Impact of an exotic earthworm on seed dispersal of an indigenous US weed.
Ambrosia trifida L. (giant ragweed), an aggressive weed of US grain crops and indigenous to North America, colonizes no-tillage crop fields and undisturbed soils despite its large seed size and susceptibility to seed predation. Secondary seed dispersal is critical to seed survival and seedling establishment, yet mechanisms of secondary dispersal are poorly understood for such large-seeded weedy species. Field experiments were conducted to determine how seed foraging by the European exotic burrowing earthworm Lumbricus terrestris L., affected A. trifida seed burial and seedling recruitment, and to determine seed selectivity by L. terrestris. Earthworms collected and buried over 90% of A. trifida seeds placed on the soil surface at a rate eightfold faster than abiotic seed burial. There was a sixfold higher concentration of seeds in burrows than in surrounding soil and a mean of 127 A. trifida seeds per burrow after a single season of A. trifida seed dispersal. Earthworms buried A. trifida seeds from 0.5 to 22 cm deep and reduced recruitment by 37% compared to seeds buried abiotically and protected from predators, due to burial of some seeds below emergence depth limits. However, seedling biomass was increased by 30%. Earthworms foraged selectively among seeds of 11 large-seeded species and collected more seeds of A. trifida than of other species. The earthworms buried small (8.5 mm) A. trifida seeds more deeply and reduced their emergence more than large (11.5 mm) A. trifida seeds. Synthesis and applications. The novel interaction of L. terrestris and A. trifida increases seed bank formation of A. trifida. Burrows of L. terrestris provide safe sites for seeds of A. trifida that may increase seedling establishment in environments with a high risk of seed predation. Control measures to prevent reproduction by A. trifida should be increased where L. terrestris is present due to the earthworms' ability to bury and protect weed seeds. Selective seed caching by this widely distributed earthworm species may change plant community composition in agricultural and natural areas in North America and influence the evolution of seed traits. Land managers should consider the effects of L. terrestris on seedling regeneration of native and exotic plant species in areas undergoing colonization by L. terrestris.