bluuebunny/crossref_metadata_2025 huggingface dataset:
Stability and Extinction of Laboratory Populations of Zooplankton Preyed on by The Backswimmer Notonecta
Authors: Murdoch William W. , Scott Mary Ann
Published: May 2006

Growth of the backswimmer Notonecta hoffmani is better on a diet of Daphnia pulex than on a diet of Ceriodaphnia reticulata. Offered a choice of these two zooplankton species, Notonecta selects Daphnia or large individuals of either species. To test whether this preference affects prey species and size—classes differently, we measured population changes when the predator and one prey species or the other were placed together in plastic tubs for 7—8 wk. Roughly half of the replicated laboratory populations of Daphnia pulex that were exposed to predation by a constant number and age—class of Notonecta were driven extinct within 8 wk. The remainder of these populations maintained densities similar to those of control populations without predators. Populations surviving at each sampling period partially compensated to mortality caused by predators by small increases in individual fecundity, which maintained their population birth rates at the same level as those of control populations. Fecundity was inversely related to the density of adult Daphnia and to the total biomass of Daphnia. In populations exposed to predation, the other compensatory mechanism was decrease in the mortality of young from factors (probably starvation) other than predation. Predation reduced the average size of individuals in Daphnia populations and consequently markedly reduced the average biomass. This presumably increased the food per capita available to juveniles and small adults in predation treatments over the level available to their counterparts in controls, and hence both reduced mortality from starvation and increased size—related fecundity. Populations exposed to predation appeared to have stable dynamics for varying periods. However, as the experiment progressed, apparently random events in some populations caused the predation rate to exceed Daphnia's ability to compensate, and in each successive intersampling period a few populations were driven extinct. Extinction was probabilistic. Thus both a deterministic and a stochastic framework are required to explain the dynamics of these laboratory populations. Only two of 18 Ceriodaphnia populations were driven extinct. Notonecta instar II had the largest effect on the density of this species and also caused its temporal variability to increase. We have less information on the dynamics of this zooplankter than on D. Pulex.
Mechanisms of Early Primary Succession in Subalpine Habitats on Mount St. Helens
Authors: Wood David M. , del Moral Roger
Published: May 2006

In the 6 yr following the 1980 eruption of Mount St. Helens in Washington state, vascular plant invasion of barren substrates in subalpine habitats has been limited despite the proximity of seed sources from resprouted vegetation. From 1983—1985 we counted 1st—yr seedlings and estimated percent cover of adults in grids of permanent plots located across the ecotone between residual vegetation and barren substrate created in 1980. We found that (1) seedling recruitment declined from 1983 to 1985, apparently in response to drought; (2) most seedlings occurred within 3 m of a conspecific adult; and (3) plots with an intermediate vegetation cover (5—72%) contained a significantly higher fraction of seedlings than did unvegetated plots. Thus, dispersal is limited in many species and nurse plants may play a key role in trapping seeds and promoting seedling establishment. We sowed 16 000 viable seeds from 22 subalpine species into 264 plots in barren substrates at two sites on the volcano. The design was a complete factorial, with species, site, and fertilizer as treatments. Survivorship through 1985 varied from 0 to 12%, with Sitanion jubatum, Stipa occidentalis, Polygonum newberryi, Eriogonum pyrolifolium, and Spraguea umbellata attaining the highest values. Survivorship was correlated with seed mass, and was higher at the site where the pre—eruption surface was exposed. Fertilizer increased the size of most seedlings but had only a marginal effect on survivorship. Species with high environmental tolerance generally dispersed short distances, whereas species that dispersed farther generally had low tolerances and apparently require site amelioration prior to establishment. The path of early succession depends upon the spatial position and dispersal abilities of species in the seed pool, and may not reflect environmental gradients.
Density‐Dependent Growth and Fecundity in the Paedomorphic Salamander Ambystoma Talpoideum
Authors: Semlitsch Raymond D.
Published: May 2006

The growth responses of larval salamanders reared under various environmental conditions were examined in relationship to adult fitness in a factorial experiment. Larvae were reared in artificial pond to measure simultaneously the effects of pond—drying regime, food level, and initial density on growth and fecundity of paedomorphic Ambystoma talpoideum. Pond—drying regime, initial density, and the drying regime x food level interaction affected early larval growth. Early larval growth was highly positively correlated with adult body size. Drying regime, food level, and initial density all significantly affected the number of ova paedomorphic females contained. The number of ova was positively correlated with body size. Removing the effects of snout—vent length by analysis of covariance revealed that variation in the number of ova was largely (84%) due to the effects of body size. Larvae grew fastest, attained the largest body size, and contained the most ova in constant water, high food, and low—density ponds. The results of this experiment demonstrate a direct relationship between phenotypic variation generated in the larval stage and adult fitness. These results suggest that the larval environment strongly affects the fecundity of adult salamanders at first reproduction and imply that behaviors influencing the selection of oviposition sites by parents can dramatically affect the ultimate reproductive fitness of the next generation.
The Influence of Food Supply on Local Density and Diversity of Sparrows
Authors: Pulliam H. Ronald , Dunning John B.
Published: May 2006

