bluuebunny/crossref_metadata_2025 huggingface dataset:
The Relationship between Seed Size and Establishment Conditions in Tropical Woody Plants
Authors: Foster Susan , Janson Charles H.
Published: May 2006

Two approaches were used to examine the relationship between seed size and typical establishment conditions in woody species of tropical rain forests. First we compared seed masses of 36 mature tropical forest tree species with differing light—gap requirements for establishment. The 14 species that become established beneath a closed canopy or in small gaps were found to have higher mean seed masses than the species that require large gaps. Though species were divided into two types of establishment requirements, plotting the mean seed masses produced a continuous unimodal distribution rather than a bimodal distribution, suggesting that either intermediate establishment conditions are most common in tropical rain forests or that other biotic or abiotic conditions confound the relationship between establishment conditions and seed mass. Second, we compared seed masses of 203 early and late successional woody plants in one Peruvian forest. Seed masses of mature forest species were found to be significantly larger than those of pioneer species, even when the effects of tree height, dispersal syndrome, and growth form were controlled for statistically. Thus, both analyses demonstrated a relationship between large seed size and establishment in shady, stable plant associations. Additional analysis of Peruvian forest data demonstrated that mammals tend to disperse larger seeds than birds do, trees tend to have larger seeds than vines, and taller plants of a given growth form tend to produce larger seeds than shorter plants of the same growth form.
Competitive Exclusion of Aphytis lingnanensis by A. melinus: Potential Role of Host Size
Authors: Luck Robert F. , Podoler Haggai
Published: May 2006

Aphytis melinus, an ectoparasitoid of California red scale, Aonidiella aurantii, replaced A. lingnanensis, another ectoparasitoid, in certain California citrus areas during the 1960's following the former's introduction in 1956—1957. We show that, although both species attack a similar range of host sizes, A. melinus utilizes a smaller threshold scale size for the production of daughters. Size differences of scales within a citrus tree (largest scales occur on fruits, smallest on wood, and intermediate on leaves), seasonal variation in scale age structure, multivoltinism of the scale and parasitoid populations, seasonal availability of citrus fruits (the substrate that supports the largest scales), differences in the temperature tolerances of the two parasitoid species (A. lingnanensis is less tolerant), and differences in their vagility (A. melinus is more vagile), probably all favor A. melinus in competition with A. lingnanensis. A. lingnanensis does not realize its fecundity advantage in female progeny when these species compete because A. melinus preempts California red scale before it grows into the size range preferred by A. lingnanensis. Collection records through 1972 show that A. melinus continued to extend its geographical range coastally in southern California through 1972.
Competitive Interactions of Two Mobile Prey Species in a Patchy Environment
Authors: Schmitt Russell J.
Published: May 2006

A 6—mo field experiment was conducted to estimate the potential for, and relative strengths of, competition within and between two species of marine reef—dwelling gastropods, Tegula aureotincta and Tegula eiseni. Both intra— and interspecific effects were detected on individuals growth and reproductive status of these snails within the range of densities naturally encountered. Survivorship was only slightly lowered even by prolonged exposure to maximum snail density. Examination of the relative strengths of the interactions revealed that each Tegula species depressed the individual growth of the other to the same degree. However, the relative strengths of intraspecific effects differed. Higher densities of T. aureotincta depressed the growth of individual T. aureotincta more than that of T. eiseni, whereas densities of T. eiseni displayed the opposite pattern: a greater influence on the other species than on conspecifics. The relative impacts of intra— and interspecific competition did not conform to expectations derived from logistic—based models of competitive coexistence. Long—term estimates of population densities of the two Tegula species revealed that both species fluctuated little in abundance over 6 yr, suggesting that the two species indeed coexist, since the span of observation was longer than the longevity of individual snails. However, detailed examination of the microdistribution of Tegula species indicated that they co—occurred within 0.1—m2 quadrats less often than expected by chance. The significant negative correlation in abundance of the Tegula species within the cobble reef is a pattern consistent with the operation of interspecific competition. Previous work has indicated that Tegula—consuming predators influence the among—reef patterns of distribution and abundance of both gastropod species: the mean abundance of both snails were inversely correlated with density of predators and, as a consequence, were positively correlated with each other among reefs. Results presented here suggest that interspecific competition, where it occurs, may lead to a negative correlation in abundance of the two gastropod species within a reef. Both predation and interspecific competition may simultaneously influence the local distribution of the Tegula species, but in opposite manners and at different spatial scales.
Historical Changes in Submerged Macrophyte Communities of Upper Chesapeake Bay
Authors: Davis Frank W.
Published: May 2006

