bluuebunny/crossref_metadata_2025 huggingface dataset:
Competition for Hummingbird Pollination and Sequential Flowering in Two Colorado Wildflowers
Authors: Waser Nickolas M.
Published: May 2006

The simultaneous flowering of co—occurring plant species with similar pollinator affinities may result in interspecific pollen transfer and consequent fecundity reductions due to wastage of pollen, stigma surfaces, and effective pollinator visits. In such cases competition for pollination occurs and may lead to or maintain sequential flowering. Two common perennials in the mountains of west—central Colorado, Delphinium nelsoni and Ipomopsis aggregata, flower sequentially in the same meadows and are visited commonly by Broad—tailed Hummingbirds (Selasphorus platycercus). Hummingbirds carry pollen of both species and their exclusion from flowers leads to significant seed set reductions. During the brief period of flowering overlap between D. nelsoni and I. aggregata in natural meadows, hummingbirds visit both species, carry mixtures of their pollen, and appear to cause interspecific pollen transfer. Flowers of both species receptive during this period suffer significant seed set reductions relative to those receptive during nonoverlap periods. Interspecific pollinator flights and pollen transfer also occur in mixtures of potted plants, and seed set reductions consistently occur for both D. nelsoni and I. aggregata in such mixtures relative to single—species controls. Finally, seed set reductions occur for both species following interspecific hand pollination of potted plants. Fecundity reductions in natural and synthetic mixtures of D. nelsoni and I. aggregata indicate that the 2 species compete for hummingbird pollination and suggest that the competitive interaction involves interspecific pollen transfer. The observed reproductive effects represent a selective force sufficient to maintain divergent flowering times of D. nelsoni and I. aggregata in nature.
Interference and Exploitation Competition Among Tadpoles of Rana Utricularia
Authors: Steinwascher Kurt
Published: May 2006

The relative importance of exploitative and interference mechanisms for intraspecific competition among tadpoles of the Southern Leopard frog was evaluated by raising sibling tadpoles in 16 experimental environments designed to alter the costs and benefits of the two mechanisms. Growth of tadpoles in water conditioned by the 16 treatments was used to assay the level of interference chemical in each treatment. Body weights of tadpoles subjected to manipulations of density, food level, food dispersion and access to their feces increased with increasing food level and access to their feces. The largest tadpole in each replicate grew as though increased density also increased the food supply. The mean weight of tadpoles in each replicate indicated that there was less food at high densities. The largest tadpole in each replicate appears to outcompete the others for food. This food is fecal material and other particular material rather than the solid food source. Body weights of single tadpoles raised in water conditioned by their siblings increased as the food level in the conditioning treatment increased. Greater dispersion of the food source in the conditioning treatment decreased the growth of single tadpoles. Other factors affected the growth of the single tadpoles by altering the food level in the conditioning treatments. The decreased growth in conditioned water is interpreted as resulting from chemical interference competition. It may account for 5—6% of the variance in the density experiment. Interference mechanisms appear to supplant exploitative mechanisms as the relative food level decreases. I suggest that this is a general phenomenon and present evidence for its occurrence in other taxonomic groups.
Prey Size Selection by the Mosquitofish: Relation to Optimal Diet Theory
Authors: Bence James R. , Murdoch W. W.
Published: May 2006

We investigated size—selective predation upon Daphnia pulex and D. magna by the mosquitofish (Gambusia). The independent variables were the relative size of predator and prey, prey density, and predator satiation. Gambusia actively chose to attack small prey (which we showed to be more profitable). Gambusia's preference increased monotonically, or decreased, or peaked at an intermediate prey size, depending upon fish size, prey size, and fish satiation level. These results contrast with the hypothesis the "gape—limited predators" select the largest prey available, and "size—limited predators" select small or intermediate—sized prey. Optimal diet theory predicts that a predator should drop less profitable prey items from the diet at higher prey densities. In contrast, Gambusia maintained a mixed diet and its preferences did not change in response to changes in either the absolute or the relative abundance of prey. Moreover, the ingestion rate would have been higher if less profitable (larger) prey types had been ignored. Gambusia did sharply alter its size—selective feeding behavior in response to its own satiation level, and we expect that this factor is correlated with prey density in the field. Well—fed fish concentrated their attacks, to a greater degree than starved fish, on small, more profitable prey. They also ingested less biomass per unit time. This last result suggests that maximizing food intake per unit time may not be the goal of well—fed fish, and that different criteria may determine the profitability of prey at different prey densities.
Spatial Variation in the Effect of Ants on Extrafloral Nectary Plant
Authors: Barton Andrew M.
Published: May 2006

