Globally, Mediterranean climate regions make up only 2% of the Earth’s land surface but contain a remarkable 20% of the world’s plant species.1Conservation International, Critical Ecosystem Partnership Fund, California Floristic Province, 2018.
Historically, the river has been both periodically dry and, at times, prone to severe flooding.2Blake Gumprecht, The Los Angeles River: Its Life, Death, and Possible Rebirth, 2001, pp 9-15. These seasonal natural disturbances supported habitat and water for numerous endemic plants and animals, as well as migratory birds resting as they traveled the Pacific Flyway.3Kimball L. Garret, “The Biota of the Los Angeles River”, 1993, pp 2. Historic maps of the region along with studies done for nearby waterways indicate that the historic flora and fauna of the LA River were likely a mix of coastal sage scrub and valley grassland ecosystems, with swaths of Southern California oak and walnut forests as the river approached the Santa Monica Mountains. As the river continued south it spread out over the alluvial LA Plain through a patchwork of riparian forests, wetlands and coastal sagebrush.4Kimball L. Garret, “The Biota of the Los Angeles River”, 1993, pp 3-10. For a description of the Los Angeles Prairie see: Schiffman, Paula M. “The Los Angeles Prairie.” From Deverell, William and Greg Hise, Land of Sunshine: An Environmental History of Metropolitan Los Angeles, 2005, pp. 38–51. For historic vegetation mapping see: Kuchkler, A. W. Natural Vegetation of California, 1977. For studies on nearby waterways see: Stein, ED, S Dark, T Longcore, N Hall, M Beland, R Grossinger, J Casanova, M Sutula, “Historical ecology and landscape change of the San Gabriel River and floodplain.” 2007.
Existing and Potential Habitat Patches and Corridors
Today, 48 of the 51 miles of the river are within heavily developed areas. Urbanization, the near complete concrete channelization of the river for flood control, and the continued impacts of a changing climate have altered the river as a native ecosystem. Within this altered context, the river’s capacity to support biological life is determined by hydraulic conditions, channel geometry, and connectivity across and along the river to adjacent patches and habitat areas. The 11.3 miles of soft bottom (portions of the channel with an earthen bottom) at Sepulveda Basin, the Glendale Narrows, and the tidal estuary are the most ecologically healthy; however, much of the river corridor continues to support algae, insects, fish, and local and migratory birds.5See for example: Kimball L. Garret, “The Biota of the Los Angeles River”, 1993; FoLAR, “The First State of the Los Angeles River Report”, 2005; FoLAR, “State of the River 2 The Fish Study”, 2008; FoLAR, “State of the River 3 The Long Beach Fish Study”, 2016
The soft bottom portions of the river also contain the most problematic invasive plant species, such as arundo (Arundo donax). These invasive species outcompete native species that might otherwise flourish in the soft bottom areas and transpire water at rates up to five times higher than native riparian plant species.6 Giessow, Jason, J. Casanova, R. Leclerc, R. MacArthur, G. Fleming, J. Giessow. 2011. Arundo donax (giant reed): Distribution and Impact Report. California Invasive Plant Council (Cal-IPC), California, USA.) Despite the current dominance of invasive species in the river channel, over 132 rare or threatened species (such as the Bell’s vireo) are associated with the river channel and adjacent areas. According to the California Natural Diversity Database (CNDDB), maintaining and enhancing habitat areas and improved connectivity through tributaries and to adjacent upland habitat areas has the potential to increase overall urban biodiversity given the high natural biodiversity occurring nearby in the region’s large inland protected areas.7See:Caltrans and CDFW, California Essential Habitat Connectivity Project: A Strategy for Conserving a Connected California, 2010 & US National Park Service, “Researchers Begin Monitoring LA River Wildlife Using Remote Cameras,” 2018. Additionally, elements of the river’s former ecology can be reintroduced where appropriate to reestablish many of the rare riparian and upland ecosystems that have been lost to urbanization. It is recommended that environmental planning efforts along the river focus on creating habitat areas large enough to support native ecosystems, interconnectivity of these habitat areas, and active monitoring and management of these areas over time against the compounding stresses of urbanization, climate change, hydrologic change, and continued competition from invasive species.