bluuebunny/crossref_metadata_2025 huggingface dataset:
Ecophysiology of Introduced Pennisetum Setaceum on Hawaii: The Role of Phenotypic Plasticity
Authors: Williams David G. , Mack Richard N. , Black R. Alan
Published: May 2006

The C4 African grass, Pennisetum setaceum (fountaingrass), has a greater altitudinal distribution than any other grass on the island of Hawaii. Clones of P. setaceum were reciprocally transplanted among coastal dry grassland, montane dry shrubland, and subalpine dry forest on the leeward side of Hawaii to evaluate the contribution of local adaptation and individual tolerance to the broad ecological amplitude of this grass. Physiological, growth, and reproductive responses differed among sites without evidence of local adaptation. Greatest tiller production and the highest photosynthetic rates were observed at the mid—altitude site, but plants at the coastal site attained greater basal areas, aboveground biomass, and number of inflorescences. Correlation among the responses of different plant characters was environmentally dependent, suggesting that the integrated expression of these characters is also plastic. Few differences in plant responses, however, were attributable to a population's origin or the site—by—population interaction; resident populations and introduced populations responded similarly at each site. Furthermore, few characters exhibited variation among clones, indicating that these population likely posses little genetic variation. Limited clonal variation was, however, detected for net photosynthesis, the ratio of internal to ambient CO2 concentration (ci/ca), and specific leaf area, but only on two of five dates over a 1—yr period. Phenotypic plasticity for both individual characters and the integration of physiological and morphological characters have apparently been most important in allowing P. setaceum to become dominant across diverse habitats on Hawaii.
Wood (delta)^(13)C as a Measure of Annual Basal Area Growth and Soil Water Stress in a Pinus Strobus Forest
Authors: McNulty S. G. , Swank W. T.
Published: May 2006

The relationships between annual wood tissue °13C, growing season soil water potential, and basal area growth were studied in a mature, white pine (Pinus strobus) stand at the Coweeta Hydrologic Laboratory, in western North Carolina. In 1992, four bolewood cores that spanned the years from 1980 to 1989 were extracted from each of ten equal—size, co—dominant white pine trees within the stand. The 1980s were a time of extreme climate with some of the hottest, driest, and wettest years recorded at Coweeta. Annual basal area growth ranged from 14.5 to 25.0 cm2°tree—1°yr—1, and modeled values of average growing season soil water potential ranged from —0.21 to —5.58 MPa, when measured to a depth of 60 cm. After correcting annual wood tissue °13C for atmospheric changes in °13C, carbon isotopic discrimination (△) ranged from 18.52 to 19.62%. The △ of annual wood tissue was positively correlated with average growing season soil water potential (r2 = 0.74, P = 0.0005, n = 10 growing seasons) and average annual basal area growth (r2 = 0.78, P = 0.0005, n = 10 seasons). Basal area growth and growing season soil water potential were also correlated (r2 = 0.64, P = 0.002, n = 10 seasons). These results suggest that annual wood tissue °13C could potentially be useful in estimating historic changes in soil water potential and basal area growth in mature forest ecosystems.
Density‐Dependent Cannibalism in Larval Dragonflies
Authors: Van Buskirk Josh
Published: May 2006

Cannibalism is a likely mechanism of population regulation in dragonflies that inhabit fish—free temporary ponds. I tested for density—dependent effects of cannibalism on survival and size structure of larvae of the dragonfly Tramea carolina in two short—term experiments in replicated artificial ponds. I was able to distinguish mortality due to cannabalism from other sources of mortality by preventing some populations from engaging in cannibalism. The first experiment included two density levels and the presence or absence of cannabilism in a factorial design. The no—cannibalism treatment was achieved by removing the labial palps from all individuals to prevent them from grasping large prey. T. carolina survived significantly better in the no—cannibalism populations at both densities, and this difference was greater at high density than low density, indicating that the proportion cannibalized was density dependent. A second experiment explored the form of the density—dependent cannibalism curve over a broad range of density. The results were adequately fitby a predator—prey model incorporating size structure, so that not all members of the population were vulnerable to cannibalism. In both experiments the survival of only the smaller instars was reduced, suggesting that cannibalism may reduce variation in the size distributions of dragonfly populations and contribute to emergence synchrony. The results demonstrated that cannibalism was strongly density dependent and may contribute to population regulation of dragonflies in temporary ponds.
Biogeographic Associations in Fish Assemblages: Local vs. Regional Processes
Authors: Jackson Donald A. , Harvey Harold H.
Published: May 2006

