bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Fire on Nutrient Movement in a South Carolina Pine Forest
Authors: Lewis William M.
Published: May 2006

The movement of major cations, nitrate, and phosphate was studied in burned and control areas of a South Carolina pine forest. Artificial leaching of litter showed that fire increased the solubility of cations by the following factors: Ca++, 20 °; Mg+?+, 10 x; Na+, 2.3 x; K+, 2.3 x. Natural leaching over a 1—mo period reduced the ion yield of burned litter by 80%—83% for divalent cations and 45%—63% for monovalent cations. Amounts of soluble nitrate and phosphate were identical in burned and unburned litter samples. Nitrate and phosphate were substantially higher in the first rainfall at the burned area after the burn, but not in subsequent rains. Collections of rainfall plus fallout from the burned area consistently contained about twice the amounts of cations present in similar collections from the unburned area. Runoff was low but contained greater amounts of cations in the burned area. Samples of superficial groundwater provide some evidence for changes in ionic balance due to burning. Biologically available N and P in the litter is apparently not increased directly by burning, but a rise in soluble nitrate and phosphate occurs some time after the burn and is probably due to increased microbial activity. Large amounts of N are probably volatilized and could exercise important effects on the productivity of systems remote from the burn if some biologically active compounds are included in the vapor. Although the upper soil layers are an efficient adsorption trap for cations, there is a potentially important movement of ions in runoff, in groundwater, or in the wind as particulate matter following a fire.
Synchrony and Periodicity of Flowering in Frasera Speciosa (Gentianaceae)
Authors: Taylor Orley R. , Inouye David W.
Published: May 2006

Synchronized flowering in populations of Frasera speciosa occurs at 2—4 yr intervals. In the East River Valley, Gunnison County, Colorado, major flowering years were 1969, 1973, 1977, and 1980; flowering occurred in alternate years at the highest site (3750 m). Some flowering in three populations in other years suggests that environmental factors can influence the periodicity of flowering. Frasera flowers are preformed at least 3 yr in advance of flowering, although the causes of floral initiation and subsequent synchronous flowering over broad areas are not clear. Progeny of a single plant produced in the same year may grow at different rates and flower over an extended period at different sizes. For Frasera this provides an alternative to iteroparity or seed dormancy as a means of persisting in an unpredictable environment. Both the increase in seed—set and the lowering of seed predation among Frasera individuals in dense neighborhoods have probably provided the selective basis for the evolution of periodic synchronous flowering.
Dynamics of Competition in a Variable Environment: Experiments with Two Diatom Species
Authors: Grover James P.
Published: May 2006

The hypothesis that environmental variability promotes phytoplankton species diversity was investigated experimentally using phosphorus—limited continuous cultures. Variability was introduced as a series of phosphorus pulses delivered at 8—d intervals. The growth of Synedra sp. and Fragilaria crotonensis was examined in cultures of natural phytoplankton, in cultures containing both species no other phytoplankton, and in monocultures. In cultures inoculated with comparable densities of both species, Synedra was competitively dominant to Fragilaria in both constatt and varying cultures, but the rate of competitive exclusion was slower in varying cultures. Synedra was able to invade both constant and varyingg monocultures of Fragilaria when injected into those monocultures, but Fragilaria could not invade monocultures of Synedra. In monocultures, the average yield of Fragilaria was increased by variability, but that of Synedra was not. During pulses, Fragilaria had a higher uptake rate per cell than Synedra, but the two species had similar specific uptake rates. Previous studies (Tilman 1981) suggest that Synedra is a better steady—state competitor for phosphorus than Fragilaria. Although Fragilaria may be better able to exploit phosphorus pulses than Synedra, this advantage is apparently too weak to prevent the dominance of Synedra under the regime of pulsed phosphorus supply used here.
Monarch Butterfly (Danaus Plexippus) Thermoregulatory Behavior and Adaptations for Overwintering in Mexico
Authors: Masters Alan R. , Malcolm Stephen B. , Brower Lincoln P.
Published: May 2006

Monarch butterflies from eastern North America migrate each fall to high altitude habitats in central Mexico and must avoid freezing, desiccation, heat stress, and predation while drawing on unrenewable lipid reserves for at least 90 d. Survival involves a heat—gaining morphology and behaviors in which thermoregulation plays a central role. Heat production by shivering permits raising of thoracic temperatures sufficiently for the butterflies to crawl up vegetation to sun—bask where they warm rapidly to flight threshold (°12.7°—16.0°C). Shivering and basking can also achieve or maintain flight temperature during partly cloudly weather. Both behaviors delay the decline of thoracic temperatures below flight threshold and consequent thermal trapping either on the ground beneath clusters of other monarchs, or while the butterflies are at water or nectar sources. The monarch thus is the first butterfly in which shivering has been shown to be of major ecological importance. At the other end of the thermal spectrum, basking in the intense insolation at this lattitude causes rapid warming at ambient temperatures as low as 9°. To avoid overheating and rapid depletion, the butterflies adopt sum—minimizing postures or fly out of insolated clusters and glide above the colony, losing heat to the cold ambient. Calculations suggest that a large proportion of the butterfies dies from depleting lipid stores. Maintenance of low body temperatures to conseve lipids is one of the major reasons why migrant monarchs select these high mountain forest as their overwinterin habitat.
Bet Hedging and the Diapause Strategies of the Cricket Allonemobius Fasciatus
Authors: Bradford Michael J. , Roff Derek A.
Published: May 2006

