Trading populations: can biodiversity offsets effectively compensate for population losses?

Abstract

Biodiversity offsetting promotes the protection or restoration of biodiversity at one site to compensate for the loss of biodiversity due to development at another site. Thus populations of species at a development site may be extirpated in the belief that offsetting elsewhere will compensate for the loss of biodiversity. In this study we tested the replaceability of roadside populations of the orchid Diuris platichila threatened by development (populations 1-5, n = 50-541 plants) with a potential offset population occurring in nearby natural vegetation (population 6, n = 143 plants). We measured differences in habitat among the populations and associated differences in flowering and fruiting. We also measured genotypic diversity within and among the populations, and the capacity of soil from each population to promote the symbiotic germination of outcrossed seed from the two largest populations (populations 1-2). An evaluation of the performance of the relevant offset policy was also undertaken which was informed by these studies. Compared to the roadside populations, the potential offset site had limited flowering (except after fire) and was genotypically less diverse. Soil from the potential offset site, and populations 2, 3 and 5, supported significantly less seed germination than soil from population 1. Translocating individuals from the most genotypically diverse populations into the offset population could help to buffer against the loss of genetic diversity if offsetting was required; however, the limited reproduction and recruitment opportunities at the offset site could result in the eventual erosion of any initial increase in diversity. The offset policy failed to secure a suitable offset for a hypothesized loss of plants. More generally we conclude that offsetting approaches which do not assess genotypic diversity and recruitment capacity may fail in their objective of protecting species and that maintaining populations in safe sites may be required.