Latitudinal gradients Species – latitude relationship of birds across the New World show the typical pattern of increased species diversity towards the equator.

Coral reef fish Labridae Pomacentridae Diversity of coral reef fish declines from their centres of diversity. There is also a strong correlation between distance and duration of the pelagic phase, which is a proxi of dispersal ability. Mora et al. 2003

Latitudinal gradient in species diversity of mollusks on North and South American Pacific shelves (Valdovino et al. 2003) Centers of diversity are often shifted north or south Species richness sharply declines towards temperate regions Tropics contain a very large proportion of total species richness Species near the center of species richness are often less dispersive

The general patterns Hillebrand (2004) conducted a meta-analysis about 581 published latitudinal gradients Basic conclusions Nearly all taxa show a latitudinal gradient Body size and realm are major predictors of the strange of the latitudinal gradient The ubiquity of the pattern makes a simple mechanistic explanation more probable than taxon or life history type specific

Counterexamples These taxa are most species rich in the northern Hemisphere Soybean aphid, Photo by David Voegtlin The sawfly Arge coccinea, Photo by Tom Murray The ichneumonid Arotes sp., Photo by Tom Murray The aquatic macrophyte Hydrilla verticilliata, Photo by FAO

Some theories that try to explain observed latitudinal gradients in species diversity. Older theories: Environmental stability or predictability (Klopfer1959) Productivity (Slobodkinand Sanders 1969) Heterogeneity (Pianka1966) Latitudinal decrease in angle of sun (Terborgh1985) Aridity (Begonet al ) Seasonality (Begonet al ) Number of habitats (Pianka1966) Latitudinal ranges (Rapoport1982) Area (Connor and McCoy 1979) Circular explanations: Competition (Dobshansky1950) Predation Paine 1966) Niche width (BenEliharuandSafriel1982) Host diversity (Rhode 1989) Epiphyte load (Strong 1977) Population size (Boucot1975) Time related explanations: Temperature dependence of chemical reactions (Alekseev 1982) Temperature dependent mutation rates Evolutionary time (Pianka1966) Ice age refuges (Pianka1988) Energy related explanations: Energy supply (Rhode 1992) Range size related explanation: Random range sizes (Colwell andHurtt1994) (Gillooly et al. 2005)

Red data points: Multihabitat gradient in ant species diversity Blue data points: Gradient for one habitat type North American grasshoppers Latitudinal gradients can also be found within single habitat types Habitat heterogeneity Energy or area per se Ant species richness is significantly correlated to mean annual temperature and mean primary production, but not to area

Refuge theory The refuge theory of Pianka tries to explain the gradient in species diversity from ice age refuges in which speciation rates were fast. This process is thought to result in a multiplication of species numbers in the tropics. In the temperate regions without refuges species number remained more or less constant.

Species diversity and temperature

Western Atlantic gastropodsEastern Pacific gastropods Biodiversity and temperature Species diversity of marine gastropods is significantly correlated with mean surface water temperature

Metabolic theory and species latitudinal gradients in species richness Body weight corrected energy use should exponentially scale to the inverse of temperature. The slope –E/k should be a universal constant for all species independent of body size. Biological times should scale to body weight to the quarter power Examples: Generation time, lifespan, age of maturation, average lifetime of a species The inverse of time are rates. Examples: Growth rates, mutation rates, species turnover rates, migration rates Hence biological rates should scale to body weight and temperature by

The rate of DNA evolution predicted from metabolic theory Body size specific metabolic rate M/W should scale to the quarter power to body weight and exponentially to temperature Now assume that most mutations are neutral and occur randomly. That is we assume that the neutral theory of population genetics (Kimura 1983) DNA substitution rate  should be proportional to M/W Body weight corrected DNA substitution rates (evolution rates) should be a linear function of 1/T with slope –E/k = Higher environmental temperatures should lead to higher substitution rates (faster evolution) Body weight corrected DNA substitution rates (evolution rates) should decrease with body weight Large bodied species should have lower substitution rates (slower evolution)

Diversity and temperature The energy equivalence rule The average abundance N of an assemblage of S species and J individuyals in areal A is N=J/SA For standard areals and species of similar body size holds therefore Species richness should increase with environmental temperature Species richness should increase with energy The slope of this relationship should be -E/k = -7541K Caveats: Mean abundance per unit area is independent of temperature. The energy equivalence rule holds at least approximately and its slope is independent of temperature.

North American trees Costa Rican trees along an elevational gradient North American amphibians Ecuadorian amphibians Fish species richness Prosobranchia species richnessEctoparasites of marine teleosts

Latitudinal gradients: Gaston K Global patterns in biodiversity - Nature 405: Allen A. P., Brown J. H., Gillooly J. F Global biodiversity, biochemical kinetics, and the energy equivalence rule. Science 297: Today’s reading