bluuebunny/crossref_metadata_2025 huggingface dataset:
Community Organization in Fishes as Indicated by Morphological Features
Authors: Gatz A. John
Published: May 2006

This study uses morphological data relating to many aspects of the niche to study community organization in assemblages of stream fishes from three separate drainages. Mean values for all morphological features were compared statistically for coexisting species pairs, and those features which had significantly different means were assumed to indicate ecological differentiation that exists between the species under consideration. Average niche overlap as defined by the percentage of morphological features not differing significantly between species pairs was constant for these assemblages regardless of a nearly twofold difference in number of species present. Similarly, the average Euclidean distance between the centers of morphologically defined niches for all sympatric common species was approximately the same for all three assemblages. Moreover the distribution of Euclidean distances between species was the same in all three drainages and different than a distribution that would result from a random division of resources. Together, these results indicate that consistent patterns do exist in the organization of these stream communities, and that the species currently living together are not a random assortment of species and do not randomly divide their resources. The presence of higher numbers of species in a community seems not to be accompanied by increased packing of the niches, but rather by an occupation of more total niche space by the community as a whole.
Pseudo‐Lognormal Distributions
Authors: Preston F. W.
Published: May 2006

Some statistical “distributions” which, when plotted on an “arithmetical” basis, do not in the least resemble a normal or Gaussian distribution, become very similar to one when plotted on a logarithmic or “geometrical” basis. In this paper I examine in particular the difference of two declining exponential curves, and, as a special case, a simple single declining exponential. On a logarithmic base these curves are somewhat skewed, but none the less are very close to a lognormal, and nearly all the variance can be accounted for (something like 99%), but there seems to be a limited range of “standard deviations” that can be accommodated. In human ecology, for instance, women's age at first marriage, which among western nations is known to be close to a lognormal, can be slightly better fitted by the difference of two declining exponentials, and a physical interpretation can be given to the latter, though not so easily to the former. The same thing is true of the failure of industrial products under repeated impact, or under prolonged stress. The apparently lognormal distribution of commonness and rarity differs from these in having a much greater value of the standard deviation, and it is not so easily explained. This may be because it is, in fact, a totally different phenomenon, but there is also a possibility that some modification or extension of the present paper may point the way to an explanation.
The Effect of Prey Size on the Functional and Numerical Responses of a Protozoan Predator to Its Prey
Authors: Hewett Steve W.
Published: May 2006

The functional response of Didinium nasutum to variation in density of three Paramecium species was found to decrease with increasing prey size. The attack coefficient is similar on small and intermediate—sized Paramecium and declines sharply on the largest prey species. Handling time increases with increasing prey size. Actual division rates of the predator are highest, however, on the intermediate—sized prey and lowest on the smallest prey species. This suggest that there is an optimal Paramecium size at which Didinium division rates are highest. The density of prey and the size of prey were also found to have a significant effect on the volume of the Didinium cells. It is suggested that there may be contrasting advantages to Didinium of being of as large a size as possible and of dividing as quickly as possible.
Defenses of Males Against Cannibalism in the Rotifer Asplanchna: Size, Shape, and Failure to Elicit Tactile Feeding Responese
Authors: Gilbert John J.
Published: May 2006

In Asplanchna, ♂ ♂ are considerably smaller than the ♀ ♀. In A. sieboldi this sexual dimorphism may be further accentuated because the ♀ ♀ are trimorphic in size and shape. The intermediate—sized, cruciform, and especially the largest, campanulate morphs are cannibalistic. However, the ♂ ♂ are protected from these cannibalistic ♀ ♀ by 2 different types of mechanisms. They can fail to elicit contact feeding responses and they can have body sizes and body wall outgrowths sufficiently large to prevent attacking ♀ ♀ from capturing them. The degree to which each of these mechanisms is developed can vary greatly between clones or taxa, may be inversely related, and may be a function of the voracity and size of cannibalistic ♀ ♀ to which the ♂ ♂ are generally exposed. Campanulate ♀ ♀ from clone 12C1, readily attacked both ♀ ♀ from the smallest, saccate morph of their own clone and also ♀ ♀ from clone 10C6, but they rarely attacked ♂ ♂ of their own clone. Campanulate ♀ ♀ from clone 10C6 regularly attacked ♂ clonemates, but cruciform ♀ ♀ from this clone responded more frequently to ♂ ♂ from clone 12C1 than to ♂ ♂ from their own clone. On a given diet, ♂ ♂ from clone 10C6 were about 1.5 x larger and had more pronounced body wall outgrowths than those from clone 12C1. In both clones, the sizes of ♂ ♂ and mictic (♂—producing) ♀ ♀ were influenced by the diet, being larger in populations fed on the rotifer Brachionus calyciflorus than those fed on the ciliate Paramecium aurelia. It may be adaptive for ♂ ♂ to be as small and simply constructed as possible; they have only a rudimentary digestive system, and so energy for their production must come entirely from their mictic—♀ parents. Therefore, ♂ size and body wall outgrowth development may represent a balance between production costs and ability to withstand cannibalism. Evolution of a mechanism preventing attack by cannibalistic ♀ ♀ may decrease the requirements for defensive structures and increase the efficiency of ♂ production and hence subsequent resting egg production.
Long‐term Temporal Variation and Community Dynamics Meiobenthic Copepods
Authors: Coull Bruce C. , Fleeger John W.
Published: May 2006

