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The Island of Mauritius bears a spectacular native biodiversity. In fact, many Mauritian species are endemic; they are unique to Mauritius. For instance, the native Mauritian wildlife is essentially composed of birds and reptiles. In the absence of terrestrial mammals, they have evolved to fulfill very different roles in the ecosystem promoting a vast diversification. As a consequence, Mauritius encloses the richest diversity of reptiles per area in the world (1). However, Mauritius has a volcanic origin and was never attached to a continent. The erosion of the island transformed a rough and inhabitable island into an environment more suitable for the establishment of life. How then did species arrive and become endemic? In the 15th century, Man arrived and provoked major changes; the Mauritian ecosystem started to decay. The birth and death of Mauritian endemic species can both be explained by Darwin's process of evolution.
Virgin Mauritius was not colonised all at once. Even if microscopic organisms started to live on the island as soon as it emerged, plants and larger animals had to wait for the environment to become more clement. Some species were stronger than others and could colonise the island sooner than others. Those robust species are qualified as "pioneer" species. They trigger the creation of an ecosystem and thus have a very important role. Because of its proximity, Madagascar was a major source of species for Mauritius (3). Plants and animals landed successively and randomly on Mauritius: some were able to colonise, others couldn't and died.
The plants were the first pioneer species of Mauritius. Their seeds were transported to Mauritius by wind, the sea or sometimes by other animals. For example, some tropical birds visited Mauritius and released seeds by excretion or simply by carrying them in their feathers. Some seeds ended up unleashed on Mauritius and were able to grow and gradually establish themselves. The settlement of some plant favoured the colonisation of others, that subsequently did the same for other species and so on. The machine of life was set up. Plants were the first living shapers of Mauritius and initiated the assembly of an ecosystem.
Animals started to colonise Mauritius after the settlement of a consistent flora. Many animals ended up on Mauritius because of sea currents. The formation of the reef barrier gave birth to the Mauritian lagoon where many marine species settled down. At the same time, many reptile species ended up on land and flourished. Equally important, air currents brought many birds that ended up adopting Mauritius as their new home. Several species arrived more or less randomly on Mauritius at different intervals. However, plants or animals had to face the same problem; they had to adapt to new conditions.
Living on Mauritius was challenging for many species. The majority of them were not used to the new physical conditions. For example, plants had to adapt to a different type of soil, in which nutrients are available in unique proportions. Many Mauritian pioneer plants known today are very robust and can grow on a very dry soil. Furthermore, animals had to change their eating habits; the first animals arriving on Mauritius had to adapt their nutrition to the vegetation. Several species also had to change their habitat. For instance, an animal that lived on a certain plant needed to find a new plant to live on. The physical conditions were not necessarily the most challenging ones to face; more specifically, species had to learn how to live together.
Animals and plants had to find their place in the new ecosystem. The species present on Mauritius could interact in many different ways. Firstly, one possible interaction they could have developed is the predator-prey relationship. A species becomes a predator for another and as a consequence both of them have to adapt their behaviours. A new prey requires to develop new means of defence and adequate habits. In this case, an animal not used to being a prey will not intuitively be aware of the danger. In contrast to what most preys do, this animal will not hunt or hide at particular times in order to avoid the predator. To illustrate furthermore, a plant can end up being eaten by an insect never encountered before. The concerned plant may have to develop a toxin. Secondly, some species need an interaction for their well being. Species relying on others for their reproduction or survival have to develop other mechanisms to circumvent the lack. For instance, a plant that relies on a particular animal for the pollination is unable to reproduce in the absence of that animal. Thirdly, Organisms can end up competing for the same niche, which is a specific role in an ecosystem. The freshly arrived organisms on Mauritius were for the most part not used to live in these conditions before. Inevitably, they ended up competing for the same resources. This competition could have concerned food or habitat and certainly created a selection in the favour of the most adapted individuals. This is what Darwin called "Natural Selection".
