Weis Arthur E, Kapelinski Audrey
Department of Ecology and Evolutionary Biology, University of California, Irvine, California, 92717.
Department of Social Services, Children's Hospital of Orange County, Orange, California, 92668.
Evolution. 1994 Jun;48(3):734-745. doi: 10.1111/j.1558-5646.1994.tb01357.x.
We examined phenotypic selection exerted by natural enemies on the gall-making fly Eurosta solidaginis in an extensive field study of 16 populations, spanning four generations. Gall-makers that induce small galls are vulnerable to the attack of Eurytoma gigantea. This imposes upward directional selection on gall size. Insectivorous birds, predominantly the downy woodpecker, are more likely to attack larvae that induce large galls than small ones, and this imposes downward directional selection. We used path analysis to explore the relative contributions of these natural enemies to the net directional selection on gall size. The path models further examined several ecological factors that influence selection intensity through their effects on parasitoid and bird attack rates. Net directional selection varied more strongly with E. gigantea attack than bird attack. Competitive interactions among birds and the three parasitoid species, including E. gigantea, were evidenced by low winter bird attack rates in fields where a high proportion of galls contained the overwintering parasitoids. Eurytoma gigantea attack was heavier in fields where mean gall size was small and bird attack heavier in fields where mean gall size was large. Neither birds nor E. gigantea showed simple density-dependent attack. Data suggested a form of frequency-dependent attack by birds but not by E. gigantea.
在一项涵盖四代、涉及16个种群的广泛田间研究中，我们考察了天敌对造瘿蝇Eurosta solidaginis施加的表型选择。诱导形成小瘿的造瘿蝇易受巨瘤瘿蜂（Eurytoma gigantea）的攻击。这对瘿的大小施加了向上的定向选择。食虫鸟类，主要是绒啄木鸟，攻击诱导形成大瘿的幼虫的可能性大于小瘿幼虫，这施加了向下的定向选择。我们使用通径分析来探究这些天敌对瘿大小净定向选择的相对贡献。通径模型进一步考察了几个生态因素，这些因素通过对寄生蜂和鸟类攻击率的影响来影响选择强度。与鸟类攻击相比，净定向选择随巨瘤瘿蜂攻击的变化更为强烈。在高比例瘿含有越冬寄生蜂的田间，冬季鸟类攻击率较低，这证明了鸟类与包括巨瘤瘿蜂在内的三种寄生蜂之间存在竞争相互作用。在平均瘿大小较小的田间，巨瘤瘿蜂的攻击更严重；在平均瘿大小较大的田间，鸟类攻击更严重。鸟类和巨瘤瘿蜂均未表现出简单的密度依赖性攻击。数据表明鸟类存在一种频率依赖性攻击形式，而巨瘤瘿蜂则没有。
