An uncertain future for California sage scrub: Implications of land use and climate change for a threatened plant community

Background/Question/Methods

The considerable diversity, endemism, and threatened status of California sage scrub (CSS) make it a conservation priority within the California Floristic Province, however, the ecosystem faces an uncertain future of habitat loss driven by climate change and continued land use.  Land development has already caused widespread loss and fragmentation of CSS, with as little as 10% of the original extent intact today, while climate change will likely drive further habitat loss and exacerbate existing threats.  Using a species distribution modeling approach, we (1) predict changes in CSS habitat and species richness driven by climate change to identify areas sensitive to change and biodiversity declines, and (2) predict the relative threats of CSS habitat loss driven by land use and climate change in California and northwestern Baja California, which differ dramatically in their patterns of land use and protection.  We modeled the climatically suitable habitat of 36 characteristic CSS species under two future climate and two dispersal scenarios using the modeling algorithm MAXENT and publically available species records and climate databases.

Results/Conclusions

We identified two patterns of habitat change driven by climate: southern habitat contraction and northern habitat expansion, though species varied in magnitude of response. We predict the greatest declines in CSS richness in coastal areas of San Diego County and northwestern Baja California; current centers of CSS diversity and endemism.  Over two thirds of the species modeled are expected to lose habitat with climate change, even under best-case unlimited dispersal scenarios.  In northwestern Baja California, there is minimal habitat protection from rapidly increasing land use in coastal areas where models also predict considerable habitat and diversity losses driven by climate change.  While both development and climate change pose strong threats in the region, land conversion may be a more immediate threat.  Although most remaining fragments of CSS in California are currently protected, the landscape has undergone heavier conversion and fragmentation compared to northwestern Baja California.  Land use will likely exacerbate the effects of climate change, impeding species’ ability to disperse to suitable habitat.  Thus, differing conservation strategies are likely needed for successful management of CSS in the two regions.  Northwestern Baja California is a particularly high conservation priority for CSS, given its diversity and lack in protected habitat.