When horticulturalists at Maui Nui Botanical Gardens want to give an extra boost to their most sensitive and critically endangered Hawaiian plants, they bring out the seabird and bat guano. Why guano? It’s the natural nutrient-packed fertilizer that Hawaiian plants are used to.
Prior to the first canoe reaching Hawaiian shores, taloned and feathered beasts ruled these Islands. Scientists estimate that seabird populations on the main Hawaiian Islands were equivalent to what the 18th century explorers found on the Northwest Hawaiian Islands–in other words, plentiful. Some accounts indicate seabirds were so abundant they blackened the sky. And if they could darken the sky with their wings, they were certainly capable of whitening the ground with their poop, aka guano.
Bird droppings may not be welcome on your car, but plants benefit from the splattered remains of a seabird’s meal. Guano is a gift for growing seedlings: high in nitrogen, phosphate, and potassium–nutrients essential for plant development. The more guano drops from the sky, the more plants sprout from the ground. This growth affects the entire ecosystem. As plants decompose, they provide plentiful and nutritious food for insects, nematodes, snails, and assorted detritivores, as well as soil bacteria and fungus.
Hawaii’s seabird populations are only a fraction of what they once were. The ‘ua‘u, or Hawaiian petrel, was once the most abundant seabird in the Islands. Today this burrowing resident is endangered. In Maui County, remnant populations exist atop the summits of Haleakalā on Maui and Lānaʻihale on Lānaʻi. The reason for the species’ decline is simple, yet irreversible. They were gobbled up by people, rats, mongoose, and barn owls; the hillsides where the birds nested are now pastures, golf courses, farms, and shopping centers. Other seabirds, such as the wedge-tailed shearwater, Newell’s shearwater, and Bulwer’s petrel once filled the air with their cries; these species now live in scattered, isolated populations.
Fewer seabirds mean fewer plant fertilizers. We may never fully understand how declines in pelagic bird populations impact the environment in Hawaiʻi, but research on islands in New Zealand offers some insight into what happens when avian populations crash due to introduced predators. Comparing islands that had been invaded by rats with those still dominated by seabirds, researchers found that the soil on rat-infested islands had a much different nutrient composition and pH. Additionally, the types and abundance of insects and other invertebrates varied, possibly due to differing rates of plant growth and nutrient uptake. The scientists concluded that reducing the seabird population triggers effects that cascade through entire ecosystems, down to the smallest microorganism.
Back in Hawaiʻi, there’s an opportunity to find out what happens in reverse–how a seabird-based ecosystem can recover when the invaders are removed. At Kaʻena point on Oʻahu, rats have been eradicated A predator-proof fence now prevents rodents from re-entering the 59-acre protected area. The resurgence of seabirds, with their increasing deposits of poop, will soon boost nitrogen and stimulate the recovery of native plants.
The loss of a single species has implications for the entire ecosystem. In this case, it’s just about the birds, it’s about what they leave behind, what lives off that, and on and on, all the way down.
You can support the restoration of Hawaiian seabird populations. Visit websites for Maui Nui Seabird Recovery Project (www.mauinuiseabirds.org/restoration/) and Hawaiʻi Offshore Islet Restoration Committee to learn more (http://www.hawaiioirc.org/about-us/).
Lissa 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.
Originally published in the Maui News, February 8th, 2015 as part of the Kia‘i Moku Column from the Maui Invasive Species Committee.