.masters.research

An Analysis of Land-use change in the Tijuana River Watershed, 1994-2006.

Masters Thesis: Introduction
The California Floristic Province, stretching from southwest Oregon to northern Baja California, is internationally recognized as one of the world's 25 biodiversity hotspots (Conservation International, 2008; Myers et al., 2000). It encompasses the South Coast Floristic Region, an area along the U.S.-Mexico border known for supporting the highest number of endemic plant species in the California Floristic Province. In the center of this floristic region lies the Tijuana River Watershed, a 4,532 km 2 basin comprised of portions of northern Baja California and southern San Diego County (Ganster, 2005).

Aside from being part of the California Floristic Province, the Tijuana River Watershed has several key physical characteristics that make it unique. First, it is a binational watershed with one-third of its area in the United States and two-thirds in Mexico (Wright, 2005). The Tijuana River is one of the many examples of shared resources, including water and ecological resources, which are bisected by an international border. The headwaters of the river begin in eastern San Diego County and travel south across the border into the city of Tijuana, eventually returning to San Diego where the river drains into the Pacific Ocean. Second, the watershed contains one of the last coastal wetlands in Southern California, the Tijuana River Estuary (Ganster, 2005; Roullard, 2005). The 1,000-hectare publicly owned reserve is located north of the city of Tijuana in San Diego County and is the endpoint for rivers and streams in the watershed as they travel through the estuary into the Pacific Ocean. The estuary also serves as an important rest stop for migratory birds traveling south along the Pacific Flyway (Roullard, 2005). Finally, the Tijuana River Watershed supports a significant number of native plant communities, including a variety of species of coastal sage scrub and chaparral (O'Leary, 2005). These globally rare plant communities provide habitat to a number of threatened and endangered wildlife species (O'Leary, 2005).

The physical characteristics of the Tijuana River Watershed make it an important site for the study of habitat and water resources, especially since in recent decades resources have become severely impacted by humans (Liverman et al., 1999). The distribution of the watershed's 1.4 million inhabitants has been the primary factor affecting the natural environment of the basin (Wright, 2005). This is because the intensity and location of inhabitants corresponds with the type of land use for residential, economic, recreational, transportation, and commercial purposes (Wright, 2005). In addition, in recent years land use planning on both sides of the border has allowed for increased urban development (Vela, 2005). This is of particular concern for areas in the Mexican portion of the watershed since urban expansion took place in areas that have a high propensity to erosion, landslides, and flooding (Vela, 2005).

Human impacts have a range of outcomes for the natural environment (Liverman et al., 1999). For the Tijuana River Watershed, water quality, water quantity, and soil erosion have been disturbed (Ganster et al., 2000). For example, in the lower Tijuana River Valley, development practices changed the cycle of erosion due to the increased amount of impervious surfaces. The effect has been that even light rains erode the bare hillsides. This, in turn, causes trash and large amounts of sediment to be carried to the Tijuana River Estuary (Roullard, 2005).

In addition to impacts on water quality and quantity, the single greatest threat to native plant communities in this region has been habitat loss and fragmentation (Conservation Biology Institute, 2004). The two dominant vegetation communities are coastal sage scrub and chaparral, which account for nearly three-quarters of all vegetation in the watershed (O'Leary, 2005). Coastal sage scrub is a low-growing vegetation community that consists of Diegan sage scrub along the coast and Martirian succulent scrub to the south (Westman, 1983 in Conservation Biology Institute, 2004). Chaparral is a hard-leaved vegetation community and in the Tijuana River Watershed it includes mainly dense thickets of shrubs such as red shank chaparral ( Adenostoma sparsifolium ) and chamise chaparral ( Adenostoma fasciculatum ) (Hanes, 1965 in Conservation Biology Institute, 2004). These two communities along with communities of juniper scrub and riparian woodland have been fragmented over time (Ojeda-Revah, 2000). The native vegetation is important because it provides habitat to species such as the California gnatcatcher ( Polioptila californica californica) , southwestern arroyo toad ( Bufo californicus) , light-footed clapper rail ( Rallus longirostris levipes) , and the quino checkerspot butterfly ( Euphydryas editha quino) . Each of these species has been listed as a federally threatened or endangered species (U.S. Fish and Wildlife Service, 2005, 2003, 1993; Zembal et al., 1989).

