Invasion Dynamics of a Fish-Free Landscape by Brown Trout (Salmo trutta)

Metapopulation dynamics over the course of an invasion are usually difficult to grasp because they require large and reliable data collection, often unavailable. The invasion of the fish-free freshwater ecosystems of the remote sub-Antarctic Kerguelen Islands following man-made introductions of brown trout (Salmo trutta) in the 1950's is an exception to this rule. Benefiting from a full long term environmental research monitoring of the invasion, we built a Bayesian dynamic metapopulation model to analyze the invasion dynamics of 85 river systems over 51 years. The model accounted for patch size (river length and connections to lakes), alternative dispersal pathways between rivers, temporal trends in dynamics, and uncertainty in colonization date. The results show that the model correctly represents the observed pattern of invasion, especially if we assume a coastal dispersal pathway between patches. Landscape attributes such as patch size influenced the colonization function, but had no effect on propagule pressure. Independently from patch size and distance between patches, propagule pressure and colonization function were not constant through time. Propagule pressure increased over the course of colonization, whereas the colonization function decreased, conditional on propagule pressure. The resulting pattern of this antagonistic interplay is an initial rapid invasion phase followed by a strong decrease in the invasion rate. These temporal trends may be due to either adaptive processes or environmental gradients encountered along the colonization front. It was not possible to distinguish these two hypotheses. Because invasibility of Kerguelen Is. freshwater ecosystems is very high due to the lack of a pre-existing fish fauna and minimal human interference, our estimates of invasion dynamics represent a blueprint for the potential of brown trout invasiveness in pristine environments. Our conclusions shed light on the future of polar regions where, because of climate change, fish-free ecosystems become increasingly accessible to invasion by fish species.入侵过程中的集合种群（metapopulation）动态通常难以厘清，因其依赖大规模且可靠的数据采集工作，而此类数据往往难以获取。1950年代人为引种褐鳟（Salmo trutta）后，偏远的亚南极凯尔盖朗群岛内无本土鱼类的淡水生态系统遭到入侵，该事件则是这一规律的例外。得益于针对此次入侵的完整长期环境监测研究项目，我们构建了贝叶斯动态集合种群模型，用以分析51年间85个河流系统的入侵动态。该模型纳入了生境斑块（patch）面积（包括河流长度及与湖泊的连通性）、河流间的多种扩散路径、种群动态的时间变化趋势，以及定殖日期的不确定性。研究结果表明，该模型可准确复现观测到的入侵模式，尤其是当我们假设生境斑块间存在沿海扩散路径时。诸如生境斑块面积的景观属性会影响定殖功能，但对繁殖体压力（propagule pressure）无显著影响。不依赖于生境斑块面积及斑块间距离的前提下，繁殖体压力与定殖功能均不随时间保持恒定。在定殖过程中，繁殖体压力逐渐升高，而定殖功能则在受繁殖体压力约束的情况下逐渐降低。这种拮抗互作所形成的动态模式表现为：初始阶段为快速入侵期，随后入侵速率显著下降。上述时间趋势可能源于定殖前沿的适应性过程或环境梯度，但无法区分这两种假说。由于凯尔盖朗群岛的淡水生态系统原本缺乏鱼类区系，且人类干扰极少，因此其可入侵性极高；本研究得到的入侵动态估算结果，可作为褐鳟在原始未受干扰环境中潜在入侵能力的参照范本。本研究结论可为极地地区的未来发展提供启示：受气候变化影响，无鱼类生态系统正愈发容易遭受鱼类物种的入侵。