones in the study area, communities composed of four galler species (stem, leaf, and petiole galls, and leaf folds), and communities composed primarily of stem galls. Clones with low ant activity levels support the former, whereas those with high activity levels support the latter. Experimental introductions of larvae of two existing species (leaf— and petiole—gallers) into and beneath natural willow clones with high and low ant activity levels showed that larvae are quickly captured by ants in and under high activity willows. Ants were not selective to larval species in these introductions, and were clearly shown to be generalist predators when experimentally offered two species of larvae. Experimental introductions of leaf— and petiole—galler larvae into four clones with high ant activity in 1987 and 1988 did not result in higher gall populations in the following year, indicating that larvae did not survive to recruit into those clones as adults. Experimental caging of leaf— and petiole—galler adults onto branches of high activity clones resulted in the establishment of modest populations of both species, followed by seemingly normal gall development patterns and larval survival rates inside galls. We conclude that the absence of leaf and petiole galls from willows with high ant activity is due to intensive larval predation by generalist ants, and not to plant resistance factors. This means that generalists and opportunistic predatory ants can significantly structure insect gall communities through differential predation that is dependent upon specific life history traits of some gallers.
Foraging Energetics of Antartic Fur Seals in Relation to Changes in Prey Availability
Authors: Costa Daniel P. , Croxall John P. , Duck Callan D.
Published: May 2006

This research examines the energy budget of breeding female Antarctic fur seals, both when food was plentiful and when it is scarce. The energy expenditure and change in body mass of lactating female Antartic fur seals, Arctocephalus gazella, foraging at sea was measured in two years using doubly labeled water at South Georgia Island. There was no difference between years in mass gain, water influx, mass—specific field metabolic rate (FMR), or absolute FMR. Mean at—sea FMR over both years was 9.52 ± 0.55 W/kg (n = 22), a value that is 6.7 times the predicted basal rate but only 1.9 times the FMR measured onshored. Comparable results have been reported for similar—sized northern fur seals. Krill, the nearly exclusive prey of breeding females, were scarce in 1984 at South Georgia. Fur seal foraging trips were twice as long in 1984 as in 1985 and total mass—specific energy expended by females during these trips was significantly greater. In addition, females were significantly lighter at parturition in 1984, and both pup mortality and the proportion of pups that died from starvation were double the 1985 values. Female condition at parturition and average foraging—trip duration (i.e., offspring—provisioning rate) appear to reflect prey availability. The similarity between years in mass increase suggests that females may have a limited ability to increase the relative time spent foraging because even in normal years only 5% of their time at sea is spent resting. This contrasts with northern fur seals, Callorhinus ursinus, which typically spend 17% of their time at sea inactive. Apparently these northern seals can increase their foraging effort by increasing the proportion of time spent foraging. This would account for the observed between—year difference in at—sea FMR of C. ursinus while foraging—trip duration remained fairly constant.
Kinship and Origin Effects on Dispersal and Space Sharing in Clethrionomys Rufocanus
Authors: Ims Rolf Anker
Published: May 2006

An experimental population of Clethrionomys rufocanus with known kinship and life history was established by introducing adult females together with their newly weaned litters onto an island. Dispersal and space use by individual voles were studied by radiotelemetry and intensive live—trapping. Dispersal occurred in relation to the process of sexual maturation. Males matured and dispersed later than females. Dispersal was highly male biased. No mature males remained at their natal site, whereas more than a third of the females were philopatric and reproduced in their natal range. Litter sex ratio accounted for 79% of the total variation in dispersal tendency among young females. Females originating from litters with a male—biased sex ratio had the greatest tendency to disperse. Dispersal in males was not related to litter sex ratio or to three other origin variables that were included in the analysis. Space sharing occurred mainly among philopatric females. Females that dispersed generally avoided contact with other females. Philopatry, coupled with general sociability rather than selective sociability among kin, was probably responsible for the occurrence of extensive space sharing between certain females. Since males dispersed in the absence of obvious external cues, dispersal is most likely triggered by some inflexible physiological mechanism in this sex. Differential exposure to andrigens during prenatal life could possibly explain the great variation in dispersal and space—sharing tendencies among females.
Recent Fire History of the Northern Quebec Biomes
Authors: Payette Serge , Morneau Claude , Sirois Luc , Desponts Mireille
Published: May 2006

