Data from: Aggregation and a strong Allee effect in a cooperative outbreak insect

Most species that are negatively impacted when their densities are low aggregate to minimize this effect. Aggregation has the potential to change how Allee effects are expressed at the population level. We studied the interplay between aggregation and Allee effects in the mountain pine beetle (Dendroctonus ponderosae Hopkins), an irruptive bark beetle that aggregates to overcome tree defenses. By cooperating to surpass a critical number of attacks per tree, the mountain pine beetle is able to breach host defenses, oviposit and reproduce. Mountain pine beetles and Hymenopteran parasitoids share some biological features, the most notable of which is obligatory host death as a consequence of parasitoid attack and development. We developed spatiotemporal models of mountain pine beetle dynamics that were based on the Nicholson-Bailey framework but which featured beetle aggregation and a tree-level attack threshold. By fitting our models to data from a local mountain pine beetle outbreak, we demonstrate that due to aggregation, attack thresholds at the tree level can be overcome by a surprisingly low ratio of beetles per susceptible tree at the stand level. This results confirms the importance of considering aggregation in models of organisms that are subject to strong Allee effects.多数在种群密度较低时会遭受负面影响的物种，会通过聚集行为来削弱该效应。聚集行为可改变阿利效应（Allee effect）在种群水平上的表现形式。我们以山松甲虫（Dendroctonus ponderosae Hopkins）为研究对象，探讨了聚集行为与阿利效应之间的相互作用。该虫是一种爆发性树皮甲虫，通过聚集来突破寄主树木的防御机制。山松甲虫可通过协同作用，使单棵寄主树上的攻击数量达到临界阈值，从而得以突破寄主防御、产卵并完成繁殖。山松甲虫与膜翅目寄生蜂存在若干共通的生物学特性，其中最显著的一点是：寄生蜂的攻击与发育过程会导致寄主必然死亡。我们基于尼科尔森-贝利（Nicholson-Bailey）模型框架，构建了山松甲虫种群动态的时空模型，该模型纳入了甲虫聚集行为与单树攻击阈值两个核心特征。通过将模型拟合至某一地区山松甲虫爆发的实地观测数据，我们证实：得益于聚集行为，林分水平上每棵易感寄主树木所需的甲虫种群比例极低，即可突破单树攻击阈值。该研究结果证实，在受强阿利效应影响的生物种群模型中，纳入聚集行为因素具有重要意义。