402 F. J. F. Maseyk et al.


reasonable proxy, although in cases of marked changes in rates of loss over time such guidance is less useful. In such situations, a more relevant time period may be used as a baseline, or a selection of plausible trends could be specified (e.g. Bull et al., 2015) and a conservative assumption made, although this carries risks that estimates of gain will be ar- tificially inflated. Furthermore, although we used a linear mean calculation in our case study, a geometric mean may be more appropriate to reflect the non-linear nature of eco- logical dynamics (Buschke, 2017). An ideal approach would be to develop a predictive


model that accounts for multiple drivers of potential loss (e.g. distance from roads and settlements, productive cap- ability of underlying soils, land tenure), as has been applied elsewhere (Sonter et al., 2014, 2017). However, such model- ling requires substantial resourcing and agency capacity that would not be feasible in many cases. Relying instead on past background rates of loss is an improvement on much of current practice, which tends to be ad hoc and less than transparent. Given the limitations of relying on past rates of loss alone


to inform future loss estimates, any assumptions should be explicit, and if over time these assumptions are proven in- correct, the loss-gain calculations underpinning the offset design need to be revisited. This highlights the need for ongoing outcome monitoring of both offset sites and other sites in the landscape that act as controls, so that the impact of averted loss measures can be estimated, and the counter- factual assumptions evaluated over time.


Incorporating site-specific influences on likelihood of loss estimates


Care is required to ensure that socio-political pressures do not have undue influence in situations where site-specific considerations are incorporated into estimates of likelihood of loss. Another risk is that a requirement for site-specific evidence at a particular offset site could create an incentive to generate threats to claim a greater amount of biodiver- sity gain using averted loss offsets. This could lead to over- inflated likelihood of loss and set precedents for unrealistic likelihood of loss estimates (Maseyk et al., 2017). To prevent this, declarations of intended development should be sub- ject to adequate scrutiny to ensure they are genuine, and not merely obtained to inflate likelihood of loss. A further site-specific issue relevant to estimating future


likelihood of loss is the chance that averting loss at a specific site will cause so-called leakage, meaning that the biodi- versity loss may be prevented at the offset site but shifted elsewhere and thus not actually averted. The likelihood of leakage occurring is dependent on the policy framework relevant to the site in question (Maron et al., 2018); i.e. whether the activity that is being shifted would trigger an offset requirement at the third site or not. If leakage is a


real risk, this would confound estimates of gains generated using averted loss offsets. Caution is thus necessary when relying solely on averted loss offsets.


Conclusion


The ability and likelihood of offset actions to deliver biodi- versity gains successfully are shrouded in uncertainty, and failures are common (Quigley & Harper, 2006;Burgin, 2010; May et al., 2016). Quantifying the gain generated by averted loss offsets relies on the accuracy of assump- tions about likelihood of loss, the uncertainty of which cannot be resolved in most cases. This inherent uncertainty in predicting the future is common to all offset actions, but is exacerbated in averted loss offsets as the variation in gain estimates is most affected by the counterfactual scenario, which is never observed and therefore can never be proven. However, improving the reasoning process for arriving at estimates of benefit from offsets, and making assumptions transparent is critical for ensuring averted loss offsets do not in fact entrench and accelerate biodiversity losses (Maron et al., 2015).


Acknowledgements This research was funded by the Australian Government’s National Environmental Science Program through the Threatened Species Recovery Hub Project 5.1 ‘Better offsets for threatened species’. MM is supported by Australian Research Council (ARC) Future Fellowship FT140100516. AG is supported by an Australian Research Council Discovery Project (DP150103122).


Author contributions Development of the guidance to the Aus- tralian Department of the Environment and Energy (the basis of this work): FM, ME, MM; conception: all authors; writing: FM; further development and revisions: all authors.


Conflict of interest None.


Ethical standards This research abided by the Oryx guidelines on ethical standards.


References


ARLIDGE,W.N., BULL, J.W., ADDISON, P.F., BURGASS, M.J., GIANUCA, D., GORHAM, T.M. et al. (2018) A global mitigation hierarchy for nature conservation. Bioscience, 68, 336–347.


AUSTRALIAN GOVERNMENT (2018) EPBC Act Environmental Offsets Policy. environment.gov.au/epbc/publications/epbc-act- environmental-offsets-policy [accessed 16 April 2018].


BULL,J., SINGH,N., SUTTLE,K., BYKOVA,E.&MILNER-GULLAND,E.J. (2015) Creating a frame of reference for conservation interventions. Land Use Policy, 49, 273–286.


BULL, J.W., LLOYD, S.P. & STRANGE,N.(2017) Implementation gap between the theory and practice of biodiversity offset multipliers. Conservation Letters, 10, 656–669.


BURGIN,S. (2010) ‘Mitigation banks’ for wetland conservation: a major success or an unmitigated disaster?Wetlands Ecology and Management, 18, 49–55.


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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