The ecological outcomes of biodiversity offsets under “no net loss” policies: A global review

Abstract

No net loss (NNL) biodiversity policies mandating the application of a mitigation hierarchy (avoid, minimize, remediate, offset) to the ecological impacts of built infrastructure are proliferating globally. However, little is known about their effectiveness at achieving NNL outcomes. We reviewed the English‐language peer‐reviewed literature (capturing 15,715 articles), and identified 32 reports that observed ecological outcomes from NNL policies, including >300,000 ha of biodiversity offsets. Approximately one‐third of NNL policies and individual biodiversity offsets reported achieving NNL, primarily in wetlands, although most studies used widely criticized area‐based outcome measures. The most commonly cited reason for success was applying high offset multipliers (large offset area relative to the impacted area). We identified large gaps between the global implementation of offsets and the evidence for their effectiveness: despite two‐thirds of the world's biodiversity offsets being applied in forested ecosystems, we found none of four studies demonstrated successful NNL outcomes for forested habitats or species. We also found no evidence for NNL achievement using avoided loss offsets (impacts offset by protecting existing habitat elsewhere). Additionally, we summarized regional variability in compliance rates with NNL policies. As global infrastructural expansion accelerates, we must urgently improve the evidence‐base around efforts to mitigate development impacts on biodiversity.

The mitigation hierarchy for sharks: A risk‐based framework for reconciling trade‐offs between shark conservation and fisheries objectives

“…The final step requires that any residual negative impacts are compensated for, through off‐site conservation actions which improve the status of the affected biodiversity elsewhere (Arlidge et al, ; CSBI, ; Milner‐Gulland et al, ). If applied successfully, the MH can lead to no net loss (NNL) of biodiversity or even net gain (BBOP, ; Bull, Suttle, Gordon, Singh, & Milner‐Gulland, ; zu Ermgassen et al, ; Gardner et al, ; Milner‐Gulland et al, ). For example, wetland mitigation banks in the United States have shown to successfully achieve no net loss of wetland area through protection, restoration or creation of wetlands in compensation for loss caused by development projects (Brown & Lant, ; zu Ermgassen et al, ).…”

“…Though in order to be true compensation, the increase in survival probability as a result of compensatory conservation must be at least equivalent to the mortality probability of the harmful gear. To address this uncertainty, high offset multipliers could be applied to bycatch taxes, as has proven to be a key success factor for delivering ecological outcomes in terrestrial applications of compensatory mitigation (zu Ermgassen et al, ).…”

The Role of “No Net Loss” Policies in Conserving Biodiversity Threatened by the Global Infrastructure Boom

“…To assess progress in achieving NNL of biodiversity from new infrastructure, we first explore the global extent of more general biodiversity compensation policies. While much past research on compensation has focused on outcomes at local scales, [32][33][34] the global implications of compensation policies are only just beginning to emerge. For example, taking just the subset of compensation represented by biodiversity offsets, an estimated 153; 679 + 25;013 À64;223 km 2 of biodiversity offsets were (as of 2018) in the process of being implemented to offset infrastructure and land-use change impacts globally, which when summed make the area of biodiversity offsets approximately equivalent in size to a country as large as Bangladesh.…”

“…Expanding the Scope of NNL Policies Many NNL policies have historically failed to achieve their intended overarching policy aim: 34 shortcomings are embedded into multiple stages of the NNL policy implementation process from policy down to project scales ( Figure 2). Perhaps the most important limitation to most existing NNL policies is that the total infrastructural impacts under their jurisdiction tend to be highly constrained: often the majority of impacts fall outside the scope of existing regulation (referred to by Maron et al 38…”

“…If NNL is only applied to a subsection of impacts, then even if project-scale mitigation is achieved, the policy will inevitably oversee landscape-scale declines in biodiversity. 34,38 There are two main sources of unmitigated infrastructural impacts: (1) deliberate policy choices that leave particular sets of impacts either entirely unaddressed or granted special exemptions from regulation and (2) illegal, noncompliant, or unreported impacts.…”