According to the EPA the most critical factor for Tijuana watershed areas on the U.S. side is the loss of species (EPA 2000). The region contains several important endangered and threatened species such as Bighorn sheep and the Arroyo toad (Cooperation 2000) and is a very important region for endangered vegetation communities such as coastal sage scrub and chaparral (Delgadillo 2000). Many of these species are migratory and known to exist on both sides of the border.
Biodiversity enhances ecosystem functioning such as cycling and fixing carbon, nitrogen. Other ecosystem benefits include water purification, air purification, and soil generation. High diversity of organisms is important for ecosystem health and resistance/recovery to disasters (Swanson 1992).
Loss of biodiversity in the TRW regions has been attributed to fragmentation, and percolating (creating holes) in vegetation (Ojeda 2000)
|
Vegetation
|
1938
% cover
|
1994
% cover
|
Patches 1938
|
Patches 1994
|
|
Chaparral
|
44.62
|
45.30
|
35
|
60
|
|
Coastal Sage scrub
|
37.08
|
29.03
|
10
|
48
|
|
Riparian
|
2.02
|
2.42
|
250
|
600
|
Percent changes in chaparral, riparian zones, and coastal sage scrub show little cause for alarm. However, the number of patches suggests these communities experienced a decrease carrying capacity with which to support their fauna. The main anthropogenic causes for fragmentation are cattle ranching, introduced species, and urbanization. Fragmentation can inhibit gene flow and cause population sinks which lead to extinction or genetic bottlenecking (decreasing resistance to disasters genetically)(MacArthur 1967; Diamond 1975; Westman 1985; Ney-Nifle 1999).
Solution: Corridors between patches such as vegetated stream-sides