Submerged macrophyte species that occurred at the head of Chesapeake Bay over the past 1800 yr have been documented by analyzing seed assemblages in sediments from a tidal embayment. To help interpret seed stratigraphy in the estuary, seed settling behavior was studied in the laboratory, and the relationship between parent vegetation and resulting sedimentary seed assemblages was studied in a tidal creek. Results suggest that estuarine seed assemblages are primarily locally derived and have high spatial variability, and that because of differences in seed output or preservation some species are overrepresented in the seed record. Seed assemblages in four cores from the estuary are accordingly treated as presence/absence data. The record of local submerged macrophytes was profiled in 7—100 yr intervals, using sediment dates obtained radiometrically and by pollen stratigraphy. Vallisneria americana, Najas flexillis, N. gracillima, and Elodea canadenis were consistently present for 1500—1600 yr prior to colonization by Europeans (1730 AD), while N. guadelupensis appeared 300—400 yr prior to European settlement. Vallisneria americana was the only local species continuously present for 150 yr following initial deforestation. After 1930, N. guadalupensis and N. flexilis appeared more regularly in the record and two local expansions of Myriophyllum spicatum were registered. Native submerged aquatic vegetation disappeared from the area between 1964 and 1972, and has shown little recovery since that time. Community changes after 1930 appear to be related to increased eutrophication.
Disturbance and Variation in Competition Between Two Stream Insects
Authors: Hemphill Nina
Published: May 2006

This study examined temporal and spatial variability in competition as well as the relative importance of competition and natural disturbance to Hydropsyche oslari and Simulium virgatum in a southern California stream. These two filter—feeding insects dominated the community studied, accounting for 99% of all individuals. I maintained four field treatments every 2 wk for a year: (1) removal of Simulium; (2) removal of Hydropsyche; (3) removal of both taxa and subsequent scrubbing of the rock surfaces; and (4) unmanipulated controls. Competition from Hydropsyche oslari restricted the spatial distribution of Simulium virgatum in all seasons except late winter in upstream but not in downstream sections of the study site. Downstream abundance of Simulium was probably limited by food availability, not competition from Hydropsyche. During the winter, when floods occurred and densities were low, competition between these insects had no significant impact on spatial distributions. Although competition was seasonally variable, it occurred frequently and was not rare. Although disturbance mediated competition, competition was more important in determining the abundance of Simulium in 1983—1984, a drought year. Disturbance caused a 13% increase, while competition caused a 34% decline in the simuliid population over the year. It was not possible, however, to rank the importance of competition and disturbance when considering both species together. The relative importances were taxon specific. The abilities of Simulium and Hydropsyche to recover quickly after disturbance were inversely related to their ability to withstand competition. In addition, the interaction between the two factors was extremely important in determining the relative abundance of both taxa through the year. Competition and disturbance both played an important role in regulating this community, at times accounting for up to 60% reductions in the numbers of individuals present in the stream. The relative importance of competition and disturbance in Refugio Creek should vary from year to year depending on the timing and intensity of winter storms.
Experiments on Mechanisms of Tree Establishment in Old‐Field Succession: Seedling Emergence
Authors: De Steven Diane
Published: May 2006

Loblolly pine (Pinus taeda) is the initial successful woody invader of abandoned old fields in the Piedmont region of North Carolina, but hardwood tree species also invade old fields and develop an understory community beneath young pine stands. The composition of this early hardwood community will be regulated by factors affecting the relative establishment success of these species in old—field habitats. Mechanisms influencing differential seedling emergence of such early successional tree species were studied in multifactor field experiments. Seeds of six wind—dispersed species (Acer rubrum, Fraxinus americana, Liquidambar styraciflua, Liriodendron tulipifera, Ulmus alata, and Pinus taeda) were sown into old fields at equal densities, and their relative germination (emergence) in two successive years was evaluated with respect to: (1) presence of old—field vegetation (tested with a weeding treatment), (2) vertebrate seed predation (tested with exclosures), (3) variation in life history traits (seed size and germination phenology), and (4) variation in physical factors (spring drought). Emergence of one species (Liriodendron) was limited by intrinsically low seed viability. In 1984, a wet spring, emergence of Pinus exceeded that of all hardwood species, and the smaller—seeded Liquidambar and Ulmus had higher emergence than the larger—seeded Acer and Fraxinus. Seed predators reduced emergence proportionately more in larger—seeded species than in smaller—seeded species. Herbaceous cover facilitated emergence of larger—seeded species by providing a moister germination microenvironment, and perhaps also by reducing visibility of seeds to predators. In 1985 an early spring drought reduced emergence of fall—dispersed species (Pinus, Liquidambar, and Fraxinus), whose earlier germination phenology coincided with the drought. Later—germinating, spring—dispersed species (Ulmus and Acer) avoided this drought period and displayed similar germination responses in both years. The prominence of drought effects in 1985 reduced the apparent impact of other experimental factors upon emergence rates. The emergence stage is an important filter on relative establishment success of tree species colonizing old—field habitats, and multiple "mechanisms" interact in complex ways to influence emergence rates.
Adaptive Significance of Egg Size in the European Starling: Experimental Tests
Authors: Smith H. G. , Ohlsson T. , Wettermark K.-J.
Published: May 2006

