bluuebunny/crossref_metadata_2025 huggingface dataset:
Mechanisms of Competition in Tropical Hummingbirds: Metabolic Costs for Losers and Winners
Authors: Tiebout Harry M.
Published: May 2006

The goal of this study was to measure directly the acquisition and allocation of energy for competing hummingbirds. In flight—cage trials providing an easily defensible food source, complete energy budgets (intake, expenditure, and storage) were obtained for two sympatric Costa Rican hummingbirds: a territorial (Amazilia saucerottei) and a low reward traplining (Chlorostilbon canivetii) species. Behavior and energy responses of solitary birds (controls) were compared to the responses of conspecific and heterospecific pairs competing for a single feeder. Relative to controls, birds in pairs generally spent more time in flight, due primarily to increased rates of visiting the feeder or to agonistic encounters, with a consequent increase in energy expenditure. Ability of birds in pairs to compensate for increased expenditures by increasing intake, thus maintaining a constant body mass over 24 h, depended upon species and dominance status. The greatest asymmetries in energy effects occurred in heterospecific pairs, in which Amazilia aggressively interfered with access to the feeder by Chlorostilbon did not increase intake and thus suffered significant loss of energy stores. Conspecific pairs of each species experienced energy effects intermediate to the extreme responses of heterospecific pairs. Individuals of either species had greater energy success when paired with a Chlorostilbon rather than with an Amazilia. For both species, individuals that were relatively more aggressive than their cagemates experienced the greater energy success, though absolute frequency of aggression of an individual was a poor indicator of its energy success. Differential energy response to competition by Amazilia and Chlorostilbon appears to be one mechanism that contributes to local species abundance and guild composition.
Life‐History Theory in the Nonbreeding Period: Trade‐Offs in Avian Fat Reserves?
Authors: Rogers Christopher M. , Smith James N.
Published: May 2006

We tested the hypothesis of optimal winter fat storage in birds. The hypothesis predicts that when the survivorship benefits (fasting capacity) but not the survivorship costs (higher cost of flight, lowered agility, increased exposure to predators) of fat are eliminated by predictable resources (e.g., tree— and shrub—borne food), species will be leaner than species exploiting unpredictable resources (e.g., ground—borne food subject to sudden covering by snow). The predicted pattern was found in an earlier study of avian communities in central North America, while snowfall is frequent. Here we tested the predictions of the hypothesis that (1) when ground—and tree—feeding guilds are compared between geographic regions of harsh and mild winter climate, only ground—feeders will have relatively high fat reserves, resulting in a significant guild ° climate interaction term in a two—way analysis of variance, and (2) ground— and tree—foraging guilds will both show low fat reserved in a mild winter environment. To test these predictions, visible subcutaneous fat class was measured in both guilds in Wisconsin and Michigan (harsh winter environments) and southwestern British Columbia, northwestern Washington, and Tennessee (mild winter environments). Both predictions were supported. We suggest that small birds wintering in North America approach local optima in energy storage strategy and winter survivorship. The energy storage strategy thus appears to be a major life—history trait in the nonbreeding period. The trade—off between the costs and benefits of winter fat has two potential implications for understanding population limitation. First, trade—offs that reduce reserves can lead to lower survival in severe winter weather than would occur at maximum fat levels. Second, our study suggest that individual birds are sensitive to, and can control, both predation and starvation risks. Recent theory of population limitation indicates that as food abundance declines, birds feed to avoid starvation, but at the expense of increase mortality from predation. Together these studies suggest that populations are limited by interacting (predation, food supply) instead of single (food supply) factors. Study of trade—offs used by individuals to maximize survival can provide unique perspectives on population limitation.
Neighbor Influences on Root Morphology and Mycorrhizal Fungus Colonization in Tallgrass Prairie Plants
Authors: Jastrow J. D. , Miller R. M.
Published: May 2006

A field study was conducted across a chronosequence of tallgrass prairie restorations to investigate the influence of plant's neighbor may have on its gross root morphology and association with mycorrhizal fungi. The root systems of a warm—season grass, Andropogon gerardii, and two perennial forbs, Coreopsis tripteris and Solidago altissima, were sampled by excavating soil blocks representing all pairwise inter— and intra—specific combinations of these species. With forb neighbors, Andropogon had higher percentages of its fibrous roots colonized by mycorrhizal fungi than with conspecific neighbors. Andropogon root morphology (as measured by average root radius, specific root length, and root branching) did not vary with neighbor growth form; however, Andropogon roots did become finer as restoration age and root densities increased. For Coreopsis and Solidago, colonization of fibrous roots did not vary with neighbor species, but both forbs had coarser roots with interspecific neighbors than with conspecifics. With interspecific neighbors, Coreopsis almost doubled the number of branches per centimetre of root observed in conspecific pairs. Although some of the variations in colonization and root morphology may be attributable specifically to neighbor root mass, others were related to the overall root mass of the target—neighbor pair. The observed variations in mycorrhizal colonization may be related to increased opportunities for colonization by the fungus that are due not only to higher root densities but also to some sort of interaction between the root systems of Andropogon and the forbs.
Soil Organic Matter Dynamics Along Gradients in Temperature and Land Use on the Island of Hawaii
Authors: Townsend Alan R. , Vitousek Peter M. , Trumbore Susan E.
Published: May 2006

