In a tropical cloud forest at Monteverde, Costa Rica, three understory shrub species overlap greatly in flowering seasons and share hummingbird pollinators (Lampornis calolaema). We conducted two field experiments to determine if a plant's pollination and subsequent reproductive output reflect local densities of conspecific and heterospecific flowers. We controlled floral composition in the neighborhoods surrounding 12 focal plants of self—compatible Besleria triflora (Gesneriaceae) and of self—incompatible Palicourea lasiorrachis (Rubiacaea), producing prescribed mixtures of flowers of the focal plants' conspecifics and of Cephaelis elata (Rubiaceae). Each experiment subjected each focal plant to four different treatments; a floral neighborhood of 10 conspecific flowers and 10 of Cephaelis within a radius of 20 m (Treatment A); 90:10 (B); 10:90 (C); and 50:50 (D). Focal plants themselves always had 10 open flowers. We assigned the sequence of treatments to each focal plant in a Latin Square crossover design using three balanced blocks of four plants each. Dependent variables were: frequency of hummingbird probes per flower; pollen tubes in styles; and seed output per flower. From standard models of competition for pollination, we predicted that pollination levels and seed production would be ranked among treatments: B(90:10) > D(50:50) > A(10:10) > C(10:90). These overall predictions were supported for both Palicourea and Besleria, based on tests for directionality showing that the rank order among adjusted treatment means did not differ from the order predicted. Separate a priori treatment contrasts indicated that in both species increasing flower density above base levels (Treatment A, 10:10) by adding conspecific flowers alone (B, 90:10) or along with Cephaelis (D, 50:50) increased the rate at which hummingbirds visited focal plants' flowers. In Palicourea, both the number of pollen tubes in styles and seed output per flower increased in the same fashion. In Besleria, the only effect was increased pollination (but not seed output) from A (10:10) to B (90:10). In contrast, enriching the basic neighborhood (A. 10:10) with Cephaelis flowers (C, 10:90) did not alter frequencies of hummingbird probes to either species of focal plant. Heterospecific enrichment did, however, decrease both pollination levels and seed output in Palicourea, and seed output in Besleria. In general, altering the floral neighborhood affected Palicourea more strongly than Besleria. Apparently the intensity of density dependence, at least in terms of female reproductive success, varies among plant species sharing pollinators. Seed outputs in self—incompatible plants such as Palicourea may closely reflect flower densities of neighbors, but interspecific effects on fertilization in self—compatible plants such as Besleria may be density vague.

At Monteverde, Costa Rica, the plants Besleria triflora (Gesneriaceae) palicourea lasiorrachis, and Cephaelis elata (Both Rubiaceae) share hummingbirds (Lampornis calolaema) as pollinators. To investigate the effect that intervening visits to Cephaelis flowers have on pollen transfer between plants of self—compatible Besleria or of self—in—compatible Palicourea, we presented captive Lampornis with series of hand—held flowers: first, 2 or 15 flowers of either Besleria or Palicourea as pollen donors, followed by 0, 2, or 10 Cephaelis flowers, followed by 20 recipient flowers of the first species. In protandrous Besleria, we used male—phase flowers as donors and female—phase flowers as recipients. In distylous Palicourea, we used pin (long—styled) flowers as donors and thrum (short—styled) flowers as recipients. We assessed pollen transfer by counting pollen tubes in recipients' styles and, in Besleria, by counting pollen grains deposited on stigmas. Intervention of Cephaelis flowers strongly reduced pollen transfer among Palicourea flowers. Increasing the number of donor Palicourea flowers significantly increased the absolute amount of pollen transferred but did not overcome the negative effect of Cephaelis. Pollen transfer among Besleria flowers also declined with the intervention of Cephaelis flowers, the decline was less strongly significant than for Palicourea, perhaps because pollen loads on stigmas of some female Besleria flowers can contain self pollen left over from their male phase. Under controlled conditions, then, strong competition may exist where plants share pollinator individuals, a situation faced by the experimental species and many others at Monteverde. Other studies on these plants reveal that despite its demonstrability in the laboratory, however, competition among individual plants varies in its ability to explain density dependence in pollination among plant populations in the field. Furthermore, community—level patterns of coexistence among plant species at Monteverde feature few "ghosts of competition past." The collective results demonstrate that studying the role of particular interactions in community structure must involve examinations of mechanisms at the level of individuals, processes at the population level, and patterns of coexistence rather than just one level of this hierarchy. Studying mechanisms alone could exaggerate their importance to community structure, whereas studying coexistence patterns along could misjudge the strength of interactions among individuals.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.Abstract. Because hummingbirds are extremely sensitive to energy stress, yet often face conditions when energy intake is reduced or energy expenditures must be increased, they should have flexible energy budgets. I experimentally measured the ability of two syntopic tropical hummingbird species to manage daytime energy budgets under conditions simulating natural foraging constraint, defined here as low nectar secretion rate, low flower density, or both. I tested a territorial species, Amazilia saucerottei, and a traplining species, Chlorostilbon canivetii, individually in a large flight cage at two rates of food delivery (HIGH = ad libitum, LOW = 64% of ad libitum) and two perch-to-feeder distances (NEAR = 4 m, FAR = 20 m). Both species increased flight time when distance was increased (FAR > NEAR by 44%) and when rate of food delivery was reduced (LOW > HIGH by 23%). However, birds on the HIGH food delivery rate did not increase food intake to compensate for increased flight expenditures to the FAR feeder. Birds on LOW food maintained total rates of energy expenditure that were less than birds on HIGH food, and LOW food birds experienced no effect of feeder distance on expenditure. These results appear to be due primarily to short-term reductions in perching metabolic rates (PMRs) by birds on LOW food or with the FAR feeder. Reduced daytime PMRs helped to mitigate the potentially negative impacts of foraging constraint: rates of mass loss of the LOW food birds were an estimated 48% less than if birds had maintained normal PMRs. Reduced PMRs also enabled birds visiting the HIGH FAR feeder to maintain rates of mass gain equal to unconstrained birds (HIGH NEAR feeder). Relative differences between species in energy management reflected their foraging modes. The low-reward trapliner paid the greatest energy costs at LOW food, regardless of distance, due to its overall tendency for high flight time. In contrast, the territorialist did poorly in the HIGH FAR treatment, due to flight expenditures associated with defending a rich but dispersed resource. Although individual birds exhibited considerable flexibility in energy management, the foraging mode represented by each species appeared to be energetically specialized for particular levels of food availability and dispersion.