Reproductive success in relation to egg size was studied in European Starlings (Sturnus vulgaris) by swapping whole clutches between nests at the stare of the incubation period. Egg size did not reflect parental quality as no measure of reproductive success was correlated with the foster mothers' mean egg size. There was a significant positive relationship between the mean size of the cross—fostered egg and the subsequent mean size of hatchlings. The mean size of cross—fostered eggs did not affect hatching success or nestling growth rates, and initial nestling size differences between broods with large and small eggs persisted for <1 wk. No effect of mean egg size on mean nestling survival could be detected. Furthermore, a partial cross—fostering experiment, where nestlings were swapped between nests the day after hatching, failed to demonstrate any lasting effect of egg size on nestling size. It is suggested that mean egg size may only influence reproductive success during particularly inferior environmental conditions.
Allocation of Energy to Growth and Respiration in Avian Postembryonic Development
Authors: Konarzewski Marek
Published: May 2006

I used published data to test three possible relationships between the allocation of energy to growth (P) and respiration (R) in chicks: (1) a negative relationship stemming from a trade—off between increased allocation for one purpose, resulting in a reduced proportion of energy available for other purposes; (2) a lack of relationship between P and R; and (3) a positive relationship reflecting functional metabolic coupling of P and R. I restricted my analysis to three periods of the chick's postembryonic development, during which the conflict between allocation of energy to P vs. R is most likely to occur: (a) when the greatest proportion of assimilated energy is allocated to growth; (b) when growth rate (in grams per day), or (c) energy allocated to growth (in kilojoules per day) are at absolute maxima. I analyzed the correlation between P and R after accounting for differences in body mass, costs of biosynthesis, mode of development, breeding latitude, and phylogeny. Under assumption of production efficiency of 0.75, the correlations were negative for periods (a) and (b), and statistically insignificant for period (c). When production efficiency of 0.9 was assumed, the correlation remained negative only for period (c), and became statistically insignificant for the other two. I also found that the positive correlation between growth rate and metabolic rate reported by Klaasen and Drent (1991) becomes insignificant when the effect of breeding latitude is removed from their data set. Thus, the results of all analyses support (1) or (2), rather than (3). I conclude that predictions stemming from (1—3) can often be difficult to distinguish, which may account for the inconsistency between the results of different studies in which the relationship between P and R was analyzed.
Functional Redundancy, Non‐Additive Interactions, and Supply‐Side Dynamics in Experimental Pond Communities
Authors: Morin Peter J.
Published: May 2006

Factorial manipulations of the abundance of two predator species (presence or absence of six adult Notophthalmus viridescens, and/or six larval Ambystoma opacum per pond) and a guild of three prey species (larvae of Pseudacris crucifer, Bufo woodhousii fowleri, and Hyla andersonii) tested the equivalence, additivity, and dependence on initial prey density of predation by both predator species in 1—m3 artificial ponds. The two predators were functionally equivalent, eliminating Bufo and Hyla, while leaving Pseudacris to numerically dominate ponds with salamanders. Effects of both predators also cascaded down to algal primary producers, which became more abundant where predators reduced the abundance of herbivorous tadpoles. Combined impacts of the two predator species on the survival of Pseudacris were less than that predicted by an additive model of the effects of each predator measured in the absence of the other, but greater than predicted by a multiplicative model of joint predator effects. Reduced growth of Ambystoma with Notophthalmus provides one possible mechanism for the observed shift to less than additive per capita predator effects. Initial variation in prey density influenced prey species composition only in ponds without predators. Increased prey density in ponds without predators reduced the survival of Hyla, apparently through increased interspecific competition among tadpoles.
Is Coexistence of Sonoran Desert Annuals Mediated by Temporal Variability Reproductive Success
Authors: Pake Catherine E. , Venable D. Lawrence
Published: May 2006

Models of annual plants with a persistent seed bank have shown that temporal variation can promote coexistence if the reproductive success of species is favored in different environments determined by temporally variable conditions. This study investigates whether this mechanism may explain the coexistence of three Sonoran Desert species (Pectocarya recurvata, Plantago patagonica, Schismus barbatus). In a 2—yr experiment, factors that vary across years (water and seedling density) were manipulated. In addition, the dominant spatial feature, presence or absence of Larrea cover, was also included as a factor. Our aim was to document fitness hierarchies in different types of years. Seedlings were mapped monthly for survival and reproductive success. To compare species, we used 10 yr of data to calculate the average value that seeds of different species have for population growth. Shifts in fitness hierarchies were found for two species pairs (Pectocarya—Schismus and Pectocarya—Plantago), depending on density and either the year or the water level. Surprisingly, all species had higher survival at higher densities in one or another treatment. Habitats were not important to the fitness hierarchies; all species had higher mean survival and fitness in the open than under shrubs.
Nutrient Dynamics and Nitrogen Trace Gas Flux During Ecosystem Development in Montane Rain Forest
Authors: Riley Ralph H. , Vitousek Peter M.
Published: May 2006