I investigated variation in the effect of ants at extrafloral nectaries on the plant reproductive success among three populations of Cassia fasciculata (Leguminosae) in northern Florida. Results show that the possession of extrafloral nectaries can lead to increased reproduction. Density of ants and removal of experimentally placed Drosophila larvae were far higher on C. fasciculata than on nonnectariferous background plants. Ants preyed on eggs and small larvae of lepidopteran herbivores. C. fasciculata plants from which ants were excluded had more larvae of the major seed pod predator Phoebis sennae (Pieridae; Lepidoptera), suffered higher pod predation (at one site), and had lower reproductive output (at two sites) than did control plants to which ants had access. Effects of extrafloral nectaries on reproduction of C. fasciculata varied among the three populations, due apparently to differences in density of ants and potential herbivores. At the site with the lowest density of ants and herbivores, pod set was not significantly different between control and ant exclusion groups. Where they were more numerous, ants had a small but significant positive effect. Only where the abundance of both ants and potential herbivores was high did ants visiting extrafloral nectaries strongly increase the reproductive success of individuals of C. fasciculata. The species of ants present and differences among herbivore species in susceptibility to ant attack may have also influenced the effect of extrafloral nectaries at any one site. Although the average effect of extrafloral nectaries throughout a plant species may be positive, the strength of this effect was found to vary widely among local populations.
Corn Monoculture Versus Old Field: Effects of Low Levels of Insecticides
Authors: Risch Steven J. , Pimentel David , Grover Herbert
Published: May 2006

The effects of very low concentrations of the insecticide carbaryl on a corn (Zea mays) monoculture ecosystem and an old—field ecosystem were studied in Ithaca, New York. Four sprayings at °3—wk intervals were applied and the effects on above— and belowground arthropod faunas and on rate of litter decomposition were investigated. The specific questions addressed were (1) are the effects of low—concentration insecticide stress different in kind or just quantity from those reported in previous studies conducted with higher concentrations? (2) is the complex old—field ecosystem more resistant to these effects than the simple corn monoculture? The insecticide caused a much greater reduction in total arthropod numbers on plants in the old field than in the corn, and this could be accounted for mainly by the effect on herbivores. Despite continued insecticide application throughout the season, the effect of spray on this herbivore reduction decreased over time in the old field. Spraying significantly increased the (predators + parasites)/herbivore ratio; thus herbivores were disproportionately more affected than their natural enemies by the insecticide. These results are in direct contrast with the previously reported studies using carbaryl at 10 times the concentration. Insecticide reduced the soil arthropod fauna much less than it did the arthropods on plants. There was no significant statistical effect of spray, field type, or date on total arthropods fauna on all groups aggregated. The only important effect of the spray on the soil anthropods was an outbreak of scavenging oribatid mites in the sprayed plots. Consistent with the above finding, rate of litter decomposition slowed significantly, but only temporarily, in the sprayed plots. There were no effects of plot type. Thus, low concentrations of carbaryl that might result from drift into fields adjacent to sprayed areas appear to have important ecological effects unpredictable from a knowledge of the effects of high concentrations. The two important findings here, that the insects in old fields were more sensitive to the insecticide and that spraying increased the enemies/herbivore ratio, suggest that the qualitatively different ecological interactions may be occurring in response to low levels of insecticides. Although pesticide droplet size and density were controlled in the present study, under natural drift conditions these factors might further complicate the already unpredictable effects of pesticide concentrations per se. The ecological effects of pesticide drift will be most important in areas dominated by small farmers and in areas where airplanes are used to apply chemicals. Because unsprayed crop and second—growth areas play an important role as refugia for both natural enemies and crop pests, further research is warranted investigating the effect of low pesticide dosages on complex ecological responses.
Bayesian Inference in Non‐Replicated Ecological Studies
Authors: Reckhow Kenneth H.
Published: May 2006

Classical hypothesis testing is founded on a long—run frequency perspective that is the basis for error rates and P values used in classical statistical inference. Thus in ecological studies involving formal hypothesis testing, it is common practice to report the P value as the summary result from the test of a point null hypthesis. However, many important ecological studies concern single, non—replicated events in which the P value has no clear interpretation. For the non—replicated study, Bayesian statistical inference provides an attractive alternative to classical statistical inference, as the results from a Bayesian analysis either may assume a long—run frequency interpretation or may be expressed as a probability of a unique event. An example concerning trends in lake acidification is used to show that the Bayesian approach is more compatible with scientific needs and scientific judgment than is classical hypothesis testing.
Plant Phenotype and Interspecific Competition between Insects Determine Sawfly Performance and Density
Authors: Mopper Susan , Whitham Thomas G. , Price Peter W.
Published: May 2006

