131 Ocean Island Colonisers

Key Idea: The biodiversity of oceanic islands often depends on the distance to the mainland and the ability of plants and animals to survive dispersal. Oceanic islands have a unique biota because only certain groups of plants and animals tend to colonise them. The animals that successfully colonise oceanic islands have to be marine in habit or able to survive long periods at sea or in the air. Plants also have limited capacity to reach distant islands. Only some have seeds and fruit that are salt water tolerant. Many plants are transferred to the islands by wind or migrating birds. The biota of the Galápagos islands provide a good example of the result of such a colonisation event.

Land mammals: Few non-flying mammals colonise islands, unless these are very close to the mainland. Mammals have a higher metabolism, need more food and water than reptiles, and cannot sustain themselves on long sea journeys.

Blown by strong winds

Reptiles: Reptiles probably reach distant islands by floating in driftwood or on mats of floating vegetation. A low metabolic rate enables them to survive the long periods without food and water.

Oceanic island

Rafting on drifting vegetation

Deep ocean

Amphibians: Cannot live away from fresh water. They seldom reach offshore islands unless that island is a continental remnant.

Active flight

Swimming

Planktonic crustacean larvae

Small birds, bats, and insects: These animals are blown to islands by accident. They must adapt to life there or perish.

Plants: Plants have limited capacity to reach distant islands. Only some have fruits and seeds that are salt tolerant. Many plants are transported to islands by wind or birds.

Seabirds: Seabirds fly to and from islands with relative ease. They may become adapted to life on land, as the flightless cormorant has done in the Galápagos. Others, like the frigate bird, may treat the island as a stopping place.

Sea mammals: Seals and sea lions have little difficulty in reaching islands, but they return to the sea after the breeding season and do not colonise the interior.

Crustaceans: Larval stages drift to islands. Crabs often evolve novel forms on islands. Many are restricted to shoreline areas. Some crabs, such as coconut crabs, have adapted to an island niche.

Charles J. Sharp cc 3.0

The flightless cormorant (above) is one of a number of bird species that lost the power of flight after becoming an island resident. Giant tortoises, such as the 11 subspecies remaining on the Galápagos today (centre) were, until relatively recently, characteristic of many islands in the Indian Ocean including the Seychelles archipelago, Reunion,

Mfield cc 3.0

Mauritius, Farquhar, and Diego Rodriguez. These were almost completely exterminated by early Western sailors, although a small population remains on the island of Aldabra. Another feature of oceanic islands is the adaptive radiation of colonising species into different specialist forms. The three species of Galápagos iguana almost certainly arose, through speciation, from a hardy traveller from the South American mainland. The marine iguana (above) feeds on shoreline seaweeds and is an adept swimmer. The two species of land iguana (not pictured) feed on cacti, which are numerous. One of these (the pink iguana) was identified as a separate species only in 2009.

1. Describe one feature typical of an oceanic island coloniser and explain its significance:

The Galápagos Islands, off the west coast of Ecuador, consist of 16 main islands and six smaller islands. They are home to 14 species of finches, each of which has evolved from a single species of grassquit, which arrived from Ecuador. A fifteenth species inhabits Cocos Island. After colonising the islands, the grassquits diversified in response to the availability of unexploited feeding niches. This adaptive radiation is most evident in the beaks of the different species, which are adapted for different purposes, including crushing seeds, pecking wood, or probing cactus flowers. A relatively recent (2005) revision of the phylogeny of the finches is shown right. It places the sharp beaked finch more distant from the other ground finches with which it was previously grouped (all are currently Geospiza). A 2015 revision based on whole genome sequencing has split the sharp beaked finches still further into three distinct groups and has suggested a reclassification. The diagram below shows the islands on which the birds are found and the age of the islands in millions of years (in brackets).

Cactus ground finches Other ground finches T ree finches

Darwin (~0.7)

Wolf (1.0)

The five species of tree finches are largely arboreal (tree dwelling). Their sharp beaks are well suited to grasping insects which form the most of their diet. One species has demonstrated tool use in extracting insects.

On Wolf Island, the sharp beaked ground finch has become a specialist blood feeder. Recent genomic analysis has suggested a classification of this species. The Cocos Island finch is the only one of Darwin's finches to be found outside the Galápagos. DNA analysis shows it is related to the warbler finches. The ancestral finch therefore colonised the Galápagos before colonising Cocos Island.

N

Pinta (0.8)

Marchena (0.7)

Genovesa (0.3)

ian finch V egeta r k ed ground finch p bea Sha r Cocos Island finch Warb ler finches Tiaris bicolor v e) (mainland relati

Cocos Is

800 km

Galápagos Is.

900 km South America

Pacific Ocean

3D

Santiago Is.(0.8)

Equator

Fernandina (0.7)

Isabela Is (0.5) Santa Cruz (1.1)

Santa Fé (2.7)

San Cristóbal (2.4)

Floreana (~1.0)

The beak of the warbler finch is the thinnest of the Galápagos finches. It uses it to spear insects and probe flowers for nectar. It is also the most widespread species, found throughout the archipelago. Española Is. (3.2)

2. How do we know the finches colonised Cocos Island after colonising the Galápagos Islands?

3. Is there a pattern to how the islands were colonised by the birds? Explain:

The cactus ground finches (left) have evolved probing beaks to extract seeds and insects from cacti.

Ground finches, genus Geospiza (left) have crushing type beaks for seed eating. Three species differ mainly in body size and in the size of their beaks. The other three species have longer beaks and supplement their seed diet with cactus flowers and pulp (the cactus finches) or the eggs and blood of other birds and reptile ticks (the sharp-beaked ground finch).