Chapter Ten

Why are there so many species in the tropics?

Why study the latitudinal diversity gradient?

Biodiversity varies enormously from region to region. One of the challenges for macroecology and macroevolution is to discover general, predictable patterns within this variation that might shed light on the processes responsible for generating biodiversity and explaining why it varies among regions. Probably the most impressive geographic pattern of biodiversity is the decline in species richness from the tropics towards high latitudes. However, even the most fundamental questions about why this pattern exists are not fully answered, so that we are still not sure whether the latitudinal diversity gradient has a historical explanation (e.g. tropical ecosystems are older) or an ecological one (e.g. there are more available niches in tropical ecosystems). Organizing, testing and comparing the multitude of possible explanations for global diversity patterns is an exercise in the careful application of scientific logic. It requires is to consider carefully how we can extract information from fossil, phylogenetic and geographic data.

What are the main points?

• The latitudinal diversity gradient (LDG) is both very general (most major groups of organisms show the pattern, and it is found in the marine and terrestrial realms) and very ancient.
• Proposed explanations for the LDG can be classified into historical (based on the accumulation of diversity through time) and ecological (based on current environmental conditions).
• An underlying assumption of historical explanations is non-equilibrium diversification: diversity is still increasing through evolutionary time. Ecological explanations assume equilibrium dynamics: diversity has levelled off to a “carrying capacity”.
• Fossil and phylogenetic data provide complementary approaches to testing hypotheses about large-scale geographic patterns in diversity, each with its own strengths and weaknesses.

What techniques are covered?

• Using fossils and phylogenies to test whether diversity is at equilibrium, or still increasing through time.
• Using fossils and phylogenies to test for differences in rates of diversification across latitudes.

 What case studies will be included?

Models that explain the LDG through the global dynamics of speciation, extinction, and geographic range expansion.