bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of Predator‐Induced Stress on Field Populations of Stoneflies (Plecoptera)
Authors: Feltmate Blair W. , Williams D. Dudley
Published: May 2006

The influence of predaceous fish in affecting components of fitness of immature and adult macroinvertebrates was examined using a predator enclosure/exclosure experiment in a southern Ontario stream. The stonefly Paragnetina media is a semivoltine species with a midsummer emergence period. After exposure of P. media to rainbow trout (Oncorhynchus mykiss) between May and August (repeated twice, 1987 and 1989), we observed significant reductions in nymphal density, size (head width), and conditions (mass/head width). Reductions in density were attributable to nymphal emigration and predation. Selective predation on larger stoneflies (direct effects), together with changes in nymphal feeding rate, activity, and distribution (indirect effects), reduced size and condition of stoneflies exposed to trout. After August—to—October (1986 and 1989) and January—to—April (1988) exposures, we observed no effect of trout on these parameters. Adults stoneflies (P. media and Alloperla mediana) cannot feed, and consequently any energy that they channel into reproductive efforts must be derived from the aquatic stage. Thus, we postulated that the impact of trout on immature P. media and A. mediana might manifest itself negatively on stonefly adults. Although size and emergence pattern of adult P. media and A. mediana did not differ between enclosure/exclosure sections of stream, in the presence of trout both species suffered significant reductions in condition, fecundity, and total number to emerge. These findings demonstrate that direct and indirect effects of a predator can cascade from immature to adult prey and from aquatic to terrestrial ecosystems.
Competition Among Refuging Sunfishes and Effects of Fish Density on Littoral Zone Invertebrates
Authors: Mittelbach Gary G.
Published: May 2006

In small lakes, juvenile sunfishes (Centrarchidae) commonly occupy vegetated habitats, reducing the risk of predation by piscivorous fish. Eight species of sunfish coexist in Lawrence Lake, Michigan, and the bluegill greatly dominates the fish fauna in terms of number sand biomass. I hypothesized that the bluegill's use of the vegetation as a predation refuge could have a significant negative effect on the growth rates of other species occupying this habitat. A "target—neighbor" experimental design was used to test this hypothesis. Twelve 3—m2 cages were arrayed linearly along the vegetated littoral zone of Lawrence Lake in 1986. Two juvenile pumpkinseeds (the "target" species) were placed into each cage, together with either 0, 2, 3, 6, 8, or 10 juvenile bluegills. Bluegill densities were randomly assigned to cages; average bluegill density in Lawrence Lake is equivalent to °5 fish per cage. Growth in mass over a 50—d period was used as a measure of competitive effect. Benthic invertebrates were also sampled from each cage to determine whether fish density significantly affected invertebrate size and abundance and whether invertebrate availability influenced sunfish growth. Growth of both pumpkinseeds and bluegills declined linearly as a function of final bluegill density in the cages, indicating that the species were competing while occupying the vegetation refuge. Growth was positively related to the density of large invertebrate prey remaining at the end of the experiment. Using an optimal foraging model, I estimated the net energy return available to bluegills in each cage. Predicted net energy gains (in joules per second of foraging time) were also positively related to bluegill growth, indicating that prey availability directly influenced growth rates. Mean invertebrate size decreased significantly as fish density increased, owing to a reduction in the number of large invertebrates; the number of small prey showed no relationship to fish density. These results are consistent with a hypothesis of exploitative competition between juvenile sunfishes and indicate that the behavioral avoidance of predators by small fish can have significant effects on invertebrate size and abundance in littoral habitats.
Peat Accumulation and the Success of Marsh Plants
Authors: Bertness Mark D.
Published: May 2006

Accumulated belowground plant debris in salt marsh habitats forms a compact peat substratum that generally increases in density from the littoral edge of marshes landward. To examine the influence of peat accumulation on the success of marsh plants, unvegetated substratum varying in peat density was transplanted over a tidal gradient in a New England salt marsh, and colonization of the plots was followed for 3 yr. Manipulated plots were established in the tall— and short—form Spartina alterniflora zones and near the littoral border of the Spartina patens zone. Substratum type markedly influenced plant colonization of plots in each zone. In the tall— and short—form S. alterniflora zones, the vigor of S. alterniflora invading plots decreased which increasing peat density. In both these zones, S. alterniflora production in experimental plots lacking peat exceeded even that in unmanipulated control plots, suggesting that natural peat accumulation in these habitats reduces plant production. Experimental plots in the S. patens zone also were colonized as a function of peat density. All plots in the S. patens zone were heavily colonized by S. alterniflora, suggesting that S. alterniflora is normally excluded from this habitat by the presence of high marsh vegetation. S. alterniflora invasion of the S. patens zone plots, however, precluded assessing the influence of substratum type on the success of the grasses normally found in this habitat. A physical parameter inventory of the experimental plots indicated that plant growth in the plots was not correlated with many edaphic conditions believed to favor marsh plant growth. For example, plots lacking peat had lower redox and total nitrogen, higher salinities, and lower permeability to water than peat substratum, yet supported more vigorous plant growth than plots containing peat. The basis of peat inhibition of marsh plant growth, therefore, is unclear and needs further work. These data suggest that peat accumulation in marsh habitats may inhibit plant growth, lead to autogenic changes in plant performance, and contribute to within— and between—marsh variation in plant production.
Stand and Soil Changes Along A Mountain Hemlock Death and Regrowth Sequence
Authors: Boone Richard D. , Sollins Phillip , Cromack Kermit
Published: May 2006

