Stratification Units

	Hawaiian High Islands Ecoregion This page last revised 23 August 2006 -- S.M.Gon III
Home Introduction Ecoregion Conservation Targets Viability Goals Portfolio TNC Action Sites Threats Strategies Acknowledgements ▫ Tables Maps & Figures CPT Database Appendices Glossary Sources Forest bird habitat occurs in three of the four stratification units of the ecoregion. This rare bog community only occurs in the Maui Nui Stratification Unit.	Stratification Units Ecoregional biodiversity distribution Biological diversity in the Hawaiian Islands is spread among the main high islands because of island-level endemism. We cannot choose to work on only one or two islands and consider our mission successful. This is because each island contains species unique to that island. For example, of 1,050 described taxa of native flowering plants, there are three found only on Ni‘ihau, 225 restricted to Kaua‘i, 157 found only on O‘ahu, 40 known only from Moloka‘i, 12 unique to Lāna‘i, 96 found only on Maui, 2 reported only from Kaho‘olawe, and 106 known only from Hawai‘i (Wagner et al 1990). Accounting for all of these island-level endemics, we are left with fewer than half the flowering plant taxa (409) that show wider distributions. Fewer than 150 can be found on all six of the higher main islands. The situation is even more pronounced among invertebrates, which comprise the majority of terrestrial species-level diversity, and show remarkable diversification and geographic endemism even within a single island setting. It becomes quickly clear that multiple examples of ecological systems across the archipelago are required to adequately represent species level biological diversity. We ensure this cross-ecoregional coverage by planning for redundant representation of major conservation targets, such as ecological systems, natural communities, and species concentrations. (continued next column)	Stratification: The insular nature of the Hawaiian Islands lends itself well to natural stratification based on island groupings. We selected four major stratification units: Kaua‘i Unit – The islands of Kaua‘i and Ni‘ihau. Kaua‘i in particular is a hotbed of endemic biodiversity. The channel separating Kaua‘i from the rest of the archipelago is the widest, leaving the oldest high island most isolated and biologically distinct. O‘ahu Unit – The island of O‘ahu is also botanically rich, bears 80% of the human population, and contains two conservation areas with many occurrences of rare and endangered species. Maui Nui Unit – The islands of Maui, Moloka‘i, Lāna‘i, and Kaho‘olawe are geologically and biologically similar, having once been united as a single large island during the Pleistocene. Hawai‘i Unit – The large, geological youngest island of Hawai‘i is comprised of six landscapes. The total contiguous native-dominated area of this unit exceeds that of all other units combined. It is fortuitous that the stratification units above also correspond with political (county level) units, so that overarching strategies need not require multijurisdictional agreements. With the exception of Maui Nui and Kaua‘i units, the stratification units also correspond with individual islands. The individual islands also form meaningful stratification subunits.
This shrubland of the silversword relative iliau (Wilkesia gymnoxiphium) is restricted to the Kaua'i Stratification Unit. Map of Stratification Units: Four stratification units reflect the biogeography of the islands: Older islands (Kaua`i and O`ahu are botanically rich, while the Maui Nui cluster share similar age and biology. The youngest and largest Island of Hawai`i is also biologically distinct.
Only the youngest, largest islands bear alpine zones, with snow during winter.	Target distribution within the stratification units above is not even. For example only the islands of Maui and Hawai‘i offer opportunities to protect occurrences of the Hawaiian Alpine System. Only islands with significant expanses of montane wet and mesic ecological system occurrences can boast forest bird concentrations, but these are spread across the archipelago (on Kaua‘i, Maui, and Hawai‘i). Even when multiple examples of ES occurrences fall across the archipelago, there are differences in their size, condition and landscape context, so that it makes sense to distinguish the largest, most intact and significant occurrences from those that are present, but smaller or more disturbed.	For example, although occurrences of the Lowland Wet Ecological System occur on six of the eight islands, on only three (Kaua‘i, Maui and Hawai‘i) are there large, exemplary occurrences that emerge as highest priority action sites for protection of this conservation target. Exemplary Lowland Wet Forest, Lumaha‘i Valley, Island of Kaua‘i

Stratification Units

Ecoregional biodiversity distribution

Stratification: