bluuebunny/crossref_metadata_2025 huggingface dataset:
Plant Characteristics Over a Century of Primary Sucession on Islands: Lake Hjalmaren
Authors: Rydin Hakan , Borgegard Sven-Olov
Published: May 2006

Between 1882 and 1886 the water level of Lake Hjalmaren, Sweden was lowered °130 cm. As a consequence, a large number of new islands arose. From 30 such islands we have complete lists of vascular plant species with notes on their abundance from 1886, 1892, 1903—1904, 1927—1928, and 1984—1985. The 112 most important species were grouped as pioneers, early—, mid—, or late—successional, depending on when in the sere they were most abundant or frequent. Several species (referred to as stayers) remained common once they had entered the succession. For the 112 species we collected literature data on height, life form, fecundity, mating system, pollination, seed size, dispersal, and relation to nitrogen and light, and the successional groups were compared with regard to these attributes. The characteristics of the pioneers were distinct from other groups. Eight of the 13 pioneers were annuals (therophytes), all of which were predominantly autogamous, indicating a strong coupling between these life history traits. After the pioneer stage there was no continuous change in these attributes. The seeds in pioneers were not smaller than in early— or midsuccessional species, and seed size did not differ between annuals and perennials. Water dispersal was very important, and therefore dispersal success put no constraint on seed size. Species with the heaviest seeds arrived late; notably they were bird dispersed. Only among wind—dispersed trees and shrubs was there a gradual increase in seed mass over succession. The Ellenberg light indicator value showed a slight decrease over time, as did a variable measuring preference for open habitats. We propose that physiology may be more important than life history for species replacements after the pioneer stage.
Regulation of an Insect Population Under Biological Control
Authors: Murdoch William W. , Luck Robert F. , Swarbrick Susan L. , Walde Sandra , Yu Dickie S. , Reeve John D.
Published: May 2006

California red scale is suppressed to very low densities by the parasitoid Aphytis melinus. The system also appears stable. We report on an experimental test of the hypothesis that stability is caused by a refuge for scale. In a grapefruit grove in southern California in 1984—1985, the bark in the interior part of the tree provided a partial refuge from parasitism. Scale were °100 times denser there than in the exterior of trees. In a field experiment, we removed Argentine ants from some blocks of trees to test whether (1) ants caused the refuge by interfering with Aphytis and (2) the expected reduction in scale density in the refuge would lead to an unstable interaction in the exterior. We also tested for density—dependent parasitism, host mutilation, and predation by analyzing data from samples and from scale placed in the field. The temporal variability of the scale was at the low end of the range recorded in field populations. The experiment provided some evidence in support of the refuge hypothesis. The population in the refuge fluctuated much less than that in the exterior. Ant exclusion led to increased parasitism and lower scale density in the interior, and to increased fluctuations in abundance in the refuge and exterior. However, these changes were relatively small and perhaps temporary, suggesting that (1) ants are not the main cause of the refuge and that (2) we did not reduce the refuge density enough to determine whether the system would go unstable in the absence of the refuge population. Parasitism, host mutilation, and prediction rates on scale showed no temporal density dependence, either direct or delayed, though detection of such patterns is difficult. Possible alternative stabilizing mechanisms include size—dependent interactions between red scale and Aphytis.
Floral Biology, Microclimate, and Pollination by Ectothermic Bees in an Early‐Blooming Herb
Authors: Herrera Carlos M.
Published: May 2006

Abiotic factors may constrain the functioning of species interactions such as plant—pollinator mutualisms. I investigated how thermal environment affects the interaction between the early—blooming daffodil, Narcissus longispathus (Amaryllidaceae) and its major bee pollinator (Andrena biclor; Andrenidae), focusing simultaneously on plant and pollinator sides of the interaction. I studied fruit and seed set, flower duration, and the intrafloral thermal environment of N. longispathus, and the thermal biology, foraging behavior, and thermoregulatory ability of A. bicolor, over a 6—yr period in southeastern Spain. N. longispathus flowers from February to April, when unsuitable weather often limits pollinator activity, yet most flowers are successfully pollinated in all years and sites. Fruit set was weakly pollen limited, but among flowers setting fruit the proportion of ovules developing into seeds was not. Individual flowers lasted for 17 d on average, remaining functional during the period. On sunny days, the air inside N. longispathus flowers was significantly warmer than outside. Mean temperature excess inside flowers was as high as 8°C, and was positively related to solar irradiance. Within flowers, air temperature was highest around the anthers; this intrafloral gradient was consistent with variation among perianth parts in radiation transmittance. Andrena bicolor foraged in N. longispathus flowering patches only on sunny days with air temperature >12°—13°C, and foraging behavior and flower visitation rate when temperature dependent. Bees were able to fly at relatively low thoracic temperatures (Tth; range 22°—31°C) and this was essential for successfully foraging at N. longispathus. Under the range of irradiance and air temperature found at foraging sites, A. bicolor individuals inside flowers were able to reach Tth suitable for flight by passive means alone. Under laboratory conditions, A. bicolor was unable to raise or otherwise regulate Tth by physiological means, but free—flying individuals thermoregulated behaviorally. Basking was used to raise Tth, and intrafloral microclimate, by influencing the proportion of foraging time devoted to basking, played an important role in thermoregulaton. Flower visitation rate was positively related to the average temperature inside visited flowers, and the probability of basking immediately after one floral visit declined with increasing flower temperature. I conclude that the favorable microclimate within N. longispathus flowers, their long duration, and the thermal biology of A. bicolor, were critical elements in this early—season pollination system.
Effect of Inflorescence‐Feeding Insects on the Demography and Lifetime of a Native Plant
Authors: Louda Svata M. , Potvin Martha A.
Published: May 2006

