Biodiversity in Madagascar’s Thorn Forest

A very special island

2 DECEMBER 2016,
Baobab (Adansonia grandidieri) © Louise Jasper
Baobab (Adansonia grandidieri) © Louise Jasper

Madagascar is the fourth-largest island in the world with a land area of 593,000 square kilometres. Mainland Britain with a land area of 209,000 square kilometres is the largest island in Europe and the ninth-largest in the world. Britain has a third of the land area of Madagascar but could not be more different. For one, Madagascar is in the tropics. As one gets closer to the equator, the number of species increases dramatically and Madagascar has several times more species than a simple multiple of the ratios of the area of land. This is because the ratio of species per unit area of land does not remain constant across a latitudinal gradient.

When a more meaningful comparison is made between Madagascar and a similar sized island such as Borneo in the tropics, Madagascar throws up a few surprises. This is because the land area and the latitudinal location alone do not explain how species rich a land would be. It also depends on its origins, geological history, climate and topographical complexity and period of isolation. Isolation is where Madagascar comes into its own. The level of endemicity (species found nowhere else in the world) is phenomenal.

Madagascar is not alone in having high rates of endemism. The Galapagos Islands are perhaps the most famous example of species evolving in isolation. With the Galapagos, the origins of its diversity are simple to understand. These are volcanic islands which originated from molten rock from beneath the earth’ s surface. They began with no life at all. Everything on it came from somewhere else. Because of the remoteness of these islands, there was little or no exchange between plants and animals that arrived on these islands and from their sources. As a result the lucky colonisers who survived ‘rafting’ across the ocean founded coloniser populations which over time, evolved into new species and at times radiated into other new species to fill vacant ecological niches. This is known as dispersal.

The high rates of endemism in Madagascar fascinate zoologists and there is much debate on vicariance versus dispersal. Vicariance refer to the process by which species present on an original land mass, have evolved over time into new species. In the case of Madagascar the original land mass is the ancient southern super-continent of Gondwana which at one time held Madagascar, Africa, Antarctica, Australia and India in its grasp. Clearly many of the species in Madagascar have their lineages in Gondwana. However, molecular phylogenetic studies are beginning to show that many species in Madagascar have originated from species that came back to it after the split of Gondwana.

It is easy to imagine Madagascar as a homogenous, lush tropical island fringed with sandy beaches and an interior of tropical rainforest. This is actually not the case and Madagascar has surprisingly different climatic and vegetative regions. Much of the island comprising its eastern half is dry deciduous forest with the western coast having a broad strip of evergreen rainforest. In between them are the central highlands with a small area of Sambrino rainforest in the North and pockets of montane ericoid forest within the central highlands. The southern tip is dry, with arid spiny bush. Whilst Borneo and New Guinea, two other large tropical islands are epitomised by the animals from rainforests, Madagascar’s signature species in the popular media comes from the spiny forest. This is the Ring-tailed Lemur; one of ten species of lemurs found in the spiny forest from a total of 103 species making up over half of the 170 species of non-flying mammals.

The great continent of Africa is 400km away and is separated by the deep Mozambique Channel. What is extraordinary about Madagascar’s mammals is that all 170 species of non-flying mammals are endemic: that is found nowhere else in the world. Madagascar separated from Africa before mammals evolved. As a result, all of its 170 species of non-flying mammals have evolved from subsequent colonisations. Each of the four groups: the tenrecs, lemurs, carnivorans and rodents have all evolved from single colonisation events.

Madagascar has been the subject of a few books, one of the best-selling being Gerald Durrell’s ‘The Aye-Aye and I’; an account of his expedition to collect animals for his zoo. This book was written with his characteristic humour. A recent book published by John Beaufoy Publishing is ‘Life Amongst the Thorns: Biodiversity and Conservation of Madagascar’s Spiny Forest’. It is written by Louise Jasper and Dr. Charlie Gardner, two naturalists who have been living and working in the spiny forest since 2006. Jasper is also a conservation photographer. They have produced a remarkable book which is extremely rich in content and provides a very detailed insight into the richness of the plants and animals in the spiny forest with comparisons across Madagascar as a whole. Part 1 of the book is a series of biodiversity chapters that focus on the flora, mammals, birds and reptiles. Part 2 of the book focuses on conservation and has chapters on history, people, conservation and protected areas. It is a large format book with stunning photography. Great design and imagery are complemented with richly detailed text.

We learn that in the birds, despite 39 genera, one subfamily, six families and three orders being endemic to Madagascar, it is relatively impoverished for birds for a tropical island of its size. The reason is not clear. Nevertheless, 37 per cent of the 280 regularly occurring birds are endemic; an impressive proportion. With plants, a staggering 14,000 species are estimated to exist with 82 percent endemic. Compare this with Britain which has around 1,300 species of native plants. Madagascar has around 400 species of reptiles. In one of my articles on Sri Lanka being super-rich for wildlife, I have drawn attention to how this island which is a tenth of the size of Madagascar, has 350 species. A key difference, one that makes Madagascar very special is that 92 per cent of Madagascar’s reptiles are endemic. With amphibians, Madagascar does even better with 99 per cent endemism of the 300 frogs described with the final number estimated at possibly 450 species of frogs. With the emphasis in the wetter parts of the island, only 14 species of frogs have been recorded in the spiny forest. Nevertheless, the comparison with Madagascar as a whole, makes the book by Jasper and Gardner one that provides a useful stock take on the whole island and not just the spiny forest.

One of the most inspiring books in my collection is the ‘Natural History of Madagascar’ edited by Steven M. Goodman and Jonathan P. Benstead. This large format book runs into just over 1,700 pages and is the collective effort of a large number of scientists producing one of the most impressive modern syntheses of the natural history of a country. Unfortunately books like this are not terribly accessible to the layman. Although more limited in scope, ‘Life Amongst the Thorns’ by Jasper and Gardener fills the need to provide a beautifully illustrated and informative account of the rich biodiversity of Madagascar, to a general readership.

References
Goodman, S. M., & Benstead, J.P. (Eds). (2003). The Natural History of Madagascar. University of Chicago Press. Pages 1709.
Jasper, L., Gardner, C. (2015). Life Amongst the Thorns: Biodiversity & Conservation of Madagascar’s Spiny Forest. John Beaufoy Publishing: UK. Pages 318.
Yoder, A. D. Yoder and Nowak, M. D. (2006). Has Vicariance or Dispersal been the Predominant Biogeographic Force in Madagascar? Only Time Will Tell. Annu. Rev. Ecol. Evol. Syst. 2006. 37:405–31