Marine populations wax and wane over time. This natural variation is caused by the natural environmental drivers of change, such as differences in conditions between seasons and years. However, sudden environmental shocks (such as major storms or flood events) can create major changes in populations and ecosystems related to the size of the disturbance. Few ecosystems affected in such major ways will ‘bounce back’ to the same state they were in before the serious shock. However, humans tend to focus on rapid change and are slow to appreciate less obvious, but not necessarily less relevant, change.95 This is sometimes caused by the phenomenon known as the ‘shifting baseline’, when managers base decisions on conditions they have personally experienced, with each successive manager relating to sequentially degraded conditions. With fishing, scientists, managers and even the general public are quick to identify and attempt to curb obvious overfishing or damage due to irresponsible fishing practices, but they have been slow to respond to less obvious signals, such as those due to climate change and fishing-induced genetic impacts.45 Both the fast (major shock) and the slow (incremental temperature shifts or genetic restructure) drivers of impacts affect resilience, and they can be equally significant.
Considering fishing as an example, species that are fished are ecologically important—they are often large, long lived and abundant, so are important in marine ecosystem functions. However, while fishing mortality plays a part in influencing resilience, other forces act on the population and its ecosystem that determine the population’s ability to recover from fishing (and other) pressures. Events that accompany overfishing often include pollution, eutrophication (a large increase in nutrients in the water, often leading to algal blooms), physical destruction of habitats and introduction of pest species. These impacts are often further complicated by social and economic responses of governments and communities that try to maintain stability in ways that have outcomes that are counter to their objectives.24 Systems that are compromised by the effects of overfishing are made more vulnerable to these additional disturbances, potentially opening the way to population collapse.96
Recovery of ecosystems can be hindered by complex and often indirect species interactions. One of the factors that helps to make ecosystems more resilient to change is high ecological redundancy (i.e. there are many species that perform similar functions), because this allows other species to potentially replace one or more key species in the ecosystem to maintain ecosystem services.97 Species-rich systems are more likely to have greater functional redundancy and flexibility, and this can provide them with a degree of ecological insurance against uncertainty,98 although this is not always the case.35 Populations in highly diverse ecosystems may therefore be more likely to be resilient to change—in diverse ecosystems, compared with systems that are naturally low in species numbers, a smaller fraction of commercially fished species have collapsed, and there has been a higher rate of recovery of collapsed species97
The natural dynamics of marine species are related to the recovery potential of healthy marine populations. Those that have high levels of spawning biomass and a natural range of ages in populations and are widely distributed across their habitat range can be considered to be naturally resilient.35 When a diversity of secure areas protected from environmental and human pressures is available, populations can capitalise on good environmental conditions with strong reproductive outputs, often creating a strong year-class (all individuals spawned in a single year) that will survive and maintain the population’s recovery potential through subsequent poor years until the next environmentally favourable year occurs. This feature can also provide fisheries with increased security of catch and a greater buffer against environmentally driven fluctuations that would otherwise reduce stability in the industry.
A recent international workshop that reviewed human impacts in the global oceans concluded that the extent and importance of the cumulative impacts of the various types of pressures (exploitation, climate change, pollution, habitat loss) have been significantly underestimated.99 In particular, the extinction threat to species is rapidly accelerating, and there is an unparalleled global rate of regional extinction of marine habitat types. The review concluded that a number of high-priority actions are required, including the proper and universal application of the precautionary principle to reverse the burden of proof (new activities that may damage the oceans should only be approved when they can show minimal and acceptable levels of impact both singly and cumulatively with other stressors). The review has also proposed that a United Nations Global Ocean Compliance Commission be established to oversee the charter of ocean protection.99