The decline of coastal sage scrub in California
The decline of coastal sage scrub communities in California
California is home to many diverse ecosystems that support numerous amounts of biodiversity and species richness. In particular, its plant communities are unique and complex, some existing nowhere else in the world. One of particular interest is coastal sage scrub (CSS), a plant community that is in rapid decline, as well as the insects, birds, reptiles and mammals it supports. Causes of decline are mostly human induced including; pollution, introduction of exotics and invasives, which increase fire regimes, as well as urbanization and agriculture, which both lead to fragmentation. Studies of the decline of CSS have raised awareness, restoration efforts, and conservation efforts.
Coastal sage scrub is generally present in a Mediterranean type climate where chaparral is generally seen. Sometimes CSS may be termed soft-chaparral due to its less woody stems. In times of drought, plants are deciduous, increasingly mesophyllous, and from about 0.5 to 2.0 m in height (Westman 1981). This plant community is usually mixed with the more typical chaparral but has a tendency to occur in more xeric sites. Much of California is known for populations of CSS plants, generally from the most northern region of the San Francisco Bay to the southern region of El Rosario in Baja California (Westman 1981). Axelrod (1978) hypothesized that CSS originated in the Early and Middle Quaternary when the environment was dry enough to support coastal sage scrub species (Axelrod 1978). He suggested that human impacts, such overgrazing for cattle, clearing for agriculture or development and fire events might have helped the community to spread to these disturbed areas (Axelrod 1978). Although some disruption may assist a plants ability to germinate, human population size was greatly reduced in the beginning of the CSS community’s birth, and the rapid increase of population is causing the community to become over-disturbed, creating barren living grounds for plants and animals alike.
Diversity had become greater through evolution of the plant community, and has since rapidly declined. The most prevalent species seen in the plant community today include; dominant shrubs California sage (Artemisia californica), California sunflower (Encelia californica), brittlebush (Encelia farinose), wild buckwheat (Eriogonum fasciculatum), lemonade berry (Rhus integrifolia), laurel sumac (Rhus laurina), white sage (Salvia apiana), purple sage (Salvia leucophylla), black sage (Salvia mellifera), and yucca (Yucca whipplei) (Westman 1981). As well as various herbs, including; yellow star thistle (Centaurea melitensis), canyon prince wild rye (Elymus condensatus), prostrate spurge (Euphorbia polycarpa), yellow clover (Haplopappus squarrosus), deerweed (Lotus scoparius), California melic (Melica imperfect), wishbone bush (Mirabilis californica), California peony (Paeonia californica), fairy mist (Pterostegia drymarioides), needle grass (Stipa lepida), and poison oak (Toxicodendron diversilobum) (Westman 1981). These species tend to be drought-deciduous, mildly woody plants with tough epidermal layers and half woody stems with shallow roots and aromatic odors (Kirkpatrick and Hutchinson 1977).
The CSS communities of California are possibly the most endangered habitat in the United States, with only 15% of its original area being intact in 1980 and most of the ecosystem being lost to agriculture and development (Rubinoff 2001). Coastal sage scrub is home to numerous threatened or endangered plant species as well as animal taxa. The southern ranges of the CSS community are home to about 100 species under legislative protection or in the proposal phase of protection, with the community being home to various endemics (Rubinoff 2001). A notable inhabitant of the CSS community is the California Gnatcatcher (Polioptila californica), whose nesting range is almost completely limited to CSS vegetation (Rubinoff 2001). Also within the CSS community is Stephens’ kangaroo rat (Dipodomys stephensi Merriam), which is endangered due to its small geographic range, which is being reduced by rapid urbanization (Price et al. 1994). Some other species that are protected include the western spadefoot toad (Spea hammondii), western pond turtle (Emys marmorata), and the California tiger salamander (Ambystoma californiense) (Fox and Nino-Murcia 2005). The numbers of species becoming rare, threatened and endangered in the CSS community is rising.