We investigated the roles of bet hedging and developmental plasticity in the diapause strategy of a partially bivoltine population of the cricket Allonemobius faciatus. When reared under naturally changing environmental conditions, first—generation females produced increasing proportions of diapause eggs over their reproductive life—span, presumably reflecting the decreasing probability over time that sufficient growing season remains for a second generation. The transition from direct—developing to diapause eggs occurred over 35 d for the population, suggesting a considerable degree of bet hedging. The analysis of individuals, however, revealed a much more rapid response and large between—female variability in the median (50%) diapause date. This rapid transition from nondiapause to diapause eggs implies that interannual variability in season length has not resulted in the evolution of substantial risk—spreading strategy for individual females. Our results highlight the need to study life history variation at the level of the individual rather than the population.
Bumble Bee Colonization, Extinction, and Reproduction in Subalpine Meadows in Northeastern Utah
Authors: Bowers Michael A.
Published: May 2006

The distributional patterns of six bumble bee (Bombus) species in discrete subalpine meadows in Utah were studied over the course of a summer. Data on the movements of individuals were obtained through mark—recapture studies of queens during nest initiation and of workers later in the summer. Censuses of the species and abundance of bumble bees and flowers in sample quadrats of 20 meadows revealed patterns of diversity, colony extinction (inferred from the absence of a species after it had been previously recorded in the meadow), and production of new reproductives. The results suggest that the number and diversity of queens that colonize a meadow in early summer is a function of meadow area. In mid to late season, after nest sites have been chosen and colonies established, meadow floristics (flower composition) governs within—meadow Bombus species composition and the persistence, ontogeny, and reproduction of bumble bee colonies. Workers appear not to utilize flowers outside the meadow where their colony is located. Specific results were that (1) mid— to late—summer bumble bee diversity was best correlated with meadow floristics, (2) flower compositions in meadows where local extinctions of Bombus occurred differed markedly from those where all species persisted, (3) the distribution of new reproductives among the meadows was independent of bombus species composition, and (4) late—season reproductive timing of Bombus flavifrons appeared to be a function of floristic composition and, particularly, nectar availability. These results underscore the importance of flower resources while downplaying the role of interactions among Bombus colonies and species in accounting for patterns of species persistence and colony reproduction.
Costs of Reproduction in a Willow: Experimental Responses Vs. Natural Variation
Authors: Fox John F. , Stevens George C.
Published: May 2006

We report on the interaction between reproduction and subsequent reproductive and vegetative growth in female plants of a dioecious, early successional, tall shrub, the feltleaf willow (Salix alaxensis). We measured natural variation among individuals in flowering, and its correlation with subsequent shoot growth and flowering. We also experimentally curtailed reproductive allocation in two ways: (1) by removing inflorescence buds in April when plants were dormant, and (2) by covering inflorescences to prevent pollination and fruit development. We hypothesized that these treatments would cause manipulated plants to retain more resources and to deploy them subsequently in shoot growth or reproduction. However, treatments had no significant effect upon shoot elongation, which decreased slightly after floral buds were removed. Treatments did significantly increase the fraction of buds differentiating as inflorescences instead of shoots. We compared the experimental outcome with the natural, phenotypic variation among plants. Among plants of either the treated or the unmanipulated groups, there was a significant negative correlation between current reproduction and subsequent growth; the corresponding regressions did not differ between the two groups. Thus, natural phenotypic variation suggested a tradeoff between reproduction and growth, but a direct test by experiment revealed only a tradeoff between current and future reproduction. Furthermore, the negative correlation seen in unmanipulaed plants was unaffected by treatments curtailing reproductive investment. We conclude that this correlation is not directly related to allocation contraints, and does not provide reliable evidence about costs of reproduction. The increased commitment to flowering that occurred after experimental reduction of current reproductive investment may be evidence for a cost of reproduction, although the response need not have been caused directly by resource allocation constraints.
Assessing Effects of Unreplicated Perturbations: No Simple Solutions
Authors: Stewart-Oaten Allan , Bence James R. , Osenberg Craig W.
Published: May 2006