Host plants not only influence herbivore performance, they can also mediate interactions between herbivores. We conducted a 5—yr field study to test the effects of pinyon pine (Pinus edulis) phenotype and competitive interactions on the colonization success, mortality, fecundity, and sex ratios of a foliage—feeding sawfly, Neodiprion edulicolis. Our experiments revealed four major patterns. (1) Sawfly survival was significantly higher on susceptible trees than on resistant trees. (2) In contrast, sawfly fecundity was significantly lower on susceptible trees than on resistant trees. (3) Interactions with a stem moth, Dioryctria albovitella, caused significantly reduced sawfly fecundity on susceptible trees. (4) Sawfly mortality, rather than sawfly fecundity, was the dominant factor influencing population growth rates: after four generations, sawflies transferred to resistant trees. A model developed with performance information from our field experiments accurately described patterns of population increase in the field: sawflies rapidly attain high densities on susceptible tress, and accumulate slowly on resistant trees. Trees displaying a susceptible phenotype are therefore better hosts than resistant phenotypes, despite the negative competitive interactions that occur between sawflies and moths on heavily infested susceptible trees.
A Field Test for the Cues of Disapause in a Freshwater Copepod
Authors: Hairston Nelson G. , Dillon Theresa A. , De Stasio Bart T.
Published: May 2006

The freshwater calanoid copepod Diaptomus sanguineus switches each year in spring from making eggs that hatch immediately to making diapausing eggs that rest in lake sediments for an extended period. In lakes and ponds containing planktivorous fish, the timing of the switch is highly consistent between years and comes at the theoretically optimal time (late March) to avoid intense summer predation. In fishless ponds the timing comes 1—2 mo later. We investigate here the environmental cues used by the copepods to time the switch to diapause. Through the use of both field and laboratory manipulations, we show that temperature and photoperiod play central roles as diapause stimuli. A field manipulation of fish density failed to reveal either a direct induction of diapause or any more subtle effects of fish occurrence on diapause timing. The copepods made immediately hatching eggs under short—day or low—temperature conditions, and diapausing eggs under long days or high temperatures. There appears to be variation between individuals in their responses to temperature and photoperiod so that similar diapause phenologies are produced by different sensitivities to the environmental cues.
Habitat Architecture and the Abundance and Body‐Size‐Dependent Habitat Selection of a Phytal Amphipod
Authors: Hacker Sally D. , Steneck Robert S.
Published: May 2006

Field and laboratory experiments were conducted on the effect of habitat architecture (the number, size, shape, and arrangement of habitable spaces and structures) created by benthic algae on the habitat selection of an abundant mobile amphipod, Gammarellus angulosus, on the central coast of Maine. Amphipod population density and body size were determined in algae of different morphologies that provided amphipods with different habitat architectures. The two primary components of habitat architecture in this system were spatial (the number and size of spaces between fronds) and structural (the number, length, and width of fronds). These were measured for algae of specific morphologies and for artificial plants that mimicked these morphologies. Field experiments using algae and algal mimic counterparts showed that there were significantly higher densities of amphipods in algae with branched and filamentous morphologies than in those algae with foliose and leathery macrophyte morphologies. There was also a significant correspondence between the body size of amphipods and both components of habitat architecture. Laboratory experiments using algae and algal mimics excluded food value, predation, competition, and physical disruption in experimental treatments. When these processes were excluded, patterns of abundance and body—size scaling to habitat dimensions were the same as those in the field. It appears, through the use of algal mimics, that the spatial component (space between fronds) is an important factor in determining amphipod demographic patterns in algae. Algal mimics of different surface rugosity and color indicate that tenacity and crypsis are also important components in habitat selection of amphipods.
Effects of Parasitism by a Haematophagous Mite on Reproduction in the Barn Swallow
Authors: Moller Anders Pape
Published: May 2006

I studied the effect of the parasitic haematophagous tropical fowl mite (Ornithonyssus bursa, Macronyssidae, Gamasida) on the reproduction of the Barn Swallow (Hirundo rustica, Hirundinidae, Passeriformes). Barn Swallows regularly reuse old nests, but avoid old nests with mites as demonstrated by experimental manipulation of mite loads. An equally large proportion of nests was infected with mites in experimental colonies, where all old Barn Swallow nests were removed, as in control colonies without such removal. Furthermore, the estimated number of mites per nest did not differ between experimental colonies, where all old nests were removed, and control colonies. These results suggest that new nests are not primarily infected from old nests harboring large mite populations. Mite population density per nest after fledging was significantly positively related to mite population density on adult Barn Swallows upon arrival at the breeding site in early spring the same year. Mate choice may significantly affect levels of ectoparasitism, if parasite loads of nests originate from inoculates provided by adult Barn Swallows. In order to mimic the effects of choice of a mate with few or many mites, respectively, I manipulated mite loads of newly built first clutch nests during the egg—laying period of Barn Swallows by either spraying nests with a pyrethrin solution or introducing an inoculate of °50 mites, or nests were simply kept as controls. Addition of mites to nests decreased reproductive success of the birds measured in terms of number of independent fledglings of first clutches and in terms of clutch size, brood size, and number of independent fledglings of second clutches. Barn Swallow nestlings in nests with mites had lower body mass but similar body size compared with nestlings in sprayed nests. Incubation periods were longer and nestling periods shorter for nests with added mites. Interclutch intervals were significantly shorter in the group of sprayed nests, mainly because more such nests were reused for second clutches and because adult swallows suffered less from mite infestations. Slightly fewer Barn Swallows had second clutches during the same breeding season if their nests held many mites. Fitness of adult Barn Swallows is thus directly related to their own mite infection level and that of their mate.
Mechanisms of Competition Between Douglas‐Fir and Red Alder Seedlings
Authors: Shainsky Lauri J. , Radosevich Steven R.
Published: May 2006

