Riihimäki Janne, Kaitaniemi Pekka, Koricheva Julia, Vehviläinen Harri
Section of Ecology, Department of Biology, University of Turku, 20014 Turku, Finland.
Oecologia. 2005 Jan;142(1):90-7. doi: 10.1007/s00442-004-1696-y. Epub 2004 Aug 19.
Numerous studies conducted in agro-ecosystems support the enemies hypothesis, which states that predators and parasites are more efficient in controlling pest densities in polycultures than in monocultures. Few similar studies, however, have been conducted in forest ecosystems, and we do not yet have evidence as to whether the enemies hypothesis holds true in forests. In a 2-year study, we investigated whether the survival of autumnal moth ( Epirrita autumnata) larvae and pupae differs between silver birch monocultures and two-species mixtures of birch with black alder, Norway spruce and Scots pine. We placed young larvae on birch saplings and monitored their survival until the end of the larval period, when we checked whether they had been parasitized. After the larvae had pupated, pupal survival was tested in a field trial. In 2002, the larvae disappeared earlier and their overall survival was lower in birch-pine mixtures than in other stand types. In 2003, survival probability was lowest in birch-pine stands only during the first week and there were no differences between stands in overall survival. Larval parasitism was not affected by tree species composition. Pupal weight and pupal survival were likewise not affected by stand type. Among the predators, wood ants were more abundant on birches growing in birch-pine mixtures than in other stand types probably because colonies of myrmecophilic aphids were common on pines. In contrast, spider numbers did not differ between stand types. Ant exclusion by means of a glue ring around the birch trunk increased larval survival, indicating that ants are important predators of the autumnal moth larvae; differences in larval survival between stands are probably due to differential ant predation. Our results provide only partial support for the enemies hypothesis, and suggest that it is both tree species composition and species diversity which affect herbivore survival and predation.
在农业生态系统中进行的大量研究支持“天敌假说”，该假说认为，与单一栽培相比，捕食者和寄生虫在混合作物栽培中控制害虫密度的效率更高。然而，在森林生态系统中进行的类似研究较少，而且我们尚未有证据证明“天敌假说”在森林中是否成立。在一项为期两年的研究中，我们调查了秋季蛾（Epirrita autumnata）幼虫和蛹在白桦单一栽培以及白桦与黑桤木、挪威云杉和苏格兰松的两种树种混合林中的存活率是否存在差异。我们将幼虫放置在白桦树苗上，并监测它们的存活情况直至幼虫期结束，届时我们检查它们是否被寄生。幼虫化蛹后，在田间试验中测试蛹的存活率。2002年，白桦 - 松树混合林中的幼虫消失得更早，其总体存活率低于其他林分类型。2003年，仅在第一周白桦 - 松树林分中的存活概率最低，各林分在总体存活率上没有差异。幼虫的寄生情况不受树种组成的影响。蛹的重量和蛹的存活率同样不受林分类型的影响。在捕食者中，白桦 - 松树混合林中生长的白桦树上的木蚁比其他林分类型中的更为丰富，这可能是因为嗜蚁蚜虫的群体在松树上很常见。相比之下，不同林分类型之间蜘蛛数量没有差异。通过在白桦树干周围设置胶环来排除蚂蚁，增加了幼虫的存活率，这表明蚂蚁是秋季蛾幼虫的重要捕食者；不同林分之间幼虫存活率的差异可能是由于蚂蚁捕食的差异。我们的结果仅为“天敌假说”提供了部分支持，并表明影响食草动物存活和捕食的因素既有树种组成又有物种多样性。
