Box 32 Islands in the ocean general features

In Box 2.1 we outlined the differing origins of three classes of island. Here we summarize aspects of their geology, geography, and biology. Oceanic islands are built over the oceanic plate, are of volcanic or coralline formation, are remote and have never been connected to mainland areas, from which they are separated by deep sea. They generally lack indigenous land mammals and amphibians but typically have a fair number of birds and insects and usually some reptiles. Continental islands are...

Box 111 The decline of Lake Victorias cichlid fishes

Lakes are 'negative islands', that is, they are more or less isolated freshwater areas surrounded by a hostile land matrix, usually connected to other lakes through rivers. Lakes thus behave as islands in many biogeographical and ecological respects. Long-lasting lakes (e.g. Lake Baikal in Siberia, the Great Lakes of the African Rift Valley, and Lake Titicaca in South America), have provided similar evolutionary opportunities to oceanic islands, and in the case of the African Rift Valley lakes...

Stochastic versus deterministic extinctions

Two main types of extinction events operating on islands can be differentiated Table 11.1 largely stochastic extinctions, inherent to the natural dynamics of island environments, and deterministic extinctions, related directly or indirectly to human activity. In the first group we include natural disasters, such as volcanic eruptions, but also the consequences of the taxon cycle dynamics, which involve species moving down an evolutionary cul-de-sac in becoming highly restricted local endemics,...

Hawaiian crickets and drosophilids

The Hawaiian endemic crickets are thought to have derived from as few as four original colonizing species, each being flightless species arriving in the form of eggs carried by floating vegetation. The ancestral forms were a tree cricket and a sword-tail cricket from the Americas, and two ground crickets from the western Pacific region Otte 1989 . Three of the successful colonists have radiated extensively, and Hawaii now has at least twice as many cricket species as the continental United...

Magnitude and frequency

Between 1871 and 1964, an average of 4.6 hurricanes per year was recorded in the Caribbean, resulting in, for example, a return time of 21 years for the island of Puerto Rico Walker et al. 1991a Fig. 2.21 . With minimum wind speeds of 120 km h and paths tens of kilometres wide, hurricanes can have a profound impact, fundamental to an understanding of the structure of natural ecosystems in the region. Moreover, the Caribbean by no means corners the market in tropical storms. Hurricanes develop...

Nauru the destruction of an island

The island of Nauru once known as Pleasant Island is located in the Pacific Ocean, about 500 km northeast of New Guinea. It has a total land area of 21 km2 and a population of about 13 000 as of 2006. The coast is fringed by a ring of sand, surrounded by a protective reef, but the interior is formed of guano deposits that have mixed and solidified with a coral limestone base. The mining of these deposits has devastated the island environmentally, and despite the initial economic returns, has...

Exploring incidence functions

One of the problems of evaluating island assembly theory as detailed above is that the biogeographical context of the island avifaunas under discussion is in each case quite complex. We now review some simpler systems, involving non-volant mammals on small, near-shore islands. Hanski 1986 discusses occurrence and turnover of three species of shrews on islands in lakes in Finland, a system in which individual species populations could be studied in some detail. The largest species, Sorex...

Darwins finches and the Hawaiian honeycreeperfinches

Sharp Billed Ground Finch

Although the Gal pagos are renowned for other endemic groups, notably the tortoises Arnold 1979 and plants Porter 1979 , the most famous group of endemics must be Darwin's finches Emberizinae genera Geospiza, Certhidea, Platyspiza, Camarhynchus, and Cactospiza . The context within which these birds and the other creatures have evolved is as follows. The Gal pagos are in the east Pacific, 800-1100 km west of South America Fig. 9.5a . Although equatorial, they are comparatively cool and average...

Introduced species

As humans have spread across the islands of the world we have taken with us, either purposefully or inadvertently, a remarkable array of plant and animal species see Box 11.2 . These anthropogenic introductions are variously called exotic, alien or non-native species. Some species persist simply as domesticated animal or cultivated plant species, while others become naturalized, i.e. they form self-sustaining populations within modified habitats Henderson et al. 2006 . Of these, some become...

Translocation and release programmes

Translocation and repatriation from ex situ breeding and rearing programmes have been used as measures to rescue highly threatened species on islands, with some success. Translocation is used to remove a species from an overwhelming local threat or to re-establish a population on an offshore island where it may be safe from exotic predators. In general, translocated wild animals have been found to establish more successfully than captive-bred animals. Griffith et al. 1989 surveyed translocation...

Filtering effects dispersal limits and disharmony

Ecological Dispersal Species

Islands tend to have a different balance of species compared to equivalent patches of mainland they are thus said to be disharmonic. There are two aspects to this disharmony Williamson 1981 . First, as Hooker noted, both the climate and the biota of islands tend to be more polar than those of nearby continents Williamson 1981 . Thus, the Canaries, located west of the Sahara, have a generally Mediterranean flora, Kerguelen in the Indian Ocean is bleak and Antarctic-like for its latitude, and the...

Box 71 Dwarf crocodiles and hominids

Dwarf Crocodile Food Webs

The dwarf crocodiles of Tagant Mauritania Seventy years after the last reports of their existence and six years after the IUCN listed them as extirpated, relict populations of the Nile crocodile Crocodylus niloticus have been rediscovered in wetlands in southern Mauritania Shine etal. 2001 . These populations appear to have been isolated by climatic change associated with the Pleistocene Holocene transition, and have persisted in association with seasonal ponds, known as gueltas. In summer,...

Box 101 Fluctuating asymmetry in a fragmented forest system

Fluctuating asymmetry FA refers to differences between the right and left sides in animal anatomical characteristics that are usually bilaterally symmetrical, and is a form of developmental instability that may have genetic and or environmental causes. Whatever the nature of the control, a high incidence of FA is regarded as a sign of a population under environmental stress. Recent work has suggested that small, isolated populations in fragmented habitats may exhibit a higher incidence of FA....

Relaxation and turnoverthe evidence

If habitat islands behave according to the expectations of the EMIB, they should be supersaturated with species immediately after system fragmentation. Subsequently, species numbers in the isolates should 'relax' to a lower level Fig. 10.5 . Immigration and extinction should continue to occur, both during the relaxation period and subsequently, when the island has found its new, lower equilibrium richness level. The time taken for relaxation to occur is called the 'lag time' and the anticipated...

Islandhopping allopatric radiations do clades respond to islands or to habitats

Oceanic island archipelagos have been described as 'speciation machines' Rosenzweig 1995 . But, as we have seen, the proportions of single-and multiple-island endemics may be highly variable between archipelagos, indicating that these 'machines' can function in rather different ways. As highlighted in Box 8.2, the application of the label 'archipelago speciation' does not tell us whether speciation is adaptive or non-adaptive. Advances in the application of molecular analyses in recent decades...

The loss of dispersal powers

Given that oceanic islands are so hard to reach, it may appear paradoxical that so many flightless forms occur on them. For a distant island popping out of the sea for the first time, there will inevitably be a strong bias towards groups that are highly dispersive, as these will, by definition, have a greater chance of colonizing. This was reflected in the bio-geographical patterns discussed in Chapter 3, of filter effects, the gradual loss of taxa with increasing distance from continental...