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Wildlife diversity can be viewed on two different levels. On one level, diversity can be viewed as the number of species that occur on a single tract of land, such as private landholdings, single fields, or woodlots. On the other level, diversity can be viewed as the number of species that occur within a larger geographic area such as large conservation areas, counties, and watersheds.

Land management focused entirely on providing abundant edge has come under recent criticism because it can exclude species that require large uniform habitat blocks or do not survive near edges.¹⁰ If most parcels are managed to increase edge, only those species tolerant of edge habitats will prosper. Species needing uniform habitat blocks away from edges can be eliminated. The result of such management will be lower wildlife diversity within large geographic regions because area-sensitive species will be lost.^9,11 Conversely, the maintenance of large habitat blocks for area-sensitive species will not result in the loss of edge species as some edges will always be present in the landscape. This is especially true in Illinois where land use patterns have resulted in an abundance of edge habitat.

In Illinois, and in many other states, we are now seeing population declines of many area-sensitive bird species (Table 1). Some of these declines have been very large. Most of these declines are very likely attributable to land use practices such as timber removal, land clearing for agriculture, mining, urban development, reservoirs, highways, and power lines, that have severely fragmented natural communities in Illinois and elsewhere. Additionally, land management has traditionally not addressed the needs of area-sensitive species. If land use patterns and management continue to favor edge species, continued population declines and possibly local or regional extinctions of area-sensitive species are likely to occur.

It is becoming increasingly apparent that a major reason many species of birds are declining is an inability to raise young successfully. The problem is not one of adult animals dying through predation or other factors. Rather, it is poor survival of nests and young animals that is causing populations to decline. In this respect, the effect of habitat fragmentation on some birds ability to raise young successfully is analogous to that of DDT. DDT did not kill adult birds. It caused females to lay eggs with such thin shells that eggs were easily broken. No young birds were added to the population to replace adults that died of other causes. In fragmented landscapes, high rates of nest predation and nest parasitism by Brown-headed Cowbirds significantly reduce many bird species' ability to successfully raise young. Research has shown that in forests and grasslands, habitat size, shape, and amount and type of edge can all negatively affect breeding bird nest success.