Mechanisms of interactions between Douglas—fir (Pseudotsuga menziesii) and red alder (Alnus rubra) seedlings were assessed in experimentally manipulated stands. The density of each species was varied systematically, creating a matrix of competitive regimes that consisted of five monoculture densities and 25 mixtures of all possible pairwise combinations of the monoculture densities. Response surfaces for growth rates, leaf area, photon flux density, soil moisture content and depletion, and plant water potential were generated within the matrix. Regression coefficients quantified the effects of species densities on response variables, and correlation analysis yielded insight into interrelationships between variables. Tree performance, leaf area per square metre of ground surface area, resources, and physiological variables were all quantitatively altered by alder density, Douglas—fir density, and the interaction between species densities. Alder was the dominant competitor and overtopped the Douglas—fir. Competition for light was mediated by density effects on the leaf area of each species per square metre of ground surface area. Increasing alder leaf area reduced the light reaching the understory Douglas—fir. In contrast, increasing Douglas—fir leaf areas increased the light penetrating through to the understory conifers, due to Douglas—fir's suppression of alder leaf area per square metre. Soil moisture limitations were also created by increasing the density of both species and resulted in increasingly negative leaf water potentials for both species. Growth rates concurrently declined as plant water stress increased. Response variables were assembled into a conceptual modeling proposing how species density regulated growth through the interactions between resource limitations and impairment of physiological function.
Nitorgen and Phosphorus Fertilization Effects on Venezuelan Montane Forest Trunk Growth and Litterfall
Authors: Tanner E. V. J. , Kapos V. , Franco W.
Published: May 2006

In a montane rain forest a °2500 m in Venezuela we fertilized plots with N, with P, or N and P for 4.5 yr. We measured trunk growth in circumference in control plots and in those fertilized with N and P, and measured litterfall in all four treatments. There were five 12 x 12 m plots per treatment. N was applied at 225 kg°ha—1°yr—1 and P at 75 kg°ha—1°yr—1 for the first 2 yr, and subsequently N was applied at 150 kg°ha—1°yr—1 and P at 50 kg°ha—1°yr—1. Rates of trunk growth in N and P fertilized plots were roughly doubled those in control plots (1.4 vs. 0.6 mm/yr). Total litterfall averaged 4.3 Mg°ha—1°yr11 in the control plots. In the 4th yr after fertilization litterfall was significantly higher in plots fertilized with N + P (1.50 g°m—2°d—1) than in control plots (1.00 g°m—2°d—1). There were no significant differences in rates of litterfall between treatments during the first 3 yr after the first fertilization. The changes of rates of litterfall with time were significantly different between treatments. Litterfall in control plots fell by 9% per year, that in P—fertilized plots fell by 5% per year, and litterfall in N— and in N + P—fertilized plots remained approximately constant. Nitrogen concentrations in litterfall were unaffected by fertilization with N or with P. Phosphorus concentrations were significantly increased by P fertilization. We conclude that N is more limiting that P in this forest.
Laboratory‐Simulated Grazing: Interactive Effects of Defoliation and Canopy Closure on Serengeti Grasses
Authors: McNaughton S. J.
Published: May 2006

A factorial experiment was performed to evaluate the relationships between defoliation and canopy closure, recognizing that a major effect of grazing in nature is to prevent that closure. Short (Eustachys paspaloides) and mid—height (Themeda triandra) grasses from the Serengeti National Park, Tanzania, were grown for 12 wk in a balanced factorial experiment. Treatments combined clipping and canopy closure, simulated by raising an opaque cardboard collar as plant height increased. At harvest, plants were separated into morphological parts, dried, weighed, and analyzed for nitrogen (N) and nonstructural carbohydrates (CHO). Simulated canopy closure of unclipped plants reduced photon flux density at the plant base by 12% perpendicular to the light source and 55% at a 45° angle from the lights. Multivariate analysis revealed that experimental treatments produced completely distinct results. Simultated canopy closure of undefoliated plants more than doubled plant height, reduced tiller number by 65% in the short grass and 90% in the mid—height grass, converted differentiation from production of many small tillers to a few very large tillers, changed allocation patterns toward more stem and sheath, and shifted the short grass from a CHO—dominated economy to a more N—balanced economy. The time course of yield from defoliation reached a decided maximum at 9 wk in the short grass but was much lower and remained comparatively constant in the mid—height grass. Comparing clipped—unshaded plants (simulating grazing) with unclipped—shaded plants (simulating no grazing) revealed that, for both grasses, compensation for defoliation was complete in total yield, yield of live tissues, crown mass, root mass, and total number of leaves. Neither species compensated in residual leaf mass, total leaf area, leaf area per unit of plant biomass, or stem mass. Defoliation shifted leaves of both species to lower CHO and higher N concentrations. In general, the short grass was modified more by canopy closure, the mid—height species by clipping. Greater defoliation tolerance of the short—statured species was due to much greater tillering, significantly greater N accumulation, greater allocation to leaf blade and sheath, less allocation to crown and root, a greater foliage concentration per unit canopy volume, a lower rate of foliage death, and the ability to sustain a considerably higher leaf area per unit of plant mass. This experiment indicated that growing unclipped plants isolated in pots grossly distorts morphology, growth, and chemical properties compared to plants experiencing the canopy closure normally encountered in ungrazed grasslands. Therefore, failure to account for the self—limiting consequences of canopy closure can mislead conclusions from laboratory experiments designed to test the consequences of defoliation.
Cone and Seed Weight Relationships in Douglas‐Fir From Western and Central Oregon
Authors: Sorensen Frank C. , Miles Richard S.
Published: May 2006