The importance of herbivorous insects for plant demography and fitness is still controversial. The fact that their effect has not be fully evaluated within the context of the complete plant life cycle contributes to this controversy. This study is the first to test directly the influence of flower and seed consumption by a guild of inflorescence—feeding insects on the demography and fitness of a native plant over its whole life cycle within its natural environment. First, we discuss the evidence required to assess the effect of inflorescence—feeding on sequential stages in the plant life cycle. Then, we present the detailed results of two experiments that quantify the effect of inflorescence—feeding insects on seeds, seedlings, juveniles, and subsequent flowering progeny of Cirsium canescens (Platte thistle), a native, monocapic perennial species of Sandhills prairie, Nebraska. Exclusion of inflorescence—feeding insects by insecticide had four main effects. Total seed output increased (P < 0.0005 in both experiments), indicating that resource were not limiting seed production. Flower heads produced later in the season contributed to the seed pool when insects were reduced (P < 0.01), suggesting that insect feeding restricted the phenology of flowering and pollination. The density of seedling increased around plants protected from florescences—feeding insects (P < 0.0005), showing that safe sites were not limiting. Finally, the increase in seedlings led to higher numbers of flowering adults (P < 0.009), demonstrating that inflorescence—feeding insects significantly reduced lifetime fitness. Neither individual compensation for seed loss, nor density—dependent compensation for increased plant population densities, was observed when insects were excluded. Thus, we conclude that the inflorescence—feeding insect herbivores limited seed production, seedling recruitment, plant density, and maternal fitness of this plant under natural conditions. This outcome challenges current theoretical models of the effect of insects on plant population dynamics. We predict that our result will be general for short—lived perennial plants with life histories in which persistence is tightly linked to regeneration from current seed, such as for fugitive perennials with transient seed banks.
The Influence of Initial Size on the Competitive Responses of Six Plant Species
Authors: Gerry Ann K. , Wilson Scott D.
Published: May 2006

Plant size may be an important determinant of competitive ability and may affect comparisons of competitive response among species. We examined the influence of initial size on plant competitive response in a field experiment using transplant in three classes of initial mass (class means: 27, 46, and 73 mg) for each of six species (Agropyron cristatum, Bouteloua gracilis, Elaeagnus commutata, Fraxinus pennsylvanica, Melilotus officinalis, and Potentilla pensylvanica.) Transplant were grown for one summer in an old field dominated by the perennial grass Bromus inermis. The transplants were grown with no neighbors present, with shoots of neighbors present, with roots of neighbors present, or with both shoots and roots of neighbors present. The existing Bromus served as neighbors. There were 10 replicates of each factor combination. Final transplant mass varied significantly with competition, species identity, and initial size. The mass of transplant grown with only neighbor roots was not significantly different from the mass of transplants grown with both roots and shoots of neighbors, suggesting that competition in the field was primarily belowground. A significant interaction between species identity and competition indicated that species differed in the extent to which they were suppressed by neighbors. There was no significant interaction between size and competition, however, suggesting that initial size had no influence on competitive response. Further, regressions of final transplant mass against neighbor mass provided no evidence that initially smaller transplants were weaker competitors. Our results suggest that the competitive responses of the six species during the first summer of growth were not influenced by their initial size under conditions of intense belowground competition.
Variation in Floral Sex Allocation with Time of Season and Currency
Authors: Ashman Tia-Lynn , Baker Irene
Published: May 2006