Six geographic regions along the Laurentian Great Lakes in Ontario, represented by 286 lakes, were examined to identify the existence of regional similarities of fish species composition and their association to geographic location and regional patterns of lake morphology and pH. Lakes differed significantly among regions with respect to surface area, maximum depth, and pH. Species presence/absence data were summarized using correspondence analysis, and the resultant scores were used in multivariate analysis of variance and canonical variates analysis. These results indicated that the fish faunas of the six geographical areas were distinct. Interregional distances based on fish community scores, lake morphology–chemistry data, and geographical distances were contrasted using Mantel's test. Regional faunal similarities were correlated significantly with geographical proximity, but not with lake morphology. We propose that post—glacial dispersal and lake thermal regimes are important determinants in structuring regional patterns of fish assemblages, whereas environmental conditions such as lake depth and pH assume greater importance in determining species compositions of individual lakes.
Habitat Selection by Pemphigus Aphids in Response to Response Limitation and Competition
Authors: Whitham Thomas G.
Published: May 2006

The leaf galling aphid, Pemphigus betae can be highly resource and habitat limited and has evolved to reduce these limitations. Over a 3—day period in spring as much as 83% of the overwintering population migrates to immature leaves of Populus angustifolia where individuals are rapidly entombed by expanding leaf tissue. The size of the mature leaf is critical to ultimate fitness of the individual colonizing stem mother. Probability of stem mother failure, body weight of stem mother and progeny, number of progeny, development rate of progeny to maturity, and number of embryos in mature progeny are all correlated with leaf size. Due to these selection pressures, stem mothers search out and colonize large leaves. Thirty—two percent of the tree's leaves were so small as to result in an 80% probability of total aphid failure. Nearly all of these leaves were avoided. In comparison, only 1.6% of the tree's leaves were so large as to result in a 0% probability of failure, and 100% of these leaves were colonized. Even though most stem mothers were forced to colonize. Even though most stem mothers were forced to colonize suboptimal leaves because competitor density (35/100 leaves) was much > the availability of optimal leaves, the average stem mother colonized a leaf 60% > the mean leaf size of the tree. Resulting from this degree of selectivity, the minimum increase in fitness was 2 times > expected if leaves had been selected at random. At low competitor densities reproductive output would still be limited by energy intake such that if there were only 1 gall on a tree, the colonizing stem mother and her progeny would still be resource limited. Apparently, no leaf produced by the tree is so large that a further increase in leaf size would not result in a further increase in fitness. Thus, even at low competitor densities, resources would still be important in the habitat selection process and in determining the parasite distribution.
A Simple Diffusion Model of Transpiration Applied to a Thinned Douglas‐Fir Stand
Authors: Tan C. S. , Black T. A. , Nnyamah J. U.
Published: May 2006

Values of forest transpiration rate, calculated from a vapor diffusion model that uses the vapor pressure defict (v.p.d.) of the canopy air, and measurements of the stomatal resistance (every 2—3 h) and leaf area index of the canopy, agreed well with those obtained from energy balance/Bowen ratio measurements. Stomatal resistance characteristics were also used in the diffusion model instead of actual stomatal resistance measurements to calculate transpiration rate. There was reasonable agreement between these transpiration values and energy balance measurements averaged over 3—h periods. Both the model, using stomatal resistance characteristics, and energy balance measurements showed that transpiration rate increased with increasing v.p.d. until a certain v.p.d. was reached after which the rate declined. The values of v.p.d. at which maximum transpiration rate occurred were 1.5, 1.3 and 1.1 kilopascals [=15, 13 and 11 mb] for soil water potential ranges of zero to —350 kilopascals [=0 to —3.5 bars], —350 to —950 kPa [=3.5 to —9.5 bars] and —950 to —1250 kPa [=9.5 to 12.5 bars] respectively. Calculations from the model, using stomatal resistance measurements (every 2—3 h) on 7 days, indicating that the fraction of transpiration from the thinned stand transpired by the salal undergrowth during daytime ours increased from approximately 45—70% during a 4—week drying period.
Regulation and Environmental Variability in Experimental Populations of Protozoa
Authors: Luckinbill Leo S. , Fenton Mark M.
Published: May 2006

Regulation of population size is described for 2 protozoan species in environments that become progressively more variable in time. the ciliate, Colpidium campylum, rapidly adjusts population levels to fixed equilibria set by bacteria food supply. As the equilibrium is varied more frequently, tracking populations of this species destabilize and eventually become extinct. A much larger species, Paramecium primaurelia, regulates slowly to fixed equilibrium but assumes more even densities in variable environments. Effects of changing environments are determined here by the frequency and not the magnitude of changing equilibria.
On Selection at High Population Density
Authors: Gill Douglas E.
Published: May 2006

Several alternative evolutionary pathways can be taken by a population subject to persistence high density conditions. The pattern of spatial and temporal predictability in habitat quality dictates whether selection will favor dispersability, diapause, competitive ability, or tolerance to crowded conditions. The latter may be indistinguishable from evolution of "Allee effects." There exist more alternatives than can be accommodated by the popular epithets "r" and "K" selection.
Single‐Tree Influence on Soil Properties in the Mountains of Eastern Kentucky
Authors: Boettcher S. E. , Kalisz P. J.
Published: May 2006

