… nature thus keeps a supply of these plants in her nursery (i.e., under the larger wood), always ready for casualties, as fires, windfall, and clearings by man.
Henry David Thoreau, Journal, 1860The landscape mosaic was presented as a whole in chapter 1. Now we begin dissecting it into its building blocks, beginning with patches. A patch is a wide relatively homogeneous area that differs from its surroundings (see glossary in chapter 1 appendix). Patches have familiar attributes, such as large or small, rounded or elongated, and straight or convoluted boundaries. These attributes in turn have widespread ecological implications for productivity, biodiversity, soil, and water.
Why are these patch characteristics so important? In forestry the size of woods affects road construction and erosion, how many trees are wasted in edge effects, and the success of tree reproduction. In suburbanization planning the size of housing developments and nature reserves are common controversial issues. In aquifer and lake protection water quality depends on a large patch of natural vegetation. In managing stream systems for fish and fishermen, the size and number of natural patches is critical. In nature reserve design, is it better to have one large patch or several small patches? How many large patches are required to maintain the biodiversity of a landscape? In managing game, small patches are frequently created. And in agriculture an optimum field size has both ecologie and economic implications.
Several of the topics included are reviewed by Forman & Godron (1986). Other useful reviews include: patch dynamics1320; the traditional species-area concept; island biogeographic theory1023,1323,302,564,1901,198,1763,1536; remnant patches in landscapes683,1490,1536; and genetics in patches1595.
This chapter begins with the origins and half-life of dynamic patches. Patch size effects are examined for ecosystem processes. Species-area curves and island biogeographic theory lead to evaluating species numbers in remnant and disturbance patches in a terrestrial mosaic. The roles of genetics and minimum viable populations are introduced. Finally we evaluate the importance of number of large patches present.
PATCH DYNAMICS: ORIGIN AND PERSISTENCE
The fortuneteller who predicts change is always right. We may dream of constancy or stability, but change, the universal law, is a key to understanding. Indeed, because change always extends over a limited area, it causes or affects a patch.