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  • Devayani Khare

Island Biogeography

When we talk of Charles Darwin and Alfred Russell Wallace's tenets of natural selection, we still consider it to be a theory. Yet there is so much empirical evidence that supports how geographic isolation, inherited variances, survival of the fittest, time and female choice (an idea that Darwin briefly mentioned yet realized was of profound importance) can lead to the diversification of species.

Islands, with their unique geographical conditions of isolation, helped finetune the theory - first, by Alfred Wallace Russell, a co-author and often forgotten proponent of the natural selection theory, later, by other ecologists like Joseph Hooker, Robert McArthur, and E.O Wilson.


It starts with a volcano - out in the middle of the ocean. Volcanoes often occur at the boundary or ridge of tectonic plates, and a volcanic ridge is connected to a nearby mainland. Over time, eruptions build up to form little islands that emerge above the surface of the ocean. Krakatau and Rakata volcanoes are connected to Indonesia, and the Hunga Tonga-Hunga Ha'apai volcano extends from New Zealand in a north-northeast direction towards Fiji.


When volcanoes first emerge as oceanic islands, they provide virgin land for the colonization of species. The pathways by which species arrive onto islands are fascinating: via rafts of floating vegetation or buoyant pumice, trapped in the muck on the feet of birds, carried in the bowels of birds or bats, blown astray by extreme weather, or by swimming across large tracts of water (the species need to be strong swimmers to overcome the ocean swell). Through geologic time, some islands were also connected to other landmasses when sea levels dipped, and allowed species to travel more easily over the isthmus, or strip of land.


Oceanic islands may also formed by corals - over time, limestone laid down by coral remains build up and emerge as atolls above the water surface, or might be elevated by volcanic or tectonic activity. Guam in the Micronesia archipelago, a territory of USA, is part limestone and part lava.


Continental islands lie on continental shelves, and are surrounded by shallow water, and life often finds its way across more easily. Oceanic islands, however, are remote and start their existence devoid of terrestrial life. Every terrestrial species of plant or animal on an oceanic island, arrived there by cross-water dispersal after the island was formed, or descended from castaway or stowed away ancestors.


Bali and Madagascar are continental islands that were cast adrift with a teeming community of terrestrial species, whereas Galápagos (Ecuador), Hawaii (USA), Mauritius (independent republic), Reunion (France), Surtsey (Iceland) are volcanic, oceanic islands, where life began in isolation.


BIOGEOGRAPHY

Biogeography is the study of facts and patterns of species distribution through geographic space and geologic time. It asks questions like which species occur where, and why do they occur where they do. And sometimes, just as importantly, it questions why not? Islands have helped us understand the laws of biogeography, which in turn, allow us to hazard at the processes of evolution.


Explainer: Biogeography - Studying the Distribution of Species Across Space

By Dr Uma Ramakrishnan, National Centre for Biological Science

For iBiology * Note: The Krakatau island off the coast of Indonesia, is among the best-known examples of island biogeography. The eruption of 1883 obliterated the biodiversity on the island, and since then, new biodiversity has been established - all within recent scientific memory. The other islands in the Malay archipelago, thanks largely to Alfred Wallace's work, are also great examples of endemism and speciation. You are sure to find a lot of articles online covering these geographies.


ISLANDS & SPECIES RICHNESS

As species make their way to islands, they compete, interact, establish ecological relationships, and an ecosystem is forged. We can draw parallels between such processes on the mainland, and in other ecosystems too. Hence understanding how species populated and diversified on islands is key to biogeography. Species richness on island ecosystems depends on 3 parameters: Old versus new islands: older islands, where there has been more time for species to interact, compete, and diversify, have more species of plants and animals than newer islands. Older islands would have had more waves of 'colonizers' too, than newer ones. Galápagos mapped as early as 1535 harbours more species than Surtsey off Iceland's coast that erupted in 1963. Smaller versus larger islands: size plays an important role in species richness, with smaller islands offering fewer niches and opportunities than larger islands. Bali, for instance, has far fewer species than Madagascar. Continental versus oceanic islands: as mentioned before, continental islands start with a thriving community, unlike barren oceanic islands. The connectedness of continental islands versus the remoteness of oceanic ones affects the species richness. These 3 dichotomies result in incredible island biodiversity, with quirks like specialization (think of Darwin's finches), gigantism, dwarfism, endemism (species that are restricted to one or a few geographic locations only), and often, extinction.


