elizabeth selig causes of the latitudinal gradient in richness
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Elizabeth Selig
Causes of the Latitudinal Gradient in Richness
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Outline of the Talk
• Background• Hypotheses explaining the
gradient• Papers• Where do we go from here
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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
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Exceptions
– Narrow latitudinal gradients
– Species w/parasitic life histories
– Aquatic floras– Marine mammals and
birds
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A Cornucopia of Hypotheses
• Historical• Spatial• Environmental• Biological
Mid-Domain
EnergyStability
Rapoport’s RuleHistorical
DisturbanceFavorableness
Heterogeneity
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I. HISTORICAL: Ecological
Problem: latitudinal gradient still exists in the oceans
Following glaciation, dispersal is ongoing and recolonization is not complete
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I. HISTORICAL: Evolutionary
Extinction and extirpation from glaciation have limited richness in high latitudes; speciation is slow and not complete
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II. SPATIAL1.Area: Tropics support more species
because they have more area
Problem: More area in high latitudes in North America and Eurasia
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II. SPATIAL2. Geometric Constraint/Mid-Domain
Effect: Bounded domain will have mid-domain
peak in species richnessProblem: Unsupported in many areas e.g. Connelly, 2003 and Zapata, 2003
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III. ENVIRONMENTAL
• Energy Availability/Productivity• Stability• Favorableness• Habitat heterogeneity• Disturbance
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III. ENVIRONMENTAL
• Energy Availability/Productivity• Stability• Favorableness• Habitat heterogeneity• Disturbance
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III. ENVIRONMENTAL
Annual input of solar radiation determines energy availability, productivity, and biomass and is inversely related to latitude
1. Energy Availability/Productivity
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III. ENVIRONMENTAL
A. Energy controls rate
of speciation (Rohde, 1992)
B. More energy allows more species to persist
Problem: Fails to provide mechanism
1. Energy Availability/Productivity
Bromham and Cardillo
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III. ENVIRONMENTAL
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
2. Stability
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III. ENVIRONMENTAL
“Favorable” environments require less energetic cost of adaptation
3. Favorableness
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IV. BIOLOGICAL
• Predation and parasitism• Mutualism• Competition Increased niche
partitioning
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Why these papers?
• Importance of scale• Focus on multiple explanations for
the gradient• Ecological and evolutionary
considerations including life history
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Dynesius and Jansson: Milankovitch oscillations
• Environmental: stability• Historical: evolutionary and
ecological
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Dynesius and Jansson: Milankovitch oscillations
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Astorga et al.: Patterns of latitudinal diversity in
crabs
• Environmental: Energy Availability/Productivity
• Spatial: Area• Rapoport’s Rule
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Astorga et al.: Patterns of latitudinal diversity in
crabs
• Importance of spatially structured SST as a primary driver of gradient
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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
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Astorga et al.: Patterns of latitudinal diversity in
crabs • Importance of larval development
on patterns of diversity
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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?
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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
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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?