Sex allocation patterns and nutrient investments in floral components were investigated in hermaphroditic morphs of Sidalcea oregana ssp. spicata utilizing a micro—technique for the determination of nitrogen and phosphorus content. A plant's investment in flowers and individual floral components decreased from early to late in the season. The primary sexual structures (gynoecium and pollen) showed less severe seasonal declines in nitrogen and phosphorus allocation than did the sepals and petals. Phenotypic gender remained relatively constant throughout the season. Sepals, gynoecium tissue, and pollen had significantly higher concentrations of nitrogen and phosphorus than did the androecium and petals. Pollen had the highest requirement for phosphorus. These results suggest that the sex functions could be limited by different nutrients. Proportional allocation to floral structures varied with time of season and currency used. A seasonal decrease in the proportion of biomass in the sepals was accompanied by an increase in the proportional allocation to each of the reproductive structures. Selection may favor preserving minimum level of allocation to the primary sexual structures if gamete (or zygote) fitness is influenced by resource availability during production and/or maturation.
Microhabitat Variation and Patterns of Colony Growth and Feeding in a Marine Bryozoan
Authors: Okamura Beth
Published: May 2006

The influences of neighbors and flow regime on growth patterns and feeding rates of the bryozoan Electra pilosa were characterized in laboratory and field experiments. In laboratory studies, neighbors inhibited growth, while flow velocity exerted a negative effect on growth rates of isolated colonies but had no influence on growth rates of colonies with neighboring Electra upstream. There were no significant effects of neighborhood or flow on zooid dimensions. Field experiments, which only tested the influence of neighbors on growth, revealed a similar reduction in growth in the presence of neighbors. The growth responses displayed by Electra do not support predictions of adaptive strategy models: despite its noted range in colony morphology, Electra did not assume more runner—like shapes when growth conditions were poor. Rather, growth of Electra appeared to be a result of local astogenetic (colonial ontogenetic) rules. Colonies that fed from similar flows to those they had experienced in the laboratory provide evidence backing the common assumption that patterns of feeding result in growth responses and so may be related to fitness in colonial animals. Evidence that growth patterns resulted from feeding rates suggests a central role for the acquisition of food in the dynamics of benthic assemblages of suspension feeders.
Thinning Reduces the Effect of Rust Infection on Jewelweed (Impatiens Capensis)
Authors: Lively Curtis M. , Johnson Steven G. , Delph Lynda F. , Clay Keith
Published: May 2006

We examined the relationship between plant density and the probability of infection by a rust (Puccinia recondita) in a natural population of jewelweed (Impatiens capensis). We also reduced plant density in field manipulations to determine whether there is any synergism between intraspecific competition and infection by the rust. The results showed no relationship between plant density and the probability of infection. However, we found that the effect of infection depends on plant density. Uninfected plants grew significantly faster than infected plants under natural conditions of high density, but infection had no significant effect on growth where density was experimentally reduced. Hence, the effect of disease on jewelweed depends on the plant's competitive environment. Such interactions between competition and disease are likely to be important to the population biology and breeding—system evolution of plants.
Variable Pelagic Fertilization Success: Implications for Mate Choice and Spatial Patterns of Mating
Authors: Petersen Christopher W. , Warner Robert R. , Cohen Sarah , Hess Helen C. , Sewell Amy T.
Published: May 2006

Fertilization success was measured in the bluehead wrasse, Thalassoma bifasciatum, a tropical reef fish with external fertilization of pelagic eggs. This species exhibits intraspecific variation in its spawning behavior; females either spawn with single males (pair spawning) or spawn with a group of at least three and often >20 males (group spawning). Fertilization success averaged °75% and did not differ between pair and group spawning, despite an estimated 80—fold increase in sperm release in group spawns. There was also no evidence that pair—spawning males suffered sperm depletion over the course of the spawning period. Thalassoma bifasciatum occurs in a variety of habitats and is exposed to varying levels of water turbulence. Fertilization success varied among days, and decreased with rougher water conditions. Within a reef, the calmer spawning sites behind the reef relative to the current had higher fertilization success than those along the side of the reef. These data suggest that while the type of spawning occurring at the site does not affect selection for fertilization success, females may gain fertilizations by selecting particular locations or periods of calmer water conditions to spawn. This is an alternative hypothesis to explain temporal and spatial patterns of mating. We provide detailed methods on how to accurately obtain data on fertilization success. Our technique can be used to study natural spawning in a wide variety of reef fishes and other marine organisms with pelagic eggs, external fertilization, and predictable spawning.
Consequences of Larval Ecology for Territoriality and Reproductive Success of a Neotropical Damselfly
Authors: Fincke Ola M.
Published: May 2006