The ecological risks of pesttcldes to wildlife are assessed by the United States Environmental Protection Agency with a simple quotient model. A risk factor is calculated by dividing the estimated dietary concentration by the chemical's dietary toxicity. The risk factor, which can be used to predict population mortality, serves as a regulatory criterion to identify compounds that could pose unacceptable risk. Under the "New Paradigm," the Environmental Protection Agency recently cut by 50% the critical risk factor, yielding more-rigorous decision-making criteria by decreasing acceptable wildlife mortality from 50 to 8.8% following one pesticide application and by increasing the number of pesticides that pose unacceptable risk to wildlife. The quotient model depends on at least one implicit and approximately 11 explicit assumptions; deviations from these assumptions may bias outputs and consequent regulatory decisions. To correct error-prone assumptions we present three modifications that improve ecological risk estimates. We used the agency's quotient model to survey risks to a regional avtfauna (Florida birds) in relation to mosquito pesticide exposure (fenthion, malathion, and naled). Certain groups (small-bodied, insectivorous passerines) were predicted to face the highest ecological risk due to taxonomy, diet, and body mass. Finally, we performed a risk assessment for Black-whiskered Vireos (Vireo altiloquus barbatulus) exposed to fenthion. The results revealed that this species could potentially suffer 42% mortality in a population exposed to one application of fenthion. Ecological risk assessments increasingly will be based on mathematical models. To improve predictions of the effects of xenobiotic compounds on wildlife, biologists--especially conservationists--should work with ecotoxtcologists to improve current models, develop new models, and provide data to parameterize models. Equally important, conservationists should participate in defining regulatory levels of ecological risk for wildlife.Evaluaci6n del riesgo ecol6gico de los pesticidas tyara los vertebrados terrestres: evaluaci6n y aplicaci6n del modelo del cociente de USEPA.Resumen: Los rlesgos ecol6gicos de los pesticidas para la fauna silvestre son evaluados por la Agencla de Protecct6n Ambiental de los Estados Untdos con un simple modelo de coctente. Se calcula un factor de riesgo dividiendo la concentraci6n dietdtica esttmada por la toxladad dletc~tica qufmica. El factor de rlesgo que puede ser usado para predecir la mortalidad poblactonal, sirve como un crtterlo regulador para identtflcar compuestos que podrfan representar rlesgos tnaceptables. La Agenaa de Protecct6n Ambtental reclentemente ha reducido en un 50% el factor de rtesgo crftico, creando un criterio rods riguroso para la toma de decistones al disminuir la mortalidad aceptable de la fauna silvestre, despuds de una aplicact6n de~ pesticida del 50% al 8.8% y al aumentar el namero de pesticidas que representan riesgos inaceptables para la vtda sllvestre. El modelo del cociente depende...

No abstract

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.

No abstract

No abstract