We address the task of determining the effects, on mean population density or other parameters, of an unreplicated perturbation, such as arises in environmental assessments and some ecosystem—level experiments. Our context is the Before—After—Control—Impact—Pairs design (BACIP): on several dates Before and After the perturbation, samples are collected simultaneously at both the Impact site and a nearby "Control." One approach is to test whether the mean of the Impact—Control difference has changed from Before to After the perturbation. If a conventional test is used, checks of its assumptions are an important and messy part of the analysis, since BACIP data do not necessarily satisfy them. It has been suggested that these checks are not needed for randomization tests, because they are insensitive to some of these assumptions and can be adjusted to allow for others. A major aim of this paper is to refute this suggestion: there is no panacea for the difficult and messy technical problems in the analysis of data from assessments or unreplicated experiments. We compare the randomization t test with the standard t test and the modified (Welch—Satterthwaite—Aspin) t test, which allows for unequal variances. We conclude that the randomization t test is less likely to yield valid inferences than is the Welch t test, because it requires identical distributions for small sample sizes and either equal variances or equal sample sizes for larger ones. The formal requirement of Normality is not crucial to the Welch t test. Both parametric and randomization tests require that time and location effects be additive and the Impact—Control differences on different dates be independent. These assumptions should be tested; if they are seriously wrong, alternative analyses are needed. This will often require a long time series of data. Finally, for assessing the importance of a perturbation, the P value of a hypothesis test is rarely as useful as an estimate of the size of the effect. Especially if effect size varies with time and conditions, flexible estimation methods with approximate answers are preferable to formally exact P values.
Size‐Based Demography of the Seaweed Ascophyllum Nodosum in Stochastic Environments
Authors: Aberg Per
Published: May 2006

Large variations in vital rates are found among individuals in populations of Ascophyllum nodosum along the Swedish west coast due to temporal variation in the environment (years with or without ice). The population growth in these stochastic environments was analyzed for two populations using model simulations. A matrix model with a set of three population projection matrices for each population described the demography during three different types of years (environments), i.e., years with (1) no ice, (2) moderate damage due to ice, (3) large damage due to ice. The frequency of ice years is known for the area studied and was higher at one site (Goteborg) compared to the other (Tjarno). These frequencies were used as the stochastic process generating the sequence of matrices. The mean mortality rates decreased with increasing size in both populations. Simulating the growth for survival matrices (no reproduction included) resulted in an average population growth rate @ls of 0.84 at Tjarno and 0.76 at Goteborg. The average growth rate @ls was set to 1 by scaling the set of matrices for each population with a fertility function. To achieve @ls = 1, °3.2 times more fertility in each size class was required at Goteborg. Because of the large environmental variability, @ls varied from 0.3 to 1.3 during a long stochastic sample path. The maximum lifetime of A. nodosum individuals was estimated to be °50—60 yr, which probably is shorter than for areas with no ice. The population growth with the sets of scaled matrices (reproduction included) was simulated by following a large number of subpopulations for 300 yr. With extinction defined as the first passage time through the point N = 1, °25% of the subpopulations "died" during the simulation, and the mean extinction time for those who died was 163 yr. An elasticity analysis of @ls to changes in the matrix elements showed that at each time step is was most important for A. nodosum individuals to survive and grow to the next larger size class or to survive and remain in the same size class.
Benefits and Costs of Defense in a Neotropical Shrub
Authors: Sagers Cynthia L. , Coley Phyllis D.
Published: May 2006

Benefits and costs are central to optimality theories of plant defense. Benefit is the gain in fitness to reducing herbivory and cost is the loss in fitness to committing resources to defense. We evaluate the benefits and costs of defense in a neotropical shrub, Psychotria horizontalis. Plants were either exposed to herbivores or protected within a cage of fine mesh in three gardens planted in large light gaps on Barro Colorado Island, Panama. Two cuttings of each of 35 clones were paired based on initial masses and assigned randomly to a garden and to either the exposed or protected treatment. Annual rates of herbivory were 37% for exposed plants and 17% for protected plants. Among the exposed plants, high tannin concentration and tough leaves reduced herbivory. Twenty months after planting, the protected plants had a 10—fold higher growth rate, and 5.5 times more leaf area and dry mass than the protected plants. Twenty—two percent of the protected plants flowered, whereas no exposed plants flowered. Within the protected treatment, both total tannin concentration and leaf toughness varied among clones, which suggests that these traits are heritable. Total tannin concentration, but not toughness, was negatively correlated with growth for protected plants, which implies a cost to producing tannins. High tannin concentration and tough leaves reduced herbivory, however, these benefits of defense were balanced by the costs such that there were no significant differences in growth among exposed plants. These results are consistent with the hypotheses that there are benefits to being defended, that defense can be costly, and that herbivores act as selective agents in the evolution of plant defense.
Competition and Allometry in Three Species of Annual Plants
Authors: Weiner Jacob , Thomas Sean C.
Published: May 2006