A long—term study of sparrows in an arid oak woodland in southeastern Arizona demonstrated that local population density of sparrows is independent of local seed abundance during years of moderate to high seed production. Experimental manipulation of the abundance and size distribution of seeds during 2 yr of moderate seed production had no discernible influence on the local abundance or diversity of sparrows. These experimental results support the hypothesis, based on previous nonexperimental observations, that sparrow populations in this habitat are locally food limited only during infrequent years of very low seed productions.
Comparisons of Seed Predation in Tropical, Tidal Forests From Three Continents
Authors: Smith Thomas J. , Chan Hung T. , McIvor Carole C. , Robblee Michael B.
Published: May 2006

An inverse relationship between the amount of seed predation and the dominance of a tree species in mangrove forest canopies has been hypothesized based on field studies conducted in Australia (Smith 1987a). Seed predation experiments have recently been performed in mangrove forests of North America (Florida and Panama) and southeast Asia (Malaysia) on several species of Avicennia, Bruguiera, and Rhizophora. The results of these experiments are compared with additional data from Australia to test the generality of the dominance—predation model. Significant differences were found in the amount of predation on four species of Avicennia. All Avicennia were consumed in greatest quantity where they were rarest in the forest canopy and in least amounts where they dominated the canopy. Consumption of Rhizophora apiculata in Australia and Malaysia followed patterns similar to that for Avicennia. For R. mangle, however, contradictory results were obtained. No R. mangle were eaten in Florida. In Panama, more were consumed where this mangrove dominated the canopy compared to a forest where it was rare. Results for Bruguiera cyclindrica in Malaysia supported the dominance—predation model, whereas results from Australia for B. gymnorrhiza did not. Data for Acicennia clearly support the dominance–predation model. Regardless of species, seed predators exert an influence on the distribution of Avicennia in mangrove forests. Variation in the amount of predation on Rhizophora among regions is partly attributable to differences in the composition of the predator guilds between the forests studied. Predator guilds in the Malaysian and Australian forests are dominated by grapsid crabs. In Florida, crabs are minor consumers of propagules and three genera of snails are most important. Seed predation is variable among forests. The effect of seed predators on mangrove forests is related to the type of mangrove tree and composition of the seed predator guild.
Intraspecific Competition and Facilitation in a Northern Acorn Barnacle Population
Authors: Bertness Mark D.
Published: May 2006

The role of density—dependent mortality in shaping adult populations of the acorn barnacle, Semibalanus balanoides, was studied on a sheltered New England shore and revealed that high recruitment densities can have both positive and negative effects on barnacle survival. Recruitment greatly exceeded the capacity of the space to support adults in the three years of this study. At low tidal heights, recruits grew rapidly, experienced severe crowding, and rarely survived to reproduce. At higher tidal heights, individuals grew more slowly and were less affected by crowding, and many (°10%) survived to reproduce. Recruit density was manipulated to examine crowding effects on survival. At low tidal heights, no recruits survived to reproduce at any density, due to intraspecific crowding, algal overgrowth, and predation. At intermediate tidal heights, lower recruitment decreased intraspecific crowding and increased survivorship, while at high tidal heights, lower recruitment decreased survivorship. In the high intertidal habitat, S. balanoides survivorship was higher on boulders than on rock cobbles due to thermal buffering, and intraspecific crowding decreased rock and barnacle temperatures and facilitated survival. Shading high intertidal cobbles from solar radiation decreased maximum rock and barnacle temperatures, dramatically increased survivorship and eliminated the survivorship advantage of high densities. High recruitment of S. balanoides can lead to massive density—dependent mortality in physically benign habitats, but in physically stressful habitats high recruitment density may buffer individuals from physical stress and facilitate survival. Variation in the nature of density—dependent interactions along environmental gradients may be important in generating pattern in many natural populations.
Nitrogen Transformations Following Tropical Forest Felling and Burning on a Volcanic Soil
Authors: Matson Pamela A. , Vitousek Peter M. , Ewel John J. , Mazzarino Maria Julia , Robertson G. Philip
Published: May 2006

