Cascading effects of earthworm invasion in tundra increase graminoid density and rodent grazing intensities

Earthworms are being introduced to numerous ecosystems through human activities. Some non-native earthworm species have the potential to ‘geoengineer’ soils and increase plant nitrogen (N) uptake, but if the increased plant N concentrations can cause increased rodent grazing is not well known. In this study, we present findings from a common garden experiment with two tundra communities, meadow (forb dominated) and heath (shrub dominated), half of them subjected to four years of earthworm presence (Lumbricus spp. and Aporrectodea spp.). Within four summers, our earthworm treatment changed plant community composition by increasing graminoid density by on average 94 % in the heath vegetation and by 49 % in the meadow. Rodent winter grazing were more intense on plants growing in soils with earthworms, an effect that coincided with higher nitrogen concentrations in plants indicating a higher palatability. Moreover, although the earthworms decreased soil moisture, our proxy for plant community photosynthesis (greenness) was not negatively affected. We conclude that earthworm-induced changes in plant composition and trophic interactions may radically alter the functioning of tundra ecosystems.

人类活动正将蚯蚓引入全球众多生态系统之中。部分非本土蚯蚓物种具备土壤工程化改造（geoengineer）的潜力，可提升植物对氮（N）的吸收效率，但植物氮浓度升高是否会加剧啮齿类的取食行为，目前尚未有明确定论。
本研究基于一项同质园实验（common garden experiment），实验设置了两类苔原群落：草甸群落（以阔叶草本为优势种）与灌丛群落（以灌木为优势种），其中一半样地持续四年接种蚯蚓（正蚓属（Lumbricus spp.）和全肺蚓属（Aporrectodea spp.））。
在四年的夏季观测周期内，蚯蚓接种处理改变了植物群落组成：灌丛群落中禾草类植物（graminoid）密度平均提升94%，草甸群落中则提升49%。在接种蚯蚓的土壤中生长的植物，其冬季被啮齿类取食的强度更高；该现象与植物体内更高的氮浓度相吻合，表明此类植物适口性更强。此外，尽管蚯蚓接种降低了土壤含水量，但我们用于表征植物群落光合能力的替代指标——绿度（greenness），并未受到负面影响。
本研究结论表明，蚯蚓引发的植物群落组成变化与营养级交互作用，可能会彻底改变苔原生态系统的功能运转。