A non-native earthworm shifts the seed predation dynamics of a native weed

Seed predators both consume and disperse seeds, with important consequences for the population dynamics of many plant species.  The net effect of multiple seed predators depends on the relative proportion of the seed pool each predator obtains, and this proportion should reflect species-specific habitat preferences.  We studied the effect of the non-native earthworm, Lumbricus terrestris, on seed loss dynamics in the native weed, Ambrosia trifida (giant ragweed).  Giant ragweed seeds are predated by mice, but L. terrestris may protect the seeds against rodent predation by caching them in its burrows.  We investigated these interactions, as well as how environmental factors affected net seed losses by competing seed predators.

A two-year field study was conducted in which we measured removal of experimentally dispersed giant ragweed seeds by earthworms and mice in habitats varying in plant cover.  We analyzed the relative proportion of seeds taken by each species under the varying experimental conditions.   

Species-specific responses to abiotic conditions and plant cover drove variation in the share of seeds taken by earthworms versus mice, with earthworms gaining relatively more seeds under warmer, wetter conditions and low plant cover habitats, and mice obtaining more seeds under colder, drier conditions and high plant cover habitats. 

Plant cover and weather conditions also determined which predator species accessed seeds first, and this conferred a competitive advantage that was compounded over time.

Earthworms cached some seeds under all experimental conditions, suggesting that L. terrestris can act mutualistically with giant ragweed by making seeds inaccessible to rodent seed predators. 

Synthesis and applications. Our results support the view that interactions among the environment and competing seed predators determine the fate of seed pools.  The data also support the hypothesis that L. terrestris facilitates giant ragweed by competing with mice for giant ragweed seeds, likely contributing to its spread across the landscape and hindering effective weed management.  Lumbricus terrestris is prevalent throughout temperate regions and may similarly affect seed predation dynamics of other large-seeded species, impacting plant communities across a range of habitats.

Methods
A two-year field study was conducted in which we measured removal of experimentally dispersed giant ragweed seeds by earthworms and mice in habitats varying in plant cover.  We analyzed the relative proportion of seeds taken by each species under the varying experimental conditions.

种子捕食者兼具取食与传播种子的双重作用，对多数植物类群的种群动态具有显著影响。多种种子捕食者的净效应取决于各捕食者获取的种子库占比，而该占比可反映物种特有的生境偏好。本研究以非本土蚯蚓正蚓（Lumbricus terrestris）对本土杂草三裂叶豚草（Ambrosia trifida，俗称巨豚草）的种子损失动态的影响为研究对象。巨豚草种子会被鼠类捕食，而正蚓可通过将种子储存在洞穴中，使其免受啮齿动物的取食。本研究探究了这类物种间互作机制，以及环境因子如何通过竞争性种子捕食者影响种子的净损失量。

本研究开展了为期两年的野外定位试验，在植物覆盖度梯度各异的生境中，量化了蚯蚓与鼠类对人工播撒的巨豚草种子的取食移除情况，并分析了不同实验条件下各捕食者获取种子的相对占比。

物种对非生物环境与植物覆盖度的特异性响应，决定了蚯蚓与鼠类的种子获取占比差异：蚯蚓在温暖湿润、低植物覆盖度的生境中获取的种子占比更高，而鼠类则在寒冷干燥、高植物覆盖度的生境中占据捕食优势。

植物覆盖度与气象条件还会决定捕食者的种子接触先后顺序，这会赋予领先者随时间累积的竞争优势。

所有实验条件下，蚯蚓均会储存部分种子，这表明正蚓可通过使种子无法被啮齿动物捕食者获取，与巨豚草形成互利共生关系。

**综合与应用**
本研究结果证实，环境与竞争性种子捕食者之间的互作过程决定了种子库的最终命运；同时验证了以下假说：正蚓通过与鼠类竞争巨豚草种子，促进了巨豚草的种群扩张，这可能会加剧其在区域范围内的扩散，并干扰有效的杂草防治工作。正蚓在温带区域广泛分布，其可能对其他大种子物种的种子捕食动态产生类似影响，进而重塑不同生境中的植物群落结构。

**方法**
本研究开展了为期两年的野外定位试验，在植物覆盖度梯度各异的生境中，量化了蚯蚓与鼠类对人工播撒的巨豚草种子的取食移除情况，并分析了不同实验条件下各捕食者获取种子的相对占比。