We measured nitrogen transformations and loss following forest clearing in a relatively fertile tropical forest site. Nitrogen mineralization, nitrification, and amounts of ammonium and nitrate increased substantially in surface soils during the 6 mo following burning, then returned to background levels. The nitrogen content of microbial biomass declined to half its original value 6 mo after clearing and remained low in the cleared sites. Plant uptake of nitrogen was substantial on cleared plots (50 g/m2), but it accounted for only 18% of 15N label added to field plots. Microbial immobilization of 15N was small relative to that in a cleared temperate site, and measurements of denitrification potentials suggested that relatively little mineralized nitrogen was lost to the atmosphere. Substantial amounts of nitrogen (40—70 g/m2) were retained as exchangeable bound nitrate deep in the soils of a cleared plot on which revegetation was prevented; this process accounted for 12% of the 15N label added to field plots.
Effects of Sex Ratio on Milkweed Beetle Emigration from Host Plant Patches
Authors: Lawrence W. S.
Published: May 2006

The effect of conspecifics on milkweed beetle, Tetraopes tetraophthalmus (Forster) (Coleoptera: Cerambycidae), emigration from patches of their host plant Asclepias syriaca was tested. The density and sex ratio of beetles was experimentally manipulated by repeatedly releasing marked individuals into two small, Tetraopes—free host plant patches. Beetle emigration rates were then monitored. Sex ratio affected the emigration rate of red milkweed beetles. Male beetles emigrated at a higher rate from host plant patches with male—biased sex ratios than from patches with female—biased sex ratios. Sex ratio had no effect on the rate of female emigration, and beetle density had no effect on the emigration rate of either sex. Male emigration rate was correlated with male flight frequency: males flew more when the sex ratio was male—biased. Males that had disappeared from a patch were more likely to return if the sex ratio was female—biased. While male behavior in the field was correlated with sex ratio, the actual determinant of male behavior was shown to be the number of receptive females in a patch. Males remained in patches with receptive females, but since not all unmated females were receptive, sex ratio was a better measure of mate availability than female density. Since the probability of mating was sex ratio dependent, male emigration patterns resulted in males leaving areas where mating was unlikely. Thus, movement patterns were directly linked to male fitness.
Effects of Crowding and Host Plant Nutrition on a Wing‐Dimorphic Planthopper
Authors: Denno Robert F. , Douglass Larry W. , Jacobs Dan
Published: May 2006

We investigated the effects of crowding and host plant nutrition on the development time and body length (a correlate of fecundity) of Prokelisia marginata, a wing—dimorphic planthopper that feeds on the phloem sap of the intertidal marsh grass Spartina alterniflora. Crowding delayed development time for males and females of both volant macropters and flightless brachypters; however, host plant nutrition did not affect development time. The body length of both sexes increased when they were raised on nutritionally superior hosts, but the nutritional effect on body size was greater for females than males. Body length exhibited a significant crowding x plant nutrition interaction; crowding had a greater reducing effect on body length if planthoppers were raised on marginal—quality hosts than on nutritionally superior hosts. These findings are discussed in the context of the performance, population dynamics, and migration behavior of P. marginata in its various habitats.
Grey Squirrel Food Preferences: The Effects of Tannin and Fat Concentration
Authors: Smallwood Peter D. , Peters Wm. David
Published: May 2006

Both tannin and fat concentration in acorns have been postulated to affect squirrel preferences and foraging strategy. Our study directly tests the effects of tannin and fat on the food preferences of free—ranging squirrels (Sciurus carolinensis). Using food items processed from white oak (Quercus alba) acorns with various amounts of tannin and fat added, we find that added tannin significantly reduces the probability that a food item will be eaten and the length of time spent eating it. The addition of fat is shown to attenuate the effects of tannin, but the overall effect is that squirrels do not forage in a manner that maximizes daily energy intake. These results are contrary to the findings of previous studies. A hypothesis to presented to account for the apparent contradiction: The squirrels may maximize energy intake over the entire fall/winter season rather than over a shorter period, such as each foraging bout or day.
Survivorship in Philopatric and Dispersing Kangaroo Rats (Dipodomys Spectabilis)
Authors: Jones W. Thomas
Published: May 2006

I use long—term mark—recapture data for banner—tailed kangaroo rats (Dipodomys spectabilis) to assess survival of philopatric and dispersing individuals. Results for 147 juveniles indicate that individuals that acquired parental resources, in this case large complex burrow systems and food catches, were significantly more likely to survive to reproductive age than individuals that successfully dispersed but did not acquire parental resources. Survival of juveniles that left their natal burrows and moved to other burrows was independent of the distance moved, which suggests that familiarity with natal areas and proximity to relatives impart no survival advantage in this species. Of the 37 juveniles that acquired natal burrows, 5 males and 7 females did so after their mothers abandoned those burrows. The frequency of burrow abandonment by adult females was independent of maternal age, and survival of abandoning mothers did not differ significantly from that of nonabandoning females
Symmetry, Change, Perturbation, and Observation Mode in Natural Communities
Authors: Lane Patricia A.
Published: May 2006