The recent fire history of northern Quebec biomes (54 000 km2), including the northern Boreal Forest, the southern and northern Forest—Tundra, and the Shrub Tundra, was documented by examining size and dates of 20th century wildfires using tree ring techniques. Results showed that pronounced south—north differences in fire properties existed, corresponding to climate and vegetation gradients. Fire frequency per biome decreased south—north from closed forest (0.7 fire/yr) to Shrub Tundra (0.4 fire/yr). Average fire size decreased south—north by 100—fold from °8000 ha in the northern Boreal Forest to 80 ha in the Shrub Tundra, while modal fire size was <50 ha in each of the four biomes. Most fires (>80%) of the northern Forest—Tundra and the Shrub Tundra were <100 ha, and fires >100 000 ha occurred only in the northern Boreal Forest and the southern Forest—Tundra. Less than 35% of all mapped fires in the Boreal Forest were <50 ha, but >30% were >1000 ha. From south to north, the fire—free interval per biome was, respectively, °2.6, 0.6, 0.6, and 2.2 yr, the Boreal Forest data being overestimated. The largest burned areas were recorded in the 1950s throughout the biomes, most likely associated with longlasting drier and warmer conditions. The fire rotation period per biome, based on the percentage of burned areas during the 1920—1984 period (or 1930—1984 in Tundra), increased south—north by 100—fold from 100 yr in the northern Boreal Forest to 9320 yr in the Shrub Tundra. The fire rotation period around the tree line, i.e., 20 km south and north of the present tree line, was estimated to be >7800 yr. Biome boundaries have developed and are maintained in response to fire by the ability of spruce to seed and regenerate. Stability of northernmost conifer sites is maintained by (1) the inability of patchy shrub and conifer cover in the northern Forest—Tundra and Shrub Tundra to carry fire and (2) failure of trees to produce viable seeds in these two biomes. Present data suggest that the area is characterized by a much higher fire frequency than expected from the fire weather index and from calculated frequencies typical of vegetation—type studies. It is concluded that size of the study area is a key element in the determination of regional fire regimes. Finally, the ecological significance of the natural fire rotation and postfire regeneration in northern environments is discussed in a paleoecological perspective.
The Role of Lupine in Succession on Mount St. Helens: Facilitation or Inhibition?
Authors: Morris William F. , Wood David M.
Published: May 2006

The barren landscape created on the north side of Mount St. Helens (Washington State, USA) by the 1980 eruption provides an excellent setting in which to examine the role of pioneer species in facilitating or inhibiting subsequent invaders in primary succession. We investigated the influence of Lupinus lepidus, a nitrogen—fixing pioneer species, on two invading species, Anaphalis margaritacea and Epilobium angustifolium. Seedlings of both invaders were initiated in the greenhouse and transplanted to four field treatments: (1) control (plots devoid of lupine), (2) live (plots with vigorous lupine), (3) mulch (lupines herbicided but left in place), and (4) no—mulch (lupines herbicided and dead aboveground biomass removed). Patches of L. lepidus exerted both facilitative and inhibitory effects on the other species. First season survivorship of seedlings planted into lupine patches was generally lower than that of seedlings planted into barren control plots. However, for both A. margaritacea and E. angustifolium, surviving seedlings within lupine patches grew larger than did controls. In addition, A. margaritacea seedlings had a much higher probability of flowering when planted within lupine patches. Comparisons among treatments indicated that both substrate alteration and the mulching effect of lupine litter mediated the effects of lupin patches on transplant performance. Our results show that both facilitation and inhibition occurred, but at different stages in the life cycle of invading species. Consequently, a complete demographic model may be needed in order to assess the net effect of a pioneer on its successors.
Determination of Species Composition in the Enallagma Damselfly Assemblages of Permanent Lakes
Authors: McPeek Mark A.
Published: May 2006

