bluuebunny/crossref_metadata_2025 huggingface dataset:
Avian Seed Dispersal of Three Neotropical Gap‐Dependent Plants
Authors: Murray K. Greg
Published: April 2006

In cloud forest at Monteverde, Costa Rica, I investigated the reproductive consequences of avian seed dispersal for three species of gap—dependent plants: Phytolacca rivinoides (Phytolaccaceae), Witheringia solanacea, and W. coccoloboides (Solanaceae). Of six bird species that consumed fruits of these plants, only three (Myadestes melanops [Muscicapidae], Phainoptila melanoxantha [Ptilogonatidae], and Semnornis frantzii [Capitonidae]), dispersed seeds in viable condition. Other species discarded most seeds before swallowing fruit pulp, or destroyed seeds in the gut. I estimated the quality of dispersal service provided by the "legitimate" disperser species by comparing the seed shadows they produced with the spatial and temporal distributions of establishment sites for the plants. I estimated seed shadows from data on gut passage rates of seeds and on movement patterns of radio—tracked birds. Seed shadows produced by all three effective dispersers were extensive, with few seeds deposited near the parent plant, and some seeds moved >500 m. Seeds of the species examined establish in forest gaps formed by treefalls or landslides. Establishment success varies with gap size and age, but the relationship is different for each species; both Witheringia species establish well in gaps as small as 15 m2 or as old as 6 mo, whereas P. rivinoides establishes well only in gaps >70 m2 or <4 mo old. Consequently, establishment sites for all three plants are both rare and ephemeral, but to differing degrees. Seeds that are not dispersed to suitable habitat patches can remain dormant in the soil until a gap is formed overhead; seed dormancy experiments showed no significant decrease in viability of seeds buried for up to 27 mo. To determine consequences of dispersal and dormancy for plant reproductive success, I developed a simulation model that uses data on seed shadows, germination requirements, seed dormancy, and forest dynamic processes to estimate reproductive output (total offspring produced during an individual plant's lifetime) and relative "fitness" (an estimator that discounts the contribution of offspring produced after a long period of dormancy). Results show that (1) dispersal by any of the three legitimate dispersers increases reproductive output 16—36 times, even without seed dormancy. (2) Dormancy capabilities up to 2 yr enhance both reproductive output and "fitness," but greater capabilities increase only reproductive output. (3) Without dispersal, dormancy has little effect on either reproductive output or fitness. Thus, both dispersal and dormancy ("dispersal" in time) are essential to these gap—dependent plants, but long—term seed dormancy may be relatively unimportant.
Population Dynamics and Life Histories of Foliaceous Corals
Authors: Hughes T. P. , Jackson J. B. C.
Published: April 2006

The population dynamics of five species of foliaceous corals (Agaricia agaricites forma purpurea, A. lamarcki, Leptoseris cucullata, Montastrea annularis, and Porites astreoides) was followed on Jamaican reefs using annual photographic censuses. Overall, population cover, size frequencies, and number of colonies were stable over the monitored period from 1977 to 1980. However, individual colonies were in turmoil: of the original 883 colonies, 315 were killed outright and 499 suffered partial colony mortality (injury) at least once during the 3 yr. Partial mortality generated an additional 189 colonies by fission, while larval recruitment added another 201, and fusion subtracted 40 colonies. The net result was a decrease of <10% in number of colonies. There was considerable variation among years and sites in measured life history parameters, as well as striking differences between species. The most stable populations were M. annularis and A. lamarcki, followed by P. astreoides, A. agaricites, and L. cucullata. Rates of partial— and whole—colony mortality were strongly dependent on colony size for all species. Typically, small colonies either were unharmed, or were killed outright, while most large colonies survived but were injured each year, often by extensive amounts. The amount of tissue lost from a population through injuries was usually much greater than through the death of whole colonies, even in a year which included a major winter storm. Frequently, large corals were split asunder by partial mortality to produce several daughter colonies, which presumably were of identical genotype. Therefore counts of physically separate colonies exceeded the number of genetically distinct individuals (genets), by at least 20%. Individual genets, measured as the lateral extent of known daughter colonies, were frequently up to 5 m across, and for M. annularis and A. lamarcki were certainly several centuries old. Colony extension rates measured in situ were very weakly dependent on depth from —10 to —55 m, and were independent of colony size. Small colonies showed much faster relative changes in area, although even the largest corals continued to grow if they avoided major injuries. Within a size—class, the fates of colonies were diverse because of differential rates of growth and shrinkage, so that size was a very poor indicator of age. Differences in the life history and "mobility" between species are reflected in the taxonomic and morphological composition of coral communities over the reef. Shallow—water assemblages of foliaceous corals are composed of more dynamic, delicately built species, while many deeper water communities are dominated by slower growing, robust species. Ironically, disturbance on coral reefs often seems to favor the organisms most vulnerable to damage.
The Development of a Tidal Marsh: Upland and Oceanic Influences
Authors: Clark James S. , Patterson William A.
Published: April 2006