Stand characteristics, dead wood, and soil carbon (C) and nitrogen (N) pools were measured through a wave—regenerated sequence of mountain hemlock death and regrowth created by Phellinus weirii. Stem density increased along the 96—yr regrowth sequence, but was low in the 225—yr—old, mature stand. Total ecosystem C (TEC) dropped upon stand death and did not recover. Net ecosystem production (NEP) was negative just after stand death and zero thereafter. The O2 horizon and the forest floor (O1 / O2) had greater mass, organic matter, and N capital in the mature stand than in the bare or regrowth zones. From the mature stand to the bare zone, forest—floor organic matter decrease 27% and C and N 24%. The forest—floor N decrease was offset by an equal N increase in the mineral soil at 0—15 cm depth. Mineral—soil C, as well as C and N for the sum of the forest floor / mineral soil, were constant across the death and regrowth sequence. The C/N ratio peaked for O1 material, and reached its minimum for fine roots, in the bare zone.
Natural Disturbance and Gap‐Phase Regeneration in a Wind‐Exposed Tropical Cloud Forest
Authors: Lawton Robert O. , Putz Francis E.
Published: May 2006

The patterns and processes of canopy tree death and replacement were studied in the elfin forest of Monteverde, Costa Rica. Natural treefalls and limbfalls in a 5.2—ha study area opened 0.8, 1.4, and 1.0% of the area in three consecutive years with about four gaps°ha—1°yr—1 larger than 4 m2. Forty—one percent of the gaps were formed by uprooted trees, 39% by snapped trees, and the remainder of limbfall, the collapse of epiphyte masses, and dead standing trees killed by lightning. Gaps were found to be spatially aggregated, with more gaps occurring within 17—20 m of one another than expected by chance. Variation among gaps was complex; the first principal component of the variation in eight important gap characteristics among 88 gaps contrasted measures of gap size with the way the gapmaker broke and the position of the gap on the slope, but accounted for only 56% of the total variation. In gaps <8 mo old, the leaf area index was 1.6, and only 8% of the area was not covered by living plants. Leaf area index increased logarithmically with time since gap formation and with gap area; 50% of the mature—forest leaf area index of 5.1 was recovered in 3 yr in gaps of 10 m2 and in 1.5 yr in gaps of 40 m2. Saplings of both shade—tolerant and shade—intolerant canopy tree species were more abundant in gaps than in the understory of mature forest. Sapling density increased with time since gap formation, but, given the effect of time, shade—tolerant sapling density decreased with gap area, while shade—intolerant sapling density increased. Saplings of eight species were concentrated on nurse logs, while those of one other species were concentrated on the mineral soil disturbed by uprooting trees. Experimental investigation of colonization of exposed soil in treefall gaps indicated that buried seeds give rise to many more tree seedlings that seeds dispersed into recent gaps. Many of the saplings in gaps, however, started life as epiphytic seedlings in the crowns of the trees that fell. The dynamics of this lower montane rain forest resemble many lowland forests in regards to the importance of gap—phase regeneration but differ in the sources of canopy gap colonists and in the importance of different substrates for seedling establishment.
A Method to Label Seeds and Seedlings Using Gamma‐Emitting Radionuclides
Authors: Primack Richard B. , Levy Charles K.
Published: May 2006

A major obstacle limiting field studies in plant population biology is that there is no way of uniquely labeling or identifying the seeds produced by a mother plant so that her seedlings can be distinguished from the offspring of other conspecific plants. This article describes experiments undertaken to develop gamma—emitting radionuclide labels that are injected into a plant and are incorporated into the developing seeds and subsequent seedlings. The nuclides were used singly and in combination since each nuclide has unique spectral peaks that can be used to distinguish it from other nuclides. Five species were used: Phaseolus vulgaris, Polygonum pensylvanicum, Impatiens capensis, Bidens frondosa, and B. connata. Easily detectable levels of 58cobalt, 60cobalt, 134cesium, 137cesium, and 75selenium were incorporated by seeds of the five species. The incorporation of 54manganese and 65zinc varied among the species. 88Yttrium, 125antimony, and 85strontium were weak or not present as labels, depending on the species, while 110silver and 46scandium were never found in the seeds. Bean seedlings showed readily detectable amounts of nuclide label, while the nuclides apparently leached out of the Impatiens seedlings. The quantities of nuclides used in these experiments were so low that there was no effect of radiation on plant performance.
Fugitive Species: Experiments with Fungi and Some Theoretical Considerations
Authors: Armstrong Robert A.
Published: May 2006

