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（invasive species）定殖事件，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，是一项极具关键意义的研究课题。数个世纪以来的外来物种跨境迁移与定殖历史，可能已耗尽了可供未来入侵的物种资源库，有观点据此认为生物入侵问题将随之缓解。然而，物种资源库耗竭对未来生物入侵的实际影响程度仍未明确。
本研究构建了一款机制模型（mechanistic model），可同时考量物种资源库耗竭与入侵路径通量（如进口量）增长的双重效应，以此预测未来新增生物入侵事件的数量。我们假设入侵路径内的物种多度分布呈正偏态，并采用对数正态分布（log-normal distribution）对其进行建模。由于优势物种拥有更高的繁殖体压力（propagule pressure），它们通常会率先完成入侵；而多数稀有物种则仅在路径通量达到较高水平时才有可能实现入侵。我们将该模型应用于美国境内小蠹亚科（Scolytinae）生物入侵的案例研究。
欧洲与亚洲的物种资源库中分别包含225种和655种小蠹亚科物种，远高于两地历史上已成功定殖的物种数量（分别为16种与32种）。模型参数化结果显示，入侵路径内的物种多度分布呈现极强的正偏态特征，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，由此解释了为何历史上仅有极小比例的物种成功完成入侵。
模型预测结果表明，尽管优势物种已从资源库中被耗竭，但随着进口量持续增长，未来仍将有更多来自上述区域的物种完成入侵。过往的统计模型由于未考虑关键的内在作用机制，在进口量增长的场景下往往会低估未来的物种定殖数量。
政策启示：本研究构建的机制模型可广泛应用于预测各类生物分类单元（taxa）的未来入侵事件，并揭示了进口量增长如何抵消物种资源库耗竭的效应，从而持续催生新的物种定殖事件。