A stratigraphic study was undertaken to determine the historic role of watershed and sea—level changes on the development of Fresh Pond Marsh on Long Island's north shore. Pollen analytic, radiometric, and macrofossil techniques were aimed at differentiating among marsh, watershed, and regional changes over the last 1000 yr. A study of surface pollen distribution was conducted to determine pollen source and tidal influences on pollen deposition and to evaluate comparability of fossil pollen records from various intertidal environments. Regional and local inputs along a transect that included several tidal regimes were identified. Pollen counts from these different environments were comparable and proved useful for marsh reconstructions. Pollen analyses from the coastal marsh and a nearby lake distinguished changes in upland vegetation and cultural patterns within the watershed from those occurring regionally. Records of local pollen, foraminifera, macrofossils, rhizomes, and lithology indicated that the history of Fresh Pond Marsh has included periods of both tidal and freshwater conditions. Close dating control provided by 210Pb measurements and historically documented changes in indicator pollen taxa identified marsh responses to external influences, including dynamics of a baymouth bar, land clearing, agriculture, mosquito ditching, and sea—level fluctuations. Results showed that (1) tidal marsh sections can provide sensitive records of both upland and marsh vegetation histories, (2) pollen and macrofossil records can be closely linked to tide—gauge records and are responsive to short—term changes in sea level with a high degree of temporal resolution, and (3) upland influences can play an important role in determining the course of plant succession in the intertidal environment. Pollen provides the most sensitive record of marsh and upland development, as long as pollen source is accounted for.
Palynological Evidence for Postglacial Change in the Position of Tree Limit in Labrador
Authors: Lamb H. F.
Published: April 2006

The postglacial history of the forest—tundra zone in Labrador is inferred from the pollen stratigraphy of sediment cores from four lakes which differ considerably in the size and relative relief of their hydrologic catchments. Local pollen—assemblage zones are established independently for each pollen sequence by numerical methods. The relative pollen stratigraphies, interpreted through multivariate statistical comparison with modern pollen spectra from the region, show an overall increase in the tundra component of the forest—tundra vegetation since °3000 BP. Decreasing absolute frequencies of Picea pollen in the sediments are interpreted as evidence of the decline in tree numbers within the lake catchments, and therefore as a potential record of lowering of the altitudinal tree limit. The records show diachronous changes between sites because of differences in catchment elevation and relief. Tree limit has apparently fallen at least 40 m between 3000 and 1000 BP. A further fall of 30 m may have taken place by 250 BP. High ground in northern Laborador makes it unlikely that any equivalent southward displacement of latitudinal tree line can be identified.
Effects of Watershed Perturbation on Stream Potassium and Calcium Dynamics
Authors: Webster Jackson R. , Patten Bernard C.
Published: April 2006

Three small streams located at Coweeta Hydrologic Laboratory, North Carolina, USA, on an old field watershed, a pine plantation watershed, and a hardwood forest watershed were investigated to determine effects of watershed perturbation on K and Ca dynamics in the stream ecosystems. Data collected included measurements of litterfall inputs, large particulate organic matter and benthic organism standing crops, large particulate organic matter and organism drift, and insect emergence. We used 85Sr and 134Cs to estimate detritivore ingestion and elimination rates of Ca and K, respectively. We found that watershed perturbations had altered stream inputs and caused accompanying changes in the stream fauna. Our results indicated that the perturbed streams had less efficient physical processing of allochthonous inputs, but greater biological utilization of inputs. The streams exhibited high resilience to perturbation with complete recovery limited by the recovery rate of allochthonous inputs.
Population Differentiation along a Flood Frequency Gradient: Physiological Adaptations to Flooding in Nyssa sylvatica
Authors: Keeley Jon E.
Published: April 2006

Throughout the southeastern United States the hardwood Nyssa sylvatica (sensu lato) is distributed along a soil moisture gradient from upland sites, which are never flooded, to floodplains, which are periodically flooded and drained to permanently flooded swamps. Population differentiation with respect to flood tolerance and related physiological attributes was investigated using 1—year—old seedlings grown in a greenhouse from seed collected along this gradient. Upland plants were very intolerant of flooded soils. Their root systems deteriorated, root respiration rates dropped and, after a year under such conditions, survival was poor and those that did remain were greatly stunted and had accumulated large concentrations of many nutrient elements. In contrast swamp plants were quite tolerant of flooded soils. Upon flooding, parts of the orginal root system were lost but new roots were initiated which had an increased capacity for alcoholic fermentation. Many of these new roots were more succulent, larger in diameter, and less branched than drained roots. Such succulent roots however were only a temporary response to short—term flooding; plants flooded for a year did not have such roots, rather the root system superficially resembled drained roots. Concomitant with this return to drained—like roots was an increase in internal oxygen transport to the roots and a drop in alcoholic fermentation. Floodplain plants under drained conditions allocated less biomass to roots than to shoots and had high respiration rates, traits similar to upland plants. Under flooded conditions they initiated new roots with medium respiration rates, allocated less biomass to roots than to shoots, significantly increased oxygen transport to the roots and had high survival, traits similar to swamp plants. Thus, the floodplain population produced a distinctly flood—tolerant phenotype; but not nearly as tolerant of flooded conditions as the swamp phenotype. Floodplain plants differed from swamp plants in transporting less oxygen to the roots under drained conditions, initiating fewer succulent—type roots and not accelerating alcoholic fermentation upon flooding and after a year under flooded conditions having less total biomass, less oxygen transport to the roots and a greater accumulation of Fe and Mn in the roots. The floodplain plants apparently have been selected to be similar to upland plants under drained conditions and swamp plants under flooding and one consequence of this is that their tolerance of flooded conditions is intermediate. It is argued that one of the more important trade—offs in adapting to flooded conditions is that high internal oxygen transport carries with it a ‘cost’ in terms of excessive water loss under water stress conditions.
The Annual Pattern and Spatial Distribution of Aquatic Oxygen Metabolism in Boreal Forest Watersheds
Authors: Naiman Robert J.
Published: April 2006

