The Five Threats
How does habitat loss affect sharks and rays?
We know that numbers of sharks and rays are declining for various reasons. Some are direct threats where sharks are killed by people – as through fishing – and others are indirect, like habitat loss. When an area of ocean or coastline is changed or degraded beyond the point that it can serve as a home for the species that live there, or is even destroyed entirely, that’s habitat loss.
How habitat loss threatens sharks & rays
There are various ways that the habitats used by sharks are altered or destroyed. Again, some are direct, like cutting down mangroves or trawling fishing nets over reefs, and others are indirect, such as pollution, which might affect water quality to the extent that the water becomes uninhabitable. Coastal development has accelerated over time as the human population grows and more and more people move to the coast, and it has been the driver of a lot of degradation in marine areas. Mining and aquaculture are other culprits.
The areas that are most accessible to people are the ones most degraded, so broadly in descending order, the most affected are terrestrial areas, freshwater habitats, estuaries, coastal areas, the inshore region and the open ocean. This means that sharks that live in fresh or brackish waters are likely to be the most affected by habitat degradation. The Ganges shark Glyphis gangeticus, for example, has almost completely disappeared from its small range in the lower reaches of the Ganges–Hooghli river system in India due to the degradation of its habitat.
Many shark species use coastal and estuarine areas as safe places away from predators to feed, give birth, mate and grow. The sharks that live in coastal areas for all or part of their lives are likely to be more affected by habitat loss than those that live in the open ocean. The lemon shark is one example of a shark that moves inshore to give birth. But scientists have found that dredging in The Bahamas for new coastal development has negatively affected the survival of young lemon sharks there. Coral reefs and mangroves are also important for various shark and ray species and both of these habitats have been in decline in terms of their extent and health.
Through research projects, such as those funded by the Save Our Seas Foundation, we can understand the effects of habitat loss on sharks and rays. What activities have the greatest and the least impact? Which species face the biggest threat from habitat loss? Where are sharks and rays affected (or unaffected) by habitat loss? The answers to these questions can guide the people who manage ocean resources, marine environments and the activities that impact on marine habitats by helping them to make decisions that protect habitats and offer sharks – and other marine life – the best chance of survival.
Frequently asked questions about habitat loss
What is habitat loss?
A habitat is a place that has the right conditions and resources for a particular species – or group of species – to survive. This means everything that a species needs to find or make food, reproduce successfully and protect itself from the outside world. These factors can vary hugely from species to species; even different sharks can have very distinct requirements that need to be fulfilled by their habitat. The world’s oceans comprise more than 90% of the habitable space on the planet. Examples of marine habitats include coral reefs, sea-grass beds, the open ocean (pelagic) and even hydrothermal vents in the deep sea.
Habitat loss refers to situations where a habitat has been altered so significantly that it no longer meets the needs of the species that live within it. There are three main types of habitat loss:
Habitat destruction: the habitat is damaged to such an extent that it cannot support the species that naturally occur there. In other words, the habitat is completely destroyed. Destruction typically results in extinction, either of local populations or, in more extreme cases, entire species. UNESCO estimates that 30–35% of critical marine habitats (e.g. coral reefs, mangroves) have been destroyed.
Habitat degradation: degraded habitats are still intact but lack quality and integrity. Species may find it harder to thrive in degraded habitats and are either forced to leave (displaced) or risk extinction. An estimated 60% of the world’s major marine ecosystems are said to be degraded.
Habitat fragmentation: larger areas of habitat are split into smaller and smaller ‘fragments’ that are separate from one another. This places species at greater risk of extinction for a number of reasons. Populations are forced to compete for fewer resources and their range and movement are restricted, which makes it harder for them to find food and a mate.
According to the Convention on Biological Diversity (CBD), habitat loss (including degradation and fragmentation) is the leading cause of global biodiversity loss.
What are the causes of habitat loss in the oceans?
The causes of habitat loss in the marine environment can be natural processes, including storms, tsunamis and typhoons. These events can be devastating, but tend to occur relatively infrequently. Unfortunately, humans are to blame for the majority of marine habitat loss we are experiencing today. A number of our activities contribute to significant habitat destruction, degradation and fragmentation:
- Climate change, accelerated by human activity, is altering marine habitats at an alarming rate. The warming of the oceans is making some habitats inhospitable to species that once thrived within them. For example, when corals are stressed by changes in temperature or nutrient level, they can expel the algae (zooxanthellae) that live inside them. These algae are the coral’s primary food source and are responsible for their colours – without them, the coral turns white. This is known as ‘coral bleaching’. Although not directly fatal, bleaching makes corals more susceptible to disease and thus more likely to die. They can also starve, due to losing their primary source of energy. This affects all species living within reef habitats. Another problem is the increased frequency of extreme climate events, which are severely impacting the communities that form coastal marine habitats.