An experimental ecosystem was constructed and used in studies of species coexistence in a patchy environment. This model system consisted of two species of fungi, Aspergillus nidulans and an unidentified species of Penicillium, competing for space on agar—filled petri dishes. Aspergillus was observed to overrun and infiltrate established colonies of Penicillium, and so was considered competitively dominant, while Penicillium produced a greater number of daughter colonies at an earlier age than did Aspergillus, and so was considered a better colonizer. It was hypothesized that these two species should be able to coexist in a patchy environment where local populations on occasion become extinct, with Penicillium playing the role of a fugitive species. Two different regimes (Annual and Perennial) for the generation of empty patches through the extinction of their resident species were compared with reference to the ease with which coexistence was obtained. Under the Annual regime, where all patches became empty (hence available for colonization) at the same time and where the (n + 1)st generation was seeded with spores from the nth generation alone, coexistence was not obtained. In the Perennial experiments, however, where only a fraction of patches became empty at any one time and where newly emptied patches were seeded from patches of several different ages, coexistence was easily obtained. Temporal differences in the use of single patches occurred under the Perennial transfer regime, with Penicillium colonizing from younger patches in which Aspergillus had not yet exerted its dominance, and Aspergillus colonizing from older patches. Temporal partitioning did not occur in the Annual experiments, and coexistence was not obtained. Since the pattern of disturbance and the life history patterns of the competing species combine to determine the extent to which temporal partitioning can occur, it is evident that for the fugitive species mechanism to be effective, the life history patterns of the species involved and the pattern of environmental disturbances must be well matched. Discussion of these problems is facilitated by the introduction of the notion of “coexistence bandwidth” a theoretical construct which relates in these experiments to the ease with which coexistence can be obtained, and in natural systems to the fragility or robustness of competitive associations in the face of changing environmental conditions.
Oscillations in Housefly Population Sizes Due to Time Lags
Authors: Taylor Charles E. , Sokal Robert R.
Published: May 2006

Oscillations of animal population size can be caused by time delays due to development. Mathematical representations of this have been studied by previous workers. We investigated the applicability of their models to laboratory populations of houseflies. Estimates of the necessary parameters–death rate, oviposition rate, development time, and the association of larval survivorship with density–were obtained from various experiments. A stability analysis of the model by the method of Maynard Smith predicted oscillations in adult population size for these parameter values. This was also predicted by numerical solutions to the equations of the model. Four population cages of houseflies maintained for periods up to 2.5 yr showed the predicted pattern. The populations fluctuated markedly and regularly in adult size, generally between 200 and 1,600, with a periodicity of 5—6 wk. The correspondence between prediction and observation was reasonably close and was improved when further complications were considered and the behavior of the populations was simulated on a computer. In another cage maintained under different conditions of husbandry, where oviposition was less but other parameters were the same, the stability and population size differed in a manner predicted by the model. Of several theories that might explain oscillations, only that of time delays is likely to be appropriate for these populations.
Translating Foraging Movements in Heterogeneous Environments into the Spatial Distribution of Foragers
Authors: Turchin Peter
Published: May 2006

The relationship between movement and population dynamics in heterogeneous environments has been approached from two directions: theoretical analyses using diffusion models and empirical studies that often employ computer simulations. In this paper I am proposing a unified framework for studying individual movements that brings both of these approaches together. The proposed framework is based on the diffusion—approximation" procedure developed by Patlak (1953a, b), which translates a probabilistic description of the pattern of individual movements (such as provided by simulation models) into a partial differential model describing the population redistribution in a patchy habitat. My first goal is this paper was to make Patlak's work accessible to ecologists interested in realistic movement models. Secondly, making some simplifying assumptions I solved Patlick's model to obtain the equilibrium distribution of organisms among patches in a heterogeneous environment. This led me to a definition of "residence index." The residence index of a patch is estimated by observing and recording movement trajectories of organisms within the patch. The pattern of variation in the residence index among different kinds of patches specifies the predicted equilibrium densities of organisms in each patch. I assessed the utility of the above approach by using data on movement tracks of Euphydryas anicia females to calculate butterfly residence indices in host—plant patches vs. nonhost environment. The predicted distribution of butterflies among host and nonhost areas differed by only 16% from the actual distribution, as documented by an independent data set. I also applied the model to data from three studies on insect movement and spatial distribution. The accuracy of model predictions varied from adequate to excellent. Finally, I argue that Patlak's model and its residence—index extension provides a general, and at the same time realistic, framework for quantifying individual movements, and for relating movement patterns to spatial population dynamics. Although this approach, as developed here, is explicitly concerned only with population redistribution within an area, birth/death or immigration/emigration terms can be added in a straightforward manner. Thus, this framework can be very useful in studies of consumer—resource (e.g., predator—prey or herbivore—host) spatial dynamics, since movement behaviors of consumers searching for prey, and prey attempting to avoid consumers, are known to be important in affecting the outcome of such population interactions.
Deactivation of Plant Defense: Correspondence Between Insect Behavior and Secretory Canal Architecture
Authors: Dussourd David E. , Denno Robert F.
Published: May 2006