The validity of the concept of "ingle—tree influence circles" was tested in a forest dominated by Tsuga canadensis and Liriodendron tulipifera on steep slopes in the Appalachian Mountains of the eastern USA. Samples of forest floors and the 0—5 cm depth of mineral soil were collected at 135 locations within plots 1.3 ha in total area. Examination of particle—size distribution and pH at 33 of these locations confirmed that parent material to a depth of 100 cm was uniform throughout the study area. Soils were sandy and generally low in nutrients. Levels of pH, Ca, Mg, K, and mineralizable N tended to be higher, and forest floor mass tended to be lower, under the crowns of L. tulipifera compared to T. canadensis. Differnces between tree species were better expressed on areas lacking Rhododendron maximum understories. These results indicate that single—tree influence on soil properties is detectable even in mixed stands on steep slopes, and that the soil landscape may be considered a mosaic of profiles reflecting the occurrence and chemical characteristics of the ground cover vegetation and of individuals of the various tree species present.
Prey Control by an Assemblage of Generalist Predators: Spiders in Garden Test Systems
Authors: Riechert Susan E. , Bishop Leslie
Published: May 2006

The hypothesis that generalist predators limit associated prey populations was tested using spiders, a group classically thought to be an insignificant component of the agroecosystem. Two habitat manipulations (addition of mulch and flowers) in separate and combined treatments were utilized to enhance spider numbers in a mixed vegetable system. Compared to control plots, significantly higher spider densities were observed in the plots to which mulch alone or both mulch and flowers had been added. Insect damage to the plants was significantly lower in the plots to which mulch had been added as well, which correlates with the lower numbers of pest insects in plots containing mulch. That spiders were the probable cause of the effect was demonstrated in plots containing both mulch and flowers but from which spiders had been systematically removed. Removing spiders from plots with mulch and flowers removed the effect of this treatment on pest numbers and plant damage. In addition, 84% of the predators observed foraging in the study areas during timed watches were spiders and 98% of the predation events observed were by spiders. In a separate experiment, spiders were added to a subset of individually bagged broccoli plants that had been infested previously with known numbers of herbivorous pests. Plant damage in bags lacking spiders averaged 93.3%, whereas damage averaged 31.8% in bags to which spiders had been added.
Invasion by the Variegated Leafhopper and Biotic Interactions: Parasitism, Competition, and Apparent Competition
Authors: Settle W. H. , Wilson L. T.
Published: May 2006

In 1980 the variegated leafhopper (VLH), Erythroneura variabilis, invaded California's San Joaquin Valley, and has since rapidly increased in numbers to become a major pest in vineyards. Correlated with the invasion by VLH are declining populations of congeneric grape leafhopper (GLH), E. elegantula. This paper explores interspecific competition and shared parasitism as possible reasons for this decline in GLH. Although interspecific competition between VLH and GLH was evident in field—cage experiments, its effects were equivalent to instraspecific competition. Instead of simple direct competition explaining the replacement of GLH by the invading VLH, the key appears to be parasitoid that is shared by two leafhoppers. In particular, the native GLH experiences higher attack rates from the shared parasitoid (Angarus epos) than does the invading VLH. This differential parasitism apparently shifts the competitive balance from one of equality to a strong disadvantage for the native relative to the invader. This is the first study, to our knowledge, to document the importance of parasitoid—mediated indirect effects in an invasion.
Aquatic Patch Creation in Relation to Beaver Population Trends
Authors: Johnston Carol A. , Naiman Robert J.
Published: May 2006

The creation of aquatic patches by beaver (Castor canadensis) in the boreal forest of northern Minnesota, USA, was studied to determine how the population dynamics of a disturbance—causing animal are linked to rates of patch formation and growth over a period of population expansion and stabilization. Using six series of aerial photographs taken between 1940 and 1986, we determined the size and growth rates of individual patches, and the numbers, area, density, and establishment rate of the patch population. The rate of patch formation was much higher during the first two decades of colonization than during the subsequent two decades. The average area of all ponds sites, which included both filled and drained ponds, remained at °ha throughout the time period, but the average area of new ponds decreased significantly over time. Ponds established by 1961 constituted 75% of the total number and 90% of the total pond—site area as of 1986. When pond sites of similar age but different pond cohort (i.e, decade of establishment) were compared, the average area per pond site was always significantly larger for the earlier cohort. Although the rate of pond creation paralleled the increase in number of beaver colonies between 1961 and 1986, the rate of new pond creation prior to 1961 greatly exceeded the increase in number of beaver colonies. We conclude that the rate of patch formation after the first two decades of beaver colonization was constrained by geomorphology, which limited the availability of sites at which a beaver dam could impound a large area of water.
Predator Satiation and Site Alteration Following Fire: Mass Reproduction of Alpine Ash (Eucalyptus Delegatensis) in Southeastern Australia
Authors: O'Dowd Dennis J. , Gill A. Malcolm
Published: May 2006