We studied soil organic matter (SOM) dynamics in allophanic soils (Udands) along independent gradients of temperature (altitude) and land use (forest—pasture) on the island of Hawaii. Using an integrated 13C signal derived from land conversion along with measurements of soil respiration and soil carbon, we separated rapid, intermediate, and very slow turnover SOM pools, and estimated turnover times for the large intermediate pool. These estimates were compared to independent estimates using either bomb—derived soil 14C or the Century soil organic matter model. All calculations based on a three—pool SOM structure yield rates of turnover that are 3 times slower than those produced by a single pool model. Accordingly, analyses of potential feedbacks between changes in climate, atmospheric CO2, and soil carbon should incorporate the heterogeneous nature of soil organic matter. We estimate that roughly three—quarters of the carbon in the top 20 cm of these soils has turnover times less than 30 yr. Turnover times for intermediate SOM double with a 10°C change in mean annual temperature, suggesting that recalcitrant pools of SOM may be as sensitive to changes in temperature as the smaller labile pools.
Acorn Production by Oaks in Central Coastal California: Variation within and among Years
Authors: Koenig Walter D. , Mumme Ronald L. , Carmen William J. , Stanback Mark T.
Published: May 2006

We measured acorn production by individual oaks of five different species at Hastings Reservation in central coastal California between 1980 and 1991. Variation in acorn production was considerable both within and among years and was generally uncorrelated between species. Compared to expected values, variance within years in the size of acorn crops was small, while variance among years was high. Crop failures occurred fairly frequently and large crops in successive years were observed, but not more than expected by chance. Individual trees masted at species—specific intervals, but these patterns did not result in regular masting cycles at the production level. We compared these patterns to predictions of four hypotheses for the evolution of seed production patterns. Observations did not support the hypotheses that production patterns track resource availability (the ”resource matching” hypothesis) or that they have evolved to attract seed dispersers (the “seed dispersal” hypothesis). However, they are generally consistent with two additional hypotheses, that masting in these wind—pollinated species evolved because of a proportional increase in fertilization and seed set during mast years (the “wind pollination” hypothesis) and that masting has evolved to maximize the probability of avoiding predation via predator satiation (the “predator satiation” hypothesis).
Temporal Variation in Humoral and Cell‐Mediated Immune Response in a Sigmodon Hispidus Population
Authors: Lochmiller Robert L. , Vestey Michelle R. , McMurry Scott T.
Published: May 2006

We examined the hypothesis that environmentally induced alteration of immunocompetence is a primary physiological mechanism by which population survival is regulated. The involvement of disease and immunocompetence in the regulation of small mammal populations is not well understood but a strong relationship between physiological stress and immunity has been demonstrated. Its involvement is further supported by several studies, which have provided evidence that survival, incidence of disease, and selected gross measures of immunity change temporally with density and season. We examined this hypothesis by documenting the existence of temporal alterations in selected measures of both humoral and cell—mediated immune function in a population of cotton rats (Sigmodon hispidus), a species known to fluctuate widely in density, inhabiting a tallgrass prairie in central Oklahoma. A total of 310 cotton rats was collected from August 1989 to May 1991 (10 collections) using conventional live—trap methods. In addition to morphological characteristics of primary and secondary lymphoid organs, the ability of splenocytes to produce specific antibody in response to a single injection of sheep erythrocytes and to proliferate in response to in vitro exposure to mitogens was measured to assess immunocompetence. Whole blood was collected for the determination of total peripheral blood leukocyte counts, packed cell volume, and total serum proteins. Statistically significant differences in all variable except total serum protein existed among the 10 collections and most of this variability was independent of specific seasonal time periods. Bimodal thythmicity was characteristic of temporal changes for several measures of immunocompetence. Changes over time in the average immunocompetence of this population could have reflected a shift in genotypic polymorphism; several other plausible explanations are offered to explain the observed temporal patterns.
Elevated CO2 and Temperature Alter Recruitment and Size Hierarchies in C3 and C4 Annuals
Authors: Morse S. R. , Bazzaz F. A.
Published: May 2006

