Hawaiian tropical shrublands

• Category: Grassland, Tundra, and Human Biomes.
• Geographic Location: Pacific Ocean.
• Summary: This ecoregion hosts a series of habitat niches defined by elevation and moisture regimes, resulting in unique assemblages of flora and fauna.

A shrubland is a plant community composed of grasses, non-woody vegetation, shrubs, and small trees, formed as a result of either natural processes or habitat alteration by humans. Shrublands can exist as a stable community type or as a step along a successional trajectory following disturbance. Because the Hawaiian Islands are the most isolated island archipelago on Earth, with the nearest continental land mass approximately 2,360 miles (3,800 kilometers) away, many of the island's plant species evolved after long-distance dispersal events and thus are endemic, or found nowhere else on Earth.

In Hawaii, shrublands are considered to be an important habitat type because they were historically widespread. Today, these shrublands exist as remnant fragments that are home to many rare species as a result of human impact on the landscape. For this reason, many shrubland communities are critically threatened.

The volcanic origin of the Hawaiian Islands has produced a mountainous environment with orographic weather patterns. As a result, elevation, humidity, precipitation, and temperature differ dramatically within and across the islands. This variation has allowed for more native terrestrial ecosystem types than generally would be found in other geographic areas of similar size. In addition, the rain shadow effect, a consequence of trade winds from the northeast, contributes to the range of habitat type. This causes drier conditions on the leeward sides of islands and wetter conditions on windward sides.

Shrubland Ecosystems

Shrubland ecosystem types are found on all eight main islands—Hawaii, Kauai, Kahoolawe, Lanai, Maui, Molokai, Niihau, and Oahu—as well as on several of the smaller northwestern Hawaiian Islands, and in greatest abundance on leeward slopes. Shrublands in Hawaii are often categorized by location or elevation (coastal, low, or high) and precipitation (dry, mesic, or wet). In the coastal zone, the two types of shrubland are dry and wet.

The Coastal Dry Shrubland communities occur on the northwestern Hawaiian islands of Lisianski, Laysan, Necker, and Nihoa, as well as all the main islands below an elevation of 984 feet (300 meters), where annual rainfall is less than 20 inches (500 millimeters). Some noteworthy genera found in these shrublands include Chenopodium, Euphorbia, Eragrostis, Gossypium, Heliotropium, Scaevola, Schiedea, Sida, and the invasive koa haole (Leucaena). The two types of Coastal Wet Shrubland habitats occur near freshwater sources on most of the main islands, as well as on Laysan; they are dominated by native Hibiscus or non-native Pluchea.

Lowland shrublands occur on most of the main islands, and are categorized as dry, mesic, or wet. Lowland Dry Shrublands occupy exposed sites from 328 to 1,970 feet (100 to 600 meters) elevation, receiving 20 to 67 inches (500 to 1,700 millimeters) of rain annually. Species composition is variable, but tends to be dominated by Bidens, Dodonaea, Sesbania, or Wikstroemia. Lowland Mesic Shrublands are found at 98 to 2,790 feet (30 to 850 meters) elevation, enduring dry summers and winter rainfall of 39 to 79 inches (1,000 to 2,000 millimeters). Dominant genera here include Dodonaea, Lipochaeta, Metrosideros, Osteomeles, Styphelia, and Wilkesia.

The Lowland Wet Shrublands are present on some exposed ridge sites from 656 to 2,950 feet (200 to 900 meters), where annual rainfall ranges from 150 to 236 inches (3,800 to 6,000 millimeters), with Metrosideros and Pipturus vegetation dominating. One significant threat to lowland shrublands is the invasive strawberry guava (Psidium cattleianum), which forms monoculture stands, preventing native plants from regenerating.

The high shrubland types include montane, subalpine, and alpine. Montane Dry Shrubland ecosystems are dominated by Dodonaea or Metrosideros, and are restricted to Maui and Hawaii from 2,953 to 8,858 feet (900 to 2,700 meters), where annual rainfall ranges from 16 to 39 inches (400 to 1,000 millimeters). Montane Wet Shrublands are often dominated by a mix of ferns and shrubs; they occur on Hawaii, Kauai, Molokai, Maui, and Oahu around 3,937 feet (1,200 meters) elevation in areas receiving annual rainfall of 98 to 197 inches (2,500 to 5,000 millimeters). Some of the important genera occupying these shrublands include Dicranopteris, Metrosideros, Rubus, Sadleria, and sometimes Gunnera and Pritchardia.

