The tropics are species-rich and: 1. In the middle (mid-domain affect) Why are the Tropics so biodiverse? 2. Bigger. More area = more species (just the interprovincial Species-Area curve again) 3. Older. More time = more species (older on two scales) 4. More stable. More specialization = more species (environmental variance) 5. Richer. More energy = more species (environmental mean) a. productivity b. rate of evolution Jetz and Fine (2012) - integrate "area" over time 4
Ecological time Predicting species richness for vertebrates using historical area Ecologically older - Post-glaciation communities less saturated (ie at nonequilibrium) Generally felt that the Northern Hemisphere is at non-equilibrium post-glaciation state. (But ask Bruce Archibald!) --Compare tree fauna of Europe and North America: (note: delta AIC is a measure of how well the model fits the data, using maximum likelihood (remember that?)) Smaller is better (higher log-likelihood) Jetz and Fine, 2012 Horizontal barriers more pronounced in Europe, and the latitudinal gradient is much stronger there. (Rosenzweig, 1995) But, some evidence that glacial cycles also promote speciation (will get to this on Wednesday) 5 The tropics are species-rich and: 4. Variation. The tropics are more "stable" 1. In the middle (mid-domain affect) 2. Bigger. More area = more species (just the interprovincial Species-Area curve again) N S max 3. Older. More time = more species (older on two scales) mean min 4. More stable. More specialization = more species (environmental variance) 5. Richer. More energy = more species (environmental mean) a. productivity b. rate of evolution This is Bruce's PhD thesis G&B,01
only slight variation across continents, oceans 4. Vague theory: Specialization is more likely in the tropics Pielou* (1969) noted that "northern" species are more widespread and eurytopic (generalists), while tropical species more restricted and stenotopic (specialists): this has become lore that there are fewer species up north because the north is more variable and unpredictable... This is coupled with "Rapoport s Rule" (now called a Rapoport "effect"), that range sizes become larger away from the equator. Sunday et al., 2010 9 * Elizabeth C. Pielou, perhaps Canada's second or third-most famous ecologist ever... Area: Rapoport s Rule does hold for birds: Southern Mexico But the generalist/specialist evidence is not that clear: --For birds, though species that move around less in the tropics have more subspecies (Wed. lecture) older data suggest that diversification rate seems higher for generalists with broad ranges than for specialists if all latitudes considered (Owens et al., 1999) -- More tropical clades of salamanders (i) inhabit more habitats collectively (are in some sense less specialized) than more temperate ones and (ii) are more speciose in the tropics (Kozak & Wiens, 2010) actual size (N and S v. different) relative to available land G&B,01 --specialist/generalist debate is ongoing...
The tropics are species-rich and: 1. In the middle (mid-domain affect) 2. Bigger. More area = more species (just the interprovincial Species-Area curve again) 3. Older. More time = more species (older on two scales) 4. More stable. More specialization = more species (environmental variance) 5. Richer. More energy = more species (environmental mean) a. productivity b. rate of evolution http://earthobservatory.nasa.gov/features/energybalance/page2.php 5. More Energy: leads to "wider resource base" a. Larger populations b. Faster evolution (more raw material for speciation) --more energy does not necessarily lead to more species (why not just one predominant species, or more individuals?) 5. Energy hypothesis (a): productivity Argument is just Area hypothesis in another guise: given any force driving incipient speciation, P(extinction) as (Pop size), and Pop size as (Energy), so P(extinction) as (Energy) more species may persist long enough to speciate again (positive feedback due to exponential nature of diversification)
Prediction: tropical species are more locally abundant? Tropics vs. temperate abundances: wildfowl temperate + tropical # individuals per species prediction Temperate species are more locally abundant 1996 0 latitude 90 Prediction: tropical species are more locally abundant? ln(individuals per species) Predicting species richness for vertebrates using historical area and a measure of "energy" or productivity Breeding Bird Survey plots of trees So, according to David Currie et al. (2005), the "more individuals" hypothesis is not supported by the data. Currie et al., 2005 Ecology Letters (note: delta AIC is a measure of how well the model fits the data, using maximum likelihood (remember that?)) Smaller is better (higher log-likelihood) Jetz and Fine, 2012 20
5. Energy hypothesis (b) - the faster evolution theory (Rohde, K., 1992) suggested faster generation time at higher T and so more adaptation per unit time... Cited >280 times More specific drivers for the faster evolution theory: 5 (b) Faster Evolution theory and mutational input: (i) if true, substitution rate per year should be higher in tropics (ii) substitution rate should be correlated with diversification rate 1. more UV, more mutations 2. faster generation time (physiology faster) 3. biotic interactions are stronger in the tropics 22 "energy" and substitution rate are correlated in ectotherms... E.g. 94 sister-pairs of amphibians lower latitude species has higher rate of substitution, p<0.02 (n=94) lower elevation species has higher rate of substitution,p<0.002 (n=16) N, or montane S, or lowland And substitution rate and diversification may be correlated in some groups Sister clade with more species (x) usually has higher substitution rate (branch length) Mutation rate is linked to diversification in birds Lanfear et al., PNAS 2010 Wright et al., GEB 2010 Spencer Myrtle (BISC440, 2013) testing this now in my lab...