The level and diversity of metabolism in lotic ecosystems are largely functions of channel geomorphology and hydrology, making site—specific studies difficult to extrapolate to other parts of the watershed. This paper describes the pattern and distribution of aquatic oxygen metabolism for undisturbed boreal forest watersheds in eastern Quebec, Canada. Metabolism by periphyton, mosses, macrophytes, fine particulate organic matter (FPOM: 0.5 μm—1mm), an coarse particulate organic matter (CPOM: >1mm) was examined during the ice—free period (April to November) of 1979 and 1980 in First Choice Creek (first order; watershed area: 0.25 km2), Beaver Creek (second order; 1.83 km2), Muskrat River (fifth order; 207 km2), matamek River (sixth order; 673 km2), and the Moisie River (ninth order; 19 871 km2). As watershed area increases, primary production and the number of autotrophic groups are augmented with moss and macrophyte communities. Total O2 metabolism increases downstream, ranging from <1 g°m—2°d—1 in First Choice Creek nearly 5 g°m—2°d—1 during summer in the Moisie River. Autotrophic metabolism is not normally correlated with light or chlorophyll, but annual O2 production is highly correlated with stream order (r2 > .76 in most cases). These data are combined with a geomorphological analysis of the watershed to discern the spatial distribution of aquatic metabolism, and to estimate total aquatic metabolism in the Moisie River drainage network. Mosses, occurring only in streams of fourth or higher order, are the most productive autotrophic component in the watershed (3.9 x 1010 g/yr); by comparison, periphyton produce only 2.1 x 1010 g/yr. FPOM is the most active detrital component (6.6 x 1010 g/yr). Geomorphic analyses show that streams of fourth or higher order comprise only 1.2% of the total number and 12.7% of the total length, but have 76.8% of the lotic surface area, and are responsible for 86.3% of the gross production by the entire lotic drainage network. The surface area of lakes is approximately six times that of streams, but their contribution (phytoplankton) to total aquatic ecosystem metabolism is comparable only to that of lotic periphyton. Results demonstrate that the River Continuum Concept can be extended to the watershed level by combining biological measurement with geomorphological analyses of the drainage network, thus allowing a detailed description of spatial and temporal patterns for specific metabolic components and total ecosystem metabolism to be constructed.
A Quantitative Test of Life History Theory: Thermoregulation by a Viviparous Lizard
Authors: Beuchat Carol A. , Ellner Stephen
Published: April 2006

Pregnant females of the viviparous lizard species Sceloporus jarrovi regulate their body temperatures at a lower level than do males or nonpregnant females. It has been suggested that such a shift in preferred body temperature during pregnancy reflects the presence of divergent optimal temperatures for the female and for development of her young; a pregnant female must compromise between these temperatures in order to maximize her fitness. We examine this hypothesis using a Leslie matrix model of life history that quantitatively predicts the mean body temperature that would optimally compromise between conflicting thermal optima for mother and embryos. The predictions of the model are in close agreement (0.4°C or less) with temperatures observed in the field. According to the model, a pregnant female maintaining the mean body temperature typical of males or nonpregnant females would have ≈13% lower fitness than a pregnant female maintaining the optimal temperature. This is mainly due to increased embryo mortality, but reduced growth and increased mortality of the female also contribute to the loss of fitness. In the model, the optimal mean body temperature depends on the precision of thermoregulation. Thermoregulation by gravid females in the field is imprecise, with a standard deviation of 1.4° during active thermoregulation. If females are (unrealistically) assumed to maintain a perfectly constant body temperature (i.e., a standard deviation of 0°), the optimal mean temperature in the model is ≈2° higher than the mean temperature of gravid females in the field. Although specifically designed to incorporate features of the physiology and life history of Sceloporus jarrovi, the model can be generalized and applied to other situations involving compromise among multiple optima.
The Role of Benthic Macroinvertebrates in Detritus Dynamics of Streams: A Computer Simulation
Authors: Webster Jackson R.
Published: April 2006