To determine if the arrangement of secretory canals in leaves affects foraging by folivorous insects, we examined the behaviors of 33 species found on diverse canal—bearing plants. Insect behaviors were categorized into three principal classes: vein cutting, trenching, and neither behavior. Canal architectures were ascertained by damaging leaves with standardized tests and measuring the secretory response Our observations document a precise correspondence between herbivore behavior and canal morphology. Vein—cutting insects occur on plants with arborescent canals (resin canals in Anacardiaceae and nonarticulated laticifers in Apocynaceae, Asclepiadaceae, and Moraceae). By severing the secretory canals in leaf veins, insects prevent the flow of secretion to distal branches of the canals. Trenching insects, in contrast, are found on plants with net—like canal systems (anastomosing articulated laticifers in Asteraceae and Caricaceae, and exuding phloem in Cucurbitaceae). To eliminate secretion outflow from these plants, the insects must transect all strands of the network by cutting a trench. The secretory canals of the Convolvulaceae (nonanastomosing articulated laticifers) differ from the preceding categories in being restricted primarily to the major leaf veins. The behavior of herbivores on this family is also distinct: all feed between the major veins without prior vein cutting or trenching. Thus, insects employ the same behavior on similar canal architectures, even when their host plants otherwise differ in taxonomy, secondary chemistry, and canal type. The occurrence of vein—cutting and trenching behaviors in multiple lineages of insects that include caterpillars, beetles, and katydids indicates that the behaviors have evolved repeatedly, apparently through convergence. Our comparative analysis supports the view that these preingestive behaviors function specifically to deactivate defensive canal systems.
Dissecting Factors Responsible For Individual Variation in Plant Fecundity
Authors: Herrera Carlos M.
Published: May 2006

This paper analyses, on an individual plant basis, the various components determining seed production in the evergreen shrub Lavandula latifolia (Labiatae), in order to estimate their relative contributions to individual differences in fecundity. The relationships between fecundity and age, size, and growth rate are also examined. Experimental pollination and watering were done in an attempt to alter natural patterns of partial fecundity components. Fecundity of individual plants is partitioned into its absolute (number of inflorescences, number of flowers per inflorescence) and relative (fruits/flower and seeds/fruit) partial components, and the following questions are addressed: (1) Are the individual differences in relative components of fecundity? (2) What are the proportional contributions of relative and absolute components of fecundity to individual variation in seed production? (3) Are relative measures of reproductive success reliable predictors of individual differences in fecundity? (4) When induced experimentally, do changes in partial fecundity components translate into modifications of the population's ranking of absolute fecundities? Fecundity depended more on plant size than age, was unrelated to growth rate, and was limited by water, but not pollen, availability. Individual plants differed in all components of fecundity examined, and differences were consistent among years. The proportional contributions of the various components to individual differences in fecundity were very unequal. Almost all variation among plants in seed production (°90% of variance) was explained by the variances and covariances of number of inflorescences and number of flowers per inflorescence. Variation among plants in the two relative components (fruit and seed set), even though statistically significant, had a negligible influence on individual differences in fecundity. These patterns remained invariant despite significant modification of flower production and fruit set induced by artificially watering a part of the study population. Fruit and seed set alone are unreliable indicators of individual differences in the maternal component of fitness in L. latifolia, and a review of relevant literature indicates that this probably applies to other polycarpic plants as well. Plant reproductive biology studies may make erroneous inferences if they consider exclusively data on relative fruit and seed set, and neglect the absolute components of individual differences in fecundity. It is proposed that, to circumvent these potential problems, demographic aspects and absolute estimates of seed output on a per individual basis should be examined in addition to the customarily employed fruit and seed set measures.
Multilevel Comparative Analysis of the Morphology, Function, and Evolution of Dalechampia Blossoms
Authors: Armbruster W. Scott
Published: May 2006