Past work in the watershed has included a brief discussion of the causes of land-use changes from the early settlement of Northern Baja California through 1991 (Minnich & Vizcaino, 1998) and from 1970-1994 during a period of industrialization (Ojeda-Revah et al., 2008). The causes for the disruption of habitat are primarily attributed to residential and industrial development, agricultural development, grazing, and the introduction of exotic species (Ojeda-Revah, 2000; Flather et al. 1998). For the time period 1953-1994, changes in land cover were primarily attributed to factors such as the conversion of lands to agricultural and urban uses (Minnich & Vizcaino, 1998; Ojeda-Revah et al., 2008). In the Mexican portion of the watershed, urban growth during this time period was driven by population growth of migrants from Michoacán, Sonora, Nayarit and Guanajuato due to job creation from national border industrialization policies, poor infrastructure investment which forced populations to move near the outer edges of urban areas, and lack of law enforcement preventing illegal land-use change (Ojeda-Revah et al., 2008). In the U.S. portion of the watershed, expansion of agricultural and urban uses was caused by population growth, socio-economic encouragement of suburban growth, policies empowering local governments combined with lower taxes in rural areas, and infrastructure construction (Ojeda-Revah et al., 2008).

Although these studies suggest potential influences on land-use change, identifying causes was not the primary purpose of these studies. Furthermore, little work has been done since 1994 to update information on dominant land-use changes that have taken place, and there has been no work done that investigates the causes of any recent changes. Further population growth, urban development, and increased transportation infrastructure in recent years on both sides of the border suggest that land use has changed in the Tijuana River Watershed and warrants further investigation. The purpose of this study is to assess the dominant types of land-use change that have occurred since 1994 and identify and analyze the causes of changes in land use.

Research has been done that is primarily focused on the city of Tijuana's growing urban population and the effects this may have for the natural environment (Higuera et al., 1993). However, the Tijuana River Watershed encompasses much land in the United States and Mexico that is outside of the city of Tijuana. It includes areas of irrigated and rainfed agriculture, grasslands, forest, scrubland, and rural development (Ojeda-Revah et al., 2008; Wright, 2005). In order to understand the complexities of changing land use in such varied areas, I studied land use throughout the watershed. There is great need for watershed scale research, especially for watersheds that span international borders, so that information regarding interconnected ecological processes and human activities is available (Brown, 2003).

The contributions of this work are twofold: 1) to advance the theoretical framework for understanding the causes of changing land use at a watershed scale, and 2) to contribute practical applications to the Las Californias Binational Conservation Initiative by identifying the causes of land-use change throughout the watershed. The Conservation Biology Institute, in partnership with Pronatura and The Nature Conservancy, initiated a habitat conservation project in 2004 titled The Las Californias Binational Conservation Initiative. The objective of their work is to develop landscape-scale conservation strategies, sustainable land use planning and long-term management programs for the border region (Conservation Biology Institute, 2004). Among the goals for the Las Californias Program are identifying threats to maintaining an interconnected conservation network, identifying large, intact wildlands that represent the region's biodiversity, linking protected areas to facilitate wildlife movement, promoting collaboration in implementing land protection strategies, and laying the foundation for a binational park system that connects the Parque Constitución de 1857 in Mexico to wilderness areas, forests and park land in the United States (Conservation Biology Institute, 2004). In order for long-term planning strategies to occur and succeed there must first be an understanding of the types of land-use change that took place in recent years and an understanding of the causes of such changes.

Past conservation planning has met with some difficulty due to the fact that land use and land ownership patterns differ markedly between the two sides of the border (Conservation Biology Institute, 2004). An initial goal of this project is to classify and update knowledge of changing patterns of land use and ownership on both sides of the border. In addition, a major goal of the Las Californias initiative is to “identify threats to maintaining an interconnected conservation network and sustaining ecosystem processes” (Conservation Biology Institute, 2004, p. 4). Assessing causes of changing land use will contribute to an understanding of the human activities that are currently playing dominant roles in disturbances to local wildlife populations and native vegetation cover.

In the following chapters, I will describe the context, approach, and results of this study. The next chapter provides an overview of the concept of ‘drivers' of land-use change, a background of the relevant characteristics of the study area, and the framework for approaching land-use change in the watershed. The third chapter gives a detailed explanation of methods that were employed. Following this, two chapters are used to present the findings and a detailed discussion of land-use change on both sides of the border. Finally, the remaining chapter ties together the themes of this research and makes recommendations for those involved with continuing the Las Californias Binational Conservation Initiative.

Thesis Research Questions
(1) What are the dominant land-use changes that took place from 1994-2006 in the Tijuana River Watershed?
(2) How have the Multiple Species Conservation Program (MSCP) and Ley Agraria influenced land-use change in the Tijuana River Watershed? How does this differ on either side of the border?
(3) In areas experiencing rapid land-use change from agriculture to urban what are the proximate causes of such changes? How does this differ on either side of the border?

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