Cone sizes and weights and seed weights were determined for 89 trees in 9 stands in the Oregon Coast Ranges, Cascade Ranges, and central Oregon. From west to east this represented a transition from mesic to xeric sites and from longer to shorter growing seasons. Differences among regions for all cone traits, and regression coefficients for change in cone and seed weights with distance from the ocean, were significant. Cone size and weight decreased, seed weight increased, and the regression coefficient of seed weight on cone weight, based on trees within regions, increased with distance from the ocean. Geographic variation in cone and seed weights was compared with variation in the phenology of cone and seed development as reported in the literature. This comparison indicated that where relatively large seeds were important to regeneration success and the growing season was short, Douglas—fir showed at least 3 changes in the timing of development as compared with milder sites. (1) The cone growth period was reduced more than the seed growth period. (2) The overlap in time of the developmental cycles of cone and seed was increased. (3) Cones and seeds grew during an increased portion of the growing season.
Presence of a Late Pleistocene Drainage System Manifested through Depredation of Red Pine by Gremmeniella Abietina
Authors: Dorworth C. E.
Published: May 2006

Residual beaches and terraces, results of wave action and sorting of glacial till at the margins of glacial and postglacial lakes prevail in the Great Lakes area of the Canadian Precambrian Shield. Relict drainage systems were identified on 2 such terraces by virtue of their dendritic pattern, gradient and relationship to the erosion sequence of an adjacent river. These minor topographic indentations became apparent through selective depredation by a fungus pathogen of pines planted within the troughs. The relict fluvial system now functions as a cold air damage system, concentrating and directing airborne spores of the pathogen. Surviving trees outline the troughs, serving as biological indicators of trough positions.
Modeling and Simulation of the Energy Flow Through Root Spring, Massachusetts
Authors: Zalucki Myron P.
Published: May 2006

Four compartment models, each postulating distinct forms of interactions among the organisms in Root Spring. Massachusetts were developed and compared. None were adequate to simulate the dynamics of energy flux in the spring. A model of the spring which adapted a detrius—processing matrix developed for stream systems, together with a more precise statement of feeding relationships, reproduced the observed data more faithfully. This model suggested that certain further information on the biology of the spring would improve understanding of its dynamics.
The Effect of Predation on Prey Resource Utilization
Authors: Glasser John W.
Published: May 2006

Recent studies suggest that predation may facilitate the coexistence of potential competitors by reducing the intensity of resource utilization. However, there are qualitative as well as quantitative dimensions to any resource utilization pattern. Although the members of populations which are resource limited should be collectively indiscriminate in their resource utilization, populations which are otherwise limited may use their resources selectively. Thus, by reducing the abundances of their prey species, predators may permit them to utilize their resources selectively, facilitating the coexistance of potential competitors. This explanation is supported by results of experiments with coupled discontinuous—flow cultures of several species of marine phytoplankton and herbivorous and carnivorous zooplankton. Additionally, they suggest that choice among alternate resources states (size and age classes of algae and herbivores respectively) is a function of their frequencies and/or rats of supply as well as other attributes. Thus, predation may increase the evenness component of species diversity as well as species richness. Furthermore, to the extent that the alternate states of energetic resources are prey species, this pattern of use implies a balance between competition and predation. Although that is the usual presumption in natural communities, the above argument suggests that resource states vary in composition from groups of species to life—history stages depending upon the relationships between resource supply and demand. Irrespective of the composition of resource states, however, such selection would stabilize the distribution of a resource among the categories into which its consumers subdivide it. Thus, the correlative relationship between species diversity and community stability may not be causal, but rather, both may result from predation.
Old‐Field Succession on a Minnesota Sand Plain
Authors: Inouye Richard S. , Huntly Nancy J. , Tilman David , Tester John R. , Stillwell Mark , Zinnel Kathlean C.
Published: May 2006

