bluuebunny/crossref_metadata_2025 huggingface dataset:
Population Dynamic Consequences of Habitat Heterogeneity: An Experimental Study
Authors: Kadmon Ronen
Published: May 2006

Population dynamic consequences of habitat heterogeneity were investigated in a population of the desert annual Stipa capensis by measuring demographic responses of subpopulations inhabiting different habitats (slopes, depressions, and wadis) to natural and experimental changes in the amount of yearly rainfall. The results indicate that rainfall fluctuations affect the dynamics of the studied population by influencing both the percentage of germination and the number of seeds produced per germinated plant. However, the effect of changes in rainfall on both demographic parameters depends on habitat conditions, with slope subpopulations exhibiting the largest, and wadi subpopulations the smallest, effects. The fact that demographic responses to rainfall fluctuations are habitat dependent has two major implications. First, subpopulations inhabiting different habitats show considerable differences in their year—to—year fluctuations in density. Secondly, since seed production per seedling is habitat dependent, the distribution of the seedling population among the various habitats is a major determinant of the total number of seeds produced by the population in a given year. The results further indicate that most of the seeds (75—99.9%, depending on rainfall conditions) are produced in the depressions and the wadis, which taken together account for only 10% of the total area. This finding indicates that the ecological conditions in these spatially restricted habitats are critical for the dynamics of the whole population. The overall results suggest that taking into account factors such as the number and types of habitats available, the relative area occupied by each habitat and the distribution of the individuals among the available habitats may be important in explaining observed patterns of populations dynamics.
Denitrification in a Tallgrass Prairie Landscape
Authors: Groffman Peter M. , Rice Charles W. , Tiedje James M.
Published: May 2006

We characterized factors controlling denitrification and quantified rates of N gas production by this process in a tallgrass prairie landscape in central Kansas. The experimental design included three land use classes (unburned, annually burned, and annually burned and grazed) in factorial combination with three slope positions (summit, back—slope, toe—slope), plus a cultivated site in a toe—slope position (10 sites total). Denitrification was measured using an acetylene—based soil core technique four times in 1987, once in early 1988, and six times in 1989. Cores were incubated under field—moist conditions and after amendment with water or water plus nitrate. Microbial biomass and nitrification and dentrification enzyme activities were also measured. Denitrification was higher (P < .05) in unburned sites than in burned, and grazed, and cultivated sites in both 1987/1988 and 1989. The cultivated site consistently had low rates of denitrification relative to the native prairie sites, even when water and nitrate were added. Levels of microbial biomass C and nitrification and denitrification enzyme activities were an order of magnitude lower in the cultivated site than in the native prairie sites. Denitrification rates were highest in the early spring of 1987 and were low at all other times. Although temporal patterns of activity were generally related to patterns of soil moisture, water additions did not stimulate activity in ungrazed prairie soils. Water plus nitrate additions consistently gave significant increases in activity. The results are consistent with previous research that has found that unburned prairie is wetter and has higher concentrations of NO3— in soil solution than burned sites. In certain years, denitrification may be significant to site fertility, landscape water quality, and atmospheric chemistry in the tallgrass prairie region.
Interactive Effects of Nutrients and Disturbance: An Experimental Test of Plant Strategy Theory
Authors: Turkington Roy , Klein Elena , Chanway C. P.
Published: May 2006

The relative impacts of nutrient availability, disturbance intensity, and interspecific competition on species distribution and growth were studied by applying five levels of nutrients and five intensities of clipping to simple artificial communities (of Dactylis glomerata, Holcus lanatus, Lolium perenne, and Trifolium repens) in a factorial design for 16 mo. This was done to test the hypothesis that the effects of interspecific competition would decline along gradients as the amount of nutrients decreased and the degree of disturbance increased. Nutrients and disturbance, alone and in interaction, produced significant effects on percent cover of all species, and all species achieved maximum percent cover in the undisturbed regime. The experimental design permitted us to distinguish between percent cover reductions caused directly by nutrients and disturbance from those caused by interspecific competition. When grown in mixture, interspecific competition produced marked changes in the distribution patterns of percent cover that were in addition to the direct impacts of nutrients and disturbance. The absolute reduction in percent cover due to interspecific competition was greatest in the undisturbed regime, and for many mixtures this competitive effect was significantly reduced or even eliminated in the highly disturbed areas. Along the nutrient gradient the responses were variable and neighbor dependent, but for all species, the absolute reduction in percent cover due to interspecific competition was generally most severe where percent cover in pure stands was highest. When the data were standardized for differences in percent cover in pure stands and competition was assessed as the percent reduction in percent cover between pure and mixed stands, then the effect of interspecific competition on Trifolium declined along both gradients. The three grasses had a reasonably constant competitive effect across the disturbance gradient, but along the nutrient gradient the highest competitive effects were generally achieved where percent cover in pure stands was highest. We argue that this standardized index is less appropriate because it obscures the obvious differences in interspecific competition effects that are clearly operating within these communities.
Suspended Clays Alter Trophic Interactions in the Plankton
Authors: Cuker Benjamin E.
Published: May 2006

