Patterns on islands • Island—a relatively small area of suitable habitat isolated from a much larger area (=source) of suitable, occupied habitat. For example, the continent nearest to an island would be considered the source. Observation • Large islands have more species than smaller islands. A general rule is that as the land area increases 10 times, the number of species doubles. • See page 429, textbook Lesser Antilles bird species S = cAz • S = number of species • c = constant measuring the number of species per unit area. Insects, for example, will have a higher c than amphibians because there will be more insects per unit area than amphibians. • A = area of the island • z = constant measuring the slope of the line relating S and A. z is dependent on the type of organism and the island group and how distant island is from mainland S = cAz • z usually equals somewhere between .15 and .35. More poorly dispersing animals have higher zs, in other words, as island size increases, poorly dispersing animals show greater responses to the increase in size than animals that disperse well. • zs are lower for terrestrial islands Smammal = 1.188A0.326, Sbird = 2.536A0.165, Brown (1978) Different z values. Area effect is smaller on mainland. Island biogeography theory • Relatively successful ecological model that predicts the influence of immigration and extinction on the equilibrium number of species that will inhabit the island. Equilibrium • Equilibrium number is a number we expect to be relatively constant over time, balanced by species immigrating to an island and other species going extinct. • Dynamic equilibrium-refers to the fact that, although the number of species will be relatively constant, the species themselves are changing (because of immigration and extinction). • MacArthur and Wilson reasoned that the equilibrium species number will be influenced by both immigration to islands and extinction on islands, which will be influenced by distance of the island to the mainland and island size, respectively. • Turnover rate (on y-axis)—the rate at which the identities of the species on the island change—is the point at which the immigration and extinction rates are equal • Near islands have higher immigration rates because likelihood of reaching a near island compared to a far island is greater • Large islands have lower extinction rates because populations of any given species will be higher on large islands compared to small islands. Larger populations have a lower risk of extinction. Wilson and Simberloff test of model in Florida Keys • First, they censused four small islands covered with red mangrove (15 m across) and at different distances from the mainland. The censuses of insects and arthropods revealed what they expected— the most species on the nearest island and the least on the farthest island. Wilson and Simberloff test of model in Florida Keys • Second, they hired a pest company to defaunate islands by covering them with rubber tents and using methyl bromide gas to kill the insects and arthropods Then they visited the islands periodically afterwards to census the islands and determine which species were there • Support for equilibria idea--In less than a year, the nearest island had 44 species, where originally it had had 43. Furthest island had 22, originally 25. Similar patterns on the other two islands. Numbers remained the same after two years. • Support for the dynamic equilibrium idea— species identities changes while numbers stayed relatively constant They also discovered differences based on species differences. E1 is most isolated island (Simberloff & Wilson 1970, Ecology 51:934-937) They also discovered differences based on species differences • Spiders arrived quickly because of their ballooning habits but tended to go extinct relatively quickly • Mites, blown in with dust, arrived more slowly but stayed longer • Cockroaches, moths, and ants recolonized islands relatively quickly and persisted • Centipedes and millipedes never recolonized over the two years of the experiment. • The researchers found higher immigration rates on the close islands, as expected • Highest turnover rates were on close islands • The size of the islands did not vary so they could not test the hypothesized relationship between island size and extinction rate More recent modifications to model • • • • • Size of the island, as well as distance from the mainland, should affect immigration rates (target area effect) Distance to the mainland, as well as size of the island should affect extinction rates because of the rescue effect Evolution and interspecific interactions will mold island biotas Different taxonomic groups will reach equilibria at different points in time Small island effect Target Area Effect: greater immigration rate on larger islands • Rescue effect—small populations of a species are rescued from extinction by the arrival of new immigrants of the same species Rescue effect (particularly on continental islands): reduced turnover due to replacement Small Island Effect: no area-diversity effect on small islands too few habitats Niering, W.A. 1963. Terrestrial ecology of Kapingamarangi Atoll, Caroline Islands. Ecological Monographs 33:131-160. • Species richness of well-dispersing taxonomic groups (wind-dispersed plants, birds) appears to