- The size of an isolated island limits its ecosystem’s species diversity, productivity, intensity of competition and resistance to species invasions.
- Ecosystem development on small islands is further limited by the smaller population size of species, which makes species vulnerable to extinction due to both catastrophic events and human action such as deforestation.
- Small and micro islands inherently have very limited carrying capacity. Therefore, human-centric island development must therefore exercise extreme caution.
- The views in the commentary are that of the author.
Charles Darwin may have been the first to observe and document evolutionary changes in island species and ecosystems. Evolutionary changes involve shifts in size, behaviour, reproduction, and physiology, driven by unique island factors such as fewer predators, limited resources and reduced competition.
According to Darwin, in any small isolated area, such as an oceanic island, the total number of species inhabiting it will be small. Yet, of these, a large proportion will be endemic. Predictable evolutionary changes are more often seen in island species than in their mainland relatives. This phenomenon is called ‘island syndrome’.
Adaptive radiation (a process through which organisms adapt and evolve) in islands far exceeds the mainland. Competition is less severe. Evolutionary innovation is lower. Vulnerability to species invasions is much higher.
Similarly, the size of islands constrains ecosystem development. Ecosystems are shaped by competition among member species for limiting resources — resources needed to grow, survive and reproduce. Competition leads to interdependence which in turn affects productivity. Since size determines the number of species that inhabit an island, it also limits competition, interdependence and productivity (amount of carbon fixed and stored in the form of biomass).
Size and ecosystem development
The size of an isolated island is the key, because it limits its ecosystem’s species diversity, productivity, intensity of competition and resistance to species invasions. Competition leads to coalition (cooperation or association), innovation, specialisation and diversification. Competition is intense only where there are many species. In islands with limited area and species diversity, competition is less intense resulting in lower interdependence and productivity. It ultimately limits ecosystem development.
Predators contribute to productivity by increasing organismal (or biomass) turnover. For example, when herbivores eat leaves, more leaves are produced to compensate for the loss, and prey animals tend to multiply faster to avoid population depletion by predators. The overall paucity of predators in islands can therefore reduce productivity and thereby limit ecosystem development.
Ecosystem development on small islands is further limited by the smaller population size of species. Smaller population size makes species vulnerable to extinction due to random catastrophic events, such as a tsunami, and deterministic human action such as deforestation. History has taught us how island species such as the hapless dodo went extinct due to human occupation and deterministic action (hunting) in Mauritius.
An island’s size not only affects the intensity of competition but also the pace of life in its ecosystem by restricting the activity level and rate of metabolism in its dominant predators. Reptilian predators such as saltwater crocodile, water monitor and reticulated python in the Andaman and Nicobar Islands are examples.
Size and ecological vulnerability
Australia is a large island-continent. Other oceanic islands are broadly classified as large islands (Greenland, Madagascar, New Guinea, Borneo), medium islands (Great Britain, Sumatra, Sri Lanka) and small islands. By definition, small islands are not more than 10,000 square kilometres in size. Such a definition, places the majority of the world’s oceanic islands to the small (Hawaiian Islands, Andaman and Nicobar Islands) and micro categories (Micronesia, Lakshadweep Islands).
Hawaiian Islands are by far the most human-impacted small, oceanic islands in the world. Polynesians first occupied the islands 1,500 years ago and started transforming ecosystems locally. They introduced plants and animals including the red junglefowl from Southeast Asia. And with other settlers from North America and Asia colonising the islands in the early 19th century, the Hawaiian Islands have witnessed the most devastating invasion of non-native species and the extermination of endemic species, especially among plants and birds. Even their native forests are being invaded by introduced fruit trees.
Land masses, including oceanic islands, develop ecosystems that resist or accommodate invaders. However, invaders that are aided by humans, find it easier to colonise habitats that are secondary (example vegetation tampered by humans). Invaders are competitively dominant and they thrive locally by taking advantage of the services and resources provided by the native biodiversity. This is particularly relevant in small islands where ecosystem development — competition, interdependency, predation and productivity — is weaker.
