Department of Botany, La Trobe University, Bundoora, Victoria 3086, Australia.
Ecology. 2011 Sep;92(9):1758-68. doi: 10.1890/11-0050.1.
In multiply invaded ecosystems, introduced species should interact with each other as well as with native species. Invader-invader interactions may affect the success of further invaders by altering attributes of recipient communities and propagule pressure. The invasional meltdown hypothesis (IMH) posits that positive interactions among invaders initiate positive population-level feedback that intensifies impacts and promotes secondary invasions. IMH remains controversial: few studies show feedback between invaders that amplifies their effects, and none yet demonstrate facilitation of entry and spread of secondary invaders. Our results show that supercolonies of an alien ant, promoted by mutualism with introduced honeydew-secreting scale insects, permitted invasion by an exotic land snail on Christmas Island, Indian Ocean. Modeling of land snail spread over 750 sites across 135 km2 over seven years showed that the probability of land snail invasion was facilitated 253-fold in ant supercolonies but impeded in intact forest where predaceous native land crabs remained abundant. Land snail occurrence at neighboring sites, a measure of propagule pressure, also promoted land snail spread. Site comparisons and experiments revealed that ant supercolonies, by killing land crabs but not land snails, disrupted biotic resistance and provided enemy-free space. Predation pressure on land snails was lower (28.6%), survival 115 times longer, and abundance 20-fold greater in supercolonies than in intact forest. Whole-ecosystem suppression of supercolonies reversed the probability of land snail invasion by allowing recolonization of land crabs; land snails were much less likely (0.79%) to invade sites where supercolonies were suppressed than where they remained intact. Our results provide strong empirical evidence for IMH by demonstrating that mutualism between invaders reconfigures key interactions in the recipient community. This facilitates entry of secondary invaders and elevates propagule pressure, propagating their spread at the whole-ecosystem level. We show that identification and management of key facilitative interactions in invaded ecosystems can be used to reverse impacts and restore resistance to further invasions.
在多次入侵的生态系统中，引入物种不仅会与本地物种相互作用，还会与其他引入物种相互作用。入侵物种之间的相互作用可能会通过改变受体群落的属性和繁殖体压力来影响进一步入侵物种的成功。入侵性崩溃假说（IMH）认为，入侵者之间的正相互作用会引发正的种群水平反馈，从而加剧影响并促进二次入侵。IMH 仍然存在争议：很少有研究表明入侵者之间的反馈会放大它们的影响，而且还没有研究证明有助于二次入侵物种的进入和传播。我们的研究结果表明，在印度洋圣诞岛上，一种外来蚂蚁的超级群落由于与引入的蜜露分泌介壳虫互利共生而得到促进，从而允许一种外来陆地蜗牛入侵。经过七年对 135 平方公里范围内 750 个地点的陆地蜗牛传播的建模显示，在蚂蚁超级群落中，陆地蜗牛入侵的可能性增加了 253 倍，但在完整的森林中则受到阻碍，那里仍然存在大量的掠食性本地陆地蟹。邻近地点的陆地蜗牛出现情况（繁殖体压力的一种衡量标准）也促进了陆地蜗牛的传播。通过杀死陆地蟹但不杀死陆地蜗牛，地点比较和实验揭示了蚂蚁超级群落破坏了生物抗性并提供了无天敌的空间。与完整森林相比，在超级群落中，陆地蜗牛的捕食压力更低（28.6%），存活率延长 115 倍，丰度增加 20 倍。整个生态系统对超级群落的抑制作用通过允许陆地蟹重新定居来改变陆地蜗牛入侵的可能性；与超级群落保持完整的地方相比，超级群落被抑制的地方，陆地蜗牛入侵的可能性要小得多（0.79%）。我们的研究结果通过证明入侵物种之间的互利关系重新配置了受体群落中的关键相互作用，为 IMH 提供了强有力的经验证据。这有助于二次入侵物种的进入，并增加繁殖体压力，从而在整个生态系统水平上传播它们的传播。我们表明，识别和管理入侵生态系统中的关键促进相互作用可以用于逆转影响并恢复对进一步入侵的抗性。