Like a kaleidoscope, nature presents continuously fleeting images of natural communities. It has been difficult to discern patterns of community structure in these elusive images. For both theoretical and applied considerations, however, holistic ecosystem properties need to be identified and quantified. An analogy of a kaleidoscope is used here to describe symmetry, change, and perturbation in the plankton community of Delaware Bay. Notions of symmetry are used extensively by physicists but much less so by biologist. For the ecological application give her, symmetry involves the notion of internal order and invariant pattern within the structure of natural communities. The concept of core structure is introduced to illustrate the inherent symmetry of plankton communities arising from consistent linkages and feedback relationships. The core is determined by using the most prevalent variables and links of a series of individual loop diagrams, each describing the community at a given point in time, to form a summary or composite model. Change refers to the seasonal alterations in community structure, whereas perturbation involves the source and entry location of environmental variation to the community network. Observing mode includes both the intrinsic (biological intuition) and extrinsic (loop analysis) models an observer uses to characterize symmetry, change, and perturbation. Loop analysis was used to model the Delaware Bay plankton community at 12 dates in an annual cycle. Loop diagrams consist of qualitative network models based on positive, negative, or zero interactions between variables in a pair for all pairs of variables. The fit of models to the data using the direct change technique resulted in 95% agreement. The core structure contained 18 variables in a three—tiered configuration with distinct subsystems of predators and small—sized algae and their associated herbivores. There was 3 nutrients, 5 algae, and 10 zooplankton variables in the core structure. It is proving to be consistent for several other marine communities. Various network properties related to community structure and stability were computed for the networks. The mean loop length varied from 1.6 to 2.8 variables of the individual networks; a network consisted of 22—31 loops of various lengths. Connectance ranged from 14 to 18%. The individual networks were stable; however, the stability of the core structure was questionable. The uniqueness problem, or verification of the single best—fit model, is presented as a concern central to all types of ecosystem modelling. The notion of three different—sized loop universes (sets of loop models) is introduced to illustrate one approach to this problem.
The Role of Microhabitat in Structuring Desert Rodent Communities
Authors: Price Mary V.
Published: May 2006

Interspecific competition is thought to be important in determining patterns of resource use and species abundances in natural communities. However, there have been few field tests of competition—based models of community structure. In this study, experiments were conducted with 4 coexisting desert rodent species to see whether competition is a sufficient explanation for their resource use and abundance patterns. Results were consistent with 3 predictions from competition theory. (1) The 4 species differed in their use of a resource, foraging microhabitat, which is potentially limiting to their populations. (2) Each species shifted its use of microhabitats in predicted directions when competitors were removed from or added to outdoor enclosures. (3) Each species was most dense where its preferred microhabitat was abundant, and augmentation of 1 microhabitat led to an increase in the density of the appropriate microhabitat specialist. These results suggest that competition maintains interspecific differences in foraging microhabitat, and that the availability of appropriate microhabitats determines species abundances on a local scale.
Fish Predation and Zooplankton Demography Indirect Effects
Authors: Vanni Michael J.
Published: May 2006

In Dynamite Lake, Illinois, the cladocerans Bosmina, Ceriodaphnia and Diaphanosoma coexist with planktivorous sunfish. An in situ enclosure experiment revealed that release of the cladocerans from fish predation resulted in an increased mean body size of Diaphanosoma and Ceriodaphnia and a slightly increased density of Diaphanosoma, the largest species. Increased body size of the cladoceran community resulted in a greater estimated grazing pressure on the phytoplankton community. The net result was greater phytoplankton density in the presence of fish, relative to when fish were excluded; there were also differences in phytoplankton community structure between treatments. The hypothesis that the increased in food (phytoplankton) arising from fish predation results in elevated survivorship and/or reproduction of the cladoceran individuals that managed to avoid predation was tested by conducting two life table experiments on the three cladocerans using, as media, water with natural phytoplankton densities from the enclosures with and without fish. The hypothesis was supported. Fecundity (in both experiments) and survivorship (in one experiment) of Diaphanosoma were increased on water from the fish enclosure, and fecundity of Ceriodaphnia was increased in one experiment. Significant increases in fecundity always led to significant increases in finite rate of increase (l). Reproduction of Bosmina was unaffected by the elevation in food; survivorship could not be accurately determined because of difficulties in handling this small species. The significance of these indirect demographic effects of fish on the age and size structure of the cladoceran populations was assessed using the life table data in Leslie matrices and obtaining stable and distributions when water from the fish enclosure (high phytoplankton) or the control enclosure (low phytoplankton) was used. For both Diaphanosoma and Ceriodaphnia, the populations projected under the conditions of fish enclosure water contained a greater proportion of younger individuals relative to the control at the stable age distribution. Since age and size are correlated in cladoceran populations, this analysis indicates that fish predators may indirectly alter zooplankton size structure through their effects on the food supply of the zooplankton, as well as directly modify size structure by removing the largest individuals.
Home Range, Time, and Body Size in Mammals
Authors: Lindstedt Stan L. , Miller Brian J. , Buskirk Steven W.
Published: May 2006