This paper explores how suspended clays alter the trophic interactions that govern composition and density of the zooplankton community in a small eutrophic North Carolina (USA) lake. Enclosures (12 m3) were used in a complete, triplicated, cross—classified design. The four treatments were; BASS (1 piscivorous fish/enclosure), BASS + CLAY, SUNFISH (16 planktivorous fish/enclosure), SUNFISH + CLAY. The +CLAY treatments received 100 g°m—2°d—1 of montmorillonitic clay. It as anticipated that turbidity from clay would release crustacean zooplankton from visual planktivores. Contrary to this expectation, crustacean zooplankton populations in the SUNFISH + CLAY were fourfold less than in the SUNFISH treatment. The reduction was not linked to deleterious effects of clay, since zooplankton populations were higher in the BASS + CLAY than the BASS treatment. Instead, crustacean zooplankton appeared to be governed primarily by Chaoborus predation. Suspended clay weakened the link between visually predaceous fish and Chaoborus, which in turn strengthened the effect of Chaoborus predation on crustacean zooplankton. Since fish selectively cropped larger instars of Chaoborus, rotifers (primarily Keratella cochlearis), which are mostly eaten by smaller instars of Chaoborus, were unaffected by clay.
Reciprocal Transplant Reveals Sources of Variation in Growth Rates of the Lizard Sceloporus Undulatus
Authors: Niewiarowski Peter H. , Roosenburg Willem
Published: May 2006

Geographic variation in life history phenotypes between populations of a single species is often assumed to reflect genetic divergence caused by natural selection. The relative contribution of genetic and environmental sources of phenotypic variation is rarely determined, especially for vertebrates. However, distinguishing between phenotypic plasticity induced by proximate environmental variation and genetic divergence is fundamental to understanding the ecological and evolutionary significance of geographic variation. We used a reciprocal transplant experiment to uncover the relative importance of population—specific (genetic) and environmental sources of variation in individual growth rates between two populations of the fence lizard, Sceloporus undulatus. Our study revealed a population ° environment interaction, consistent with a genotype ° environment interaction that would result if differences between populations were genetically based. The growth rates of Nebraska lizards, normally twice that of New Jersey lizards, were reduced by half when the animals were transplanted to New Jersey. However, New Jersey lizards showed no increase in growth rates when transplanted to Nebraska. Comparisons of relative food availability (arthropod abundance) indicated that more food was available in New Jersey during the experiment. Estimates of potential activity day length using hollow copper models were from 2 to 2.5 h longer in Nebraska. This result suggests that the thermal biophysical environment may have reduced growth rates of Nebraska lizards transplanted to New Jersey, perhaps reducing foraging time or by limiting physiological processes supporting growth. Our results are consistent with previous studies of S. undulatus that assumed genetic differences in life histories between populations and implicated the thermal biophysical environment as an ecological source of variation.
Leg Morphology and Feeding Postures in Four Parus Species: An Experimental Ecomorphological Approach
Authors: Moreno Eulalia , Carrascal Luis M.
Published: May 2006

The foraging behavior of four Parus species feeding at artificial feeders was studied, while controlling for ecological variables related to patch characteristics (food quality, food access, and escape distance to the nearest refuge). Hindlimb morphology (osteology and myology) was analyzed and compared with foraging postures at feeders. Using the Long—Tailed Tit as an appropriate outgroup for comparison, and considering functional changes associated with morphological changes, we demonstrated the existence of clear ecomorphological patterns relating foraging postures and hindlimb morphology in the four Parus species studied. The Blue Tit uses hanging postures preferably and its hindlimb morphology is modified for helping leg flexion. The Crested Tit more often stands and its hindlimb morphology is modified to aid leg extension. Great and Coal Tits are ecologically and morphologically intermediate between the two former species. Our results show morphology as a determinant of locomotion mode. Since foraging postures are tightly associated with substrate use, then morphology should be considered when studying pressures determining community organization. Our proposed integrative method for dealing with ecomorphology can be valuable in demonstrating the adaptiveness of morphological structures in phylogenetically and ecologically related species.
Plasticity, Constraint, and Optimality in Reproductive Timing
Authors: Morgan Steven G. , Christy John H.
Published: May 2006

