Earthworm fear for forest orchids

Worms are great for your garden, but a recent study shows that non-native species may be wreaking havoc on orchids in forests along the east coast.

A group of scientists from the Smithsonian Environmental Research Center (SERC) and Johns Hopkins University’s Department of Earth and Planetary Sciences recently published a study that shows the damage non-native earthworms, which creep their way into forests thanks to human activities such as fishing and gardening, may cause to one of the world’s favourite flowers, the orchid.

Of more than 20,000 orchid species, the study focused on Goodyera pubescens, a tall, erect plant with white flowers common in America’s east coast forests, including those around the SERC campus in Edgewater, Maryland. The problem with earthworms, the scientists found, is that they reduce Goodyera pubescens’ numbers by ingesting their seeds, which are the size of dust specks and fall into the soil surrounding orchids when the plants flowers. As earthworms eat through the soil, they swallow the microscopic seeds, preventing germination in two ways: either the ingestion process kills the seeds before they make it out the earthworm’s other end, or the seeds survive ingestion but are reintroduced into the soil too deeply to access upper-level fungi nutrients required for growth.

The team determined that almost 80 per cent of the seeds ingested in a six-week period could no longer grow, and almost a third were buried too deeply to flourish. By a conservative estimate, the study concludes, older forests – 120 to 150 years old – around SERC would lose 49 per cent of Goodyera orchid seeds to earthworm ingestion in a year, and younger forests – 50 to 70 years old, where non-native earthworms flourish – would lose 68 per cent.

These numbers do not suggest that earthworms are inherently bad for orchids. On the contrary, native earthworms keep the plants’ ecosystems in balance, and allow plenty of room for growth. What the numbers do show is that the unchecked introduction and proliferation of new earthworm species in forests has a dramatic effect that defies the conventional wisdom that earthworms always are great for soil health.

As part of a forest ecosystem, orchids actually are relatively insignificant, however, earthworms also might disrupt the distribution and diversity of the fungi on which the orchid seeds feed, which would have a much more fundamental effect on the forest, because many plants depend on them. 

By Pamela Kelt