Morphological and ecological data from Dalechampia flowers were subjected to analysis of covariance and component regression analysis, a technique that allows separate estimation of within— and among—group relationships. They yielded results consistent with hypotheses stating that bee—flower relationships are influenced by several morphological parameters of the blossoms (pseudanthial inflorescences). The following correlations emerged: (1) the area of the gland that secretes the pollinator reward (and hence the amount of reward) and the size of the largest bees that visit the flowers; (2) the distance between gland and stigmas and the size of the smallest floral visitor contacting the stigmas; and (3) the distance between gland and anthers and the size of the smallest floral visitor contacting the anthers. Together these variables appear to determine which subset of the bee fauna will be effective in pollination. The distance between the anthers and stigma was correlated with the rate of autopollination and hence probably affects the breeding system. Anther—stigma distance, gland—anther distance, gland—stigma distance, and gland area were correlated with, and may have evolved in response to the size of the principal pollinator. In general, patterns reflecting ecological relationships between plants and pollinators were similar whether comparisons were made among populations within species or among species. However, patterns reflecting the evolution of floral morphology in response to pollinators were generally different at population and species levels; the regressions reflecting the evolutionary response of floral morphology to bee size were consistently steeper in among—species comparisons than in among—population (within—species) comparisons.
Light Responses of a Submersed Macrophyte: Implications For Survival in Turbid Tidal Waters
Authors: Goldsborough William J. , Kemp W. Michael
Published: May 2006

Responses and acclimation of the submersed vascular plant Potamogeton perfoliatus L. to changes in total irradiance were investigated by growing replicate plant populations under neutral density screens to create three treatment levels (11, 32, and 100% of ambient). Changes in the relationship between photosynthesis and irradiance (P—I) were monitored during a 17—d treatment period and to 16—d "recovery" period, as were concentrations of photosynthetic pigments and several morphological features. Both initial slope of P—I relations and leaf chl a content increased significantly within 3 d after the beginning of shade treatment. These responses, which represent mechanisms of increasing photosynthetic efficiency at low irradiance, were also reversed within 3 d after treatment removal. Significant morphological responses to the recovery from shade were evident within 10 d, including: elongation of stems, thinning of lower leaves, and canopy formation at the water surface. Preliminary calculations indicate that both photosynthetic and morphological acclimations to shade conferred substantial improvements in P. perfoliatus production at experimentally reduced irradiance compared to pretreatment conditions. Significant decreases in plant stem density, biomass, and reproduction, as well as increases in mortality, were observed for plants at low (but not medium) growth irradiance. The inability of populations treated at low irradiance to exhibit any recovery (i.e., posttreatment increases) in these variables after 16 d of full ambient light suggests that 11% of ambient irradiance was below the minimum level needed for survival of this plant. Time scales for significant shade acclimation responses were comparable to the temporal scales of changes in light climate for aquatic ecosystems such as Chesapeake Bay. It is argued that, although stem elongation is a beneficial response to shade for P. perfoliatus in turbid lakes, it may be nonadaptive in turbulent tidal waters because of increased susceptibility to fragmentation.
Habitat Suitability and Local Population Persistence of the Sea Palm Postelsia Palmaeformis
Authors: Paine R. T.
Published: May 2006

Sporophytes of the sea palm Postelsia normally occur on a variety of intertidal surfaces. An experimental technique was developed permitting spores to be sown onto natural substrata whose relative suitabilities were evaluated using three criteria. All major surface categories (bare rock, animal, plant) were equivalent if judged by the appearance the following spring of a descendant sporophyte. However, the proportion of these plants surviving to maturity and continuing into the following year was highly substratum dependent. Bare rock surfaces are the most suitable for Postelsia as judged by probability of successful occupancy, sporophyte density, and inter—annual persistence. Single fertile plants can establish or maintain a population. The probability of local extinction is a function of population size: only 36% of small populations (1—30 plants) tended to continue to the next year, whereas all those >120 individuals did. Postelsia seems incapable of persisting in the presence of the turf—like algae Corallina or Holosaccion. Thus sea palms may require the presence of competitively superior mussels: mussels out—compete algae of low stature; waves remove mussels, thereby generating Postelsia's most suitable substratum.
Testing For Indirect Effects of Predation in an Intertidal Soft‐Bottom Community
Authors: Kneib R. T.
Published: May 2006

The killifish Fundulus heteroclitus and the grass shrimp Palaemonetes pugio are abundant predators in salt marshes along most of the United States Atlantic coast. Both species feed on benthic invertebrates, but large (>70 mm total length) killifish also prey on grass shrimp. As a test of the hypothesis that killifish have a positive indirect effect on some benthic invertebrates by controlling the abundance of grass shrimp, I maintained both predators together and separately in 5—m2 cages on the intertidal marsh surface for 10 wk. Predation by killifish significantly reduced the survival of grass shrimp in cages to which both predators were added, and the numerical responses of the dwarf mud anemone Nematostella vectensis provided unequivocal evidence for a positive indirect effect of killifish in this system. However, most benthic prey taxa were not significantly affected by the predator treatment. Either predation is generally unimportant in this community or another intermediate predator has a greater effect than P. pugio. Larval killifish and post—larval grass shrimp, which invaded the cages and became abundant in predator—exclusion treatments, may have confounded the predator effects.
Grazer Control of Algae in an Ozark Mountain Stream: Effects of Short‐Term Exclusion
Authors: Power Mary E. , Stewart Arthur J. , Matthews William J.
Published: May 2006

