Development is the most serious threat to biological communities—it replaces them, usually irreversibly, with buildings, parking lots, and maintained landscaping or agricultural fields. Cal Poly Pomona is unusual in having a substantial amount of undeveloped land on campus, roughly divided between pastureland and walnut woodland on the south side of campus and coastal sage scrub and oak woodland on the north. The former still exists because of the agricultural roots of the campus, and the latter because much of it is too steep to build on.
Development has indirect effects, as well. One important effect is through fragmenation of habitat into smaller and smaller "islands". Species have different needs for contiguous habitat, and as the "islands" become smaller, more and more species are eliminated as their needs are no longer met. Fragmentation also increases the effects of other impacts: a fire in continuous habitat will be bordered by unburned areas, but a small "island" can be entirely consumed.
Another indirect effect comes from increasing human use of adjacent undeveloped areas. Vehicle use can have substantial impact, especially in hilly terrain. At various times, and especially after the 1989 fire, vehicles have been driven across the undeveloped areas. The most serious consequence of vehicle use is increased erosion. By crushing the vegetation, or removing the ash crust after a fire, vehicles allow rain to reach the soil below, and gullies result. The 1989 fire uncovered gullies spaced a truck-tire-track apart that had been around for decades, and beside them were newly formed gullies from mountain bike tracks.
The indirect effects of development can often be so severe, and create so much habitat degradation, that little remains to protect from further development.