LECTURE 26 CH 23-24
SPECIES DIVERSITY, HISTORY, AND
BIOGEOGRAPHY
Multiple scales of species diversity
Local
Regional
Latitudinal
Continental
Global
Local-scale: Factors affecting distributions/presence of species
Abiotic factors, biological interactions, dispersal limitation, human introduction, chance
Equilibrium theory: number of species is a balance of
Additions (e.g. immigration and speciation)
Subtractions (e.g. emigration and extinctions)
E.g. at continental scale: affected by balance of speciation and extinction
E.g. Theory of Island Biogeography
Number of species at equilibrium results from balance of
immigration on regional scale and extinction on local scale
Distance from mainland: affects immigration rate so no. species > near than far
Island area: affects extinction rate so no. species > large than small islands
Applicability to ‘terrestrial habitat’ islands too
Regional-scale: Factors affecting patterns species diversity
Habitat heterogeneity
Suitability of physical conditions
Isolation from centers of diversity
Effect of regional-scale on local-scale
Compare diversities in similar habitats in different regions  reveals regional effects
Saturation of local communities: test how local and regional diversity relate
Variation in local diversity depends on regional diversity
Comparisons of tree diversity among regions:
effects of loss of habitat during glaciation
gains due to building stepping stones for dispersal
regions with more species have species in greater ecological range
Latitudinal-scale
Increase in diversity with decrease in latitude
Hypotheses to explain (see earlier lecture)
Time Hypotheses: tropics are older; more time to accumulate species
But…what happened to species diversity in tropics during glacial era?
Continental/global-scale
How do history and biogeography influence local communites and species diversity?
Diversity has not been constant over geological time
Catastrophes have caused major changes in direction of evolution
Historical roots affect current species distributions
Continental drift
Altered positions of continents; some separations; some joining
Changed routes of dispersal
Land bridges connected land masses
Exchanges of biotas possible; affected migration and extinctions
Drift separated lineages onto different continents subsequent divergence
Result = organisms not distributed uniformly over earth
(Phylogenetic effects: traits shared by lineage irrespective of environment
e.g. marsupials limited to Australia because of evolution lineage, not env. of Aust.)
Zoogeographic regions reflect long-term evolutionary isolation (e.g. 6 regions)
Contrast: similar environments among regions has led to convergent evolution
Climatic history determines species distributions
Glaciations
Caused extinctions in Europe because of dispersal barriers; no refuges
Species in N.A. behaved individualistically  shifts in community composition
Miocene cooler/drier in NA  major shifts in community and species diversity