If the alternatives presented here are to be considered by the regional stakeholders and their decision-making processes, there is significant benefit in doing so soon. The spatial patterns and magnitudes of the impacts of the several alternative scenarios have greater variance in the short term between 1990+ and 2010, than in the longer period toward build-out.
When considered in a longer time frame, in which the scenarios are carried forward to their build-out, all of the alternatives cause serious impacts, as was seen in figure 132. All will lose agriculturally productive soils and cause increased erosion and sedimentation. All of the development scenarios will cause a change in soil moisture, a reduction in water tables in the river basins and a dramatic increase of flood discharges. The net effect will be a reduction of biological diversity in the region of Camp Pendleton as the hydro-period of the riparian zone becomes shorter, and floods wash out flood plain sediments and associated native vegetation. Re-colonization by native vegetation, which once happened naturally, will be inhibited by introduced naturalized plants such as the giant cane, Arundo donax, that can opportunistically dominate a flood plain to the exclusion of native vegetation. This will radically change the riparian zones which are (or were) the areas of greatest species richness. Upland biodiversity will also be altered in all scenarios due to reduced soil moisture and increased fire suppression associated with rural residential development. In all scenarios, the landscape will lose most of its visually preferred character.
The direct consequences on biodiversity of all the scenarios are also all negative. In all of them, but to different degrees, the landscape ecological pattern is increasingly fragmented. Natural areas are decreased in area and increasingly isolated in an urbanized region. Some of the most important single species, such as the gnatcatcher and cougar, will be seriously impacted, and their long-term regional survival will be in doubt. The pattern of species richness will be dramatically altered by invasive species.
On the positive side, the population forecast for 2010 can be easily accommodated and the build-out populations for all scenarios are potentially several times larger. If one assumes the continued importation of water (as the regional plans and alternative scenarios do), the region can accommodate growth into the longer-range future.
When the several regional alternatives are assumed to be developed to build-out, all of the environmental measures upon which this research has focused will decline dramatically. While all of the alternative futures converge to a generally similar pattern of impacts caused by the transformation of a predominantly "natural" regional landscape to a predominantly "urbanized" pattern, the variance among the alternatives, both spatially and in magnitude, is almost entirely the result of policies which (are assumed to) take effect between 1990+ and 2010. The Private Conservation alternative, which proposes to maintain the integrity of the region's larger natural areas and their connections along the riparian network, is also the most protective of the long-term regional biodiversity. As was seen in Spread with Conservation 2010, waiting fifteen years will not be effective; it will be too late to make a difference.
There are many opportunities at the sub-regional level to manage or improve the conditions for high biodiversity, several of which are demonstrated in this study. There are many more opportunities for habitat restoration such as those shown at Camp Pendleton. The barrier to wildlife movement now caused by Interstate-15 can be "bridged." There are ways for rural residential development to be compatible with species movement and habitat protection, as shown in the "Oak Grove" study. An area can accommodate development and still maintain biodiversity by acknowledging some common sense constraints to development on steep slopes and flood-prone riparian zones.
The dependence of species richness on the precise style of rural residential development underscores an important point: the individual owners of these lands can have a critical impact on the long-term level of species richness. If they retain the native vegetation, they will maintain substantial numbers of species; if they use the land for other purposes, they will not.