In order to understand the implications of changes in global CO2 concentrations and temperature for the growth and fitness of individual plants, performance must be investigated in relation to the performance of other plants within a population. In this study we examined patterns of recruitment, mortality, and size structure of monospecific stands in response to ambient (400 μL/L) and elevated CO2 concentrations (700 μL/L) across three temperature regimes; 18°, 28°, and 38°C. We created experimental populations of two annual plants that differ in their photosynthetic pathway and water use patterns: Abutilon theophrasti (C3) and Amaranthus retroflexus (C4). The effects of CO2, temperature, and their interactions on population structure were complex and species dependent. For both species increasing temperature resulted in higher germination and faster initial growth rates. These initial temperature responses increased the intensity and role of competition in determining stand size and structure. Postemergence responses to elevated CO2 differed markedly between the two species. For Abutilon, the C3 species, self—thinning and the mean biomass of the survivors increased under elevated CO2. For Amaranthus, survivorship, but not growth, increased under elevated CO2 conditions. We attribute differences in response between species not only to photosynthetic pathway, but also to differences in the onset of competition mediated through differences in plant form and in resource uptake and deployment. The patterns of stand development in response to CO2 and temperature suggest that the effects of changing CO2 and temperature may be understood within mechanistically based models of resource use. Temperature regulates the rate of resource use and the onset of interference among plants, while CO2 functions both as a resource and a resource regulator. Although mortality was concentrated later in stand development for Abutilon than Amaranthus, overall patterns of stand size and structure were similar for both species; mortality and size inequalities increased with increasing temperature and CO2. Because size is often correlated with fecundity, and increase in size hierarchies in response to elevated CO2, in conjunction with a decrease in survivorship, may result in a smaller effective population size. Our ability to predict changes in effective population size due to changing size due to changing size hierarchies alone, however, should also consider developmental shifts in response to elevated CO2 that may result in, as in this study, a decrease in the minimum size at the onset of flowering.
Increasing CO2: Comparative Responses of the C4 Grass Schizachyrium and Grassland Invader Prosopis
Authors: Polley H. Wayne , Johnson Hyrum B. , Mayeux Herman S.
Published: May 2006

The woody C3 Prosopis glandulosa (honey mesquite) and C4 perennial grass Schizachyrium scoparium (little bluestem) were grown along a gradient of daytime carbon dioxide concentrations from near 340 to 200 μmol/mol air in a 38 m long controlled environment chamber. We sought to determine effects of historical and prehistorical increases in atmospheric CO2 concentration on growth, resource use, and competitive interactions of a species representative of C4—concentration of C4—dominated grassland in the southwestern United States and the invasive legume P. glandulosa. Increasing CO2 concentration stimulated N2 fixation by individually grown P. glandulosa and elicited in C3 seedlings a similar relative increase in leaf intercellular CO2 concentration, net assimilation rate, and intrinsic water use efficiency (leaf net assimilation rate/stomatal conductance). Aboveground biomass of P. glandulosa was not altered by CO2 concentration, but belowground biomass and whole—plant water and nitrogen use efficiencies increased linearly with CO2 concentration in seedlings that were grown alone. Biomass produced by P. glandulosa that was grown with S. scoparium was not affected by CO2 concentration. Stomatal conductance declined and leaf assimilation rates of S. scoparium at near maximum incident light increased at higher CO2 concentration, but there was no effect of CO2 concentration on biomass production or whole—plant water use efficiency of the C4 grass. Rising CO2 concentration, especially the 27% increase since the beginning of the 19th century, may have contributed to more abundant P. glandulosa on C4 grasslands by stimulating the shrub's growth or reducing the amount of resources that the C3 required. Much of the potential response of P. glandulosa to CO2 concentration, however, appears to be contingent on the shrub's escaping competition with neighboring grasses.
Calcium Carbonate and Secondary Metabolites in Tropical Seaweeds: Variable Effects on Herbivorous Fishes
Authors: Schupp Peter J. , Paul Valerie J.
Published: May 2006

On coral reefs where herbivorous fishes are both numerous and diverse, many seaweeds combine mineral and chemical defenses. We hypothesized that combined defenses affect a greater number of herbivore species than either single defense, and in this study we experimentally tested this hypothesis. We first incorporated calcium carbonate (aragonite form) alone at a range of concentrations into an artificial diet composed of freeze—dried samples of the noncalcified, green alga Enteromorpha calathrata, carragenan, cornstarch, and water. In field experiments where diets were made available to a natural fish community, aragonite had variable effects: in some trials aragonite deterred feeding, in others it had no significant effect, and in one trial it even simulated feeding. Parrotfishes (Scaridae) and the surgeonfish Ctenochaetus striatus (Acanthuridae) showed a significant preference for food containing aragonite in field observations, while the surgeonfish Naso vlaminigii and three damselfishes (Pomacentridae) were observed to feed preferentially on food without added aragonite. In the laboratory, the surgeonfishes Acanthurus nigrofuscus, A. triostegus, A. xanthopterus, and Naso unicornis and the rabbitfish Siganus spinus (Siganidae) significantly avoided food with added aragonite. However, the parrotfishes Scarus sordidus and S. schlegeli consumed significantly more food containing aragonite the plain food. Ctenochaetus stiatus avoided aragonite only at the highest aragonite concentration and showed no preference at lower aragonite concentrations. Artificial diets containing natural concentrations of aragonite (80% dry mass), dieter—penoid secondary metabolites from Halimeda macroloba (1% dry mass), and a combination of both defenses (80% aragonite and 1% aragonite—free dry mass of diterpenes) were tested in the laboratory against four fish species. Scarus sordidus and Ctenochaetus striatus avoided secondary metabolites but not aragonite, and Naso lituratus and Signaus spinus were deterred by aragonite but not by secondary metabolites. All fishes were deterred by one defense or the other, and all fishes but Scarus sordidus avoided food containing the combined defenses. Combinations of aragonite and secondary metabolites increased the number of fish species that were effectively deterred from grazing relative to either single defense.
Radio‐Tracking Confirms a Unique Diurnal Pattern of Spatial Occurrence in the Parasitic Brown‐Headed Cowbird
Authors: Rothstein Stephen I. , Verner Jared , Steven Ernest
Published: May 2006

