Biological diversity is not evenly distributed across the Earth; certain latitudes, ecosystems, and regions contain more diversity than others. There are several factors that influence how many species a given ecosystem supports and these factors lead to predictable patterns of diversity across the globe.
Some regions are deemed biodiversity hotspots (Fig 2.3.1); these regions contain more than 1,500 endemic (found only in that region) plant species and have lost at least 70% of their original area to anthropogenic uses (Myers et al 2000). Consequently, hotspots contain a high number of species that are unique to that ecosystem and which face serious conservation threats. The United States contains two biodiversity hotspots. The California Floristic Province stretches from southern Oregon to northern Baja Mexico, covers the majority of the state of California, contains ~2,125 endemic plant species and has lost over 75% of its area to agriculture and urban expansion. Nearly 40% of the Province is under federal protection to preserve the remaining biodiversity. The other biodiversity hotspot in the United States is the state of Hawai’i, which is part of the Polynesia-Micronesia hotspot incorporating the majority of the southern Pacific Ocean island systems.