rainforest, making it a unique habitat as well. The mountain supports approximately 155 native species of vertebrates, crustaceans, mollusks, and plants. A diverse complexion of fungi, liverwort, and mosses further add to the variety. In fact, up to a quarter of the plants in the forest are mosses and ferns. These work to capture the water from clouds, in turn providing microhabitats for invertebrates and amphibians, and their predators. Estimates on the water capacity of the forest range from . The mountain is also home to several bogs, which exist as breaks in the cloud forests. It is believed that bogs form in low-lying areas where clay in the soil prevents proper water drainage, resulting in an accumulation of water that impedes the root systems of woody plants. Kohala's bogs are characterized by sedges, mosses of the genus Sphagnum, and oha wai, native members of the bellflower family. Other habitats include rain forest and mesophytic (wet) forests. The same isolation that produced Kohala's unique ecosystem also makes it very vulnerable to invasive species. Alien plants and feral animals are among the greatest threats to the local ecology. Plants like the kahili ginger (Hedychium gardnerianum) and the strawberry guava (Psidium cattleinum) displace native species. Prior to human settlement, many major organisms such as conifers and rodents never made it onto the island, so the ecosystem never developed defenses against them, leaving Hawaii vulnerable to damage by hoofed animals, rodents, and predation. Kohala's native Hawaiian rain forest has a thick layer of ferns and mosses carpeting the floor, which act as sponges, absorbing water from rain and passing much of it through to the groundwater in aquifers below; when feral animals like pigs trample the covering, the forest loses its ability to hold in water effectively, and instead of recharging the aquifer, it results in a severe loss of topsoil, much of which ends up being dumped by streams into the ocean. Human history There is evidence of pre-modern agriculture on the leeward slopes of Kohala. From 1400 to 1800, the principal crop grown at Kohala was sweet potato (Ipomoea batatas), although there is also evidence of yams (Dioscorea sp.), taro (Colocasia esculenta), bananas (Musa hybrids), sugarcane (Saccharum officinarum), and gourds of the family Cucurbitaceae. The optimal rainfall level for the sweet potato lies between per year. A combination of factors makes the rainfall at Kohala variable both from location to location