In this study I investigated the important ecological interactions that contribute to maintaining a striking species distributional pattern for the Enallagma damselflies (Odonata: Coenagrionidae) among lakes that do and do not support fish populations. One group of Enallagma species is found as larvae only in lakes containing fish, while the remaining Enallagma species are found as larvae only in fishless lakes. I performed observational and experimental studies to evaluate the importance of water chemistry, competition among the Enallagma species, and predators of the Enallagma species in maintaining the two discrete assemblages. The results of observational studies suggested that water chemistry could not account for the pattern of species distributions, but that differences in predator compositions and competition among the Enallagma species may. Quantitative sampling indicated that dragonfly larvae were the most abundant invertebrate predator group in both lake types, with each lake type supporting a characteristic set of dragonfly species, as in the damselflies. Fish were also potentially major predators in lakes containing fish. Also, Enallagma densities in both lake types were among the highest reported in the literature, suggesting that density—dependent competitive interactions may potentially contribute to maintaining the Enallagma species distributions. The results of laboratory and field experiments demonstrated that predation by large dragonflies in fishless lakes and predation by fish in fish—containing lakes are two major environmental factors maintaining the Enallagma species distributions. When simultaneously offered two Enallagma species in the laboratory, one from the fishless lake group and one from the fish—containing lake group, fish consumed more individuals of species from the fishless lake group, but all dragonfly species, regardless of lake type affiliation, consumed more individuals of species from the fish—containing lake group. The results of field experiments in which I transplanted Enallagma species between the lake types showed that large dragonflies in fishless lakes and fish impose much greater mortality on Enallagma species with which they do not coexist than on those with which they do. Other field experimental results indicated that density—dependent competitive interactions among the Enallagma species were only apparent in the fishless lakes, affected species from both groups similarly, and altered their growth but not their survival. The results of this study are discussed in the context of overall community organization and the consequences to the evolution of species.
Influence of Oxygen and Sulfide Concentration on Nitrogen Uptake Kinetics in Spartina Alterniflora
Authors: Bradley Paul M. , Morris James T.
Published: May 2006

The effects of sulfide concentration and hypoxia on NH4@?—uptake kinetics in Spartina alterniflora were examined in a laboratory culture experiment. Both factors significantly influenced the Michaelis—Menten parameters, Vmax and Km, which characterize the nitrogen uptake rate as a function of nitrogen concentration. Under oxygen—saturated conditions, the mean NH4@? Vmax per unit root dry mass (DM) was 12.91 @mmol°g—1°h—1 and the Km was 1.05 @mmol/L NH4@?+. Under hypoxia, Vmax decreased to 8.14 @mmol°g—1°h—1, while Km increased to 2.54 @mmol/L NH4@?+. Dissolved sulfide concentrations as low as 0.25 mmol/L inhibited the NH4@?+—uptake kinetics of S. alterniflora significantly, and to a greater extent than hypoxia alone. Exposure to 1.0 mmol/L sulfide resulted in a Vmax per unit root DM of 2.28 @mmol°g—1°h—1 and a Km of 8.79 mmol/L NH4@?+. No significant NH4@?+ uptake was found with exposure to 2.0 mmol/L sulfide. The results are consistent with a conceptual model of changes in productivity of S. alterniflora in the salt marsh as a function of environmental modification of NH4@?+—uptake kinetics.
Role of Ecosystem Development and Climate Change in Bog Formation in Central Sweden
Authors: Foster D. R. , Wright H. E.
Published: May 2006

Bog development, in terms of the rates of horizontal and vertical accumulation of peat and the timing of landform development of open—water pools, was examined on two concentric raised bogs in central Sweden. The results are compared with three models (allogenic, autogenic, and neutral models) of bog development in order to evaluate the relative contribution of environmental factors vs. ecosystem control of developmental processes. Both mires began to grow on discrete upland sites °6000 BP and then spread concentrically and accumulated peat vertically at approximately continuous rates to the present. Radiocarbon dates from pool sediments are progressively younger from the center to the margin of the mire, suggesting that pool formation is triggered autogenically by changes in hydrology. The results conform to hydrological models of mire formation based on groundwater mound equations and suggest that autogenic processes exert a major control over bog expansion, landform development, and the formation of conspicuous features in the stratigraphic record.
Susceptibility to Predation for Different Grasshoppers: An Experimental Study
Authors: Belovsky Gary E. , Slade Jennifer B. , Stockhoff Brian A.
Published: May 2006