This study examines seedfall, seed predation, and seedling recruitment in a relatively fire—sensitive tree, the alpine ash (Eucalyptus delegatensis), in the Brindabella Range, Australian Capital Territory (A.C.T.), Australia. An experimental high—intensity fire was used at Bushrangers Creek (BC) to determine whether massive, synchronized seedfall and subsequent seedling recruitment are linked to (1) satiation of postdispersal seed predators and/or (2) reduction of the physical and biotic resistance to seedling growth and survivorship. Results were contrasted with findings at Moonlight Hollow (MH), a site which served as a natural, unburned control. Overall results wee consistent with the predator—satiation hypothesis. High—intensity fire killed 93% of the alpine ash trees at BC and caused a massive, synchronized release of seeds, averaging 405 viable seeds/m2 (4021 total ovular structures/m2) in the 3 wk after fire. Fire precipitated an immediate drop in weekly removal of seeds from trays, from an average of 65.3% prefire to 13.6% postfire. Postfire seed removal at BC dropped to significantly lower levels than corresponding measures at unburned MH. Caging experiments demonstrated that ants were the only important removal agents of alpine ash seed at BC and MH. Although abundance and species richness of ants trapped on the seedbed increased following fire, the number of ants trapped per available seed significantly declined due to the massive input of seed. Fire and subsequent seedfall resulted not only in a greater absolute number of emergent seedlings (66.1 seedlings/m2 at BC; 0.8 seedlings/m2 at MH) but also in a significantly greater proportion of emergence from seed input from the canopy (14.7% at BC; 2.7% at MH). Seedling mortality was great, and after 72 wk mean density at BC was 1.9 seedlings/m2. Successful recruitment did not occur at unburned MH. A phytometer experiment demonstrated that fire neither enhanced the physical environment for early growth and survivorship of seedlings nor reduced the effect of natural enemies on E. Delegatensis seedlings. Greater frequency of grazing on seedlings growing in burned soil and a more rigorous physical environment at BC appeared to account for lack of enhanced growth and survivorship at postfire BC. Massive, synchronized release of stored seed from shrub and tree species following fire is a common feature in many fire—prone environments. Parallels in seed storage, postdispersal seed predation, and regeneration following fire in Eucalyptus and Pinus suggest that postdispersal seed predators are important selective agents in the evolution of seed storage and induction of mass seed release by fire.
Anuran Size at Metamorphosis: Experimental Test of a Model Based on Intraspecific Competition
Authors: Travis Joseph
Published: May 2006

Size at metamorphosis and length of larval period are often, but not invariably, correlated within cohorts of larval anurans. Collins (1979) proposed intraspecific competition as one causative agent for such a correlation in nature, and this hypothesis was supported by observational data. This study reports the results of an experimental test of that hypothesis. Isolated larvae were reared at one of two food levels (high or low) and, during the period in which metamorphosis occurred, were maintained at either constant per capita food levels, or at increasing per capita levels designed to mimic effects of continual release from scramble—type, exploitative competition. Low food levels decreased average larval growth and average size at metamorphosis and increased average length of larval period. A reduced level of food had no effect on the relative variation in length of larval period. Only competitive release affected the relative variation in size at metamorphosis. All four treatments displayed a positive correlation between size at metamorphosis and length of larval period, but only in the combination of low food levels and competitive release was the correlation more than barely significant. Size at metamorphosis displayed a positive allometric relationship to length of larval period in the low food—release treatment, and a negative allometric relationship in the other three treatments. These results support Collins' (1979) model for intraspecific competition as one causative mechanism for such relationships. Certain aspects of these results support each of two distinct models for the timing of amphibian metamorphosis, but the data as a whole do not fit well with either model.
Analysis of Parentage for Naturally Established Seedlings of Chamaelirium Luteum (Liliaceae)
Authors: Meagher Thomas R. , Thompson Elizabeth
Published: May 2006

The genealogical relationships among naturally established seedlings and flowering individuals were analyzed for a large population of the dioecious plant species Chamaelirium luteum. Genetic likelihoods based on 11 electrophoretic markers of all possible parent pairs within flowering seasons from 1974 to 1981 preceding observed establishment of seedlings from 1976 to 1982 were evaluated; and most—likely parents for 283 seedlings were thus identified, enabling a partial reconstruction of the genealogy of this population. This information, combined with the map location of each plant, was used to analyze realized gene—flow patterns. Intermate distances showed more nearby matings than would the kind of pollen dispersal profile that could be expected under random mating. Overall, seedlings tended to undergo establishment at locations between their maternal and paternal parents, indicating localized genetic adaptation along environmental gradients. Finally, a negative correlation was observed between inflorescence size (reproductive effort) and number of progeny observed (reproductive success). Techniques of genealogy reconstruction have previously been limited to human populations. As seen in the present study, such techniques also show great promise for revealing the evolutionary behavior of natural populations.
Switching and the Pattern of Host use by Battus Philenor Butterflies
Authors: Rausher Mark D. , Odendaal Francois J.
Published: May 2006