Angiosperms We can put them together... Environmental energy and evolutionary rates in flowering plants T. Jonathan Davies, Vincent Savolainen, Mark W. Chase, Justin Moat and Timothy G. Barraclough Proc. R. Soc. Lond. B (2004) 271, 2195 2200 (slides by Carolyn Duckham, 04) Major radiation of recent geological times (100 mya) Francis & Currie 2003 Are the dominant primary producers of terrestrial environments Completely dependent on solar energy input Significant correlation between molecular evolution rates and species richness Environmental Energy Direct Measures: UV, Temperature, AET Indirect Measures: Area, Elevation, Latitude Faster Evolution Theory 86 sister-family comparisons of angiosperms A B C D Species Richness Rate Environmental Energy Rate Molecular Rate SR MR SR EE EE MR Soltis 1999
Species Richness measure log (# of species in A) log (# of species in B) A sr B Environment Energy measure Xa-Xb A ee B Molecular Rate measure Molecular Branch Length representative taxon per family (node density effect) rbcl, atpb, 18S rdna Maximum Likelihood branch length leading to each species Xa Xb / mean of branch lengths A mr B Now have 86 data points for Species Richness rates, molecular rates, Environmental energy rates of UV, AET, Temperature, elevation, area, latitude, and their interaction terms. Least Squares regressions through the origin ( to explore relationship between SR, MR, EE) Focused on rbcl and atpb third position sites That leave amino acid sequence unaffected when a substitution occurs. (Neutral Theory) SR MR SR EE EE MR -same as exercise with the birds (except EE and MR vs Lat. and area) Test 1 Species Richness as Response Variable all measures of energy (plus area occupied by family) as explanatory variable Test 3 Species Richness as the response variable with Molecular rate as the Explanatory variable. SR rate Results: Temperature alone: r 2 =0.19 of the variation in species richness (!) SR rate MR rate Result : r 2 = 0.08, p= 0.004 EE rate Test 2 molecular rate (or substitution rate) as response variable and environmental variables as explanatory variables. MR rate EE rate Results: indirect measures Latitude r 2 = 0.15 UV seems the most important underlying EE variable. EXCITING: all are consistent with the faster evolution theory Test 4 Species Richness as response variable, Environmental Energy AND molecular rate as explanatory variables - multiple regression SR rate MR and EE rate If MR most important: support the FET as the main explanation for relationships (hard to believe) If EE most important: Direct effect of energy on SR IF both important: need further investigation to determine relative importance of direct and indirect factors.
Does rate of molecular evolution explain species richness after controlling for EE? (Ie. look at residuals on this plot) Species Richness No... Molecular Rate dropped out of the model - SRichness explained by Environment. Therefore main effect of energy on richness is direct, rather than via an intermediate effect on molecular rates. Molecular rate does not mediate the relationship between energy and species richness The relationship between SR and MR appears to be an artifact of both variables being correlated with energy. Environmental Energy 33 T.J Davies et al, 2004 Is speciation rate higher in tropics? Jason Weir (2007) birds sister species ages mammals Sister species are older in the tropics estimated rates from sister-species ages speciation oldest haplotype Species contain OLDER genes the tropics extinction oldest phylotype Species contain OLDER subclades the tropics so more bird species in south because they don t go extinct; but not due to larger populations...so we re still stuck.
Current state of affairs (using similar, tree-based approaches) These sister species divergence dates come from a molecular clock and mtdna Do you see a problem? Energy has strong effect on plant diversification rate Davies et al. PRSLB 2004 (seen this) Latitude alone has marginal effect on Squamate diversification Ricklefs et al., JEB 2007 Latitude alone does not predict one measure of diversification rate in birds (shorter edge lengths): Jetz et al., 2012 37 38 Bottom 25% Median diversification rate of all species in assemblage on a 110*100 km grid Jetz et al., 2012 39 Jetz et al., 2012 40
Current state of affairs (using similar, tree-based approaches) Energy has strong effect on plant diversification rate Davies et al. PRSLB 2004 (seen this) Latitude alone has marginal effect on Squamate diversification Ricklefs et al., JEB 2007 Latitude alone does not predict one measure of diversification rate in birds (shorter edge lengths): Jetz et al., 2012 232 Genera of mammals: no evidence of faster diversification nearer the equator: Soria-Carrasco and Castresa, 2012 If looking at the whole tree, evidence for faster speciation in tropical mammals: Rolland et al., 2014 42 Why are there more species in the tropics? Doesn't hold for all Orders, e.g. rabbits and pikas (which are steppe-associated), nor Carnivores 1. It is bigger - Species Area Curve (mechanism unknown*) 2. It has more energy - productivity (mechanism unknown)? 3. It is older (on two time scales) (mechanism known) Probably a combination of these three speciation extinction net diversification 43 *perhaps via lower extinction?
Marine latitudinal gradients Tittensor et al., Nature 2010 like most sharks! Averaged across 13 taxa (so not raw numbers) 45 12000 spp, all taxa (inverts, plants, verts)