A study was conducted in western Montana to determine the impact of different predators on grasshopper populations. By placing grasshoppers of different species, sexes, ages, and body masses on monofilament tethers in the field, rates of predation could be estimated. These rates are consistent with other estimates obtained from population studies in the same environment, but provide a far more detailed assessment of which grasshoppers are most vulnerable to predators and which predators are most important. Immature individuals are more vulnerable at small body sizes and adults are more vulnerable at large body sizes; this is because the principal predators on immature individuals are arthropods, and on adults, vertebrates. Male grasshoppers were 2—3 times as vulnerable as females. The subfamily Oedopodinae was more vulnerable than the Gomphocerinae or Melanoplinae. Predation rates indicate that predators are only able to impact seriously (reduce life expectancy below the suitable abiotic period for existence) the largest Oedopodinae, making predation of little consequence for most grasshopper populations in this environment.
Trade‐Off between Interspecific Competitive Ability and Growth Rate in Bacteria
Authors: Kurihara Yasushi , Shikano Shuichi , Toda Masahiko
Published: May 2006

In an attempt to clarify the significance of interspecific competition in evolutionary ecology, the growth characteristics of bacterial populations were investigated. Bacteria from aquatic microcosms were subjected to serial transfer at 12—h and 3—d intervals in both single— and two—species culture. Selection in single—species populations under serial transfer at 3—d intervals favored an increased growth rate.In mixed culture, serial transfer at 3—d intervals led to coexistence of the two species, and resulted in the r values of both species undergoing slight reductions or remaining unchanged in comparison to the parental strain; serial transfer at 12—h intervals led to extinction of the species with lower r. These results can be viewed as evidence for a trade—off between interspecific competitive ability and growth rate. This interpretation was examined by computer simulation.
Food Thresholds in Daphnia Species in the Absence and Presence of Blue‐Green Filaments
Authors: Gliwicz Z. Maciej , Lampert Winfried
Published: May 2006

Three Daphnia species of different body sizes were grown on a single food resource in steady—state conditions. They differed in food threshold concentrations necessary for body growth and reproduction. The largest species, D. pulicaria, had the lowest thresholds and the smallest, D. cucullata, had the highest thresholds, while D. hyalina was intermediate. This implies competitive superiority of the largest species and inferiority of the smallest one. The sequence, however, was reversed in the presence of blue—green algal filaments, when the thresholds became lowest for D. cucullata and highest for D. pulicaria. Thus the inhibitory effect of filaments is body—size dependent. High densities of filaments that mechanically interfere in food collection may cause shifts in competitive ability among congeneric cladocerans. This may be the reason why, in Europe, D. pulicaria is most successful in ultraoligotrophic lakes while D. cucullata is most successful in eutrophic lakes.
The Behavior of Temperate Cave Bats in a Subtropical Environment
Authors: McNab Brian K.
Published: May 2006

This study seeks to determine how temperate bats react to subtropical climates in which flying insects are available throughout the year and in which cave microclimates in winter are appreciable warmer than those typical of temperature climates. The cave temperatures selected by bats in a cold climate are negatively correlated with their body weight; clusters of bats select even colder temperatures. The temperature differential that a bat (or a cluster) maintains with the environment is also a function of body weight (and thus cluster size), rate of metabolism and Q10. Only bats that are both small and nonclustering are capable of hibernating at environmental temperatures above 14°C. Therefore, the small Pipistrellus subflavis is the only cave bat that hibernates for an extended period in Florida. Myotis grisescens, a large species, is limited to caves in northernmost Florida, and much of that population migrates northward to winter in Alabama, where the cave temperatures are low enough for large clustering bats to remain in torpor for extended periods. Myotis austroriparius, an intermediate—sized species, remains active for the entire year in Florida, although it probably spends more time in torpor in the northern than in the central part of the state. The seasonal pattern of fat deposition in bats is correlated with their hibernal behavior: large fat deposits are found both in M. grisescens and P. subflavus, whereas those in M. austroriparius are larger in northern than in central Florida. Some temperate bats, such as M. grisescens and P. subflavus, do not seem able to eliminate hibernation to take advantage of the nearly continuous food supply in warmer climates. Rather, these species appear to be obligatory hibernators. In these species most of the copulation occurs in fall and winter, which in turn requires that the females remain torpid if pregnancy is to be delayed to spring. In warm climates this behavior is impossible for large or clustering species that require extended periods of torpor. Such species are thus excluded from a subtropical environment.
The Distribution and Dispersal of Pikas: Consequences of Insular Population Structure
Authors: Smith Andrew T.
Published: May 2006