The two subalpine shrubland types are dry and mesic, both of which are restricted to Maui and Hawaii, within the range of 5,906 to 9,843 feet (1,800 to 3,000 meters). Subalpine Dry Shrublands receive 20 to 59 inches (500 to 1,500 millimeters) of rainfall annually and contain Chenopodium, Dodonaea, Styphelia, and Vaccinium species. The Subalpine Mesic Shrubland community type is known from only one location on East Maui, where annual rainfall ranges from 51 to 75 inches (1,300 to 1,900 millimeters); it is vegetated predominantly by Sadleria and Vaccinium.

Finally, Alpine Dry Shrublands receive 30 to 49 inches (750 to 1,250 millimeters) of rainfall annually, and are restricted to elevations of 9,843 to 11,155 feet (3,000 to 3,400 meters) on East Maui and Hawaii. These extremely exposed habitats are home to species of silverswords (Argyroxiphium) and Dubautia. Plant species growing in these high-elevation sites have adapted to survive harsh and variable conditions. The silverswords exhibit a classic plant adaptation to such environments, having densely pubescent leaves that give the plant a silvery appearance and reflective property, protecting it from intense irradiance. Notable fauna include birds such as the Nene goose (Branta sandvicensis) and the Hawaiian dark-rumped petrel, an endangered bird that nests in these high elevations.

Biodiversity and Threats

Hawaii is famous for encompassing a range of biotic communities, each with unique characteristics that have allowed for spectacular biotic diversification over time. Hawaii has some of the highest rates of endemism in the world and also the greatest number of threatened and endangered species in the United States.

The increasing prevalence of invasive species in these shrubland communities, however, has shifted ecological processes and threatened native species. Many native shrubland habitats have lost a large number of indigenous and endemic species due to human activity. Some of the most critical threats to these habitats are introduced mammals, invertebrates, plants (especially several fire- and drought-adapted species), and human land use.

Additional threats to the Hawaii Tropical Shrublands biome include overgrazing by livestock and even seemingly benign human activities such as hiking, which can damage delicate plants and lead to erosion. Climate change impacts are still being studied. Rising sea levels are already threatening the long-term existence of some of the outlying, smaller islands, which could impact various bird species that circulate among the shrublands habitats across the archipelago.

Twenty-first century studies have shown decreasing rainfall and increased average air temperature. However, projections of these trends into the future is an unknown. As a rule, endemic species in such variegated habitat niches are more vulnerable to climate pattern alteration than species that thrive broadly across less unique habitats. Therefore, the species adapted to Hawaii Tropical Shrublands biome niches will tend to be more at risk as the climate continues its warmer, drier trends.

Bibliography

Ainsworth, Alison, and Donald R. Drake. "Classifying Hawaiian Plant Species along a Habitat Generalist-Specialist Continuum: Implications for Species Conservation under Climate Change. Plos One, 7 Feb. 2020, doi.org/10.1371/journal.pone.0228573. Accessed 6 Aug. 2022. Barton, Kasey E., et al. "Hawai‘I Forest Review: Synthesizing the Ecology, Evolution, and Conservation of a Model System." Perspectives in Plant Ecology, Evolution and Systematics, vol. 52, 2021, p. 125631, doi.org/10.1016/j.ppees.2021.125631. Accessed 23 Oct. 2024.

Berio Fortini, Lucas, et al. "Estimating the Impact of Climate and Vegetation Changes on Runoff Risk across the Hawaiian Landscape." Conservation, vol. 3, no. 2, 2023, pp. 291-302. doi.org/10.3390/conservation3020020. Accessed 23 Oct. 2024.

“Hawaiian High Islands Ecosystem.” The Nature Conservancy, 2007, . Accessed 23 Oct. 2024.

Olson, David. “Hawai'i Tropical High Shrublands.” One Earth, www.oneearth.org/ecoregions/hawaii-tropical-high-shrublands/. Accessed 23 Oct. 2024.

Wagner, Warren L., et al. Manual of the Flowering Plants of Hawaii, Revised Edition. University of Hawaii Press, 1999.

Ziegler, Alan C. Hawaiian Natural History, Ecology, and Evolution. University of Hawaii Press, 2002.