Regional diversity What factors operating at regional scales account for local patterns of species diversity? Dispersal Range expansion/contraction Movement into new areas Speciation Vicariance, ecological speciation, speciation by polyploidy, etc. Extinction Together, these are often called historical processes or biogeographical processes (the terms are synonymous) that determine patterns of diversity. Processes that determine the pool of species for a region.

The Equilibrium Theory of Island Biogeography (ETIB) Robert H. MacArthur and E. O. Wilson (1963) What is an island? What is biogeography? What is dispersal? Write on the board: Biogeography is the study of the factors that determine the dispersal and distribution of species through geological time. Dispersal is the movement of offspring from their birth site to a new site where they can grow and reproduce.

What is an island? Ecological island: habitat surrounded by an inhospitable matrix Oceanic islands surrounded by saltwater Continental islands e.g. patches of habitat Other islands “sky islands” = montane habitats surrounded by desert or forest Lakes and ponds surrounded by terrestrial habitat Rivers and streams, separated by dry drainage divides All of these “islands” share in common the property of discrete boundaries… whereas so many boundaries are “fuzzy” in biological systems, including entities within these islands, such as species , communities, and ecosystems (all pretty fuzzy).

Alexander von Humboldt (1769-1859) Russian naturalist & explorer Father of biogeography 1799-1804 travelled throughout Latin and S. America Documented several key biogeographic patterns What is the background to the ETIB? Bioigeography and dispersal… as well as the species-area relationship (SAR), all of which dates back to von Humboldt… A Russian naturalist and explorer wrote the first book on biogeography. For five years between 1799 and 1804 he travelled through Latin and South America. Universities, Rivers, Currents, and ships are named after him. Investigated how species changed with elevation up a mountain and identified specific zones: alpine, montane, foothills. Also, observed the species-area relationship on islands.

The species-area relationship (SAR) The SAR is one of the “laws” of ecology. What could cause this pattern? Island size and isolation. Think about how we could use these ideas to conserve the maximum number of species. Discuss.

General interacting explanations Habitat diversity hypothesis The likelihood of encountering new habitat types and thus new niches for different species increases as area increases. Dispersal hypothesis Large areas are likely to receive more colonists than small areas. Area-Extinction hypothesis The population sizes of species are greater in large areas. Thus, fewer species go extinct than in small areas where there are more rare species.

ETIB immigration and extinction How are I and E modeled in the ETIB?

ETIB immigration and extinction How are I and E modeled in the ETIB? Immigration I Rate P No. species on island (S)

ETIB immigration and extinction How are I and E modeled in the ETIB? Immigration I Rate P No. species on island (S) P = total no. of species in the regional species pool

ETIB immigration and extinction How are I and E modeled in the ETIB? Immigration Extinction I E Rate Rate P P No. species on island (S) No. species on island (S) P = total no. of species in the regional species pool

ETIB: equilibrium species number What is the equilibrium species richness for an island?

ETIB: equilibrium species number What is the equilibrium species richness for an island? This is produced by the interaction of the I and E curves, determine equilibrium number species (S’) I E Rate S’ is the equilibrium number of species… at the equilibrium point, there is also ONE turnover rate where I = E at equilibrium, where the lines cross. This is not a stable equilibrium, it’s an equilibrium because I = E. Unlike competition and predation models, this model predicts species composition is constantly changing due to ongoing I and stocahstic E, although species number remains constant. P S’ No. species on island (S) P = total no. of species in the regional species pool

ETIB: theory on area and isolation Large islands should hold larger populations than small islands, so large islands should have reduced extinction rates. This chart shows how immigration and extinction change over time but time is implicit…like phase-plane charts. McArthur and Wilson created a theory based on a balance between two opposing processes. Both the size/area and isolation of an island will determine it’s diversity through their effects on immigration and extinction rates.