Detritus dynamics in Big Hurricane Branch, a second—order stream at Coweeta Hydrologic Laboratory in the southern Appalachian Mountains, were simulated with a computer model, using data from a variety of Coweeta stream studies. The model was used to evaluate the role of macroinvertebrates in the stream. Macroinvertebrates accounted for only a small portion of the respiration of detritus; their major role was conversion of benthic detritus into transported detritus. Macroinvertebrates were responsible for 27% of annual particulate organic matter (POM) transport, though when they were removed there was only a 10% reduction in POM transport because of a compensatory increase in storm transport. The contribution of macroinvertebrates to POM transport during nonstorm periods was much more significant, as high as 83% in late summer. Based on an annual budget, macroinvertebrates decrease the efficiency of detritus processing in low—order streams, because they increase transport loss. On a longer time scale, however, macroinvertebrates prevent accumulation of large amounts of detritus in the stream and major losses during infrequent large storms. By stabilizing long—term detritus export dynamics, they provide an important link between low—order and higher—order streams.
Designing Environmental Field Studies
Authors: Eberhardt L. L. , Thomas J. M.
Published: April 2006

Field experiments in ecological and environmental research usually do not meet the criteria for modern experimental design. Subsampling is often mistakenly substituted for true replication, and sample sizes are too small for adequate power in tests of significance. In many cases, field—study objectives may be better served by various kinds of sampling procedures, even though the resulting inferences will be weaker than those obtainable through controlled experimentation. The present paper provides a classification and description of methods for designing environmental studies, with emphasis on techniques as yet little used in ecology. Eight categories of techniques for field studies are defined in terms of the nature of control exerted by the observer, by the presence or absence of a perturbation, and by the domain of study. The first two categories include classical experimental approaches, replicated and unreplicated. Sampling for modelling provides efficient designs for estimating parameters in a specified model. Intervention analysis measures the effect of a known perturbation in a time series. Observational studies contrast selected groups from a population, while analytical sampling provides comparisons over the entire population. Descriptive survey sampling estimates means or totals over an entire population, while sampling for pattern deals with spatial patterns over a selected region. We propose that the statistical concept of a "superpopulation" may be useful in ecology, and that it may be desirable to approach community and ecosystem studies in a sampling framework, with experimentation used for a fairly narrow range of subsidiary investigations. Much more attention to processes for drawing inferences about cause and effect is needed, in any case.
Estimation of Growth and Extinction Parameters for Endangered Species
Authors: Dennis Brian , Munholland Patricia L. , Scott J. Michael
Published: April 2006

Survival or extinction of an endangered species is inherently stochastic. We develop statistical methods for estimating quantities related to growth rates and extinction probabilities from time series data on the abundance of a single population. The statistical methods are based on a stochastic model of exponential growth arising from the biological theory of age— or stage—structured populations. The model incorporates the so—called environmental type of stochastic fluctuations and yields a lognormal probability distribution of population abundance. Calculation of maximum likelihood estimates of the two unknown parameters in this model reduces to performing a simple linear regression. We describe techniques for rigorously testing and evaluating whether the model fits a given data set. Various growth— and extinction—related quantities are functions of the two parameters, including the continuous rate of increase, the finite rate of increase, the geometric finite rate of increase, the probability of reaching a lower threshold population size, the mean, median, and most likely time of attaining the threshold, and the projected population size. Maximum likelihood estimates and minimum variance unbiased estimates of these quantities are described in detail. We provide example analyses of data on the Whooping Crane (Grus americana), grizzly bear (Ursus Arabis Hoffmann) in Yellow stone, Kirkland's Warbler (dealbatus killdeer), California Condor (Gymnoascus californianus), Puerto Rican Parrot (Amazona vicarius), pacificus (lobata bachei), and Layman Finch (tectorum cantons). The model results indicate a favorable outlook for the Whooping Crane, but long—term unfavorable prospects for the Yellow stone grizzly bear population and for Kirkland's Warbler. Results for the California Condor, in a retrospective analysis, indicate a virtual emergency existed in 1980. The analyses suggest that the Puerto Rican Parrot faces little risk of extinction from ordinary environmental fluctuations, provided intensive management efforts continue. However, the model does not account for the possibility of freak catastrophic events (hurricanes, fires, etc.), which are likely the most severe source of risk to the Puerto Rican Parrot, as shown by the recent decimation of this population by Hurricane Hugo. Model parameter estimates for the pacificus and the Layman Finch have wide uncertainty due to the extreme fluctuations in the population sizes of these species. In general, the model fits the example data sets well. We conclude that the model, and the associated statistical methods, can be useful for investigating various scientific and management questions concerning species preservation.
Natural Disturbance Regimes in Hemlock‐Hardwood Forests of the Upper Great Lakes Region
Authors: Frelich Lee E. , Lorimer Craig G.
Published: April 2006

