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What is a Keystone Species?

A keystone species is a species which has a disproportionately large effect on its natural environment relative to its abundance.

Every ecosystem has certain species that are critical to the survival of the other species in the system. The keystone species could be a huge predator or an unassuming plant, but without them the ecosystem may not survive.

In any arrangement or community, the “keystone” is considered one of the most vital parts. In a marine ecosystem, or any type of ecosystem, a keystone species is an organism that helps hold the system together. Without its keystone species, ecosystems would look very different. Some ecosystems might not be able to adapt to environmental changes if their keystone species disappeared. That could spell the end of the ecosystem, or it could allow an invasive species to take over and dramatically shift the ecosystem in a new direction.

Our 3 Main Species of Focus

Seychelles Flying Fox

Pteropus Seychellensis

Seychelles Flying Fox (Pteropus Seychellensis)

IUCN Global Conservation Status: Least Concern (revision needed, outdated census)

In Seychelles, Flying Foxes are the only native mammals. As a key stone species, The Flying Fox plays a vital role in the Seychelles ecosystem, as it contributes to seed dispersal and pollination of many endemic, indigenous and fruiting tree species.
They can cross pollinate flowering plants and disperse seeds of various vegetation, amidst a wide range of habitats. Healthy Flying Fox populations are imperative for the conservation of the region’s biodiversity. They are highly susceptible to human induced environmental degradation particularly through the removal of feeding and roosting sites by land clearing for development. They feed on pollen, flowers, seeds, leaves and fruit.

Their wingspan can measure 1m, adult males can weigh up to 800g. It has been recorded that flying foxes can live up to 30 years in captivity and around 10 years in the wild. They reach sexual maturity after 2 years of age.

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They give birth to one pup a year, between November and April, gestation is 6 months long. The mother will carry her pup for the first 6-8 weeks, pups learn to navigate at this stage whilst attached to their mums, maps are created of available feeding and roosting trees. After 8 weeks the pup will then be left at the nursery tree at night which is near the colony tree. Pups are weaned by 6 months of age and become fully dependent at this stage. They form strong close family bonds.
Pups become orphaned in bad weather when they can fall off their mums or the nursery tree. Slightly older pups who are first time fliers can also get lost or injured and require rescue.

Flying Foxes do not echolocate but have excellent vision. They have large eyes that are placed forward on their face. The size and placement of their eyes gives them binocular vision. They also have an excellent sense of smell. They are highly intelligent; scientists have found that they have a high “encephalization quotient”. Meaning that their brains are very large relative to their body size. Their intelligence level can be compared to that of domestic dogs.

Threats

Hunting for consumption

Habitat loss due to rapid development and human encroachment. Removal of roost, nursery, and bat colony trees.

Climate Change, which affects rainfall and flower, pollen, fruit production and heat stress.

Collision with power lines.

Inappropriate fruit tree netting

By Tamara Dreyer

Aldabra Giant Tortoise

Aldabrachelys gigantea

Aldabra Giant Tortoise (Aldabrachelys gigantea)

IUCN Global Conservation Status: Vulnerable

The Aldabra Giant Tortoise is a species unique to Seychelles. It can adapt to a wide range of habitat types. People associate it with the desert-like conditions of Aldabra, and its skull is specially structured to enable it to drink through its nose (like a drinking straw). This means it can take advantage of tiny pools of water in rock crevices. But in fact, given a choice, Aldabra Giant tortoises very much enjoy having access to fresh-water pools and marshes in which they love to soak themselves. Aldabra tortoises are grazers that feed primarily on vegetation found at ground level–especially grass, leaves, fruit, and flowers. But they will supplement their diet with meat, given the opportunity. Giant tortoises will strip the meat off carcasses of dead animals at Aldabra; and their ability to kill and eat small animals and birds has been documented.

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At Aldabra, giant tortoises can take 20-25 years to reach adulthood.  Adult males are larger than adult females and can be distinguished by their large size, long tail, and concave plastron (lower shell). Adult females and immature males and females are all characterized by a short tail and a flat plastron. Females can lay egg clutches ranging from 5-30 eggs depending on the conditions where they live. At drier locations (such as parts of Aldabra) adult females remain small and lay fewer eggs per clutch; but when food and water are abundant the females grow larger and can lay more eggs.

Giant tortoises perform many critical roles in shaping a healthy ecosystem, such as dispersing seeds of native plants. At Aldabra giant tortoises play the role that would normally be expected of large grazing mammals on the African continent. This is one of the features that makes Aldabra unique. Tortoises and their habitats have evolved to accommodate each other on Aldabra. For example, “Tortoise turf” is an assortment of some 20 species of plants that have co-evolved with tortoises in such a manner that their fruiting bodies remain close to the ground and are not destroyed by heavy tortoise grazing. This in turn also benefits the tortoise population by ensuring it doesn’t destroy its food source.  Aldabra Giant Tortoises in Seychelles have been documented to live more than 150 years.