The population structure of a small boreal mammal, the pika (Ochotona princeps), was analyzed from demographic data and the observed spacing of populations on discrete islands of habitat in the Sierra Nevada of California. These islands of habitat varied in size and distance from one another. Not all of the islands were occupied and most seemed to contain fewer individuals than they could hold. The populations apparently represent dynamic equilibria between extinction, which was directly related to island size, and recolonization, which was inversely related to interisland distance. Distances greater than 300 m appeared to pose difficult barriers to dispersing juveniles. The study site is near the distributional boundary of pikas in this area. The birth rate is higher, adult death rate lower, and juvenile death rate higher than for other pika populations found near the center of the altitudinal or geographic range of the species. It appears that the probability of successful colonization decreases with progressively lower elevations, until ultimately colonization is unable to offset extinction and the limit of the species range is determined.
Population Dynamics of Yellowstone Grizzly Bears
Authors: Knight Richard R. , Eberhardt L. L.
Published: May 2006

Data on the population of grizzly bears in the environs of Yellowstone National Park suggest that the population has not recovered from the reductions following closure of garbage dumps in 1970 and 1971, and may continue to decline. A computer simulation model indicates that the risk of extirpation over the next 30 yr is small, if the present population parameters continue to prevail. A review and further analysis of the available data brings out the importance of enhancing adult female survival if the population is to recover, and assesses various research needs. In particular, a reliable index of population trend is needed to augment available data on the population.
Some Consequences of Leaf Pubescence in the Andean Giant Rosette Plant Espeletia Timotensis
Authors: Meinzer Frederick , Goldstein Guillermo
Published: May 2006

The effects of leaf pubescence on leaf energy exchange and transpiration were evaluated for Espeletia timotensis, a caulescent giant rosette plant that occurs in the paramo zone of the Venezuelan Andes at elevations up to 4500 m. Computer simulations using standard energy balance equations and field and laboratory measurements showed that the thick leaf pubescence (up to 3 mm) modifies energy balance mainly by increasing boundary layer thickness and resistance to convective and latent heat transfer. Effects of the pubescence on solar radiation absorption are minor compared to its influence on boundary layer resistance. The degree of coupling between leaf temperature and incident solar radiation is thus greater in pubescent than in nonpubescent leaves, despite the greater absorptance to solar radiation of the later. Vertically oriented expanding leaves, which have the thickest pubescence, also have higher stomatal resistance than the other leaves in the rosette. The higher stomatal resistance enhances coupling between leaf temperature and incident radiation. Pubescence—induced increases in leaf temperature of 5°C or more can be attained without significant increases in transpiration if stomatal resistance is maintained at the proper levels. These features may be advantageous in the native habitat of E. timotensis where incoming radiation can reach a high level but mean air temperature is °3°C and maximum air temperatures are usually below 10°C. Process of assimilate translocation and leaf growth, which normally would be severely inhibited at prevailing environmental temperatures, may be favored by the higher leaf temperatures. The ability to increase leaf temperature while at the same time avoiding increases in transpiration is also of adaptive importance, since water availability in the habitat of E. timotensis is often limited by low temperatures.
Pollinator Sharing and Seed Set of Stellaria pubera: Competition for Pollination
Authors: Campbell Diane R.
Published: May 2006