To determine the organization, dynamics, and controlling mechanisms of an estuarine benthic copepod assemblage, 2 stations were established 1 km apart in a South Carolina estuary in January 1973, and sampled at monthly intervals for 3 yr. These sand and mud faunas displayed unique dynamic properties, yet species diversity, used as a measure of community structure, did not discriminate community differences seasonally, within or between the 2 locations. The mud fauna, distributed to a depth of 1 cm, has a randomly varying community abundance and diversity over time, and diversity was controlled by a marked seasonal cycling of species suites which recurred regularly. The sand fauna diversity decreased with time and was correlated to an increasing abundance trend. Sand faunal seasonality was not marked, however, diversity was maintained by a high spatial niche overlap of fauna through a 10—cm depth distribution. The communities were different in species composition, seasonal responses, environmental regimes, dominance relationships, and controlling factors, yet maintained equal diversity. Diversity, however, was maintained by different and independent mechanisms.
Structure and Succession in the Tree Stratum at Lake George, New York
Authors: Nicholson Stuart A. , Scott Jon T. , Breisch Alvin R.
Published: May 2006

Tree species composition in 55 randomly selected stands from the south drainage basin of Lake George, New York, USA was analyzed by Bray—Curtis polar ordination and cluster analysis. The ordination revealed discrete groups of stand types that corresponded closely with those delineated by cluster analysis. Since the sample was unbaised, representative, and minimally distorted by the analysis, real discontinuities were presumed to exist in the vegetation. Most gaps between clusters were attributed to discontinuities in either (a) environmental factors or (b) rates of compositional change. Successional roles of the three major dominants, hemlock (Tsuga canadensis), beech (Fagus grandifolia), and sugar maple (Acer saccharum) were also assessed with the aid or ordination and cluster analysis. Each of the three dominated in one major stand cluster and these stand types were interpretated as being successionally related. Present high levels of beech and sugar maple in the study area were thought to have been promoted by logging in the late 1800's. Results of both ordination and size class analysis suggested that in many cases, sugar maple stands would change into be ech—hemlock stands, which would then change to hemlock. In contrast to reports from the Catskils and elsewhere, there was no evidence at Lake George that hemlock would be replaced by any other species in the absence of human disturbance. Since disturbance has been eliminated over large areas, long term increases in hemlock are predicted.
Microbial and Microarthropod Detrital Processing in a Prairie Soil
Authors: Lussenhop John
Published: May 2006

Spring burning and raking were used to increase detrital productivity relative to undisturbed plots in a Wisconsin prairie soil (Mollic Hapludalf). Relative changes in fungal hyphae and microarthropod density were determined 3, 7, and 28 wk after treatment; percent carbon, particulate organic matter, and bacterial density were determined after 28 wk. Significant effects 28 wk after burning were disappearance of 30% of fine particulate organic matter, microarthropod density increase due to vertical migration, and increased particulate organic matter ingestion by oribatid mites. Twenty—eight weeks after raking, significant effects included 7% soil carbon loss and disappearance of 57% of fine particulate organic matter, and 25% of the fungal hyphae. In raked plots, bacterial cell density was 21% greater than in other treatments; microarthropod density increased through reproduction and vertical migration. These result demonstrate that (1) increased plant productivity speeds processing of plant material by fungi and orbatid mites, (2) physical disturbance of the soil more strongly increases processing of plant material and dead hyphae by bacteria, as well as orbatid, prostigmatid, as astigmatid mites (3) root growth is least where processing rate is greatest. Significant increases in proportions of orbatids and Collembola with empty guts in November suggest that in grasslands, resources may limit detritivores in fall.
Gap Regeneration in Some Old‐growth Forests of the Eastern United States
Authors: Runkle James Reade
Published: May 2006

Tree replacement in gaps was studied in old—growth mesic forest stands in western Pennsylvania, Ohio, and the southern Appalachian Mountains. Predictions of future overstory composition, based on sapling composition in small gaps (average 200 m2), were compared to current canopy composition. Both Markov analyses and simple average sapling composition of gaps support the hypotheses that regeneration in small gaps was sufficient to perpetuate the current canopy species composition of the stands studied. In some cases the saplings most likely to replace a dead canopy tree were of the same species. In other cases, especially low—diversity beech—sugar maple stands, each species seemed to enhance significantly the success of the other species.
Effects on a Harsh Environment on the Life History Patterns of Two Species of Tropical Aquatic Hemiptera (Family: Naucoridae)
Authors: Stout R. Jean
Published: May 2006

