accelerating valley development. This abundance of surface water inspired the first of Kohala's three massive irrigation diversion "ditches", the Upper Hamakua Ditch, although the water was initially used for transporting ("fluming") sugarcane to the mills rather than irrigation. Surface water supplies, however, were not stable enough for the large scale plantation economy developing in Hamakua and North Kohala, so later ditches tapped the spring fed base flows of the valleys and larger gulches. Only Waimanu Valley escaped with none of its tributaries tapped, along with the 16 smaller streams between Waikoloa Stream and the Wailoa river in Waipi'o Valley. Ecology The natural habitats in the Kohala district range across a wide rainfall gradient in a very short distance—from less than a year on the coast near Kawaihae, to more than a year near the summit of Kohala Mountain, a distance of just . At the coast are remnants of dry forests, and near the summit lies a cloud forest, a type of rainforest that obtains much of its moisture from "cloud drip" in addition to precipitation. These large cloud forests dominate its slopes. This biome is rare, and contains a disproportionate percentage of the world's rare and endemic species. The soil at Kohala is nitrogen-rich, facilitating root growth. The forest lies perpendicular to the prevailing trade winds, a characteristic that leads to frequent cloud formation and abundant rainfall. Thus the rain forests carpeting Kohala are vital to the supply of groundwater on the island. The role of Kohala's ecosystem in supporting water flow has long been recognized. As early as 1902, US Forester E. M. Griffith wrote "Forest protection means not only increasing the rainfall, but more important still, conserving the water supply. The future welfare and agricultural prosperity of the Hawaiian Islands depends upon the preservation of the forest." Before humans arrived, the wildlife population at Kohala and on the rest of the island was isolated from other species, as it was separated from the nearest major landmass by over of open water. The early colonizers of Kohala came by wind, storm or floating on the ocean. Successful colonizations occur on the magnitude of once every 35,000 years. The 2,000 successful ancestors that arrived at Kohala evolved into about 8,500 to 12,500 unique species, resulting in a very high ratio of the species present being found nowhere else on Earth. Only about 2.5% of the world's rainforest is cloud