Island Biogeography

Darwin on Biogeography “Neither the similarity or dissimilarity of the inhabitants of various regions can be wholly accounted for by climatic and other physical conditions.” “Barriers of any kind…are related in a close and important manner to the differences between the productions [organisms] of various regions.”

Biogeography Definition: The study of how species are articulated on the landscape in space and time. Image credit: http://www.success.co.il/

Island Biogeography Definition: A subdivision of biogeography that relates the manner in which species distributions are influenced and restricted by “islands.” The “island” is any area of habitat surrounded by an inhospitable matrix to the species occurring on that island. Image credit: http://www.okstate.edu/artsci/botany/bisc3034/lnotes/islands.htm

General Island Types Oceanic Islands – Never connected to a mainland source Ex. Hawaiian Islands Continental/ Land bridge Islands – Connected to mainland during glaciations or prior to flooding Ex. Tasmania or Barro Colorado Islands, Panama Virtual Islands - Isolated communities separated via some sort of barrier Ex. Sky Islands (southeastern Arizona/ southwestern New Mexico) Ex. Discontiguous habitats created via fragmentation Ex. Caves!

Island Size Richness = island size and distance from mainland Small islands Less habitat Smaller populations Higher rates of extinction (intra,inter-specific competition)

Island Distance Richness = island size and distance from mainland Distant islands Lower rates of colonization However, this does depend on dispersal mechanism of the species!

Dispersal vs. Vicariance Hypotheses Dispersal Hypothesis: Species originated in one area and dispersed to other areas. Vicariance Hypothesis: Areas were formerly contiguous, and were occupied by a common ancestor. Speciation occurred once barriers arose.

“Theories, like islands, are often reached by stepping stones…” MacArthur and Wilson (1967)

Theory of Island Biogeography MacArthur and Wilson (1963): The number of species of a given taxon that become established on an island represents a dynamic equilibrium controlled by the rate of immigration of new species and the rate of extinction of previously established species. Image Credit: http://www.sscnet.ucla.edu/anthro/faculty/arnold/california-lab.htm

Biogeography Theory in a Nutshell Equilibrium number of species, but constant turnover Migration slows as richness increases Extinction increases with richness Equilibrium = rate of extinction, rate of colonization intersect Colonization balances extinction Equilibrium no. of species

Theory of Island Biogeography Immigration varies with the distance of the island from the mainland (or the pool of potentially colonizing species). More distant islands are colonized less frequently. The rate of colonization declines as species richness increases, because there are fewer potential colonists and fewer unexploited niches.

Theory of Island Biogeography Extinction varies with the size of the island. Big islands can support larger population sizes, and large populations are less prone to extinction. So, the smaller the island, the greater the probability of extinction. Extinction rates rise as colonization increases; interspecific competition displaces of some species and lowers population sizes when species can coexist.

Theory of Island Biogeography At equilibrium between immigration and extinction, the number of species remains stable, although the composition of species may change. This is the dynamic equilibrium. The rate at which some species are lost and others becomes established is the turnover rate.

Theory of Island Biogeography At the same time as colonisation is occurring, some species will become extinct. As new species arrive they will compete for limited resources and the extinction rate will increase. The rate at which one species is lost and a replacement is gained is the turnover rate.

Theory of Island Biogeography Islands further from mainland have lower immigration rates More distant islands have lower species diversity

Theory of Island Biogeography Smaller islands have lower total populations Probability of extinction increases with lower population Smaller islands have lower species diversity

Equilibrium Theory of Island Biogeography Immigration rate decreases as island diversity increases Extinction increases as island diversity increases Species equilibrium on islands is a balance of immigration and local extinction

Equilibrium Theory of Island Biogeography Larger islands should have more species Consistent with the species area relationship

Equilibrium Theory of Island Biogeography Equilibrium model may explain the common observation that species richness decreases with distance from mainland. Diamond, J. M. (1972) Avifauna of the eastern highlands of New Guinea. Publ. Nuttall Ornith. Club 12: 1-438. Species richness decreases with distance from New Guinea (mainland) – from Diamond (1972).