Patterns of nitrogen trace gas emissions, soil nitrogen flux, and nutrient availability were evaluated at five sites that form a chronosequence in Hawaiian montane rain forest. The estimated age of basaltic parent material from which soils developed at the Kilauea site was 200 yr, 6000 yr at the Puu Makaala site, 185 000 yr at the Kohala site, 1.65 ° 106 yr at the Molokai site, and 4.5 ° 106 yr at the Kauai site. Peak net N mineralization and nitrification values were found in soils from the 185 000—yr—old Kohala site. Nitrogen content of foliage and leaf litter was highest in the intermediate age sites (Puu Makaala and Kohala) and N and P retranslocation was lowest at the Puu Makaala site. Soil cores fertilized with nitrogen had significantly higher rates of root ingrowth than control cores at the two youngest sites (200 and 6000 yr old) but not in older sites (185 000 and 4.5 ° 106—yr—old sites) and total fine root growth into control cores was greatest at the Kohala site. The highest N2O emissions were found at the 185 000—yr—old Kohala site, while the highest combined flux of N2O + NO was observed at the 4.5 ° 106—yr—old Kauai site. While overall N2O emission rates were correalted with rates of N transformations, soil water content appeared to influence the magnitude of emissions of N2O and the ratios of emissions of NO vs. N2O. N2O emissions occurred when water—filled pore space (WFPS) values were >40%, with highest emissions in at least two sites observed at WFPS values of 75%. Among sites, high NO emissions were associated with high soil N transformation rates. Large NO fluxes were observed only at the Kauai site when WFPS values were <60%.
Recruitment of Seedlings and Vegetative Sprouts in Unburned Chaparral
Authors: Keeley Jon E.
Published: May 2006

Age structure of 12 stands of chaparral, unburned for 56—120 yr, was investigated. All shrubs produced discernible growth rings, and ring counts on stems from stands of known age, plus synchrony in annual growth ring width among species in the same stand, were taken as evidence that growth rings represented annual rings. Species that survive fire by vegetative regeneration from the root crown were capable, in the absence of fire, of continuously regenerating their canopy with basal sprouts. These multistemmed resprouting species had an uneven age structure, indicating continuous recruitment of new stems into the population. The predicted age distribution for resprouting shrub stem populations illustrated very different patterns for species within the same stand and for the same species in different stands. In most resprouting shrubs, <15% of the stems dated back to the first decade after fire, and in many species the majority of stems were <10 yr of age. This contrasts with obligate seeding shrubs that do not initiate new stems from the root crown, and for which, in undisturbed chaparral, 100% of the stems date back to the years immediately following the last fire. Seedling recruitment was abundant in seven of these old chaparral communities. Species with abundant seedling populations, ranging from 1000 to 37 500 individuals/ha, were Quercus dumosa, Q. wislizenii, Rhammus crocea, Prunus ilicifolia, Heteromeles arbutifolia and Cercocarpus betuloides. These species share the characteristic of regenerating after fire solely by resprouting, and seldom recruit seedlings during the first few decades after fire. Thus, it is concluded that these postfire obligate resprouters require long fire—free periods for successful reproduction and potential population expansion. For all but the last species, seedlings were most abundant in stands with a closed canopy and deep litter layer. Safe sites for establishment were not in gaps, although gaps may be important for successful recruitment of saplings into the adult population. For all sprouting species the actual age distribution of seedlings and saplings indicated that recruitment did not occur every year; mortality was high but most populations had some saplings that survived beyond the first decade. For all postfire seeding taxa, viz., Adenostoma, Arctostaphylos, and Ceanothus, no significant seeding recruitment was observed in these ancient communities. Seedlings of woodland tree species had colonized several of these old chaparral communities; however, even in stands a century old, successional replacement of chaparral was not imminent.
Effect of Diaspore Characteristics on Removal of Seeds Adapted for Dispersal by Ants
Authors: Hughes Lesley , Westoby Mark
Published: May 2006

