Elizabeth Selig

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Causes of the Latitudinal Gradient in Richness

• Elizabeth Selig

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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 biotas 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

Rapoports Rule
Mid-Domain
Historical
Energy
Favorableness
Disturbance
Stability
Heterogeneity
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I. HISTORICAL Ecological
Following glaciation, dispersal is ongoing and
recolonization is not complete
Problem latitudinal gradient still exists in the
oceans
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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. SPATIAL

• 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. 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
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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
1. Energy Availability/Productivity

• Annual input of solar radiation determines
  energy availability, productivity, and biomass
  and is inversely related to latitude

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III. ENVIRONMENTAL
1. Energy Availability/Productivity

• Energy controls rate of speciation (Rohde, 1992)
• More energy allows more species to persist
• Problem Fails to provide mechanism

Bromham and Cardillo
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III. ENVIRONMENTAL
2. Stability

• Temporal variation in the environment causes
  higher extinction and lower speciation
• Low latitudes, stability results in finer niche
  division and lower extinction
• High latitudes, environmental variation results
  in biota that are speciation and extinction
  resistant

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III. ENVIRONMENTAL
3. Favorableness

• Favorable environments require less
  energetic cost of adaptation

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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
• Rapoports 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 lt5 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?