Comparisons between competing and noncompeting populations of three annual plant species demonstrate that plant allometry is altered by competition. When plants are grown in isolation, relationships between stem diameter, height, and plant mass generally show simple allometry (i.e., the relationships are linear on log—log scale). When plants are competing, however, these relationships are curvilinear or discontinuous. We attempt to clarify the relationship between interindividual allometry (the usual source of allometric data) and allometric growth. When plants are not competing, the allometric relationships among individuals of different sizes at one point in time and the allometry of individual plants as they grow appear to be similar, but these two classes of allometric relationships are very different for competing plants. We present a simple model that explains both static and dynamic patterns of plant allometry in terms of (a) the allometric responses of individual plants to competition, and (b) the size dependence of growth after the onset of competition. Our results illustrate the importance of reciprocal interactions between competition and allometry, and emphasize the difficulty in making inferences from one size measure to another. We conclude that the commonly held assumption that plant size is a single entity, which can be reflected by any of several measures, may not be justified.
Turnover of Breeding Birds in Small Forest Fragments: The "Sampling" Colonization Hypothesis Corroborated
Authors: Haila Yrjo , Hanski Ilpo K. , Raivio Suvi
Published: May 2006

A sampling view of the colonization of small habitat fragments by birds is based on the assumption that individual birds acquire territories in the fragments randomly, and their occurrence frequencies follow from abundances in the regional species pool. We tested the following predictions of this view using birds census data from a fragment "arhchipelago" in the southern Finnish taiga: (1) the location of breeding pairs varies randomly among the fragments from year to year, and (2) the pattern of species accrual with increasing sample size is similar to radom sampling. The data agreed with the predictions. For most species the distribution of pairs in the fragments agreed with the Poisson prediction. The pattern of species turnover in single fragments agreed with the sampling view so that an average fragment had about 10 pairs of 7 species in a single breeding season, and about 40 pairs of 13 species over the 4—yr period. The sampling model is a "null" assumption that should be considered when assessing the occurrence of birds in small habitat fragments. This requirement defines methodological imperatives that have often been violated in ornithological studies; we discuss such methodological problems. The sampling model implies that local turnover in small fragments may be mechanical reflection of change in territory location from year to year and disconnected from population dynamics. In such case, apparent "extinctions" and "recolonizations" in single fragments are ecologically trivial. Assessing the interface between sampling colonization and genuine metapopulation dynamics is an urgent challenge for conservation ecology, particularly in temperate and boreal areas.
Species‐Dependent Effects of Zooplankton on Planktonic Ecosystem Processes in Castle Lake, California
Authors: Brett M. T. , Wiackowski K. , Lubnow F. S. , Mueller-Solger A. , Elser J. J. , Goldman C. R.
Published: May 2006

Freshwater zooplankton communities typically undergo pronounced seasonal succession and often show dramatic responses to external factors such as changes in zooplanktivore abundance. For this reason it is important to assess how common zooplankton species differ in their grazing impacts on planktonic ecosystems. To accomplish this we used single—species treatments with Diacyclops bicuspidatus thomasi, Daphnia rosea, Diaptomus novamexicanus, and Holopedium gibberum in situ in Castle Lake, California. These taxonomically diverse zooplankters differ markedly in feeding modes and typical seasonal population dynamics. We measured the response of nutrient concentrations, bacterioplankton abundance, phytoplankton species composition and biomass, primary production, a grazing index (phaeophytin/chlorophyll a), and microzooplankton to our single species treatments. The filter—feeding cladocerans Daphnia and Holopedium and the raptorial filter—feeding calanoid copepod Diaptomus showed several effects typical of herbivorous zooplankton. These included increasing dissolved nutrient concentrations, decreasing algal biomass and the abundance of several common algae, increasing a grazing index, increasing the ratio of bacterial to algal biomass, as well as depressing ciliate microzooplankton abundance. The raptorial cyclopoid copepod Diacyclops was apparently exclusively predaceous as it decimated the ciliate and rotifer microzooplankton, but had no notable effect on the other measured parameters relative to zooplankton—free controls. Diacyclops had the greatest effect on the microzooplankton and Daphnia and Diaptomus had the greatest effect on inorganic nutrients and characteristics of the phytoplankton. Holopedium had qualitatively similar but weaker impacts compared to Daphnia and Diaptomus. None of the zooplankton treatments had an effect on bacterioplankton abundance, nor did grazing by any of these zooplankters increase total algal primary production. Our results suggest differences in the grazing effects of common freshwater zooplankton can be pronounced and indicate that both seasonal succession and long—terms shifts in the zooplankton community structure should have marked effects on microzooplankton competitors and prey, the phytoplankton, and nutrient cycling.
Loop Analysis: Evaluating Life History Pathways in Population Projection Matrices
Authors: van Groenendael Jan , de Kroon Hans , Kalisz Susan , Tuljapurkar Shripad
Published: May 2006

