There is more science involved in designing reserves than just choosing the right place and the right size. To attain its conservation goals, the reserves in a three-component system should be designed using guidelines based on the following criteria. (Note that some of these apply to all three component types, some to only one or two.)

1. Bigger is better.
   • Large reserves work better for many reasons. They protect complete ecological units, allowing natural functions to take place in an undisturbed way. They avoid the problems of habitat fragmentation. They more easily resist catastrophes-by sheer size and because distinctive ecosystems are repeated inside their borders (duplication provides fail-safe insurance). They contain larger populations of species-making them less susceptible to extinction and better protecting sources for species re-colonization in other locations. And they serve as more reliable benchmarks against which scientists can measure environmental changes taking place elsewhere.
2. Circular shapes are best.
   • The most-protected area in any reserve is its centre, the area farthest from outside disturbances. Roughly circular reserves isolate this protected core area more effectively than elongated reserves. (The greater the radius of a circular reserve, the better the core is protected.) With circular-shaped reserves, the vulnerable outer edge of circular reserves-the area most affected by outside events-is also relatively much smaller compared to the reserve’s overall area than it is in elongated or multi-pronged designs.
3. Reserves should include areas with the richest biological diversity.
   • “Ecotone” areas contain elements of all bordering ecosystems, and so have the highest degree of biodiversity. Large reserves should try to include such areas. Smaller reserves can protect specific biological hot spots-seabird colonies, for example, or salt marshes.
4. Adequately protecting an ecoregion requires more than one reserve.
   • This is the universally understood principle of not putting all the eggs in one basket. Duplicating ecoregion protection reduces the ecological cost if a reserve is lost, damaged, or fails to sustain itself. Duplication can also preserve diverse genetic stocks and increase the potential for naturally occurring animal or plant (via seeds and pollen) movement between areas.
5. Reserve area and design should be based on ecological criteria.
   • Reserves must include enough critical habitat to sustain viable populations of plants and animals. Reserve boundaries can often reflect input from public consultations and discussions, but they must first respect the needs of the ecological systems they need to protect. And because nature cannot respond to humanity’s concept of borders, some reserves may need to be joined by corridors or connecting areas of protected land or water, to facilitate wildlife movement.
6. No reserve is an island.
   • Even with the best planning, protected areas never function in isolation. Nearby land-, water- and resource-use always have an influence. Reserve success may depend on the continued, integrated management of some resource-use and developmental activities. Some reserves may require buffer zones around their borders, in which the range of human activity is limited to those that do not damage the ecological integrity of the reserve. And networks of linked reserves may sometimes be needed to provide dynamic natural systems with adequate protection.