We investigated the role of ant behavior as a selective influence on the presentation and morphology of elaiosome—bearing seeds in Australian sclerophyll vegetation by manipulating characteristics of natural and artificial diaspores. We compared the response to these diaspores of three seed—removing ant species, two of which (Rhytidoponera "metallica" and Aphaenogaster longiceps), produce more favorable seed fates than the third species (Pheidole sp. 1). We measured the effect on removal rate of diaspore arrangement (degree of clumping), elaiosome removal, diaspore size (i.e., mass), elaiosome/load ratio, and composition of both the elaiosome and seed. Clumped seeds were removed significantly faster than single seeds by Pheidole sp. 1, a group forager. A. longiceps, a solitary forager, removed single seeds faster while R. "metallica" removed single and clumped seeds at the same rate. The presence of elaiosomes on seeds increased removal rates, especially by A. longiceps and R. "metallica." Both A. longiceps and R. "metallica" responded positively to elaiosome/load ratio while Pheidole sp. 1 responded mainly to diaspore size. These species—specific responses were such that diaspores with high elaiosome/seed ratios are more likely to be removed by A. longiceps or R. "metallica" than by Pheidole sp. 1. Diaspore characteristics other than size had inconsistent effects on removal rates. The differential response of the ant species to such characteristics as seed arrangement and elaiosome/seed ratios demonstrates the way in which ant behavior may have been an important selective force in the evolution and maintenance of myrmecochory.
Brood Size and Food Provisioning in Masked and Blue‐Footed Boobies (Sula Spp.)
Authors: Anderson David J. , Ricklefs Robert E.
Published: May 2006

Food limitation has been thought to explain the evolution of single—chick broods in pelagic seabirds. We compared the meal sizes, feeding rates, and time budgets of Masked Boobies (Sula dactylatra), typical pelagic seabirds, which raise one chick per breeding attempt, with those of sympatric Blue—footed Boobies (S. nebouxii), which often raise two or more chicks to fledging. Breeding adult Masked Boobies forage farther from land than do Blue—footed Boobies. If longer transit times caused Masked Boobies to deliver less food per day than did Blue—footed Boobies, and thus limited brood size, we expected Blue—footed Boobies, but not Masked Boobies, to be able to provision a two—chick brood. Analysis of chick meal size and adult foraging effort (time spent foraging) indicated that Blue—footed parents could typically feed two, but not three, chicks; brood size thus matches feeding capacity in this species. Masked Boobies, however, appear to have sufficient excess foraging capacity to raise an additional chick, yet do not. The different brood sizes of these species are frequently cited to support the idea that foraging biology determines brood size in seabirds. In contrast, we conclude that Masked and Blue—footed Boobies have similar capacities to provision two—chick broods; some other factor limits Masked Boobies, but not Blue—footed Boobies, to fledging single chicks. Obligate siblicide clearly limits brood size at the proximate level in Masked, but not Blue—footed Boobies. A role for sibling competition also seems likely at the ultimate level, either through an interaction with delivery frequency or by driving genetic parent—offspring conflict over brood size.
Niche Overlap and the Potential for Competition and Intraguild Predation Between Size‐Structured Populations
Authors: Wissinger Scott A.
Published: May 2006

Many populations are heterogeneous collections of different sizes or stages of conspecifics. Existing overlap indices do not account for the size—/stage—structured nature of these populations. In this paper I present new overlap indices that use information about the sizes of individuals that co—occur in time and space to predict the potential for interactions in size—structured guilds. An index of the opportunity for competition (IOC) calculates the frequency with which similar size classes of two species encounter each other, whereas an index of the opportunity for intraguild predation (IOP) calculates the frequency of encounters among disparate size classes of the same two species. To illustrate that these indices are more appropriate for size—structured populations than conventional indices, I calculated overlap, IOP, and IOC for all species pairs in a diverse assemblage of dragonfly larvae. The new indices revealed size—specific patterns of overlap that were not detected by the conventional index, including that (1) some species with high overlap values should interact mainly as competitors, others mainly as intraguild predators, and many as both competitors and predators, (2) subtle differences in phenology and/or size—specific shifts in habitat distribution can lead to the potential for asymmetric interspecific interactions, (3) some species with low pairwise IOP and IOC values are nonetheless vulnerable to the effects of diffuse competition or intraguild predation, (4) seasonal segregation reduces competitive overlap but at the same time increases the opportunity for intraguild predation. The indices are general in form and should be useful for analyzing distributional data for any size—structured assemblage in which the type and intensity of interaction varies as a function of relative size.
Predator‐Induced Morphological Defenses: Costs, Life History Shifts, and Maternal Effects in Daphnia Pulex
Authors: Tollrian Ralpoh
Published: May 2006