Male territorial behavior and female oviposition behavior of Megaloprepus coerulatus, a damselfly that develops in water—filled tree holes, is interpreted in light of its larval ecology. Sexual selection favors large males because they win more territorial contests than do smaller males, and females mate only in the vicinity of defended tree holes (i.e.,passive female choice). Small males behave as territory holders until displaced, and/or act as subordinate satellites at large territories. Females are more predictably found in forest light gaps, even though they oviposit in both gap and understory tree holes. Males defended only 14% of the sites used by ovipositing females. Large tree holes are relatively rare, but support a greater number of emerging adults per season and produce larger adults than tree holes containing ≤1 L of water, which rarely produce more than one individual per season. By defending only large tree holes in gap areas, a male increases his chances of mating, and of producing multiple offspring and reproductively successful sons. Results from a field experiment suggest that both mate acquisition and procurement of superior larval habitats have been selective pressures in the evolution of male territory choice. Because even large holes are inadequate to support a female's entire clutch, and the presence of conspecific and heterospecific odonate larvae makes offspring survival uncertain, selection favors a female that partitions her clutch among multiple tree holes.
On the Spatial Spread of Insect Pathogens: Theory and Experiment
Authors: Dwyer Greg
Published: May 2006

The mathematical theory of animal diseases has seen explosive growth in the past decade, yet most of the existing theory examines only temporal disease spread, ignoring the effects of patchy host or pathogen spatial distributions. Here I present a model for the within—season spatial spread of insect pathogens that incorporates host movement in an otherwise conventional insect host—pathogen model. Mathematical analysis of the model reveals that the pathogen will spread through the host population in a moving wave front of disease, known as a "travelling wave." This analysis shows how the spatial rate of spread of the pathogen depends upon the transmission rate of the disease, the rate of production of the pathogen by infected hosts, the initial population of the host, the decay rate of the pathogen, and the death rate of infected hosts. To test the predictions of the model, I performed a series of field experiments with the nuclear polyhedrosis virus (NPV) of Douglas—fir tussock moth, Orgyia pseudotsugata. First, I estimated each of the parameters of the model in the field with a series of small—scale experiments, and used the parameter estimates to predict the spatial rate of spread of the NPV through a population of tussock moth larvae (NPV diseases, like many insect pathogens, do not infect adults). To test this prediction, I then performed an experiment in which I measured the rate of spread of the NPV in an experimental population of tussock moth larvae on linear arrays of Douglas—fir seedlings. The model predicts the rate of spread of tussock moth NPV fairly accurately, suggesting that one can use this type of model to extrapolate individual behavior and localized transmission patterns to broader—scale spatial dynamics.
Interactions of Intraspecific, Interspecific, and Apparent Competition with Host‐Pathogen Population Dynamics
Authors: Grosholz Edwin D.
Published: May 2006

I investigated the interactions of intra— and interspecific competition with parasitism for populations of two terrestrial isopods, Porcellio scaber and Porcellio laevis, infected with a lethal virus. In field experiments, I varied densities of one or both isopod species in enclosures in which food and virus presence were also manipulated. I estimated the effects of intra— and interspecific competition, additional food, and virus infection on both the survival of P. scaber and the prevalence of the virus. In uninfected populations, intraspecific and interspecific competition negatively affected isopod survival, although food resources had no effect. In infected treatments, interspecific competition produced higher levels of virus prevalence than did intraspecific competition. Virus infection and reduced food interacted to produce higher overall mortality in interspecific competition treatments than in intraspecific competition treatments. The lack of significant effect of food on competition suggests that interference interactions rather than resource competition predominated. This mechanism is supported by a laboratory study in which total food availability was controlled. These results indicate that competition and parasitism interacted to produce the observed mortality patterns, and that ignoring mortality due to the virus would produce apparent interspecific competition. The data suggest that the impact of P. laevis on P. scaber involves both direct and indirect effects.
Functional Response During Winter of a Herbivore, The Moose, in Relation to Age and Size
Authors: Andersen Reidar , Saether Bernt-Erik
Published: May 2006

This study was undertaken to provide an estimate of the functional response curve of the moose during winter. The functional response consists of two components: the intake rate while foraging and the time available for food searching. In generalist herbivores an interaction may exist between those two components, because the quality of food eaten (expressed as digestibility) may limit the period available for grazing. Thus, variation in intake rate while foraging in relation to food availability may give a biased estimate of functional response in such animals. We studied the relationship between the different components of functional response during winter for a generalist browser, the moose Alces alces, by comparing the intake rates of free—ranging individuals foraging on three Norwegian winter ranges of different quality. The intake rate was determined by collecting feces defecated during a 24—h period by radio—collared individuals. The amount of food eaten was found by multiplying the dry mass of feces with the inverse of the digestibility of the twigs eaten during that period. No relationship was found between intake rate and either snow depth or temperature. Body mass accounted for a large proportion of the variance in intake rates. The slope in the regression of intake rate on body mass (b = 0.73) was close to the one expected from Kleiber's law. However, when calves and adults were analyzed separately, a significant allometric relationship was found only for the calves. The intake rates of both age groups increased linearly with resource availability. This relationship also remained after accounting for the effects of body mass in a multiple—regression analysis. The intake rate also increased with the digestibility of the twigs eaten. However, twig digestibility was closely correlated with resource abundance in the presence data set, so the separate effects of quality and quantity were difficult to estimate. In the present data set no significant relationship appeared between resource availability and the time used for foraging. Thus, the functional response of winter—foraging moose was linearly related to the resource availability. However, we suggest that the quality of the food also should be included in the computation of functional response curves of large herbivores. Consequently, the functional response should be described in three dimensions as a function of quality and quantity of the food resources.
Stable Isotopes and Planktonic Trophic Structure in Arctic Lakes
Authors: Kling George W. , Fry Brian , O'Brien W. John
Published: May 2006

