The islands of the world’s oceans represent the diversity of life on Earth in all of its forms: geological, biological and cultural. There are at least 20,000 islands of an area greater than 1 km2, and millions if all sizes are considered. These include oceanic islands in a strict sense (i.e., islands that emerged as volcanoes from the seabed), atolls (the last stage of a former tropical volcanic island now represented by only a coral reef), land-bridge islands (continental peninsulas that due to interglacial sea-level rise lost their connection to continents), other islands on a continental shelf, and continental fragments or micro-continents (originally continental areas but now isolated in the ocean through continental drift, e.g., Madagascar or Seychelles).
Further, below sea level, seamounts rise from the seabed but do not reach the ocean’s surface. They are important habitats for sea-life and some of them emerge above the ocean’s surface in times of lower sea levels (e.g. during glaciation periods) and are therefore important for understanding dispersal of species to isolated islands via stepping stones. Islands can be found in all oceans of the planet, at all latitudes, and consequently in all climate zones: from the Mediterranean Sea, via Macaronesia and across the Atlantic to the Caribbean Sea, across the vast Pacific from the East Pacific, to Polynesia, Melanesia, and Micronesia and further to South-East Asia, and back to Africa across the Indian Ocean. Not to forget the Arctic and Subantarctic islands at high latitudes.
Oceanic islands are renowned for the many and diverse scientific breakthroughs that their fascinating biotas have enabled during the past two centuries. They have served as model systems for research in biogeography, ecology, evolution and conservation. Charles Darwin and Alfred Russel Wallace independently discovered the principles of evolution after extended travels through the island archipelagos of the world. MacArthur and Wilson’s dynamic theory of island biogeography has become the most influential theory in biogeography, and has major relevance to other biological fields including conservation biology. Peter and Rosemary Grant’s work on the dynamic adaptation of beak size and form of Galápagos finches to variation in food sources has become a textbook example of rapid evolution that happens within years in nature.
In recent years island biology has gained new momentum as a major research avenue in ecology, evolutionary biology and biogeography as exemplified by a series of international island biology conferences (2014 in Hawaii, 2016 in the Azores). Such new dynamism in island biology is stimulated by the emergence of a truly global island research community that today works on most islands of the world (Kueffer et al. 2014). This enables big-data analyses of island biodiversity patterns and multi-island comparisons. In addition, modern research approaches such as genomics, phylogenetic and functional ecology, and palaeoecology, are also increasingly used on islands enabling interdisciplinary work across levels of biological organisation from genes to ecosystems and integrating ecological and evolutionary processes.
At the same time, islands urgently require major additional conservation efforts. Many species face extinction, natural areas are small and fragmented and alien species dominate most ecosystems. Although the world’s islands together make up only ca. 5% of the Earth’s land area, it has been estimated that up to one-quarter of global plant diversity is endemic to islands. A majority of these species might be threatened by extinction through habitat loss, invasive species, small population sizes, loss of their mutualists (e.g. seed dispersers or pollinators) and climate change. The situation is so severe that many conservationists believe that traditional conservation strategies, such as the establishment of protected areas, do not suffice to save island biodiversity. As a consequence, islands have become places where new conservation solutions for the twenty-first century are being tested (Kueffer and Kaiser-Bunbury 2014).
Novel ecosystems that are dominated by non-native species are increasingly accepted as a new and inevitable reality by many conservationists. Sometimes non-native species are even deliberately released into the wild in the hope that they will replace extinct species and take on their lost ecological functions; for instance giant tortoises on the islands of Mauritius and Rodrigues in the Western Indian Ocean. And rare species are moved from the wild to park-like protected areas, e.g. on small offshore islands, where they can be supported through intensive management. These biodiversity parks are a mix between nature reserves, botanic gardens and zoos. Whether or not such innovative new conservation solutions will suffice to save the many unique species that existed for thousands of years in isolation from any other biota on tiny land fragments in the oceans of the world, will be decided in the next few decades.
Islands are like thousands of unique pearls sprinkled across the globe. Their unique biological treasures will disappear first if we do not find solutions to growing environmental problems. Threatened from the surrounding oceans by sea level rise as a result of climate change, and threatened from within through unsustainable use of resources and land. Will they be remembered as the early warning systems that helped to trigger action just in time, or the beginning of the destruction of millions of years of evolution?
Featured image credit: From Tenerife with an endemic Echium in the foreground, by José María Fernández-Palacios.