Abstract
The combined effects of a mounting number of threatened animal taxa and limited resources for conservation compel priorities setting. Therefore, a unit for conservation must be identified. Although the original concept of a conservation unit (Evolutionary Significant Unit: ESU) was proposed in 1986 (Ryder 1986), various definitions proposed by various authors created confusion. This paper is aimed to examine the inconsistencies with its usage by reviewing ESU papers published from 1996-2004. The number and trend in publication of papers indicate that the ESU concept has gained support, particularly in studies of mammals and fishes. Although the original concept includes two aspects (genetic and ecological), these are often not satisfied. Due to practical difficulties in obtaining ecological data, Moritz's (1994) definition, which is biased to the genetic aspect, is widely used. The main cause of the confusion is that the ESU concept has been applied in cases where populations were examined from only one aspect. Only when sufficient genetic and ecological data are available, should the ESU designation be applied. We propose Genetically Significant Unit (GSU) and Ecologically Significant System (ESS) as aspectual units constituting the ESU. GSU or ESS should be used instead of ESU, when only one of them is examined.