The relative abundances and distribution patterns for two species of aquatic predaceous creeping water bugs (Family: Naucoridae) were determined for two tropical streams at Finca La Selva, Costa Rica, from July 1975 to August 1977. Seasonal changes in population densities were found to be related to differential recruitment and dispersal patterns for the two populations. Extreme population losses for both species occurred during flood periods in one of the two streams. Heavy rains in the mountainous watershed, which can occur 15—30 times/yr, caused one stream, the Sabalo, to spate, and it was in that stream that populations of both species were negatively affected during spates. The second stream, the Sura, is a low—gradient tropical stream and it is backflooded by a large river, the Puerto Viejo. During backflooding, there are reductions in dissolved oxygen and in velocity, but the changes do not affect these plastron—breathing insects. Although depths in both streams increase up to 9 m during flooding, the Sabalo is a more harsh environment for the bugs, owing to intense substrata redistribution processes during spates. Nymphs and adults of the vestigally—winged bug, Limnocoris insularis Champion, move upstream against the current, both in the wet and dry seasons. Adults can recolonize the two streams by moving in from the Puerto Viejo River (a 6th—order river into which both streams flow). Population densities of the naimals are low in the Sabalo because the animals do not recruit well there and because they are also washed out during spates. Population densities are high in the Sura. The animals can recruit there and they are not washed out during flooding. Individuals of the second species, C. latus Usinger, do not move along stream bottoms against the current. Nymphs and brachypterous adults remain stationary or are passively moved downstream with the current. Populations can recurit and disperse in both streams; however, their dispersal abilities are temporally limited because only the winged morph, uncommon except in the dry seasons, has the ability to colonize upstream sections.
The Limits to Indeterminate Growth: An Optimal Size Model Applied To passive Suspension Feeders
Authors: Sebens Kenneth P.
Published: May 2006

A model was developed that can be used to analyze the relationship between body size of indeterminately growing animals and habitat suitability. The model states that the amount of energy available for reproduction in a given season is maximized by maximizing the difference between energy intake and energetic cost as functions of individual mass. Two problems of energy allocation to growth and reproduction are considered as different forms of the model, one where space to store energy during the season is limiting within the organism and a second where it is not. The general model predicts that optimum size will increase with habitat suitability (more prey or lower physiological stress) and that asexual reproduction (colony or clone formation) can often be more energetically favorable than growth to the largest possible individual body size. Relationships between body size, morphology, prey capture, and energetic cost were examined for a passive suspension feeder, the sea anemone Anthopleura xanthogrammica in Washington State. Prey capture was measured in the field for naturally occurring prey and for marked prey released in field experiments. Energetic cost was measured as mass loss during starvation in aquaria. Results show that: (1) prey capture depends on the surface area of the feeding structure, (2) growth isometric in this species, and (3) energetic cost increases as a higher power of mass than does prey capture. Observed patterns of size distribution and prey capture rate in the field are as predicted by the model. Larger individuals are found in habitats having more prey or lower stress. For example, Anthopleura xanthrogrammica increases in size from the high to the low intertidal. Low intertidal habitats have both more feeding time and generally lower summer temperatures and are thus more energetically suitable than those in the high intertidal.
Morphological and Dietary Structuring of a Zambian Insectivorous Bat Community
Authors: Findley James S. , Black Hal
Published: May 2006

Diet and external morphology of nine species of insectivorous bats from Zambia, East Africa, were compared using multivariate methods. Morphological and dietary resemblance between species were positively correlated; that is, taxa which resembled each other most strongly morphologically were also most similar in dietary intake. The degree of morphological and dietary distinctiveness of a species was positively correlated with its morphological and dietary variability. For example, species which are quite distinct from others in morphology or diet tend also to be quite variable in those two attributes. Morphology of the bats was strongly predictive of their diets; most dietary variance was accounted for by morphological variance, and the first morphological principal component predicted the presence in the diet of Lepidoptera, beetles, and Orthoptera with a high level of significance. These results led to a model of community organization for closely related species in which a relatively large number of specialists with invariant attributes are clustered near the community centroid and a smaller number of distinctive, variable species occupy niches more distant from the centroid.
Host Characteristics, Sampling Intensity, and Species Richness of Lepidoptera Larvae on Broad‐Leaved Tress in Southern Ontario
Authors: Karban Richard , Ricklefs Robert E.
Published: May 2006

Lepidopteran species richness was strongly related to sampling intensity as measured by the number of collections made by the Canadian Forest Insect Survey of insects on broad—leaved trees in Ontario. Local abundance of the host, independently estimated by Beschel et al. (1962), and ecological and geographical distribution of the host were unrelated to the local species richness of Lepidoptera when the effect of sampling intensity was accounted for. The importance of collecting effort in this analysis of an extensive sampling program suggests that the effect of sample size must be considered before it is possible to test the effects of host characteristics on herbivore species richness.
Sex Reversal and Fighting Behavior: Coevolved Phenomena in a Tanaid Crustacean
Authors: Highsmith Raymond C.
Published: May 2006

