Biodiversity offsetting in dynamic landscapes: Influence of regulatory context and counterfactual assumptions on achievement of no net loss
Introduction
Biodiversity offsets aim to achieve no net loss of biodiversity by counterbalancing residual biodiversity loss from development with equivalent gains at an offset location (ten Kate et al., 2004). While their use is increasing globally (Maron et al., 2016a, Maron et al., 2016b), detailed evaluations of offset policies remain few. Indeed, in most cases, their outcomes will only be evident after several decades (Maron et al., 2012, Gibbons et al., 2015), limiting our ability to assess directly whether no net loss is being achieved. Thus, ex-ante evaluation of alternative offsetting approaches is crucial for pinpointing how offset scheme design influences biodiversity outcomes and achievement of no net loss (Sonter et al., 2014).
Almost all existing offset policies involve some component of averted loss (Gibbons and Lindenmayer, 2007, Maron et al., 2015). This involves generating biodiversity ‘gains’ by protecting and/or maintaining biodiversity that would otherwise have deteriorated in condition or been lost, for example, due to deforestation or other pressures (that would not themselves trigger offset requirements; (Gibbons and Lindenmayer, 2007, Maron et al., 2013)). To determine the biodiversity gains such protection and maintenance generates, the ‘with protection’ outcome must be compared to a counterfactual scenario—i.e. what would be expected to occur in absence of development and offsetting (Maron et al., 2013, Bull et al., 2014). Such counterfactual scenarios, although never observed directly, strongly influence the biodiversity outcomes from offset exchanges (Maron et al., 2015).
Despite their fundamental importance to achieving no net loss, counterfactual scenarios are often neglected in decision-making and rarely explicitly stated (Maron et al., 2015, Maron et al., 2012). Nevertheless, all offset decisions imply a counterfactual, the nature of which can be inferred post-hoc. Both implicit and explicitly-stated counterfactuals used to calculate equivalence in offset schemes tend to assume that the ‘background’ rate of biodiversity change – that is, without the impacts and offsets – is one of biodiversity decline. This assumption may often be invalid, meaning that offsets do not avert enough loss, and thus enable ongoing biodiversity decline (Gordon et al., 2015, Maron et al., 2015).
Often, the assumed counterfactual trajectory of biodiversity loss is implausibly steep, meaning that the expected biodiversity gains from offsetting are unrealistically large (Maron et al., 2015). In some cases, trajectories of net biodiversity gain may be more realistic. For example, landscapes with regrowing native vegetation (sensu Guariguata and Ostertag, 2001) may gain biodiversity, both in terms of vegetation extent and habitat quality (Bowen et al., 2007). Nevertheless, even in such naturally recovering ecosystems, biodiversity loss tends to occur in some places, so opportunities to avert loss probably still exist. In these cases, spatially-explicit counterfactual scenarios that account for heterogeneous biodiversity losses and gains are required, if averted loss offsetting is to be possible at all.
Because counterfactual scenarios are best-guess descriptions of future biodiversity trends, plausible counterfactuals must also account for their surrounding regulatory context—including both biodiversity management policies and offsetting requirements (Githiru et al., 2015, Maron et al., 2016a, Maron et al., 2016b). For example, different ecosystems may be legally protected to various degrees, which in turn affect biodiversity gains achieved through conserving a site as an offset. As such, a one-hectare offset can yield widely different biodiversity gains depending on where it is, what ecosystem it contains, and the set of regulations that apply to it. For example, in Brazil's Quadrilátero Ferrífero mining region, allocating offsets to highly threatened ecosystems would likely avert nine times more biodiversity loss than allocating the same area of offsets to ecosystems deemed biologically equivalent to those damaged by development (Sonter et al., 2014).
Such regulatory context is also often dynamic over time. For example, in Queensland, Australia, changes in land clearing regulations over the past decade and a half have altered the degree to which remnant vegetation and certain types of regrowth are protected from being cleared. As a consequence, land clearing declined dramatically from 2003 to historically low levels in 2009, followed by resurgence during 2012–2014 (DSITI, 2015). In such a volatile regulatory environment, selecting appropriate counterfactuals is likely to be fraught. Understanding the sensitivity of offset outcomes to the regulatory context and accompanying policy settings is important for developing robust offset approaches that effectively achieve desired outcomes (Gordon et al., 2015).
In this study, we modelled expected biodiversity outcomes of averted loss offsetting in a dynamic ecosystem—the endangered brigalow (Acacia harpophylla) woodlands of central Queensland, Australia. This ecosystem underwent huge regulatory change over the past two decades, affecting vegetation clearing rates. It also has the capacity to recover following disturbance, resulting in natural biodiversity gains. Therefore, we used data on clearing rates to simulate offsets and their biodiversity gains—in terms of vegetation extent and habitat quality—under different counterfactual and offsetting assumptions. Our results reveal major implications for achieving no net loss of biodiversity in dynamic landscapes.
Section snippets
Study region
Our study region is defined by the northern extent of pre-clearing brigalow woodlands (Fig. 1; SI Table 1). This ecosystem has been extensively cleared over the past century (Seabrook et al., 2006) and continues to face pressures from multiple competing land uses. They also are characterised by a capacity to regrow following disturbance (Butler, 2007), where habitat structural complexity and species richness of birds improve with regrowth age (Scanlan, 1991, Johnson, 1997, Bowen et al., 2009),
Results
Vegetation clearing rates more than doubled between 2006–2009 and 2009–2011 (Table 1). Remnant clearing increased from 356 to 3076 ha yr− 1 and regrowth clearing increased from 1297 to 3055 ha yr− 1. Clearing rates also differed between vegetation types (Table 1). Regrowth clearing was nine times greater than remnant clearing during 2006–2009; while remnant clearing was greater than regrowth clearing during 2009–2011. Projecting counterfactual vegetation change to 2040 caused a decline in remnant
Discussion
No-net-loss of biodiversity was not achieved under any combination of counterfactual and offsetting scenarios that we considered. However, biodiversity outcomes were highly sensitive to the time period used to inform counterfactual scenarios and to differences in clearing pressures among vegetation types used for offsetting. Our results illustrate major challenges for developing plausible counterfactual scenarios and quantifying averted loss potential in dynamic landscapes.
Acknowledgements
M.M. is supported by Australian Research Council Future Fellowship FT140100516 and the National Environmental Science Program's Threatened Species Recovery Hub. L.J.S. received support from USDA McIntire-Stennis award #2014-32100-06050 to the University of Vermont.
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