Seeking Ferns, Finding Worms

Zlonis, K. J. & Henderson, B. W. (2018). Invasive earthworm damage predicts occupancy of a threatened forest fern: T Implications for conservation and management. Forest Ecology and Management, 430, pp. 291-298. https://doi.org/10.1016/j.foreco.2018.08.004

Armed with GPS coordinates, field notes, and one or two maps, researchers from the University of Minnesota set out to find goblins. Or, more precisely, goblin ferns (Botrychium mormo). These rare ferns are one of many plants that some agencies are required to protect. And it is difficult to find, not only because of its limited range, but also because it does not emerge every year.

But there’s a deeper problem: invading earthworms, who are making the ecosystem uninhabitable for these ferns. This means habitats are shrinking for ferns and other threatened, endangered, and sensitive plants (TES). How can land managers better track these plants, and preserve the uninvaded habitat?

To answer this, the researchers looked for the rare fern in historic locations for three consecutive years, and used occupancy models to understand this information. Because the fern does not emerge every year, those who try to detect the fern can overestimate the plant’s absence. The plant is not absent; it is just dormant that season, perhaps due to drought. By returning to the same sites for 3 years, the researchers hoped to gain more accurate numbers and model better ways to collect data for managers.

But sometimes there are external factors that make the ferns much less likely to be found in that area, even if they had large populations there in the past.

Unwelcome Worms and Ecosystem Change

Earthworm invasion was one of these factors. You may think worms are a natural fixture of your backyard, but there are no native earthworms to Minnesota and the northeast because they were moved out in the last glacial period. That means for the past 11,000 years, the forests and understory plants (including ferns) have developed without these soil creatures. All the worms in the study area were non-native worms, often accidentally brought by fishermen as fish-bait or introduced on logging equipment.

But why are they a problem? Worms thrive on the thick bed of leaf-litter in these rich hardwood forests. They eat it all up, changing the soil structure and chemistry, which has impacts on the whole ecosystem. In our case, the goblins are left exposed. Without the organic matter and leaf litter, the fern loses its warm, moist blanket of protection and cannot emerge.

The scientists also thought that an open canopy might play a role, for similar reasons. Losing a closed canopy means that the soil is hotter and drier– an unwelcome environment for these drought-sensitive ferns.

Find the Drivers

With the ferns detected, and data on the worms and canopy in hand, the scientists looked to see how well their models fit the results. What factors made the detection more accurate? And which was a better predictor for occupancy: earthworm invasion stage, canopy closure, or both factors together?

First, detectability: the ferns were more likely to be detected if they had been detected there before. This makes intuitive sense; if it’s found once, it’s more likely to be found in the same place again.

Next, occupancy: While canopy closure gives important moisture and darkness for the fern, it wasn’t helpful in determining if the fern would be detected there or not. Instead, the best predictor for occupancy was the stage of worm invasion.

An Uninhabitable Understory

As the worm invasion progresses, finding the goblin fern becomes less likely. The ferns can survive at a stage 2 invasion, characterized by a worm that feeds on leaf litter but doesn’t reduce the thickness of the forest floor (Dendrobaena octaedra). But the following stages of invasion brings a drop in fern occupancy. Once invasion reaches stage 5, a rapid leaf-litter and organic matter eater arrives on the scene (Lumbricus rubellus). At this stage, there was no evidence that the site could support the ferns. The worm’s effect on the ecosystem makes the site uninhabitable for them.

The issue is bigger than the goblin ferns; worms also have similar negative effects to other threatened plants. This makes it more and more important to protect quality habitat. The researchers suggested that managers could monitor earthworms (using an invasive earthworm rapid assessment tool) to know if they should expect any ferns at that site, and more broadly, to know which areas to prioritize for conservation. Managers could also educate foresters and users of forest roads on equipment use and cleaning, to limit further spread of worms.

By understanding why the plants are where they are (and aren’t where they aren’t), we can better protect goblin ferns, and other threatened or rare plants in general.

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Featured Image: Earthworm. https://pixabay.com/photos/earthworms-the-frog-s-perspective-2773457/