Vegetation and soils were sampled in 22 old fields ranging in age from 1 to 56 yr since abandonment. Soil nitrogen concentration increased significantly with field age. Vegetation cover, total aboveground plant biomass, and litter cover increased significantly with soil nitrogen. Light penetration to the soil surface was negatively correlated with total plant biomass. Field age and soil nitrogen concentration were used as independent variables in simple regression and partial correlation analyses to determine the relative importance of such time—dependent processes as dispersals vs. the availability of a limiting resource (nitrogen) as predictors of patterns in species richness or the abundance of various plant groups. Species richness per field and within—field heterogeneity in species composition increased with field age. Local species richness decreased with increasing soil nitrogen. Cover of annuals and introduced species decreased with field age and nitrogen; however, annuals contributed an important part of total vegetative cover even n 25—yr—old fields. Cover of perennials and woody species increased with soil nitrogen and field age. Although the fields were bordered by woods, woody species contributed @<15% cover even in the oldest fields. For several plant groups the relationship between cover and soil nitrogen within individual fields was the opposite of that among all fields. These patterns suggest that while soil nitrogen is an important determinant of local species composition and abundance, dispersal and colonization, which are dependent on field age, determine which species are present in a field.
The Ecology of Seed Heteromorphism in Heterosperma Pinnatum in Central Mexico
Authors: Venable D. Lawrence , Burquez Alberto , Corral Gabriela , Morales Eduardo , Espinosa Francisco
Published: May 2006

Heterosperman pinnatum is an annual composite that produces heteromorphic achenes, with the proportion of achene morphs varying geographically. Field, greenhouse, and laboratory experiments were used to examine the ecological consequences of the heteromorphism. Achenes were classified into three types based on morphology. Central, intermediate, and peripheral morphs, named for the positions occupied within the fruiting head, range in shape from long, thin—beaked achene with barbed awns (central) to short wide achenes without beaks or awns (peripheral). Considerable within— and between—population phenotypic variation in achene and head characteristics was documented. Field experiments using artificial dispersal agents showed that central achenes are the most likely to adhere to animals and that the proportion of a population's achenes presented for dispersal that adhere depends on the proportion of central achenes produced in the population and on the proportion of achenes with adhesive awns. Once achenes had reached the ground, mean and median secondary dispersal distances were short (@<20 cm). Central achenes lose innate dormancy earlier than peripheral achenes during the period between autumn achene production and the onset of the summer rainy season. Germination is inhibited by darkness, though darkness becomes less inhibitive at the onset of the germination season (earlier for central achenes). In the laboratory, peripheral achenes germinated over a slightly narrower range of temperatures than central achenes. Percent germination was greater for central than intermediate achenes, and for intermediate than peripheral achenes, in two natural germination events in the field. Seed bank samples indicated that few if any achenes remain dormant between years. No significant differences in the growth rates of young seedlings were found in the greenhouse. A demographic experiment documented trade—offs resulting from the dormancy differences: central achenes tended to germinate earlier than intermediate and peripheral ones, and early germination resulted in greater mortality, but survivors that had germinated earlier attained greater size and produced more seeds. Intermediate achenes, while intermediate in all components of fitness, had the highest total fitness. This was because they resembled peripheral achenes in germinating late and having high survival, but they resembled central achenes in having higher percent germination. The results of these experiments suggest that a achene behavior ranges from "low—risk" (peripheral achenes) to "high—risk" (central achenes) within the progeny of a single individual, while individuals and populations vary in the proportion of offspring exhibiting each type of behavior.
Analysis of Local Variation in Plant Size
Authors: Mitchell-Olds Thomas
Published: May 2006

Attempts to quantify local competitive interactions by examining size and growth of neighboring plants may suffer from several statistical difficulties: (1) when individuals are analyzed both as focal plants and as neighbors of other individuals, observations are not independent and tests of statistical significance may be invalid, (2) effects of competition and local site quality may be confounded. The techniques of path analysis and randomization tests are described; use of these procedures may permit more accurate estimation and testing of influences on individual plant growth. Analysis of results of field and greenhouse experiments with Impatiens capensis showed that conventional statistical tests may yield serious errors in estimates of significance, and that these errors are of unpredictable magnitude and direction. The results demonstrate that it is essential to measure or estimate the effects of site quality on variation in plant size.
Optimal Life Histories and the Maximization of Reproductive Value: A General Theorem for Complex Life Cycles
Authors: Caswell Hal
Published: May 2006

The theorems on the maximization of reproductive value in optimal life histories were originally stated and proven for age—classified populations. Many organisms, however, are more appropriately classified by other variables, such as size, developmental stage, instar, spatial location, etc. Their life cycles may be considerably more complex than those for which classical demographic theory was devised. In this paper, I show how to calculate reproductive value for such populations, and give conditions for its maximization. The theory for age—classified populations follows as a special case.
The Application of a New Method for the Determination of Particle Deposition onto a Lake
Authors: Banks Hugh H. , Nighswander James E.
Published: May 2006