Carrying capacity and island development
The majority of the world’s oceanic islands qualify as small and micro. In India for example, islands are all small or micro; the largest being the Middle Andaman Island which is merely 1,536 sq km in area. Given the fact that size limits ecosystem development and productivity, it is crucial that utmost care is exercised in planning and implementing development projects on small islands.
It is well-known that human occupation and the resultant development have together devastated many oceanic islands. Prehistoric human occupation as that in the Andaman and Nicobar Islands may have had minimal impacts on the archipelago’s fragile ecosystems.
Nevertheless, relatively more recent human occupation (approximately 3,500 years ago) as that in the Lakshadweep Islands has evidently caused more damage to the biodiversity-rich archipelago. One of the reasons for the more drastic erosion of biodiversity in the Lakshadweep Islands, as compared with the Andaman and Nicobar Islands, may be the geology and the micro size of the islands in the former.
Small and micro islands inherently have very limited carrying capacity, especially that which is required to support large human populations. Sustaining humans in such islands will require heavy imports of resources. Imports contribute to waste generation and thereby open a pandora’s box of ecological perils.
Lessons to be learnt
A considerable amount of knowledge has been accumulated during the past 100 years on island ecology through dedicated research on the Barro Colorado Island (BCI). This is the result of collective effort, put in by hundreds of researchers, young and old, who worked under the umbrella of the Smithsonian Tropical Research Institute (STRI), and its predecessor (Canal Zone Biological Area), and visionaries, including the Governor of Canal Zone, the first Director of STRI — Martin Moynihan, and S. Dillon Ripley, the former Secretary of the Smithsonian Institution and a close associate of India’s legendary birdman Salim Ali. BCI, a 15-sq km fragment of tropical forest in the Panama Canal, has since 1923 provided answers to many questions on how insular ecosystems changed over time. Although BCI is for all practical purposes micro, and not a natural island, it has served as a ‘natural experiment’ through which some of the world’s most compelling ecological theories and other ecological insights have emerged.
As development-pressures on India’s islands are increasing, planning and decision-making must take into consideration the many lessons learnt from islands in general and BCI in particular. In BCI, for instance, an estimated 12,000 sloths, about eight individuals/hectare, survived during 1975. In less than 50 years, sloths more or less disappeared from the island. Sloth numbers plummeted so sharply that in 2003, a camera-trap-based survey failed to locate any. The case of the sloth forewarns that large population size is no insurance against local extinction. This, and many other lessons that island ecology has taught us should therefore guide planning and decision-making in small and micro island human-centric development.
Citation:
- Darwin, C (1859) On the origin of species by means of natural selection. John Murray, London.
- Leigh Jr, et al (2007) The biogeography of large islands, or how does the size of the ecological theatre affect the evolutionary play? Rev. Ecol. (Terre Vie) 62: 105-168.
- Leigh Jr, E G, Vermeij, G J and Wikelski, M (2009) What do human economies, large islands and fragments reveal about the factors limiting ecosystem evolution? European Society for Evolutionary Biology: Journal Compilation 22: 1-12.
- Daniels, R J R (2024) A Voice for the Great Nicobar Island. Current Science 126 (4): 424-425.
- Daniels, R J R (1996) The Vanishing aborigines of the Andaman and Nicobar Islands. Current Science 70:775-776.
- Daniels, R J R (1991) Island biogeography and the birds of the Lakshadweep Archipelago, Indian Ocean. Journal of the Bombay Natural History Society 88: 320-328.
- Leigh Jr, E G and Zeigler, C (2025) One hundred years of science on an island: how a fragment of tropical forest hosted a century-long explosion of scientific discovery. Independently Published/Printed in the European Union.
Banner image: A saltwater crocodile. An island’s size not only affects the intensity of competition among species for limiting resources, but also the activity level and rate of metabolism in its dominant predators. Reptilian predators such as saltwater crocodile in the Andaman and Nicobar Islands is an example. Representative image by Richard.Fisher via Wikimedia Commons (CC BY 2.0).
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