Where to concentrate one's efforts is a critical issue facing conservation groups and government agencies. The uniqueness of an ecosystem, the number of species, especially endemic species (species found only in that area) it supports, and the imminence of the threats to its survival all play a role in the targeting of conservation activities. The World Wildlife Fund has identified 25 ecosystems around the world for highest priority.

The size and placement of existing protected areas around the world is determined by many factors, and not necessarily primarily by conservation needs. Many are located in remote and unproductive lands, or in areas of great scenic beauty, or as a result of a conservation-minded national leader, philanthropist, or member of royalty. Some are in logged-over areas, and are only beginning to recover to their original splendor. A map of US wilderness areas is a useful reminder that only a few percent of rivers and total land area, at most, are protected. In the US, there is more protected land in the West, partly due to its scenic beauty, but partly because much of this area is less productive.

At least until recently, the system of protected areas has been almost entirely haphazard. Gap analysis is a new approach based on mapping of vegetation, animals (usually terrestrial vertebrates) and land ownership in order to identify gaps in the network of parks, reserves, and public lands that hopefully protect the biodiversity they contain.

Gap analysis relies on three primary data layers. These are:

1. The distribution of actual vegetation types, delineated from satellite imagery.
   
2. The distribution of terrestrial vertebrates, predicted from the vegetation distribution by associating individual species with the vegetation that characterizes their habitat.
   
3. The distribution of land ownership.

The process can be as simple as placing layers of transparent mylar over a base map, such as a topographic map, and tracing the information onto separate layers of transparencies. The first gap analysis looked at the distribution of three species of endangered Hawaiian honeycreepers (forest birds) on the island of Hawaii. It was possible to see the distribution of each bird species, and also locations where all three species coincided. Logically, these areas would be conservation priorities. Although the island had a number of nature reserves, none overlapped the birds' distributions. These maps determined the site for a new reserve.

Extending this approach to a large scale depends on computer mapping of satellite images. Landsat TM (thematic mapper) or other remotely sensed imagery may be used to construct vegetation maps. Landsat TM receives seven spectral bands of reflected infrared light, in individual cells or pixels of 30m x 30m. Image classification uses those spectral data to develop a map of vegetation classes, which is compared to ground measurements to improve accuracy. The resulting vegetation map is geo-referenced, meaning that every location has a latitude and longitude or some other X-Y grid location.

Specialized computer software, along with these spatial data and other information, make up a Geographic Information System (GIS).

Using these data layers and a GIS, one can ask:

• What fraction of threatened species occurs within existing reserves?
• What fraction of each major vegetation type falls within existing reserves?
• Are areas of highest species richness found within existing reserves?
• Are areas of high endemism found within existing reserves?

This map of the distribution of a desert reptile, the gila monster, shows that its distribution falls mainly outside protected land areas.

USGS Gap Analysis Program

Distribution of Gila monster and protected areas.

Design of nature reserves
Many factors influence the design as well as the placement of nature reserves. These include the specific habitat needs of species, the climate/soil/spatial requirements of ecosystems that one wishes to protect, and possible conflicts with human use of the target location. We will focus on some biological issues.

Reserves must be of sufficient size to maintain "large enough" populations of all "important species."

• What is large enough? This depends on the species of course. But as a general rule of thumb, a minimum "viable population" contains at least several hundred reproductive individuals. Several thousand is more desirable. A reserve should be large enough to preserve a viable population of the most wide-ranging species found therein.
  
  
• What are the important species? The answer to this question will vary. (Perhaps, all species?) But species of conservation concern (rare, endangered), keystone species and economically important species should certainly be considered.

Reserves should protect at least several populations, with a metapopulation structure. A metapopulation is a "population of populations," or set of subpopulations that are spatially distinct, but have some migration between them. Over a time span of a hundred years or more, a single population might experience catastrophic bad luck--a storm, fire, disease outbreak, etc. The existence of multiple populations, with limited dispersal and genetic exchange, provides insurance for the survival of the population as a whole, while maintaining genetic diversity.

• Reserve design can encourage natural migration through the use of corridors and stepping-stones for connectivity.
  
• Individual plants and animals can be physically transferred if necessary.

Reserves should also minimize edge and fragmentation effects.

• Round shapes, which minimize edge, should be favored over elongated shapes, which maximize edge.
  
  
• Internal fragmentation of a reserve due to logging, farming, roads, power lines, etc., should be avoided.
  
  
• Small areas can often be aggregated to create larger conservation blocks by including surrounding land, even if some areas are given lower conservation priority. This can create a land mosaic containing areas of varying conservation status, but under integrated management.
  
  
• Entire ecosystems should be incorporated if possible.

Reserves should consciously include the matrix of unprotected land in which they are embedded. Non-protected areas make up more than 95 percent of the landscape, so parks and sanctuaries necessarily exist in a matrix of developed land. Species wander outside of parks routinely. Future climate change raises the alarming likelihood that the entire flora and fauna of parks will migrate outside their current sanctuaries, as the climate within the park boundaries becomes unsuitable.

Priorities for reserve management include:

• Forming partnerships with public (agencies) and private (citizens) landowners.
  
• Education of landowners and the general public.
  
• Establishment of mechanisms to resolve land-use conflicts.