Cumulative effects

The Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act) stipulates that direct, indirect and offsite impacts (specifically upstream, downstream and facilitated impacts) on matters of national environmental significance should be considered when planning activities and undertaking broadscale strategic assessments.

However, it does not explicitly address cumulative effects (Dales 2011, Dunstan et al. 2019); redressing this is a recommendation of the recent EPBC Act review (Samuel 2020). Moreover, the increasing focus on implementing ecosystem-based management approaches requires appreciating how human activities influence and reshape ecosystems (Levin et al. 2009), which requires taking a systemic and integrated view of all pressures on marine and coastal systems (Ocean Panel 2020).

Cumulative effects result from multiple pressures exerted by a single activity, together with the interacting pressures from multiple activities (Foley et al. 2017). They often occur when pressures occur at the same time, but it is possible for effects to arise even if the pressures occur at different times, as long as they occur in the same space (due to the long-term effects of past pressures). These compounding effects erode ecosystem resilience, which can make a system more susceptible to future change (Roberts et al. 2017).

Scientifically meaningful estimates of cumulative effects, and accurate attribution of observed effects to activities, are essential for effective management, but are extremely difficult to achieve. The cumulative nature of interactions and pressures means that they do not always accumulate linearly. Synergistic (working together) and antagonistic (working against each other) interactions are both found about as commonly as additive outcomes (Crain et al. 2008, Stockbridge et al. 2020).

Estimating cumulative effects in simple systems (e.g. small geographic areas with a small number of, ideally additive, pressures) is possible (e.g. Sutherland et al. 2009). More complex systems can be dealt with by making simplifying assumptions, but this may compromise the robustness of estimates (Halpern & Fujita 2013). Estimating cumulative effects in complex systems with feedbacks requires sophisticated models informed by large amounts of data, as well as in-depth ecosystem knowledge embedded in these analyses (Dunstan et al. 2019). In complex systems, full estimation and attribution is possible, but it requires carefully designed elicitation (Martin et al. 2012, Hosack et al. 2017, Hemming et al. 2018), observation (Hayes et al. 2019) and model-based (statistical and/or mechanistic) analysis (Foster et al. 2015, Large et al. 2015, Uthicke et al. 2016, Fulton et al. 2017).

The environmental decline of natural systems has motivated increasing numbers of assessments of cumulative effects. These may be completed by development proponents through project-related environmental impact assessments, or by national and state or territory government agencies as part of strategic or regional assessments, such as Parks Australia’s Monitoring, Evaluation, Reporting and Improvement framework (Hayes et al. 2021); the New South Wales Marine Estate Management Strategy (MEMA 2018); or Victoria’s Marine and Coastal Policy (DELWP 2020). Whereas spatial additive assessment frameworks are readily available (e.g. Halpern et al. 2008, Stelzenmüller et al. 2018) and widely applied in Australia (Figure 23) and overseas, assessment of nonadditive (combinatory) effects is still so challenging that it is rarely attempted.

Expertise for the implementation of an assessment and management framework for cumulative effects is hampered by funding and capacity constraints (Gurran et al. 2013). As a result, many cumulative impact assessments provide only a relative measure of effect, and there is still little formal jurisdictional capacity (at an agency or interagency level) to manage cumulative effects. Instead, state and territory governments approach the issue via planning processes and development schemes, confirming alignment between local government planning scheme and state goals (Queensland Government 2018). Although this allows some cross-agency coordination (e.g. the New South Wales Marine Estate Management Authority), pressures are typically still treated individually by sector. However, climate change is emerging as a sufficiently broad motivating force to see more attention to multistressor considerations.

An exception to the jurisdictional issue is the Great Barrier Reef, where the Reef 2050 Long-Term Sustainability Plan sets out key actions for managing the cumulative pressures on the Reef. The Cumulative Impact Management Policy and the Net Benefit Policy provide the basis for managing cumulative effects.

Clearly articulated management objectives, and appropriate forms of assessment and monitoring data form the basis of adaptive management and a systematic way to address cumulative effects. But carefully designed long-term monitoring strategies are most often missing from attempts to effectively manage cumulative effects. Without these, there is little certainty around the assessment result itself (Halpern & Fujita 2013, Stockbridge et al. 2019) or the success of interventions. Such monitoring schemes not only provide information on the performance of management but can identify emerging issues and form the basis for forward-looking management actions aimed at reducing or, better still, avoiding cumulative effects.