A model is presented that accounts mechanistically for the seasonal change in host—searching behavior exhibited by pipevine swallowtail butterflies (Battus philenor) in eastern Texas. The model assumes that the proportion of females searching for broad— (or narrow—) leaved hosts represents an equilibrium determined by the rates at which females switch search modes. In the field, females switched search modes every 1—3 h. Observed proportions of females using the two search modes were, on average, within 10% of the proportions predicted by the model. The results were consistent with the assumption that switching occurs primarily when females alight on and oviposit on the host species for which they are not currently searching. The seasonal change in proportions of broad— and narrow—leaf searchers seems to be a consequence of a seasonal change in the rate at which females alight on hosts for which they were not currently searching. The latter change is possibly due to seasonal changes in the relative abundances of the two hosts and to changes in the degree to which surrounding vegetation interfers with a butterfly's ability to detect host plants.
Nitrogen and Lignin Content as Predictors of Litter Decay Rates: A Microcosm Test
Authors: Taylor Barry R. , Parkinson Dennis , Parsons William F. J.
Published: May 2006

Decomposition rates of leaf litter have been predicted from the leaves' lignin or nutrient (N or P) contents, the C:N ratio, and more recently the lignin:N ratio. But tests of these predictors have been based on groups of substrates each spanning only part of the natural range of lignin contents, and neglecting low—lignin (<10%) species. We allowed leaf litter from eight species of tree, shrub, or herb, ranging in lignin content from 3.4 to 20.5%, to decompose in laboratory microcosms for up to 4 mo (equivalent to 1.5—2 yr decay in the field) to test two hypotheses: (1) that the lignin: nitrogen ratio would have a better correlation with mass loss rates than would the C:N ratio, nutrient content, or other substrate quality indexes, and (2) that correlations of mass loss with initial N content would decrease, while correlations with lignin content would increase, as decay proceeded. Contrary to the first hypothesis, nitrogen content and the C:N ratio were the best predictors of mass loss rate, and were substantially better than the lignin:N ratio. We could find no better predictor of decomposition rate than the C:N ratio, and no better regression model than the simple linear one. However, when regressions were tested using pine needles (lignin content 26.2%), the C:N ratio and N content badly underestimated mass remaining (by 10—16%), while lignin content and the lignin:N ratio overestimated it by <2%. In accordance with the second hypothesis, regressions of initial lignin content or lignin:N ratio on mass remaining improved (higher R2) from 2 to 4 mo decomposition, while those of N content grew worse, illustrating succession of nitrogen to lignin control of decomposition rate. Reported correlations of the lignin: N ratio with decomposition rate for some litter types arise as a special case of this two—phase mechanism of control by nutrients and lignin. For substrates low in lignin, or where a broad range of lignin contents is being considered, the C:N ratio is a better predictor of decomposition rate than the lignin:N ratio.
Island Biogeograhy of Montane Forest Mammals in the American Southwest
Authors: Lomolino Mark V. , Brown James H. , Davis Russell
Published: May 2006

Information from packrat (Neotoma spp.) middens and other paleoenvironmental indicators was used to reconstruct Pleistocene distributions of macrohabitats in the American Southwest (Arizona, New Mexico, and southern Utah and Colorado). This revealed a system of southern montane forests that were isolated during the Pleistocene (primarily by woodlands) as well as today by grasslands, chaparral, and desertscrub). Based on the presence of at least five species of nonvolant forest mammals on these montane islands, we inferred that these mammals immigrated across woodlands during the Pleistocene. Because the great majority of montane forests in the American Southwest are now isolated by woodlands, but not grasslands, chaparral, or desertscrub, we hypothesized that post—Pleistocene immigrations may influence the structure of mammalian communities on these montane islands. This hypothesis was supported by the highly significant correlation of mammalian species richness with current isolation as well as area (P < .01 for simple correlations and for partial correlations after removing effects of latitude and area or isolation: 26 species, 27 islands). When isolation was partitioned into distance to be traveled across woodland vs. grassland—chaparral habitats, species richness was significantly correlated with the latter measure of isolation (P < .01) but not with the former, indicating that woodlands do not represent major barriers to immigration. Seventeen of 26 species considered are reported with at least 10% of their record locations in woodlands and other low—elevation habitats. Analyses of patterns of occurrence of seven species of forest mammals inhabiting between 7 and 16 islands revealed that insular distribution was significantly (P < .01) affected by area in two species, by area and isolation in three species, and by isolation alone in the remaining two species. Based on our reconstruction on Pleistocene macrohabitat distributions, analyses of community— and species—level patterns, and evidence on the ability of forest mammals to inhabit and disperse across woodlands and other low—elevation habitats, we conclude that community structure of montane forest mammals in this region of the American Southwest is influenced by post—Pleistocene immigrations as well as extinctions.
Floral Development and Chemical Defense Allocation in Wild Parsnip (Pastinaca Sativa)
Authors: Nitao James K. , Zangerl Arthur R.
Published: May 2006

