The concept of resilience is not straightforward, with definitions varying across social and scientific disciplines. Complexities in the level of detail also vary, depending on the system of focus. Definitions of resilience can contain any combination of 3 major principles (Folke et al. 2002, Bernhardt & Leslie 2013):
- the magnitude of shock or pressure that a system can absorb while remaining within a given state
- the degree to which the system is capable of self-organisation in light of the shock or pressure
- the degree to which the system can build capacity for learning and adaptation.
Depending on the interpretation, self-organisation and adaptation can be associated with the absorption of shocks without a fundamental change (e.g. returning to an original ideal state). Self-organisation and adaptation can also include readjustment to a new state (e.g. adaptation without sacrificing the provision of ecosystem services; Folke et al. 2004). Ultimately, at the core of any definition of resilience, is the capacity of a system to keep functioning even when disturbed (Levin & Lubchenco 2008). Protection of the ecosystem services provided by the marine environment requires that the resilience of marine ecosystems is maintained.
Organisms have developed mechanisms for coping with both short-term variability (e.g. waves, tides) and longer-term variability (e.g. seasonal changes in temperatures, occasional extreme events). Natural selection across long timescales has operated to ensure resilience to variability within historical environmental conditions. However, there is no reason to expect that this same resilience will be maintained as conditions change on shorter timescales, such as those associated with anthropogenic pressures (Hughes et al. 2005, Levin & Lubchenco 2008). The maintenance of resilience is therefore driven by 2 essential components: the adaptive capacity of individual organisms and the timescale at which changes in their environments are occurring.
Similarly, the resilience of ecosystems to disturbance depends on the spatial scale at which both disturbance and resilience operate (Hughes et al. 2005). Improving resilience at small scales might, ultimately, be little defence against pressures that operate at larger scales. For example, climate change is a global pressure with the potential to overcome resilience built at the local scale. The high connectivity of the marine environment means that additional remote pressures, including invasive species, marine debris and coastal run-off, also have the potential to threaten local resilience.
Recent research indicates that local management, including the establishment of marine reserves (see Managing for resilience), can build resilience to external pressures, perhaps buying some time for more effective regional or global management measures to be put in place (Abelson et al. 2016). Understanding and managing the coupled socio-ecological system may increase resilience to changes that the ecological system itself cannot resist. This will require a clear understanding of the properties and value of future environmental conditions, so that managers can help direct the trajectory of change.
Current understanding of the resilience of Australia’s marine environment is sparse. This is because it is difficult to monitor the environment across the timescales relevant for assessing resilience, as a result of the vast spatial extent of Australia’s marine ecosystems, their complexity, and the many and varied sources of pressures exerted on them. It is accepted that even well-managed systems such as the Great Barrier Reef are unlikely to resist anthropogenic pressures originating outside the management area, making their future prognosis poor (GBRMPA 2014a, Hughes et al. 2015).
Ecosystems that have relatively high diversity tend to be more resilient to external pressures, largely because of high variability in population densities and their ability to maintain aggregate properties, such as nutrient cycling or trophic functioning (Levin & Lubchenco 2008, Hughes et al. 2010). Studies of shallow-water marine reserves around Australia have shown an increased stability in fish populations compared with waters outside the reserve (Babcock et al. 2010). High abundance of larger predators in marine reserves in Tasmania has been shown to reduce the numbers of range-extending black-spined sea urchins by 2–10 times (Ling & Johnson 2012), and increased herbivore abundance and coral recruitment have increased the resilience of habitats within marine reserves to coastal run-off associated with flood events in eastern Australia (Olds et al. 2014). However, determining direct effects on populations takes at least 5 years, and more than 10 years before cascading indirect effects on other species are identified (Babcock et al. 2010).
Understanding the resilience of the Australian marine environment currently provides indications of how local resilience can be built successfully, but not whether this will be enough to support resilience in the longer term or at the broader scale. Further, there are substantial gaps in the understanding of the thresholds beyond which ecosystem functioning cannot be maintained, particularly where the responses to pressures are highly uncertain (e.g. those associated with climate change).