Cumulative pressures

 

Cumulative pressures are pressures that are acting in combination with other past, present and future pressures. Multiple pressures interact with each other to produce additive, synergistic or antagonistic effects. Cumulative impacts may arise even if the pressures occurred at different times but in the same space (due to the long-term effects of past pressures). A key challenge for the sustainable management of Australia’s marine and terrestrial environments is cumulative impacts from the interactions and feedbacks among multiple pressures.

Ecosystems and species seldom respond to pressures in isolation, and the most abrupt changes in ecological systems frequently arise from interactions among multiple pressures rather than changes to a single pressure. Compounding effects erode ecosystem resilience, leaving a system more susceptible to future change. The cumulative effect of multiple pressures over many decades across whole regions and landscapes and seascapes, especially within and around intensive land-use and marine-use zones, exacerbates fragmentation and further degrades the quality of remnant native habitats, which support many threatened plants and animals. Extreme events can also provide a tipping point that overwhelms systems under multiple pressures.

Research has shown that the combination of pressures leads to most of the declines in our threatened species (see Threatened species). Impacts are compounded by the current and forecast impacts of climate change – for example, on land, more heat extremes; more time in drought; more intense, short-duration storms; continued decreases in cool-season rainfall; and a longer fire season for southern and eastern Australia. Rainfall declines result in a decrease in river flows, with implications for water availability for all purposes. Reduced availability of surface water can increase demand for groundwater, with negative impacts on groundwater-dependent ecosystems. In the ocean, the cumulative impacts of climate change affect the condition of Australia’s rocky reefs and algal beds – for example, rising temperatures and heatwaves; increasing flows of warm, nutrient-poor tropical waters into temperate regions; and overgrazing by sea urchins and tropical fish herbivores. These impacts are facilitated and compounded by the removal of urchin and fish predators by commercial and recreational fishing.

More and more we are discovering that most threatened species and ecosystems cannot be recovered by managing a single threat; 86% of Australia’s threatened species are subject to multiple threats that result in habitat destruction and degradation, such as logging, mining, urbanisation and agriculture. To deal with these cumulative pressures, the key conservation response is habitat retention and restoration (Kearney et al. 2020). Increasingly, species require multiple integrated management responses to address their threats (Figure 24). However, there remains much uncertainty about how to define and predict the ecological impacts from cumulative pressures.

Figure 24 Number of Australian threatened species that would benefit from different management responses

Cumulative losses of habitat are critical for understanding the overall impact, yet actions referred under the EPBC Act are currently individually assessed. Of the 7.7 million hectares of land habitat cleared between 2000 and 2017, 7.1 million hectares (93%) was not referred to the Australian Government for assessment. Between 2000 and 2017, only 4 of 3,058 referred actions to remove land habitat have been deemed ‘clearly unacceptable’ (0.1%), 2,252 have been deemed ‘not a controlled action’ (i.e. not requiring approval to proceed; 74%), and 806 have been deemed a ‘controlled action’ (26%). Since the commencement of the EPBC Act, a significant amount of habitat destruction has either not been assessed or has been approved, albeit sometimes with conditions, resulting in considerable cumulative habitat loss.

The interaction between weed and feral animal invasion and fire is of increasing concern as climate change continues to alter fire regimes. Invasive grasses such as gamba grass (Andropogon gayanus) and buffel grass (Cenchrus ciliaris) increase fuel loads and fire intensities, sometimes dramatically altering ecosystem structure and function, and forming dense infestations that increase fire connectivity. Intense fires can also exacerbate other pressures – for example, feral cats are more abundant and hunt more successfully in areas that have experienced recent or severe fires (Davies et al. 2020, Legge et al. 2020).

Many pressures, historical and current, impact our inland waters and coasts – including habitat fragmentation and degradation; clearing of catchments; the volume and timing of extractions for use; dams and weirs altering flow patterns and blocking movement of fish; rivers disconnected from their floodplains; changes in nutrients and temperature; contaminant pollution; introduced weeds and pests; and the disruption of overland flows and aquifers. In the remote and northern parts of the country, these impacts are less significant or more localised, but aquatic ecosystems in the south are highly affected. Understanding these cumulative impacts requires long-term detailed studies, combined with integrated monitoring methods (Sparrow et al. 2020, Taylor & Lindenmayer 2020).

Run-off is affected not only by total rainfall, but also by changes in seasonal patterns and catchment condition. It has been demonstrated that run-off following rain is significantly reduced during droughts. In the northern Murray–Darling Basin, run-off appears to have been more severely reduced during recent droughts than in previous droughts, compounding the impacts on downstream long-term water availability (Vertessy et al. 2019). Low water inflows throughout the system may persist long after the end of the dry weather patterns.

The drying out of wetlands and areas of acid sulfate soils can affect the quality of water and soil. More frequent and severe bushfires also severely impact water catchments and cause water quality impacts if there are subsequent large rainfall events – from ash, sediment, nutrients and debris. Although storage inflows from deforested catchments may increase in the short term, regrowth of catchment vegetation will result in changes to water yields for many years. Some of these impacts may be the consequence of failures of knowledge, management or regulation, demonstrating that the challenges of water management are complex, interconnected, and increasingly amplified by climate change.

Assessment Cumulative pressures
2021
2021 Assessment graphic showing that pressures are high, meaning they moderately degrade the state of the environment, over a moderate extent and/or with moderate severity. The situation is deteriorating.

The great majority of Australia’s ecosystems and threatened species are impacted by multiple pressures, which often accumulate and amplify each other. Despite awareness of the importance of cumulative pressures, management actions often target a single pressure, resulting in declines in threatened species populations and loss of important habitats.
Assessments of impact are high
Assessments of trend are deteriorating
Related to United Nations Sustainable Development Goal targets 11.3, 11.b, 12.2

Legend
How was this assessment made
Infographic showing that the environment – in this metaphor, a tree branch – can cope with 1 or perhaps 2 pressures (imagined as weights hanging on the branch). But too many pressures (weights) at once break the branch irrevocably. If we remove pressures before the limit is reached, things can recover.