Non-native earthworms have invaded ecosystems around the world but only recently received attention after invading previously earthworm-free habitats in northern North America. Earthworms can affect plants by ingesting seeds and burying them in the soil. The effects, which can be either positive or negative, are expected to become more negative with decreasing seed size.
Because orchids have some of the smallest seeds of any plants, McCormick et al. hypothesized that earthworm consumption would decrease seed viability in these plants and lead to burial of ingested seeds. They used a combination of mesocosms and field measurements to determine whether native and non-native earthworms would affect the seed germination of the North American native orchid Goodyera pubescens by decreasing seed viability through digestion or burial. To determine soil depths at which seed burial would decrease chances of germination, they used field measurements of the abundance of mycorrhizal fungi needed for G. pubescens germination with soil depth.
The researchers found that the combined effects of earthworm ingestion and burial are expected to result in a substantial loss of orchid seeds. Their models estimated that 49% of orchid seeds in mature forests and 68% of those in successional forests would be lost to earthworm ingestion over an average year. The combined effects of earthworm ingestion and burial have the potential to result in a substantial loss of orchid seeds, particularly in successional forests. This effect may slow the ability of orchids to recolonize forests as they proceed through succession. Further testing will determine whether this strong effect of earthworms on G. pubescens viability and germination also applies to other orchid species.