- Waste, chemicals and other pollutants from industry, urban development and agriculture can enter marine systems in a process known as ‘run-off’. Pollutants are either emptied directly into the sea – via a sewage pipe, for example – or are carried into the nearest body of water by rainfall or irrigation systems. Important coastal marine habitats, like coral reefs, are increasingly threatened by terrestrial run-off. One particular problem linked to run-off is eutrophication, which occurs when too many nutrients enter the marine environment, leading to excessive plant and algal growth. Nutrients like nitrogen and phosphorus occur naturally in marine ecosystems and are responsible for life in the oceans, but when more is added through run-off – from man-made fertilisers and sewage, for example – it tips the system out of balance. Marine plants and algae explode in numbers, forming enormous ‘blooms’ and limiting the amount of dissolved oxygen in the water as they die and decompose.
Deoxygenation (low levels of oxygen)
- Ocean deoxygenation is a less talked-about side-effect of human-induced climate change. It occurs when the amount of available oxygen is reduced, making the water ‘hypoxic’ and less habitable for marine species. This can be caused by eutrophication, the warming of our oceans and increased stratification. The latter describes a process where water masses with different properties are separated into layers, with warmer, lighter, less salty water on top of colder, saltier and therefore heavier water. Naturally, some mixing occurs between these layers and oxygen is distributed throughout the ocean. But as the surface waters get warmer and lose more salt due to freshwater input from the global ice melting (both due to human-induced climate change), the difference between these layers grows. Mixing can’t occur as easily, meaning less oxygen reaches the cooler waters below.
- Scientists have named these low-oxygen waters ‘dead zones’ because they cannot support the majority of marine life. There are now almost 500 dead zones around the world.
Destructive fishing and industrial activities
- Some intensive fishing methods are highly destructive. Bottom trawling, which involves a large net being dragged along the sea floor, and techniques that use explosives instantly destroy entire habitats. Another damaging activity is dredging, where sand, silt and other materials from the sea floor are removed and deposited elsewhere. This is a bit like taking an enormous plough to the seabed and causes significant and irreversible damage to sea-floor habitats and the communities that depend on them.
- The expansion of marine-based tourism and recreation, although great for coastal economies, can contribute to habitat loss. Snorkellers, boaters, swimmers and scuba divers can inadvertently damage fragile marine life, like reef systems. Even sunscreen can contain chemicals that have a detrimental effect on corals. Scientists have documented significant damage to tropical reefs after a busy summer of sea-dwelling tourists.
What are the effects of habitat loss on sharks and rays?
Like all species, sharks and rays depend on healthy habitats to survive, reproduce and find food. Habitat loss therefore has severe consequences for shark and ray populations around the world. Coastal habitats like mangroves, kelp forests and reef systems are important nursery and feeding grounds for many shark and ray species, providing essential shelter for their young and supporting a diversity of prey species. However, the close proximity of these habitats to human settlements makes them vulnerable to destruction and degradation, meaning that sharks and rays are losing areas that are central to their life cycle.
What can be done about marine habitat loss?
Terrestrial habitats we can see being destroyed in real time – the Amazon rainforest being cut down, for example. But extensive habitat loss beneath the ocean’s surface is hidden from us – and we still don’t know the full effects. In recent years there has been a big push from international organisations to expand ocean science and better understand our impacts on the marine environment, and how we can manage it more sustainably. The United Nations has declared the next decade (2021 –2030) the decade of Ocean Science for Sustainable Development, in recognition of the severe decline in ocean health and the impact on the natural world and human lives and livelihoods. The Save Our Seas Foundation regularly funds projects that examine critical habitats for sharks and the effects of habitat loss and degradation on certain species. One example is a project led by Lauran Brewster, which studies the effect of harmful algal blooms on bull sharks in an important nursery area in the Indian River Lagoon, Florida. The more we know about the extent of marine habitat loss and its effects on sharks and rays, the better we can inform policy-makers about the types and level of protection needed for critical habitats.
Unless drastic action is taken on a worldwide scale, we will continue to see the adverse consequences of habitat loss on our marine life and productivity. This means that world leaders and governments must commit to slowing climate change, reducing marine pollution and limiting destructive fishing and dredging activities, as well as putting resources into the restoration of important marine habitats.
Convention on Biological Diversity, Definitions
Airoldi, L., Balata, D. and Beck, M.W., 2008. The gray zone: relationships between habitat loss and marine diversity and their applications in conservation. Journal of Experimental Marine Biology and Ecology, 366(1-2), pp.8–15.
United Nations Ocean Conference 2020, Facts and Figures
Convention on Biological Diversity, Targets
National Geographic, Encyclopedia, runoff
Fabricius, K.E., 2005. Effects of terrestrial runoff on the ecology of corals and coral reefs: review and synthesis. Marine Pollution Bulletin, 50(2), pp.125–146.
National Ocean Service, NOAA, Facts, eutrofication
IUCN Issues briefs, Ocean deoxigenation
National Ocean Service, NOAA, Facts, deadzone
Ospar Commission, Assessments, Dredging
Marine Safe, Sunscreen pollution
Worm, B. and Lotze, H.K., 2021. Marine biodiversity and climate change. In Climate Change (pp. 445–464). Elsevier.
National Ocean Service, NOAA, Facts, coral bleaching
Save Our Seas Foundation, Of algal blooms and bull sharks
Russel C. Babcock. et al, 2019, Severe continental-scale impacts of climate change are happening now: Extreme climate events impact marine habitat forming communities along 45% of Australia’s coast Frontiers in Marine Science.