The relationship between home range area and body size of terrestrial mammals is reconsidered in light of the concept of biological time. Biological time is an internal, body—mass—dependent, time scale to which the durations (of rates) of biological events are entrained. These events range from purely physiological (e.g., muscle contraction time) to purely ecological (e.g., time to traverse home range). Evidence is presented that home range size scales linearly to body mass for carnivores as it does for herbivores. This scaling supports the hypothesis that animals select their home range areas to meet metabolic demands integrated over biologically critical periods. Confounding variables in the home range—body mass regression include habitat productivity and methods of location. Data on home ranges derived from telemetry studies of terrestrial carnivores are presented and used to derive allometric equations for home range area. The exponents of these equations are shown to approximate 1.0, although intercept values vary with latitude and, presumably, habitat productivity. Social organization and behavior may also influence the relationship of home range area to metabolic needs for different sex and age categories within a species.
Density‐Dependent Population Regulation in a Texas Grassland
Authors: Fowler N. L.
Published: May 2006

This study examines the extent to which plant population dynamics are affected by density—dependent processes, and at what stage(s) in the life cycle density—dependent effects occur. A natural population of Bouteloua rigidiseta, a perennial grass, was subjected to two changes of density, the addition of seeds and the removal of 30—40% of the adult plants. Censuses were conducted before and after these treatments, one year apart. Survival, growth, and reproduction were all highly size dependent. Seed additions increased proportionately the number of new recruits to the population, indicating that the size of this age class was limited by seed input; seed additions decreased the size of new recruits. Adult removal tended to increase the sizes of both new recruits and adults. Thus competition among adults, among recruits, and by adults against recruits, but not competition by recruits against adults, affected this population in a density—dependent fashion. No density—dependent treatment effects involving reproduction were found. There was a high level of environmental heterogeneity in the magnitude of treatment responses. The magnitudes of all responses to changes in density were much less than the magnitudes of the changes themselves, indicating that density—dependent effects were quite weak. The most probable cause was the decline of the B. rigidiseta population following a drought after the first census and before the treatments were effectively imposed. The highly variable climate of this region may make the relaxation or absence of density—dependent population regulation common. The size of new recruits to the population of Aristida longiseta, another perennial grass, was decreased by the increase in the number of B. rigidiseta new recruits, and the survival of A. longiseta recruits may have been increased. No other effects of the treatments on A. longiseta were found, and the treatments had no effect on the total densities of three other grasses. These results may also be a product of the decline in the B. rigidiseta population. The changes in abundances of the five species suggest that temporal partitioning of niche space is important in this community.
Invasion of Polygyne Fire Ants Decimates Native Ants and Disrupts Arthropod Community
Authors: Porter Sanford D. , Savignano Dolores A.
Published: May 2006

The fire ant Solenopsis invicta Buren invaded southeastern United States from South America >50 yr ago. Urban and agricultural consequences of this invasion are well documented; however, ecological effects are still poorly understood. Increasing frequencies of polygyne or multiple—queen fire ants in Texas and other areas of the Southeast are disturbing because nest densities of this new form are often ten times as great as those of the more familiar monogyne form. We studied the ecological impacts of a polygyne fire ant invasion on ants and other surface—active arthropods at a field station in central Texas. Arthropod abundance and species richness were assessed using a combination of baits, pitfall traps, and litter samples. This invasion decimated the indigenous ant fauna. Competitive replacement appears to be the primary mechanism behind this effect. Species richness of ants in infested areas dropped by 70%, while the total number of native individuals dropped by 90%. Of 35 species of ants collected in this study, 23 were either significantly less common or absent from infested sites; only S. invicta was more common at infested sites. The most dramatic effect of the invasion was a 10—30 fold increase in the total number of ants at infested sites–of which >99% were the imported fire ant S. invicta. The impact of this invasion on other surface—active arthropods was less severe, but still substantial. The abundance of isopods, erythraeid mites, and tumblebug scarabs declined significantly, while the abundance of ground crickets, a brachypterous roach, and a symbiotic scarab increased significantly. Overall, the species richness of non—ant arthropods was 30% lower in infested sites, and individual numbers were 75% lower. Total arthropod species richness (including ants) was 40% less at infested sites. These data indicate that polygyne fire ants pose a substantial threat to the biodiversity of native arthropod communities.
Life After Death: Site Pre‐Emption by the Remains of Poa Annua
Authors: Bergelson Joy
Published: May 2006

