Latitudinal gradients in diversity

What changes from the tropics to the poles?

Climate:

• Solar energy

• Temperature

• Precipitation

• Evapotranspiration (terrestrial primary productivity)

• The tropics are older

• The tropics have suffered fewer harsh events

Area

What are 4 general hypotheses for the mechanism that leads to the latitudinal diversity gradient?

• Null models

• Ecological hypotheses

• Historical hypotheses

• Evolutionary hypotheses

What are null models for the mechanism that leads to the latitudinal diversity gradient?

• Mid-domain effect

• Neutral theory

What is an ecological hypothesis for the mechanism that leads to the latitudinal diversity gradient?

More individuals hypothesis / species-energy model

What are historical hypotheses for the mechanism that leads to the latitudinal diversity gradient?

• Time

• Area

• Time-integrated area

What are evolutionary hypotheses for the mechanism that leads to the latitudinal diversity gradient?

• Climate stability

• Evolutionary speed theory

• Biotic interactions hypothesis

What is the mid-domain effect?

• Random set of species ranges + bounded domain

• Ranges overlap more at centre than at limits

• More species in tropics

What is the more individuals hypothesis / species-energy model?

• Climate sets limit to richness

• Climate in tropics (temperature + precipitation) → higher net primary productivity → more individuals → more species with viable populations → less extinction

What is the area hypothesis?

• Tropics have a larger “area” now

• Larger areas → larger populations → less extinction

• Larger areas → more barriers → more speciation

What is the time hypothesis?

• Speciation occurs at the same rate

• Extinction is faster at higher latitudes

What are issues and ideas with latitudinal diversity gradient hypotheses?

• Biologists should consider mechanism

• Hypotheses for patterns based on lower-level processes will proliferate

• Should consider higher-level processes:

  • Selection

  • Ecological drift

  • Dispersal

  • Speciation

• Measuring speciation and extinction is difficult

What is selection?

Selective loss of species (extinction)

What is ecological drift?

Chance loss of species (extinction)

Which taxa differ in diversification rates across latitude?

• Foraminifera

• Marine bivalves

What is the time-integrated area hypothesis?

• The tropics covered an even larger area 66 million years ago

• Larger areas → larger populations → less extinction

• Larger areas → more barriers → more speciation

• More time → more speciation

What is the climate stability hypothesis?

• Tropics: predictable and mild

  • Greater specialization

  • Smaller niches

  • More speciation

• High latitudes: unpredictable and severe

  • Selection for flexibility

  • Larger niches

  • Less speciation

  • Less species per habitat

  • More extinction

What is the evolutionary speed theory?

• Higher temperature and solar radiation

  • Elevated mutation rate → faster molecular evolution ←→ higher speciation rate

  • Faster growth of individuals → shorter generations → faster selection → higher speciation rate

What is the biotic interactions hypothesis?

Biotic interactions are a stronger selective force than abiotic factors in the tropics → higher speciation