Actual food—web structure or function is difficult to determine based on visual observation, gut analyses, or the feeding interactions expected from a given list of species. We used C and N stable—isotope distributions to define food—web structure in arctic lakes, and we compared that structure with results based on more traditional analyses. Although zooplankton species composition was similar across the eight lakes studied, the food—web structure varied greatly. In some lakes the copepod predator Heterocope fed on the herbivorous copepod Diaptomus as expected in a conventional food web. In most lakes, however, °15N data were consistent with Heterocope functioning as an herbivore rather than a predator. These inferences were supported by evidence from carbon isotopes and energy—flow data. Our study indicates that only two or three trophic levels exist in the macrozoopolankton of these lakes, in comparison to five or six trophic levels reported in temperate lakes. Isotope analyses showed that actual food—web structure is poorly predicted from simple consideration of species lists and potential trophic interactions.
Carbon Dynamics and Estimates of Primary Production by Harvest, ^(14)C Dilution, and ^(14)C Turnover
Authors: Milchunas D. G. , Lauenroth W. K.
Published: May 2006

Large plots of native shortgrass steppe were labeled with 14C to assess short—term patterns of carbon allocation and the long—term process of herbivory, death, and decomposition, and to compare estimates of net aboveground, crown, and root primary production using 14C dilution, 14C turnover, and traditional harvest methods. Stabilization of labile 14C via translocation, incorporation into structural tissue, and respiration and exudation required one growing season. Exudation was 17% of plant 14C after stabilization. Estimates of turnover time for leaves, crowns, and roots by 14C turnover were 3, 5, and 8 yr, respectively, yielding estimates of belowground production that were much lower than previously thought. Estimates of aboveground production by 14C turnover were close to those obtained by harvest of peak—standing crop, but lower than reported values obtained by harvest maxima—minima. Estimates of root production by harvest maxima—minima were zero in 2 of 4 yr. 14C turnover appeared to provide reliable estimates of aboveground, crown, and root production. In contrast to reliable estimates by 14C turnover, 14C dilution estimates of root production were anomalous. The anomalous estimates were attributed to a nonuniform labeling of tissue age classes resulting in differential decomposition/herbivory of 14C:12C through time, as well as movement and loss of labile 14C through the first growing season. Isotope—dilution methodologies may be unreliable for any estimate of pool turnover when the labeling period is not as long as pool—turnover time. Problems and biases associated with traditional harvest maxima—minima methods of estimating aboveground primary production are well known, but are greatly exacerbated when the method is used to estimate root production. Estimates of root production by 14C dilution were unrealistic. 14C turnover methodology provided reliable estimates of production in this community.
Role of Polyphenolic Molecular Size in Reduction of Assimilation Efficiency in Xiphister Mucosus
Authors: Boettcher Anne A. , Targett Nancy M.
Published: May 2006

Feeding deterrent effects of polyphenolic compounds from both terrestrial and marine environments have been well studied. However, the effects of this compound class on herbivore digestion and assimilation are less well known, particularly with respect to marine herbivores. This study found that the reduction of assimilation efficiency in Xiphister mucosus, a marine herbivorous fish, fed algae treated with marine polyphenolics (phlorotannins) was affected by phlorotannin molecular size (mass). In general, phlorotannins > 16.60 x 10—21 g(= > 10 kDa) significantly decreased the assimilation of X. mucosus; those <8.30 x 10—21 g (= <5 kDa) rarely, if ever, had an effect. To determine the ecological relevance of these findings, phlorotannin molecular size profiles were completed for several temperate and tropical brown algal species. In all but one of the algal species examined, the phlorotannins were concentrated in the > 16.60 x 10—21 size range. Thus, the size range of phlorotannins that significantly reduced assimilation was also the size range in which they were most concentrated. This work supports the idea that polyphenolics as a chemical class do not all have the same bioactivity, but differ in their activity in a size—dependent manner. It also addresses some of the discrepancies seen in feeding—preference studies, providing an alternative explanation for results.
Quantifying the Effect of Predation Risk on Foraging Bullies: No Need to Assume an IFD
Authors: Kennedy Martyn , Shave Cathy R. , Spencer Hamish G. , Gray Russell D.
Published: May 2006