Brood—parasitic Brown—headed Cowbirds (Molothrus ater) in the eastern Sierra Nevada of California, breed and feed in almost totally disjunct areas that reflect local optima for finding host nests and food, respectively. Radio—tracking showed that five females and four of eight males spent mornings in host—rich habitats such as forests and then commuted 2.1—6.7 km to one or more prime feeding sites such as horse corrals and bird feeders for the rest of the day. The four noncommuting males, which were all yearlings and possibly socially subordinate, also showed high mobility bud did not occupy the same area each morning. Since cowbirds lay eggs in the morning and were rarely seen feeding then, the disjunct areas visited by commuters can be characterized as morning—breeding (egg—laying) and afternoon—feeding areas. We found little evidence of territoriality on morning ranges, nor did we find evidence of prolonged pair bonds. The morning ranges of commuters averaged 68 ha, and their total home ranges, including afternoon—feeding areas, averaged 442 ha. These are among the largest breeding home ranges described for passerines, and they equal those of certain raptors. Raptors require large areas to provide a sufficient prey base, and cowbirds require similarly large areas to provide a sufficient number of host nests. The cowbird's commuting pattern, which is unique among passerines, involves a shift from largely asocial behavior in the morning to extreme sociality in the afternoon. Unlike related nonparasitic icterids that disperse from central breeding sites to feed, cowbirds do the reverse, dispersing from centralized feeding sites to breed. The cummuting behavior of cowbirds is yet another example of the flexibility of a species' behavioral ecology in response to the dispersion of resources essential for maintenance and reproduction.
Fungus Consumption by the Siskiyou Chipmunk within a Variously Treated Forest
Authors: McIntire Patrick W.
Published: May 2006

Mycophagy (consumption of fungi) by the Siskiyou chipmunk (Eutamias siskiyou) was studied during the summers of 1978 and 1979. Fecal pellets were sampled from livetrapped chipmunks in two sites within a shelterwood—logged coniferous forest in Jackson County, Oregon. Slash (logging residue) was mechanically piled and then burned (treated) in one area but not in another. Habitat analyses indicated that huckleberry (Vaccinimum membranaceum) was severely reduced, and exposure of mineral soil increased in the treated study site. Sixteen genera of hypogeous, mycorrhizal fungi, representing three classes, were identified in fecal samples taken from animals in both sites. Spores of Rhizopogon spp. were most frequently encountered in both sites. The frequencies of occurrence for 11 of the 16 fungal genera were greater for samples from the untreated site, 2 significantly so. Endogone spp. occurred exclusively in fecal samples from the treated site, but no fungal type from this site exhibited a significantly greater frequency. The reduction of spores in samples from the treated site is thought to be a function of diminished sporocarp availability that resulted from slash treatment. Differential microhabitat selection by the sexes may also account for some differences in the observed spore frequencies.
Interhabitat Differences in Energy Acquisition and Expenditure in a Lizard
Authors: Karasov William H. , Anderson Roger A.
Published: May 2006