Synchronous release of gametes or larvae by marine animals may be controlled as many as four environmental cycles thereby enabling an array of reproductive timing patterns. Cohesive scenarios that account for the diversity of reproductive patterns are rare and tests of their adaptive significance have been even rarer. By exploiting plasticity in the timing of larval release, we isolated proximate factors regulating reproductive synchrony and provided evidence that predation ultimately may best explain the diversity of hatching patterns by brachyuran crabs. Tides entrain reproductive rhythms by many crabs, and therefore spatial and temporal variation in tides produces intraspecific variation in reproductive patterns. This was demonstrated by determining the timing of larval release of the same or sibling species of intertidal crabs in Pacific semidiurnal and Caribbean mixed semidiurnal tidal regimes on the two coasts of the Republic of Panama. The time of larval release varied during the year in the Caribbean, where entraining physical cycles exhibited complicated changes in phase, but not along the Pacific coast, where the phase relationships among physical cycles varied little year—round. Crabs timed larval release relative to the light—dark, tidal phase, and tidal amplitude cycles, but not the lunar cycle, suggesting that three rhythms determined when larvae were released. For each species we ranked these rhythms by the degree to which larval release kept phase with their entraining physical cycles. The species—specific hierarchies of rhythms we observed match those expected if the time of larval release minimizes predation on females, embryos, and newly hatched larvae. Such hierarchies enable crabs to track phase shifts of cycles in variable tidal environments and may enhance reproductive success across tidal regimes. However, larval release may be timed better in some tidal regimes than others due to differences in the phasing of environmental cycles. In some tidal regimes, larval release cannot be synchronized with all three physical during the year, and hierarchies of rhythms may determine the timing and duration of breeding.
Analysis of Larval Trematode Communities
Authors: Lafferty K. D. , Sammond D. T. , Kuris A. M.
Published: May 2006

We can compare natural communities with null models of communities to indicate how they differ from random assemblages of species (i.e., how much structure is present). However, because null models draw on observed values of species' prevalences, whatever structure already exists in natural communities affects the composition of a null model and weakens its comparative power. To address this, we developed formulae to estimate "pre—interactive" species prevalences permitting a more sensitive quantification of community structure. Nonetheless, if a null model deviates from the community that we base it on, it is difficult to separate the effects of heterogeneity in recruitment from competition. We have developed a method to test for each independently. Applying our analytical techniques to a well—studied guild of larval trematodes in the salt marsh snail Cerithidea californica revealed that competitive interactions among species were the most significant structuring force. Interestingly, spatial heterogeneity acted to significantly intensify species co—occurrences. This differs from previous studies, which argued that the isolating effects of spatial heterogeneity, not competition, structure these communities by reducing co—occurrences.
Costs of Inducible Defense: Protein Limitation, Growth, and Detoxification in Parsnip Webworms
Authors: Berenbaum May R. , Zangerl Arthur R.
Published: May 2006

Although xanthotoxin, a linear furanocoumarin, is toxic to a wide variety of lepidopterans, the parsnip webworm, Depressaria pastinacella (Lepidoptera: Oecophoridae), is restricted to feeding on only two genera of Apiaceae, both of which contain large quantities of this toxin. The cost of cytochrome P450—mediated detoxification of xanthotoxin by this oligophagous species was examined in two artificial diet experiments. In the first experiment, growth rates, silkspinning rates, and detoxification capacity of larvae were measured after they had consumed diets ranging in protein content from 0 to 100% of the amount required for maximum growth. As protein became progressively more limited, growth rates declined progressively while silkspinning rates and detoxification capacity remained unchanged; only when dietary protein was completely absent did silkspinning rates and xanthotoxin metabolism rates decline. These findings suggest that xanthotoxin detoxification capacity is maintained at the expense of growth. In the second experiment, growth rates, silkspinning rates, and xanthotoxin metabolism were measured in larvae that consumed diets with or without xanthotoxin in the presence or absence of protein. As in the previous experiment, the absence of protein led to significant reductions in growth, silkspinning, and detoxification. The presence of xanthotoxin in the diet resulted in an increase in the rates of xanthotoxin metabolism and silkspinning but did not have any effect on growth rates. Inasmuch as there were no interactions between protein and xanthotoxin treatments, induction of xanthotoxin metabolism by xanthotoxin was not affected by protein limitation. The fact that xanthotoxin metabolism is induced almost threefold in the absence of dietary protein, conditions that result in reduction of growth rates by almost 80%, is also consistent with the greater importance of detoxification capacity relative to growth over the short term. Due to its highly oligophagous habits, the parsnip webworm is unlikely to encounter host plants tissue entirely lacking furanocoumarins; circumstances under which the inability to respond to the presence of xanthotoxin in the diet by increasing rates of its metabolism may never exist. Thus, maintaining high and inducible levels of cytochrome P450—mediated xanthotoxin metabolism may be regarded as one of the costs associated with the highly specialized diet of D. pastinacella.
Gap Partitioning among Maples (Acer) in Central New England: Shoot Architecture and Photosynthesis
Authors: Sipe T. W. , Bazzaz F. A.
Published: May 2006