Optimal defense theory predicts that within—plant allocation of defensive compounds should reflect the value of plant tissue in terms of its relative contribution to fitness. This theory was tested by examining the quantity of furanocoumarins in wild parsnip, Pastinaca sativa (Umbelliferae), in relation to the value of a floral unit as the unit develops. P. sativa is andromonoecious and protandrous, each hermaphroditic flower sequentially passing through bud, male, and female stages. Resource commitment to the primary umbel increases as flowers and seeds mature; if resources are limiting, the timing of primary umbel defloration due to herbivory can be expected to affect a plant's ability to compensate for lost floral parts. The degree of fitness reduction following defloration of the primary umbel and compensatory growth in higher order umbels was used as a measure of floral stage value. Primary umbels were artificially deflorated at various stages and resulting seed set was compared with undamaged control plants. In an experiment with small plants (with a mean total biomass of 5.838 g), the loss of developmentally more advanced floral units resulted in greater fitness reduction. Total seed production, both in terms of biomass and number, followed the pattern: control > female deflorated > fruit deflorated plants. A similar experiment using larger plants (mean total biomass = 21.78 g) and defloration at bud and female floral stages produced a parallel but statistically nonsignificant trend for seed biomass. Mean seed mass was significantly reduced by defloration in both experiments. The furanocoumarin content and composition of floral units were measured as primary umbels developed sequentially from bud stage through male, female, and mature seed (schizocarp) stages. Total and individual furanocoumarin levels increased with advancing floral development over the course of the season. In addition, co—occurring developmentally asynchronous floral units on the same primary umbel were compared for furanocoumarin content to control for passive allocation patterns or for differences in floral unit apparency. Within umbels containing developmentally asynchronous stages, advanced floral units had higher levels of furanocoumarins than less advanced floral units. These results were consistent with optimal defense theory: defensive compounds were allocated in higher quantities to floral units in developmental stages at which their loss would have a greater negative impact on fitness.
Uinta Ground Squirrel Demography: Is Body Mass a Better Categorical Variable Than Age?
Authors: Sauer John R. , Slade Norman A.
Published: May 2006

We compared the relative impact of age and body mass on fecundity and survival in a population of Uinta ground squirrels, and modeled the population using body—mass classes as stage categories of Lefkovitch stage transition matrices. Body mass was a better predictor of survival than was age, and was nearly as good a predictor of litter size. Mass—based stage transition matrices provided results similar to age—based transition matrices, but also indicated that larger and subsequently more fecund young were a consequence of a population reduction. We believe that mass—based analysis of ground squirrel populations is a viable alternative to traditional age—based analyses. Body mass is easy to measure in small—mammal populations and is linked to a variety of life—history characteristics.
Extinction of an Island Forest Avifauna by an Introduced Snake
Authors: Savidge Julie A.
Published: May 2006

The island of Guam has experienced a precipitous decline of its native forest birds, and several lines of evidence implicate the introduced brown tree snake (Boiga irregularis) as the cause of the range reductions and extinctions. The range expansion of B. irregularis correlated well with the range contraction of the forest avifauna. Exceptionally high predation by snakes on bird—baited funnel traps occurred in areas where bird populations had declined. Little or no snake predation occurred in areas with stable bird populations. Several factors have contributed to Boiga's success in decimating the avifauna on Gaum. Although most common in forests, B. irregularis has occupied a variety of habitats on Guam, and few effective barriers to its dispersal exist. Prey refuges are present only in urban areas, on concrete or metal structures, and in the savanna. Boiga's nocturnal and arboreal habits and an apparent keen ability to locate prey, make roosting and nesting birds, eggs, and nestlings vulnerable. Besides birds, B. irregularis feeds on small mammals and lizards. By including the abundant small reptiles as a major component in its diet, Boiga has maintained high densities in forest and second—growth habitats while exterminating its more vulnerable prey. This is the first time a snake has been implicated as an agent of extinction.
Ecological Correlates of Incubation Feeding: A Comparative Study of High Arctic Finches
Authors: Lyon Bruce E. , Montgomerie Robert D.
Published: May 2006

