A principal impact of invasive species is that they reduce local species richness. However, it is unknown whether the magnitude of the richness decrease has been consistent over the past two decades of published research. We used cumulative meta-analysis to synthesize evidence from 240 articles evaluating whether this cumulative evidence base generally supports, or refutes, the association between invasive species presence and richness declines. First, we determined whether evidence accumulation lowered the mean effect size of invasive species on local native richness through time; termed the 'decline effect'. Then, as mean effect sizes changed over time, we identified when accumulated evidence reached sufficiency, indicating that the mean effect direction (positive or negative) was unlikely to be reversed by unpublished research. We also assessed whether the mean effect size reached a threshold of stability over publication years. To date, no research has tested mechanisms of the decline effect, and here we determine whether publication bias, sample size, time since invasion or invader trophic position are driving a decline effect in the published evidence base. We found a clear decline in the cumulative mean effect of invasive species on local native species richness as published evidence accumulated between 1999 and 2016. Despite this decline, an average negative association was stable and sufficiently robust to unpublished studies by 2007, showing a 21% mean richness decrease by 2016. Contrary to our expectation, the decline effect manifested consistently regardless of invasive species trophic position, time since invasion, or journal rank. Within taxonomic subgroups, trees, insects, herbaceous plants and insects exhibit a decline effect, yet still show sufficient and stable negative impacts on richness. However, many other taxonomic subgroups (e.g., crustaceans, fish, mammals) lack evidence for average negative impacts on richness, or have not met sufficiency or stability thresholds.

中文翻译：

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累积荟萃分析确定了 20 年后入侵物种对丰富度的下降但负面影响

入侵物种的一个主要影响是它们减少了当地物种的丰富度。然而，在过去二十年发表的研究中，丰富度下降的幅度是否一致尚不清楚。我们使用累积元分析来综合来自 240 篇文章的证据，评估该累积证据基础是否普遍支持或反驳入侵物种存在与丰富度下降之间的关联。首先，我们确定证据积累是否随着时间的推移降低了入侵物种对当地本土丰富度的平均影响大小；称为“衰退效应”。然后，随着平均效应大小随时间变化，我们确定积累的证据何时达到充分，表明平均效应方向（正面或负面）不太可能被未发表的研究逆转。我们还评估了平均效应大小是否达到了发表年份的稳定性阈值。迄今为止，还没有研究测试过下降效应的机制，在这里我们确定发表偏倚、样本量、自入侵以来的时间或入侵者营养位置是否正在推动已发表的证据库中的下降效应。根据 1999 年至 2016 年间积累的已发表证据，我们发现入侵物种对当地本地物种丰富度的累积平均影响明显下降。尽管有这种下降，但平均负相关是稳定的，并且对 2007 年未发表的研究来说足够稳健，显示为 21%到 2016 年，平均丰富度下降。与我们的预期相反，无论入侵物种的营养位置、入侵时间或期刊排名如何，下降效应始终如一。在分类亚群内，树木、昆虫、草本植物和昆虫表现出下降效应，但仍对丰富度表现出足够而稳定的负面影响。然而，许多其他分类亚群（例如甲壳类、鱼类、哺乳动物）缺乏证据表明对丰富度的平均负面影响，或者没有达到充分性或稳定性阈值。