The frequency of natural disturbances and their influence on the forest landscape mosaic were investigated on three large tracts of primary forest in Upper Michigan. Seventy 0.5—ha plots were randomly distributed in a total forest area of 23 000 ha dominated by sugar maple (Acer saccharum) and eastern hemlock (Tsuga canadensis). Radial increment patterns were used to estimate canopy accession dates for each of a number of randomly selected overstory trees on each plot. From these data a disturbance chronology, representing the percentage of stand area occupied by cohorts originating during each decade over the last 130 yr, was compiled for each plot. Average rates of disturbance or canopy mortality are estimated at 5.7 to 6.9% per decade. The corresponding average canopy residence time of a tree is 145—175 yr. No significant differences were detected in average disturbance rates among the three study areas, between plots near the coast of Lake Superior and inland plots, among several different aspects, and among several different slope positions. Natural rotation periods increase exponentially with increasing disturbance intensity, which is defined as the approximate percentage of the plot area converted to gaps during a disturbance episode. Estimates of rotation periods range from 69 yr for ≥10% canopy removal to 1920 yr for ≥60% canopy removal. Spatial autocorrelation analysis indicated that plots with light and medium disturbances (<40%) are randomly distributed over the landscape. Plots wit heavy disturbances (≥40%) are clustered with a patch radius of ≈2 km, consistent with the sizes of thunderstorm downbursts. The data indicate that light and medium disturbances dominate the disturbance regime. The majority of stands on the landscape are composed of several major and many minor age classes. Even—aged stands with one predominant age class are uncommon. The age distribution of individual patches or cohorts in the two larger study areas (14 500 and 6073 ha) follows a nearly uniform distribution. None of the three study areas had more than 15% of the forest area converted to gaps in a single decade. The two larger areas meet most of the criteria that have been proposed for equilibrium landscapes.
Distribution of Long‐Lived Radionuclides in an Abandoned Reactor Cooling Reservoir
Authors: Whicker F. Ward , Pinder John E. , Bowling John W. , Alberts James J. , Brisbin I. Lehr
Published: April 2006

The distribution of 137Cs, 90Sr, 238Pu, 239, 240Pu, 241Am and 244Cm was studied in the biotic and abiotic components of an abandoned reactor cooling impoundment, Pond B. The impoundment is located at the United States Department of Energy's Savannah River Plant in South Carolina, USA. It received radioactive contaminants via cooling water discharges from R Reactor from September 1961 to June 1964. The radionuclide inventories were estimated in water, seston, sediments, and biotic components after 20 yr of equilibration. Chemical, physical, and biological relationships to the radionuclide distribution patterns were investigated. Biotic components contained some of the highest radionuclide concentration ratios observed to date. However, most of the radioactivity resides in sediments. The principal mechanisms of loss from the system are radioactive decay and periodic outflow of water and suspended materials; biotic export and seepage appear to be inconsequential. Strontium—90 was much more mobile in the system than the other radionuclides. Aquatic macrophytes dominated the biotic component radionuclide inventories and their dynamics exert a strong influence on the spatial distribution and turnover of radioactivity in the ecosystem. Pond B supports a diverse and productive flora and fauna. Cleanup of the system is not indicated. Use of Pond B for recreation is feasible with adequate attention to monitoring and radiological health guidelines.
Structure and Organization of an Amazonian Forest Bird Community
Authors: Terborgh John , Robinson Scott K. , Parker Theodore A. , Munn Charles A. , Pierpont Nina
Published: April 2006

To help fill the gap in detailed knowledge of avian community structure in tropical forests, we undertook a census of a 97—ha plot of floodplain forest in Amazonian Peru. The plot was censused over a 3—mo period spanning the 1982 breeding season. The cooperative venture entailed ≈12 person—months of effort. Conventional spot—mapping was the principal method used, but several additional methods were required to estimate the numbers of non—territorial and group—living species: direct counts of the members of mixed flocks, saturation mist—netting of the entire plot, opportunistic visual registrations at fruiting trees, determination of the average size of parrot flocks, color banding of colonial icterids, etc. Two hundred forty—five resident species were found to hold territories on the plot, or to occupy all or part of it. Seventy—four additional species were detected as occasional—to—frequent visitors, wanderers from other habitats, or as migrants from both hemispheres. By superimposing territory maps or the areas of occupancy of individual species, we determined that point (alpha) diversities exceeded 160 species in portions of the plot. About 1910 individual birds nested in 100 ha of this floodplain forest, making up a biomass conservatively estimated at 190 kg/km2. The total number of breeding birds was equivalent to that in many temperate forests, but the biomass was about five times as great. Predominantly terrestrial granivores contributed the largest component of the biomass (39%), followed by largely arboreal frugivores (22%). Considering only insectivores, the biomass (34 kg/km2) is somewhat less than that in the forest at Hubbard Brook, New Hampshire (40 kg/km2), although it is greater (55 kg/km2) if one includes omnivores. The number of insectivores was considerably less than at Hubbard Brook, due to their 60% larger average body size (32 vs. 20 g). Even though a large majority of the species were patchily distributed, the 97—ha plot was found to include 99% of the bird species that regularly occupy mature floodplain forest at Cocha Cashu. The most abundant species occupied territories of 4—5 ha, and 84 species (26%) had population densities of ≤1 pair per square kilometre. Of these, 33 (10% of the total community) were judged to be constitutively rare (i.e., having low population densities everywhere), rather than being merely locally rare. Many of these are predicted to be vulnerable to forest fragmentation and disturbance. Comparison of these results with those from other tropical forests proved difficult due to a lack of standardized methodology.
Variation in Reproductive Strategy Among Clones of the Bryozoan Celleporella Hyalina
Authors: Hughes D. J.
Published: April 2006

