Components of a Hawaiian Forest Ecosystem

The Hawaiian Islands are volcanic islands in the middle of the Pacific Ocean. Because of their remote location, the living parts of ecosystems that formed on the islands derive from a few species that flew or washed up on the island, and more recently, species that came with humans. Hawaii has both dry-land forests and rainforests with unique biotic and abiotic ecosystem components, and its forests house 10,000 species that are found nowhere else.

The islands of Hawaii were formed by volcanic activity along a plate in the ocean floor. The islands emerged as the magma cooled and the soils formed as the volcanic rocks weathered. Depending on the age of the island, soils can be anything from magma that cooled yesterday to material that is millennia old -- though still young by soil standards. The islands are surrounded by water, but fresh water can be in short supply. The combination of trade winds and mountainous islands means there are a wide variety of climate zones. Regions can be dry or wet, with as little as 20 centimeters (8 inches) to as much as 1,027 centimeters (404 inches) of annual rainfall. Temperatures also vary widely by region, with average annual temperatures between 26 degrees Celsius (78 degrees Fahrenheit) and 7 degrees Celsius (45 degrees Fahrenheit). There is very little annual variation in temperature or precipitation in any given area.

The producers in the Hawaiian forest ecosystem form the basis of the food chain. The dominant tree in both rainforests and dry-land forests is the amazingly adaptable Metrosideros polymorpha, or ‘O- hi‘a lehua. These can take the form of desert shrubs or 31-meter (100-foot) tall rainforest giants and two or more phenotypes can grow right next to each other. In rainforests, the understory is dominated by uluhe tree ferns. The rainforests also contain the silversword alliance, a group of over 50 divergent species in the sunflower family. Dry forests contain 22 percent of Hawaii’s plant biodiversity with notable entrants like wiliwili, lama, and ʻOhe kukuluāeʻo.

Prior to humans, the fauna of Hawaii was limited, and a few original species evolved to fill ecological niches that include herbivores, predators, and decomposers. Birds, insects, spiders and slugs were the only animals capable of making or surviving the long journey over the ocean to the Hawaiian Islands. The rainforest ecosystems are home to the Hawaiian honeycreepers, a diverse group of birds that likely evolved from a single species to eat insects, fruits and nectar. The rainforests are also home to predatory caterpillars, Hawaiian land snails, representing decomposers, and one third of the known species of fruit fly. The dry forests are notably home to the nene, an endemic goose.

Hawaii’s forest ecosystems evolved in the absence of mammals and predators like snakes and the species did not have to compete with invaders for an ecological niche. That changed with the arrival of the Polynesians and changed dramatically when the Europeans arrived. Snakes, rats and mongooses are efficient predators that have devastated the native bird populations. Invasive plants like ginger and strawberry guava crowd the forest understory. The island was light on predators prior to colonization, and their addition has been rippling down the entire food chain.