Patterns on islands

1. Patterns on islands

2. 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.

3. 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

4. Lesser Antilles bird species

6. 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

7. 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

8. Smammal = 1.188A0.326, Sbird = 2.536A0.165, Brown (1978)

9. Different z values. Area effect is smaller on mainland.

10. 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.

11. 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).

12. 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.

14. 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

17. 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.

18. 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.

19. 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

20. 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.

21. E1 is most isolated island (Simberloff & Wilson 1970, Ecology 51:934-937)

22. 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.

23. 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

24. 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

25. Target Area Effect: greater immigration rate on larger islands

26. Rescue effect—small populations of a species are rescued from extinction by the arrival of new immigrants of the same species

27. Rescue effect (particularly on continental islands): reduced turnover due to replacement

29. 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.

30. Krakatau Islands Biogeography: Differential immigration rates for plants with different dispersal mechanisms

31. 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

32. Patterns regarding three processes on islands • Immigration • Establishment • Extinction

33. 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

34. Birds and the plants they eat are well-represented on oceanic islands, as are bird parasites

35. 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.

36. 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

37. 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.

38. 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

39. 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).

40. 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

41. Individuals with high fecundity rates, i.e. large clutch or brood sizes, will likely become established more readily than other types of species

42. Islands that are large with a diversity of habitats and resources may be more hospitable to populations for establishment

43. 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

44. 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

45. 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

46. 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.

47. 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

48. Hypotheses to explain these patterns? • Energy conservation • Advantages to individuals of site fidelity (staying near their natal site)

49. Reduced dispersal ability is also evident in land snails and plants found on islands

50. 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