Biogeography
Learning goals:
* Be able to describe the different spatial scales of diversity and how they relate to each other
* Learn the possible underlying causes for observed latitudinal patterns of diversity
* Understand the theory of island biogeography and what predictions it makes about the influence of area and distance from source of colonization

Global to regional patterns of diversity

Species diversity and distribution vary at global and regional spatial scales in association with geographic and evolutionary processes
Diversity will vary according to the spatial scale, reflecting sampling effect and incorporation of diversity from local to regional scale (all species in pool samples)
alpha (α) diversity- local scale
gamma ( γ) diversity- regional scale; regional species pool
beta (β)diversity is the turnover of species across the regional scale- i.e. how different is species composition among the different communities within a region
Landscape diversity (= topographic and environmental features of a region) influences relationship between levels of diversity

Global patterns of species diversity and composition are controlled by geographic area and isolation, evolutionary history, and climate
Major biogeographic regions correspond with tectonic plates; The plates are sections of Earth’s crust that move or drift (continental drift) through the action of currents generated deep within the molten rock mantle
Past orientation of the continents has influenced the groups of species found within a region
Isolation in some areas (e.g. Neotropical, Ethiopian, and Australian regions) has resulted in unique distinctive floras and faunas)
The legacy of continental drift can be found in the fossil record and in existing taxonomic groups.  Vicariance —evolutionary separation of species due to a barrier such as continental drift.
Oceans also have significant impediments to the exchange of biota, in the form of continents, currents, thermal gradients, and differences in water depth.  Identification of marine biogeographic regions has been hindered by the extra complicating factor of water depth and by the basic lack of knowledge of the deep oceans.

Latitudinal gradients in species richness
For many taxa, diversity is highest in the tropics, and declines toward the poles
Global patterns of species richness should be controlled by three processes: Speciation, extinction, and migration
If we assume migration rates are similar everywhere, then species richness should reflect a balance between extinction and speciation
Both speciation and extinction rates should increase with species richness – the more species, the higher the probability of either occurring
As the number of species increases, we would expect more species to evolve from them (a positive feedback loop).
The probability of extinction would increase due to a higher potential for resource depletion and thus competitive exclusion and eventual extinction.

Why should speciation and/or extinction rates be different at different latitudes?
Proposed explanations:

Greater land mass, in combination with warmer, more stable temperatures in the tropics increases population sizes and geographic distributions, lowering the probability of extinction, and enhancing the possibility of speciation over parts of the geographic range

Evolutionary history hypothesis:  Tropical regions have longer histories, they have been climatically stable and thus had a lot of time for evolution to occur- a higher degree in niche specialization in the tropics relative to other zones. At higher latitudes, large scale disturbances such as ice ages would increase extinction rates and hinder speciation.

Productivity hypotheses: For terrestrial systems, species diversity is higher in the tropics because productivity (energy supply) is higher.Higher productivity should promote larger population sizes, which will lead to lower extinction rates.

Regional differences of species diversity are controlled by area and distance due to a balance between immigration and extinction rates
At this regional scale, common observation is that species diversity varies with the size of the area; islands make a good unit to examine this relationship
Additionally, distance from other areas (e.g. mainland) influences the richenss of some species
Observations explained by the theory of island biogeography:
1) extinction rates increase as the number of species increase, but are controlled by the size of the island: larger islands have lower extinction rates
2) immigration rates decrease with increasing species richness (i.e. probability of a new species arriving decreases); immigration rates are controlled by distance to source of species (e.g. mainland): islands closer to the mainland will have higher immigration rates

How does the biogeography of mainland areas differ from islands?
Immigration rates are greater because there are fewer barriers to dispersal and extinction rates are lower because of continual immigration.
Species on mainlands will always have a good chance of being “rescued” from local extinction by immigration from other populations.