Once there were at least nine species of giant tortoises inhabiting the islands of the Western Indian Ocean. Due to over-exploitation by people and destruction caused by animals brought to the islands by humans (i.e., cats, rats, pigs, dogs, etc.) they all went extinct except the tortoise population living on Aldabra.  Genetic data indicate that all the tortoises that now occur in Seychelles are derived from Aldabran ancestors.

Threats

Illegal pet trade and trafficking

Climate change

Being kept as pets in unsuitable and small enclosures

Habitat loss

Attacks from domesticated animals

By Dr Jeanne Mortimer

Hawksbill Turtle

Eretmochelys Imbricata

Hawksbill Sea Turtle (Eretmochelys imbricata)

IUCN Global Conservation Status: Critically Endangered

Legal conservation status in the Seychelles: Legally protected since 1994

Hawksbills are, compared with the other six species of sea turtles, a medium-sized turtle species, and the species which is the most linked to coral reef ecosystems. Fully grown they can reach a length of around 1m and a weight of 80 kg. Their name is derived from their sharp beak-like mouth which is looks quite like a hawk’s beak.

Compared to loggerheads, hawksbills have a slender head, and the sharp beak distinguishes them very well from other sea turtle species. The scutes on their oval, elongated carapaces are overlapping at the rear end, giving this part of the body a serrated look.

Each hawksbill individual can be identified by its facial pattern, the pattern of the scales on each side of the head is unique for an individual, no other hawksbill shows the same facial pattern.

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Seychelles is a habitat hotspot for Hawksbills and many breed and nest on our beaches. They prefer warm water, thus are primarily found in the tropics.

A hawksbill’s diet consists of 70 – 95% sponges, the rest are macroalgae, cnidarians (which are also toxic!), jellyfish, comb jellies, mollusks, crustaceans and even fish. Hawksbills are omnivorous!

Only very few animals eat sponges because they are toxic, some of them even lethally toxic, somehow hawksbills were able to adapt to this during their evolution. But they only feed on selected species of sponges while ignoring other sponges. Life cycle: Little is known about it. Like all sea turtle’s hawksbills hatch from eggs that their mother lays into a nest on a beach. After hatching they run down the beach to the sea and disappear there for years before they reappear on coral reefs or other foraging grounds as juveniles. It is assumed that hawksbills spend their first months and years out in the open ocean, maybe around drifting seaweed, or drifting wood like it was observed for other species of sea turtles, but there’s no scientific proof for this theory until now.

Major hawksbill nesting sites are located on some larger Caribbean islands, in Guyana’s, in the Red Sea, in the Seychelles and Maldives, in Indonesia, and around northern Australia and southern New Guinea. Strangely, major nesting sites seem to be missing in the whole central and eastern Pacific where this species also occurs. It takes 20 to 30 years for hawksbills to reach sexual maturity, their active reproductive phase lasts for ten years. Every 1 to 5 years, female hawksbill turtles return to nest on beaches in the general areas where they hatched decades earlier. Hawksbills generally lay three to five nests per season, which each contain an average of 130 to 160 eggs. The eggs incubate for around 2 months.

These facts combined with the very low natural survival rate of the hatchlings make it easy to understand why hawksbill sea turtle populations are highly sensitive to any threats of human origin. The maximum age of hawksbills is unknown, generally estimated to be around 60 to 65 years for sea turtles.

Another fun fact about hawksbills: Although they are seen at cleaning stations in reefs, like other species of turtles, their carapaces still manage that their carapaces look ‘dirty’. Lots of different algae grow on them, barnacles and other sessile marine fauna like to settle on them. Maybe they don’t visit cleaning stations often enough? Green turtles seem to be far more successful with keeping their carapaces clean.

And how do hawksbills sleep? Underwater, not on the surface as they can hold their breath for up to 5 – 7 hours. They like to hide in caves on the reef for sleeping. When they are tired and find it difficult to find a cave which is big enough, they simply stick their head into a small cave and let their body, stick out, since their bodies are protected by their shell.

Threats

Illegal poaching

Boat strikes

Dirty living environment from boat cleaning chemicals and pesticide toxic run off from towns

Plastics, and micro plastics

Climate change, beach erosion, rock armoring on nesting beaches

By catch

Ghost gear and FAD entanglement

Interference from humans and domesticated animals while nesting or nests are hatching

By Thorsten Albrecht, Marine Life Protectors