I used field experiments to examine the effect of pollinator sharing on seed set in the understory herb Stellaria pubera. In piedmont North Carolina the solitary bee and fly pollinators of this species are also frequent visitors to Claytonia virginica. Removals of nearby flowers of C. virginica increased seed set of S. pubera. Examination of seed set in populations of potted plants indicated that the effect was due to change in species composition rather than plant density. Variation in seed production was high, however, and even when flowers were provided with excess pollen through supplemental hand pollinations, only 20% of ovules produced seeds. Competition for pollination appears to be one of several selective forces that act on blooming time of S. pubera. Stellaria pubera blooms slightly later than C. virginica. Whereas the seeds set of early flowers was increased 13% by removal of C. virginica and 19% by hand pollination, seed set of late flowers was unaffected. Plants forced to bloom early also tended to be more pollinator—limited than control plants. Removal of C. virginica did not, however, increase the seed set of S. pubera plants forced to bloom early significantly more than that of control plants. Other forces are probably also involved in the maintenance of blooming time.
The Mechanism of Competition for Pollination between Two Forest Herbs
Authors: Campbell Diane R. , Motten Alexander F.
Published: May 2006

The primary mechanism of competition for pollination between the two forest herbs Stellaria pubera and Claytonia virginica in piedmont North Carolina is interspecific pollen movement. The most common visitor, the bee fly Bombylius major, forages indiscriminately among flowers of the two species. In only one of five experiments did the presence of C. virginica reduce the pollinator visit rate per S. pubera flower, and in other experiments addition of C. virginica enhanced visit rate. Thus these plant species exhibit little or no competition through pollinator preference. In some natural populations, however, visits to S. pubera are frequently immediately preceded by a visit to C. virginica, and a flower receives less conspecific pollen and produces fewer seeds following such an interspecific visit than if the visitor has arrived directly from a conspecific flower. Interspecific pollen movement is responsible for most, if not all, of the reductions in seed set of S. pubera due to pollinator sharing. Although insects deposit a substantial amount of S. pubera pollen on stigmas of C. virginica, little C. virginica pollen is found on S. pubera stigmas. Moreover, application of foreign pollen to the stigma does not influence seed production of S. pubera. The effect of interspecific pollen movement is due to loss of conspecific pollen, not stigmatic interference.
Use of an Improved Statistical Method for Group Comparisons to Study Effects of Prairie Fire
Authors: Zimmerman Gregory M. , Goetz Harold , Mielke Paul W.
Published: May 2006

This paper describes an improved method for performing statistical comparisons among experimental groups. This technique, termed multi—response permutation procedures (MRPP), is similar in purpose to the t test and one—way analysis of variance F test. However, in contrast to these, the new method features very relaxed requirements on the data structure, is easily applied to multivariate problems, and makes it possible to relate the analysis visually to the perceived data space. The MRPP test statistic is based on the within—group average of pairwise distance measures between object response values in a euclidian data space. The null distribution of the test statistic is based on the collection of all possible permutations of the objects into groups having specified sizes. For large group sizes, this distribution is approximated by a continuous distribution satisfying three exact moments. The advantages and applications of MRPP are illustrated using both artificial examples and empirical data on total August standing crop in mixed prairie following an October wildfire. The MRPP analyses of the empirical data revealed that there were no differences in standing crop between burned an unburned areas after the first postfire growing seasons, standing crop was significantly greater in the previously burned areas.
Patterns of Community Dynamics in Colorado Engelmann Spruce‐Subalpine Fir Forests
Authors: Aplet Gregory H. , Laven Richard D. , Smith Frederick W.
Published: May 2006

A five—stand chronosequence spanning >500 yr is used to characterize changes in age structure, overstory mortality, recruitment, and understory growth in developing Colorado Engelmann spruce (Picea engelmannii)—subalpine fir (Abies lasiocarpa) forests. Stand development follows a postdisturbance sequence of colonization, spruce exclusion, spruce reinitiation, and second—generation forest. This model of spruce—fir forest development reflects a range of disturbance intensities from large conflagrations to small—scale tree deaths. Catastrophic disturbance initiates stand development, and canopy gap replacements occur at predictable times during stand development as the life—spans of the two species are expressed. Previous, spruce—fir forest have been described as either equilibrium or nonequilibrium systems. In the equilibrium coexistence model, spruce and fir constitute a stable, climax community through balanced life history strategies. Nonequilibrium coexistence relies on periodic catastrophic disturbance to disrupt competitive exclusion of spruce by fir. Neither of these models, taken alone, adequately explains the range of stand behavior observed in Colorado spruce—fir forests. Instead, stand development appears to involve processes from both models. The stands we investigated are undergoing postfire development and, therefore, support the nonequilibrium model, but a successful spruce cohort late in stand development insures the continued presence of both species, as predicted by the equilibrium model.
Energy Constraints on Avian Distributions and Abundances
Authors: Root Terry
Published: May 2006