Cnemidophorus hyperythrus, a small (°4—g) teiid lizard, occurs along an elevational thorn scrub–thorn woodland–thorn forest habitat gradient in the cape region of Baja California. We compared body size, daily energy expenditure (DEE, measured with doubly labeled water), relative feeding rate (as reflected by H2O influx rate), behavior, and abundance of this species at two sites along the gradient. At the inland thorn woodland site C. hyperythrus were more abundant (°100 lizards/ha) than at the thorn scrub site near the ocean (°50 lizards/ha). Mean body mass of woodland site lizards was 13% greater than that of scrub lizards. The DEE of the thorn woodland lizards, 330 J°g—1 d—1 , and their H2O influx, 99 mm3°g—1°d—1 , were also higher than the thorn scrub lizards', 219 J°g—1°d—1 and 52 mm3°g—1°d—1. Diets at the two sites were similar. There were no differences between sexes in diet, DEE, or H2O influx. Daily maintenance energy costs were calculated based upon laboratory measures of O2 consumption of resting lizards at a series of temperatures that represented the daily range of body temperatures experienced by lizards in the field. Activity costs (=DEE minus maintenance) were three times higher in the woodland lizards. Behavioral observations showed that the woodland lizards were active most of the day (°9 h/d) whereas scrub lizards were active primarily in the morning (°3.5 h/d). Thus, the higher activity cost, DEE, and feeding rate of woodland lizards can be explained by their longer daily activity period. We suggest casual factors for the difference in daily activity period, and discuss implications of length of daily foraging period for adult body size, population density, and various life history parameters of lizards.
Ecological Determinants of Stability in Model Populations
Authors: Mueller Laurence D. , Huynh Phuc T.
Published: May 2006

Detailed studies of the density—dependent interactions of larval and adult life stages of Drosophila melanogaster suggest that the stability of populations is dependent on environmental variables. Models of Drosophila population dynamics predict that the levels of food supplied to the adults and larvae will have important effects on population stability: stability is enhanced when adults are given low amounts of food and larvae are given high amounts of food. Experimental results presented here are consistent with these general predictions. These observations have implications for the management of biological populations and present an opportunity for studying the evolution of population stability.
Feeding Behavior of Free‐Ranging King Penguins (Aptenodytes Patagonicus)
Authors: Putz Klemens , Bost Charles A.
Published: May 2006

The foraging ecology of free—living King Penguins (Aptenodytes patagonicus) was studied during October and November 1991 on Possession Island (46°25' S, 51°45'E), Crozet Archipelago. Feeding was detected with stomach temperature sensors, which logged feeding activity as indicated by temperature drops. The penguins fed mainly on myctophid fish. Stomach temperature of 16 birds at sea was recorded for a total of 146 d. During 125 d a total of 16 474 food ingestion events was recorded, a mean of 132 events per day. Here, calculated food intake averaged 2320 g/d (range 11—12 889 g) with >80% (range 71—95%) of prey caught during daylight. During the remaining 21 d stomach temperature dropped below the detection limit of 20°C, which made determination of the number of ingestion events impossible and mass ingested determination inaccurate. We propose that there is selection pressure for King Penguins to ingest and digest large quantities of food quickly because these birds alternate fasting periods on land, during which time they care for their chicks, with foraging trips, during which time they must feed on patchily distributed prey.
Vertical Hydrologic Exchange and Ecological Stability of a Desert Stream Ecosystem
Authors: Valett H. Maurice , Fisher Stuart G. , Grimm Nancy B. , Camill Philip
Published: May 2006

The influence of hydrologic linkage between hyporheic and surface subsystems was investigated in sand—bottomed reaches of a desert stream. Direction of hydrologic exchange was measured as vertical hydraulic gradient (VHG) using mini—piezometers. Maps of VHG indicated upwelling (discharge from the interstitial regions into surface water) at the base of riffles and heads of runs; downwelling (infiltration of surface water into the hyporheic zone) occurred at the bases of runs. Dissolved NO3—N in surface water was higher over or immediately downstream from upwelling zones. Loss of continued supply from the hyporheic zone and intense assimilatory demand by surface autotrophs generated longitudinal declines in NO3—N and lower nutrient concentrations in downwelling zones. Algal standing crop (as chlorophyll a) was significantly higher in upwelling zones than in areas without positive VHG. Postflood trajectories of chlorophyll a indicated that algae at upwelling zones recovered from disturbance significantly faster than those at downwelling zones. Recovery rate was related to supply of NO3—N from enriched interstitial water in the hyporheic zone. Hydrologic linkage integrates surface and hyporheic subsystems and increases ecosystem stability by enhancing resilience of primary producers following flash flood disturbance.
Shape Divergence Associated with Size Convergence in Sympatric East African Jackals
Authors: Van Valkenburgh Blaire , Wayne Robert K.
Published: May 2006

