Let’s look at an example of the effects these have.
Why are we losing coral reefs?
Coral reefs are home to hundreds of thousands, if not millions, of species
. This makes them some of the most biodiverse places on Earth.
Just like trees in rainforests, corals give reefs their structure. Corals are invertebrate animals that allow algae to live inside them. In return, algae pay their rent by providing corals with food. This is called symbiosis.
Coral reefs directly support around 500 million people across the world in several ways:
- They provide homes for a quarter of all fish species. Although they only produce 2-5% of global fishery catches yearly, fish associated with reefs contribute 25% of the total fish catch in developing countries.
- They protect people from floods and storms by breaking waves.
- They provide income to at least 94 countries by attracting tourism, contributing significantly to many of their economies.
While buffering by the ocean may be good for us on land, corals are taking the hit underwater. When stressed by temperature increases, algae produce molecules that can damage their coral hosts. This causes the corals to get rid of the algae.
As well as providing food, algae are responsible for the corals’ colour. Corals without algae, therefore, turn white and starve. This is called coral bleaching.
Since the 1870s, we have lost around half of the live coral in the ocean! Severe bleaching events, where entire areas of coral are bleached, used to occur once every 25 years. Now they happen every 6 years.
We have already reached 1.0°C warming. Coral reefs are so sensitive that if temperatures reach 1.5°C above pre-industrial levels, it is predicted that 70 to 90% of them will be lost. If we reach 2°C, nearly all coral reefs will be lost.
This is all made worse by an issue known as ocean acidification: when CO₂ reacts chemically with water (H₂O), it results in an increase in the number of hydrogen ions (H⁺) that are found in the water. This makes the ocean more acidic, like a lemon, although not as much. Higher acidity slows the chemical reactions that corals need to build their hard skeletons, making them even more vulnerable.
The challenges that marine species face
Normally, marine species are more widely distributed than land species and can travel further distances more easily. This means that complete extinctions in the ocean are rarer and harder to keep track of. However, there are several ways in which population reductions or movement in species are felt throughout an entire ecosystem.
Let’s look at an example of how the abundance of a species can matter to an ecosystem:
Southern sea otters are very important in kelp forests because they eat sea urchins, thus keeping their population in check. However, sea otters are endangered. When sea otter populations are low, urchins are left to overgraze the kelp, and the kelp forest as a whole eventually disappears.
The consequences of warming on interactions between species
Deeper water, and water close to the poles, is colder. Therefore, many species are moving towards them. Since the 1950s, we have seen the area in which species live move by up to 50 kilometres per decade!
However, some species are less able to move, and the size of the area they can live in has decreased. This is because regions become too warm or experience stronger heatwaves. As we saw in the previous chapter, higher temperatures at earlier times of the year can alter species’ life cycles and food availability.
Changes to where and when species are found in the ocean don’t necessarily happen at the same time or speed for different species. As a result, some species that would normally interact fail to find each other because they look in the wrong place or at the wrong time. This is called mismatching.
Disruptions to species interactions change the arrangement of marine ecosystems. Predator populations shrink: when they mismatch with their prey or when their prey declines they have less food to eat, and the dominant species in the ecosystem can change.
Let’s look at one example of how temperatures affecting food availability for one species can have effects on many other species along a food chain:
What other threats does the ocean face?
The ocean is threatened by many other human activities which combine to make the situation worse.
We continue to use the ocean to dump waste, including sewage, industrial chemicals, fertilizers from agriculture, and around 8 million tonnes of plastic every year.
Still, the biggest threat facing life in the ocean today is exploitation through fisheries. To learn more about this, you can check out our chapter on Fish Farming in the Agriculture course.
Essential Biodiversity Variables are important factors for describing the state of ecosystems, such as whether populations are stable, and aspects like the length and complexity of food chains. Fisheries cause 29% of changes to these variables in marine systems, while climate change accounts for 16% and pollution for 15% of the changes.
Does this mean you shouldn’t worry about the effects of climate change on the ocean?
No, you should worry! As you saw with coral reefs, the impact of climate change is worsening, and as important ecosystems disappear, so does the habitat for many species.
The vastness of the ocean has fascinated us for hundreds of years. Let’s take care of it like it has taken care of us!
Go to quiz!
