Download Latitudinal Gradients in Richness

Causes of the Latitudinal
Gradient in Richness
Elizabeth Selig
Outline of the Talk
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Background
Hypotheses explaining the gradient
Papers
Where do we go from here
Background
• Among the oldest and broadest patterns
in ecology
• Peak in species richness at the equator
is generally true regardless of biota’s
taxa, geographic context, or time
domain
• Gradient has existed for at least 250
million years
Exceptions
– Narrow latitudinal gradients
– Species w/parasitic life
histories
– Aquatic floras
– Marine mammals and birds
A Cornucopia of Hypotheses
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Historical
Spatial
Environmental
Biological
Historical
Energy
Disturbance
I. HISTORICAL: Ecological
Following glaciation, dispersal is ongoing and
recolonization is not complete
Problem: latitudinal
gradient still exists
in the oceans
I. HISTORICAL: Evolutionary
Extinction and
extirpation from
glaciation have
limited richness in
high latitudes;
speciation is slow
and not complete
II. SPATIAL
1. Area:
Tropics support more species because
they have more area
Problem: More area in high latitudes in
North America and Eurasia
II. SPATIAL
2. Geometric Constraint/Mid-Domain Effect:
Bounded domain will have mid-domain peak in
species richness
Problem: Unsupported in many areas e.g.
Connelly, 2003 and Zapata, 2003
III. ENVIRONMENTAL
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Energy Availability/Productivity
Stability
Favorableness
Habitat heterogeneity
Disturbance
III. ENVIRONMENTAL
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Energy Availability/Productivity
Stability
Favorableness
Habitat heterogeneity
Disturbance
III. ENVIRONMENTAL
1. Energy Availability/Productivity
Annual input of solar
radiation determines
energy availability,
productivity, and
biomass and is
inversely related to
latitude
III. ENVIRONMENTAL
1. Energy Availability/Productivity
A. Energy controls rate of
speciation (Rohde,
1992)
B. More energy allows
more species to persist
Problem: Fails to provide
mechanism
Bromham and Cardillo
III. ENVIRONMENTAL
2. Stability
Temporal variation in the environment causes
higher extinction and lower speciation
A. Low latitudes, stability results in finer niche
division and lower extinction
B. High latitudes, environmental variation results
in biota that are speciation and extinction
resistant
III. ENVIRONMENTAL
3. Favorableness
“Favorable” environments require less
energetic cost of adaptation
IV. BIOLOGICAL
• Predation and parasitism
• Mutualism
• Competition  Increased niche
partitioning
Why these papers?
• Importance of scale
• Focus on multiple explanations for the
gradient
• Ecological and evolutionary
considerations including life history
Dynesius and Jansson:
Milankovitch oscillations
• Environmental: stability
• Historical: evolutionary and ecological
Dynesius and Jansson:
Milankovitch oscillations
Astorga et al.: Patterns of
latitudinal diversity in crabs
• Environmental: Energy
Availability/Productivity
• Spatial: Area
• Rapoport’s Rule
Astorga et al.: Patterns of
latitudinal diversity in crabs
• Importance of
spatially
structured SST as
a primary driver of
gradient
Astorga et al.: Patterns of
latitudinal diversity in crabs
• Scale: <5° SSTs do
not explain gradient
in diversity
• Circulation patterns,
geography, nutrient
etc. may diversity at
small scales
Astorga et al.: Patterns of
latitudinal diversity in crabs
• Importance of larval development on
patterns of diversity
Discussion Questions
• Does the paper by Astorga et al. identify
a causal mechanism to explain the
latitudinal gradient?
• What does their conclusion about SST
and larval development mean for finding
a general explanation for the gradient?
• Dynesisus and Jansson assume that
speciation is slow. Is this a fair
assumption?
Where do we go from here?
Discussion Questions
What are the drivers? What are the
modifiers? Can we create a hierarchical
model to explain the pattern?
Jansson, 2003
Where do we go from here?
Discussion Questions
• How can we incorporate scale into the
model?
• Is there a silver bullet? Is it important to
look for one?