We measured shoot architecture and photosynthesis by three species of maple (Acer pensylvanicum, A. rubrum, A. saccharum) in response to understory and small canopy gaps in the mixed deciduous forests of central New England. Trees were felled to create six cleared gaps of two sizes (8 ° 12 m, 75 m2; 16 ° 24 m, 300 m2). Seedlings of the three species (2160 total, 720 per species) were transplanted into five plot locations (center plus northwest, northeast, southwest, and southeast gap edges) within all gaps and additional understory sites 1 yr before gap creation. Measurements of microclimates, architecture, photosynthetic performance and seedling survival and growth were made over 1 yr before, and 2 yr following, gap release. Architectural variation increased greatly over the 3—yr period. Striped maple (A. pensylvanicum) and red maple (A. rubrum) increased branch numbers, leaf numbers, and total leaf areas in gaps, especially large gaps, while sugar maple (A. saccharum) showed much smaller changes. Red maple tended to increase the number of leaves while leaf size decreased; striped maple increased leaf number but held leaf size constant. Diurnal patterns of photosynthesis by these species differed within and between gap and understory sites. Red maple showed higher photosynthetic rates per unit leaf area than striped and sugar maple in all site/plot combinations except the large—gap south plots, where striped maple exceeded red maple. Estimated diurnal shoot—level assimilation differentiated species more than unit area assimilation rates and also altered the rank order of performance, with striped maple > red maple > sugar maple in all microsites except the large gap north. Population—level assimilation vs. irradiance response curve exhibited a similar pattern, with red maple dominating unit area rates in most microsites. In contrast, shoot assimilation curves showed striped maple > red maple > sugar maple in all microsites except the large—gap north, where red maple > striped maple. Architectural variation among these species interacted with leaf—level assimilation rates to produce some differences among these species in shoot—level assimilation across the gap—understory microclimatic gradient. Since survival and growth patterns are usually correlated with differences in whole—plant carbon assimilation, our results suggest that there is some photosynthetic potential for gap partitioning among these three species of Acer.
Amino Acid Absorption by Arctic Plants: Implications for Plant Nutrition and Nitrogen Cycling
Authors: Kielland Knut
Published: May 2006

Recent studies of nitrogen (N) cycling in arctic tundra have indicated that inorganic N supplied to plants by mineralization is not sufficient to meet the annual requirement of N by many tundra species. Whereas N mineralization is slow in tundra soils and concentrations of inorganic N are low, these soils have large stocks of both structural and soluble organic N. In light of these observations, kinetics of absorption of three amino acids (glycine, aspartic acid, and glutamic acid) were measured in dominant vascular plant species of the four major ecosystems types in arctic Alaska and compared with concentrations of free amino acids in soils. Absorption rates were measured on roots using 14C—labeled substrates. Concentrations of free amino acids in soil were measured on water—extracted samples by high pressure liquid chromatography. All species had higher capacity (Vmax) for ammonium uptake (measured using methylamine as an ammonium analogue) than for any amino acid. However, at concentrations observed in the field, uptake rates estimated for amino acids were similar to (glycine) or less than (aspartic and glutamic acids) that for ammonium. On the basis of these comparisons, uptake rates of the three amino acids together may account for between 10 and 82% of the total N uptake in the field, depending on species and community. Deciduous shrubs had higher uptake rates than the more slowly growing evergreen shrubs, suggesting that new growth created a sink that strongly influenced capacity for amino acid uptake. In general, ectomycorrhizal species had higher amino acid uptake than did non—mycorrhizal species. In species that were sampled from more than one community, amino acid uptake rates were highest in the community where a given amino acid was most abundant in the soil. The results indicate that, in arctic tundra, plants short—circuit the mineralization step of decomposition by directly absorbing amino acids. This implies that in the organic soils of these tundra systems (1) inorganic nitrogen is an inadequate measure of plant—available soil nitrogen, (2) mineralization rates underestimate nitrogen supply rates to plants, (3) the large differences among species in capacities to absorb different forms of N provide ample basis for niche differentiation of what was previously considered a single resource, and (4) by short—circuiting N mineralization, plants accelerate N turnover and effectively exert greater control over N cycling than has been previously recognized.
Shoot Morphology and Reproduction in Jojoba: Advantages of Sexual Dimorphism
Authors: Kohorn Loraine U.
Published: May 2006

