Okland Bjørn, Bjørnstad Ottar N
Norwegian Forest Research Institute, As, Norway.
Ecology. 2006 Feb;87(2):283-90. doi: 10.1890/05-0135.
Detailed analyses of thresholded ecological interactions can improve our understanding of the transition from aperiodic to periodic dynamics. We develop a threshold model of the population dynamics of outbreaking bark beetle populations that alternate between non-epidemic and epidemic behavior. The model involves accumulation of resources during low-density periods and depletion during outbreaks. The transition between the two regimes is caused by disturbance events in the form of major tree felling by wind. The model is analyzed with particular reference to the population dynamics of the spruce bark beetle (Ips typographus) in Scandinavia for which a comprehensive literature allows full parameterization. The fairly constant outbreak lengths and the highly variable waiting time between outbreaks that are seen in the historical records of this species agree well with the predictions of the model. The thresholded resource-depletion dynamics result in substantial variation in the degree of periodicity between stochastic realizations. The completely aperiodic tree colonizations are partly predictable when the timing of the irregular windfall events are known. However, the predictability of inter-outbreak periods is low due to the large variation of cases falling most frequently in the middle between the extremes of purely nonperiodic (erratic) and periodic (cyclic) fluctuations.
对阈值化生态相互作用的详细分析可以增进我们对从非周期性动态向周期性动态转变的理解。我们建立了一个爆发性树皮甲虫种群动态的阈值模型，该种群在非流行和流行行为之间交替。该模型涉及低密度时期资源的积累和爆发期间资源的消耗。两种状态之间的转变是由风导致的大树倒伏形式的干扰事件引起的。我们特别参照斯堪的纳维亚云杉树皮甲虫（Ips typographus）的种群动态对该模型进行了分析，针对该物种有全面的文献资料可供进行完整的参数化。该物种历史记录中呈现出的爆发长度相当恒定以及爆发之间等待时间高度可变的情况与模型的预测非常吻合。阈值化的资源消耗动态导致随机实现之间周期性程度的显著变化。当不规则风倒事件的时间已知时，完全非周期性的树木定殖在一定程度上是可预测的。然而，由于在纯非周期性（无规律）和周期性（循环）波动这两个极端之间最常出现的情况变化很大，爆发间期的可预测性较低。