The geographic ranges of three extant species of medium—sized canid, the sidestriped (Canis adustus), the golden (C. aureus), and the blackbacked (C. mesomelas) jackal, overlap only in East Africa. Elsewhere each species exists alone or in sympatry with one other jackal species. The fossil record suggests that the golden jackal evolved outside of Africa and joined the other two species in East Africa within the last 500 000 yr. Previous research showed that unlike other sympatric canids, all three jackal species in East Africa are extremely similar in size and overall morphology and thus exhibit no obvious evidence of ecological character displacement. Moreover, the three species do not form a monophyletic group, are distantly related, and thus may have had ample time to diverge morphologically. Here, we explore in further detail the possibility of character displacement in the three species by assessing variation in 62 measures of the skull and teeth for populations of all three species throughout their ranges. The expanded measurement and population sample allows us to explore four possible manifestations of character displacement: (1) divergence in size, (2) divergence in shape, (3) reduced morphological variability, and (4) reduced sexual dimorphism. There is no evidence of size divergence among jackals in East Africa; by contrast the three species appear to converge on a similar size. However, divergence in shape is apparent. The recent insinuation of golden jackals into the East African fauna has resulted in character divergence and probable niche compression of blackbacked jackals as evidenced by adaptations for increased carnivory, reduced sexual dimorphism, and reduced variability. The response has been asymmetric as the golden and sidestriped jackals have not diverged functionally or shown niche compression. The pattern of character displacement is likely a function of starting conditions at the time the three became sympatric. The golden jackal exhibits dietary adaptations intermediate to those of the other two species but more similar to those of blackbacked jackal. Thus character displacement was more likely to occur in goldens and blackbacks. Selection for increased carnivory in the blackbacked jackal might have been favored because of the abundance of vertebrate prey in the East African plains. The success of the golden jackal as an insinuator into East Africa suggests that the carnivore guild might not be saturated.
Effects of Herbivory on Growth and Biomass Allocation in Native and Introduced Species of Lonicera
Authors: Schierenbeck Kristina A. , Mack Richard N. , Sharitz Rebecca R.
Published: May 2006

We compared growth and biomass allocation patterns of Lonicera japonica, an invasive vine in the southeastern United States, and its native congener, L. sempervirens, among three herbivory treatments. Growth and biomass variables were calculated at nine harvest dates across 14 mo under no herbivory, insect herbivory, and mammal and insect herbivory. In the absence of herbivory, L. sempervirens accumulated greater biomass and had higher growth rates than L. japonica. Lonicera sempervirens, however, experienced more herbivore damage than L. japonica, particularly due to the combined effects of insect and mammal herbivory. Total biomass accumulation and greater allocation to leaves and stems indicate a compensatory response to herbivory by L. japonica. These attributes in response to herbivory coupled with lower herbivory in its new range compared to L. sempervirens could give L. japonica a substantial advantage over its native congener.
Synergisms in Plant Defenses against Herbivores: Interactions of Chemistry, Calcification, and Plant Quality
Authors: Hay Mark E. , Kappel Quaker E. , Fenical William
Published: May 2006

Many tropical seaweeds and benthic invertebrates produce both secondary metabolites and calcium carbonate (CaCO3) particles or spicules that serve as possible defenses against consumers. To evaluate the relative defensive value of CaCO3, secondary metabolites, and the potential synergistic or additive effects of the two, we made artificial agar—based “seaweeds” in which we manipulated algal organic content, CaCO3, and the secondary metabolites produced by the calcified green seaweeds Rhipocephalus phoenix, Udotea cyathiformis, and Halimeda goreauii, all of which are relatively resistant to herbivores. The effects of these manipulations on herbivore feeding were evaluated using three different types of herbivores, the sea urchin Diadema antillarum, the amphipod Cymadusa filosa, and a mixed—species group of small parrotfishes. Addition of finely powdered CaCO3 as 69% of food dry mass had no effect on feeding by parrotfishes, deterred feeding by Cymadusa, and deterred Diadema when food organic content was low but not when it was higher. Although calcification of algal tissues has generally been considered a structural defense that hardens seaweed thalli and makes them more resistant to attack, the decreased feeding on CaCO3—containing foods in our assays occurred without any measurable alternation of food toughness. At natural concentrations, semipurified secondary metabolites from Rhipocephalus or Udotea deterred feeding by all three herbivores. In most assays, feeding was depressed more by the addition of metabolites from Rhipocephalus or Udotea than by the addition of CaCO3 even though CaCO3 was added at 1.3—2.2 times the natural concentration for these plants. In contrast, the major metabolite from Halimeda goreauii, when tested alone, did not affect feeding by any of the herbivores. In two of our nine assays, the synchronous combination of CaCO3 and secondary metabolites acted synergistically and deterred feeding significantly more than the sum of the effects of each tested separately. Mechanisms producing these synergisms are unknown, but it is possible that calcification could also be acting as a chemical defense by altering gut pH in ways that increase the potency of the secondary metabolites. It is common for chemical, structural, morphological, and nutritional deterrents to co—occur in individual prey species. For some plant—herbivore interactions, the combined effects of these characteristics can be much more than the sum of their separate effects.
Multiple Indirect Effects of Plant Litter Affect the Establishment of Woody Seedlings in Old Fields
Authors: Facelli Jose M.
Published: May 2006