The reproductive success of an individual plant is in part a function of its capacity to assimilate resources, and this in turn is partially a result of plant morphology. Simmondsia chinensis, jojoba, a dioecious desert shrub, is dimorphic in secondary sexual characters in some desert populations, where females on average have larger leaves and more open canopies than males. The function of this sexual dimorphism was examined by investigating the importance of shoot morphology to reproductive components of plant fitness. The reproductive behavior of individuals tended to be associated with different morphological attributes for both males and females. Males with more “male—type” morphology tended to have more inflorescences and a greater likelihood of flowering than did males structured more like females, though they also had smaller inflorescences. Females with more “female—type” construction produced more flower buds in one of the study years and also produced heavier seeds than females with more male—like structure. Females from four different morphological classes allocated 1.5–4.5 times as much to reproduction as did males, but showed wide variability in whether or not they set fruit. Therefore females either had a much higher or a much smaller reproductive allocation than most males. Fruit set in the population studied is not limited by pollen availability, but is presumably constrained by abiotic resource availability. The asymmetry between sexes in the morphology of reproductively more successful individuals points to a potential for a resource—based evolutionary origin of sexual dimorphism in jojoba, arising from different morphological optima for each sex.
The Allocation of Resources to Reproduction in Daphnia Galeata: Against the Odds?
Authors: Boersma Maarten
Published: May 2006

Variation in offspring is a size common phenomenon in many organism. In cladoceran zooplankton large offspring are known to have a high starvation resistance. One could, therefore, expect offspring to be large at low food levels, whereas at higher food levels the production of more, but smaller, offspring would yield the highest parental fitness. However, in Daphnia galeata I found that individual offspring were smallest at a low food level and largest at intermediate food levels. Moreover, in contrast to the predictions made by several theoretical models, I found that larger mothers produce larger offspring. The first discrepancy between data and theory could be explained by the existence of a maximum offspring size, combined with the difficulty of Daphnia to produce one egg less than was done in reality. The dependence of offspring size on maternal size could be explained by the higher likelihood of intraspecific competition when large (=old) females are present, and hence higher starvation risks for the offspring. It was found that embryonic respiration was lowest at the lower food levels and hence the smaller individual egg size seemed to be compensated by lower carbon losses during development.
Can Small Rare Prey be Chemically Defended? The Case for Marine Larvae
Authors: Lindquist Niels , Hay Mark E.
Published: May 2006

Marine larvae are consumed by a wide variety of generalist fishes and particle—feeding invertebrates, but larvae of any particular species probably constitute a small and variable of the diet for these consumers. Because virtually all consumers can ingest small quantities of noxious compounds with minimal detrimental effects, it is uncertain that toxic chemicals in larvae could be consumed in quantities sufficient to select for predator recognition and avoidance. Despite this, chemically defended larvae do occur. We show that, at low doses, secondary metabolites (the didemnins) from adults and larvae of the Caribbean tunicate Trididemnum solidum induced vomiting in fish, resulting in rapid learned aversion of the didemnin—defended food. The particle—feeding anemone Aiptasia pallida did not learn to avoid the chemically defended food. When anemones ingested the chemical equivalent of 15 larvae/d, representing <2% of the mass of their total daily diet, the didemnins in the "larvae" significantly reduced: (1) growth of adults by 82%, (2) combined growth of adults and daughter clones by 76%, (3) production of daughter clones by 44%, and (4) average mass of individual daughter clones by 41%. At higher water temperatures, anemones cloned more rapidly, but the negative effects of consuming didemnins also occurred more rapidly. Significant differences in the number of daughter clones produced between treatment and control anemones occurred after only 4 d at seawater temperatures of 27°—29°C vs. 32 d at seawater temperatures of 18°—21°C. Thus consumption of even very small quantities of secondary metabolites can decrease consumer fitness substantially and select for predators that recognize and avoid chemically defended larvae, as do many consumers that co—occur with Trididemnum solidum larvae. This is the first rigorous demonstration that consumption of marine secondary metabolites can decrease consumer fitness when ingested at ecologically realistic doses.
Indirect Effects of Citrus Cultivars on Life History Parameters of a Parasitic Wasp
Authors: Hare J. Daniel , Luck Robert F.
Published: May 2006

We determined sex ratio, initial egg complement, and size of adult Aphytis melinus, an ectoparasitoid of California red scale, Aonidiella aurantii. Host scales were reared on leaves of four citrus cultivars previously shown to differ in suitability for scale growth and survival. Wasps from scales reared on leaves of lemon (Citrus limon) produced nearly twice the proportion of female progeny as did wasps from scales reared on leaves of grapefruit (Citrus paradisi), orange (Citrus sinensis), or mandarin (Citrus unshiu). Moreover, female wasps from lemon—reared scales had highest initial egg complement, followed by wasps reared from grapefruit, mandarin, and orange. The quality of scales for the production of fecund, female wasp progeny also varied seasonally over all cultivars pooled. Scales reached largest size when reared in the cooler autumn temperatures and were more than twice as suitable for the production of fecund, female A. melinus as scales reared in early summer or midsummer. Results suggest that the mechanisms allowing persistence of the interaction between California red scale and A. melinus may vary among different citrus cultivars.
Moose Browsing on Scots Pine along a Gradient of Plant Productivity
Authors: Danell Kjell , Niemela Pekka , Varvikko Tuomo , Vuorisalo Timo
Published: May 2006

