• With almost every arge taxon group, species richness is highest near the tropics and decreases towards th poles. Why?

Hypotheses for LDG

• Fosil record shows that the current LDG only began to form (or become this extreme) around 30 to 40 Mya!
• The modern LDG only occurs when we have "icehouse" worlds with cold poles and narrow tropical band
  • Periods like the Cretaceous had a much wider tropical band, and didn't have so much latitudinal variants in species richness
  • This can helpus understand how things will change in the last 30 Mya

Foraminefera

• Oceanic fossils containing foraminefera over the last 40mya (2023) showed that the LDG emerged around 34 Ma, as the climate cooled especially around the poles

Explanations of LDF

• Alexander von Humboldt was the first to explicitly observe the LDG, studied also by Darwin and Wallace
• Different theories on why:

Historical explanations

• The world is not yet full of species, we are in non-equilibrium
• Explanations that emphasize time or speed

Ecological explanations

• Ecological niches/carrying capacity

Rate of Diversification

• Rate of diversification is the difference between speciation ($\lambda$) and extinction ($\mu$) within a lineage.
• The relationship btween these tells us if the world is already "Full of species", asnwering if historical or ecological explanatinos work for the LDG
• J. Sepkoski diversity curves seem to indicate that we have already reached the upper limit of diversity and the Ecological explanation is dominant
• Although it's a bit more complicated, and both explanations work together

Historical explanations

Wallace: time and stability (1878)

• Tropical climates allow species to prosper, which puts them in constant competition with each other and don't allow a few species to dominate
• In high latitudes the species compete with harsh climates
• Since tropical environments were more common in history, this has also allowed

Rohde: speciation rates

• Higher solar radiation int he tropics literally results in more mutations in DNA, generating more variation for specis to act on
• Also, higher temperatures lead to faster physiological processes and shorter generation times, allowing for faster diversification

Intrinsic vs Extrinsic factors in historical speciation:

• We can think of tropical areas as "cradles" of diversity, since they support higher evolutionary speed or lower rate of extinction
• Or we can think of them as "museums" of diversity - they are generally more stable than higher latitudes and are not subject to "reset" events that wipe out many taxa
  • Reset events (extinctions) are more likely in areas with short growing seasons and cold temperatures

Out-of-the-tropics (OTT) model

• This postulates that the rate of speciation is higher in the tropics and the rate of species movement between the tropics and extratropical areas is high, sort of indicating that the "cradle" and "museum" are maybe both true
• Suggests that this is driven by "bridge" species that are able to expand beyond their niche and expand their temperature tolerance
  • However, these events are quite rare but they are more likely to occur in taxa with high speciation

Testing evolutionary speed and time hypotheses

• Jetz et al 2012 showed no association btwen diversification rates and latitude, high species richness in the tropics is likely dirven by the antiquity of wet tropical areas (museum/evolutaionry time hypotheis)
• Rabosky et al 2018 showed that marine fish have higher rates of speciation in the poles - despite having very slow pace of life (slow develoment, high parental care), they show high diversification rate
  • Friedman & Muñoz (2023) show rapid diversification along a depth gradient could be a mechanism, shallow tropical seas have more similar niches to each other

Capacity for dispersal

• High-latitude species have higher capacity for dispersal because they need a broad thermal tolerance to survive in both summer and winter, while tropical species typically can only tolerate a narrow band of temperatures

Ecological Explanations

• Based on the idea that the upper limit of speices diversity has already been reached, and these are just about the differences between current conditions in the regions.
• These could be the result of:
  • Different selective pressures for evolutionary innovation (Dobzhansky)
  • Differences in the way resources are partitioned (MacArthur)

Dobzhansky: ecology and evolution of innovation

• Dobzhansky's explanation is about different types of "struggle" in different latitudes
• Low latitudes promote specilization to cope with strong competition, parasitism, predation
• High latidudes select for increasd rate of development or reproductionin short growing/breeding seasons

MacArthur: ecological niches and carrying capacities (1972)

• Tropical environments have a higher carrying capacity compared to temperate environments
• High diversity in tropical ecosystems is explained by:
  1. High numbers of resources available
  2. Wider resource dimensions (diversity of food size)
  3. Low avg. amount of resources utilized - can be divided more along a greater number of species
  4. High degree of overlap in available resources beween species (more species can be tightly packed in resource space)

Further reading:

An overview of the Latitudinal Diversity Gradient explanations:

Pontarp, M., L. Bunnefeld, J. S. Cabral, R. S. Etienne, S. A. Fritz, R. Gillespie, C. H. Graham, O. Hagen, F. Hartig, S. Huang, R. Jansson, et al. (2019). The Latitudinal Diversity Gradient: Novel Understanding through Mechanistic Eco-evolutionary Models. Trends in Ecology & Evolution 34:211–223.