Bedoya-Pérez Miguel A, Issa Daniel D, Banks Peter B, McArthur Clare
School of Biological Sciences, The University of Sydney, Sydney, NSW, 2006, Australia,
Oecologia. 2014 Aug;175(4):1167-77. doi: 10.1007/s00442-014-2980-0. Epub 2014 Jun 13.
While trying to achieve their nutritional requirements, foraging herbivores face the costs of plant defenses, such as toxins. Teasing apart the costs and benefits of various chemical constituents in plants is difficult because their chemical defenses and nutrient concentrations often co-vary. We used an approach derived from predator-prey studies to quantitatively compare the foraging response of a free-ranging mammalian herbivore, the swamp wallaby (Wallabia bicolor), through three feeding trials with artificial diets that differed in their concentrations of (1) the terpene 1,8-cineole, (2) primary constituents (including nitrogen and fiber), and (3) both the terpene and the primary constituents. Applying the giving-up density (GUD) framework, we demonstrated that the foraging cost of food patches increases with higher dietary cineole concentration and decreases with higher dietary nutrient concentration. The effect of combined differences in nutrients and cineole concentrations on GUD was interactive, and high nutrient food required more cineole to achieve the same patch value as low nutrient food. Our results indicate that swamp wallabies equate low nutrient, poorly defended food with high nutrient, highly defended food, providing two contrasting diets with similar cost-benefit outcomes. This behavior suggests that equal concentrations of chemical defenses provide nutrient-poor plants with relatively greater protection as nutrient-rich plants. Nutrient-rich plants may therefore face the exacerbated problem of being preferred by herbivores and therefore need to produce more defense compounds to achieve the same level of defense as nutrient-poor plants. Our findings help explain the difference in anti-herbivore strategy of nutrient-poor and rich plants, i.e., tolerance versus defense.
在努力满足其营养需求的过程中，觅食的食草动物面临着植物防御的代价，比如毒素。区分植物中各种化学成分的成本和益处很困难，因为它们的化学防御和营养浓度常常共同变化。我们采用了一种源自捕食者 - 猎物研究的方法，通过三项喂食试验，使用人工饲料定量比较自由放养的哺乳动物食草动物——沼泽小袋鼠（Wallabia bicolor）的觅食反应，这些人工饲料在以下方面的浓度有所不同：（1）萜烯1,8 - 桉叶素；（2）主要成分（包括氮和纤维）；（3）萜烯和主要成分两者。应用放弃密度（GUD）框架，我们证明了食物斑块的觅食成本随着饲料中桉叶素浓度的升高而增加，随着饲料中营养浓度的升高而降低。营养成分和桉叶素浓度的综合差异对GUD的影响是相互作用的，高营养食物需要更多的桉叶素才能达到与低营养食物相同的斑块价值。我们的结果表明，沼泽小袋鼠将低营养、防御性差的食物等同于高营养、防御性强的食物，提供了两种具有相似成本效益结果的对比性饮食。这种行为表明，相同浓度的化学防御为营养贫瘠的植物提供了相对于营养丰富的植物更大的保护。因此，营养丰富的植物可能面临被食草动物偏好的更严重问题，因此需要产生更多的防御化合物以达到与营养贫瘠的植物相同的防御水平。我们的发现有助于解释营养贫瘠和丰富的植物在抗食草动物策略上的差异，即耐受与防御。