ETIB SUMMARY Lowest diversity: small-distant islands Intermediate diversity: small-near and large-distant islands Highest diversity: large-near islands

How does island isolation affect extinction and immigration? The distance of an island from the mainland (a source of colonists) should affect the immigration rate and thus, species diversity. Isolation should also affect how frequently declining populations are “rescued” from extinction. In E Rate If P Sf Sn No. species on island (S) “The rescue effect”

How does island size affect extinction and immigration? Extinction rates on small islands would be higher because small islands have smaller populations than large islands.  Larger islands may accumulate more species by chance just because they are larger targets. I Es Rate El P Ss Sl No. species on island (S) “The target effect”

Extending area and distance effects in the ETIB Immigration (I) Extinction (E) Target Effect ETIB ETIB Rescue Effect

Ultimately, however, the ETIB predicts the SAR In Es Sln Rate El If Log no. species Slf Ssf Ssn P Ssf Ssn Sln Island area (A) Slf No. species on island (S) … even without target and rescue effects.

Species turnover Definition: change in species composition from one community to another through time Island biogeography theory makes 2 general predictions: Species diversity is affected by size and isolation Community composition is dynamic

Dan Simberloff (1942-present), University of Tennessee Tests of island biogeography theory Dan Simberloff and E.O. Wilson’s test on mangrove islands in the Florida Keys Jared Diamond’s test on California’s Channel Islands Dan Simberloff (1942-present), University of Tennessee These are two famous ecologists who provided some of the earliest and most well known tests of the ETIB. Jared Diamond was a PhD student of XYZ, while Dan Simberloff was a PhD student of E.O. Wilson’s. Jared Diamond (1937-present) University of California at Los Angeles Guns, Germs, and Steel: The Fate of Human Societies Collapse: How Societies Choose to Fail or Succeed

Insects and spiders in the Florida Keys Tests of isolation determining immigration and richness: Insects and spiders in the Florida Keys Results: So, what did they do? This is a summary of Dan Simberloff’s work with McArthur and Wilson to test island biogeography theory. They had this crazy idea… there are 1000s of islands in the Florida keys, many of small size making them more manageable for field work / studying their communities (e.g. accurate surveys). Simberloff and Wilson went out and carefully characterized the number and abundance of insect and spider species on a number of study islands (how many?). RESULT: Used defaunation studies… found results consistent with the predictions of the model. They calculated an average 0.67 per day of species turnover. Later study by Simberloff, including a re-analysis of the data after taking singletons out of the dataset, suggested much of the turnover observed in their earlier study owed to extinction of “transient species” rather than permanent resident taxa… and suggested only 1.5 species turnover per year (1.5 species extinctions per year)!! Nonetheless, the data in both cases suggest turnover is frequent and that communities can stabilize at short-term equilibria (if not long-term ones)… as predicted by the ETIB.

Tests of turnover and equilibrium prediction: Extinctions were balanced by immigration despite turnover of species The Channel Islands of California are a chain of eight islands located in the Pacific Ocean off the coast of Southern California along the Santa Barbara Channel. They are along the coast from Santa Barbara to LA all the way down to San Diego. Five of the islands are part of the Channel Islands National Park. Diamond went out and surveyed all 8 islands/island groups in 1968, visiting each island 3 times… and compared his results to those of a record of survey data up til 1917 by Howell. Why did the Channel Islands provide a nice system for testing the equilibrium hypothesis of the ETIB? They vary in size (1-96 sq miles) and distance from the mainland (8-61 km), and had an earlier baseline of survey data, thus provide a nice model. Prediction: rates of species turnover should decrease with island size and distance from the mainland (lower I and E on large far islands)… Result: over 51 years (since Howell 1917), there were many extinctions and immigrations but richness remained the same. So, equilibrium seems to exist and turnover is common… community composition was dynamic. However, diversity was not very closely linked to size and isolation… indicating no evidence for an “isolation effect”. Instead, negative relationship between turnover and # species. The second aspect we will talk about in the community lecture begins here. Results:

Assumptions of island biogeography theory? No speciation Equilibrium dynamics prevail Problems? Both are unrealistic Near, large islands may have the same number of species or fewer species than small, distant islands.

Non-equilibrium dynamics Should we always expect an equilibrium or steady balance between immigration and extinction? Non-equilibrium in this context means that the slopes of the colonization-extinction curves for an island varies over time. Thus, where the two lines cross varies and the island diversity varies.