The Deoxyribo Nucleic Acid (or DNA) constitutes the genetic material of an individual and determine its traits. This genetic material is segmented into parts that are expressed (called genes) and some that are not expressed (junk DNA). Each gene codes for a specific protein that will carry a very specific function in the body of a living organism. Therefore, a change in the DNA may change the organisation of the individual and the way it functions. The diversity among all living creatures is only possible if the DNA mutates. DNA mutations happen constantly and randomly: there is absolutely no purpose for a particular mutation to appear. Random mutations within an individual give birth to new genetic material. This new genetic material can have two opposite discrete consequences. On one hand, it may have no effect on the genes of the individual which will keep the individual's traits or phenotype unchanged. On the other hand, mutations may occur within a gene and therefore give rise to a new version of the gene, called an allele. When expressed, the new allele will cause the development of a new trait. Random mutations in DNA initiates diversity and consequently may strongly affect the survival of an individual.
A new trait can be deleterious or advantageous. An allele is considered deleterious if the trait it codes for provides a disadvantage to the individual. The individual will hence suffer from this trait and its chances of survival will automatically decrease. Effectively, the individual will die and so will the allele. On the contrary, an advantageous allele will provide the individual with an adaptation. An adaptation is a useful trait in a particular context that increases the chances of survival of the adapted individual. For example, adaptations can provide the organism with the ability to digest a compound toxic to the species. The individual will then be able to eat this toxin and perhaps access more food sources compared to others. Both adaptive and deleterious traits participate in the process of evolution by natural selection.
Natural selection directs the evolution of species. Natural selection is defined as the action of the environment on an individual which thereby changes the frequency of particular traits within a population. Depending if arising traits are deleterious or advantageous, the individuals are selected against and die, or have an advantage and survive respectively. In other words, an adaptation drives the evolution of species. An adapted organism lives longer, in better health and thus tends to reproduce more often. Consequently, the adaptive trait is passed on to the next generation. Effectively, the frequency of the adaptive allele increases in the population as the old allele decreases. The effect of this process causes the new allele to replace the old allele in the population. The species is said to have evolved by natural selection.
Endemic species eventually resulted from Natural Selection. After hundreds of thousands of years of evolution, the colonising species became so different from the original species that they were not the same anymore. The genetic gap was so important between them that inter-reproduction became impossible. This process is called speciation which results from natural selection coupled with the creation of a barrier. In the case of Mauritius, the isolation represented a major barrier: it prevented the arrival of too many foreign species. As the new generated genetic material stayed in Mauritius, many species became unique, the only representatives of a specific gene pool: they became endemic. The high endemism in Mauritius is the result of Natural Selection and isolation.
The Dodo bird Raphus cucullatus is a famous example of Mauritian endemic species. Formerly, we should know that DNA studies proved that the Dodo belonged to the Columbidae family, together with pigeons and doves (2). We can then hypothesise that the Dodo's ancestors were similar to the common pigeon. Assuming the preceding statement, we can roughly and hypothetically retrace its emergence as a species. First, pigeons arrived on Mauritius a few hundred thousand or million years ago. Within that population of pigeons, a trait of gigantism appeared. Obviously, Natural Selection favoured this trait. The overall population size gradually increased until it reached the size of the Dodo. Meanwhile, the Dodo lost its capacity to fly and gained the ability to store larger quantities of fat. This substitution was favoured by the environment for two possible reasons: first, the Dodo itself was placed at the top of the food chain and did not fear any predators. Second, food was abundant on the floor, there was thus no need to fly. In addition, the energy invested in fat storage provided the bird with a better adaptation to fight against seasons of drought or low food availability. The way the Dodo evolved was unfortunately a reason why Man killed this species so easily. The example of the Dodo helps us understand how a species evolves and how fragile an ecosystem can be.
Humans extensively shaped Mauritius and caused the extinction of many species. The first to walk on Mauritius were the Portuguese in 1512. The first settlers were the Dutch in 1598, followed by the French (1722) and the British (1810). The colonisation has radically changed Mauritius. The first impact humans had on Mauritius was the direct removal of the resources. The colonists destroyed the forest to settle down, and implemented agriculture. They also exploited the precious woods, that ended up table-shaped in Europe. For example, a huge number of ebony trees (mostly Dyospyros tesselaria) were taken down and sold throughout the world. As another example, the hearts of Latania lodigesii were used to make rum. The second impact was the direct consumption of native animals, such as the Mauritian Giant Tortoise, the Dugong, the Dodo etc... Many native animals were also removed from their habitat to be exported. Removing a species from an ecosystem or destroying habitats had vast effects on the endemic flora and fauna of Mauritius. It caused massive extinctions because many species rely on others for survival. On top of the direct action of Man on Mauritius, introduced species have had tremendous effects on the extant wildlife.
