Extinction

been more or less constant through geological time, because all habitats have an essentially full quota of species, with the result that adding new species to the total inventory is a one-in-one-out process. Over 150 years later, we inevitably have a more nuanced view of how global diversity has changed through time, which shows that there have been periods of rapid increases of species numbers, especially after mass extinctions, but conversely there are also long intervals of little change, as Darwin originally envisaged. In many ways, Darwin, a Victorian gentleman, was a man of his time who saw long-term progress as both the product of human endeavour and nature's selection process. Thus, he did not think extinction could be caused by environmental changes because such physically driven losses would likely lead to rather random evolutionary trends instead of the long-term improvement of life that he saw. Also, as an adherent of Lyell's gradualist school, he thought it unlikely that any abrupt, Cuvier-style mass extinctions had happened, although he acknowledged that, in special circumstances, multiple extinctions may occur. For example, the sudden immigration of many species into an area could cause losses among the local species. Overall, though, Darwin's view of evolution and extinction was one of inexorable improvement to an essentially constant total pool of species that saw the gradual demise of inferiors in the struggle for existence in favour of species that were higher in the scale of nature. Fast forward to today and our ideas about the causes of extinction have changed substantially, while Darwin's theory of evolution is of course well established and has stood the test of time. Darwin's dismissal of catastrophic extinction events was still current up to 1980. Such mega-crises are now well documented and they have given life's history a much more random appearance than Darwin ever envisaged. We now know that successful dominant groups, such as the dinosaurs, can disappear and be replaced by obscure animals, such as the mammals. Darwin's inter-specific competition is still considered a valid process of extinction, but habitat loss is now thought to be much more important. SPECIES-AREA RELATIONSHIP In a celebrated series of experiments in the 1960s, Robert MacArthur and E. O. Wilson showed that the number of species found on a series of small islands was related to the area by the following relationship. S equals Caz, where S is the number of species, C is constant, A is the habitat area where the species live, and Z is the slope of the curve when plotted on a log-log scale. This quantified a long-recognised fact that larger areas tend to have more species present than smaller areas, but it is not as simple as saying double the area, double the species. As the area increases, the rate of addition of new species flattens off, giving a species-area curve a convex up-appearance. In the natural world, this power-law relationship, as it is known, makes sense because once an area has attained a large size, it is unlikely that there will be many new habitat types to appear for new types of species to occupy. Plotting a species-area curve using logarithmic axes produces a straight line with a constant slope, Z, which is easily measured. MacArthur and Wilson provided an explanation for the species-area effect. They suggested that the number of species present on an island represents a balance between new species arriving, immigrating, either by flying there or as seeds dispersing in the wind, and local species going extinct. The immigration rate is dependent on how far the island is from other islands and the mainland, both of which provide the pool of new species. The rate of extinction is dependent on an island's area, which controls the population size. You can only squeeze so many individuals onto an island. Larger islands can also accommodate more species because they are likely to have a greater diversity of habitats. The smallest islands may just have a few palm trees, as depicted in cartoons of shipwrecked sailors, while large islands may have volcanic uplands as well as coastal forests.