have reached equilibrium on Krakatoa but the richness of more poorly-dispersing taxonomic groups (animal-dispersed plants, non-flying mammals) has not. Krakatau Islands Biogeography: Differential immigration rates for plants with different dispersal mechanisms Island patterns • Insular refers to island • Ecological release— expansion of a species’ niche in the absence of competitors • Harmonic insular biotas— proportions of different types of organisms are similar on island and source • Disharmonic insular biotas— proportions of different types of organisms are different on island and source Patterns regarding three processes on islands • Immigration • Establishment • Extinction Immigration • Bats are well-represented on oceanic islands while many nonvolant mammals are not • Bats colonized New Zealand and the Hawaiian islands while these areas have no other native mammals • Birds and the plants they eat are wellrepresented on oceanic islands, as are bird parasites • Amphibians and freshwater fish are poorly distributed on oceanic islands (New Zealand has no native freshwater fish) • Rana cancrivora (crab-eating frog) and Bufo marina (marine or giant toad) have high tolerances for salt water both as tadpoles and adults and so are found on oceanic islands much more frequently than other amphibians. • Slugs are very intolerant of salt water and so are infrequently found on oceanic islands while land snails, which often thrive in dry habitats, are frequently found on such islands — land snails are able to raft to islands • Large species, and those that stay active year-round are more likely to be found on islands (not necessarily distant oceanic islands). These types of species can use ice for travel. • Islands that are large and in archipelagos may be more likely to be found by dispersers, or islands that are in the route of particularly strong wind or water currents Establishment • Species that are generalists are more likely to become established on islands than specialists (for ex. dung beetle generalists tend to have more successful introductions than specialists). • A study with land snails found that species with individuals that could self-fertilize were more likely to become established that species that could not do so • Individuals with high fecundity rates, i.e. large clutch or brood sizes, will likely become established more readily than other types of species • Islands that are large with a diversity of habitats and resources may be more hospitable to populations for establishment Extinction • Large animals, carnivores, and specialists are more likely to become extinct on islands than small generalist herbivores. Smaller generalists will have larger population sizes than larger specialists and with larger population sizes there is less probability of extinction Evolutionary patterns on islands • Reduced dispersal ability--so, ironically, the ancestors who dispersed well have descendants who don't disperse well • Changes in body size Reduced dispersal ability • Flightless birds and insects are common on oceanic islands • Flightlessness has evolved in at least 8 orders of birds: ostriches, ducks and geese, parrots, owls, doves, rails, storks and herons, and passerines • New Zealand--25-35% of land and freshwater birds are (or were) flightless, 24% of Hawaii's endemic bird species Evolutionary scenario to account for flightlessness? • First of all, why do most birds fly? • Predation is probably very important • Those individuals who invested less in costly flight muscles would have more energy for other activities (like producing young) and would not suffer the losses from predation important on the mainland because many islands lack their traditional predators. • Flightlessness has evolved repeatedly in insects: beetles, butterflies and moths, flies, ants bees and wasps, grasshoppers and crickets, true bugs • On Madeira Island--off coast of Africa and Portugal--200 of 550 beetle species are flightless • Insects also tend to be wingless at higher latitudes and in mountains Hypotheses to explain these patterns? • Energy conservation • Advantages to individuals of site fidelity (staying near their natal site) • Reduced dispersal ability is also evident in land snails and plants found on islands Changes in body size • Woolly mammoth range shrunk from much of the northern Palearctic 20,000 ya to only Wrangel Island 10,000 years ago • Size of woolly mammoths also shrunk from 6 tons to 2 tons by 2,000 years ago • Size change must be positive for individuals to evolve but then may have positive consequences for the population • Individuals of small species tend to get larger on islands and individuals of larger species tend to get smaller—why? Advantages of large size • Larger individuals within a species can use more types of resources • Larger individuals can produce more offspring • Larger individuals tend to win in intraspecific competition • Larger individuals have more stored energy to make it through times of food shortage Investigators suggest that these advantages will be more important for individuals of small species rather than individuals of large species • Even a small elephant can use a variety of resources while a large rat may have a significant advantage over small rats in the variety of resources it can use • Individuals of small