What are coral reefs?
Tropical coral reefs, the ‘rainforests of the sea’, represent one of the most biologically rich ecosystems on our planet. Though they cover less than 1% of the ocean floor, they support around 25% of all marine life. These diverse collections of species thus offer a range of ecosystem services which benefit humankind, including but not limited to fisheries, coastal protection and income from tourism. Despite difficulties in quantifying the exact economic value of coral reefs, estimates range between 30 and 375 billion dollars each year. Despite flourishing for millions of years, like many of our planet’s ecosystems coral reefs are under increasing stress and the capacity to provide critical ecosystem services is being eroded. This article will briefly explore what coral reefs are, their importance to humans, the factors behind their decline and potential solutions to this.
Building blocks of coral reefs
The structural foundation of tropical coral reefs is provided by scleractinian or hard corals. Despite often appearing as stones or rocks, corals are actually animals belonging to the phylum Cnidaria, along with jellyfish and sea anemones. Each individual coral is a collection of colonial animals known as polyps, consisting of a mouth surrounded by tentacles. A common feature of cnidarians (as many of us know well from the closely-related jellyfish) is that their tentacles contain stinging cells known as cnidocytes, which they use for capturing their prey.Scleractinian coral polyps secrete a calcium carbonate skeleton which forms protective cups. The skeleton grows as polyps ‘bud’ (asexually reproduce by splitting in two), and new polyps continue to secrete calcium carbonate for their own protective cups. Secreting this skeleton is highly energy intensive, and to maximize the energy available to them, corals live in symbiosis (a mutually beneficial relationship) which single-celled algae known as zooxanthellae. The zooxanthellae photosynthesize and produce sugars for the coral host to use, while they in turn gain a safe and stable environment within the coral tissue. Up to 95% of the sugars produced by the zooxanthellae are used by the coral host, significantly increasing the energy available to them compared with if they relied solely on heterotrophy, i.e. feeding on small animals in the water column.
Darwin’s Paradox; Coral reefs are oases in the desert of the ocean
Ever since Charles Darwin described coral reefs as oases in the desert of the ocean, we have been striving to understand what allows such diverse ecosystems to thrive in warm and oligotrophic (nutrient-poor) oceanic waters. Though this mystery is not entirely unraveled, we now understand that complex feedbacks and interactions allow highly efficient recycling of nutrients and energy within the ecosystem. One example is the symbiotic relationship between corals and zooxanthellae (as described above), which maximizes the energy available to the framework corals which are so essential to reefs.
Another is the newly described “sponge loop” (de Goeij et al. 2013). Sponges are very basic filtering animals which were previously understudied on reefs, despite finding that they are the major inhabitant of hidden coral reef cavities (which represent a large proportion of total reef surface area). However, recently scientists found that these simple animals play an integral role in recycling dissolved sugars and making them bio-available to the food chain once more. Whilst filtering water, they take up large amounts of dissolved organic carbon (DOC); a compound released by many coral reef organisms and is then generally inaccessible. Due to the quick cell turnover of the filtering cells, this DOC is then released in the form of particulate organic matter (essentially sponge poop), which is then readily accessible to higher trophic levels. The amount of DOC recycled through this sponge loop is ten times higher than via the microbial loop which was previously the only known means for DOC to be recycled.
Different roles on a reef
Corals come in a variety of shapes and sizes, from massive corals that look like boulders, to branching and tabulate corals and even corals that look like mushrooms. Generally speaking, corals can be divided into soft corals and hard corals. Each type of coral has a different function on the reef, ranging from providing shelter for fish to absorbing wave energy. Growth rates vary from less than 1cm per year to 20-30cm per year depending on the coral. That means some of the large colonies you observe when you dive may be hundreds of years old!Coral reef fish also provide a wide range of functions which are all critical to the maintenance of a healthy system. Some fish, such as the colorful parrotfish and surgeonfish, are herbivores which graze algae and keep more free space for new corals to settle into as well as reducing the overgrowth of corals by algae. Piscivores, such as snapper and grouper, are critical in supporting healthy fish stocks by maintaining a balanced system. Similar to other mesopredators (predators that prey on other fish and are themselves preyed on), they remove sick and weak individuals. Some invertivore fish (that feed on invertebrates) act as ‘cleaner fish’ and feed on parasites from the bodies and mouths of larger fish - which is why you can find cleaning stations on reefs where visitors may include large predators and majestic creatures such as turtles and manta rays. 
Problems facing coral reefs
Despite persisting in the oceans for millions of years, coral reefs are now seriously in decline. Studies from around the world have claimed that coral cover is around 50% what it was a few decades ago (Wilkinson et al. 2008). The Caribbean region has shown one of the worst declines, with coral cover being reduced by 80%, from 50% to only 10% (Gardner et al. 2003).Unfortunately, it is difficult to establish what the main factors are driving this decline due to the multitude of stressors to which coral reefs are now exposed. Some stressors are found on a local scale, with overpopulation in many areas driving changes in fishing, habitat degradation and declining water quality. More than 60% of reefs are directly threatened by these local stressors (Burke et al. 2012). Fishing in coastal populations has changed from traditional to more destructive, modern methods. For example, dynamite fishing destroys the structural foundation of coral reefs while either stunning or killing the fish, making them easy to catch, and nighttime spearfishing with flashlights gives fishermen the advantage over fish by allowing them to select the largest and most important individuals while they are sleeping on the reef. 83% of fished reefs are now missing over half of their fish biomass (Macneil et al. 2015). Habitat degradation of coastal areas often leads to increasing sediment in the usually clear water, resulting in stressful conditions for corals. This often comes hand-in-hand with an increase in nutrients from sewage and land-based fertilisers which increase the productivity of algae in the water allowing them to gain competitive strength over the weakened corals. One of the major ecosystems affected by habitat destruction is mangrove areas, which both act as nursery areas for juvenile coral reef fish and filter the land-driven wastewaters – clearly amplifying the difficulties for reefs to recover. 
