Patterns of plant endangerment on California’s Channel Islands

Background/Question/Methods

The conservation status of individual plant species in the Channel Islands flora has been dictated to a major part by historic and current land use impacts as well as differing management policies between islands. These impacts together with traits of plant life history, growth form, and morphology characteristic of the island floras have produced patterns of endangerment that differ from those on the mainland.  These oceanic islands include a northern (Anacapa, Santa Cruz, Santa Rosa and San Miguel) and southern group (San Clemente, Santa Catalina, Santa Barbara, and San Nicolas) subject to varying rainfall patterns but sharing many rare plant species. Island species require special attention to vulnerabilities related to introduced ungulate grazers and invasive plant species, as well as the potential for significant impacts from climate change with few available mitigation options.  A review of listed status, lifeform type, and noted threats for Channel Island endemic or listed plant species has been conducted to highlight informative patterns.

Results/Conclusions

With over seven hundred native plant taxa, nearly one hundred of which are endemic, the California Channel Islands comprise a significant component of diversity within the California Floristic Province. Despite a relatively small total area (908 km²), these taxa include more than 10% of the known flora of the state of California. The California Rare Plant Ranking (CRPR, formerly CNPS ranking) contains 148 listed taxa occurring on the Channel Islands. Twenty-two plant species occurring on the islands are listed as endangered or threatened federally. Differing from the mainland, nearly one third of the CRPR listed taxa and more than half of the federally listed taxa are woody shrubs, consistent with patterns on other oceanic island systems. Of the 148 CRPR island taxa, threats from introduced herbivores are highlighted in 53 instances, and 28 taxa have had recovery subsequent to herbivore removal.  As of this year, only one island remains with a population of introduced ungulates.  Species not responding favorably since ungulate removal may be more sensitive to other threats such as competition with invasive plants, climate change, and other human caused impacts, despite the preserved status of the majority of area of this island chain.