A long—standing hypothesis within ecology states that external environmental factors, such as extreme temperatures, are the primary forces shaping species' biographic ranges. On a continentwide scale, the correlative evidence presented in this study strongly indicates that the locations of winter distribution and abundance patterns of several avian species are directly linked with their physiological demands. I calculated the metoblic rate at the nortern boundary of the distribution (NBMR) for 14 of 51 passerines known to have range boundaries associated with a particular average minimum January temperature isotherm. Based on physiological measures taken from the literature, a regression analysis reveals that the northern boundary metabolic rate (NBMR) is 2.45 times the basal metabolic rate (BMR). The NBMR to BMR ratio shows little interspecific variation; the mean of this ratio is 2.49 with a standard error of @+0.07. Thus, the scalar multiple of BMR ranges from 2.42 to 2.56. This strong association between NBMR and BMR implies that the winter ranges of these 14 birds are restricted to areas where the energy needed to compensate for a colder environment is not greater than °2.5 times the BMR. Body masses of 36 of the remaining 37 birds, those with their range limits associated with isotherms and without physiological measurements, were used to estimate the various physiological parameters. Even when using such crude estimates of these parameters, the approximation of the metabolic rate of individuals at the northern boundaries of their distribution was 2.64 times their BMR. Furthermore, high—density populations of sever of the 14 species with accurately measured physiological parameters were limited to regions where homeostasis requires energy output of no more than 2.13 times their BMR. The range of this scalar extends from 2.08 to 2.34. These findings strongly suggest that on a broad scale the winter ranges of a large number of passerines are limited by energy expenditures necessary to compensate for colder ambient temperatures.
Spatial Pattern Analysis of Seed Banks: An Improved Method and Optimized Sampling
Authors: Bigwood Douglas W. , Inouye David W.
Published: May 2006

Analysis of spatial pattern of the seed bank of an old field in Maryland showed that the seed bank was composed primarily of annual and short—lived perennial species. Tree and shrub species were poorly represented. Seed abundance decreased with depth in the soil. The seeds of all species generally had clustered distributions in the soil. However, there was no trend in mean cluster size within or among species, or at different soil depths. The lack of definite trends indicates that the spatial distributions of seeds are governed primarily by environmental factors, and to a lesser degree by biological factors. The method of spatial pattern analysis developed for this study represents a significant improvement over traditional contiguous quadrat methods. In this method, individuals' positions are mapped in the field using cartesian coordinates, and the quadrat data are subsequently derived mathematically. The resolution of pattern is greatly improved, and several problems that reduce the effectiveness of other methods are eliminated. This method can be used to advantage in any study in which spatial patterns analysis is performed, particularly if the objects can be mapped easily. A comparison of three sampling techniques revealed that the precision of seed—number estimates can be improved by taking a large number of small samples, or by subsampling large whole units with very small subunits. The smaller number of large samples favored by many seed—bank researchers generally results in imprecise seed estimates. This result has implications for spatial studies of any clustered organism.
The Indeterminacy of Ecological Interactions as Perceived Through Perturbation Experiments
Authors: Yodzis Peter
Published: May 2006

Press perturbation experiments, in which individuals of a particular species are continually added to or removed from an ecosystem and the responses of other species' densities are observed, are discussed in the context of whole systems. When the sizes of direct interactions are determined to within an order of magnitude, the long—term outcomes of press pertubations are highly indeterminate, in terms both of whether species densities increase or decrease and of which interactions have the largest effects. It is emphasized that short—term observations of systems subject to press perturbations (e.g., toxin additions to ecosystems, or experimental manipulations of species using exclosures) are not useful for estimating long—term impacts. The difference between long— and short—term responses is detailed, and a rule of thumb is given for what constitutes "long—term" in this context.
Late‐Holocene Development of Subarctic Ombrotrophic Peatlands: Allogenic and Autogenic Succession
Authors: Payette Serge
Published: May 2006

