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Chapter 10
The Geography of Diversity
Species Diversity
Richness – The number of species
in a census
► Species
§ Rare species are as important as dominant
§ Working with census lists
► Shannon
Diversity Index
Scales of Diversity
Diversity – Species richness of a local
ecological community (e.g. defined as a study plot
such as a hectare)
► Beta Diversity – Change (or turnover) in species
composition between two distinct communities
► Gamma Diversity – Total species richness over a
large geographic area such as a biome.
► Alpha
Patterns in Biogeography and
Latitudinal Gradient – Increasing
diversity towards the equator
► The Peninsula Gradient – Decreasing
diversity away form the mainland
► The Elevation Gradient – Species diversity
decreases with elevation
► The Aridity Gradient – Species diversity
decreases with diminishing water availability
► Aquatic Environments – Similar patterns
► The
Species diversity studies reveal a pattern of higher diversity
near the equator. The following are a few hypotheses
attempting to explain this pattern.
The Peninsula Effect - Variation in species richness of three
difference groups of organisms in Baja California, showing a
variety of patterns
Causes of these Patterns
► Nonequilibrium
§ Glaciation
§ Climate Change
§ Plate Tectonics
► Equilibrium
Harshness and Abiotic Stress
Climatic Stability
Habitat Heterogeneity
Biotic Interactions
Ecogeographic Rules
Bergmann’s Rule (1847) – Animals with larger body forms
occur at high latitudes. Larger animals have a lower
surface area to volume ration.
Allen’s Rule (1877) – Endothermic vertebrates that live in
warmer climates have longer appendages.
Gloger’s Rule (1883) – Coloration of related forms is
correlated to humidity with darker coloration occurring in
more humid regions. Most likely driven by crypsis
Bergmann’s Rule and adaptation to temperature in the
Bushy-tailed woodrat (Neotomoa nicerea), over both
geographic space and evolutionary time.
Allen’s Rule showing ear length in rabbits and foxes where
the organism on the left occurs in a hot desert habitat and the
one on the right occurs in a cold tundra environment
Macroecology: Assembly of
Continental Biotas
– A quantitative and statistical
approach that tries to identify general
ecogeographic patterns and to understand the
underlying mechanisms of the distributions of
ecological particles (organism, species, biotas, or
replicated sample plots; Brown 1995)
► Macroecology
Relationship between area of range and body size showing
that there are few large species with small geographic ranges
Natural Experiments
► The
Great American Interchange
§ Isolated from 160 million years before present
§ Formation of the Central American landbridge at
3.5 million years ago
§ See the effects of dispersal , interspecific
interaction, extinction, and evolution
§ Half of South American species are derived
from North American while only 10% of North
American species are derived from South
The Great American Interchange
► Northern
species had three advantages
§ They were better migrators
§ The were better survivors and speciators
§ The were better competitors
Organisms that crossed or were filtered out by the Central
American Landbridge