Lecture 19 SPECIES DIVERSITY 2 - REGIONAL TO GLOBAL LEVELS
1.Equilbrium theories of diversity (e.g. Island Biogeography Theory) balance factors that
add (speciation + immigration) and remove species (extinction + emigration).
2. Regional patterns of diversity reflect habitat heterogeneity, climate change, tolerance
of physical conditions, and isolation from centers of diversity.
2.Numerous hypotheses explain the increase in species diversity with decreasing latitude.
3.History, including continental drift and building of land bridges, account for patterns
of species distribution on continents.
4.Biogeographic regions reflect the long-term evolutionary isolation of large areas.
5.Catastrophes have caused major changes in the direction of evolution.
6.Climate changes have shifted the distributions of plants and animals.
I. Regional Scale Diversity, cont.
A. Equilibrium theory: number of species is a balance of
Additions (e.g. immigration and speciation) minus
Subtractions (e.g. emigration and extinctions)
E.g. Theory of Island Biogeography 426-8
Number of species at equilibrium results from balance of
immigration on regional scale and extinction on local scale
Island area: affects extinction rate so no. species > large than small islands
Distance from mainland: affects immigration rate so no. species > near than far
Applicability to ‘terrestrial habitat’ islands too
B. 1. Other factors affecting patterns of regional species diversity 416-20
Habitat heterogeneity and range of conditions used
Suitability of physical conditions
Isolation from centers of diversity
2. Patterns of species richness over regional areas differ among taxa
(beta) diversity: difference in number of species (
) between habitats 420
Turnover more rapid in N-S (than E-W) direction; steeper climate gradient
Decreases more sharply with changing longitude in S than N latitudes in North America
A. Increase in diversity with decrease in latitude 416
B. Hypotheses to explain high species richness in tropics (see biotic factors at local scale)
Add one more = tropics have been stable for longer time; time to add species and
Greater solar energy input and precipitation enhances species richness 418-19
How do history, biogeography, and climate influence species diversity?
A. Equilibrium Theory of continental communities: # species affected by balance of
speciation and extinction on continental scale 427-8
B. Diversity has not been constant over geological time
Communities not in constant equilibrium
Catastrophes (e.g. meteorite hits; volcanic explosions) have caused major
changes in species diversity
C. Greater diversification (additions-extinctions) occurs in ecological zone of origin
May have been greater + faster in tropics than temperate area –> more species
D. Historical roots affect current species distributions
1. Continental drift/Plate tectonics 444-5
Altered positions of continents; some separations; some joining
Changed routes of dispersal
2. Biogeographical result = organisms not distributed uniformly over earth
Zoogeographic regions reflect long-term evolutionary isolation (e.g. 6 regions) 445-6
Drift separated lineages onto different continents subsequent divergence
New land bridges arose connecting land masses
Exchanges of biotas possible; affected migration and extinctions
Comparisons of diversity among regions with unique geographic positions 454-55
Gains due to building stepping stones for dispersal + connections to tropics
3. Climatic change influences species distributions 447-50
Glaciations and intolerance of low temperatures
Europe: extinctions because of dispersal barriers as moved N to S
Some species had limited dispersal and could not expand post-glacial
N.A.: species migrated individualistically
shifts in community composition
E. Processes on large geographic + temporal scales influence biodiversity 456-459
1. Age and area of biomes: greatest = tropics
2. Speciation and extinction
Higher speciation rates in tropics
Diversification (speciation – extinction) greater in tropics?
Shifts to new ecological zones more difficult and infrequent
Most diversification occurs within ecological zone of origin (e.g. tropics)
Hard to adapt to more stressful environments
Summary: 1, 3-7, 11-13