Pines growing in unproductive habitats experience more damage by moose than those growing in more productive habitats. This suggests that trees in unproductive habitats are qualitatively different, e.g., more palatable to moose, and/or less able to recover from browsing. Moose browsing was compared on four phenotypes of Scots pine taken from a gradient of forest productivity. The trees were exposed, side by side, at test stations during winter. Consumption of twig biomass was highest on pines from the most productive habitat and lowest on those from the moderately productive habitat with an overstory of mature pines. Consumption was intermediate on pines from the least productive habitat and from the moderately productive habitat without an overstory. Pines from the highly productive habitat had greater nitrogen, potassium, and phosphorus content and more readily degradable dry matter (when incubated in the rumen of a cow) than did pines from the less productive habitats. Even though they provided a lower quality food for moose, pines in the less productive habitats generally suffered most from moose browsing. The slender twigs and the slow growth rate of these pines allow the moose to browse the main part of the needle—bearing twigs, in contrast to pines from more productive habitats. We also recorded the mortality response to simulated winter browsing in living pines on this same forest productivity gradient. Pine mortality was greater in the less productive habitats than in the more productive ones. These findings largely explain why pines growing in habitats of low and medium productivity suffer more than pines in highly productive habitats, where the possibility for growth is better. Severe damage to stands growing on poor soils most probably arises where moose density is high and where moose have already harvested most of the available twigs from pines growing in the more productive habitats. The moose are then forced to utilize the suboptimal food resources available in less productive habitats.
Population‐ and Community‐Level Analyses of Species Occurrences of Central Amazonian Rainforest Tadpoles
Authors: Gascon Claude
Published: May 2006

In this paper I examine occurrence patterns and habitats use of a New World tropical tadpole species assemblage from primary tropical terra firme forest. First, I describe phenology of reproduction. I then undertake a population approach to examine abiotic characteristics associated with sites used for reproduction by each species. Finally, analysis at a more general level tests to see if some correlative structure exists at the community level, i.e., do sites with similar tadpole species also have similar abiotic characteristics? A significant correlation would indicate, for example, that sites with similar abiotic characteristics also share species assemblages. Fifty—three independent aquatic sites were sampled across two consecutive rainy seasons (two 9—mo periods). Twenty—five tadpole species were encountered during the entire study. Significant differences existed among four types of habitats (terra firme pools, peccary wallows, streamside pools, and streams), all located within primary undisturbed forest, with respect to abiotic characteristics such as site permanency and mean depth of water. Most of the rainfall was concentrated between December and May, and phenology of reproduction was strongly affected by the rainfall pattern. For each of the 11 most common species, discriminant analyses revealed the major abiotic characteristics that differed between sites used and not used by those species. On a broader scale of analysis, Mantel tests showed no significant correlations among sites in the similarity of their abiotic characteristics and indices of similarity in their tadpole assemblages. There was also no concordance among sites between either type of similarity and actual proximity of locations. These results indicate that although individual species are responding to habitat characteristics and breed in sites having specific attributes, there are no large—scale species assemblages that covary in habitat use.
Density‐Dependent Population Regulation in a Salamander
Authors: Van Buskirk Josh , Smith David C.
Published: May 2006

We studied population responses to variation in larval density in a salamander (Ambystoma laterale) that breeds in splash pools on exposed rocky shorelines at Isle Royale, Michigan. Population densities and size distribution were sampled in the field with little error because the water is clear and the pools are kept free of sediment and vegetation by waves from winter storms on Lake Superior. We experimentally manipulated salamander densities in 12 natural pools (three replicates each of four densities) and simultaneously monitored salamander populations in 27 control pools on two islands. The experiment demonstrated that increasing density significantly reduced survival and growth, and increased the skew in the size distributions of individuals. Each of these responses was also observed in the undisturbed natural populations. Density effect were probably due to interference because (1) natural variation in the densities of prey taxa did not influence salamander performance in the experiment, (2) salamander density had no effect on prey availability, and (3) tail damage from attacks by conspecifics was widespread and positively correlated with density. We conclude that density dependence is commonplace in salamander larvae, and probably makes an important contribution to regulating the population size of Ambystoma laterale at Isle Royale.
Mechanisms of Benthic Algal Succession in Lotic Environments
Authors: McCormick Paul V. , Stevenson R. Jan
Published: May 2006