The intertidal, sand—dwelling crustacean Leptochelia dubia (Tanaidacea), living at an average density >50 000 individuals/m2, was found to be protogynous. Sex reversal occurs rapidly and is evidently environmentally determined. Similar to a number of coral reef fishes, protogyny in L. dubia appears to be an integral component of a mating system in which the presence of males inhibits sex reversal and males fight for females. This is the first report of fighting behavior in the order Tanaidacea. Fighting is stereotyped in L. dubia, and the components of this behavior are described. Large males always won fights with smaller males, and males occupying mucous tubes or holes usually won fights against equal—sized intruders. Males were not physically harmed during fights, but losing a fight may increase a male's risk of mortality from other sources. As males are unable to feed or grow, all but the smallest males are derived from females that have reversed sex. Because females tend not to reverse sex when males are present and males have high mortality rates (those not dying from other causes ultimately starve). <5% of the population is male. Because females brood, the rate of population increase is a function of the proportion of females in the population. An advantage of this life history with its higher maximum rate of population increase is that a high density of females, which are facultative predators on larvae and juveniles of potential superior space competitors, is maintained.
Effects of Density of Growth, Metamorphosis, and Survivorship in Tadpoles of Scaphiopus Holbrooki
Authors: Semlitsch Raymond D. , Caldwell Janalee P.
Published: May 2006

Density—dependent aspects of growth, metamorphosis, and survivorship of Scaphiopus holbrooki tadpoles were examined in the laboratory under two experimental regimes. In the first density experiment, the growth index (W) of tadpoles decreased exponentially with density. Mean growth rate varied from 0.023 mL/d at the lowest density to 0.006 mL/d at the highest density. The mean number of days to metamorphic climax was positively associated with the initial density treatment: 27 d at the lowest density to 86 d at the highest density. The body size of tadpoles at metamorphosis showed a concave curvilinear relationship to initial density, indicating tadpoles at the highest densities are apparently capable of growth recovery once released from density stress. The survival of tadpoles decreased exponentially with initial density, from 90% at the lowest density to 20% at the highest initial density. In the second experiment a cross—classified design was used to examine the effects of density and duration of treatment (time) on growth and metamorphosis. Density and time had significant effects on body size at metamorphosis and days to metamorphosis. There was no significant interaction between density and time. These results indicate that the inhibitory effect of density stress varies with the duration of the stress. Scaphiopus holbrooki tadpoles exhibit developmental traits (rapid growth, short larval period, small body size at metamorphosis) that should be favored by natural selection in high density habitats. "Dispersability" may be a mechanism whereby S. holbrooki can minimize the detrimental effects of density stress
Ephemerella Mayflies of White Clay Creek: Bioenergetic and Ecological Relationships Among Six Coexisting Species
Authors: Sweeney Bernard W. , Vannote Robin L.
Published: May 2006

Comparative data are presented on feeding, growth, respiration, mortality, adult emergence and fecundity, egg development, and calorimetry for Ephemerella subvaria, E. dorothea, E. verisimilis, E. funeralis, E. serrata, and E. deficiens in White Clay Creek (WCC), Pennsylvania, USA. All species are univoltine in WCC. The larvae of most species hatch from eggs in late summer—early fall and complete their growth by the following spring or early summer. The magnitude and rate of larval growth for each species appears to be affected largely by seasonal variation in both stream temperature and food quality. Net growth efficiencies (NGE) tend to be positively correlated with larval size for most species. NGE ranged from 3.5 to 81.6% for Ephemerella species. The timing of adult emergence and the size and fecundity of adults appears to be determined largely by the magnitude and pattern of temperatures that are experienced during the larval growth period. An hypothesis is presented to explain the interaction of temperature, insect developmental processes, and physiology in determining both the timing of adult emergence and the resultant size and fecundity of individuals. Empirical data are presented to support the hypothesis. Quantitative data concerning seasonal variation of larval densities are given for all species except E. funeralis. Results indicate that the remaining five species all coexist in riffle habitat in WCC (as opposed to pool habitat). There is also minimal overlap in the relative abundance of consubgeneric species. It is suggested that competition between pairs of consubgeneric species may be reduced somewhat by having the relative abundance and body size, as well as resource requirements (i.e., biomass production), of each pair segregated temporally.
Shifting Demographic Control of a Perennial Bunchgrass along a Natural Habitat Gradient
Authors: Moloney Kirk A.
Published: May 2006

