An incredible biogeographical journey

By Ifham Nizam

‘The Ecology and Biogeography of Sri Lanka: A Context for Freshwater Fishes’

by Dr. Rohan Pethiyagoda and Hiranya Sudasinghe is a guide to the origins of Sri Lanka’s fauna and flora that strings together different fields of research to shed light on Sri Lanka’s remarkable biodiversity, with a history of over 100 million years.

The book provides a comprehensive context for the island’s plants and animals with a special focus on the ecology and biogeography of freshwater fishes. It contains more than 200 diagrams, photographs and maps including underwater photographs of freshwater fishes, perhaps photographed for the first time in their natural habitat.

It is a source book intended for scientists, students and biodiversity enthusiasts and would help to understand and appreciate the historical and evolutionary context of Sri Lanka’s unique biodiversity.

Although the island’s unique and spectacular biodiversity is celebrated, one rarely stops to think how this incredible diversity arose in Sri Lanka. Remarkably, few scientists seem to have pondered this question. Such studies as there have been, are limited in scope and are scattered across scientific literature.

It was perhaps only a matter of time before Rohan Pethiyagoda, whose name is synonymous with biodiversity exploration in Sri Lanka, lent his mind to this question. Aiding him is Hiranya Sudasinghe, a young biologist who graduated from the University of Peradeniya, presently reading for his PhD in the University of Bern, Switzerland. Together, these authors have put together a definitive text in the form of a richly-illustrated book of some 270 pages. In it, they provide a compelling and comprehensive account of Sri Lanka’s ecology and biogeography.

Few Sri Lankans are aware that our island began its biotic history as a part of the ancient supercontinent of Gondwana, 120 million years ago. Sri Lanka, India and Madagascar broke away from Antarctica and began drifting northwards. The ‘Indian Bloc’ kept on drifting northward until it collided with Asia around 55 million years ago. That gave rise to the Himalayas and the monsoonal weather pattern that still characterises our climate.

Sri Lanka lies within the rainy Intertropical Convergence Zone, and its Southwest quadrant, which we know as the wet zone, has remained perhumid (a wettest type of climate) for about past 30 million years. This means that this region not only receives high rainfall, but also has no dry season. The authors show that this is what facilitated Sri Lanka’s amazing biodiversity: The wet zone is the only perhumid region between Equatorial Africa and Southeast Asia. Its mixed-dipterocarp rainforests contain some of the richest biodiversity on Earth.

As a result, Sri Lanka has ancient evolutionary lineages of animals with Gondwanan origins, such as the familiar spiders, beetles, scorpions, blind snakes and land-snails. The authors illustrate many of these with more than 200 stunning colour photographs. No less remarkable are the plants. They show that dozens of genera of plants that are found in Sri Lanka are wholly absent from India but occur further afield, in Madagascar, the Seychelles, Borneo and even New Guinea. How did this happen? Each case tells a particular story, helping to piece together a puzzle of intricate complexity.

Using tools of molecular biology, Pethiyagoda and Sudasinghe reconstructed the history of their favourite group of fauna: Freshwater fishes. Compared with the Western Ghats of India, Sri Lanka’s freshwater-fish is poor indeed. But the authors use this ‘poverty’ as an opportunity to explore why this is the case. Using DNA-based tools, they reconstruct the ‘molecular phylogeography’ of the fishes, showing how they dispersed through the island. Despite the somewhat technical nature of this particular chapter, it yields some fascinating results which identify, for example, which river basins served as drought refuges during periods of aridification. They show that despite the island’s incredible biotic wealth, it suffered major climate-driven extinctions that decimated its fauna and flora.

The first of these probably took place 65 million years ago, when the impact of a meteor with Earth caused the Cretaceous-Paleogene mass extinction. It was that event that led to the extinction of dinosaurs and resulted in the rise of mammals, including ourselves. By the time of India-Asia contact 55 million years ago, the authors show, very little was left standing in Sri Lanka except for those resilient Gondwanan lineages, such as spiders, snails and possibly a single lineage of frogs that somehow managed to survive. And then, a flood of Asian species swept into Northeastern India and down the peninsula into Sri Lanka. But wait! They couldn’t enter Sri Lanka until sea levels were low enough to give rise to a land bridge between India and Sri Lanka in the vicinity of Adam’s Bridge.

With detailed graphs that reconstruct sea levels over the past several million years, the authors show that Sri Lanka was frequently, if intermittently, connected to India during this time, leading to both immigration and emigration of plant and animal lineages, such as fish, freshwater crabs, amphibians and scorpions. The island’s biotic story then, is one of species evolving prolifically between periodic extinctions. The last of these extinctions occurred only quite recently, probably around 20,000 years ago. Many large mammals, including two species of elephant, two species of rhinoceros, a species of hippopotamus, the tiger, the lion and much else suddenly disappeared. The arrival in Sri Lanka of the charismatic dry-adapted large mammals we cherish today, such as the leopard and sloth bear, was very recent indeed.

Pethiyagoda and Sudasinghe’s ‘Ecology and Biogeography of Sri Lanka’ is nothing short of a masterpiece. It is not just ‘the’ science book of 2021, it ranks among the best science books ever to come out of Sri Lanka, able to stand proudly beside, for example, Professor P. G. Cooray’s ‘Introduction to the Geology of Sri Lanka’ (1984). This is a book that every Sri Lankan nature lover should read in order to understand the context of our island’s biodiversity and just how fragile it is. It is a monumental piece of unadulterated scholarship.