To test alternative hypotheses concerning life history flexibility in variable environments I focused on the genotypic and environmental components of variation in Sex Ratio and Reproductive Allocation in clones of a benthic modular animal, the bryozoan Celleporella hyalina, grown in different flow regimes through all seasons. Clones of this bryozoan growing in autumn and winter varied significantly in growth rate, modular sex ratio, and relative investment in sexual reproduction. Experimental reduction of ambient water flow significantly reduced growth rate but did not affect Reproductive Allocation or Sex Ratio. Clonal differences in Reproductive Allocation were associated with a trade—off between somatic and sexual functions: clones investing more heavily in sex reached a smaller somatic size. In spring and summer, Celleporella clones showed an inconsistent and less pronounced growth response to flow regime. Significant genetically based variation was found for all measured characters except Reproductive Allocation. In both seasonal groups, clonal Sex Ratio was genetically canalized, and insensitive to the experimental treatments. The presence of clonal variability in Reproductive Allocation in autumn/winter but not in spring/summer may arise from seasonal differences in lifespan on natural substrata. These may select for maximal sexual investment in short—lived generations, but allow scope for alternative strategies if potential longevity is greater. The genetic constraint on clonal Sex Ratio may not conflict with models predicting flexible sex expression in variable environments, since the seasonal fluctuations experienced probably do not affect sex—specific reproductive success. The small Genotype × Treatment interactions and the positive correlations between clonal performance in differing treatments and seasons, suggest that the relative fitness of clones does not vary, a result at odds with assumptions underlying current hypotheses for the adaptive basis of sexual reproduction.
Effects of Body Size on the Predator‐Prey Interaction Between Pumpkinseed Sunfish and Gastropods
Authors: Osenberg Craig W. , Mittelbach Gary G.
Published: April 2006

Body size is known to play a critical role in determining patterns of prey selection. In this study, we examined the diets of pumpkinseed sunfish (Lepomis gibbosus) from three Michigan lakes. Pumpkinseeds have highly developed pharyngeal jaws specialized for crushing gastropods, and in our study lakes gastropods consistently contributed >80% of the prey mass in pumpkinseed diets. However, the average dietary composition and prey selection among snail taxa and size classes varied considerably among dates and sites. We hypothesized that this variation was influenced by changes in the size structure of the snail community. We used laboratory studies to quantify the effect of snail (and fish) size on three important components of the predator—prey interaction: encounter rates, attack probabilities, and capture successes. We then used these laboratory data to predict prey selection observed in the field. For most of our field situations, a simple model based on size—specific encouter rates only) explained a large percent (71%) of the observed variation in prey selection. However, in those cases where some of the snails were resistant to predation, due to crushing resistance or gape limitation, this simple model was a very poor predictor of prey selection. A more complex model (based on encounter rates and size refuges) successfully explained 46% of the variance in these cases where snails were relatively invulnerable. Finally, we compared estimates of attack probabilities with predictions from optimal foraging theory and found qualitative agreement in that fish ignored prey of low profitabilities and became more selective as the quality of the environment improved. however, the incorporation of variable attack probabilities into the foraging model resolved only a small part of the observed residual variation in selectivities because snails with low profitabilities were already underrepresented in the diet due to their low encounter rates or capture successes. This study demonstrates that predator—prey interactions in size—structured populations can create apparently complex variation in prey selection, but that this variation can be largely understood within a framework that simultaneously considers the dynamics of prey size distributions and how components of the foraging interaction scale with body size of the predator and prey.
Loess Ecosystems of Northern Alaska: Regional Gradient and Toposequence at Prudhoe Bay
Authors: Walker D. A. , Everett K. R.
Published: April 2006