The spatial pattern of plants in one generation may alter the competitive interactions between plants in two ways: (1) it may affect the survival or fecundity of competing individuals in that generation, or (2) it may influence the success of seedlings in the following generation. I performed field experiments, spanning two generations, that allowed me to measure how the spatial pattern of the grass Poa annua influences the performance of two species of dicotyledonous annual weeds (Capsella bursa—pastoris and Senecio vulgaris). Randomly distributed weeds were introduced into plots with either a random or a patchy distribution of Poa. I found that seedling numbers in the following generation were four to six times as high for Capsella and Senecio when Poa was patchily rather than randomly distributed. This was not due to differences in the fecundity and survival of first—generation Capsella and Senecio between the two Poa distributions. Instead, the improved success of the subsequent generation in the patchy distributions was due to increased emergence or survival of seedlings in areas with a low density of dead Poa adults. In a second experiment to investigate why seedling survival increased in the absence of dead Poa, I found that interference by dead grass blades was responsible for high seedling mortality.
An Experimental Test of Plant Strategy Theory
Authors: Campbell Bruce D. , Grime J. Philip
Published: May 2006

Two of the central hypotheses of the triangular model of primary plant strategies were tested by a movel technique involving seven grasses of contrasted ecology grown in pure stands and an additive mixture on an experimental matrix of crossed gradients of mineral nutrient stress and vegetation disturbance. The experimental design allowed reductions in vegetative and reproductive vigor resulting from interspecific competition to be distinguished from those arising from direct effects of nutrient stress and vegetation disturbance. It was also possible to determine the extent to which competitive suppression of each species was affected by stress and disturbance. In isolation, all species showed maximum vegetative and reproductive vigor at high soil fertility and low disturbance. In the mixture, absolute reductions in biomass and flowering due to competition were greatest at high soil fertility and low disturbance, and the species of most extreme strategy became restricted to areas of the matrix broadly consistent with those predicted by strategy classification. When standardized for differences in biomass in pure stands, the effect of competition remained relatively constant across the stress—disturbance matrix for all species except Poa annua, which was less restricted by competition at high intensities of stress. There were marked and consistent differences between species in their susceptibility to competition. At both high and low soil fertility, two species of natural occurrence on infertile soils (Festuca ovina, Bromus erectus) were poor competitors relative to Arrhenatherum elatius, a widespread dominant of productive grasslands. The effect of competition was least severe on flowering of annuals in low—stress portions of the matrix. If competition is assessed simply as the percentage of reduction in biomass between pure and mixed stands it appears that competition intensity is constant across different intensities of stress and disturbances. However, observations that maximum reductions in biomass coincided with low stress and low disturbance, that competition decreased in importance as a factor reducing yield and flowering (relative to stress and disturbance) as stress and disturbance intensities increased, and that there was a consistently inferior competitive ability of plants from infertile soils at all positions on the matrix all support the hypothesis that competition declines in importance as a vegetation determinant in the vegetation of infertile soils.
Spatial and Temporal Variability in Defoliation of Australian Eucalypts
Authors: Lowman M. D. , Heatwole Harold
Published: May 2006

Insect defoliation is commonly associated with and assumed to be a cause of mortality in Australian eucalypts, particularly in rural regions where trees suffer from the eucalypt dieback syndrome. To test this, leaf growth and defoliation were measured in the canopies of Eucalpyptus trees from June 1982 to June 1986, and related to tree health and eucalypt dieback. Over 5000 leaves were monitored, including replicates of branches, canopy heights, individual trees, species, and sites. Three types of sites were selected, representing the most common conditions in the eastern Australian tablelands: woodlands (comprised of healthy trees but with slight signs of dieback), healthy trees in pastures, and dying trees in pastures. Within each site the commonest native tree species were selected for study; species were not always the same between adjacent sites because of interspe_kw canopy
A Bite is a Bite is a Bite? Constraints on Response to Folivory in Piper Arieianum (Piperaceae)
Authors: Marquis Robert J.
Published: May 2006