Another of the main problems is climate change, which increases sea surface temperatures and changes weather patterns. Prolonged elevated sea surface temperatures lead to coral bleaching, where the symbiotic zooxanthellae leave the coral tissue thus leaving a white (bleached) coral, and may result in eventual coral mortality. Widespread bleaching events are now commonplace around the world, including right now (see here coralreefwatch.noaa.gov/satellite/index.php). Another weather pattern influenced by climate change is storm intensity, which can seriously affect the integrity of coral reefs if the intensity or frequency is higher than normal. With increasing frequencies of these events, reefs sometimes don’t have the chance to recover from the last event before being hit by the next one, which is why we have to focus on increasing their recovery potential. By controlling the local stressors outlined above, we can increase the resilience of coral reefs and hence their ability to withstand and recover from climate disturbances. 
These factors (as well as many more) are driving coral reefs into different systems which are dominated by alternative organisms such as algae or soft invertebrates. Dominance by these organisms is often considered less desirable as the altered systems offer seriously reduced ecosystem services – for example these systems are often less favourable for fish and have much less potential to absorb wave energy.
"83% of fished reefs are now missing over half of their fish biomass"
Macneil et al. 2015
What is being done to protect coral reefs?
There are several different ways that local stakeholders, non-Governmental organisations and Governments are managing coral reefs around the world. One of the most popular methods is by closing parts of the reefs to fishing by creating Marine Protected Areas (MPAs). Coral reefs in MPAs are considered to be six times more likely to recover from disturbances than non-protected reefs (Mumby et al. 2014). However, up to now less than 1% of coral reefs in the coral triangle (the center for biodiversity of coral reefs) are considered to be ‘effective’ MPAs. Fishing restrictions (i.e. quotas, gear restrictions) can also help improve coral reef health by keeping an intact fish community and supporting the role of herbivorous fish which maintain a balanced system. Given protection from fishing, fish populations do have the potential to recover but this will take time (i.e. 35 years on average – Macneil et al. 2015). However, it is difficult to enforce these in many remote regions and particularly when fish are targeted by subsistence fishermen (not to be sold at market). Managers are also targeting the problem of land-based pollution, but this is a slow process and comes with many difficulties in both remote regions where infrastructure and funds are low, as well as largely populated areas where it is challenging to control the ever-growing population’s waste. 
What can you do to help protect coral reefs?
Now the good news: though coral reefs may seem so distant, your actions can affect coral reefs directly and indirectly. Here is a list of things you can do to help this threatened ecosystem so that humans can continue to receive their benefits into the future:Avoid eating coral reef fish – whilst 500 million people depend on coral reefs for their protein and livelihoods, people from other regions should not add to the exploitation of these resources. People can further avoid the ornamental trade which encourages the unsustainable removal of coral reef organisms.Alert yourself to your personal footprint in climate change (i.e. turn out lights when you leave a room, take a bicycle and not the car) in order to minimize the destructive effects on coral reefs as well as many other threatened ecosystemsDispose of your trash properly wherever you are – litter pollution will eventually affect our oceansSupport reef-friendly businesses when you are planning a vacationBuy reef-friendly sunscreens to avoid reducing the water qualityPractice responsible diving and/or snorkeling if you visit coral reefs – “take only photos and leave only bubbles”Read some of the readily accessible information on coral reefs available online, or watch some of the beautiful documentaries about themFinally, there is always the option to directly support some of the many organisations which are tackling the problem of coral reef decline
_______________________________________[su_spoiler title="References" style="simple"]Burke L, Reytar K, Spalding M, Perry A (2011). Reefs at Risk Revisited. World Resources Institute, Washington DC. Available for download: http://www.wri.org/publication/reefs-risk-revisited.Goeij JM De, van Oevelen D, Vermeij MJ, Osinga R, Middelburg JJ, de Goeij AF, Admiraal W. Surviving in a marine desert: the sponge loop retains resources within coral reefs. Science. 2013 Oct 4;342(6154):108-10. Gardner TA, Côté IM, Gill JA, Grant A & Watkinson AR (2003). Long-term region-wide declines in Caribbean corals. Science 301 (5635): 958-960. MacNeil MA, Graham NA, Cinner JE, Wilson SK, Williams ID, Maina J, Newman S, Friedlander AM, Jupiter S, Polunin NV, McClanahan TR (2015) Recovery potential of the world's coral reef fishes. Nature 520 (7547): 341-4. Mumby PJ, Wolff NH, Bozec YM, Chollett I, Halloran P (2014). Operationalizing the resilience of coral reefs in an era of climate change. Conservation Letters 7 (3): 176-187 Wilkinson C (2008). Status of coral reefs of the world: 2008. Global Coral Reef Monitoring Network and Reef and Rainforest Research Center, Townsville, Autsralia, 296 pp.[/su_spoiler]