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.

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.