Sagers C L, Ginger S M, Evans R D
Department of Biological Sciences and Arkansas Center for Environmental Research, University of Arkansas, Fayetteville, AR 72701, USA e-mail:
Oecologia. 2000 Jun;123(4):582-586. doi: 10.1007/PL00008863.
Mutualisms, biological interactions from which each species benefits, are thought to be evolutionarily stable only under a limited set of circumstances. Underlying all mutualisms is an intrinsic conflict between the parties, in that each is under selection to further exploit the other. Conflict between mutualists will lead to instability unless each species receives a net benefit from the interaction. To understand how mutualisms persist, then, it is essential to document the exact nature of the interaction. The relationship between Cecropia trees and Azteca ants has been a model for the study of mutualism since its description more than 100 years ago. Ants live in Cecropia's hollow stems and harvest specialized food bodies produced by the host. In return, ants defend the plant from leaf-feeding herbivores and encroaching vegetation. Central to modeling this relationship is understanding the exchange of nutrients between symbionts. Here we present evidence that the flow of nutrients has been misjudged. Although ants consume plant products, they provide more nutrients than they receive. Using stable isotope analysis, we calculate that only about 18% of worker ant carbon is derived from Cecropia, whereas 93% of the nitrogen in ant-occupied host plants is derived from debris deposited by ants. Ants rely on sources other than their host for food and, in doing so, make a rich source of nitrogen available to the host.
互利共生是指每个物种都能从中受益的生物相互作用，人们认为它只有在有限的一组情况下才在进化上是稳定的。所有互利共生关系的背后，各方之间都存在内在冲突，因为每个物种都在选择进一步利用对方。互利共生者之间的冲突将导致不稳定，除非每个物种都能从这种相互作用中获得净收益。因此，要理解互利共生关系是如何持续存在的，就必须记录这种相互作用的确切性质。自100多年前被描述以来，塞克罗皮亚树和阿兹特克蚂蚁之间的关系一直是互利共生研究的一个模型。蚂蚁生活在塞克罗皮亚树中空的树干里，收获宿主产生的特殊食物体。作为回报，蚂蚁保护植物免受食叶食草动物和入侵植被的侵害。对这种关系进行建模的核心是理解共生体之间的营养交换。在这里，我们提供证据表明营养物质的流动被误判了。虽然蚂蚁消耗植物产品，但它们提供的营养比它们获得的更多。通过稳定同位素分析，我们计算出只有约18%的工蚁碳来自塞克罗皮亚树，而被蚂蚁占据的宿主植物中93%的氮来自蚂蚁沉积的残骸。蚂蚁依靠宿主以外的来源获取食物，这样一来，它们就为宿主提供了丰富的氮源。