A new way of analyzing life histories is presented, based on the use of elasticities as derived from the limit properties of population projection matrices. In general there is not a simple relation between life history traits and matrix elements and their corresponding elasticity values. By recognizing the fact that a life history trait always is part of a life history pathway that can be contrasted to other pathways, this problem can be overcome. The various life history pathways form loops within the life cycle graph, the graphical representation of the projection matrix. It is shown that one can calculate the elasticities of such loops from the single elasticity values in the matrix in an unequivocal way. This presents a new and meaningful decomposition of total elasticity in population projection matrices and provides a potentially powerful tool to evaluate the importance of alternative life history options in demographic studies. This point is demonstrated using two previously published sets of data.
Competitive Responses of Eight Old‐Field Plant Species in Four Environments
Authors: Wilson Scott D. , Tilman David
Published: May 2006

Variation in relative competitive abilities among environments has been proposed to control the species composition of plant communities. We tested whether the competitive responses of eight species varied with fertility and disturbance in an old field. We also tested whether the competitive effects of vegetation varied with fertility and disturbance in an old field. We also tested whether the competitive effects of vegetation varied with fertility and disturbance. Plots received one of four combinations of nitrogen (added or not) and disturbance (tilled or not) in each of five years. There were 10 replicates of each treatment. In the fifth summer, transplants of eight species comprising annual and perennial forbs and grasses were grown in all four combinations of the N and disturbance treatments. Transplants were grown in subplots with all neighbors, with only roots of neighbors, or with no neighbors. Transplant roots and shoots were harvested after one summer and individual growth rates were determined. Growth rate varied significantly with the interaction among species, competition, and N, suggesting that species differed in their competitive responses and that these differences varied with N availability. Comparisons among competition treatments at each level of N showed six distinct responses in the eight species examined. The competitive effect of neighboring vegetation shifted from roots to shoots as N increased, but decreased in disturbed plots. The total effect of competition increased significantly with neighbor mass in 4 of the 32 examined (eight species in four combinations of N and disturbance). All four cases involved annuals in annual neighborhoods. The results for eight species confirmed earlier single—species diverged in their responses to competition as soil N availability varied.
Differential Herbivory in an Oak Population: The Role of Plant Phenology and Insect Performance
Authors: Mopper Susan , Simberloff Daniel
Published: May 2006

Individual trees in a North Florida population of Quercus geminata, (sand live oak) vary widely in rates of herbivory by a leaf—mining moth (Stilbosis quadricustatella). Some oaks are always heavily infested while neighboring trees remain lightly attacked. We conduct field experiments in 1991 and 1992 to determine if leafminer performance on heavily and lightly attacked trees contributed to this pattern. We also compared the leaf production phenology, leaf area, and stem length of heavily and lightly attacked trees to determine if tree phenotypic traits influenced herbivory. Our results indicate that: (1) Light attacked trees produced leaves earlier in the spring than heavily attacked trees, and the difference among trees in leaf production phenology explained 61% of the variation in leafminer densities (P = 0.008). (2) Lightly attacked trees produced significantly smaller leaves than heavily attacked trees, but leaf size explained only 13% of the variation in herbivory (P = 0.048). (3) There were no differences in larval performance on heavily and lightly attacked trees in either 1991 or 1992, with the exception of egg mortality, which was significantly higher on lightly attacked trees in 1991 (P = 0.04). (4) Host plants caused greater leafminer mortality than natural enemies, but neither differed significantly between heavily and lightly attacked trees. Our transfer experiments and observations of wild leafminers revealed little difference in leafminer performance on heavily and lightly attacked Quercus geminata trees. However, tree phenotypic traits differed, especially the timing of spring foliation, which occurred significantly earlier among trees with low rates of herbivory. These results suggest that leafminer ovipositional preference, rather than performance, could be an important determinant of differential herbivory. There may be an unknown selective advantage to preferential oviposition on trees with delayed spring leaf production.
Host Dispersal and the Spatial Spread of Insect Pathogens
Authors: Dwyer Greg , Elkinton Joseph S.
Published: May 2006

