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- Species well distributed across their native range are less susceptible to extinction than species confined to small portions of their range.
A widely distributed species will be unlikely to experience a catastrophe, disturbance, or other negative influence across its entire range at once. The broader the species’ distribution, the less likely that a catastrophic event will devastate the population. Additional support for this principle comes from consideration of the important ecological roles that many species play in local plant and animal communities, and the genetic distinctness of many local populations.
- Large blocks of habitat, containing large populations, are better than small blocks with small populations.
The principle of “bigness” is perhaps the best-accepted generalization of conservation biology. The larger the area, the more habitats and species it will contain. Furthermore, a larger block of suitable habitat will usually contain larger populations of species. All else being equal, large populations are less vulnerable than small populations to extinction.
- Blocks of habitat close together are better than blocks far apart.
In the absence of impenetrable barriers, habitat blocks that are close together will experience more interchange of individuals of various species than will blocks far apart. If enough interchange occurs between habitat blocks, they are functionally united into a larger population that is less vulnerable to extinction. Determining the nature of the intervening habitat relative to the dispersal behavior of the species in question is highly desirable. Blocks of habitat that appear close together to human eyes may be completely isolated from the standpoint of organisms that are physically or behaviorally unable to cross the intervening habitat.
- Habitat in contiguous blocks is better than fragmented habitat.
Fragmentation involves a reduction in size and an increase in isolation of habitats. Effects of fragmentation at population, community, ecosystem, and landscape levels are well documented. Problems include edge effects, increased human trespass and disturbance, and disruption of natural disturbance regimes, hydrology, and other natural processes. The end result of fragmentation is often a landscape that has lost sensitive native species.
- Interconnected blocks of habitat are better than isolated blocks.
Connectivity—the opposite of fragmentation—is another well-accepted principle of conservation planning. Although arguments over the benefits versus costs of particular corridor designs continue, few conservation biologists disagree that local populations functionally connected by natural movements are less subject to extinction than populations isolated by human activity. Planners should base connectivity designs on the needs of species most sensitive to fragmentation.
- Blocks of habitat that are roadless or otherwise inaccessible to humans are better than roaded and accessible habitat blocks.
This principle applies to species especially sensitive to harassment, persecution, or sometimes even the simple presence of people. Roads and other forms of human access often lead to high mortality rates for sensitive species. Furthermore, dirt roads contribute sediments to streams, roads serve as invasion routes for introduced plants, and roads are barriers to movement of some small animals. For these and other reasons, roadless areas should be protected, roads should be closed whenever possible, and busy highways should be equipped with underpasses or other wildlife movement passages.
- The fewer data or more uncertainty, the more conservative (i.e., causing less reduction or disruption of natural habitats) a conservation or development plan should be.
- Maintaining viable (i.e. undegraded, fully functioning) ecosystems is usually more efficient, economical, and effective than a species-by-species approach.
Although many sensitive species require individual attention in order to avoid extinction, focusing on every species individually is impossible. Ecosystem conservation has the advantage of being potentially proactive, by protecting habitats and assemblages before any single species declines to endangerment. If many species associated with an ecosystem are already imperiled, habitat protection and restoration based on their collective needs will be more efficient than single-species recovery actions.
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