Stony substrata in streams of the southwestern Ozark Mountains are covered with cyanobacterial felts dominated by Calothrix sp., an active nitrogen fixer. To study the effect of grazers on these producer assemblages, we exposed or protected periphyton from gazers for periods ranging from 4 to 33 d. When protected from grazing fishes and invertebrates, cyanobacterial felts are overgrown by turfs of benthic diatoms within 4—10 d. Diatom turfs also develop on bare substrata that are incubated in sites in the stream inaccessible to grazers. Diatom turfs exposed to grazing minnows are stripped off in 3—5 min, and when left exposed to grazers, are replaced by cyanobactrial felts within ll d. Basal regeneration of Calothrix trichomes may contribute to their persistence under intense grazing.
Host Quality and Sawfly Populations: A New Approach to Life Table Analysis
Authors: Preszler Ralph W. , Price Peter W.
Published: May 2006

In the field, population density of a shoot—galling sawfly Euura lasiolepis was on eighth as great and mortality was five times as great in dry habitats as in wet habitats. The effect of host water stress on Euura populations was experimentally tested by initiating Euura populations on potted Salix lasiolepis willow plants given various amounts of water. As in wild populations Euura mortality was much higher (2.5 times) on waterstressed Salix. The life table analyses of experimental Euura cohorts began with the potential number of eggs in the adult females of the preceding generation. This enabled us to describe the effects of female reproductive responses to host quality on Euura densities. These responses, failure to initiate a gall or to release an egg during gall initiation, resulted in a 27.5% reduction of the Euura cohort on hosts given a high—water treatment and an 85.1% reduction on low—water hosts. Excluding behavioral limits on the potential number of eggs laid, first—instar larval death was the largest and most variable Euura mortality factor of the experimental populations. Host water treatment accounted for 51.9% of the variance in first—instar larval mortality (P ° .001), with 13.6% of the first instars dying on hosts given high—water treatments and 54.5% dying on the low—water hosts. The threefold decrease in gall initiation and egg release on water—stressed hosts has probably been selected for by the fourfold increase in first—star larval mortality on these water—stressed hosts. From an evolutionary perspective the reproductive behaviors have amplified the effects of the first—instar mortality. From an ecological perspective the reproductive responses of the adult females were the critical factors regulating Euurra densities. Understanding the effect of variation in host quality on herbivore survivorship and behavior is essential to understanding the densities of local herbivore populations and should be routinely incorporated into life table analyses.
A Paleoecological Test of a Classical Hydrosere in the Lake Michigan Dunes
Authors: Jackson Stephen T. , Futyma Richard P. , Wilcox Douglas A.
Published: May 2006

Aquatic vegetation varies along a chronosequence of dune ponds at Miller Woods, Indiana Dunes National Lakeshore. Submersed and floating—leaved macrophytes dominate the vegetation of the youngest ponds. Older ponds contain mixed assemblages of submersed, floating—leaved, and emergent plant taxa. The oldest ponds are dominated by emergent plants, especially Typha angustifolia. We conducted paleoecological studies at one of the oldest ponds to test the hypothesis that the modern vegetational array along the pond chronosequence represents a hydrarch successional sequence. Macrofossil stratigraphy of 3000—yr—old pond indicates no significant changes in pond vegetation following early colonization until <150 BP. Pond vegetation before 150 BP consisted of a diverse assemblage of submersed, floating—leaved, and emergent macrophyte taxa. Pollen and macrofossil data indicate a major, rapid vegetational change at <150 BP, evidently in response to local human disturbance. Pollen data reveal that the extensive Typha stands in the older ponds have developed recently, following postsettlement disturbance. Modern vegetational differences along the chronosequence reflect differential effects of disturbance rather than autogenic hydrarch succession. This study illustrates a major pitfall in inferring successional trends from spatial sequences of vegetation.
What is a Safe Site?: Neighbor, Litter, Germination Date, and Patch Effects
Authors: Fowler Norma L.
Published: May 2006

To investigate some of the factors that determine the fate of a seedling, censuses of seedlings of two native grasses, Aristida longiseta and Bouteloua rigidiseta, were made at frequent intervals in a Texas grassland for 2 yr. Seedlings of both species with neighboring seedlings or juvenile plants of either within 2 cm had higher rates of survival and growth than those without such neighbors, implying that the effects of aggregation in favorable microsites outweighed the effects of competition among these plants. Neighboring adults of these species, and neighboring juveniles of other species, had positive effects in some instances, negative in others. Litter, even in amounts too small to cover the surface, reduced the survival rates of seedlings of both species, while surface rocks may have increased them. Overall, seedlings that germinated earlier in the autumn were more successful. The rank order of quadrat favorableness for seedlings varied in a complex fashion between species, between years, among census intervals, and among germination cohorts. This variation in patch favorableness must prevent these populations from reaching a stable spatial distribution and from reaching a stable age distribution in any given patch.
Pattern of Self‐Thinning in Jack Pine: Testing the Random Mortality Hypothesis
Authors: Kenkel N. C.
Published: May 2006

