The provocative aspect of the study is in its relatively accepting attitude toward nonnative, noninvasive plant species, often the traditional nemesis of ecologists.

By Leah Sherwood.

Researchers at the University of Hawai‘i at Hilo were recently awarded a Bradshaw Medal for their provocative paper questioning a fundamental assumption of the field of restoration ecology, which is the science of restoring natural habitats that have been subject to anthropogenic disturbances.

The Bradshaw Medal, named after British ecologist and restoration pioneer Tony Bradshaw, is given by the Society for Ecological Restoration, in recognition of a scientific paper published in the Society’s major journal, Restoration Ecology, which advances the field of restoration ecology.

The paper, which is the product of observations from multiple studies done over several years, is titled, “Quandaries of a decade-long restoration experiment trying to reduce invasive species: beat them, join them, give up, or start over?” (2016). Lead author is Susan Cordell, director of the U.S. Department of Agriculture’s Institute of Pacific Islands Forestry, Hilo, with coauthors Laura Warman, also with the USDA Institute of Pacific Islands Forestry, and Rebecca Ostertag and Jené Michaud of UH Hilo.

Cordell is also an affiliate faculty member at UH Hilo who serves in an advisory role for the tropical conservation biology and environmental sciences graduate program. Coauthors Ostertag, an ecologist, and Michaud, a hydrologist, are both professors and researchers at UH Hilo. Warman is a plant ecologist with the USDA forestry institute who also teaches at UH Hilo.

A provocative approach to native forest restoration

The provocative aspect of the paper is in its relatively accepting attitude towards nonnative, noninvasive plant species, often the traditional nemesis of ecologists. The authors argue that in some cases it is better to “give up” on the traditional goal of restoring disturbed ecosystems to their pristine native state, and instead pursue a “hybrid” approach that incorporates both native plant species and nonnative (but noninvasive) plants.

“Our perspective is that in many cases we cannot keep these areas all native,” says Ostertag. “It is just not feasible or pragmatic.”

The focus of the paper is a multiyear, multistudy restoration project called Liko Nā Pilina, which in Hawaiian means roughly “growing or budding novel relationships.” The project is an ongoing effort to restore an area of Hawaiian lowland wet forest, an ecosystem found on the northeastern sides of the Hawaiian islands and that is particularly susceptible to loss of native plant species biodiversity and domination of invasive plant species. Hawaii’s native lowland wet forests were first altered by the arrival of the Polynesians and later exploited by Western colonists for agricultural and housing purposes. The result was an altered ecosystem and loss of biodiversity. Today, remnants of the forests remain on Hawai‘i Island in patchy forest reserves in Puna and East Hawai‘i, but they remain threatened by development.

In practice, ecologists want to restore ecosystems back to their original state because the native species evolved over time to fill certain niches or functions in the overall system. This was the original goal of the research team in the Liko Nā Pilina project.

“We had originally done an experiment where we removed all the invasives from our ten-by-ten meter plots,” explains Ostertag. “We thought by removing the highly invasive species we would able to improve the germination of the native species and get them to regenerate. However, that is not really what we got. And the amount of weeding we had to do to keep out the invasives was really really intense. We estimated about 40 person hours per meter squared to do all the weeding to keep it native.”

“Weeding will kill you!” agrees Michaud, the hydrologist whose primary role was studying water flow in the study area. She and Ostertag, along with rest of the team, started to realize that the ecosystem would never return to an-all native state, and even if this were possible, the cost would be too high and payoff too low.

“We realized we needed a different strategy,” Ostertag says. “Just removing the invasives, just doing a passive restoration, was not going to work, the effort was too great. We decided that we needed to do a more active restoration that involved planting the specific species we wanted. This led us to this idea of planting a hybrid forest, making hybrid ecosystems of the native and nonnative species grow together, using nonnative species that were not invasive but that could fill important functional roles. This hypothesis led us to collecting really important data that showed that one problem is that the native community is missing certain functional roles. Therefore, by including nonnative, noninvasive species that can fill these functional roles that are currently missing, we might have more success.”

An example of a functional role that can be filled by a nonnative species is providing shade.

“We found that we were missing fast-growing species with large leaves that create a lot of shade,” explains Ostertag. “We need the shade in the environment because that’s what keeps out the highly invasive seedlings. We need to manipulate the light environment to the goldilocks level where it is just right. We needed species that closed the canopy faster and helped produce shade to keep out the undesirable invasive species but that still allowed native species’ seedlings to regenerate.”

Ostertag adds that they plan to continue to manage the forest indefinitely to support the growth of native species and prohibit the spread of invasives under their new strategy of mixing native and nonnative species to fill functional roles.

“If you are in it for the long game you can start to see real changes,” she says. “After five years, we are starting to see the canopy getting darker and starting to close, and we are really reducing our weeding effort.”

Fine tuning the approach

Ostertag emphasizes that the researchers’ hybrid restoration strategy is not appropriate in every case.

“Our strategy for mixing native and nonnative is less palatable at higher elevations, which are more native-dominated,” she says. “And if there is already high native cover in an area you may not need this method. However, at the lower elevations, which are completely dominated by these highly invasive species, we think this is a realistic approach.”

Ostertag says that winning the Bradshaw Medal was a surprise considering that the team had originally written a completely different type of data-rich paper focusing on weeding and invasive species reoccurrence. The original idea was not reviewing well and instead a new paper emerged.

“We decided we would morph our study into a story format and a lessons learned paper,” says Ostertag. “It took on more of a narrative structure. I think people like the paper because we explain our experience over a decade of work, and the trials and tribulations of this lowland wet forest restoration project.”

Their mixing of native and nonnative species may raise the eyebrows of some conservation ecologists, but Ostertag says her colleagues in Hawai‘i have been very receptive to the hybrid approach.

“Ecologists who work in Hawai‘i were enthusiastic and encouraging because they understand the huge problem that we have with invasive species here,” says Ostertag. “Hawai‘i is like an endpoint on the conservation continuum. Half of our flora is nonnative, we have these highly disturbed systems in the low elevations, and if you go to most places you don’t see native species. They are completely altered, modified systems. Once people realize this, they understand that this is a potentially viable strategy that deserves to be tested.”

About the author of this story: Leah Sherwood is a graduate student in the tropical conservation biology and environmental science program at UH Hilo. She received her bachelor of science in biology and bachelor of arts in English from Boise State University.