Empirical studies of the spatial spread of insect pathogens have emphasized the importance of high dispersal rates, but typically rely strictly on observational data. In contrast, existing mathematical models on the spatial spread of infectious diseases have suggested that a highly infectious disease can spread rapidly even if the dispersal rate of its host is low; such models, however, are largely untested. To understand how host dispersal and disease infectiousness affect the spatial spread of infectious disease, we performed a field experiment in which we released a nuclear polyhedrosis virus (NPV) into two experimentally established, disease—free gypsy moth (Lymantria dispar) populations. We simultaneously and independently measured the long—distance dispersal of ballooning, first—instar, gypsy moth larvae, the stage in which dispersal is by far the greatest. Surprisingly, dispersal by ballooning was a good predictor of NPV spread only in the first few weeks of spread. Subsequently, the virus spread much farther and less directionally than did ballooning larvae. No infections appeared in control plots that started with only uninfected larvae, confirming that the virus we released caused the epizootic in our experimental populations. On the theory that additional spread was due to a combination of small—scale larval dispersal and a high rate of disease transmission, we compared our data to the predictions of a mathematical model for the spatial spread of insect pathogens that combines disease transmission and small—scale host dispersal. By using independent estimates of each of the model parameters, we used the model to make post hoc predictions of the rate of spread of the virus from an initial distribution dictated by larval ballooning. Although the model included both initial long—distance dispersal by larval ballooning and subsequent short—distance dispersal by larval crawling, it poorly matched the observed distribution of the virus. An alternative and as yet untested hypothesis is that the observed spread of the virus may be due to mechanical vectoring by a parasitoid fly, a mechanism for which there are as yet no available mathematical theory or relevant field data.
Primary Production in the Orinoco River
Authors: Lewis William M.
Published: May 2006

Autotrophic carbon fixation was studied from 1982 to 1985 along a 500—km reach of the Orinoco River, Venezuela, and in three major tributaries (Apure, Caura, Caroni). Among the tributaries, the annual mean gross primary production (measured as carbon) was highest for the Apure River (26 mg°m—2°d—1), a whitewater river that drains from the Llanos region to the north of the Orinoco. Mean annual production was lower in the blackwater tributaries draining from the Guayana Shield to the south (Caura, 13 mg°m—2°d—1; Caroni, 4 mg°m—2°d—1). In the Orinoco main stem, mean production varied among stations from 19—43 mg°m—2°d—1. Both in the main stem and the tributaries, production per unit volume and per unit area peaked during the period of low water. Seasonal variations at a given station were almost entirely explained by variations in the phytoplankton biomass per unit volume rather than by seasonal variations in transparency. However, differences in mean transparency among the tributaries partly explained differences in mean production per unit area: suppression of potential production by dissolved and suspended substances was 85% in whitewaters (Apure), 40% in blackwaters (Caura, Caroni), and intermediate in the Orinoco main stem. Autotroph biomass (as chlorophyll a) was highest for the Apure River (mean, 0.36 mg/L), lowest for the blackwater tributaries (0.01—0.04 mg/L), and intermediate for the Orinoco River (0.11—0.19 mg/L). Transport of biomass had a different seasonal pattern than either production per unit area or biomass per unit volume. All stations along the main stem of the Orinoco River and all tributaries except the Caroni, which is impounded by a large dam, showed a significant rise in transport of algal biomass at high discharge, particularly on the declining limb of the hydrograph. Significant transport also occurred during the period of low water, well after the period of contact between the ricer and the floodplain. A mass—balance method was used in estimating the yield of phytoplankton biomass from the floodplain to the river. The annual carbon yield per unit floodplain area from 3660 km2 of floodplain was only 75 mg°m—2°yr—1, or < 1% of estimated floodplain phytoplankton production. The data indicate that non—floodplain sources, including within—channel and near—channel stagnant or slow—flowing areas, account for 63% of the annual transport of phytoplankton carbon (2.4 x 106 kg/yr), while the floodplain accounts for only 37%. The floodplain, although productive, conserves algal production, and in this respect functions as a closed system, even though it is physically an open system.
A Model of Algal Exports in Some Iowa Streams
Authors: Swanson Clayton D. , Bachmann Roger W.
Published: May 2006

The rivers of central Iowa carry high densities of suspended algae. Evidence is presented to support the hypothesis that these plants are derived from benthic populations on the streambed. In an analysis of several streams, a significant correlation was found between the amounts of algal chlorophylls being exported downstream and the areas of the wetted streambed upstream from the sampling points. In the warm months the algal exports amounted to 4.5 mg°m(—2)°day(—1) of chlorophyll a.
Are Autecologically Similar Species Also Functionally Similar? A Test in Pond Communities
Authors: Harris Patricia M.
Published: May 2006

The study of community dynamics often requires that species by grouped into a smaller number of system components. Species are typically grouped on the basis of their autecological traits (sensu lato), but this assumes that autecology is an adequate predictor of species' functional roles are in the community. I tested whether autecological similarities correctly predict similarities among species' effects using experimentally reconstructed temporary pond communities. I individually manipulated six taxa (two predators, two herbivorous snails, and two herbivorous tadpoles) to assess the effects of each on a complex background community. The species' effects were all best explained as direct effects of a consumer on its food; no clear evidence of indirect effects was found. The six species could be divided into three statistically distinct groups based on their effects on the background community: "Ambystoma," "Gambusia," and "herbivores." The pattern of similarities among species' effects was well predicted by the pattern of similarities among their autecological traits. The results suggest that, at least over the spatial and temporal scale of this experiment, autecological traits are indeed a useful guide to grouping species for studies of system dynamics.
Epibenthic Predators and Marine Meiofauna: Separating Predation, Disturbance, and Hydrodynamic Effects
Authors: Palmer Margaret A.
Published: May 2006

