2.4.4.2 Coral reef degradation and small-scale fisheries

Shallow-water tropical coral reefs are an example of an ecosystem that is already being heavily affected by climate change (Hughes et al., 2018; Hughes et al., 2017). Coral loss in today’s oceans can tip entire reefs into less desirable states in which other kinds of benthic cover (e.g. macroalgae, rubble, algal turfs) become dominant (see CLD, Figure 2.4.7) (Tebbett et al., 2023). These long-lasting shifts can have significant consequences for people who depend on reef-based fisheries and tourism for their livelihoods.

Two elements are critical in understanding the nature of ecological tipping points on coral reefs. First, many of the more ecologically significant coral species are slow-growing, and may take years to reach their full reproductive potential; after a mortality event, recovery is initially dominated by weedy, faster-growing corals that are also more vulnerable to bleaching (Cannon et al., 2021; Darling et al., 2013). Second, corals must compete for space with other species (e.g. algae, sponges and sessile invertebrates such as giant clams) and their growth and survivorship are strongly influenced by water quality (Cooper et al., 2009). Corals typically favour clear, low-nutrient waters. Human activities in coastal environments (e.g. dredging activities in harbours, fertiliser-rich nutrient runoff from agriculture, over-fishing of keystone species such as parrotfish) can tip the balance of ecological conditions such that coral mortality is high and growth rates are slow (Cooper et al., 2009). These changes in turn often mean that either corals can no longer survive in degraded habitats, and/or other taxa are able to out-compete them.

The social and economic elements of coral reef tipping points arise through the reliance of many coastal communities on coral reefs and the resources they provide. It is estimated that a billion people live within 100km of a coral reef (~13 per cent of the global population) – a number that has significantly increased in the last 20 years (Sing Wong et al., 2022). Reef fish and invertebrates provide a year-round source of critical nutrients in locations where other sources of protein may be scarce (Mellin et al., 2022). Both artisanal fishing and gleaning are important activities in many Indigenous cultures, providing a wide range of social, economic and psychological  benefits (Grantham et al., 2021). Some reef fish are harvested commercially (e.g. coral trout) and coral reefs contribute to local and regional income more generally through tourism and related industries. Some estimates state that coral reefs provide up to US$9.9trillion/year through ecosystem services and goods (Costanza et al., 2014). On the Great Barrier Reef in Australia, for example, coral reefs in 2012 were estimated to support the employment of more than 68,000 people and provide a benefit of AUS$5.7billion per year, mainly from tourism (Deloitte Access Economics, 2013). Coral reefs also support other industries, such as the provision of tropical fish and coral pieces for aquaria, and the harvesting and sale of snail shells (e.g. ‘Triton’s Trumpet’). 

The impacts of coral loss on fish communities are still poorly understood. Negative impacts have been documented for coral-dependent species such as butterfly fish and parrot fish (Magel et al., 2020; Thompson et al., 2019). Loss of these species further increases the challenge of restoring coral reefs because of the important role that herbivorous fishes play in keeping reefs clear of algae; overfishing has been blamed independently for declines in coral cover and algal overgrowth on reefs. Conversely, other research has shown limited effects of past bleaching events on fish communities (Wismer et al., 2019a). Based on these findings, some scientists have argued that the loss of corals may have little impact on net fish biomass production if habitat structure (benthic complexity) remains (Wismer et al., 2019b). The potential time lags between coral loss and impacts of coral loss on the fish community make these debates harder to resolve; and it is also possible that threshold effects exist whereby the fish community has a form of resilience and only exhibits marked changes beyond the loss of a particular proportion of coral. Most available evidence, however, points to a potentially significant impact of coral loss on fish communities. Documented impacts of coral cover declines include a loss of fish species, reductions in overall fish biomass and productivity, and potential destabilisation of the food web (Magel et al., 2020; Bellwood et al., 2019).

If reefs are forced by climate change into low-productivity states, and if these states in turn force fish communities across a tipping point into a less diverse and less productive state, many coastal human communities will be forced to modify their lifestyles in significant ways (Strona et al., 2021; Lam et al., 2020; Hoegh-Guldberg et al., 2019). These changes may in turn lead to tipping points in socioeconomic systems. In many coastal cultures, fish and corals are central to nutrition, income streams, social dynamics and established cultural practices and traditions (Eddy et al., 2021). Changes in fish species composition and abundance will also lead to shifts in the interactions of coastal communities with external actors, such as overseas markets, tourists and fisheries companies (Bartelet et al., 2023). These interactions in turn are likely to create further changes in the interactions between people and ecosystems, potentially leading coral reef social-ecological systems along new trajectories. Coral reefs may also provide coastal protection against storm surges, which may increase exposure to climate change impacts. For reef-dependent human communities in isolated locations, the options for adaptation (e.g. fishing open-water fish stocks or importing protein) may be dangerous or unviable. Available evidence suggests that tourists have flexible baselines, with degraded reefs still providing benefits (Bartelet et al., 2022); but dive tour operators, for example, may switch into other kinds of business, leading to a potential loss of expertise and local knowledge (Bartelet et al., 2023).

Reductions in coral reef fish diversity and biomass have significant implications for the nearly one billion people globally who depend on tropical seascapes, and particularly their reef-based small-scale fisheries, for nutrition and livelihoods (Cumming et al., 2023). Coral reefs provide a wide range of economically valuable ecosystem services, including provisioning services, regulating and supporting services, and cultural services (Eddy et al., 2021). 

Thresholds and tipping points may occur in coral reef social-ecological systems in numerous different ways (Figure 2.4.7, adapted from Van de Leemput et al. 2016). Coral reefs may exhibit at least five different states that appear to be relatively stable: hard coral-dominated, soft coral-dominated, macroalga-dominated, rubble and algal turf (Bellwood et al., 2019). These each have different values for fisheries and tourism. Van de Leemput et al. (2016) show how even relatively weak effects acting in concert can lead to shifts between some of these states. Hard coral-dominated reefs offer the highest values for most ecosystem services, but are vulnerable to bleaching.  

There are again numerous pathways by which coral reef degradation may cascade into social and economic tipping points. For example, Crona et al. (2016) show how the interactions of small-scale fisheries with the global seafood trade may shift between different economic states. Small-scale fisheries may be exporters of seafood; competitors with the global trade; or victims whose livelihoods are destroyed by commercial over-harvesting (Figure 2.4.7). Shifts between these economic arrangements will have profound consequences for local communities. Another pathway by which social-ecological tipping point cascades may occur in coral reef systems is via the effects of coral reef degradation on tourism (Figure 2.4.7) (Bartelet et al., 2023; Bartelet et al., 2022). If a region that has been known for its snorkelling and diving opportunities loses much of its coral, it may gradually lose business to other areas with more intact ecosystems. Lower income from tourism will place greater pressure on local livelihoods and drive either a shift into other activities, some of which may have consequences (e.g. harbour enlargement or dredging) that are harmful to coral remnants. In either case, investment into the conservation and management of corals and other marine ecosystems is likely to decrease and the perceived value of coral reefs to local people is likely to decline, leading to lower levels of stewardship and enforcement and potentially resulting in further knock-on effects via overfishing and pollution (CLD, Figure 2.4.7). In this way it is plausible that an entire social-ecological system shifts into a self-reinforcing state in which coral recovery becomes increasingly difficult and unlikely, even in the absence of pressure from climate change.