For any material liberated from a watershed and deposited by air onto a lake, there is likely to be a relationship between distance from shore and quantity of particle deposition. A new method of estimating total particle deposition onto a lake surface was developed. The method is based on the distance vs. quantity relationship, and uses planimetry to integrate lake surface areas at various distances from shore. The method was applied to Tsuga canadensis and Pinus spp. pollen deposition onto a small lake on the Candian Shield. General agreement was found to exist between total pollen deposition values derived by this new method and values determined by more common methods not involving the relationship between particle deposition and distance from shore.
A Model for Fruiting Display: Seed Dispersal by Birds for Mulberry Trees
Authors: Stapanian Martin A.
Published: May 2006

The effectiveness of fruiting displays of mulberry trees (Morus) for seed dispersal by birds was studied in eastern Kansas. Alternative models were developed in which the number of fruits eaten by birds and the number of seeds dispersed from a tree were hypothesized to be functions of the number of fruits on a tree, reward energy in an individual fruit, and total reward energy on the tree. The null hypothesis was that birds eat fruit randomly on trees regardless of these numerical or energetic characteristics. The models predict an increase in seeds dispersed with an increase in fruiting display, with each of the three models predicting a different behavioral response to increases in fruiting display. When empirical data were analyzed on a weekly basis, the null hypothesis was rejected and total reward energy on the tree did a much better job of explaining the number of fruits eaten by birds and the number of seeds dispersed than did number of fruits on a tree or reward energy per fruit. The latter two variables are components of the first, more significant, variable. In successive weeks smaller displays resulted in the same number of seeds being dispersed. Decreasing availability of insect food probably is at least partially responsible for this increase in fruiting display efficiency. Mulberry trees are monoecious with highly variable sex ratios, and the results predict selection for reproduction at an early age in Morus and equal fitness for all sex ratios.
Distributions of Galapagos Ground Finches Along An Altitudinal Gradient: The Importance of Food Supply
Authors: Schluter Dolph
Published: May 2006

I investigated the abundance, diet and food supply of ground finch species (Geospiza Gould) at different altitudes over a calendar year on Isla Pinta, Galapagos. The primary purpose of the study was to evaluate which of three alternative hypotheses best explains the present distributions of species along the vegetation gradient: habitat structure, food supply, or current interspecific competition. Isla Pinta was specifically chosen because Lack (1947) thought that on this island one species, G. difficilis, was competitively restricted to the highlands by the morphologically very similar species, G. fuliginosa. Four of the five species present were broadly distributed among habitats, and their altitudinal ranges overlapped considerably. The fifth, G. scandens, was rare and little was learned of its habit. G. difficilis ate mainly litter arthropods and gastropods, especially at high elevations. The remaining three species, G. fuliginosa, G. fortis, and G. magnirostris, were primarily granivorous, but each species consumed a different range of seed sizes. Diet overlap between species was highest when food production was high, and lowest when food was scarce. Seasonal declines in overlap seemed to be mainly due to finches' response to asynchronous food production and a gradual broadening of the available food spectrum. In the dry season the abundance and distribution of the four common species matched to varying degrees the abundance and distribution of the foods they consumed. Effects of current competition on these distributional patterns were not detected. Most Geospiza species did not seem to possess strict habitat requirements, and it is unlikely that variation in habitat structure greatly influenced distributions. One possible exception to this is G. difficilis, where litter supply may be important. Overall, the data suggest that while competition may be occurring between species, variation in food supply is currently a far more important determinant of distributions on Pinta.
Individual Variation in Nectar Production and Its Effect on Fitness in Kalmia Latifolia
Authors: Real Leslie A. , Rathcke Beverly J.
Published: May 2006

During summer 1986, we assessed the effect of individual variation in per flower nectar production rate on one component of plant reproductive success (percent fruit set) mediated through differential pollinator attractiveness. Thirty—two individual mountain laurels, Kalmia latifolia, were monitored for average 24—h nectar production in individual flowers, average per flower pollinator visitation rate, average floral density, and percent fruit set. Significant positive correlations occurred between (1) average 24—h nectar production by individual flowers and average visitation rate per flower, and (2) average visitation rate per flower and percent fruit set. Pollinators appeared to be differentially attracted to individual shrubs that produce larger quantities of nectar per flower, and the increase visitation rate promoted a higher percentage fruit set because K. latifolia is pollination limited. However, there was no correlation between an individual shrub's 24—h nectar production in 1986 and the same individual's 24—h nectar production in 1985. Thus, while pollinator behavior appears to influence this component of plant reproductive success in Kalmia, any selective advantage to individuals may vary between flowering seasons.
Outcrossing Distance Effects in Delphinium Nelsonii: Pollen Loads, Pollen Tubes, and Seed Set
Authors: Waser Nickolas M. , Price Mary V.
Published: May 2006