I studied the effects of oak leaf litter, herb competition, and insect herbivory on the establishment of seedlings of Ailanthus altissima in New Jersey, USA. I performed two experiments in an early—successional site in two consecutive years. In the first experiment I tested the effects of litter and competition, and their interaction, on establishment and growth of tree seedlings. In the second experiment I studied the effect of insecticide application in combination with litter and herb competition on seedling establishment and growth, and on arthropod abundance. In the absence of competition, litter had no significant effect on the biomass of woody seedlings, though it delayed seedling emergence. Herb competition reduced the growth of the woody seedlings, but the addition of leaf litter reduced the density and biomass of the herbs, indirectly improving the growth of the woody seedlings. Litter increased both invertebrate damage to seedling cotyledons and seedling mortality. The application of insecticide increased seedling emergence and reduced seedling mortality, and more so in the presence of litter. The total number of arthropods and the number of plant—chewing arthropods were increased by the addition of litter and reduced by application of insecticide. Thus, litter had, simultaneously, a positive indirect effect on individual biomass through reduced competition, and a negative indirect effect on the number of individuals through increased seed/or seedling predation, and a negative direct effect on seedling mortality.
Plant Competition in Relation to Neighbor Biomass: An Intercontinental Study with POA Pratensis
Authors: Reader R. J. , Wilson S. D. , Belcher J. W. , Wisheu I. , Keddy P. A. , Tilman D. , Morris E. C. , Grace J. B. , McGraw J. B. , Olff H. , Turkington R. , Klein E. , Leung Y. , Shipley B. , van Hulst R. , Johansson M. E. , Nilsson C. , Gurevitch J. , Grigulis K. , Beisner B. E.
Published: May 2006

A standardized neighbor removal experiment was conducted in 12 plant communities located on three continents to test the null hypothesis that competition intensity (CI) was independent of the amount of plant biomass present. Six plots were chosen in each community to cover the range of local variation in plant biomass. In each plot the relative growth rate (RGR) of transplanted Poa pratensis (Poaceae) seedlings was compared in the presence and absence of neighbors. Neighbors were removed experimentally using herbicide. Removing neighbors increased RGR of transplants significantly in most plots. CI increased with an increase in the amount of neighbor biomass present in one community where the range of neighbors biomass was greater than in any other community. In contrast, CI did not change significantly with an increase in neighbor biomass in other communities where the range of neighbor biomass was smaller. For the communities combined, CI was not related to neighbor biomass in a consistent fashion. These results indicate that competition may reduce growth over a wide range of habitat productivity, but the relationship between CI and neighbor biomass may differ among communities.
AIC Model Selection in Overdispersed Capture‐Recapture Data
Authors: Anderson D. R. , Burnham K. P. , White G. C.
Published: May 2006

Selection of a proper model as a basis for statistical inference from capture—recapture data is critical. This is especially so when using open models in the analysis of multiple, interrelated data sets (e.g., males and females, with 2—3 age classes, over 3—5 areas and 10—15 yr.) The most general model considered for such data sets might contain 1000 survival and recapture parameters. This paper presents numerical results on three information—theoretic methods for model selection when the data are overdispersed (i.e., a lack of independence so that extra—binomial variation occurs). Akaike's information criterion (AIC), a second—order adjustment to AIC for bias (AICc), and a dimension—consistent criterion (CAIC) were modified using an empirical estimate of the average overdispersion, based on quasi—likelihood theory. Quality of model selection was evaluated based on the Euclidian distance between standardized ° and ° (parameter ° is vector valued); this quantity (a type of residual sum of squares, hence donated as RSS) is a combination of squared bias and variance. Five results seem to be of general interest for these product—multinomial models. First, when there was overdispersion the most direct estimator of the variance inflation factor was positively biased and the relative bias increased with the amount of overdispersion. Second, AIC and AICc, unadjusted for overdispersion using quasi—likelihood theory, performed poorly in selecting a model with a small RSS value when the data were overdispersed (i.e., overfitted models were selected when compared to the model with the minimum RRS value). Third, the information—theoretic criteria, adjusted for overdispersion, performed well, selected parismonious models, and had a good balance between under— and overfitting the data. Fourth, generally, the dimension—consistent criterion selected models with fewer parameters than the other criteria, had smaller RSS values, but clearly was in error by underfitting when compared with the model with the minimum RSS value. Fifth, even if the true model structure (but not the actual parameter values in the model) is known, that true model, when fitted to the data (by parameter estimation) is a relatively poor basis for statistical inference when that true model includes several, let alone many, estimated parameters that are not significantly different from O.
Comparative Population Dynamics of Two Goldenrod Aphids: Spatial Patterns and Temporal Constancy
Authors: Cappuccino Naomi
Published: May 2006