Because resource movement is restricted within and among branches of woody plants, different patterns of folivory are likely to differentially affect plant fitness. I examined experimentally the effects of folivory pattern on growth and seed output in the understory tropical wet forest shrub Piper arieianum (Piperaceae) in Costa Rica. I removed 10% of a plant's leaf area in two constrating manners, either from a single reproductive branch or throughout the canopy. I also conducted the experiment at two times of year, 3 mo before flowering and during flowering, to determine the effects of damage relative to the phenology of reproduction. Leaf area was removed to simulate damage by natural folivores. I followed the effects of the removal for 1 yr, both for experimental plants and for control plants from which no leaf area was removed. Response to experimental folivory varied by treatment both at the branch and whole plant level. When 10% of a plant's leaf area was removed from single reproductive branches 3 mo before the main flowering season, those damaged branches grew less and produced 80% fewer viable seeds than did similar branches on control plants and plants that lost 10% of their leaf area, but with the losses scattered throughout the plant. These differences among treatments for individual branches translated into differences over the entire plant: plants with damage concentrated on one reproductive branch grew less overall and produced fewer seeds than plants of the other two treatments. Thus, restricted movement of resources in P. arieianum following leaf damage to single reproductive branches resulted in decreased whole plant fitness. When this same experiment was conducted at the time of flowering, there were no differences among treatments in seed production or growth, either at the branch or plant level. Results of these two experiments demonstrate that the effects of folivory are dependent on the timing of the loss relative to the reproductive phenology of the plant; resources necessary for fruit production are accumulated before flowering and not subsequent to it in P. arieianum. In 32% of plants in which damage was concentrated, seed production was reduced due to either abortion of the damaged branch (17%) or aborption of its bud inflorescences (15%). In the remaining 68% of branch—defoliated plants, neither level nor timing of flowering was affected by the experimental folivory, suggesting that reduction in seed production was due to a decrease in resource availability and not to a lack of pollinator visitation. Together these results add to the evidence that resource movement is restricted in woody plants, even in the face of a stress such as herbivory. The results further suggest that relatively small amounts of folivory can have major detrimental effects in woody plants and that damage pattern as well as damage level must be measured.
Greater Maternal Investment Can Decrease Offspring Survival in the Frog Bombina Orientalis
Authors: Kaplan Robert H.
Published: May 2006

This study quantifies the influence of the persistent variation in maternal investment in individual offspring on larval fitness in a population of the oriental fire—bellied toad Bombina orientalis. Offspring fitness was evaluaed by exposing newly hatched larvae to interspecific tadpole predators and assessing the effect of ovum size on the probability of surviving predation. The experiment incorporates natural variation in temperature that occurs during development. Embryos that developed in colder ponds took longer to hatch and hatched at a larger size. Larger size afforded larvae significant protection from predation. In addition, Iarvae that developed from large eggs in cold ponds had a higher probability of surviving predation than larvae that developed from small eggs. A significant interaction between egg size and developmental temperature, however, resulted in larvae that developed from large eggs in warm ponds having a lower probability of surviving predation. Large eggs resulted in large snout—to—vent lengths at all temperatures. In warm ponds, however, tails of larvae from large and small eggs were of equal length. Thus, diminished locomotory ability due to larger amounts of inert yolk can be invoked to explain the increased susceptibility to predation of larvae that developed from large eggs in warm ponds. In cold ponds tail length was substantially greater than in warm ponds, and larvae from large eggs tended to have longer tails than those that developed from small eggs. This allowed larvae from large eggs to achieve the advantages of increased body size. Thermally induced plasticity during early development resulted in an uinpredictability regarding the adaptive value of a particular egg size. This result fits within a growing theoretical literature that indicates that environmental uncertainty can result in the natural selection of development plasticity in egg size, even though the fitness ramifications are realized in subsequent stages. Thus, increased maternal investment in offspring is not necessarily associated with an increase in offspring fitness, and the evolution of developmental plasticity in egg size may be the result of environmental uncertainty during the larval stages of life.
Estimating Transition Probabilities for Stage‐Based Population Projection Matrices Using Capture‐Recapture Data
Authors: Nichols James D. , Sauer John R. , Pollock Kenneth H. , Hestbeck Jay B.
Published: May 2006

In stage—based demography, animals are often categorized into size (or mass) classes, and size—based probabilities of surviving and changing mass classes must be estimated before demographic analyses can be conducted. In this paper, we develop two procedures for the estimation of mass transition probabilities from capture—recapture data. The first approach uses a multistate capture—recapture model that is parameterized directly with the transition probabilities of interest. Maximum likelihood estimates are then obtained numerically using program SURVIV. The second approach involvesa modification of Pollock's robust design. Estimation proceeds by conditioning on animals caught in a particualr class at time i, and then using closed models to estimate the number of these that are alive in other classes at i + 1. Both methods are illustrated by application to meadow vole, Microtus pennsylvanicus, capture—recapture data. The two methods produced reasonable estimates that were similar. Advantages of these two approaches include the directness of estimation, the absence of need for restrictive assumptions about the independence of survival and growth, the testability of assumptions, and the testability of related hypotheses of ecological interest (e.g., the hypothesis of temporal variation in transition probabilities).
The Effect of Extra Food on Fitness in Breeding Carrion Crows
Authors: Richner Heinz
Published: May 2006