Previous studies of the effects of predation risk on patch choice have assumed the distribution of foragers to be Ideal Free. However, recent investigations have revealed systematic departures from the Ideal Free Distribution. In this study we do not assume that the distribution of foragers fits an Ideal Free Distribution. Instead we use a method that separates the effects on patch choice of predator avoidance (site bias) from effects due to the resource ratio. Our experiment examines the distribution of foraging upland bullies (Gobiomorphus breviceps) between two resource sites over a series of five different food input ratios. We had two conditions, a predator—free and a predator condition. The bullies exhibited a strong bias (preference) toward Site 1 when there was no predator (quinnat salmon, Oncorhynchus tshawystcha). When a salmon was present at Site 1 they exhibited a strong bias towards the alternative resource site (Site 2). We estimated that it would be necessary to change the relative food availabilities by a factor of °28 to remove the effect of the presence of the predator. The slope of the distribution of bullies (their sensitivity to resource profitability) deviates from the Ideal Free prediction for both conditions. A trend in the slope of the distribution of the bullies between the predator—free and predator conditions suggested that the presence of a predator may alter the behavior of the bullies. By acknowledging that the distribution of foragers may deviate from Ideal Free, we demonstrate that it is possible to separate the effects of predator avoidance from a change in their ability to discriminate between resource sites caused by the presence of a predator.
Local Adaptation in the Armored Scale Insect Pseudaulacaspis Pentagona (Homoptera: Diaspididae)
Authors: Hanks Lawrence M. , Denno Robert F.
Published: May 2006

We investigated adaptation of the armored scale insect Pseudaulacaspis pentagona (Targioni—Tozzetti) to individual mulberry trees (Morus alba L.) in Maryland, USA. We tested local adaptation by transferring eggs within and between trees and by measuring the subsequent survival of developing scales. Survival of P. pentagona was significantly higher when raised on their natal host tree compared to rearings on novel trees. This finding is consistent with local adaptation of scale populations to individual host trees. However, we found evidence for differences in local adaptation of scale populations only between populations on distantly separated mulberry trees. There was no evidence for genetic differentiation of scale populations on neighboring trees, suggesting that gene flow inhibited local adaptation. By adapting to individual plants, few populations of herbivores appear to sacrifice their ability to colonize new conspecific hosts. While local adaptation may fine tune herbivore populations to individual host phenotypes, it is unlikely to play a general role in explaining the patchy distribution of herbivorous insects on their hosts plants.
Gender Specialization Across a Climatic Gradient: Experimental Comparison of Monoecious and Dioecious Ecballium
Authors: Costich Denise E.
Published: May 2006

The ecological basis for breeding—system evolution was explored in Ecballium elaterium (L.) A. Rich. (Cucurbitaceae). Two subspecies are parapatric on the Iberian Peninsula: dioicum (dioecious) is found in southern, more arid regions and elaterium (monoecious) is found in northern, wetter regions. Six experimental gardens were set up in three sites that span the climatic gradient of the species' natural range in Spain in order to address two issue: (1) gender specialization in reproductive characters and life history, and (2) ecological specialization of the two subspecies across environments. Overall, the dioecious subspecies outperformed the monoecious subspecies. Monoecious plants, on average, were shorter—lived in all environments. Females of the dioecious subspecies produced similar numbers of female flowers, but more fruits and seeds, than did monoecious plants (cosexuals). The fruits of females contained more, but smaller, seeds than the fruits of cosexuals. Males always produced more male flowers than did cosexuals, disproportionately so with increased plant size (canopy volume); no such disproportionate increase in seed production was found in females. A geographic analysis of gender (proportion of female flowers) in the monoecious plants revealed a striking pattern: dimorphism with increased gender differentiation in drier sites, in contrast to monomorphism with respect to gender in wetter sites. Differences between the subspecies and between habitat types can be related directly to breeding—system differences. The higher overall performance of dioecious plants may be due to enhanced heterozygosity resulting from outcrossing enforced by dioecy. On the other hand, the gender differentiation of the monoecious plants in the drier sites may reflect gender allocation "decisions" enforced by limited resource availability.
Style Morph Ratios in Tristylous Decodon Verticillatus (Lythraceae): Selection vs. Historical Contingency
Authors: Eckert Christopher G. , Barrett Spencer C. H.
Published: May 2006