Ombrotrophic peatlands that developed on islands in Clearwater Lake, a large subarctic lake in northern Quebec, provide evidence of long—term community stability and successional change. Repeated alternations of regeneration cycles of Sphagnum cushions and black spruce (Picea mariana [Mill.]BSP.) over the last 5050 14C years of peatland development suggest climate, self—building mechanisms, and cyclic—autogenic succession as the dominant active factors operating in the absence of external disturbance such as fire. During the last several thousand years, self—perpetuating vegetation cycles have occurred at variable periodicity and have been controlled primarily by local ecological conditions whose existence is marked by differential peat accumulation rates. Water supply for peat production, in the absence of water table in most sites, was controlled by snow availability, precipitation, and atmospheric humidity from the large lake. Allogenic succession, illustrated by the community shifts from Sphagnum girgensohnii—Picea to S. russowii—Picea, S. fuscum—Picea, and lichen—heath, has been initiated by climate, and more particularly by continuous peat accumulation enhancing, as time passed, the ecological gradient from moist, snow—protected conditions to snowless, exposed conditions. In contrast, absence of such changes at some sites characterized by a uniform, self—regenerated S. fuscum—Picea assemblage suggests that autogenic succession has been an active driving mechanism in community persistence over several thousand years. Long—term trends in peatland development toward topographic equilibrium and late Holocene cooling best explain the gradual phasing—out of the vegetation cycles. Natural deforestation over the last 2000 14C years may have triggered a negative feedback process by changing the winter snow environment and the associated thermal soil regime. The ensuing peat fossilization, permafrost aggradation, and ice—wedge development at the peatland surface are all events consistent with the occurrence of this process. Present growth of the peatland pproceeds mostly along the margins, where levelling of the topographic profile is not yet completed.
Pollen Removal From Tristylous Pontederia Cordata: Effects of Anther Position and Pollinator Specialization
Authors: Harder Lawrence D. , Barrett Spencer C. H.
Published: May 2006

Stamens produce male gametophytes and expose them to the pollination process, so that stament characteristics should reflect selection on male function. For animal—pollinated plants, restricting removal in favor of using all available pollinators should generally promote pollen dispersal, so that staminal characteristics should limit pollen removal by individual pollinators. Here we describe the influences of anther position on pollen removal from Pontederia cordata, a tristylous plant with anthers at three distinct positions: exserted from the flower (long—level anthers), at the mouth of the flaring tubular perianth (mid—level anthers), and near the perianth based (short—level anthers). To measure pollen removal, we presented unvisited flowers to freely foraging bees and counted the number of pollen grains left in flowers after 1—4 visits. The pollinators included two generalist bumble bees (Bombus impatiens and B. vagans: Apidae) and Melissodes apicata (Anthophoridae), a specialist with morphological and behavioral adaptations for collecting pollen from P. cordata. Analysis of pollen removal incorporated the effects of bee species, anther position, pollen size, flower length, and the number and cumulative duration of bee visits. Anther position and bee species significantly influenced pollen removal, although the differences between bee species were not related to pollinator specialization. Long—level anthers dehisced early, lost an average of 39% of their pollen during their first visit, and did not consistently contact small pollinators, such as M. apicata. Short—level anthers restricted pollen removal more (28% lost) than long—level anthers, but dehisced more slowly and were subject to unpredictable pollen removal. Mid—level anthers combine the advantages of more exserted and inserted positions without suffering the corresponding disadvantages as they restricted removal (25% lost), dehisced relatively early, and seemed to consistently contact all pollinators. The overall benefits of positioning anthers within the perianth mouth, the position occupied by mid—level anthers in P. cordata, may explain the prevalence of this morphology among bee—pollinated plants with flaring, tubular, monomorphic flowers.