We compared the ecology of two high arctic finches to determine which ecological factors might influence male parental roles during incubation. Male Snow Buntings (Plectrophenax nivalis) feed their mates at the nest throughout the incubation period, while male Lapland Longspurs (Calcarius lapponicus) usually do not. We identified and tested seven ecological factors that could potentially explain the difference between these two species; (1) risk of nest predation, (2) nesting synchrony, (3) potential for polygyny, (4) timing of breeding, (5) diet, (6) distance between nest and foraging areas, and (7) nest microclimate. Of these, only predation rate and nest microclimate differed significantly between longspurs and buntings. Buntings nest in rock cavities and suffer lower overall nest predation than longspurs (28 vs. 51%, respectively), suggesting that this factor could influence the occurrence of incubation feeding. However, since male longspurs could feed their mates away from the nest, risk of nest predation can only explain why incubation feeding at the nest does not occur, not why incubation feeding per se is rare in longspurs. Due to differences in nest microclimate on sunny days, eggs cool more rapidly in bunting nests, suggesting that female longspurs can take advantage of solar radiation to warm their eggs and can consequently spend a higher proportion of their time foraging than female buntings. A comparison of female incubation schedules supports this idea; female longspurs spent more total time off their eggs (X = 25.9 min/h) than female buntings (X = 15.9 min/h). We conclude that, of the factors we investigated, the colder nest microclimate associated withhole nesting has been the most important ecological factor influencing the occurrence and high frequency of incubation feeding in Snow Buntings. We discuss the possibility that hole nesting favors incubation feeding in passerine birds in general.
Nitrogen Availability and Nitrogen Use Efficiency in Loblolly Pine Stands
Authors: Birk Elaine M. , Vitousek Peter M.
Published: May 2006

The effects of nitrogen availability on N cycling and N use efficiency (NUE) were examined in natural and fertilized loblolly pine stands on the upper coastal plain of South Carolina. Indices of N availability, based on potential rates of N mineralization, ranged from 1.6 to 11 kg°ha—1 (8 wk)—1 in the stands, and concentrations of N in foliage, wood, fine roots, and needlefall increased with greater N availability. Litterfall dry masses, net aboveground production, and litterfall nitrogen were all positively correlated with N availability, while indices of NUE decreased with increased N availability. Mechanisms that could explain increased NUE in low—N sites were examined in the field and in a phytotron study. First, nitrogen retranslocation on a per needle basis did not change significantly across the N—availability gradient, and thus could not account for the change in NUE efficiency. For the stands as a whole, however, substantially more N was retranslocated at the highest levels of N availability. Second, an increase in N uptake efficiency could not account for an increase in NUE with low N availability, since phytotron—grown seedlings fertilized with N had significantly higher rates of N uptake per unite root mass and lower root: shoot ratios than N—limited seedlings. Third, net production per unit N within pine seedlings was significantly higher in the N—limited plants, suggesting that an increase in the amount of carbon fixed per unit of tissue N could account for the observed increased in NUE in loblolly pine stands at low levels of N availability.
Internal Factors Controlling Peatland‐Lake Ecosystem Development
Authors: Kratz Timothy K. , DeWitt Calvin B.
Published: May 2006

To examine nonclimatic factors controlling small—peatland development we expanded the classical model of peatland development by hypothesizing the existence of three horizontal zones. We then tested five predictions derived from the expanded model. Prediction 1: Peat Strata. There are two vertical strata of peat, an upper layer consisting of peat formed in the floating or grounded mat (mat peat) and a lower layer of peat formed in the floating mat, but dropped from the side or bottom of the mat (debris peat). Underlying these strata are lake sediments originating in the water column. Prediction 2: Strata Boundary. Chamaedaphne calyculata provides the framework for mat growth, and therefore stems and roots of this plant form the boundary between the two peat strata. The lower boundary of mat peat is continuous from the lake edge to the upland. Prediction 3: Strata Thickness. Thickness of mat peat increases as a function of the depth of the original basin and the length of time peat has been accumulating. Prediction 4: Peat Density. The density of peat increases with distance from the lake edge up to a zone in which peat has reached a maximum density and mean density is constant. Prediction 5: Vertical Accumulation Rate. The long—term rate of vertical accumulation (in grams per square metre per year) decreases with distance from the lake edge. To test these predictions we used data collected on peat stratigraphy, lake—edge vegetation, and peat age and density in Fallison Bog, a 5.5—ha peatland—lake system in northern Wisconsin, and to a lesser extent from 13 other peatlands in the region. Each prediction was confirmed, except that thickness of mat peat appears to be independent of peat accumulation time. We estimated the vertical growth rate of the mat by radiocarbon dating of C. calyculata stems. By dating twigs at different locations in the mat we estimated that the rate of vertical peat accumulation ranges from 34—75 g°m—2°yr—1, depending on location within the peatland. The relationships among peat densities, vertical accumulation rates, and distance from the lake edge suggest that three horizontal zones occur in Fallison Bog: (1) a zone of thickening near the lake edge where vertical accumulation of organic matter thickness the floating mat, (2) a zone of compaction farther from the lake edge where vertical accumulation compacts underlying peat, and (3) a zone of equilibrium farthest from the lake edge where peat has reached a maximum density (100—110 kg/m3 in Fallison Bog) and, in the absence of a perched water table, no peat accumulates. Our results underscore the importance of spatial dynamics in peatland development.
Behavioral Interactions Among Four Species of the Salamander Genus Desmognathus
Authors: Southerland Mark T.
Published: May 2006