Endangerment of the CSS communities of California has been caused by many factors. A serious issue is pollution, especially pertaining to the more inland areas of habitat. Pollution disrupts and disturbs chemical processes in the community and, creates habitable conditions for invasive or exotic species of plants. A serious concern is the type-conversion of coastal sage scrub to grasslands, which are forged by exotic grasses. Talluto and Sudling (2008) compared a 76 year span of time, and showed that CSS cover had been reduced by 49%, and has been replaced with exotic grassland species. Non-native grasses such as those in the genera Bromus and Avena thrive in polluted conditions. Nitrogen deposition in Los Angeles is the greatest in the nation, and is mostly caused from air pollution (Talluto and Sudling 2008). Nitrogen deposition is a key by-product of the burning of fossil fuels, mostly to power vehicles in the automobile congested areas of the greater Los Angeles area, as well as pollution from industrialization and close proximity construction. Exotic grasses in California favor nitrogen rich environments and native perennials cannot survive in the N-rich environment and are outcompeted by the invasive species (Talluto and Sudling 2008). Type conversion of CSS is a serious problem, one that lessens biodiversity. Grasslands, especially ones that are facilitated by increased nitrogen also pose a greater fire risk.
Fire is a natural occurrence in the lifecycle of California plant communities and has been disrupted due to human influence. Increased air pollution creates high intensity of flammable material within the native plants, as well as invasive species. Chaparral (CSS’s sister community) is well adapted to fire, and those traits have also been associated with CSS due to the communities’ similarities. These shrub lands are more flammable due to the compact structure of the community, low moisture availability, and very low decomposition rates (Syphard et al. 2006). Current fire regime studies found that there should be strategic efforts nearest fragment lines to keep domestic fire issues away from the wild community (Syphard et al. 2006). The increase of fires has facilitated the spread of exotic grasses, increasing their overall dominance. Due to the low ignition temperatures and high nitrogen contents, invasive grasses burn readily having the longest fire season in California (Syphard et al. 2006). The increase in fire regimes is decreasing the activity of crown sprouting and fire-germinated seedlings that are being outcompeted by exotic grasses.
Urbanization is another factor in the disappearance of native CSS communities. Morrison and Bolger (2002) show that only 10% of CSS’ natural community remains, due to high rates of urbanization. Not only are communities being completely overrun by urbanization, but the edges where urban and wild systems meet are also creating unique ‘ecotone’ locations. Ecotones are zones of transition between neighboring ecological systems (Gosz 1993). Although urbanization is not a ‘natural’ ecosystem, it functions on its own with unique characteristics. It is especially important to understand the environmental issues within urband-wildland boundaries, where an artificial fragmented ecosystem is created (Morrison and Bolger 2002). Increasing urbanization has created a fragmented system of native ecosystem islands surrounded by mostly urban areas. This has left very few places undeveloped, most of which are steep sloped canyons that are hard for any organism to live in (Crooks and Soule 1999).
The Mediterranean climate and ocean breeze facilitate the growth of many agricultural crops, and is reducing the populations of CSS communities. Not only does agriculture create problems of habitat loss and fragmentation, but it also causes pollution. A study concerning the endangered red-legged frog (Rana aurora draytonii) found that pesticides are being transported in residues of organophosphates to different elevations and areas all together and are contributing to declines of the red-legged frog (Davidson et al. 2001). The decline of one species, especially by chemical mechanisms will likely contribute to the decline of various other species, both plant and animal. Although more research is needed to show the exact effects of agrochemical quantity in a living organisms system and its effect, it is known that the chemicals interfere with hibernation and immune system response and suppression (Davidson et al. 2001). Overall agriculture increases fragmentation and pollution. Davidson et al. (2001) also indicates that agricultural and ornamental species may escape into the wild by natural seed movement, and can create shaded areas at ecotone boundaries that reduces photosynthetic activity.
