Facilitation of tiger moths by outbreaking tussock moths that share the same host plants

Ecologists have argued about the commonness and strength of interspecific competition between insect herbivores, but facilitation between herbivores has received much less consideration. We previously found that when two species of folivorous caterpillars co-occurred on a shared host plant, feeding by early season tiger moth caterpillars reduced the growth and reproduction of later season tussock caterpillars. However, densities of tussock caterpillars in summer were positively correlated with densities of tiger moth caterpillars the following spring. 2. In this study, we experimentally manipulated numbers of feeding tussock caterpillars and found that they facilitated tiger moth caterpillars. 3. The depth of the litter layer beneath host lupine bushes was positively correlated with the number of tussock caterpillars feeding on each bush. Experimental additions of litter beneath lupine canopies during summer resulted in increased numbers of tiger moth caterpillars in the following spring, indicating a causal role of litter. Litter potentially provides food, habitat and protection from desiccation and predation. We failed to find evidence that tussock caterpillars facilitated tiger moth caterpillars by mechanisms independent of litter. 4. Our study demonstrates that facilitation may operate between insect herbivores, across life-stages through indirect interactions that are non-trophic. Facilitation operated by a novel mechanism, the accumulation of litter which was a by-product of feeding by one species was valuable to a second species. Facilitation persisted in time and space far beyond the creation of litter by tussock caterpillars which should be considered important ecosystem engineers from the point of view of tiger moths. Facilitations that involve habitat modification may generally connect species that do not interact directly or trophically, and have not previously been considered to affect one another. Journal of animal ecology (2012) volume: 81 issue: 5 page: 1095-1102生态学界曾围绕昆虫植食者（herbivores）间种间竞争（interspecific competition）的普遍性与强度展开过广泛争论，但植食者之间的促进作用（facilitation）却鲜有关注。此前我们的研究发现，当两种食叶幼虫（folivorous caterpillars）共享同一种寄主植物（host plant）时，早季灯蛾幼虫（tiger moth caterpillars）的取食会抑制晚季丛毒蛾幼虫（tussock caterpillars）的生长与繁殖。但夏季丛毒蛾幼虫的种群密度，与次年春季灯蛾幼虫的种群密度呈正相关。 2. 本研究中，我们通过实验调控取食的丛毒蛾幼虫种群数量，结果发现丛毒蛾幼虫可对灯蛾幼虫产生促进作用。 3. 寄主羽扇豆（lupine）灌丛下方的枯落物层（litter layer）厚度，与每丛灌丛上取食的丛毒蛾幼虫数量呈正相关。夏季在羽扇豆冠层下方人工添加枯落物，会导致次年春季灯蛾幼虫的种群数量增加，这表明枯落物发挥了因果性作用。枯落物可为幼虫提供食物、栖息场所，并帮助其抵御干燥与捕食者。我们未发现证据表明，丛毒蛾幼虫可通过不依赖枯落物的其他机制对灯蛾幼虫产生促进作用。 4. 本研究证实，昆虫植食者之间可通过非营养型间接相互作用，跨生活史阶段产生促进作用。本次发现的促进作用由一种全新机制介导：某一物种取食产生的副产物——枯落物的积累，对另一物种具有生存价值。这种促进作用在时间与空间上的存续，远超丛毒蛾幼虫产生枯落物的周期；从灯蛾的视角来看，丛毒蛾可被视为重要的生态系统工程师。涉及生境改造的促进作用，通常可连接原本不存在直接相互作用或营养级相互作用的物种，而这类物种间的相互影响此前从未被关注。 《动物生态学杂志》(Journal of Animal Ecology) 2012年，第81卷第5期，第1095-1102页