Four sympatric species: Desmognathus quadramaculatus, D. monticola, D. fucus, and D. ochrophaeus, showed different substrate preferences in experimental trials. In addition, the choices of substrates and of cover objects were influenced by the presence of other salamanders. Significant differences in substrate choice were found for each pair of species. The ratio of rocky to woody substrate chosen was lower for the more terrestrial species. The ecologically intermediate species, D. monticola, exhibited the broadest choice of substrate and shifted its choice when confined with congeners. Adults of D. monticola became more active and avoided the preferred substrate of D. quadramaculatus when confined with that species. Juvenile of D. monticola became less active but also avoided preferred substrates of the larger D. quadramacultatus. In 1—m2 arenas with four cover objects, small individuals of the three more terrestrial species avoided cover occupied of large individuals of these species. No aggressive behavior patterns were observed in 100 encounters of different individuals. These results suggest that this assemblage of salamanders is structured by species—specific microenvironmental preferences that are affected by interspecific interactions. Avoidance of other salamanders is the common response that segregates individuals and species. This is likely an adaptation to the severe risk of predation faced by small salamanders.
Growth Ecology of a Tropical Lizard, Basiliscus basiliscus
Authors: van Devender Robert Wayne
Published: May 2006

Linear growth in the tropical lizard Basiliscus basiliscus fits the von Bertalanffy growth model of the form: GR = a / bSVL (SVL = snout—vent length). Analysis of covariance was used to compare data from 2 sites in Costa Rica, wet and dry seasons, and several years. Initial growth rates were between 0.3 and 0.4 mm/day for both sexes, but asymptotic sizes were much larger in ♂ ♂ (212—238 mm SVL versus 167—176). Female growth rates were sensitive to seasonal and annual differences in rainfall, varying inversely with reproductive activity. Male growth differed between sites, perhaps in response to different social milieux. Asymptotic size was highest at the site with more large, dominant ♂ ♂. The basilisk results suggest a novel reproductive strategy including facultative responses to both social and environmental cues.
A Lesson in the Limitations of Field Experiments: Shrubsteppe Birds and Habitat Alteration
Authors: Wiens John A. , Rotenberry John T. , Van Horne Beatrice
Published: May 2006

We manipulated the habitat composition and structure within territories of birds breeding in the Artemisia—dominated shrubsteppe of central Oregon in order to assess how closely individuals track habitat in features such as territory placement of size or behavioral budgeting and space. We removed 75, 50, 25, and 0% of the shrub individuals from 625—m2 blocks in a checkerboard designs. Over the following 7 yr we monitored territorial locations and sizes and breeding densities of Horned Larks (Eremophila alpestris), Sage Sparrows (Amphispiza belli), and Brewer's Sparrows (Spizella breweri) on the manipulated area and adjacent unaltered control area. Activity and substrate use patterns and intensity of use of the different treatment blocks within the manipulated area were also determined for the two sparrow species. Sage Sparrows, which from our previous studies appear to be more closely linked to shrub (sagebrush) coverage than the other species, did not respond to the manipulation at a local population level. The manipulation apparently did affect territory placement and densities of Brewer's Sparrows and Horned Larks in 1980—1982, but these trends were not consistent over the entire postmanipulation period. Densities of all species varied among years. For Brewer's Sparrows and Horned Larks these variations did not parallel those in nearby census locations, but Sage Sparrow densities varied in the same ways over all the locations. These variations appeared to parallel variations in bioyear (October—April) precipitation, with a 1—yr time lag. Individuals of both sparrow species altered the details of their activity budgeting, but not their substrate—use patterns, in response to the manipulation. Both species clearly expressed an affinity for unmanipulated blocks within the manipulation area. This experimental manipulation is complicated by influences of time lags on individual and population responses that may stem in part from site tenacity by breeding adults, leading them to return to previous breeding locations in years following the manipulation despite the habitat changes. Further, the spatial scale on which the manipulation was conducted may have been inappropriate to gauge responses at the population level. We suggest that these complications may plague many field experiments in ecology.
Population Dynamics of a Montane Conifer Forest, Western Cascade Range, Oregon, USA
Authors: Stewart Glenn H.
Published: May 2006

Size and age structure were used to reconstruct the population dynamics of two forested stands in the western Cascade Range, Oregon, USA. Trees with different growth rates and size—age relationships occurred in the same stand. The relationships between diameter and age of 939 aged trees of four species, although statistically significant, were often weak. Data on both age and spatial dispersion added critical information on stand dynamics not available from size alone. The population structures and regeneration patterns described were strongly influenced by natural disturbances and species' life history characteristics. Periodic fires of variable intensity and extent have produced a mosaic of relatively even—aged patches of different conifer species. The early establishment and dominance by a species on a site was a determinant of subsequent forest regeneration. If Pseudotsuga menziesii dominated early, regeneration of Tsuga heterophylla, and often Abies amabilis, was rapid. If, however, Tsuga heterophylla established first, further regeneration of other species was absent or minimal until canopy openings formed.