Tristylous plant populations should exhibit equal frequencies of the three style morphs at equilibrium. In contrast, New England and central Ontario populations of Decodon verticillatus (Lythraceae) show a marked deficiency of the mid—styled (M) morph. This pattern was also evident in an independent sample of 30 populations from eastern Ontario: all nine cases of morph loss involved the M morph, and this morph also occurred at low frequencies, especially in large trimorphic populations. The hypothesis that a fitness disadvantage to the M morph accounts for its low frequency was not supported by data from a series of field and glasshouse experiments. Measures of reproductive success from two populations showed no disadvantage to the M morph, but revealed substantially higher seed set in the mid— (M) and short—styled (S) morphs than in the long—styled (L) morph. A pollen addition experiment showed that low seed set in the L morph can only partly be explained by reduced pollen receipt. Comparison of morph frequencies between adult plants and their open—pollinated progeny in three populations failed to reveal any morph—specific fitness differences. Analysis of growth and flowering of open—pollinated progeny from three populations in a 2—yr glasshouse experiment revealed differences among morphs for some parameters in some populations, but no consistent patterns were evident. The deficiency of the M morph may have resulted from an historical accident during post—glacial colonization of parts of the northern range. Computer calculations examining the return of morph frequencies to equilibrium after major perturbations indicated that populations of long—lived clonal species like D. verticillatus may preserve skewed morph ratios for >10 000 yr. Moreover, the distribution of fossil seeds indicates that this species has occurred in its post—glacial range in southern Ontario for only °5000 yr. It is important to distinguish between ecological and evolutionary time scales when testing selective interpretations of microevolutionary patterns involving clonal taxa in glaciated regions.
Seed and Seedling Demography of Four Co‐Occurring Carpinus Species in a Temperate Deciduous Forest
Authors: Shibata Mitsue , Nakashizuka Tohru
Published: May 2006

In order to clarify regeneration traits and their fitness, we compared seed fall, seedling emergence, and first—year seedling of four co—occurring species. Carpinus tschonoskii, C. japonica, C. laxiflora, and C. cordata, in an old—growth temperate deciduous forest in Ogawa Forest Reserve, central Japan. Seedling emergence ratio of C. cordata (5.5%), whose seeds mostly germinated 1.5 yr after seed fall, was lower than the other species (8.5—11.2%). First—year seedlings of Carpinus tschonoskii and C. japonica had much lower survival rates (2.7%, both species) than C, laxiflora and C. cordata (7.5—8.5%) under closed canopy, suggesting that the former two are more disturbance dependent than the later two. They are characterized by their seed and seedling performances as seed rain type fundamentally; however, C. cordata and C. laxiflora are able to establish a seedling bank, and C. cordata has a seed bank too. These regenerative strategies partly explain the abundance and population structure of the four species in the old—growth forest. Multiple regression analyses showed that topography was important for survival of all species but C. cordata. This result was consistent with the adult distribution of these species in this forest. Wind dispersal of Carpinus is effective, both to escape from density— or distance—dependent high mortality and to increase the chance of arriving at canopy gaps, where seedling survival rate is high. However, escaping high mortality seems relatively more important for C. laxiflora and C. cordata (seedling bank strategy), and arriving at a canopy gap is relatively more important for C. tschonoskii and C. japonica (seed rain strategy).
Interactions Between Individual Plant Species and Soil Nutrient Status in Shortgrass Steppe
Authors: Vinton Mary Ann , Burke Ingrid C.
Published: May 2006

The effect of plant community structure on nutrient cycling is fundamental to our understanding of ecosystem function. We examined the importance of plant species and plant cover (i.e., plant covered microsites vs. bare soil) on nutrient cycling in shortgrass steppe of northeastern Colorado. We tested the effects of both plant species and cover on soils in an area of undisturbed shortgrass steppe and an area that had undergone nitrogen and water additions from 1971 to 1974, resulting in significant shifts in plant species composition. Soils under plants had consistently higher C and N mineralization rates and, in some cases, higher total and microbial C and N levels than soils without plant cover. Four native grasses, one sedge, and one shrub differed from one another in the quantity and quality of above— and belowground biomass but differences among the six species in soil nutrient cycling under their canopies were slight. However, soils under bunchgrasses tended to have higher C mineralization and microbial biomass C than soil under the rhizomatous grass, Agropyron smithii. Also, the one introduced annual in the study, Kochia scoparia, had soils with less plant—induced heterogeneity and higher rates of C and N mineralization as well as higher levels of microbial biomass C than soils associated with the other species. This species was abundant only on plots that had received water and nitrogen for a 4—yr period that ended 20 yr ago, where it has persisted in the absence of resource additions for 20 yr, suggesting a positive feedback between plant persistence and soil nutrient status. Plant cover patterns had larger effects on ecosystem scale estimates of soil properties than the attributes of a particular plant species. This result may be due to the semiarid nature of this grassland in which plant cover is discontinuous and decomposition and nutrient availability are primarily limited by water, not by plant species—mediated characteristics such as litter quality. That local plant—induced patterns in soil properties significantly affected ecosystem scale estimates of these properties indicates that consideration of structural attributes, particularly plant cover patterns, is critical to estimates of ecosystem function in shortgrass steppe.
Light Limitation in a Stream Ecosystem: Responses by Primary Producers and Consumers
Authors: Hill Walter R. , Ryon Michael G. , Schilling Elizabeth M.
Published: May 2006

