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 Energy Stability Rapoport’s Rule Historical Disturbance Favorableness 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. 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 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?