Although seasonal patterns of algal succession have received considerable attention, few studies have attempted to elucidate the processes that contribute to short—term algal succession following disturbance. We framed the present study around four general mechanisms of succession in order to investigate the contribution of species—specific autecologies and interspecific interactions to observed short—term patterns of algal succession in a productive third—order stream in Kentucky, USA. Observations in different current environments in the stream during two consecutive winters were used to classify dominant species as early or late successional based on changes in relative abundance through time on newly exposed substrates. Assemblages of the same age but differing in the relative abundance of early— and late—successional species were developed in streamside channels recreating both current environments to measure growth parameters (e.g., reproduction) of dominant species and to test for interspecific interactions. Of the five dominant species of algae in the winter assemblage, three species were consistently classified as early successional (i.e., decreased in relative abundance with increasing assemblage age) and two species as late successional. Early—successional species differed in their growth form and strategy for initially dominating the substrate, having high densities in the water column, high probabilities of attaching, or fast early reproductive rates. Late—successional species exhibited a more extended growth form and had the highest per capita reproductive rates during later growth. Late—successional species reduced the reproductive rate of early species as succession proceeded in both current environments but were not themselves inhibited by biovolume increases. While differences in growth strategies between early— and late—successional species indicated the importance of passive tolerance mechanisms of succession, density—dependent interactions during community development were consistent with active tolerance mechanisms. As in terrestrial plant communities, successional patterns in benthic algal assemblages appear to result from several processes that defy explanation by a single mechanistic model.
Postfledging Survival of European Starlings Exposed as Nestlings to an Organophosphorus Insecticide
Authors: Stromborg Kenneth L. , Grue Christian E. , Nichols James D. , Hepp Gary R. , Hines James E. , Bourne Henry C.
Published: May 2006

To test the hypothesis that exposure to organophosphorus (OP) insecticides reduces postfledging survival of altricial birds, 16—d—old European Starlings (Sturnus vulgaris) were weighed and orally dosed with corn oil containing 6.0 mg of dicrotophos per kilogram of body mass or an equivalent exposure of pure corn oil (controls). Two days later, each survivor was weighed again and patagially tagged for identification after fledging. Resightings of marked fledglings were made during weekly 2—d intensive observations yielding four estimates of postfledging survival. Before fledging, only OP—dosed birds died (18.5%), and OP—dosed survivors lost more mass (5.2%, P = .001) than controls (1.4%) but their masses on day 18 were only slightly lower (2% of control mean, P = .10). Brain cholinesterase activity, a sensitive indicator of OP exposure in birds, was depressed and average of 93% in OP—dosed nestlings that died compared to controls, and an average of 46% in OP—dosed nestlings alive on day 18. Age at fledging, postfledging survival, flocking behavior, and habitat use, however, did not differ between OP—dosed and control birds. The effects of the OP on the nestlings appeared to be rapid, to be reversible in survivors, and did not extend into the postfledging period. In addition, we found no relationship between body mass at fledging and postfledging survival.
Larval Predation by Barnacles: Effects on Patch Colonization in a Shallow Subtidal Community
Authors: Young Craig M. , Gotelli Nicholas J.
Published: May 2006

Laboratory studies and gut—content analysis suggest that barnacles should be important predators on invertebrate larvae. To determine if predation on larvae limits settlement, we recorded recruitment in artificially produced patches of clear substratum surrounded by monocultures of either living or dead barnacles. Living barnacles inhibited recruitment of colonial ascidians and bryozoans, but had no detectable effect on recruitment of barnacles, serpulid polychaetes, or solitary ascidians. Patch size did not influence the recruitment of any species. The presence of dead barnacle shells influenced spatial pattern of settlement in ascidians and serpulids. Comparison of blank control plates with barnacle—covered plates indicates that these spatial patterns are better explained by larval behavior than by small—scale hydrodynamic effects. Ascidians, which are dominant competitors in this community, settle primarily on shells of dead barnacles rather than on clear primary substratum. Although predation on larvae by barnacles appears to inhibit recruitment of some species, its effects are variable and easily confounded with other determinants of settlement distribution.
A Comparison of Recent and Historical Populations of the Colonial Hydroid Hydractinia
Authors: Buss Leo W. , Yund Philip O.
Published: May 2006

The hydroid Hydractinia sp. nov. occurs as an epibiont on shells occupied by the hermit crab Pagurus longicarpus. There is a seasonal pattern of abundance, with maximum reproduction, recruitment, and competition occurring during the summer months. Recruitment occurs in stereotypic positions on the undersurface of shells. Since recruits are clustered, intraspecific competitive encounters occur most frequently when hydroid colonies are small. The hydroid size—frequency distribution is bimodal, suggesting high mortality at small colony size and low mortality at large colony size. A refuge from intraspecific competition exists on small shells. Interspecific competition between the bryozoan Alcyonidium polyoum and H. sp. nov. is also seasonal, and static observations of overgrowth interactions imply that A. polyoum is the dominant competitor. Predation on H. sp. nov. is rare in Long Island Sound. Samples of five museum collections dating from 1875 to 1916, supplemented by a limited collection of Miocene material, display patterns of site—specific recruitment, size—frequency distribution, and shell—size dependence of recruitment that are largely congruent with patterns observed in modern samples.
Productivity and Nutrient Cycling of Alaskan Tundra: Enhancement by Flowing Soil Water
Authors: Chapin F. Stuart , Fetcher Ned , Kielland Knut , Everett Kaye R. , Linkins Arthur E.
Published: May 2006