Spatial statistics were used to examine the pattern of self—thinning in a 0.25—ha even—aged (65 yr old), pure stand of jack pine in the boreal forest near Elk Lake, Ontario, Canada. The positions of 459 living and 916 dead trees were recorded and refined nearest neighbor analysis [G(W)] and second—order spatial statistics [L(t)] were used to examine distributional deviations from both the Poisson expectation and the hypothesis of random mortality. The results indicate that the initial (live + dead, n = 1375) distribution was locally random. By contrast, the distribution of live trees was locally highly regular, while the dead trees were significantly more clumped than random mortality would dictate. It is suggested that the development of a strong regular pattern in the survivors is attributable to differential mortality involving two distinct competitive phases: an early scramble phase involving two—sided competition for soil resources, and a lager contest phase involving one—sided competition for light. Analysis of L(t) for the live trees indicated a mean "area of influence" for each individual of an °3.5 m radius, suggesting that trees may compete directly only with their immediate neighbor.
Herbivore‐Caused Greenfall in the Southern Appalachians
Authors: Risley Lance S. , Crossley D. A.
Published: May 2006

Freshly fallen green leaves (greenfall) were collected from plots on four forested watersheds differing in aspect and treatment history at the Coweeta Hydrologic Laboratory in southwestern North Carolina. Greenfall was categorized as "herbivore—caused" or "unexplained." In almost all cases herbivore—caused greenfall biomass was significantly greater than was unexplained greenfall. Herbivore—caused greenfall was subdivided into "clipped" (caused by petiole—clipping caterpillars), "petiole—damaged" (caused primarily by petiole borers), "mined" (primarily by microlepidoptera on hickories), and "orts" (discarded leaf fragments). Leaves from 25 species or species groups of deciduous trees and woody vines (°35 species) were represented in foliar litterfall collected from litter traps; all of these contributed to herbivore—caused greenfall. Despite differences in aspect and stand age (10 yr, 23 yr, mature), patterns of greenfall were similar among watersheds. Seasonal inputs of greenfall to the forest floor were <5% of total foliar production but were greater than combined inputs of insect fecal pellets and insect body parts. Greenfall occurred continuously during the growing season and resulted from the activities of numerous species of insect herbivores. We suggest that greenfall is a widespread phenomenon that may be important as a resource for decomposers and that it should be included in measures of herbivory.
Population Fluctuations in Complex Life Cycles: An Example From Pemphigus Aphids
Authors: Moran Nancy A. , Whitham Thomas G.
Published: May 2006

The population dynamics of organisms with complex life cycles are rarely examined due to the number of potentially interacting variables. Experimental manipulations of conditions and long—term data on abundances were used to investigate the bases of year—to—year fluctuations in population size in the aphid Pemphigus betae, which has a complex life cycle. Spring generations may live in leaf galls on cottonwood trees, whereas summer generations obligately inhabit roots of herbaceous Rumex and Chenopodium species. Experimental manipulation of irrigation levels of root colonies in both field and garden experiments showed that soil moisture affects aphid survival and reproduction on roots. The importance of this single factor in governing population size is further supported by 6 yr of gall—density and weather data showing a strong relationship between gall abundance and the amount of rainfall the previous season. Censuses of autumn migrants arriving at cottonwoods during three of these years corroborate this. Thus, weather affects success on summer hosts and consequently determines numbers of migrants returning to cottonwoods and gall abundances during the subsequent spring. This study demonstrates that conditions during one phase of a complex life cycle may largely determine the population size of future generations using alternate niches.
A Graphical Model for Shell‐Species Selection By Hermit Crabs
Authors: Lively Curtis M.
Published: May 2006

Shell—species preferences by hermit crabs offer a unique opportunity to study a form of habitat selection by animals. A graphical model is presented in the present study, which considers the interacting effects of shell mass, shell aperture width, and shell depth on hermit crab shell—species selection. The model predicts that the most preferred shell species will be the one that maximizes the room for retreat into the shell (estimated as the ratio of shell depth to aperture width) such that a certain mass threshold (estimated as the ratio of shell mass to hermit crab size) is not exceeded. A good qualitative fit was observed between the predictions of the model and a test of the model using the hermit crab Clibanarius digueti from the Gulf of California.
Rapid Measurement of Fine Root Length Using Photoelectronic Image Analysis
Authors: Burke M. K. , LeBlanc D. C.
Published: May 2006