This study was designed to measure and separate the physiological costs of inducible defenses from life history trade—offs and maternal effects in the waterflea Daphnia pulex. Juveniles of D. pulex produce morphological changes ("neckteeth") and undergo life history shifts as defenses against predatory Chaoborus (phantom midge) larvae. These traits are induced by a chemical cue (kairomone) released by the predator. I performed life history experiments with and without Chaoborus kairomones at different food levels to quantify the induced changes and their potential physiological costs. The Daphnia clone used in this study also increased its body depth in response to the predator substance. Life history shifted toward a larger body size (both length and depth) and higher fecundity, which was balanced by an increased time to reach maturity and by increased adult instar durations. Reproductive effort was higher in the typical morph in the first adult instar, indicating resource allocation shifts towards growth in the protected morph. However, even in the absence of predation the chemically induced protected morph tended to show an increased intrinsic rate of population growth (r). The longer time to reach maturity was not a direct physiological cost of neckteeth production, but a trade—off for larger body size. The life history shifts are independent of neckteeth formation. Developmental mechanisms leading to life history changes occurred after neckteeth were induced and could thus be uncoupled from neckteeth formation and its direct costs. In this study no direct costs were found. Carbon incorporation rates for the two morphs, at high and low food, were not different. As a maternal effect, the large females of the induced morph produced larger neonates which, in turn, matured at a larger size. Morphological changes, life history shifts, and maternal effects acted in concert to form defenses against Chaoborus. This study shows that the often assumed high physiological costs resulting from the formation or maintenance of the defenses are not necessary to explain the evolution of inducible defenses. As morphological changes increase the visibility of Daphnia pulex, a fitness disadvantage can be caused by a changing predator regime (e.g., fish). The results of this study suggest that environments with changing predator selectivities favor the evolution of inducible defenses.
Competition between Predator and Prey: Resource‐Based Mechanisms and Implications for Stage‐Structured Dynamics
Authors: Olson Mark H. , Mittelbach G. G. , Osenberg C. W.
Published: May 2006

In predator—prey interactions between size—structured populations, small (young) predators may compete with species that ultimately become their prey. We tested experimentally whether such competition occurs between young—of—year (YOY) largemouth bass and their eventual prey, bluegill. In a divided experimental pond, target densities of YOY bass and juvenile bluegill were subjected to different densities of bass and bluegill neighbors to examine the dependence of juvenile growth on fish density and species composition. After 7 wk, clear differences in growth rates of both species existed across densities. Bass growth was reduced in the presence of both bass and bluegill neighbors, whereas bluegill growth was primarily affected intraspecifically. Bluegill had strong competitive effects on bass despite substantial resource partitioning between the two species. These effects were mediated through changes in the size—structure of important invertebrate prey in both open water and vegetation habitats. Data collected from a set of seven lakes in Michigan indicate that intraspecific competition between juvenile bluegill and YOY bass is an important process in natural populations. However, once bass become piscivorous, bluegill become a significant resource for bass. As a result, bass populations are split into two functionally distinct stages that respond differently across a gradient of bluegill density. Because of the competitive stage between predator and prey, dynamics of the interaction differ from predictions based on classical predator—prey or competition theory.
Ecological and Demographic Effects on Intraspecific Variation in the Social System of Prairie Dogs
Authors: Travis Steven E. , Slobodchikoff C. N. , Keim Paul
Published: May 2006

Gunnison's prairie dogs (Cynomys gunnisoni) are social North American ground squirrels whose social system has been shown to vary with food resource distributions, as predicted by the habitat variability–mating system model. We expanded this model to include the effects of variations in population densities, in addition to resource distributions, on both the social system and the individual mating strategies of Gunnison's prairie dogs. Specifically, we predicted that monogamy would be associated with uniform resources, regardless of population density, giving way to polygyny with increasing resource patchiness at intermediate densities, and to multiple males and females at high population densities. In addition, we predicted inbreeding at low densities and outbreeding at high densities on the basis of naturally low levels of genetic eity in this species. We tested these predictions by comparing social systems and mating strategies at two prairie dog colonies near Flagstaff, Arizona, during a 3—yr period of population increase. We observed variations in sociality to vary in the predicted fashion at both colonies, providing support for the plant patchiness–population density model.
Experimental Removal of Insectivores from Rain Forest Canopy: Direct and Indirect Effects
Authors: Dial Roman , Roughgarden Jonathan
Published: May 2006

This study considered the effects of insectivorous Anolis lizards on a large, complex food web of arthropods and associated herbivory in a tropical rain forest canopy. We excluded Anolis lizards for 6 mo from 20—30 m high tree crowns in Puerto Rican rain forest. Simultaneous with lizard exclusion, we sampled orb spiders, airborne arthropods, and leaf arthropods in lizard removal crowns and in controls. We also sampled herbivory at the end of the experiment. Lizard removal had strong, statistically significant, positive effects on arthropods >2 mm in length and weak negative effects on arthropods <2 mm. Parameters of arthropod body size distributions differed between removals and controls for leaf arthropods, but not for airborne arthropods. Among arthropod taxa >2 mm, both predatory, i.e., orb spiders and parasitic Hymenoptera, and nonpredatory forms, i.e. Diptera, Coleoptera, Orthoptera, and Blattaria, showed strong significant and positive responses to lizard removal. Large Psocoptera, Homoptera, leaf spiders, and ants did not show significant overall responses to lizard removal. Frequency of herbivore damage on new leaves was positively correlated with abundance of Orthoptera and Blattaria. This damage was significantly greater in lizard removal crowns than in controls, indicating an indirect effect of anoles on plants. The indirect effect of lizards on small arthropods through the predatory anthropod pathway appeared weak. Results of lizard removal shown by this study corroborate other lizard removal studies from more xeric, ground—level habitats with simpler food webs in the West Indies, particularly with respect to orb spiders and herbivory. Taken together with the results of similar experiments performed in trophically less complex systems, this experiment suggests that food web size is less important than body size in determining interaction strength between community members.
Convergence and Divergence of Old‐Field Vegetation After 11 yr of Nitrogen Addition
Authors: Inouye Richard S. , Tilman David
Published: May 2006

