THE MASCARENE ISLANDS BIOMES AND ECOSYSTEMS Endangered Endemic Fauna and Flora of the Indian Ocean Island of Mauritius Dr. R. Venkatasamy Consultant EnviroSolutions Ltd Mauritius Summary: Oceanic islands ecosystems are unique in that most islands remained uninhabited for a long time, and fauna and flora species evolved undisturbed and very differently from continental species. However, the advent of human settlements and introduction of exotic species quickly caused irreparable damage to fragile island ecosystems, leading to the extinction of many interesting and rare species, as has been the case with endemic fauna and flora species that existed on the Indian Ocean island of Mauritius. The Mascarene Islands and Island Ecosystems The Mascarene group of Islands form a distinct eco region which contains habitats and species that differ from their assigned biome. Originally, these islands were covered with tropical moist broadleaf forests, habitat to a diverse range of fauna and flora. The common transition included coastal wetlands and swamp forests near seacoasts, windward rain forest, leeward lowland dry forest, to palm savannas, montane deciduous forests, and sometimes montane heathlands on high peaks. Figure1: The Mascarene Islands Prior to the coming of settlers, these islands were home to many unique endemic plants and animals, assumed to be originally derived from nearby Madagascar and the East African coastal countries. However, these islands have never been connected to either Madagascar or mainland Africa. Consequently, one different theory is that the flora and fauna of the Mascarenes must have either arrived from over the sea, or used other islands of the original Mascarene Plateau, now disappeared under the sea, as 'stepping stones' to hop from one land mass to another. The uniqueness of the Mascarene islands is in that they form systems in which biological diversity varies as a consequence of remoteness and size, but not necessarily because of environmental factors. On the whole, they are generally simpler systems than conventional continental ecosystems and biomes. Islands may thus provide an opportunity to determine how island biological diversity may affect ecosystem functions. Islands have been known to present two specific paradoxes: • Diversity, and • Stability. 1 Although being highly species-poor, existing species have represented considerable biological interest in terms of unusual endemic genera and taxonomically isolated groups. They have enjoyed stability, as they have persisted for millions of years through whatever adverse or extreme climatic events they may have had to face. However, once exposed to extrinsic (anthropogenic) disturbances, they quickly lose that stability. These apparent paradoxes are resolved when it is realized that all these features are consequences of the same island characteristics: • Biotic isolation, and • Oceanicity. As a result of these two characteristics, island systems are quantitatively and qualitatively different from continental systems in the nature of their ecological processes, which appear to give rise to an interspersed equilibrium model of evolutionary change. Rare and unique island endemics may be either the remnants of ancient continental endemics displaying prolonged stasis and persistence, or products of adaptation and evolution of localised species representing a series of rapid but irreversible events. A process-based definition of a relict endemic (palaeoendemic) is one whose founding lineage (i.e. the original continental source taxon) having left no descendents on the continent of origin, or the continental lineage may have evolved into totally different forms. The time of separation between an island endemic and its continental sister-group should predate the colonization of the island by the now endemic lineage. These island relict endemics are important representatives of remnants of the last segments of biodiversity that have vanished from continental ecosystems. For island conservation strategies to be successful, an integrated understanding of both sides of the diversity and stability paradox needs to be understood and appreciated so that both island processes and island organisms can be conserved. Oceans of the world have isolated volcanic islands, generally formed from hotspot activities underlying the ocean’s crust. The character of the terrestrial biota of these islands cannot be dependant on the ocean, but rather on whichever source continent has ultimately been responsible for providing the colonists. The African continent provides a good example, as it is surrounded by many small islands, including Reunion, Mauritius and Rodrigues, forming part of the Mascarenes. These islands may have sampled the African biota from different geological times, preserved it, and re-invented some through normal processes of evolution. Each island has its particular biota due to age, size, geographical isolation and such other factors as climate and exposure. A number of factors have to be considered when examining island ecosystems, including: • Components responsible for biological diversity on islands and their patterns of variation, • Threats and risks in maintaining such biological diversity, • Intricacies of island biology and consequences of modifications by men over ecosystem functions, • The global necessity for islands biodiversity studies in conservation, and • How we can learn from islands in understanding the processes that induce changes on continental ecosystems. It is known that introduced (exotic) species have better chances of survival on island environments than on mainland areas, and have been shown to have greater negative impacts there, since island ecosystems are known to be fragile, and island species weak. Island species have been termed, wrongly or rightly, “backwaters and deadends,” whereas island ecosystems 2 are seen as presenting less “biotic resistance” to invaders or introduced exotic species than do continental ecosystems. However, genetic weaknesses in island species, if any, are debatable. Island endemic species have evolved in isolation, and it is expected that natural selection would have adapted them better to their environment than invasive or introduced species. But that has not been the case. The percentage of introduced species (not to be confused with invasion) has always been higher on isolated, uninhabited islands than on continental masses,. Many islands have been regarded as ideal environments for the introduction of exotic species, partly to enrich the less dense populations of endemic species, or to provide sources of sustenance for settlers. Though island species are not basically unadaptative weaklings, there exist an aspect of island biota that predisposes them to be particularly susceptible to introduced species, especially predators of the aggressive, competing and scavenging types, since most island environments have lacked such predators. As a consequence, native species evolved without any need to fend off predators, or develop specific defensive or aggressive strategies. The absence of such characteristics, not necessarily their weaknesses, was often the cause for their decimation and rapid extinction. Another factor that predisposes island native communities to be vulnerable lies in the fact that islands have smaller surface areas than continents, and it is unlikely that space occupied by native species would have enough room for introduced species that will compete for food, nesting spaces, and territories. Other factors that have also contributed to the disappearance of native species include hurricanes, fire, and sometimes, changes in climatic conditions. But species lost through these agents must have been few. The dodos of Mauritius must have survived innumerable hurricanes and other factors adverse to life, but only disappeared when human colonisers, together with exotic species and predators, landed on the island. There is ample evidence that species extinction inexorably followed the advent of human colonisation of islands, and that is well documented. The disappearance of the Dodo on the island of Mauritius, and the rapid extinction of the Moa bird in New Zealand are two of the many instances where human colonisation has led to rapid disappearance of fauna and flora on islands. Several reasons have been attributed to this human-induced extinction: • Human colonisers/settlers were more interested in valuable timber species, clearfelling large areas, at the same time destroying habitats of native endemic species. • Settlers on newly colonised islands soon started clearing forests and shrubland for agricultural purposes, further destroying habitats. • Settlers brought with them domestic animals, some of them known predators, and farm animals, and both further engaged into habitat destruction and species reductions. • Settlers introduced new exotic species that may have been useful to them in several ways, but increasing competition with and aggression to native species. • Settlers themselves turned into predators, hunting local species for food, skins, plumage, or for exportation. The Island of Mauritius Location The island of Mauritius lies about 800 kilometres East of Madagascar between longitudes 57 18' and 57 49' East, and latitudes 19 59' and 20 32' South. It is sixty-one kilometres long and forty- six kilometres wide at the extremes and has a total land area of some 1,865 square kilometres. 3 Figure 2: Mauritius - Location Mauritius is believed to be the oldest of the Mascarene Islands