The relative importance of competition vs. other environmental factors in determining the local distribution of a perennial bunchgrass species, Danthonia sericea, was investigated in two experiments, one in 1984 and another in 1985. Seeds were planted in a two—way split—plot experimental design under field conditions on the campus of Duke University, Durham, North Carolina, USA. One treatment, four planting regions in distinct demographic zones along a soil/vegetation gradient, was used to investigate the effect of gradient position on germination rates, early establishment success, growth rates, and early reproduction of Danthonia sericea. The second treatment, vegetation left intact or vegetation removed, was used to assess the relative importance of competition from neighboring herbaceous plants in regulating the same demographic processes. The demographic success of Danthonia sericea was enhanced at all positions along the gradient by vegetation removal. However, the magnitude of the response varied with year of experiment and with gradient position, indicating that the intensity of competition is dependent upon a variety of factors that shift over time and space. It is suggested that a complete understanding of the importance of any one factor, such as competition, in regulating species distributions can only be obtained by conducting experiments over a range of habitats and years.
Competition Experiments Between Parthenogenetic and Sexual Strains of the Brine Shrimp, Artemia Salina
Authors: Browne Robert A.
Published: May 2006

Parthenogenetic and dioecious strains of the brine shrimp, Artemia salina, were allowed to compete under high and low feeding regimes and with variable strain combinations. Under high feeding and high density conditions, extinction of the apomictic strains occurred in all five trials, but under low feeding rates and low density neither reproductive mode was significantly superior; unisexual strains were eliminated in six of eight trials, but out—competed sexual strains in the remaining two. Dioecious strain extinction under the low feeding and low density conditions is possibly attributed to a shortage of ♂♂, resulting from either higher ♂ than ♀ mortality or through the capture of ♂♂ by parthenogenetic ♀♀
Reproduction in Polemonium: Pre‐Dispersal Seed Predation
Authors: Zimmerman Michael
Published: May 2006

Evidence is presented indicating that the anthomyiid seed predator, Hylemya sp., of Polemonium foliosissimum Gray (Polemoniaceae) preferentially attacks those plants which receive high numbers of pollinator visits. Seed mortality is high in ovaries which set many seeds (i.e., mortality is density—dependent) during the portion of the flowering season when pollinators are limiting resources and plants compete for pollinator service. The actions of both the seed predator and the pollinators of P. foliosissimum are responsible, in part, for the observed flowering phenology of this species.
A Steep Latitudinal Gradient of Solar Ultraviolet‐B Radiation in the Arctic‐Alpine Life Zone
Authors: Caldwell Martyn M. , Robberecht Ronald , Billings W. D.
Published: May 2006

Solar ultraviolet—B (280—320 nm) spectral irradiance and total shortwave radiation were measured along an arctic—alpine life zone gradient from the Arctic (70°N) at sea level to equatorial latitudes at high elevations above sea level. This series of measurements portrays the radiant flux to be anticipated during periods of maximum annual solar radiation. Along this arctic—alpine gradient, maximum daily total shortwave irradiance varies only by a factor of 1.6 and total daily shortwave radiation by less than 15% for cloudless conditions. In contrast, the maximum integrated effective UV—B irradiance can vary by a full order of magnitude and by seven—fold for total daily effective radiation. The steep UV—B radiation gradient is the result of a natural latitudinal gradient in total atmospheric ozone column thickness, prevailing solar angles at different latitudes, elevation above sea level, and an optical amplification effect, which results from a combination of highly wavelength—dependent radiation attenuation in the atmosphere and the pronounced wavelength dependence of biological action spectra. Forty to 70% of the solar UV—B flux is in the form of scattered radiation. Because of the large proportion of diffuse UV—B radiation, steeply inclined foliage would still receive a sizable fraction of the global UV—B irradiance in most situations. Snow cover can add substantially to the radiation load.
The Coexistence of Weeds and Restricted Native Plants on Serpentine Barrens in Southeastern Pennsylvania
Authors: Hart Robin
Published: May 2006

Three plant species restricted to serpentine outcrops in southeastern Pennsylvania, USA, coexist with weedy congeners on these outcrops. The contrasting strategies by which each of the two types persists on the outcrops were examined by comparing growth rate, competitive ability, seedling mortality, mineral uptake, phenology, and leaf structure. In general, weeds had higher potential growth rate, more mesic leaf structures, lower seedling mortality on normal soil, lower Ca uptake, and earlier and/or more abundant seed production. Both types showed poor competitive ability when grown with ryegrass. Some, but not all, serpentine populations of weeds differed from nonserpentine populations in traits that may be adaptive to the serpentine habitat. The growth patterns of weeds are suited not only to the temporally open conditions of disturbed areas, but also to physically severe sites where the greatest stresses are seasonal. When moisture and nutrients are most abundant, weeds grow faster than serpentine—restricted plants and allocate biomass to structures allowing rapid growth and early reproduction. During periods when moisture and nutrient supply are low, growth of serpentine—restricted plants is favored over weeds. Within a particular range of moisture and nutrient availability, both types can coexist.
Competition between a Planktivore, a Benthivore, and a Species with Ontogenetic Diet Shifts
Authors: Bergman Eva , Greenberg Larry A.
Published: May 2006