Two aphid species, Uroleucon nigrotuberculatum and U. caligatum, that feed on the goldenrod Solidago altissima, are similar in life histories and feeding habits, but differ markedly in their spatial distribution and temporal variability. In this study, U. caligatum typically occurred in small colonies scattered throughout goldenrod fields, and its numbers remained relatively constant through time, whereas U. nigrotuberculatum occurred in dense colonies and exhibited greater fluctuations in population size. The aggregated spatial pattern of U. nigrotuberculatum resulted from both active aggregation of alates and lack of dispersal by apterae and nymphs. Field experiments in which colony size was held constant revealed that U. nigrotuberculatum enjoyed no clear reproductive advantage that might explain its greater tendency to outbreak. In addition, U. nigrotubercultatum was more susceptible to generalist predators (such as cantharid beetles, mirid bugs, and mites) than U. Caligatum. Aphid dispersion was then manipulated in the field to determine how the different spatial patterns of the two species influenced reproduction and mortality, and hence the relative tendencies of their populations to irrupt. Feeding in large colonies provided no reproductive advantage to either species. Aggregation did, however, enhance aphid survivorship when generalist predators were the main mortality agents. The advantage was reversed later in the season when the aphids' fungal pathogen was present; aggregation then led to a decrease in survivorship. Thus, aggregation allowed a numerical escape from generalist predators but also promoted fungal epidemics, so that the temporal variability in U. nigrotuberculatum populations was largely a consequence of its spatial distribution.
Avian Fruit Removal: Effects of Fruit Variation, Crop Size, and Insect Damage
Authors: Jordano Pedro
Published: May 2006

Avian dispersal of seeds of the wild olive tree (Olea europaea var. sylvestris) was studied in Mediterranean shrubland, southern Spain. Fourteen species of small frugivorous birds in the genera Sylvia, Turdus, Sturnus, and Erithacus accounted for 97.4% of the fruits consumed by birds. The significance of each bird species as an Olea fruit consumer was closely related to its abundance in the area and was not associated with its dependence on the fruit for food; this resulted in a highly asymmetric interaction between the plant and its dispersers. Fruit production differed greatly between two consecutive seasons. During an extremely dry year most trees aborted their entire fruit crop just after flowering. During the 2nd yr, both flower production and fruit set increased, and this resulted in larger crops of ripe fruit. Most fruits that ripened (96.2% of the final—sized fruits) during the year of low fruit production were consumed by dispersers (X = 93.9%), and incidence of fruit—damaging agents was low (6.0%). During the 2nd yr, fruit loss to the two main fruit predators, Dacus oleae (Tephritidae) and Prays oleae (Yponomeutidae), increased (X = 27.1%, range 1.1—52.3%). A satiation process took place during this year since fruit production exceeded the energy demand of the disperser assemblage, and this increased the potential for fruit loss to insect frugivores. Fruit removal by dispersers decreased (X = 52.4%), but the increase in fecundity during the 2nd yr for most of the trees compensated for this difference and resulted in a greater absolute number of seeds removed. Most variation in ripe fruit removal by birds was attributed to the interaction of dispersal—related plant traits with insect frugivores that determine post—ripening fruit losses. The types of higher order interactions that result from the action of birds, the pulp—damaging fly, and the seed—eating moth larva may damp, or reverse, any selective effect of a single system component on plant traits related to seed dispersal.
Interspecific Competition and the Abundance of Nest Sites: Factors Affecting Sexual Selection
Authors: Breitburg Denise L.
Published: May 2006

The abundance of suitable nest sites and competition with other species for such sites appears to affect the intensity of intersexual selection in Coryphopterus nicholsi, a temperate goby that is a protogynous hermaphrodite. Field manipulations demonstrated that, within a population, the proportion of males that breed, and therefore the intensity of sexual selection, depends on the number of suitable nest rocks. The abundance of nest sites may also affect the timing of sex change in this species. Collections from areas with many suitable nest sites contained a higher proportion of males than collections from areas with few suitable nest sites. Aggression among males probably determines which individuals within the population can monopolize a suitable nest site and therefore are able to mate as a male. Field observations indicated that aggressive interactions occur frequently and that the larger individual of an interacting pair almost inevitably chases off the smaller individual during aggressive encounters. In field collections, males guarding eggs averaged larger in size than males not guarding eggs, and large males guarded nest sites that were superior to those guarded by small males. Competitors, including a large fish and several invertebrates, monopolize rocks otherwise suitable for Coryphopterus' use and thereby limit the number and proportion of breeding males in the goby's population. Field and laboratory observations and experiments indicated that a larger fish, Porichthys notatus, displaces Coryphopterus from nest sites and prevents Coryphopterus from using many of the large rocks on the reef. Other animals, including encrusting invertebrates and sea urchins, may also reduce the suitability and availability of rocks as nest sites. Such competition for nest sites reduces the opportunity of breeding by small males and increases the number of mates per large breeding male. Interspecific competition should, therefore, increase selection for protogynous hermaphroditism in Coryphopterus by increasing the reproductive advantage of large over small males. I discuss the possibility that interspecific competition may be an important selective force in resource—based mating systems of other animals and present potential examples from the literature.