Equilibrium Theory of Island Biogeography Because travel to the near island is easier, the maximum immigration rate, to this island should exceed that of a more distant island Closer islands should have more species Consistent with the distance effect

Equilibrium Model of Island Biogeography - Summary The species richness of an island represents a balance between extinction and colonization Species turnover is continuous Larger islands have a greater species richness at equilibrium Islands closer to the mainland have a greater species richness at equilibrium

Species Turnover Rates at Different Equilibriums Mainland MacArthur, R.H. (1972) Geographical ecology. Harper and Row, New York.

Tests of Theory – Species Turnover Wilson and Simberloff (1968, 1969), Florida Keys Exterminated all arthropods (insects and arachnids) on islands Repeated census over 2 yrs. Species turnover constant Lowest rates of turnover on distant islands

Simberloff: Experimental Test

Wilson and Simberloff (1968, 1969)

The Species Area Curve Articulation of the relationship between the number of species and the area of habitat. de Candolle (1855): “the breakup of a large landmass into smaller units would necessarily lead to the... extinction of (some) species and the preservation of others.” Darlington (1957): A tenfold increase in habitat area, loss of 90% of land area results in the loss of 50% of the species.

The Species Area Curve Relationship between the number of species and the area of habitat. Species diversity increases with increasing area

The Species Area Curve S = species richness c = constant, regionally specific richness A = sampling plot size z = constant, shape of curve (slope)

The Species Area Curve Species richness increases rapidly with increasing area, then levels off. More specifically: S = cAz where c and z are constants that vary with habitat.

The Species Area Curve Empirical studies have shown that z typically falls with the range of 0.20 - 0.35 z-values from continental areas of different size tend to be lower than for true islands within a comparable size range. As the size of an area increases, species richness will increase up to a maximum value.

Variation in Species Area Relationships Continents have more species than islands for same area Islands close to continents have more species Older islands have more species than newer islands

Species Area Curves Species Area

Area Influences Species Richness

Tests of the Theory (Species Area Curves) Boreal mammals on sky islands Mollusks in New York lakes Arthropods on Spartina islands Breeding birds on remnant woodlots, Il.

The SLOSS Debate If only a limited amount of habitat can be protected in parks and reserves, which strategy is better: a Single Large reserve Or Several Small reserves (SLOSS)?

Habitat Conservation Reserve Design SLOSS Vs. Single Large Or Several Small? Vs.

Habitat Conservation Reserve Design vs vs SLOSS Single Large Or Several Small? Corridors = Matrix area easier for Dispersal vs vs

Habitat Conservation Reserve Design vs vs SLOSS Single Large Or Several Small? Corridors = Matrix area easier for dispersal Buffers = Compensate for edge effects vs vs

Implications for Protected Area Design… Several small islands (land and water) lose species faster than one single island of equivalent size (Burkey 1995). Burkey, T.V. (1995) Extinction rates in archepelagoes: implications for populations in fragmented habitats. Conservation Biology, 9, 527–541.

SLOSS Take home message: Jury is still largely out on this debate. It will depend on the objectives of the reserve, which will dictate the ultimate design. Also, species dependent – different species are more susceptible to different disturbance regimes Most importantly – it’s highly politically. We often have to play the cards we’re dealt. Thus, we may not always have the “best” reserve design to maximize multiple species protection and conservation.

Island Biogeography But what is Island Biogeography Theory (or any Theory) good for: Provides a conceptual approach for understanding our increasingly fragmented, island-like terrestrial habitats. Provides theoretical basis for explaining the relatively impoverished island biotas. Offers a predictive framework for estimating species diversity. Serves as a foundation (or at least a starting point) for more realistic models. The Role Of Theory: better understanding of a Complex system.

Theory of Island Biogeography What are its Weaknesses: No mechanistic foundation to explain patterns Does not account for which species will be present Tendency to over-interpret and over-apply Ignores addition of species in-situ by evolution

Species-Area Relationships and Biogeography Critically important for studying patterns and processes of biodiversity Conservation, reserve design, extinction rates Spatial dependence of species richness Relation of scale to process