Your primary concern is biological diversity: in other words, to prevent as many native species as possible from becoming extinct.
Here's a hypothetical puzzle for you: It's the year 2000 and the U.S. Congress has been seized by a rare case of nonpartisan common sense. They've consolidated all the land conservation agencies into one. We'll call it the Department of Nature Reserves. You've been placed in charge of it (congratulations) and given the mission of preserving five percent of the nation's land mass. (That's about a twofold increase, incidentally, over what's currently protected in wilderness, national parks, and large state parks.)
Your primary concern is biological diversity: in other words, to prevent as many native species as possible from becoming extinct. It's a great job, but you soon find there's one fundamental question that's going to make your decisions very difficult: Will more species be saved by setting aside a small number of very large natural areas or by preserving a large number of tiny, disconnected ones?
That is a question being asked in a sub-discipline of ecology called island biogeography. As the phrase implies, it is the study of the ecology of islands. It has become, in addition, the study of the degree to which the ecology of protected lands may be analogous to that of islands as the habitat outside their boundaries becomes increasingly altered. Island biogeography may help solve our hypothetical puzzle.
The seminal event in this field was the publication, in 1967, of The Theory of Island Biogeography by Robert MacArthur and Edward O. Wilson. In that book, the authors reported two predictable principles of islands. The first is that larger islands support more species of animals than smaller ones. The second is that remote islands support fewer species than less remote islands. Over geologic time, islands experience both extinctions and colonizations. But based mainly on these two factors--size and remoteness--each island has a number of faunal species it will tend toward.
MacArthur and Wilson took the second principle, remoteness, and expanded it to include insularity--a form of remoteness not caused necessarily by distance but by anything that effectively tends to insulate the island from other lands. They also predicted that their theory would apply not just to islands but to isolated land-based habitats:
"Insularity is moreover a universal feature of biogeography. Many of the principles graphically displayed in the Galapagos Islands and other remote archipelagos apply in lesser or greater degree to all natural habitats" (MacArthur & Wilson, 1967, p. 3).
Around the same time, a scientist named Preston (1962a, 1962b) studied distributions of common and rare species in natural areas. One of his conclusions, simply put, was that many nature reserves were too small: "If what we have said is correct, it is not possible to preserve in a State or National Park, a complete replica on a small scale of the fauna and flora of a much larger area (1962b, p. 427)."
The late 1960s and 1970s witnessed a rapid growth in interest in island biogeography. Jared Diamond, researching bird diversity and equilibrium theory in the tropics, became concerned about the effects of rain forest destruction. Referring to attempts in New Guinea to save habitat samples, he wrote "If these plans succeed, the rain forests, instead of disappearing completely, will be broken into "islands" surrounded by a `sea' of open country in which forest species cannot live" (1972, p. 3203).
Continuing to apply island biogeography to conservation problems, Diamond (1975) offered a set of "design principles" for an ecologically sound park ("nature reserve") system. Among his principles were that large units will hold more species than small ones; that a unit close to another unit will hold more species than an isolated one; that a group of reserves adjacent to each other or clustered close together will hold more species than the same units isolated from each other; that a round unit will hold more species than a long, narrow one; and that corridors between fragmented habitats might alleviate the island effect (1975) (our emphasis). …