Along a gradient of increasing productivity in lakes, the abundance of perch (Perca fluviatilis) first increases and then decreases, whereas the abundance of ruffe (Gymnocephalus cernuus) and that of roach (Rutilus rutilus) continue to increase. Perch, unlike roach and ruffe, undergoes dramatic ontogenetic diet shifts. The decrease in the abundance of perch along the productivity gradient may be due to competition from roach at its plankton—feeding stage and from ruffe at its benthic—macroinvertebrate—feeding stage. Previous studies of these three species have focused on the effects of roach on the other species. To further examine the competitive relationship among these species, we studied the effect of increasing the density of ruffe, while maintaining a relatively high, constant density of roach, on the diet and growth of perch. Food resources were also monitored during this 2—mo experiment, which was conducted in 10 enclosures in a pond in southern Sweden. Based on prior experiments we expected roach, an efficient planktivore, to reduce the biomass of zooplankton in all enclosures. Moreover, we expected that the increased density of benthic—feeding ruffe would force perch to increase its consumption of zooplankton, resulting in reduced growth. We found, as predicted, that the biomass of cladocerans of copepods decreased over time but was not affected by ruffe density. The biomass of the commonly consumed macroinvertebrates Sialis (a megalopteran), trichopterans, and ephemeropterans was lower in enclosures with ruffe than in enclosures without ruffe. Moreover, the biomass of Sialis decreased and that of chironomids and the clam Pisidium increased over time. High ruffe densities were related to an increased proportion of zooplankton in the diet of perch in August, but not in September. The diet of ruffe changed little along the ruffe gradient. Only the proportion of trichopterans (in August) and Pisidium (in September) in the diet of ruffe increased along the ruffe gradient. The growth of both perch and ruffe decreased with increasing ruffe density, whereas the growth of roach was constant. These data suggest that one reason perch decrease in abundance along a productivity gradient may be because they are competitively sandwiched between plantivore and benthivore specialists.
Bluefish Mediation of Foraging Competition between Roseate and Common Terns
Authors: Safina Carl
Published: May 2006

The presence and foraging activities of predatory bluefish (Pomatomus saltatrix) differentially affect the foraging success of Roseate (Sterna dougallii) and Common Terns (S. hirundo), and depress the foraging activities of Roseate Terns. In the absence of bluefish, Roseate Terns had higher mean fishing success frequencies than Common Terns. This advantage was lost in the presence of bluefish, and Roseate Terns foraging success was inversely correlated with bluefish feeding intensity. Roseate Terns left foraging flocks more frequently as predatory fish activity increased and flock density increased. Conversely, the rate at which Common Terns left flocks was inversely correlated with predatory fish feeding and was unrelated to flock density. Bluefish may be considered keystone competitors in this system.
Light Beneath the Litter in a Tropical Forest: Effect on Seed Germination
Authors: Vazquez-Yanes C. , Orozco-Segovia A. , Rincon E. , Sanchez-Coronado M. E. , Huante P. , Toledo J. R. , Barradas V. L.
Published: May 2006

The spectral composition of the light transmitted through the forest canopy, through dry and moist litter leaves, and through intact forest litter was measured in a tropical rain forest in Mexico. Germination of photoblastic seeds of some pioneer trees and shrubs was studied over a light quality gradient, and in field germination experiments on the surface of the forest floor and beneath the litter layer. There is a shift to longer wavelengths inside the forest that is even greater beneath the litter. Light filtered through dead leaves strongly inhibits germination as demonstrated with field and laboratory germination experiments. The role of litter as a light filter might be important in maintaining a reserve of dormant seeds in the soil in tropical rain forests.
Soil Algae in Field and Forest Environments
Authors: Hunt Margo E. , Floyd Gary L. , Stout Benjamin B.
Published: May 2006

The prokaryotic and eukaryotic algal components of soil were quantified and identified generically over a 17—mo period in 2 successional fields and a climax forest. A correlation of the quantitative and qualitative changes was sought with other biological components (bacteria, actinomycetes and fungi) and major chemical and physical parameters. The highest counts obtained for the eukaryotic algae were 3.3 x 107, 2.2 x 107, and 1.2 x 105 cells/g soil for a 1—yr old field, an 11—yr old field and the forest, respectively. The highest counts obtained for the prokaryotic (bluegreen) algae were 8.2 x 105, 2.3 x 105 and 1.6 x 104 cells/g soil for the same sites. The total number of algae in all 3 sites declined during the summer. There was no further decline in winter. The greatest number algal genera was found in the 1—yr old field, 28 of a total 35 isolated or 80%; fewer were observed in the 11—yr old field and the forest, 57% and 63%, respectively. A predictive model for numbers of soil algae was formed by multiple regression analysis. The significant factors that showed a relationship with the bluegreen algae were pH, time, Mg, precipitation and the eukaryotic algae, R2 = 45%; significant factors with the eukaryotic algae were time, Mg and the bluegreen algae, R2 = 37%. Prediction equations were tested against additional data with success.
Woody Stem Production in Alaskan Tundra Shrubs
Authors: Shaver Gaius R.
Published: May 2006

