Orivel Jérôme, Malé Pierre-Jean, Lauth Jérémie, Roux Olivier, Petitclerc Frédéric, Dejean Alain, Leroy Céline
CNRS, UMR Ecologie des Forêts de Guyane, AgroParisTech, CIRAD, INRA, Université de Guyane, Université des Antilles, Campus Agronomique, BP 316, 97379 Kourou Cedex, France
UMR Evolution et Diversité Biologique, Université de Toulouse, 118 Route de Narbonne, 31062 Toulouse Cedex 9, France.
Proc Biol Sci. 2017 Mar 15;284(1850). doi: 10.1098/rspb.2016.1679.
Species engaged in multiple, simultaneous mutualisms are subject to trade-offs in their mutualistic investment if the traits involved in each interaction are overlapping, which can lead to conflicts and affect the longevity of these associations. We investigate this issue via a tripartite mutualism involving an ant plant, two competing ant species and a fungus the ants cultivate to build galleries under the stems of their host plant to capture insect prey. The use of the galleries represents an innovative prey capture strategy compared with the more typical strategy of foraging on leaves. However, because of a limited worker force in their colonies, the prey capture behaviour of the ants results in a trade-off between plant protection (i.e. the ants patrol the foliage and attack intruders including herbivores) and ambushing prey in the galleries, which has a cascading effect on the fitness of all of the partners. The quantification of partners' traits and effects showed that the two ant species differed in their mutualistic investment. Less investment in the galleries (i.e. in fungal cultivation) translated into more benefits for the plant in terms of less herbivory and higher growth rates and vice versa. However, the greater vegetative growth of the plants did not produce a positive fitness effect for the better mutualistic ant species in terms of colony size and production of sexuals nor was the mutualist compensated by the wider dispersal of its queens. As a consequence, although the better ant mutualist is the one that provides more benefits to its host plant, its lower host-plant exploitation does not give this ant species a competitive advantage. The local coexistence of the ant species is thus fleeting and should eventually lead to the exclusion of the less competitive species.
如果参与每种相互作用的性状相互重叠，那么参与多种同时进行的互利共生的物种在其互利共生投资方面会面临权衡，这可能导致冲突并影响这些共生关系的持久性。我们通过一种三方互利共生关系来研究这个问题，这种关系涉及一种蚁栖植物、两种竞争的蚂蚁物种以及一种真菌，蚂蚁培育这种真菌以在其寄主植物的茎干下建造廊道来捕获昆虫猎物。与在叶子上觅食这种更典型的策略相比，利用廊道是一种创新的猎物捕获策略。然而，由于蚁群中的工蚁数量有限，蚂蚁的猎物捕获行为导致在植物保护（即蚂蚁在叶片上巡逻并攻击包括食草动物在内的入侵者）和在廊道中伏击猎物之间进行权衡，这对所有共生伙伴的适合度产生连锁反应。对共生伙伴的性状和影响进行量化表明，这两种蚂蚁物种在互利共生投资方面存在差异。对廊道（即真菌培育）投入较少转化为植物在较少食草和较高生长速率方面获得更多益处，反之亦然。然而，植物更大的营养生长在蚁群大小和有性个体产生方面并没有对互利共生关系更好的蚂蚁物种产生积极的适合度效应，其蚁后更广泛的扩散也没有补偿这种共生关系。因此，尽管更好的蚂蚁共生者是为其寄主植物提供更多益处的一方，但其对寄主植物较低的利用并没有赋予该蚂蚁物种竞争优势。因此，这两种蚂蚁物种的局部共存是短暂的，最终应该会导致竞争力较弱物种的排除。