species may show ecological release because of lack of competitors on islands. • Advantages of small size to escape predation may be less useful in absence of typical predators Advantages of small size • Smaller individuals can get by with less food and other resources • Smaller individuals often use food more efficiently than larger individuals (assimilate energy from food) • Smaller individuals can use smaller shelters and hiding spots than larger individuals Investigators suggest that these advantages will be more important for individuals of large species rather than individuals of small species • A small elephant will be able to get by on much less of a resource base than a large elephant and resources for elephant-sized animals are more likely to be limited on islands • The resources necessary to rats may not be as limited and so there may be no selective advantage to small rats vs. large rats. Island body size may also be a by-product of the characteristics of immigrators • Successful active dispersers may tend to be the larger individuals of a species (because, for example, larger individuals would have more energy reserves and hence be more likely to survive the journey to an oceanic island) Island body size may also be a by-product of the characteristics of immigrators • Successful passive dispersers may tend to be the smaller individuals of a species (smaller individuals are more likely to make it by being pushed by wind or water). • These patterns of successful immigrants may not be maintained in the island populations over time if there are counterselective pressures for reasons discussed above and/or if immigration to the island is rare • In short, ecological release drives small animals to become bigger while resource limitation drives big animals to become smaller • These patterns may not hold if the immigrant size patterns we just discussed are more influential than ecological release and resource limitation—for example when immigration is very common Birds and reptiles show some similar patterns to mammals with many exceptions • Exceptions may result because there is much more data for these taxonomic groups • It is possible that birds and reptiles are under different selective pressures than mammals on islands. • Reptiles are ectothermic and so resource limitation may not be a problem • The lack of many medium-sized and large mammals on many islands may have allowed birds and reptiles to show ecological release Homo floresiensis discovered on Flores Island, Indonesia, 2003 The skull of Homo floresiensis can only hold a brain that's about 380 cubic centimeters in size. The modern human skull, at right, holds a brain that measures between 1,400 and 1,500 cubic centimeters. (Peter Brown) Homo floresiensis possible history • Arrived on island as Homo erectus (first large-brained hominid from Africa and Asia) • Unclear how they arrived—boats, swimming, and rafting all seem unlikely • Homo erectus probably averaged 5’10” • After arrival on the island, the smallest individuals may have survived best because of resource limitation (Flores island is only 31 sq. miles in area) • Hot, humid weather of the region may also have favored small individuals, who, with a greater surface area to volume would have been able to cool off faster and would have generated less heat when they moved • Appears the individuals lived in caves • Remains of female individual found with remains of miniature Stegodon sp., Komodo dragon, and burned bones of birds, rats, and fish, and stones tools, in cave. • Since publication describing the new species was submitted to Nature, authors have found remains of more individuals that appear to be 95,000 to 13,000 years old • Some other anthopologists are skeptical of calling it a new species • The new species may have been killed off when a volcano erupted on the island 12,000 years ago • Signs of modern humans on the island are 11,000 years old Very controversial today • Oct 2005, Nature published descriptions of bones of 9 individuals of Homo floriensis, some thousands of years apart in age—scientists argue that the number of specimens and the time span, refute the idea that they simply discovered abnormal individuals. • Several studies argue that the skull is probably from a small-bodied modern human who had a genetic condition, microcephaly. Individuals with microcephaly have small heads. 2007 papers • Homo floresiensis were not microencepahlic (based on comparisons with truly microencephali individuals, Falk et al. 2007) • Hand bones of H. floresiensis more similar to chimps or early hominids than to modern humans (Tocheri et al. 2007) • Falk, D et al, (2007). "Brain shape in human microcephalics and Homo floresiensis". Proceedings of the National Academy of Sciences 104 (7): 2513. doi:10.1073/pnas.0609185104. PMID 17277082. Retrieved on 2008-03-05.; "lay summary" (2007-01-29). Retrieved on 2008-03-05. • Tocheri et al, (2007). "The Primitive Wrist of Homo floresiensis and Its Implications for Hominin Evolution". Science 317 (5845): 1743. doi:10.1126/science.1147143. PMID 17885135.; "lay summary" (2007-09-20). Retrieved on 2008-03-05. As of 2009, debate continues • Weston and Lister 2009 suggest that small brain size of H. floresiensis may be consistent with brain size evolution of other mammalian groups on islands.