Invasive Species and Biodiversity Crises: Testing the Link in the Late Devonian

During the Late Devonian Biodiversity Crisis, the primary driver of biodiversity decline was the dramatic reduction in speciation rates, not elevated extinction rates; however, the causes of speciation decline have been previously unstudied. Speciation, the formation of new species from ancestral populations, occurs by two primary allopatric mechanisms: vicariance, where the ancestral population is passively divided into two large subpopulations that later diverge and form two daughter species, and dispersal, in which a small subset of the ancestral population actively migrates then diverges to form a new species. Studies of modern and fossil clades typically document speciation by vicariance in much higher frequencies than speciation by dispersal. To assess the mechanism behind Late Devonian speciation reduction, speciation rates were calculated within stratigraphically constrained species-level phylogenetic hypotheses for three representative clades and mode of speciation at cladogenetic events was assessed across four clades in three phyla: Arthropoda, Brachiopoda, and Mollusca. In all cases, Devonian taxa exhibited a congruent reduction in speciation rate between the Middle Devonian pre-crisis interval and the Late Devonian crisis interval. Furthermore, speciation via vicariance is almost entirely absent during the crisis interval; most episodes of speciation during this time were due to dispersal. The shutdown of speciation by vicariance during this interval was related to widespread interbasinal species invasions. The lack of Late Devonian vicariance is diametrically opposed to the pattern observed in other geologic intervals, which suggests the loss of vicariant speciation attributable to species invasions during the Late Devonian was a causal factor in the biodiversity crisis. Similarly, modern ecosystems, in which invasive species are rampant, may be expected to exhibit similar shutdown of speciation by vicariance as an outcome of the modern biodiversity crisis.

晚泥盆世生物多样性危机（Late Devonian Biodiversity Crisis）期间，生物多样性下降的核心驱动因素为成种速率的急剧降低，而非灭绝速率升高；但此前学界尚未对成种速率下降的成因展开研究。成种作用，即由祖先种群演化形成新物种的过程，主要通过两类异域成种机制实现：其一为分隔成种（vicariance），即祖先种群被动分裂为两个大型亚种群，后续二者发生遗传分化并形成两个子代物种；其二为扩散成种（dispersal），即祖先种群的一小部分个体主动迁移至新环境，随后分化形成新物种。针对现生类群与化石支系的研究通常显示，分隔成种的发生频率远高于扩散成种。为厘清晚泥盆世成种速率下降的机制，本研究针对三个代表性支系，基于地层约束的物种级系统发育假说计算了成种速率；同时对三个门类（节肢动物门Arthropoda、腕足动物门Brachiopoda、软体动物门Mollusca）的四个支系的分支成种事件的成种模式进行了评估。所有分析结果均显示，从中泥盆世危机前时段至晚泥盆世危机时段期间，泥盆纪类群的成种速率呈现一致的下降趋势。此外，危机时段内几乎完全不存在分隔成种事件；此期间的多数成种事件均由扩散成种导致。此期间分隔成种作用的停滞与广泛的跨盆地物种入侵密切相关。晚泥盆世分隔成种作用的缺失，与其他地质时段观测到的模式截然相悖，这表明晚泥盆世期间由物种入侵引发的分隔成种作用丧失，是本次生物多样性危机的关键成因之一。类似地，在入侵物种泛滥的现代生态系统中，或也将出现类似的分隔成种作用停滞，这或许是现代生物多样性危机的一项潜在结果。