398 F. J. F. Maseyk et al.


FIG. 2 A process for determining future likelihood of loss (total loss of area) under a counterfactual scenario (Pwo). ‘Foreseeable future’ may be defined by the relevant policy or legislation but can be considered to be the life of the offset or a generation. The time horizon is the period over which the outcome of the offset is being calculated (e.g. benefit achieved at 20 years).


FIG. 3 A process for determining future likelihood of loss (total loss of area) under an offset scenario (Po). The time horizon is the period over which the outcome of the offset is being calculated (e.g. benefit achieved at 20 years).


Estimating likelihood of loss under a counterfactual scenario


There are three important factors to guide estimation of the likelihood of loss of a proposed offset site under a counter- factual scenario: (1) site-specific influences on likelihood of loss, (2) policy and legislative requirements likely to be trig- gered by any impact to the site, and (3) recent rates of loss of similar sites. The four pathways in Fig. 2 help determine how these factors should guide estimates for both known and unknown type I and type II impacts. PathwayAillustrates that even if the loss of a site is highly


likely, but those losses result from type I impacts, the rele- vant likelihood of loss is negligible (0%in Fig. 2). Pathway B shows that without credible evidence for likely future development at a proposed offset site, and when any un- anticipated losses would be type I impacts, the relevant like- lihood of loss is also 0%. In both cases, this is because, assuming compliance with policy, any losses at the site would themselves have to be offset. Pathway C describes si- tuations where development impacts would not be sufficient to trigger offset requirements, for example when the magni- tude of the impact falls below a policy threshold that would


trigger an offset requirement. In such situations, the likeli- hood of loss at the proposed offset site is considered to be greater than the calculated recent rates of loss. Pathway D, however, illustrates that in the absence of credible evidence that development will occur specifically at the proposed off- set site, and where any unforeseen impacts would be type II impacts, it can be assumed that the site is subject to the same level of threat as other sites in the landscape. Therefore, the likelihood of loss can be derived using the calculated recent rates of loss without any adjustment.


Estimating likelihood of loss under the offset scenario


The pathways in Fig. 3 describe factors to take into account when estimating the likelihood of loss of a proposed offset site under an offset scenario, in which the site receives additional protection. In addition to the factors that need to be considered under the counterfactual scenario, a fur- ther important consideration is the strength of protection (e.g. through change in tenure) that will be placed on the site to avert future losses.


Oryx, 2021, 55(3), 393–403 © The Author(s), 2020. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605319000528


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