ISOLATION, ENDEMISM & EXTINCTION

Islands get colonized via dispersed, cast away or stowed away species. Yet that is only the first crucial step, establishment is the second. New arrivals have to find food, protect themselves, procreate, and establish a thriving population of their own kind, to be deemed successful in their conquest. Distance from nearby landmasses (isolation) will dictate how many species arrive and establish themselves, and the failures are effaced from history. Time will dictate how species thrive or succumb, the degree of competition between them, the number of niches they diversify into, and how they speciate or evolve. As species occupy more specialized niches - they tend to become endemic, where they can no longer tap into other resources, nor survive anywhere else. Endemism is the quickest route to extinction, as any disturbance or threat to a population would mean we've lost them to gene pools for good. (Robert H. MacArthur and E.O Wilson's Theory of Equilibrium offers other insights into the balance of species on islands).


THE ANDAMAN & NICOBAR ISLANDS

Geologically, the Andaman and Nicobar island chain is believed to have been originated during Tertiary (between 66 - 2.6 million years ago), when the Indian plate collided with the southern edge of Asia.


The arc shape of the Andaman and Nicobar island chain is typical of convergent plate boundaries - where two or more tectonic plates collided, with one plate eventually subducts, or slides under the other. (Much the same process of subduction occurs beneath the Himalayas).


While the Andaman and Nicobar islands, and the subduction system they represent, are among the world's most seismically active regions - they are not volcanic islands. They are formed due to the accretion or slow accumulation of sea sediment and oceanic crust - that has been eroded from the Himalayas, transported by the Ganges and Brahmaputra, and deposited as fans under the sea. Over time, these sediments and crust fragments built up and were exposed as islands.


While Barren island and some other older, inactive volcanoes are present, this is not a classic volcanic arc.


The Andaman and Nicobar chain is a classic example of a rifted arc-raft islands - found in supra-subduction zones. Supra-subduction zones, where there is intense vertical plate motion, have the geochemical characteristics of island arcs but the structure of oceanic crust and are thought to have formed by sea-floor spreading directly above subducted oceanic lithosphere.

This complex submarine activity that shaped the Andaman and Nicobar islands.

Image credit: Singh, Satish C. et al. Sourced from here. (*This is a great paper for anyone keen on a more detailed geological overview of the A&N subduction system).


From the perspective of biogeography, as a rift arc-raft island chain, the coloniser ancestors of native terrestrial animals would have either arrived over water through dispersal, or walked over across ice-bridges. So the biodiversity evolved largely in isolation, but there were some overland colonisers every now and then, which accounts for the incredible biodiversity.

This map shows the accretion of submarine sediments and oceanic crust that has shaped the Andaman and Nicober island chain. (Map credit: Raj Bhagat (Twitter: @rajbhagatt). Sourced from here.)


Interestingly, the indigenous tribal population on the Andaman islands may have been isolated from other populations as far back as 30,000 years, and have since evolved culturally and linguistically. The Nicobar tribes speak another language, which has Austroasiatic roots, but is different from those of the Andamanese.


The insular nature and physical isolation between islands have contributed to the evolution of rich biological diversity in the region, with a variety of rare and unique flora that are highly endemic to the region.


Of over 9,100 species of flora and fauna (both terrestrial and marine) recorded in and around the Andaman and Nicobar islands, a 1032 species are endemic - which means they are only found here, and nowhere else in the world. Such a high percentage (11.3% in this case) of endemism isn't rare on island chains, and it is in part because of the isolation. The diverse adaptations of island fauna allow us to understand how species evolved, and the modifications that lent them better advantages in survival.


Some studies indicate that species on the Andaman islands are more closely related to those found in southwestern Burma, whereas those on the Nicobar islands bear resemblance to Sumatran species. Other studies disagree, and believe both island groups have similar species that evolved from those found on the mainland/southwestern Burma. There is great scope for research, as vast datasets from different species can help us better understand the biogeography of these islands.



A pair of endemic Andaman cuckoo-doves (Macropygia rufipennis) photographed at Mt Harriett National Park on the Andaman islands. These birds are threatened by habitat loss.

Image credit: M V Shreeram (CC BY-SA 4.0)


Endemic species on Andaman and Nicobar islands face the threats like introduction of exotic or invasive species, deforestation (for development), natural disaster, and climate change. If these species are lost, we not only lose a valuable genetic heritage, but also the evidence of the Earth's geologic and biogeographic past - as their arrival and establishment on these islands may indicate which landmasses they once travelled from, what past climate regimes held sway, as well as understanding the effects of habitat fragmentation. FOOD FOR THOUGHT: While most species on the Andaman and Nicobar island chain may have arrived via dispersal or via the land bridge from Myanmar during low sea levels, the Andaman Day Gecko has its closest ancestors in Madagascar! This tantalizing paper by Dr Ashwini V. Mohan et al. doesn't draw biogeographical inferences yet, but imagine the clues to our past held by the species on this island chain?

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