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5 keystone species crucial to balance of the ecosystem


The reason sea otters are a key species is that if they were to die out, sea urchins would boom in numbers

The reason sea otters are a key species is that if they were to die out, sea urchins would boom in numbers

As we travel the world watching wildlife, we often seek out the largest and most impressive creatures – lions and tigers, whales and sharks, bears and elephants, writes Stephen Moss. But these are not just the most noticeable species, but may also be the most crucial to the survival of the whole ecosystem where they live.

These animals are known as ‘keystone species’: a term that comes from architecture, and refers to the stone at the very top of an archway that, if it is taken away, will cause the whole structure to collapse. Keystone species play the same critical role in he natural world: if they disappear, this risks setting off a domino effect of local and global extinctions, as their very presence is what keeps the ecosystem where they live in balance.

Sadly, in the past century or so we have seen the decline of many of these crucial creatures. And where the keystone species has been removed, we often see an imbalance in the remaining wildlife. For example the loss of wolves in the Scottish Highlands means that deer no longer have any predators to keep their numbers in check, and are now causing all sorts of problems because of overpopulation. 

Conservationists are now working hard to secure the future of keystone species, in the knowledge that money spent on saving tigers or elephants also benefits a host of other wild creatures, and indeed the entire habitat where they all live.

We can play our part too: by travelling to see these animals, we alert local people to their importance, and bring much needed tourist revenues that can then be used in conservation projects.


The sight of this magnificent beast – the largest and heaviest of all the big cats – is always the highlight of a trip to India. And yet encounters are becoming harder and harder to achieve: tiger numbers have plummeted in the last century or so due to hunting, habitat loss and unlawful killing – with a population of more than one billion people, India simply doesn’t have the room for tigers to co-exist with human beings.

Today the world population of tigers may be as low as 3200 individuals, of which about half are in India, making this the most important population from a global point of view. This is down from about 100,000 at the turn of the twentieth century: a devastating loss.

Tigers are what are known as ‘apex predators’: at the very top of the food chain. So if they disappear this upsets the delicate balance of the forest ecosystems where they live, allowing herbivorous animals such as deer to increase in numbers, which in turn leads to habitat destruction.

You can still see tigers at special reserves in India such as Corbett and Ranthambore National Parks, and various reserves in Madyha Pradesh to the south of Agra. But be prepared to make several game drives to increase your chance of connecting with this spectacular animal.


 These delightful and endearing creatures are a different species from our own European otter – larger and greyer in colour – and are found along the Pacific west coast of South America from Canada in the north to California in the south, and also in eastern Russia. They feed on sea urchins, which they pick up from the ocean floor by diving, and then using a stone or rock break them open on their chest.

The reason sea otters are so important is that if they were to die out, the sea urchins would boom in numbers, and because they feed on kelp, the underwater forests of this giant seaweed, home to a wide range of marine species, would start to die back, reducing the biodiversity of the whole coastline. Other species that depend on kelp for food, such as crabs and abalones, would also suffer if sea urchin numbers were to increase.

Once very common, sea otters declined dramatically during the twentieth century until only about 1000-2000 individuals were left. Thanks to careful conservation measures, their numbers have bounced back, and they are now fairly easy to see. One hotspot is off the coast of Monterey in northern California, or in the creeks and estuaries nearby, where special boat trips often allow a very close approach.


Although many keystone species are top predators, others are herbivores rather than carnivores. These maintain the ecosystem not through preying on other animals but by creating new opportunities through changing the habitat.

Beavers – both the North American and European species – both play this vital role. North American beavers famously make dams by cutting down huge trees with their sharp teeth and then using the logs to hold back the water of rivers and other waterways. This can lead to problems with flooding, but also creates new habitats by changing the flow of the water.

European Beavers, although slightly larger and heavier than their American cousins, are far less destructive. They too cut down trees and saplings, but do not make the dams; instead they cordon off smaller areas of water on the edge of rivers and lakes. This has massive benefits for a wide range of creatures, including amphibians such as frogs and newts, and insects including dragonflies and damselflies, which lay their eggs in the pools created by the beavers. Many woodland species, including birds, bats and butterflies, also benefit from the thinning out of the trees, which allows sunlight to reach the forest floor.

Having declined dramatically, beavers have been reintroduced to many European countries, including Scotland, where despite opposition from farmers, landowners and anglers the species has been shown to have many beneficial effects. The River Tay holds the largest population, but beavers can be very tricky to see; so patience if vital. There are also semi-captive beavers at experimental sites at Aigas Field Centre in Scotland and in various places in England.


As the third tallest and second heaviest bird in the world, after the ostrich and emu, the cassowary is a pretty formidable creature: two metres tall, and able to kill a human being with one kick of its powerful legs with their razor sharp claws. But it is also a crucial species for the coastal Queensland rainforest in its native Australia; because of the way it enables certain plants to complete their complex lifecycle.

Despite their huge size, cassowaries live mainly on fruit, though they are opportunist feeders and will take a range of other plant and animal food including rats, frogs, flowers, fungi and even carrion. But their place as a keystone species is because not only do they distribute the seeds of the fruits they eat over the forest floor, but their dietary processes also enable some seeds to germinate – without the cassowaries, the seeds might never be able to do so. The bird’s dung also acts as a convenient ready-made fertiliser, enabling the seeds it contains to grow and flourish.

Cassowaries can be seen at special reserves and lodges in northern Queensland, but with only about 2500 individuals remaining they can be surprisingly hard to find for such a huge creature. They are suffering from habitat loss but are also frequently run over by cars, putting pressure on an already small and declining population.


Some creatures are ‘keystone species’ in another crucial sense: in that their presence in or absence from a particular location has a major cultural and economic importance, because they attract visitors to see them. One such is Europe’s tallest bird, the common crane.

Cranes are elegant waterbirds, found across a wide range of Northern and Eastern Europe, Asia and the Middle East. Like all waterbirds, they suffered major declines during the past few centuries, but have bounced back with the protection of wetlands, and the creation of new ones.

In Britain cranes went extinct as breeding birds almost 500 years ago, due to persecution and the draining of their watery homes. A small population returned to breed in Norfolk in the late 1970s, and since then the species has been reintroduced onto the Somerset Levels, where more than 100 birds have been released.

Cranes can be seen around the West Sedgemoor area south of Glastonbury; but to enjoy the true spectacle of this magnificent waterbird you need to head north to Lake Hornborga in southern Sweden, where in spring hundred of cranes perform their spectacular courtship dance; or south-east to the Hula Valley in northern Israel, where up to 20,000 cranes spend the winter. In all these places, cranes have helped conservationists flag up the importance of the wetland habitat, and so preserve it for other wildlife.