Food limitation during the nestling stage was experimentally investigated in an urban population of Carrion Crows. Parents were offered supplemental food during the nestling period and the following variables compared with those from non food—supplemented pairs: (1) nesting success, (2) fledgling number, (3) fledgling mass and linear body size, (4) proportion of fledglings resighted (a) after transition to independence from the parents, and (b) after the first winter postfledging, and (5) the probability that a fledgling became a breeder. Food—supplemented pairs had a higher nesting success and produced more fledglings. The fledglings of food—supplemented pairs were heavier and bigger in tarsus length than controls, but experimental and control fledglings were seen in equal proportions both after transition to independence and after the first winter. The production of potential breeders wa increased ninefold in food—supplemented pairs. The study demonstrates that in this urban habitat food limitation causes low fitness not through lowered survival, but through the production of a low number of potential breeders.
Community and Population Level Responses to Fertilization in an Old‐Field Ecosystem
Authors: Bakelaar R. Gary , Odum Eugene P.
Published: May 2006

The effects of nutrient enrichment by 2 applications of N—P—K pebble fertilizer (November and April) on the plant community of an 8—year old—field successional ecosystem were studied in 2 adjacent 1—acre [=.4047 hectares] experimental plots in Georgia. Net primary production, standing crop biomass, turnover and dominance increased, while all components of diversity decreased as a result of the treatment. These responses generally support Odum's (1969) model of ecosystem development, if the assumption is that eutrophication should involve a "set back" in development to earlier (younger) stages of autogenic succession. However, species composition was not "set back" since species characteristic of earlier successional stages (i.e., Ambrosia) exhibited reduced importance in the treated areas, while dominants more characteristic of the 8—year stage increased in importance. Results of this study suggest that fertilization has its major effect through competition—altered changes in the organization of realized niches of primary producers. A few opportunistic species, already established in the community, expand their niches by preempting some niche space of subordinates, thus reducing overall diversity. At the population level, productivity and niche width were increased for the dominant, Solidago, but greatly reduced for Aster which was codominant in control plots. Some 21 species were stressed by the nutrient enrichment (in that their net production was reduced), but a positive response by 16 species produced an overall subsidy effect for the vegetation as a whole.
A Mutualistic Interaction Between a Sessile Marine Clam and Its Epibionts
Authors: Vance Richard R.
Published: May 2006

The jewel box clam, Chama pellucida, lives attached to shallow marine rock surfaces. The shell is normally covered by a dense growth of sessile plants and animals of diverse phyla. Removal of these epibionts seems to make detection and/or attack of Chama by the predatory starfish Pisaster giganteus more often successful in the laboratory and substantially increases Chama mortality through Pisaster in the field. Thus, Chama benefits from the presence of these attached organisms. Chama occurs commonly only in areas characterized by low mortality rate of sessile organisms. The organisms attached to Chama therefore enjoy freedom from the higher mortality rates which characterize some areas not inhabited by Chama such as sea urchin (Centrostephanus coronatus) foraging ranges. Thus, attached organisms benefit from their association with Chama. Accordingly, the interaction is mutualistic. Naturally rugose Chama shells placed in the field appear to accumulate certain epibiotic species faster than do artificially smooth shells. This result suggests that natural selection on one or both participants has favored mechanisms that facilitate their association. The interaction very likely affects the abundances of the participants. Also, by the resulting creation of the physical habitat of a Chama bed, the interaction probably affects the abundances of many other species in the community as well. Influences of mutualism in general on community composition and stability are discussed.
Cherries, Ants and Tent Caterpillars: Timing of Nectar Production in Relation in Relation to Susceptibility of Catepillars to Ant Predation
Authors: Tilman David
Published: May 2006

The North American black cherry, Prunus serotina Ehrh., has extrafloral nectaries which are most active during the first 3 wk after budbreak. These nectaries attract large numbers of the ant Formica obscuripes Forel, especially to trees within °20 m of an ant colony. The average number of ant visits per bud is highest just after budbreak, decreasing as the number of active extrafloral nectaries decreases. These ants are predacious on many species of insects found on P. serotina, including the eastern tent caterpillar, Malacosoma americanum Fabricus, the major defoliator of black cherry. Formica obscuripes is only able to prey upon M. americanuum within °3 wk of budbreak, after which time, tent caterpillar larvae are large enough that most escape ant predation. Tent caterpillar survivorship was found to be positively related to distance from colonies of F. obscuripes. It is suggested that the ant—cherry relationship is a faculative mutualism and that nectar production is timed so as to maximize the chance of successful ant predation on tent—caterpillar colonies.