Eleven years of data from experimental nitrogen gradients in three old fields and in native savanna are used to test the hypothesis that similarity of resource availability should influence similarity of plant species composition. Vegetation became less similar on plots that received different amounts of nitrogen. The difference in vegetation between plots was a function of both the difference in added nitrogen and the absolute levels of added nitrogen. Changes in similarity on plots that received the same amount of nitrogen were influenced by initial species composition, the amount of added nitrogen, and the rate of colonization by species not initially present on plots. Replicates of many treatments initially became less similar, only to increase in similarity after 3—6 yr. Data are generally consistent with predictions made in 1988, and clearly illustrate the importance of long—term responses to experimental manipulations.
Ecosystem Gas Exchange in a California Grassland: Seasonal Patterns and Implications for Scaling
Authors: Valentini Riccardo , Gamon John A. , Field Christopher B.
Published: May 2006

We used the eddy covariance technique to measure exchanges of water vapor, energy, and CO2 between California serpentine grassland and the atmosphere. Even though the system was built around an inexpensive, one—dimensional sonic anemometer and a closed—path CO2 analyzer, energy balance closure was accurate to +20% at a net radiation of 500 W/m2, and the spectra and cospectra indicated only modest information loss from incompletely resolved high—frequency turbulence. In the early and middle parts of the growing season, net radition, latent heat, and sensible heat all had similar diurnal dynamics, with latent heat accounting for °60% of the net radiation. Late in the growing season, energy dissipation by latent heat dropped dramatically, even though the vapor pressure gradient remained high. The factor, an index of the role of canopy conductance in regulating transpiration (Jarvis and McNaughton 1986), decreased from 0.8 early in the growing season (indicating predominant control of transpiration by net radiation) to 0.1 late in the growing season (indicating a shift to control of transpiration by canopy conductance and vapor pressure deficit). Canopy conductance was a linear function of the product of net photosynthesis and relative humidity, divided by the CO2 concentration, as predicted by Ball (1988). The slope of the relationship, however, was greater early in the growing season at other times. Whole—ecosystem carbon exchange rates were modest, with midday net photosynthesis reaching maximum values of 6—8 μmol°m—2.s—1 in early April. Diurnal variation in photosynthesis roughly paralleled variation in photosynthetically active photon flux density (PFD), but with daily maximum increasing with canopy development early in the growing season and decreasing with drought at the end of the growing season. Photosynthesis did not clearly saturate at high levels of PFD. Ecosystem dark respiration increased strongly (Q10 = 4.6) with increasing soil surface temperature. The efficiency with which absorbed radition was used in ecosystem photosynthesis, integrated over entire days, was 0.0115 + 0.0015 mol CO2/mol PFD (°20% of the values measured for healthy, single leaves under low light conditions) until late in the growing season, when the efficiency fell sharply. Using simple assumptions to extrapolate measurements from 11 d to the entire growing season, we estimate ecosystem annual gross primary production to be 11.1 mol CO2/m2, yielding an annual net primary production of °133 g biomass/m2. This is near the center of the range of published values from above—ground harvest studies.
Causes and Consequences of Dietary Specialization in Surfperches: Patch Choice and Intraspecific Competition
Authors: Holbrook Sally J. , Schmitt Russell J.
Published: May 2006

The existence, causes, and consequences of dietary specialization were explored for a temperate reef fish, striped surfperch (Embiotoca lateralis). Analyses of diets of adults from Santa Cruz Island, California, revealed three dietary regimes within the same population: specialization on caprellid amphipods, specialization on gammarid amphipods, or a mixed (generalized) diet composed of both caprellids and gammarids. Individuals that had specialized on caprellids at the time of capture had obtained a greater intake of prey than did generalists or specialists on gammarids. Different diets were associated with marked differences on body size within most adult year classes; specialists on caprellids were the largest and specialists on gammarids were the smallest for a given age. Generalists were intermediate in size. Females in the different dietary categories were projected to experience different expected lifetime reproductive outputs. Fitness was calculated to be °10% lower for generalists and °20% lower for specialists on gammarids compared with caprellid specialists. The different diets did not arise from active choice of prey taxa, but from selective use of available foraging patches (i.e., types of benthic algae from which prey are harvested). The degree to which an individual fed from the red alga Gelidium robustum was an excellent indicator of the taxonomic composition of its diet. Gelidium was the only alga harboring substantial members of caprellid amphipods; more importantly, it was the most profitable foraging microhabitat. Access to Gelidium was unequal among striped surfperch due to strong interference, and only competitively dominant individuals appeared able to consistently harvest crustaceans from this microhabitat. The least aggressive individual fed from other, less profitable patches that contained mostly gammarid amphipods. "Dietary" specialization was a passive consequence of patch selection dictated by a strong competitive hierarchy, and was not the result of active choice among available types of prey items.
Adaptive Radiation in Sticklebacks: Size, Shape, and Habitat Use Efficiency
Authors: Schluter Dolph
Published: May 2006

