Size of selection units for future reserves and its influence on actual vs targeted representation of features: a case study in western New South Wales

Most systematic assessments of future conservation areas rely on selection units-parts of the landscape that are analysed as the potential building blocks of an expanded system of reserves. Selection units can be natural, administrative or arbitrary subdivisions of the landscape. They differ widely in size between studies and within regions. The paper begins with a review of the role of selection units in conservation planning and the implications of using them. The review is followed by quantitative analyses on a large regional data set. We show that the total extent of new reserves needed to represent all land types (land systems in this case) to different targeted levels depends strongly on the size of the selection units. Differences in required total areas are related to the extent to which some land types are represented above target levels. The results indicate that some degree of inefficiency is inevitable in any reserve selection exercise based on units that are large enough to function as viable reserves or to be amalgamated realistically into viable reserves. We also show that the actual representation of land types in selected reserves is related to their distributional parameters, so that the extent of above-target representation is predictable to some extent. Finally, we show that patterns of actual vs targeted representation from a reserve selection algorithm are very different from those arising from random selection of the same number of areas. Selection algorithms introduce a degree of above-target representation which is the price of guaranteeing that all features are represented at least to target levels. (C) 1998 Elsevier Science Ltd. All rights reserved.