Heavy shade presents serious challenges for primary producers and food—limited herbivores in forest streams. In this study, we examined the response of periphyton and grazing snails (Elimia clavaeformis) to summer shade in White Oak Creek (WOC), a second—order stream in a Tennessee deciduous forest. Three experiments were performed: (1) in situ manipulation of light and snail density to test the effects of light limitation and grazing; (2) construction of photosynthesis—irradiance (P—I) curves to test for shade adaptation by periphyton; and (3) measurements of snail growth vs. irradiance, to qualify the indirect relationship between grazers and an abiotic constraint on photosynthesis. In the first experiment, light and snail densities were manipulated in a 2 X 2 factorial design: two light treatments were created by removing streamside vegetation from four sites in WOC and by pairing each of these sites with an adjacent, shade site; two snail density treatments at each site were created by adding snails at normal (970 individuals/m2) and low (50 individuals/m2) density to the two sides of Plexiglas channels. Snails at normal densities cropped periphyton biomass to low levels regardless of light regime, but periphyton productivity was higher at the open sites where snails grew faster and accumulated more lipid. Snail growth and lipid accumulation were strongly affected by intraspecific competition in both light regimes. In the second experiment, photosynthesis—irradiance curves for periphyton from shaded and open sites illustrated considerable shade adaptation: shaded periphyton was 2 times more efficient at low irradiance than was periphyton from open sites. Despite the greater efficiency of shade periphyton at low irradiance, integrated primary production estimated with photosynthetic models was 4 times greater in the open because shade adaptation provided only partial compensation for the very low irradiances in the shade. In the third experiment, in situ snail growth again increased with decreasing shade. The growth vs. irradiance response resembled a P—I curve: snail growth increased almost linearly with increased light and then leveled off at a photon flux density of °7 mol°m—2·d—1. If this curve primarily reflects rates of food supply, then periphyton production and grazer growth in WOC and similar streams is light—limited at a photon flux density <7 mol·—2·d—1. Bottom—up effects of light limitation were propagated very strongly in WOC, where the invertebrate fauna is dominated by a grazer that appears to escape top—down control.
Patch Dynamics of a Phytophagous Mite Population: Effect of Number of Subpopulations
Authors: Walde Sandra J.
Published: May 2006

I conducted a field test of the hypothesis that the dynamics of a predator—prey interaction should be affected by the number of interacting subpopulations within an assemblage. I used a "successful" biological control system, the phytophagous European red mite (Panonychus ulmi) and its predator, the phytoseiid mite Typhlodromus pyri. An orchard was established where apple trees were arranged in groups of 1, 4, or 16 and the dynamics of the mite populations on the trees followed over a season. The pests reached highest densities and were most persistent on trees in the largest groups. These results are explained in terms of an interaction between prey immigration/emigration and predation.
Ecological Interactions Among Predators in Experimental Stream Communities
Authors: Resetarits William J.
Published: May 2006

Field observations on the effect of brook trout on an assemblage (sometimes described as a guild) of predatory salamanders suggest that the interactions among predators in small, headwater streams may be important in determining the structure of this assemblage of predators and thereby, the predation pressure felt by species at lower trophic levels. The interactions among predators and the potential role of those interactions in headwater stream communities was examined in 16 replicated experimental streams. I examined the ecological interactions among four species of co—occurring predators, adult Salvelinus fontinalis (98—122 mm standard length [SL]), larval Gyrinophilus porphyriticus (36—58 mm snout—vent length [SVL]), larval Eurycea bislineata (13—21 mm SVL), and Cambarus bartonii (6—17 mm carapace length [CL]), in an experiment that began in mid—June and ran for 3 mo. Brook trout affected both survival and growth of the larger salamander species, Gyrinophilus, and Salvelinus and Gyrinophilus affected the growth of the smaller salamander Eurycea and the crayfish Cambarus. The presence of Gyrinophilus had no effect on relative condition or fecundity of Salvelinus. Larger predators caused Cambarus and Eurycea to alter their activity levels and habitat, but Gyrinophilus made no adjustments to the presence of Salvelinus. Results demonstrate that Gyrinophilus is vulnerable to predation/aggression from brook trout, but that Eurycea Cambarus are able to behaviorally avoid predation by both large predators at a significant cost to growth. The effect of brook trout on the two species of salamander in the experimental streams parallels the observed differences between salamanders in trout and trout—free waters in the field. Thus, interactions among predators in headwater streams directly and indirectly influence population dynamics of the interacting species by affecting survival, growth, behavior, and habitat use. The interactions among these predators can determine, in part, the predation environment experienced by individuals at lower trophic levels.