Heyneman Amy J
Department of Zoology, University of California, 90027, Berkeley, CA, USA.
Oecologia. 1983 Nov;60(2):198-213. doi: 10.1007/BF00379522.
A model is developed to elucidate the determinants of sugar concentrations in flower nectars. This model analyses the efficiency of sugar intake, or energy flux, which for nectarivores closely approximates the rate of net energy gain. For both steady state and some non-steady flows of nectars, this energy flux is shown to be maximal at particular sugar concentrations referred to here as the maximum flux concentration. Higher concentrations actually yield lower energy intake rates because the concomitant rapid increase in viscosity sharply reduces the rate of fluid intake. For pure sucrose solutions, the maximum flux concentration is 22%. For flower nectars, which are chemically more complex, the maximum flux concentration is predicted to be closer to 26%, using the first viscosity measures obtained for flower nectars. This concentration is shown to be essentially independent of the pollinator's feeding organ morphology and of the type of potential inducing nectar flow. It is proposed that this concentration applies for virtually all pollinators that select nectars with maximal energy flux.However not all pollinators are expected to select such nectars because this 26% concentration is not necessarily "optimal". The model predicts that optimal sugar concentrations vary for particular pollinators as a function of two primary factors: (1) the energy flux derived from the nectar, as discussed above, as well as (2) the relative contribution of transit costs to overall foraging costs. Relatively "dilute" nectars, with sugar concentrations close to the maximal flux value, are predicted for flowers pollinated by organisms that minimize feeding time to reduce high feeding costs, such as that of hovering or of exposure to enhanced predation while feeding. More concentrated nectars are predicted for flowers pollinated by nectarivores that incur high foraging transit costs relative to feeding costs.Flowers pollinated by hovering pollinators, including many hummingbirds, hawkmoths and bats, have nectars with mean sugar concentrations in close accord with the 26% maximum flux concentration predicted. Moreover, these nectars have relatively low concentrations of nonsugar constituents, which increase viscosity and thereby decrease sugar flux. Over 75% of the flowers examined in this study, which are pollinated primarily by territorial hummingbird species, provide nectars that allow sugar uptake with an efficiency of 90% or greater of the maximal value. According to the model, these data suggest that feeding costs of these pollinators far outweigh foraging transit costs. In contrast, the model suggests that flower nectars taken by traplining hummingbirds and by bees, with sugar concentrations significantly above the maximum flux value, reflect the higher costs of foraging flight relative to costs of feeding for these pollinators.Increasing temperature decreases nectar viscosity, and thereby increases absolute nectar uptake rates sharply. This leads to a number of predictions regarding foraging behavior as well as flower location, orientation, and color. However, the maximum flux concentration is shown to be practically invariable over a wide range of temperatures-increasing by only 2% sugar from 10°C to 30°C. Thus, contrary to previous expectations, little change in average sugar concentrations of flowers pollinated by particular groups of nectarivores is expected from cooler to warmer regions.
建立了一个模型来阐明花蜜中糖浓度的决定因素。该模型分析了糖摄入效率或能量通量，对于食蜜动物来说，这与净能量获取率密切相关。对于花蜜的稳态和一些非稳态流动，这种能量通量在特定的糖浓度下显示为最大值，这里称为最大通量浓度。实际上，较高的浓度会导致较低的能量摄入率，因为随之而来的粘度快速增加会大幅降低液体摄入率。对于纯蔗糖溶液，最大通量浓度为22%。对于化学成分更复杂的花蜜，根据首次获得的花蜜粘度测量值，预计最大通量浓度更接近26%。该浓度基本上与传粉者的取食器官形态以及潜在诱导花蜜流动的类型无关。有人提出，这个浓度适用于几乎所有选择具有最大能量通量花蜜的传粉者。然而，并非所有传粉者都预计会选择这样的花蜜，因为这个26%的浓度不一定是“最优的”。该模型预测，特定传粉者的最优糖浓度会因两个主要因素而有所不同：（1）如上所述，来自花蜜的能量通量，以及（2）转运成本对总体觅食成本的相对贡献。对于由那些尽量减少取食时间以降低高取食成本（如悬停成本或取食时暴露于更高捕食风险的成本）的生物传粉的花朵，预计会有相对“稀释”的花蜜，其糖浓度接近最大通量值。对于相对于取食成本而言觅食转运成本较高的食蜜动物传粉的花朵，预计会有更浓缩的花蜜。由悬停传粉者传粉的花朵，包括许多蜂鸟、天蛾和蝙蝠，其花蜜的平均糖浓度与预测的26%最大通量浓度非常接近。此外，这些花蜜中非糖成分的浓度相对较低，非糖成分会增加粘度，从而降低糖通量。在这项研究中检查的超过75%的花朵主要由领地性蜂鸟物种传粉，这些花朵提供的花蜜能使糖的摄取效率达到最大值的90%或更高。根据该模型，这些数据表明这些传粉者的取食成本远高于觅食转运成本。相比之下，该模型表明，由游走蜂鸟和蜜蜂采集的花蜜，其糖浓度明显高于最大通量值，这反映了这些传粉者的觅食飞行成本相对于取食成本更高。温度升高会降低花蜜粘度，从而大幅提高花蜜的绝对摄取率。这导致了一些关于觅食行为以及花朵位置、方向和颜色的预测。然而，最大通量浓度在很宽的温度范围内实际上是不变的——从10°C到30°C，糖浓度仅增加2%。因此，与之前的预期相反，预计从较凉爽地区到较温暖地区，由特定食蜜动物群体传粉的花朵的平均糖浓度变化不大。