In many marine soft—bottom systems, natant predators disturb the bottom while foraging for infaunal prey. This study was designed to determine if the predatory fish Leiostomus xanthurus exerts its effect on meiofaunal prey primarily via predation (consumption) or via disturbance and if flow modifies this effect. Experiments were conducted in a laboratory flume that allowed for both the addition and removal of fish and the mimicking of predator—induced disturbance, while controlling flow. Prey species differed with respect to the relative effects of predation and predator—induced disturbance. Disturbance accounted for 30—55% of the predator—associated decreases in copepods and foraminiferans, while disturbance accounted for virtually all of the nematode mortality. The latter was significant only when flow was absent. For other prey taxa, mortality was not influenced by flow; however, predator—induced drift (down—stream transport) of all meiofauna was significantly increased when flow was present. If experimental sites are not in equilibrium between upstream and downstream transport, then ignoring this drift could result in an overestimation of fish consumption on meiofauna by as much as 50%. Thus, the mechanisms by which fish predators influence meiofauna prey included direct consumption, disturbance—induced mortality, and enhanced water—column transport.
Importance of Belowground Herbivory: Pocket Gophers May Limit Aspen to Rock Outcrop Refugia
Authors: Cantor Lisa F. , Whitham Thomas G.
Published: May 2006

We hypothesized that pocket gophers, Thomomys bottae, represent keystone herbivores in slowing down or preventing aspen, Populus tremuloides, invasion of mountain meadows in northern Arizona. Observations at the edge of three aspen clones bordering meadows showed that where pocket gopher disturbance was relatively low (0.70 mounds/m2) aspen mortality was <20%, but where disturbance was high (5.6 mounds/m2), aspen mortality increased to 90%. To separate cause and effect, aspen performance was compared in three replicate pocket gopher removal and control plots. Aspen release from pocket gopher herbivory on roots was rapid; for the single replicate monitored only 4 mo after the initial removal of pocket gophers, aspen survival increased to a value 3.5 times, vegetative reproduction 2.5 times, and average tree growth rates 3 times that of the control. At the end of 16 mo all three replicates showed similar increases. Because of these findings we reasoned that aspens might be limited to rock outcrops that are largely inaccessible to pocket gophers and act as refugia from belowground herbivory. Consistent with this hypothesis, in 32 aspen—meadow associations, we found that distributions of aspens and pocket gophers were nonoverlapping 93% of the time. Pocket gophers were almost always associated with the deep soils of mountain meadows while the centers of aspen clones were almost always associated with rock outcrops. Although tree ring analyses indicate that aspens grow best in the deep soils of meadows in the absence of pocket gophers, when pocket gophers encounter aspens, tree mortality can be high.
Direct Evidence for Meteorologically Driven Long‐Range Dispersal of an Economically Important Moth
Authors: Showers William B. , Whitford Fred , Smelser Richard B. , Keaster Armon J. , Robinson John F. , Lopez Juan D. , Taylor S. Elwynn
Published: May 2006

Ecological and evolutionary research on lepidoptera migration has traditionally concentrated on the long—range dispersal of day—flying butterflies. Nocturnal flights at high altitudes and the difficulty in species identification have resulted in a paucity of evidence supporting long—range dispersal of moth species attacking economic crops. Black cutworm (Agrotis ipsilon [Hufnagel]), moths were reared on diet containing a marker dye and were released at Crowley, Louisiana, and College Station, Texas. Traps baited with A. ipsilon sex pheromone were stationed in a line east to west over 618 km. There were six trap lines 108 km apart north to south and two more trap lines 720 km further south. Following release, six male moths were captured after two to four nights in Missouri, Kansas, and Iowa. The climatological conditions that allow this rapid long—range movement were quantified and the data suggest that the moths were traveling at altitudes of <1500 m. These results may provide information on the mechanism for long—range migration of many moth species into the north central United States. These findings also strengthen the compatibility of other components of A. ipsilon physiology and behavior with "oogenesis—flight syndrome" (sexual maturity after long—range flight).
Interaction Between Food Availability and Predation Mortality Mediated by Adaptive Behavior
Authors: Anholt Bradley R. , Werner Earl E.
Published: May 2006

