Forest birds on the island of Hawaii are responding positively to being restored in one of the largest, ongoing reforestation projects at Hakalau Forest National Wildlife Refuge, according to a new study released July 10 in the journal Restoration Ecology.
Serving as pollinators and seed dispersers, birds have an important role in ecosystem function and their presence in restoration areas can be a measure of success for conservation efforts.
“The study results show the birds are responding positively to restoration efforts faster than anyone thought possible,” said U.S. Geological Survey researcher and the study’s lead author Eben Paxton. “Birds are now in parts of the refuge where they weren’t found 10 to 20 years ago.”
Since the project’s establishment, restoration efforts in the refuge have included reforestation with the large, fast growing native koa tree and, more recently, the planting of understory shrubs that provide a variety of food resources for birds.
Researchers used bird survey data collected over 26 years at the refuge to document how a diverse community of birds responded to the nearly 30 years of restoration efforts on the refuge. Their analysis revealed that most bird species increased in number throughout the restoration area, with the greatest increases detected in areas closest to intact forest where the density and diversity of understory shrubs was greatest.
“We now have a better understanding of the time it takes for the Hawaiian birds to respond to our restoration efforts on the refuge and move into reforested areas,” said Steve Kendall, a U.S. Fish and Wildlife Service Wildlife Biologist at the refuge and coauthor of the study. “Given the time lag for bird arrivals, it is clear that our continued reforestation efforts now are needed to ensure there is habitat for these birds in the future.”
The Hakalau Forest National Wildlife Refuge, established in 1985, encompasses large tracts of intact native forest but also includes large areas that were degraded and largely deforested by logging and grazing before becoming part of the refuge. The refuge serves as an important sanctuary for Hawaii’s native forest birds that are faced with many threats including habitat loss, invasive predators and introduced avian diseases. The refuge protects high-elevation forests on the slopes of Hawaii’s tallest volcano that are largely disease-free because the cool mountain temperatures prevent disease-transmitting mosquitoes from becoming established, allowing native birds that are not resistant to the introduced disease to thrive.
The broad range of bird species on the refuge, both native and introduced, rely on different plants for food. Some rely mostly on flower nectar, others on small fruits and seeds, and others on insects. The species also have a range of requirements for where they build their nests, with the endangered Hawaii Akepa being the most specialized, requiring cavity nests that are usually only found in old-growth trees. Given this wide range in habitat needs, reforestation varied among the birds.
Reforestation and the ensuing bird colonization take time. The restoration areas closest to the intact forest areas, where reforestation can occur naturally now that the refuge is largely protected, had the greatest diversity and density of plants and showed the greatest increases in bird population densities. But bird numbers dropped off sharply about a mile away from the edges of the intact forest where forest restoration requires more human assistance. Fast-growing koa trees that have been planted in many of these areas are providing some needed habitat for birds, but Paxton says, “as the plant community in the reforestation areas becomes more diverse, the benefit to more avian species will be stronger.”
As temperatures rise with climate change, the area where the transmission of avian malaria is hindered by cooler temperatures will shrink, meaning the restoration of high-elevation forests may be one of the most important conservation tools to protect Hawaii’s native birds.
“We have a clearer picture of how we can facilitate expansion of particular bird species into reforestation areas by creating particular plant communities; this will help us ensure that Hakalau Forest National Wildlife Refuge can continue to be a stronghold for Hawaiian forest birds,” said Kendall.
The journal article, “Rapid colonization of a Hawaiian restoration forest by a diverse avian community,” was published July 10 in the journal Restoration Ecology with lead author Eben Paxton, Stephanie Yelenik, and Eli Rose of the USGS, Tracy Borneman and Richard Camp of Hawaii Cooperative Studies Unit at the University of Hawai‘i at Hilo and Steve Kendall of the USFWS.