The five threats
Threat 5: Habitat destruction
Coral reefs are home to 25 percent of all marine life. —IPCC 2007 report
Over 80 percent of coral reefs in Southeast Asia – the most species-rich on earth – are at risk. —“Reefs at Risk”, World Resources Institute
Coastal areas are home to over 90 percent of all marine species, which thrive in ecologically-rich and diverse shallow water habitats. These habitats – chief among which are coral reefs, mangrove forests, and seagrass meadows – are being lost at alarming rates.1
Overfishing, pollution, and climate change all contribute to the degradation of coastal ecosystems. Another major factor is the accelerating development of coastlines for industry, agriculture, and tourism. According to the UN, nearly two-thirds of the world’s population now lives within 60 kilometres of the coast.2 Overdevelopment and aquaculture – fish and shrimp farming – have been responsible for severe declines of these productive habitats over the past decades, and particularly of mangrove forests, which protect coastlines by absorbing storm forces and supply nutrients vital to marine life. As these habitats disappear, both people and the environment suffer: marine animals lose both their food and shelter, leading to the collapse of food webs from the bottom up, and human settlements become increasingly exposed to the effects of storms and flooding.
Coral reefs are some of the most biologically productive and diverse ecosystems on the planet. They occupy less than 1% of the ocean surface, yet are home to a quarter of all marine life.3 Reefs are extremely fragile and slow-growing, and face a number of threats: ocean acidification, warming waters, and outright destruction through overdevelopment and unchecked tourism are a few of them. Less known, but equally dangerous, is eutrophication – the introduction of excess nutrients from fertilizers used in agriculture into the ecosystem. Rich in nitrogen and phosphorus, this waste ultimately flows into the oceans, where the increased levels of nutrients stimulate the growth of algae, causing what are known as “algal blooms”. These blooms can smother reefs when they occur over them, as they block the sunlight required by the zooxanthellae in the coral to complete photosynthesis. The algae also sink to the bottom when they die, covering reefs in slime.
Case study: Dead zones
If they sound scary, it’s because they are. Scientists at the University of Gothenburg in Sweden recently counted 400 “dead zones” around the world – areas of ocean along the coast where the water at the sea floor has become completely deprived of oxygen, essentially killing off all life.4 They range in size from expansive (in the Baltic and Gulf of Mexico) to sporadically-appearing small ones; together, they cover an area of about 250,000 square kilometres.
Dead zones are closely-tied to eutrophication (an increase in nutrients). When nitrogen and phosphorus, the key ingredients in fertilizer, enter the environment, they stimulate the growth of phytoplankton at the surface. This negatively affects the ecosystem in several ways. While the phytoplankton (algae) produce oxygen through photosynthesis, as they begin to die they sink and accumulate at the bottom, where they are broken down by bacteria. As the dead algae biomass increases, oxygen-consuming bacteria multiply, eventually using up all the dissolved oxygen in the water, causing fish and bottom-dwelling shellfish to suffocate.
Dead zones occur along the most developed coastlines in the world, often around estuaries. One of the biggest dead zones is a 22,126 square kilometre region of the Gulf of Mexico – an area equivalent to the US state of New Jersey – where the Mississippi river dumps its fertilizer-rich runoff.
Mangrove forests are an extremely important coastal habitat under increasing pressure from human development. Mangrove trees fulfill a variety of roles: their roots, which are specially-adapted to filter salt water, collect sediment and protect the coastline by preventing erosion. They also serve as buffers, limiting the destructive force of storms and tidal waves – studies have shown that during the 2004 tsunami in Southeast Asia, regions with mangroves suffered less damage than those without.5
Mangroves are home to a vast number of land and marine species, and serve as a nursery for many fish and shellfish. Unfortunately, the ecological importance of mangroves went unrecognized until relatively recently. As a result, roughly 35% of mangrove forests worldwide vanished over the past several decades.6 Commercial shrimp farming is responsible for about a quarter of all mangrove destruction, augmented by agriculture (rice fields) mining, harbour development, and other activities.
“Roughly 35% of mangrove forests worldwide vanished over the past several decades.
Seagrasses (not to be confused with seaweeds) are a unique group of flowering plants that have adapted to exist fully-submersed in the sea. Like coral reefs and mangrove forests, seagrass meadows are key marine habitat that has come under threat from human development. A recent study found that seagrass declines over the past century are comparable to those reported for mangroves and corals, spurred on by degraded water quality, climate change, and coastal development. It’s estimated that 29 percent of seagrass meadows have disappeared over the past hundred years, placing them amongst the most threatened ecosystems on Earth.7
Seagrasses are the primary food source for endangered dugongs, manatees, and green turtles. They are also a vital element of the Earth’s carbon cycle – while occupying a mere 0.1 percent of the seafloor, they store 11 percent of all the carbon in the ocean. In addition to this, seagrass meadows, as producers of oxygen, limit the effects of eutrophication.7
As the world’s population continues to grow, further coastal development is inevitable. What can be controlled, however, is the sustainability of development. Some of the damage done over the past century is reversible, at least in part.
The Black Sea dead zone temporarily disappeared after fertilizers became prohibitively expensive immediately after the fall of the Soviet Union. This demonstrates that dead zones are reversible. Indonesia, following the destruction of half of its mangrove forests to make room for unsustainable shrimp farms in the 1980s, has implemented programs to replant mangroves.
Broadly, protecting and restoring our planet’s marine habitats requires the following:
- Removing subsidies on fertilizer use, which would limit their use and reduce the problem of marine dead zones.
- Rapid legislative protection of vital ecosystems, like coral reefs and mangrove forests, in the form of Marine Protected Areas (MPAs), biosphere reserves, and marine parks.
- Reducing the conversion of important and valuable coastal ecosystems to aquaculture facilities and real estate.
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2. UN Atlas of the Oceans: Coasts and coral reefs.
5. Half of Indonesia's mangroves gone in less than thirty years. Jeremy Hance. mongabay.com (March 23, 2010).
7. Waycott et al. Accelerating loss of seagrasses across the globe threatens coastal ecosystems. PNAS July 28, 2009 vol. 106 no. 30 12377-12381.