Increased activity rates in larval anurans rates are associated with both higher growth rates and higher predation mortality. Models of adaptive foraging behavior in the face of predation risk predict that at higher resource levels, foraging activity should be reduced. Thus, at higher resource levels predation mortality should also be reduced. We manipulated the resources available to Rana catesbeiana tadpoles and then measured the activity of tadpoles in the presence of caged dragonfly larvae and the mortality rate of the tadpoles when the dragonflies were free to forage. At low food levels the tadpoles moved more often and more quickly. Similarly, at low food levels the tadpoles suffered higher predation mortality. The dependence of predation mortality on resources available to prey underlines the futility of characterizing population regulation as being due to predation or resources. Adaptive variation in behavior responds to both pressures simultaneously. These results suggest the possibility that adaptive variation in behavior may lead to density—dependent population regulation. Density—dependent depletion of resources by prey should lead to increased activity levels, which will result in higher per capita predation rates. The generality of the trade—off between growth rate and mortality rate argues that this mechanism may be widespread. If adaptive variation in behavior is as widespread as it appears, incorporating this variation into population dynamic modelling may improve our ability to predict the outcome of interactions within ecological communities.
Seasonal Succession and the Strength of Intra‐ and Interspecific Competition in a Daphnia Assemblage
Authors: Hu Shixi S. , Tessier Alan J.
Published: May 2006

Species of the genus Daphnia are generalized filter—feeders and potentially strong competitors, yet several species often coexist in the open waters of lakes. We investigated the population dynamics and competitive interactions of two Daphnia species in Gull Lake, Michigan. In each of 3 yr D. pulicaria was always common, while D. galeata mendotae became abundant only in late summer. The summer invasion of D. galeata mendotae was associated with a decline in abundance of D. pulicaria in some, but not all years. Decline of D. pulicaria was strongly predicted by the loss of oxygen from deep water during summer, which resulted in greater spatial overlap between the species, and by changes in the quality of the phytoplankton resources. We employed in situ enclosures to manipulate the density of each species and quantify the intensity of intra— and interspecific competition. Repeated competition experiments demonstrated that the relative exploitative ability of species reversed from early to late summer. Strong interspecific competition was often observed, but its strength varied with juvenile or adult stage, and with season. Daphnia pulicaria juveniles were always affected strongly by interspecific effects, while D. galeata mendotae juveniles were impacted largely by intraspecific effects. The relative strength of intra— and interspecific competition on adults reversed from early to late summer for each species. The magnitude of intra— and interspecific effects on D. pulicaria adults was similar during early summer, but by late summer interspecific effects were much stronger than intraspecific effects. The reverse was true for D. galeata mendotae adults; interspecific effects greatly decreased in late summer. This seasonal shift in the nature of competitive interactions provides an explanation for seasonal succession in this Daphnia assemblage.
Ecological Correlates of Regional Variation in Life History of the Moose Alces Alces
Authors: Saether Bernt-Erik , Andersen Reidar , Hjeljord Olav , Heim Morten
Published: May 2006

The food limitation hypothesis of population regulation states that a stable equilibrium will exist between a population and its food resources due to a density—dependent decrease in fecundity and/or increase in mortality. This hypothesis was tested for the moose (Alces alces) by comparing regional variation in life history characteristics in four Norwegian study areas, chosen to represent a gradient both in summer and winter range conditions. The rate of body growth was most rapid in the northern study area with the best summer ranges. Lowest body growth occurred in the population living under the poorest winter conditions. After snow—free winters the rate of body growth increased substantially, leading to large annual variations in selective regimes. The peak timing of ovulation of old females in the autumn showed a latitudinal delay. Females in the alpine population, with the poorest winter conditions, had significantly later mean calving dates and produced fewest calves per year. Gestation length appears to be dependent on nutritional condition of females during pregnancy. Mortality was highest in the northern study are where most of the deaths occurred during the summer. Very few calves died during the winter. These results suggest that a stable high—density equilibrium between moose and their food resources as expected from the food limitation hypothesis is unlikely. The decrease in fecundity and the increase in mortality under poor nutritional conditions during the winter has only a small effect on the population growth rate and is therefore unlikely to have a strong regulatory effect. In the absence of large predators, this will lead to large fluctuations in population size that will overshoot the carrying capacity.
Monitoring Long‐Term Population Change: Why are there so Many Analysis Methods?
Authors: Thomas Len
Published: May 2006

Monitoring long—term population change is an integral part of effective conservation—oriented research and management, and is central to the current debate on the status of Neotropical migrant land birds. However, the analysis of count data such as the Breeding Bird Survey is complicated by the subjective nature of trend estimation, and by limitations inherent to extensive, volunteer—based surveys, such as measurement error and missing data. A number of analysis methods have been used that differ in their approach to dealing with these complications and produce different estimates of population change when applied to the same data. There is, however, no consensus as to which method is the most suitable. Many analytical issues remain unresolved, such as model of trend, observer effects, treatment of missing observations, distribution of counts, and data selection criteria. These issues make it difficult to evaluate the relative merits of the methods, although a number of new approaches (nonlinear regression, Poisson regression, estimating equations estimates) offer promising solutions to some problems. I suggest the use of Monte Carlo simulations to empirically test the performance of the methods under realistic, spatially explicit scenarios of population change, and provide an example of the approach.