Coral reef ecosystems are being damaged all over the world through a number of direct and indirect anthropogenic (i.e. human) threats, including coral bleaching, coral disease, coral predator outbreaks, ship groundings, and anchor damage, destructive fishing, and land-based runoff or sedimentation. Almost 50% of all coral reefs are experiencing a high level of impact due to anthropogenic activities. These impacts are disrupting the ability of coral reefs to provide ecosystem services, such as food provision, protecting coastlines and preventing floods. This had led to reef ecosystems being less resilient to other threat events, such as ship groundings or pollution. In turn, this exacerbates the disturbance experienced by these ecosystems and results in reefs being unable to recover if the threat subsides. Overall, the health of coral reefs becomes a negative feedback loop.
Researchers have been becoming increasingly aware that humans can alter how often threat events occur and how the area is that the event effects. This is important to help us understand how our impact affects coral reefs and their potential to recover following these threat events. In some cases, the relationship between a human-caused threat and the impact on the coral reef ecosystem is clear, such as unsustainable exploitation of coral reefs from destructive ﬁshing methods that use cyanide or dynamite ﬁshing. These explicit human disturbances can also be seen where there is ineffective habitat management that fails to be integrative and large-scale. For example, coral reefs are often affected by decisions taken in their drainage basins. Clearing coastal forests for intensiﬁed land use and urbanization increases run-off of pollutants, nutrients and sediment particles, resulting in problems with algal and coral growth and competition. Logging activity can prolong sedimentation, creating disturbances that are long-term, even if logging activity is no longer going on. Longer-term, a small increase in sedimentation may lead to changes in coral reef community structure in response to the stress of these disturbances.
Anthropogenic disturbances have even altered naturally occurring stress events to the point at which now even natural disturbances may not be considered as such. For example, predator outbreaks, mass bleaching and disease are directly or indirectly linked to anthropogenic impacts. To summarize these threats:
The vast majority of coral species have a narrow temperature range tolerance. If a coral becomes overly stressed than this will cause the coral to bleach. This is predominantly done when ocean sea surface temperatures become too warm. Coral bleaching is where the coral expels the symbiotic algae (called zooxanthellae) that live inside them. As a result, corals lose their algae and their associated color and turn white. They also lose a crucial food source, which the algae provide by photosynthesis. Within the last 25 years there have been three global coral bleaching events in 1998, 2010 and 2014-2017, and many other localized bleaching events. The most recent bleaching event was the most widespread and longest lasting event since records began and resulted in the death of two-thirds of Australia’s Great Barrier Reef. Corals are able to survive a bleaching event, but without their associated algae, they will eventually die if the stress is prolonged.
Destructive fishing practices
The use of cyanide and dynamite for blast fishing is largely illegal in many countries. However, policing this is difficult in many developing countries with limited resources. Other fishing practices such as bottom trawling and muro-ami (banging on the reef with sticks) have also devastated reefs in a short period of time. Bottom-trawling is known to be one of the greatest threats to cold-water coral reefs, with scientists calling to end the practice.
Live coral is removed from reefs for use as bricks, road-fill, or cement for new buildings. Corals are also sold as souvenirs to tourists and to exporters who don’t know or don’t care about the longer term damage done, and harvested for the live rock trade.
Fish play a vital role in keeping coral clean by removing harmful algae that can smother the corals. By removing these fish, it upsets the ecological balance of coral reef communities, causing cascading impacts through the rest of the food chain and has implications far beyond the directly overfished population.
Pollution takes many forms. From urban and industrial waste such as sewage, agrochemicals, and oil pollution as well as plastic pollution. These artificial chemicals effectively poison coral reefs. Worryingly, they are sometimes directly dumped into the ocean or leach into the water through river networks upstream and are then carried into the ocean. Often pollution including sewage or pesticides increase the nitrates in the water, causing toxic algal blooms, which effectively smother reefs by creating a dense mat that blocks out the sunlight.
Erosion caused by development (both on the coast and inland), mining, forestry and farming contributes to increased erosion in waterways. This ends up in the water, where they can ‘smother’ corals by depriving them of the light they need to live. The loss of mangrove forests, which generally trap vast quantities of sediment, is exacerbating the crisis.
Careless boating, swimming, snorkeling and fishing is taking place around the world, with people touching the beaches, stirring up mud, catching coral and throwing anchors on the reefs. Any tourism resorts and facilities were constructed right on top of the reefs, and some resorts emptied their sewage or other waste directly into the water around the coral reefs.
Anthropogenic disturbances (e.g., coastal development, poor land-use practices, and overfishing) have eroded natural processes that regulate the population of coral predators. This has led to largely uncontrolled predator population fluctuations. Therefore, the severity and incidence of predators such a crown-of-thorns starfish (COTS) has dramatically increased.
Human-induced eutrophication has been linked to diseases such as black-band disease. Human pathogens have also been shown to directly cause coral disease. Additionally, the increased incidence of mass bleaching has resulted in reduced reef resilience owing to loss of species diversity favoring temperature-tolerant species. Coral disease outbreaks typically follow bleaching events as stressed corals are more susceptible to infectious diseases.
A recent special report by the Intergovernmental Panel on Climate Change (IPCC) has shown that, with an additional 0.5 °C of warming above current levels, tropical coral reefs will face “very frequent mass mortality” while coral adaptation is possible. If temperatures increase by a further 2 °C above pre-industrial levels, coral reefs are in danger of disappearing permanently. Our climate is becoming warmer and the oceans are turning more acidic. Globally integrated conservation and proper management is needed to help reefs recover and continue to provide the essential ecosystem services they generate.