Seed production of a flower depends both on the quantity of pollen received and on its quality as manifested in rates of germination, ovule fertilization, and seed maturation. Disentangling effects of these two factors is complicated by the fact that seed set is a decelerating function of pollen quantity. One potential element of quality is the genetic similarity of pollen donor and recipient. In ten experiments with Delphinium nelsonii, seed set varied with outcrossing distance, a correlate of genetic similarity. Over all experiments, a 10—m distance significantly (P < .02) outperformed shorter and longer distances by 23—33%, suggesting quality differences. To explore this possibility, we fit decelerating negative—exponential regressions to relationships among stigma pollen load, pollen tube number reaching the ovary, and seed set in a subset of experiments having 1—, 10—, and 100—m treatments. Regression parameters suggested that 10—m pollen is most efficient at producing tubes and seeds, and analysis of residuals from pooled regressions (a nonlinear ANCOVA) showed that 10—m residuals were most positive. The latter effect was significant for pollen load—seed set relationships (P = .009, five experimental replicates) and pollen tube—seed set relationships (P = .021, two replicates), but not pollen load—pollen tube relationships (P = .128, three replicates). These results illustrate the utility of nonlinear regression in distinguishing pollen quantity and quality. They reinforce the conclusion that outcrossing distance affects pollen quality in D. nelsonii, and that an optimal outcrossing distance between 1 and 100 m maximizes seed set on average in pollinations using one donor per carpel. Insofar as quality differences persist in natural pollen mixtures that arrive on stigmas, outcrossing distance should affect reproductive success through both paternal and maternal sexual functions.
Food Web Graph of a Coevolutionarily Stable Community
Authors: Matsuda Hiroyuki , Namba Toshiyuki
Published: May 2006

A model community, which consists of multiple trophic levels (several species of prey, and several predator species including omnivores) is considered from the viewpoint of coevolutionary stability. Some properties of community digraphs are studied and resource utilization patterns of predators are given as necessary conditions for a coevolutionarily stable community (CSC). If a member of a predator species can stimultaneously search for multiple prey species, a set of the prey species is interpreted as a single "prey trophic species." (1) For any subgraph of a CSC food web, the number of links between predator and prey trophic species is less than the sum of the number of predator species and the number of true prey species. (2) If an predator member searches for only one prey species, then for any CSC food web, the number of links connecting that predator and its prey is less than the sum of the number of predator species and that of prey species, where a species at an intermediate trophic level is doubly counted as prey and also as predator. (3) The maximum number of links in a CSC digraph is proportional to the number of species in the community. With our model we attempt to explain intervality of a niche overlap graph and lack of a hole in a common enemy graph. Finally, we examined two examples of real communities to confirm validity of our results.
Competition Between Tadpoles of Hyla Femoralis and Hyla Gratiosa in Laboratory Experiments
Authors: Wilbur Henry M.
Published: May 2006

All combinations of 0, 10, 20, and 40 hatchlings of two species of frogs, Hyla femoralis and H. gratiosa, were raised in a replicated experiment in which food and space were controlled. Models of density—dependent growth and metamorphosis can be extended to include the effects of competing species by inclusiion of linear, additive terms. Hyla gratiosa has a strong, negative effect on the rate of metamorphosis and size at metamorphosis of Hyla femoralis. Hyla femoralis has no effect on the survival or mean size at metamorphosis of H. gratiosa, but it does lengthen both the minimum and mean larval period. Interspecific density effects on growth may increase both the risk of predation and the risk of desiccation in natural populations.
Patterns of Nested Dispersion in a Tropical Ground Ant Community
Authors: Levings Sally C. , Franks Nigel R.
Published: May 2006

Ant nests are frequently reported to be overdispersed. Less commonly, interspecific interactions have been shown to affect nest spacing. We here report on the pattern of dispersion of 15 ground ant species in four plots located on Barro Colorado Island, Panama. All plots showed significant evidence of both intra— and interspecific overdispersion. We hypothesize that the observed pattern of dispersion is the result of competition among all members of this set of species with similar requirements. Mechanisms which could produce this pattern are nest movement and the preferential predation on conspecific founding queens and incipient colonies by established colonies.
Planktivore Preference by Prey Size
Authors: Eggers Douglas M.
Published: May 2006

The reactive field volume model of prey encounter was shown to give a close approximation to the apparent size model of prey encounter rate for prey size distributions consisting of many prey types. The two models of prey encounter give almost identical predictions for the Lake Washington zooplankton community. The prey encounter models do not account for observed patterns of prey selection by Lake Washington juvenile sockeye salmon. This suggests an active preference for large nonevasive prey. This preference is dynamic. Small as well as evasive prey are pursued and eaten at times in the year when the large nonevasive prey are rare or absent from the water column.