Elizabeth Selig

Causes of the Latitudinal Gradient in Richness

Outline of the Talk

• 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

• Historical• Spatial• Environmental• Biological

Mid-Domain

EnergyStability

Rapoport’s RuleHistorical

DisturbanceFavorableness

Heterogeneity

I. HISTORICAL: Ecological

Problem: latitudinal gradient still exists in the oceans

Following glaciation, dispersal is ongoing and recolonization is not complete

I. HISTORICAL: Evolutionary

Extinction and extirpation from glaciation have limited richness in high latitudes; speciation is slow and not complete

II. SPATIAL1.Area: Tropics support more species

because they have more area

Problem: More area in high latitudes in North America and Eurasia

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

III. ENVIRONMENTAL

• Energy Availability/Productivity• Stability• Favorableness• Habitat heterogeneity• Disturbance

III. ENVIRONMENTAL

• Energy Availability/Productivity• Stability• Favorableness• Habitat heterogeneity• Disturbance

III. ENVIRONMENTAL

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

1. Energy Availability/Productivity

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

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

III. ENVIRONMENTAL

“Favorable” environments require less energetic cost of adaptation

3. Favorableness

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?