Functional habitat area as a reliable proxy for population size: case study using two butterfly species of conservation concern

Accurate estimates of population size are essential for effective conservation and restoration management of threatened species. Nevertheless, reliable methods to estimate population size, such as mark-release-recapture studies (MRR), are time and labour consuming and may generate negative impact(s) on both the habitats and organisms studied. This may complicate their use if several sites need to be studied concurrently. Consequently, there is a strong interest to develop reliable proxies of population size, e.g., to be used in Population Viability Analysis. Habitat area has often been used as an obvious proxy. For butterflies, many studies focused on the area of host plant patches, but resource-based definition of the habitat (i.e., the area containing the different ecological resources and conditions needed by the individuals) has recently gained much attention. Using two peat bog butterflies, we tested the reliability of these two measures of habitat area as proxies for population size by (1) predicting population sizes based on the product of larval habitat area by the number of emerged butterflies per spatial unit of habitat (eliminated by ground cover traps) and (2) comparing these predictions to accurate population size estimates inferred from MRR studies. Results on both species showed that: (1) adult population size was strongly related to larval habitat availability and quality when habitat was accurately defined according to functional resources, (2) resources other than the host plant have to be included in the habitat definition, (3) after careful control of its similarity, the resource-based habitat delineation can be reasonably well transferred among populations of the same species in a wider region.