What are we measuring? A review of metrics used to describe biodiversity in offsets exchanges
Introduction
Biodiversity offsets are becoming increasingly popular as a regulation and conservation tool aimed at reducing the impact of developments on biodiversity (BBOP, 2012). Around the world over 45 offset programs have been established and as many as 108 public policies now incorporate no net loss of biodiversity principles, which is often the key objective of biodiversity offsets (Madsen et al., 2011; Bull and Strange, 2018). Biodiversity offsets are highly critiqued, largely because it is unclear how effective offsetting policies are in practice (Bull et al., 2013), and whether no net loss is achievable using current frameworks (Bezombes et al., 2019). Achieving no net loss through offsetting requires implementing conservation actions that aim to match the environmental losses caused by development with biodiversity gains (Bull et al., 2016; Birkeland and Knight-lenihan, 2016). Quantification of biodiversity values may happen at several stages in the impact assessment and offsetting process, as well as during monitoring of proposed actions and outcomes (Geneletti, 2002). One of the major challenges in the implementation of offsets is how to quantify the trading of biodiversity losses due to development for appropriate gains delivered through an offset action (Bull et al., 2013; Ives and Bekessy, 2015).
Methods and metrics used to evaluate biodiversity have important impacts on conservation strategies and resource allocation (Davies and Cadotte, 2011). However, measuring biodiversity is notoriously difficult in all fields of ecological research and generally cannot be summarised using a single-metric approach (Williams and Araújo, 2002; Liu et al., 2018). ‘Biodiversity’ is used as a catch-all term which encompasses any of the multiple levels of biological complexity (Ferrier, 2002). To simplify the task of measuring biodiversity, ecologists generally measure a small subset of it to act as surrogates for other features not explicitly assessed, usually based on habitat attributes (Davies and Cadotte, 2011; BBOP, 2012). Commonly used offsetting metrics tend to focus on a measure of habitat condition which is calculated and weighted across several habitat features. This is combined with the area impacted and a ratio or multiplier value which may increase offset requirements so as to deliver equitable or greater biodiversity gains (Institute for European Environmental Policy, 2014). The final value used for the trade is generally a summed habitat condition score which determines the amount of area of a particular quality or condition that is required to offset the losses expected through development (Gibbons et al., 2018). For example, in Australia the ‘habitat hectare’ has been developed specifically for use in offsetting and incorporates seven habitat features and three landscape metrics into a weighted habitat score which is combined with site area to compute a quality-adjusted area of habitat (Parkes et al., 2003; The State of Victoria Department of Environment, Land W and P, 2017a,2017b). Similarly, in the United States, wetland mitigation ratios are based on the type of wetland affected and the size of the impact (US Army Corps of Engineers, 2014). In this approach, the habitat type and area of impact determine how extensively a developer must offset their environmental impacts. This can influence both the size of the offset required and the type of offset activity implemented (May et al., 2016a; Bull and Strange, 2018).
The assumption in using metrics based on habitat attributes or vegetation types in offsetting programs is that by protecting or restoring these features, there will be both a direct benefit to habitat and a corresponding, but indirect benefit to plant and animal species (Cristescu et al., 2013). This, however, will not always be the case (Bedward et al., 2009). Several studies have demonstrated that metrics based on habitat attributes and vegetation type tend to be overly simplistic and do not fully capture individual species’ ecological needs (Cristescu et al., 2013; Kujala et al., 2015a; Hanford et al., 2016). These metrics assign low scores to ecologically important sites which may occur in a degraded condition or in small patches (Hobbs, 2016; Maseyk et al., 2016). Moreover, smaller or more degraded sites are often considered of lesser conservation value (Wintle et al., 2019), and therefore may not be prioritised for offsetting since they are presumed to deliver fewer gains. Resulting offset sites can therefore deliver markedly different biodiversity values from those lost (Price et al., 2019), with the risk of trading away critical habitat, such as large old-growth trees, which may support rare or threatened species (Maron et al., 2012; Le Roux et al., 2015, 2016; Wintle et al., 2019). Consequently, we must understand the ramifications of using metrics which are uncoupled from the biodiversity values they are intending to capture (Cristescu et al., 2013), and identify transparent and fungible methods for assessing biodiversity impacts and offsetting requirements.
Despite increased efforts to incorporate ecological processes into metrics that support offsets, such as through the use of landscape measures (Gibbons et al., 2016; The State of Victoria Department of Environment, Land W and P, 2017a, 2017b), most currently used offsetting metrics largely fail to capture landscape level impacts on populations and species (Bekessy et al., 2010; Crouzeilles et al., 2015). The inclusion of species biology or population processes (e.g. dispersal, Allee effects) adds an additional layer of complexity to biodiversity assessment (Ferrier and Drielsma, 2010) and offset calculations. When the objective of offsetting is to ensure the persistence of particular species in a region, offset metrics should incorporate measures of variables that directly mediate population persistence (Cristescu et al., 2013; Drielsma et al., 2016), such as species-specific dispersal measures, or estimates of the carrying capacity, expected survival and fecundity of species in a habitat patch. Testing current biodiversity offsetting metrics and identifying realistic alternatives is not yet fully addressed in research on offsets (ten Kate et al., 2004; Maron et al., 2016).
Here, we reviewed the offsetting, conservation planning and ecology literature to identify the most common metrics being used in offsetting compared to those used for measuring or assessing biodiversity in the broader fields of conservation planning and ecological research. Understanding how biodiversity is treated in other conservation and environmental management activities may provide valuable insights into current offset metrics, where they fall short and how they could potentially be improved. The purpose of this review is therefore to highlight potential gaps in current offset metrics and identify where future research could contribute to testing alternatives.
Section snippets
Review design
We used a cross-disciplinary review approach which followed a step-wise search and assessment procedure (Fig. A.1: Pickering and Byrne, 2014). The purpose of this design was to capture the most commonly used measures across multiple disciplines. We used Scopus to collect publications from three fields; offsetting, conservation planning and ecology (See Appendix A for detailed definitions of each category). The intention of this review was to examine and characterise how these different fields
Extent of the literature reviewed
Across 255 publications we identified 24 metric sub-categories (Table 1). Of the 255 papers reviewed, 158 came from the ecology literature, 54 from conservation planning and 43 from offsetting. The number of publications in all three fields increased from 1999 to April 2017, and followed the same trends, with a spike in publications between 2012 and 2017 (Fig. A.2). The literature in all three fields was widely distributed around the world. However, as expected, developed regions tended to be
Discussion
Our results demonstrate that the definition of biodiversity remains notably narrower in commonly-used offsetting metrics compared with metrics used in the broader fields of conservation and ecology (Fig. 2), which is concerning given how widely offsets are now applied (Gordon et al., 2011; Bull and Strange, 2018). The primary implication of our research is that current offset metrics are likely to be limited in their capacity to capture all the biodiversity values that are generally of interest
Conclusion
Despite the increasing use of biodiversity offsets worldwide there remains little quantitative evidence to support that they deliver their claimed benefits. Achieving no net loss of biodiversity depends strongly on how biodiversity is defined and measured. We found that within the offsetting literature the definition of biodiversity remains much narrower, in terms of the complexity and breadth of biodiversity features measured, than in the closely related fields of ecology and conservation
Declaration of Competing Interest
The authors confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.
Acknowledgements
This study was supported by the Australian Government’s National Environmental Science Program’s Threatened Species Recovery Hub and a Melbourne University Research Scholarship. We thank two anonymous reviewers for their insightful comments that improved this work.
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