Data from: Depletion of heterogeneous source species pools predicts future invasion rates

Predicting how increasing rates of global trade will result in new establishments of potentially damaging invasive species is a question of critical importance to the development of national and international policies aimed at minimizing future invasions. Centuries of historical movement and establishment of invading species may have depleted the supply of species available for future invasions, and it has been suggested that the problem of invasions will diminish as a result of this. However, the extent to which source pool depletion affects future invasions remains unclear. Here we describe a mechanistic model that captures the simultaneous effects of depletion of source species pools along with increases in pathway rates (e.g. imports) to predict future numbers of new invasions. We assume that the distribution of species abundance within invasion pathways is positively skewed, which is modelled using a log-normal distribution. Given their high propagule pressure, the most abundant species are likely to invade first, while the many rare species are likely to invade only under high pathway volumes. We apply this model to the case study of bark beetle, Scolytinae, invasions in the USA. Source species pools in Europe and Asia (225 and 655 species of Scolytinae, respectively) are much larger than numbers that have historically established (16 and 32). Parameterization of the model indicates a highly skewed species abundance distribution in the pathway and this is confirmed by species frequencies in port inspection records, thus explaining why only a small fraction of species has historically invaded. Forecasts from the model indicate that with increasing rates of imports, more species from these regions are likely to invade in the future despite the depletion of the most abundant species from source species pools. Previous statistical models tend to underestimate future establishments in the presence of increasing import rates due to their failure to account for key underlying mechanisms. Policy implications. The mechanistic model developed here is widely applicable for predicting future invasions of all taxa and provides insights into how increases in rates of imports counteract the species pool depletion effect, resulting in the continued establishment of new species.预测全球贸易增速将如何催生更多具有潜在危害性的外来入侵物种定植事件，对于制定旨在降低未来生物入侵风险的国家及国际政策而言，是一项至关重要的核心议题。数个世纪以来的外来物种跨境扩散与定植事件，或许已经耗竭了可供未来入侵的物种种群资源；有观点据此认为，生物入侵问题将因此得到缓解。然而，物种种群库耗竭对未来生物入侵的实际影响程度，目前仍未明确。 本研究构建了一款机理模型，可同时考量物种种群库耗竭与入侵路径通量（如进口规模）增长的双重效应，以此预测未来新增入侵事件的数量。我们假设入侵路径内的物种种群丰度分布呈正偏态，该分布可通过对数正态分布（log-normal distribution）进行拟合。鉴于丰度较高的物种具有更高的繁殖体压力，其往往会率先完成入侵；而多数稀有物种则仅在入侵路径通量较高的条件下才有可能实现入侵。我们将该模型应用于美国境内小蠹亚科（Scolytinae）树皮甲虫的生物入侵案例研究中。 欧洲与亚洲的物种种群库（分别包含225种和655种小蠹亚科昆虫）规模远大于历史上已完成定植的物种数量（分别为16种和32种）。模型参数化结果显示，入侵路径内的物种种群丰度分布呈显著偏态，这一结论得到了港口检疫记录中物种出现频率数据的验证，由此可解释为何历史上仅有极小比例的物种成功完成入侵。 模型预测结果显示，尽管核心入侵物种种群库中的优势物种已出现耗竭，但随着进口规模的持续增长，未来来自上述区域的入侵物种数量仍将增加。此前的统计模型因未纳入关键底层作用机制，在进口规模增长的场景下，往往会低估未来的物种定植事件数量。 政策启示：本研究构建的机理模型可广泛应用于预测各类生物类群的未来入侵事件，同时揭示了进口规模增长如何抵消物种种群库耗竭的效应，进而推动新物种持续完成定植。