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Earthworms could be supporting invasive plants, animals

Kia‘i Moku

Hawaii has no native earthworms, but they have spread throughout the forest in Hawaii. Research shows more worms where pigs are present, indicating that they benefit each other with the combination increasing damage to the forests of Hawaii. This fence on Hawaii island prevents pigs from accessing the forest on the left. The rooting activity of pigs as they search for food is apparent on the right side of the photo. Photo courtesy of Nathaniel Wehr

If you garden, you’ve gained an appreciation for the relationship between soil and plant health. From soil pH to mineral content or drainage, there are countless indicators of soil quality and one size doesn’t fit all; the right soil for a cactus won’t support a lily. As with climate, rainfall and temperature, soil type and composition play a key role in determining what grows where and if it will thrive.

Gardeners look to earthworms as an indicator of soil aeration, and they’re often thought of as a beneficial critter, but earthworms introduced to Hawaii don’t necessarily benefit the plants that evolved in Hawaii.

Earthworms were among the first living creatures on earth, predating dinosaurs. Until relatively recently — when people began moving plants and soil great distances — worms remained in the warmer regions of the continents where they had been for billions of years, breaking down organic matter, aerating the soil, and living their fossil-like existence, engineering the soil to support the surrounding ecosystem.

The Hawaiian Islands have an entirely different, and earthworm-less, evolutionary history. The islands are geologically young, arriving in the middle of the Pacific long after dinosaurs had come and gone. The plants that evolved for life in Hawaii are adapted to grow in iron-rich volcanic soils free of worms. For millions of years, the work of decomposition was done, slowly, by leaf-shredding insects, springtails, fungi, bacteria and the native kahuli snails.

So, when earthworms arrived, thanks to people, they started to change the soil. We don’t know when they arrived, but the first earthworms in Hawaii were collected in 1896. They’ve nonetheless successfully spread throughout Hawaii, both with the help of humans and gradually moving from gardens and homes through the soil.

Stephanie Joe, alien invertebrate research specialist with the Oahu Army Natural Resource Program, wanted to research the impacts of earthworms in Hawaii but couldn’t find any forested areas that lacked earthworms, from the summit of Puu Kukui in the West Maui Mountains to isolated kipukas (pockets of vegetated land surrounded by newer lava flows) on Hawaii island.

“Given that we don’t have a place without earthworms in Hawaii, it’s hard to quantify the impacts, but it’s not good — earthworms are definitely changing the forest floor,” says Joe.

Earthworms are not native to the parts of the continental Mainland that were covered in glaciers. Researchers look at the earthworm invasion front in temperate forests in Canada and the northern United States comparing areas that do not have earthworms to those that do. They have found a correlation between invasive worms and invasive plants — in particular, grasses — suggesting that worms — known ecosystem engineers — are creating conditions that favor invasive plants.

The little research on earthworms in Hawaii has found that they alter nutrient cycling, increasing nitrogen content in the soil by breaking down plant matter. In other experiments, researchers have shown that additional nitrogen increases the growth of invasive plants in Hawaii.

There is also evidence that earthworms support feral pigs in Hawaii.

Earthworms are food for wild pigs, and in search of dinner, pigs will dig up acres of the rainforest.

For his graduate research, Nathaniel Wehr looked at the relationship between soil macroinvertebrates (animals that lack a backbone) and pigs. He compared pig-free sections of rainforest in Volcanoes National Park on Hawaii island to sites where pigs were still present. There are more worms where pigs are present, and he detected particularly high numbers of worms where pigs had been rooting in the dirt.

Wehr suspects the reason is not necessarily because pigs are good at finding worms, but because pigs aerate the soil and press organic material into it — conditions that worms favor. The pigs then cycle back to find more worms.

“It’s termed invasional meltdown,” he explains. “Pigs and worms interact to create a constant cycle, ultimately benefiting each other.”

Given how widespread they are, nothing can be done on a landscape scale to address earthworms in Hawaii. They may be good in gardens and compost piles, but the presence of worms in Hawaii has altered the ecosystem forever. Preventing impacts from species yet to arrive and spread in the state is critical.

Find out more about efforts to increase biosecurity by looking at the Hawaii Interagency Biosecurity Plan at dlnr.hawaii.gov/hisc/plans/hibp/. The Great Lakes Worm Watch website at greatlakeswormwatch.org/ is a good resource for information about the impacts of invasive earthworms.

* Lissa Fox Strohecker is the public relations and education specialist for the Maui Invasive Species Committee. She holds a biological sciences degree from Montana State University. Kia’i Moku, “Guarding the Island,” is prepared by the Maui Invasive Species Committee to provide information on protecting the island from invasive plants and animals that can threaten the island’s environment, economy and quality of life.