I examined habitat use efficiencies of two morphologically distinct threespine sticklebacks (Gasterosteus spp.). The species make up one of several coexisting pairs that apparently formed in the past 13 000 yr in coastal British Columbia lakes. Previous comparative work has shown that morphology and habitat use are correlated within and among populations. Sticklebacks occurring alone in lakes are intermediate in morphology and habitat use, suggesting that differences between the sympatric species result from ecological character displacement. I tested two hypotheses to explain these patterns, and which are implicit in general views of the causes of adaptive radiation: (1) Each resource (habitat) subjects its species to unique selection pressures, owing to the advantages of certain combinations of traits for foraging; (2) Phenotypes intermediate between habitat specialists suffer a competitive disadvantage. These hypotheses were evaluated by measuring the foraging rates of the two species and their interspecific hybrids in the two main habitats (benthos and open water) provided in the laboratory. Efficiencies of habitat exploitation matched obeserved morphological differences. In the benthic habitat the larger, deeper bodies species (having also a wide mouth and few short gill rakers) was superior to the smaller, more slender species (having a narrow mouth and many long gill rakers). Success of hybrids was intermediate. This rank order of species efficiencies was reversed in the open water habitat. These results support the hypothesis that different traits are favored in different habitats, and that adaptation to one habitat has occurred at the expense of feeding rate in the other. Average foraging success of hybrids in the two habitats fell near to below the average of the two parental species, suggesting natural selection against intermediate phenotypes. Foraging efficiency in open water was greatest in the smaller size classes of fish, hinting that small size of the open water species is the result of selection for high foraging efficiency. However, size could not explain most differences in feeding efficiency between species, and it is likely that many traits contribute.
Gender‐Specific Physiology, Carbon Isotope Discrimination, and Habitat Distribution in Boxelder, Acer Negundo
Authors: Dawson Todd E. , Ehleringer James R.
Published: May 2006

In the semiarid Intermountain West, boxelder, Acer negundo, var. interior, a deciduous, dieocious tree, exhibits significant habitat—specific sex ratio biases. Although the overall sex ratio (male/female) does not deviate significantly from one, the sex ratio is significantly male biased (1.62) in drought—prone habitats, while it is significantly female biased (0.65) in moist, streamside habitats. The causes underlying gender—specific habitat associations in this species are not known. We hypothesized that spatial segregation of the sexes is maintained by differences in gender—specific photosynthetic behavior, water relations characteristics, and both instantaneous and integrated water—use efficiency. Gender—specific physiological characteristics were measured and related to growth, reproduction, population age structure, and habitat distribution of male and female trees. Under both field and controlled—environment conditions, males and females differed significantly in a number of physiological traits. Males maintained lower stomatal conductance to water vapor (g), transpiration (E), net carbon assimilation (A), leaf internal CO2 concentration (ci), carbon isotope discrimination (△ an index of time—integrated ci and water—use efficiency), and higher instantaneous (A/E) and long—term (△) water—use efficiency than females. Furthermore, male trees exhibited greater stomatal sensitivity to both declining soil water content and increasing leaf—to—air vapor pressure gradients, a measure of evaporative demand. Higher rates of carbon fixation in female trees were correlated with higher g, higher leaf nitrogen concentrations, and greater stomatal densities. For females growing in both wet and dry habitats, vegetative shoots had higher growth rates than reproductive shoots, while for males, growth rates of the two shoot types did not differ. In streamside habitats, female trees exhibited significantly greater vegetative shoot growth when compared to male trees. In contrast, males showed slightly greater vegetative and much greater reproductive shoot growth in non—streamside habitats. Regardless of habitat or growing conditions, females allocated proportionately more of their aboveground biomass to reproduction than did males. These results suggest that (1) gender—specific physiological traits can help explain the maintenance of habitat—specific sex ratio biases in A. negundo along a soil moisture gradient, and (2) that the combination of the gender—specific physiology, growth, and allocation differences contribute to differences in the size (=age) structure of male and female plants within the population. Gender—specific physiological differences may have evolved as a product of selection to meet significantly different costs associated with reproduction in male and female plants.