Island biodiversity

Biodiversity on the islands

Islands are home to an incredible wealth of biodiversity. The isolation of islands over millennia has led to the evolution of unique species that are found nowhere else on Earth. Islands hold higher concentrations of endemic species than continents, and the number and proportion of endemic species increases with the isolation of the island, its size, and the variety of its habitats. Over 90 percent of Hawaiian island species are endemic.

In Mauritius, about half of all higher plants, mammals, birds, reptiles and amphibians are endemic, and the Seychelles has the highest level of amphibian endemism in the world. Cuba is home to 18 endemic mammals while mainland Guatemala and Honduras have only three each. Madagascar, the fourth largest island in the world, is home to more than 8000 endemic species.

The unique characteristics that make island biodiversity so exceptional also make it particularly fragile and vulnerable. Island species often develop survival strategies based on interdependency, co-evolution, and mutualism rather than defense mechanisms against predators as competitors.

Many island species have become rare or threatened, and islands have a disproportionate number of recorded species extinctions compared to continental ecosystems. Of the over 700 recorded animal extinctions that have occurred in the last 400 years, about half were of island species. At least 90 % of the bird species that became extinct during this period were island species.

In the Caribbean, where the distance between islands is relatively small compared to the Pacific, groups of islands link biodiversity corridors especially for birds. A biodiversity ‘hotspot’, the Caribbean islands support a wealth of biodiversity within its diverse terrestrial ecosystems, with a high proportion of endemicity –making the region biologically unique. It includes about 11 000 plant species, of which 72 % are endemics.

For vertebrates, high proportions of endemic species characterize the herpetofauna probably due to low dispersal rates, in contrast to the more mobile birds and mammals. Much less diversity in marine species exists because of the high degree of connectivity. The Caribbean Current runs through the basin all year long transporting larvae between the islands. As a result, marine habitats share many of the same species–in contract to the regions terrestrial biodiversity with its high rates of endemism. [1]

However, the Caribbean’s biodiversity is at serious risk of species extinction. More than 700 species are threatened, making the Caribbean one of the top hotspots assessed for globally threatened species. The region has by far the highest percentage of threatened or extinct amphibian species. In fact, the top five countries in the world with the highest levels of threatened and extinct amphibians are all in the Caribbean.

Biodiversity is crucial to food security in many small isolated islands and especially among the SIDS. The continental shelves and coastal ecosystems of many SIDS are of major economic importance for settlements, subsistence and commercial agriculture, fisheries, and tourism.

In addition to hurricanes and cyclones—which devastated several  Caribbean islands in 2017, changes in sea level are a clear threat to terrestrial species that live on low-lying islands or which inhabit low-elevation coastal zones.

In what is believed to be the first instance of an extinction caused solely by human-induced climate change, a small rodent called the Bramble Cay melomys—an animal that lived on a small island in the eastern Torres Strait of the Great Barrier Reef, disappeared from the island in 2016.

The melomys was already hugely vulnerable: it had the most isolated and restricted range of any Australian mammal.  It is estimated that the area of the cay above high tide decreased from 4 ha in 1998 to 2.5 ha in 2014. In addition, over the last ten years, 97% of the rodents’  habitat was destroyed by severe weather compounded by sea level rise [2].

 

 

 

 

 

For more information check out :

[1] See : Critical Ecosystem Partnership Fund: The Caribbean Islands Biodiversity Hotspots, at  https://www.cepf.net/sites/default/files/finaldraft_caribbean_ep.pdf

[2] See : First mammal species goes extinct due to climate change. http://news.nationalgeographic.com/2016/06/first-mammal-extinct-climate-change-bramble-cay-melomys/.
And also : A national disgrace: Australia’s extinction crisis is unfolding in plain sight. https://www.theguardian.com/environment/2018/feb/13/a-national-disgrace-australias-extinction-crisis-is-unfolding-in-plain-sight?