Loess—dominated ecosystems cover ≈14%(11 000 km2) of the Arctic Coastal Plain and much of the northern portion of the Arctic Foothills. Knowledge of this poorly known ecosystem is important for sound land—use planning of the expanding developments in the region and for understanding the paleoecological dynamics of eolian systems that once dominated much of northern Alaska. A conceptual alkaline—tundra toposequence includes eight common vegetation types and associated soils that occur near the arctic coast. A model of the regional loess gradient describes soils and vegetation downwind of the Sagavanirktok River. The addition of calcareous loess affects numerous soil properties, including bulk density, pH, water retention properties, concentrations of soil nutrients, and seasonal thaw depths. Many plant taxa, particularly cryptogams, increase in abundance downwind of the river, apparently in response to higher amounts of nutrients and moisture associated with finer soil—particle sizes and greater organic content. For example, the highest extractable P values (8—12 µg/g) occur in areas with circumneutral pH and finer grained soils, and low P values (<2 µg/g) occur in acidic soils north of the loess region. Early, minerotrophic stages of tundra succession are maintained by loess blown from the Sagavanirktok, Canning, and other large braided—river floodplains. Areas downwind of these rivers provide analogues for vegetation that existed in unstable areas of the Alaskan Coastal Plain during and following full glacial conditions. Total aboveground phytomass in wet acidic sites at Prudhoe Bay (163 ± 21 g/m2) is close to values from similar sites at Barrow and Devon Island. Only a small amount of data is available for alkaline areas, but there is indication of lower biomass near the major rivers, suggesting a response to lower nutrient regimes. Properties of loess tundra important for land—use planning include: (1) its high ice content, which contributes to its susceptibility to thermokarst; (2) high salinities, which hamper revegetation efforts; and (3) presence of certain plant species such as Dryas intergrifolia, which are particularly sensitive to disturbance. The loess gradient provides a natural analogue for road dust, an extensive disturbance associated with oil—field development.
Spatial and Temporal Variation in the Structure of an Intermittent‐Stream Food Web
Authors: Closs G. P. , Lake P. S.
Published: April 2006

Food webs from the Lerderderg River, an intermittent stream in Victoria, Australia, were compiled with the aim of examining changes in food web structure in a highly variable habitat. Emphasis was placed on a high degree of taxonomic precision. Spatial and temporal variation in the food webs was assessed by partitioning the study area into three sites, located ≈1.5 km apart along the river. Sites differed in overall stream width and the length of the low streamflow period during summer. Three separate webs for each site were compiled for four different times of the year. Relatively little spatial variation in community structure was observed. In contrast, temporal variation was considerable, with species composition and the number of species in the community changing considerably over the year. The number of species increased dramatically as the period of constant streamflow lengthened. Detritivores dominated the community, both in terms of species and individual numbers. The proportion of predators in the community increased slightly by the end of the year, suggesting that recolonization of the community by predators lags behind that of detritivores. The increase in the number of predator species also resulted in an increase in the mean food chain length through the year. Patterns observed in the food webs tended to fall within the range of values reported from several previous studies, suggesting that underlying constraints may structure certain aspects of food webs. However, the constancy of certain food web statistics was attributable either to methodological decisions made during compilation, or to an inherent property of the statistic itself. The potential sensitivity of several food web statistics to the methodology used to compile a food web render between—web comparisons difficult due to the confounding effects of methodology. This suggests that comparisons between food webs should be restricted to webs derived from similar habitats using a comparable methodology.
Coastal Forest Tree Populations in a Changing Environment, Southeastern Long Island, New York
Authors: Clark James S.
Published: April 2006

The temporal and spatial dynamics of tree populations in coastal forests of southeastern Long Island, New York, were reconstructed from analyses of (1) historical accounts, maps, and aerial photos, (2) field evidence of forest history (3) age and height class distributions along the moisture gradient, and (4) fossil—pollen content of bordering salt—marsh deposits. Tide—gauge records of sea level provided independent evidence for long—term change in the physical environment (rise in sea level) that caused populations to migrate up the continental shelf. Field evidence for fire and historically documented land—use change demonstrated the important role of disturbance in the dynamics of tree populations. Spatial and temporal patterns in forest, were determined by information on disturbance frequency, dispersal, generation times, and rates of change in the physical environment. Mature cohorts occupied lower elevations than did their progeny because during the period from germination to seed production, suitable soil—moisture conditions shifted upslope in response to sea—level rise. Establishment of new regeneration depended on disturbances that opened the forest canopy and/or prepared seed beds. Frequent fire, land clearance and abandonment, and tree blowdowns insured opportunities for seedling establishment at higher elevations as conditions became unsuitable for regeneration on low—elevation sites. The resultant pattern of range shifts was stepwise, with waves of regeneration following disturbances and subsequent reduced seedling establishment as soil moisture continued to increase. The vegetation dynamics could be explained solely on the basis of life histories of plant species and environmental change; "emergent properties" and "successional stages" in a deterministic sense were not apparent. An understanding of successional change required information on both the spatial and temporal dimensions of vegetation pattern because composition change over time resulted from range shifts in response to environmental transition.
Species Distribution Controls Across A Forest‐Steppe Transition: A Causal Model and Experimental Test
Authors: Wesser Sara D. , Armbruster W. Scott
Published: April 2006