The contribution of woody stems to the aboveground net primary production (NPP) of several species of tundra shrubs was documented at four sites in northern and central Alaska. To measure stem secondary growth, a new method was developed based on the ability to measure the changes in mass per unit length of stem segments or known age. Net accumulation of N and P in woody stems was also estimated, from additional measurements of changes in element concentration with stem age. Results showed that stem secondary growth was an important component of NPP in all species and sites, ranging from 13 to 49% of total aboveground NPP for individual species. The most productive species and sites had the highest proportions of NPP accounted for by secondary growth. Accumulation of N and P in woody stems occurred at slower rates, presumably due to internal reuse of these elements, but still accounted for 5—19% of the annual net aboveground N accumulation and 6—24% of the P accumulation. One implication of these results is a faster rate of N and P turnover in the vegetation than previously believed, and a greater annual nutrient demand. The relative contribution of each age class of stem segments to the total woody biomass was due to an interaction between the number of stem segments surviving to a given age and the annual percentage increase in mass per unit length. Older stem segments also tended to be longer than younger segments.
Environmental Instability, Competition, and Density‐Dependent Growth and Survivorship of a Stream‐Dwelling Salamander
Authors: Petranka James W. , Sih Andrew
Published: May 2006

To investigate whether growth in amphibians is density dependent, annual variation in larval growth rate, length of the larval period, size at metamorphosis, and survivorship to metamorphosis were documented in a population of small—mounted salamanders (Ambystoma texanum) for 6 yr. Peak hatchling densities were similar during all years, and varied from 18.2 to 24.5 hatchlings/m2 of bottom surface. Severe flooding during at the last 2 yr of study eliminated 90% if the larvae within 2 wk after hatching. The few larvae that survived these floods grew faster, had shorter larval periods, and were larger at metamorphosis than larvae in years when flooding was less extensive and larval densities were higher. Mean size at metamorphosis was inversely related to, and mean length of the larval period directly related to, larval density at day 15 (the earliest date at which postflooding censuses were available for both years). In a flood year, a larva of any given size contained about twice the volume of food in its stomach as a larva of the same size in a nonflood year. In a nonflood year food was limiting and average larval mass was inversely related to larval density in stream pools. Average air temperatures during the larval period in flood years and other years did not differ significantly. These data strongly support the hypothesis that growth was density—dependent because of competition for food. Survivorship to metamorphosis did not differ significantly in flood and nonflood years because of the manner in which flooding, stream drying, and competition interacted. In nonflood years competition for food lengthened the average time metamorphosis by °17 d relative to flood years. This, coupled with the fact that streams dried relatively early in the season, resulted in °3% survivorship to metamorphosis. In flood years larvae metamorphosed earlier and suffered only slight mortality from stream drying so that survivorship from day 15 to metamorphosis was an order of magnitude higher than in nonflood years. Increased postflooding survivorship compensated for catastrophic losses during floods; consequently, overall survivorship to metamorphosis was greater than in most nonflood years.
Rocky Intertidal Community Organization: The Impact of Avian Predators on Mussel Recruitment
Authors: Marsh Christopher P.
Published: May 2006

In the Pacific Northwest, mussels (Mytilus spp.) are major prey items of Surfbirds (Aphriza virgata), gulls (Larus glaucescens and L. occidentalis), and Black Oystercatchers (Haematopus bachmani). The impact of this predation is potentially great, as mussels are the competitively dominant members of the mid—intertidal community. I tested the effect of these predators on mussel recolonization of 50 x 50 cm clearings, using bird exclusion cages. Three of five exclosure experiments showed that birds significantly reduced recruitment of juvenile mussels (M. californianus and M. edulis) into clearings in established mussel beds. A sixth experiment indicated that birds were responsible for the absence of mussels from an area with smooth substrate lacking mussel beds. Here clumps of Mytilus edulis 10—20 mm in length became established in all exclosures, but not in any controls. These experiments indicate that the long—term impact of avian predators is greatest in patches where invertebrate predators are uncommon and larval settlement, rather than adult encroachment, is the major form of recruitment. The results of this and other studies suggest that avian predation increases the patchiness of a sedentary prey in heterogeneous habitats, but decreases patchiness where the habitat is sufficiently homogeneous that birds have access to all microhabitats.
A Test of Competition in Two Sympatric Populations of Desmognathine Salamanders
Authors: Kleeberger Steven R.
Published: May 2006

The effect of competition on foraging success and density in two sympatric populations of salamanders (Desmognathus monticola and D. quadramaculatus) was examined by density and cover—object manipulation experiments from June 1980 to April 1982. A second—order mountain stream in southwestern North Carolina was divided into experimental and control sections, and four treatments were performed: (1) addition of D. monticola, (II) addition of D. quadramaculatus, (III) addition of cover objects (rocks), (IV) addition of rocks and D. monticola. Treatments III and IV were performed for D. monticola only. Salamanders were marked individually for mark—release—recapture data. After experiments were terminated, salamanders were collected from control and treatment plots. Total lipid contents were determined and were used as an indirect measure of foraging success. Additions of D. monticola increased the total number of conspecific salamanders on plots and decreased their lipid content; congeners were not affected by the additions. Additions of D. quadramaculatus did not affect the total lipid content or the density of conspecifics or congeners. Addition of rocks significantly increased the density of D. monticola but did not affect their lipid content. Addition of rocks and D. monticola increased conspecific density and total lipid content. Results of salamander additions indicated that intraspecific competition for limited cover objects and energy resources may be important for adults and juveniles of D. monticola but not for D. quadramaculatus; interspecific competition was not detected.
Habitat Effects on Reproductive Success in the Coral Reef Fish, Acanthochromis Polyacanthus (Pomacentridae)
Authors: Thresher Ronald E.
Published: May 2006