Natural ecosystem functions are being altered, especially predation and food web patterns. The disappearance of scrub birds is related to fragmentation and urbanization. Crooks and Soule (1999) that found that 32% of homeowners on the cusp of San Diegen urban fragments each had an average of 1.7 domesticated cats. This finding led to another statistic, that 77% of these land owners allow the felines to recreationally hunt outdoors, which leads to 84% of cats bringing back kills to their residence (Crooks and Soule 1999). If domesticated cats continue to recreationally hunt scrub-breeding birds, the decline will be swift, and the food web disturbance will increase to a higher trophic level. Even very slight increases in fragmentation as well as mesopredator pressure are driving native prey to rarity and extinction (Crooks and Soule 1999).
Issues surrounding conservation and restoration of CSS communities include policy issues, property rights and democratic participation as well as public awareness. There have been many milestones helping to conserve natural lands since the enacting of the Endangered Species Act (ESA) of 1973. The beginning of ESA guidelines generally had interests only in conserving species that were valuable for commercial, recreational and economic value (Watt et al. 2004). The Committee on Rare and Endangered Wildlife was established, and produced the first list of endangered species, though this was limited to mammals and birds- no plants or insects were included (Watt et al. 2004). Later though, insects and plants joined the list and some became game-changers for these laws, including the Mission Blue Butterfly. This butterfly was found on a proposed multi-million dollar residential site in San Bruno, CA and helped to establish the first Habitat Conservation Plan or HCP (Watt et al. 2004). Preservation of land due to the presence of certain species became the biggest reason for areas to be protected. Landscape protection is complicated due to varying definitions of what a landscape really is and what the definition should include (Watt et al. 2004).
Conservation has been focused on single species, which by default helps to protect whole ecosystems due to its presence. Much of the conservation efforts in California have been within San Diego, Riverside and Orange counties, that are home to the threatened California gnatcatcher (Polioptila californica). It has been found that more protection is needed in northern parts of the state as well, where numerous sensitive, rare or endangered species exist (Davis et al. 1994). The government needs to support different types of environmental ethic ideals, as well as the politics of the situation (Price et al. 1994). Conservation takes economic planning as well as regional goal setting. There is continued pressure due to urban development and private property, where rare or endangered species have been found, and where protection is ambiguous (Price et al. 1994).
Optimistically, restoration efforts have shown some promise. Expanding coastal sage scrub sites, rather than only protecting isolated patches has helped to increase conservation (Bowler 2000). Continual grass control by mowing and grass specific herbicide reduced the reoccurrence of grasses for a six-year span, and was also shown for exotic forbs (Cox and Allen 2008). Although volunteer groups and conservation groups have restored many sites, there is little research on long-term survival of these restored communities. Data on post-fire succession and community behavior are not generally part of mitigation or restoration plans (Bowler 2000). It seems that data on restoration would be of the utmost interest to those restoring plant communities. Without clear data sets, the possible success of these sites will go unnoticed, and any possible problems would also be overlooked. Hierl et al. (2007) have created a solid plan for ecological conservation monitoring. They have proposed guidelines based on extent, representativeness, fragmentation, and endangerment (Hierl et al. 2007). This study assesses the state of communities in need of restoration and helps other conservationists have a plan that assists in prioritization. Overall, a need for management of restored sites is required, even at the government level, to help sustain the CSS communities of California.
The many issues associated with decline of the CSS communities of California are difficult to overlook. With such rapid degradation of the community over time, it is amazing that more has not been done to assist in its recovery and protection. Pollution, exotic species, fire disruption, urbanization, agriculture and fragmentation are at the forefront of the issue. While the situation is dire, there have been efforts made not only by conservationists, but also policy makers to help remedy the situation. More laws are needed to prevent increased development and disturbances of the CSS communities. If this community were to disappear altogether, the fate of other plant communities, particularly chaparral and riparian would be of great concern. While there is no way to predict the future of wild conditions in California, there can be predictable means of conservation, and restoration in the areas of need.
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