Introduced species can destroy an endemic population. Introduced species may predate endemics and compete against them. In both cases, introduced species generally win and cause many species to be endangered. Many introduced plants grow and reproduce mush faster than endemics: they are invasive. Invasive plants cause the surface of endemic forest to decrease, hence the habitat of many endemic species disappears. Some introduced animals such as rodents can cause much rapid and drastic downfall of the endemic population. Perhaps the most vicious animal accidentally introduced by Man was the common rat, Rattus rattus. Being a mammal, the rat basically stepped over various niches, thus competing against several reptiles for food and habitat. Moreover, the rat ate any bird egg it could find. On top of that, the rat reproduces very often. In this way, many species perished and became endangered because of introduced species (3).
The environmental pressure has radically changed on Mauritius since the arrival of Man. Natural Selection has automatically followed, and the process of evolution re-starts. The most adapted species (most of the time introduced)survive while others, endemics, perish. Only one known endemic species adapted to the new occupants of the island: the Grey white eye (Zosterops mauritianus), a bird that has adapted and is not threatened. Nevertheless, the great majority of endemics cannot cope because the environmental pressure changed too quickly. For instance, species cannot find alternative habitats if theirs is destroyed. They simply do not have enough time to adapt. The endemic forest of Mauritius is reduced to only 1% of its original population (1). Several remarkable species disappeared more or less shortly after the arrival of Man: The Dodo (Raphus cucullatus), the Giant Skink (Didosaurus mauritianus), the Blue Pigeon (Alectroenas nitidissima), The Broad-billed Parrot (Lophopsittacus mauritianus), the Red Rail (Aphanapterix bonasia) just to name a few (3). Each of these species displayed unique characteristics. Thankfully, a good number of species survived, even if they were very close to extinction.
A few organisations intensively struggle to save Mauritian wildlife from extinction. Many rare endemic plants are as much as possible propagated and replanted in order to restore an acceptable population. The Mauritian Kestrel, Falco punctatus, was the rarest bird in the world in the late 1980's (3). Only four specimens remained. The Mauritian Wildlife Foundation was first founded in order to save this species. Admiringly, the scientists succeeded and the species counts nowadays about 700 individuals. Similarly, The Pink Pigeon (Columba mayeri) and the Echo Parakeet (Psittacula eques echo) nearly disappeared were saved from extinction. Conservation measures roughly consist of taking care of the species by removing in a given territory all possible introduced predators. Round Island and Ile aux Aigrettes are islets entirely managed to bear only native and endemic species. Round Island was connected to the mainland and because the sea-level rose, Round Island was preserved from the establishment of invasive species. Many endemic species were saved, such as the Round Island Boa (Casarea dussumieri), many different skinks (Telfair's skink, Buton's skink...) and geckos (Phelsuma). Gratefully, some organisations mobilise many scientist and workers to save the extant Mauritian wildlife.
Plants and animals that colonised Mauritius evolved to become endemic. They displayed unique characteristics maintaining and supporting the remarkable uniqueness of the Mauritian ecosystem. Humans colonised Mauritius and destroyed a fragile equilibrium. Several species became extinct and many endemics have suffered from the human presence. Nowadays, a few organisations struggle to maintain the identity of the Mauritian flora and fauna. The importance of maintaining Nature needs to be understood by everybody. In my opinion, diversity is important because we cannot judge whether a certain species is more or less important than another. Humans do not have the right to choose what should live and what should not. Conserving the biodiversity is essential to keep the planet healthy. If the same species were present everywhere, the machine of life would simply not work. Conversely, the importance of wildlife conservation is unfortunately not understood by everybody and Mauritian wildlife is all the more so a worldwide unique heritage. The public awareness should be increased and children should be educated about the importance of biodiversity as they represent the future of our planet.
(1) Cole, N. (2009). A field guide to the reptiles and amphibians of Mauritius. Mauritian Wildlife Foundation, Mauritius.
(2) Hillary Mayell (February 28, 2002) Extinct Dodo Related to Pigeons, DNA Shows, National Geographic News. Accessed from: http://news.nationalgeographic.com/news/2002/02/0227_0228_dodo.html
(3) Staub, F. (1993). Fauna of Mauritius and associated flora. Precigraph Limited, Mauritius.