Current hypotheses advanced to account for the maintenance of the diversity of coral reef fishes differ fundamentally in the assumed role of competitive processes. The equilibrial hypothesis is based primarily upon an assumption of extensive competitive interactions and subsequent niche partitioning; the lottery hypothesis, instead, assumes populations limited to levels well below carrying capacity due to the high larval mortality, a large effect of chance patterns in larval recruitment, and few, if any, significant competitive interactions. To date, no direct evidence for or against the presence of such interactions has been produced, with the role of competition, if any, inferred from community characteristics. To obtain such data, reproductive success of pairs of the planktivorous reef fish Acanthochromis polyacanthus located on isolated habitat patches was recorded over a 2—yr period at One Tree Island, Great Barrier Reef. Correlations were sought between each of six measures of individual fitness and 20 physical and social characteristics of the inhabited reefs. Significant correlations were found with only four characteristics: the number of individuals of all species inhabiting the reefs, the presence of the dominant lagoonal piscivore, current strength, and, most often, the number of heterospecific planktivores inhabiting the reef. Increased numbers of these apparent food competitors correlated with delayed spawning, reduced initial brood size, and reduced adult (and possibly juvenile) rates of growth. Results of a brief series of experimental manipulations of community structure, involving selective of apparent food competitors, appear to support these observations. The results suggest that food abundance is a limiting resource for A. polyacanthus and that significant interspecific competition for food occurs. The potential therefore exists, despite possible external constraints on population size, for selection to favor niche partitioning and for resources in short supply to affect year—class success and, subsequently, community structure.
Latitudinal Differentiation in Copepod Growth: An Adaptation to Temperature
Authors: Lonsdale Darcy J. , Levinton Jeffrey S.
Published: May 2006

We demonstrate the presence of significant differentiation in development rate, adult body length, and stomatic growth rate in the estuarine harpacticoid copepod Scottolana canadensis (Willey) collected from a broad range of latitudes (27°—43° N) and reared in the laboratory for several generations under the same conditions (15 g salts per 1000 g seawater, and 15°, 20°, 25°, or 28°C). The changing pattern of differential growth with increased temperature suggests local adaptation to maximize scope for growth under prevailing temperatures conditions; northern—derived individuals grew faster at low but not at high temperatures.
Response of Metrosideros Polymorpha Seedlings to Experimental Canopy Opening
Authors: Burton Philip J. , Mueller-Dombois Dieter
Published: May 2006

Twenty 100—m2 plot in a forest dominated by tree fern (Cibotium spp.) and ohia (Metrosideros polymorpha) on the island of Hawaii were subjected to different degrees of canopy removal. Seedlings growing in the control plots received an average of 10% full irradiance, whereas those growing in completely cleared plots received an average of 43% full irradiance. Invasion of Metrosideros tree seedlings into the completely cleared plots averaged more than three times that in the undisturbed forest. These open—born seedlings grew in height by an average of 3.5 cm/yr, 80% more than comparably sized seedlings that had started life in the shade and were than subjected to higher irradiances. The average height increment of all shade—born seedlings was 2.5 cm/yr in the control plots and 6.6 cm/yr in open plots; in terms of relative growth rates, this represents an increase from 0.19 to 0.47 cm°cm—1°yr—1. Height growth was optimal (10.5 cm/yr) at microsites receiving 55—60% relative irradiance. During the course of monitoring the growth and irradiance of 1130 seedlings that were tagged before canopy disturbance, it was found that these shade—born seedlings suffered mortality at a rate of 57%/yr when growing at <5% full irradiance. This death rate was reduced to 13% for seedlings grown at relative irradiances of 5—45%. Very heavy mortalities and a decline in the growth rate at irradiances >°50% indicate that many of these Metrosideros seedlings cannot be readily conditioned to open habitats. It therefore appears that shade—born seedlings of this variety of Metrosideros are intermediate in shade tolerance. Although seedlings may survive in the shade for a year or two, dense canopies of tree ferns of older Metrosideros trees are able to suppress their growth effectively, so that the Metrosideros population may have to depend on canopy gaps or canopy diebacks to maintain itself in mature rain forests. Some individuals were nevertheless able to acclimate successfully to very high light intensities, and this same species is a successful colonizer of barren volcanic deposits, suggesting that Metrosideros polymorpha exhibits broad acclimation ability and/or a wide range of genetic variability.