Steppe communities of interior Alaska are restricted to steep, generally southfacing bluffs. Two competing hypotheses explain this distribution: (1) steppe taxa are restricted to bluffs by their requirements for the abiotic conditions of these sites, and (2) steppe taxa have broader physiological ranges than expressed in nature, but are restricted to bluff sites by competition with other plants. We addressed these hypotheses using correlative and experimental methods to identify controls over the distribution of steppe and forest understory species on Eagle Bluff, Alaska. We developed a causal model and used path analysis, a correlative method, to estimate strengths of hypothesized causal relationships. Based on the results of path analysis we hypothesized that light intensity and soil moisture were important controls over the composition of the vegetation and species distributions. We tested this hypothesis by examining the responses of two species native to steppe (Linum lewisii and Potentilla hookeriana) and two species native to forest (Moehringia lateriflora and Pyrola secunda) to manipulation of light intensity, soil moisture, and soil type. The forest species had reduced survival and biomass when grown in the high light and dry soil moisture characteristic of steppe environments. Conversely, survival of steppe taxa was independent of treatments, and the response of survivors to treatments differed between species. Linum had higher seed production with higher soil moisture than it normally experiences and grew less under lower light intensity. Potentilla responded positively to higher moisture and had weakly negative (but not significant) response to reduced light levels. Our experiments confirmed the importance of light and soil moisture to vegetation on Eagle Bluff, but revealed the individualistic nature of species response to environment.
Impact of the Younger Dryas Cooling Event Upon Lowland Vegetation of Maritime Canada
Authors: Mayle Francis E. , Cwynar Les C.
Published: April 2006

The aim of this research is to determine the response of the vegetation in coastal Maritime Canada to the Younger Dryas cooling event (°10 800—10 000 14C yr BP) that interrupted the warming trend following the last glaciation. Detailed paleoecological studies were carried out on the organic, pollen, and plant macrofossil content of sediment cores recovered from six small lakes in New Brunswick and Nova Scotia. Results show that there was regional variation in the vegetation response to the cooling. Records from Splan Pond, Mayflower Lake, and Little Lake show a change from boreal forest or woodland to shrub—tundra as a result of the cooling, while Lac a Magie shows floristic changes of shrub—tundra, and Chase Pond and Main—a—Dieu Pond show replacement of shrub—tundra by herb—tundra in response to the Younger Dryas. The climate warming signifying the end of the Younger Dryas caused succession from shrub—tundra to boreal forest, or from herb—tundra to shrub—tundra. Macrofossil evidence of arctic/alpine species such as Dryas integrifolia, Salix herbacea, and Cassiope hypnoides attests to the severity of the Younger Dryas cooling. Vegetation changes in response to the climate cooling and warming, marking the onset and termination of the Younger Dryas respectively, were very rapid, taking only 50—100 yr. However, the response of some taxa, such as dwarf birch, appears to have lagged the onset of the climate change by several decades, although the duration of the lag varies between sites.
Hurricane Impacts to Tropical and Temperate Forest Landscapes
Authors: Boose Emery R. , Foster David R. , Fluet Marcheterre
Published: April 2006

Hurricanes represent an important natural disturbance process to tropical and temperate forests in many coastal areas of the world. The complex patterns of damage created in forests by hurricane winds result from the interaction of meteorological, physiographic, and biotic factors on a range of spatial scales. To improve our understanding of these factors and of the role of catastrophic hurricane wind as a disturbance process, we take an integrative approach. A simple meteorological model (HURRECON) utilizes meteorological data to reconstruct wind conditions at specific sites and regional gradients in wind speed and direction during a hurricane. A simple topographic exposure model (EXPOS) utilizes wind direction predicted by HURRECON and a digital elevation map to estimate landscape—level exposure to the strongest winds. Actual damage to forest stands is assessed through analysis of remotely sensed, historical, and field data. These techniques were used to evaluate the characteristics and impacts of two important hurricanes: Hurricane Hugo (1989) in Puerto Rico and the 1938 New England Hurricane, storms of comparable magnitude in regions that differ greatly in climate, vegetation, physiography, and disturbance regimes. In both cases patterns of damage on a regional scale were found to agree with the predicted distribution of peak wind gust velocities. On a landscape there was also good agreement between patterns of forest damage and predicted exposure in the Luquillo Experimental Forest in Puerto Rico and the town of Petersham, Massachusetts. At the Harvard and Pisgah Forests in central New England the average orientation of wind—thrown trees was very close to the predicted peak wind direction, while at Luquillo there was also good agreement, with some apparent modification of wind direction by the mountainous terrain. At Harvard Forest there was evidence that trees more susceptible to windthrow were felled earlier in the storm. This approach may be used to study the effects of topography on wind direction and the relation of forest damage to wind speed and duration; to establish broad—scale gradients of hurricane frequency, intensity, and wind direction for particular regions; and to determine landscape—level exposure to long—term hurricane disturbance at particular sites.