Patterns of biomass, productivity, and nutrient cycling were compared between water—track and nontrack areas in Alaskan tussock tundra. Water tracks (channels of subsurface water drainage) sampled in Alaskan tundra were 2.4—fold more productive than adjacent nontrack areas due almost entirely to the 10—fold greater aboveground production of the sedge Eriophorum vaginatum L. in the track. Nitrogen and phosphorus cycled more rapidly in water tracks than in nontrack areas due to slightly warmer soil temperature, deeper thaw, higher soil phosphatase and protease activities, and more rapid nitrogen mineralization. The importance of E. vaginatum in stimulating productivity and nutrient cycling in the water track may result from its deep—rooting habit, which enabled it to exploit flowing subsurface water. Observed lateral subsurface soil water flow rates in the track (0.57 ± 0.09 cm/h) were 6 and 8 times as rapid as diffusion of phosphate and ammonium, respectively, in water and are of the right magnitude to explain the 10—fold enhancement of Eriophorum productivity in the track. Roots of E. vaginatum growing in the track had a high capacity to absorb ammonium and phosphate and thus could exploit nutrients brought to the root surface by flowing soil water. The low degree of mycorrhizal colonization of these roots makes E. vaginatum particularly sensitive to the rate of nutrient movement to the root surface. The enhanced productivity and' nutrient cycling in the water track compared with adjacent tundra probably reflect increased nutrient flux to the root surface caused by flowing water in the water track.
Patterns of Ramet Survivorship in Clonal Fragments of the Stoloniferous Plant Potentilla Anserina
Authors: Eriksson Ove
Published: May 2006

One of the suggested adaptive benefits of clonal propagation is the reduced mortality risk experienced by genets that produce several ramets each capable of suffering independent mortality. The hypothesis that the mortality of each ramet of a clonal plant is independent of the fate of other ramets on that plant was tested by use of demographic data from a 4—yr study of a seashore meadow population of Potentilla anserina. This plant produces vegetative daughter ramets that become physiologically independent during their 1st yr of life. Frequency distributions of the number of surviving ramets originating from the same mother ramet were not significantly different from what was predicted by a model that assumed independent mortality of ramets. This result is consistent with the initial hypothesis. However, other results imply that the fate of ramets is influenced by the fate of other ramets of the same clone. Ramet survivorship was related to clone size during one of the year. Furthermore, nodal position of daughter ramets on stolons affected survivorship. In contrast, the distance between consecutive ramets on the same stolon, and the fate of sibling ramets on the same stolon, did not significantly influence ramet survivorship.
Modules of Production and Reproduction in a Dioecious Clonal Shrub, Rhus Typhina
Authors: Doust John Lovett , Doust Lesley Lovett
Published: May 2006

The hypothesis that male—biased sex ratios result from the greater costs of reproduction for females was examined at various modular levels in a study of the dioecious clonal shrub, staghorn sumac (Rhus typhina). The population of flowering trunks was male—biased, though male and female clones proved to be of similar ages. Trunks within apparent female clones were more likely to be vegetative or dead than in male clones, though the density of living plus dead trunks was comparable in male and female clones. Females of given ages had the same mean trunk diameter as similarly aged males. Overall, the average female trunk diameter was the same as the average male diameter. Differential costs and patterns of reproduction for males and females were revealed through destructive harvests and separate analysis of trunks and branches. An abrupt discontinuity n the performance (growth) of male trunks and branches. An abrupt discontinuity in the performance (growth) of male trunks was found; those with a diameter less than °4 cm had greater total numbers of leaves, branches, and inflorescences than females of comparable diameter, whereas larger males and females did not differ from each other. Flowering males branches had significantly fewer leaves than those of flowering females. The number of leaves per branch was highly correlated with the number of fruits produced in the corresponding terminal inflorescence. The average (±SE) male inflorescence had 4099 ± 312 flowers, whereas the average female inflorescence bore 1575 ± 67.2 fruits. There was a sixfold difference in the biomass of these terminal reproductive units (female infructescences being heavier.) This study suggests the importance in a clonal species of defining the modular level at which costs are assessed. The hypothesis that females suffer greater reproductive costs as a consequence of fruit production is supported by the finding of diminished survivorship of daughter trunks in female clones. However, it is not supported by data on annual trunk diameter increments. Reproductive costs in sumac are shown at the level of the clone (ramet survivorship) or the branch, rather than at the level of the flowering trunk module.