Root—length estimates obtained using three different methods of a commercially available image analyzer were compared. Precision, accuracy, and the speed of obtaining estimates differed significantly among the Semirandom, Parallel, and Right Angle methods; this was a result of root orientation effects and analyzer calibration limitations. A new rapid, accurate, and precise method of obtaining root—length estimates using the analyzer was developed. The procedure interfaces the image analyzer with a microcomputer and used a program that standardizes orientation effects during calibration.
Predator‐Prey Relationships in an Estuarine Littoral Copepod Community
Authors: Fulton Rolland S.
Published: May 2006

Seasonal divergences of planktonic copepod communities in a marine eelgrass bed and adjacent unvegetated channel habitats were associated with increases in predator abundance in the littoral habitat. During midsummer, when planktivorous silversides, Menidia menidia, were abundant in the eelgrass bed, pelagic copepod densities were significantly reduced in the eelgrass bed, relative to channel abundances. Differences declined overnight, when fish were not feeding, and were reestablished after dawn. In late winter, coincident with the arrival of large numbers of postlarval fish in the eelgrass bed, there was selective reduction of juvenile Centropages spp. in the eelgrass bed, relative to channel abundances. Field and laboratory experiments indicate that these differences were due to intense predation by planktivorous fish. However, the littoral copepod Pseudodiaptomus coronatus appeared unaffected by the intense predation in the eelgrass bed. Laboratory experiments demonstrated that its epibenthic habit and cryptic coloration allow P. coronatus to utilize refuges in the littoral habitat, and that prey selection by two fish species differed due to differences in foraging behavior.
Population Experiments with the Anolis Lizards of St. Maarten and St. Eustatius
Authors: Pacala Stephen W. , Roughgarden Jonathan
Published: May 2006

During 1980 and 1981 population experiments were performed with two two—species guilds of Anolis lizards: A. gingivinus and A. wattsi pogus on the island of St. Maarten (Netherlands Antilles) and A. bimaculatus and A. wattsi schwartzi on the island of St. Eustatius (Netherlands Antilles). These experiments were designed to test the hypothesis that the strength of interspecific competition is inversely related to the degree of interspecific resource partitioning. The experimental design consisted of four field—enclosures (12 x 12 m) on St. Maarten and four on St. Eustatius. On each island two enclosures contained only the larger native species (A. gingivinus or A. bimaculatus), and two contained both native species. Resource partitioning was measured inside each of the two—species enclosures. There was more interspecific partitioning of space and prey (arthropods) on St. Eustatius than on St. Maarten. A comparison of the one— and two—species enclosures on St. Maarten revealed five competitive effects of A. wattsi pogus on A. gingivinus. In the presence of A. wattsi pogus, A. gingivinus individuals had lower growth rates and increased perch heights. Also, dissected A. gingivinus from the two—species enclosures contained smaller volumes of developing eggs per female, smaller volumes of insect prey per lizard, and smaller mean prey sizes than A. gingivinus from the single—species enclosures. In contrast, no consistent and significant competitive effects of A. wattsi schwartzi on A. bimaculatus were observed on St. Eustatius. These results support a central assumption of competition theory: the strength of interspecific competition increases as the amount of interspecific resource partitioning decreases.
Coping with a Capricious Environment: A Population Study of a Rare Pierid Butterfly
Authors: Cappuccino Naomi , Kareiva Peter
Published: May 2006

Found in small, scattered colonies, the West Virginia white butterfly (Pieris virginiensis) is absent from many areas where its food plant, Dentaria diphylla, is common. To better understand the demographic processes contributing to this butterfly's rarity, we studied the natural history and population ecology of a colony of Pieris virginiensis in Connecticut. Survival to instar IV for P. virginiensis is comparable to that observed for its superabundant congener, P. rapae. However, for P. virginiensis problems begin with instar IV and V larvae, which appear to be victimized by unpredictable host phenology. In 1982, early senescence of Dentaria stranded most fourth and fifth instar caterpillars; stranded caterpillars were unsuccessful at locating secondary host plants. P. virginiensis adults also encounter difficulties because they attempt to oviposit in the spring, when weather conditions are rarely suitable for flight or oviposition. Furthermore, when the females do fly, they are not very effective at laying eggs rapidly. Buildup of granulosis virus through soil contamination may also contribute to the butterfly's rarity. By coating Dentaria plants with water—and—soil slurries from woodland areas with and without P. virginiensis populations, we documented significant vertical transmission of granulosis virus through the soil (i.e., higher virus mortality on plants coated by slurries from P. virginiensis locales). Finally, because P. virginiensis appears reluctant to fly across open fields, recolonization of extinct colonies is an unlikely event. We speculate that P. virginiensis may have been more abundant in the past, when butterflies would have had less trouble dispersing among Dentaria stands because fewer woodlands were dissected by open habitats.