The tropics are species-rich and: 4. More stable. More ... · is better (higher log-likelihood) ... (slides by Carolyn Duckham, ‘04) ... Least Squares regressions through the origin

Why are the Tropics sobiodiverse?

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

The tropics are species-rich and:

Jetz and Fine (2012) - integrate "area" over time

4

5

Predicting species richness for vertebrates using historical area

Jetz and Fine, 2012

(note: delta AIC is a measure of how well the model fits thedata, using maximum likelihood (remember that?)) Smalleris better (higher log-likelihood)

Ecologically older - Post-glaciation communities lesssaturated (ie at nonequilibrium)

Generally felt that the Northern Hemisphere is at non-equilibriumpost-glaciation state. (But ask Bruce Archibald!)

--Compare tree fauna of Europe and North America:

Horizontal barriers more pronounced in Europe, and thelatitudinal gradient is much stronger there. (Rosenzweig, 1995)

But, some evidence that glacial cycles also promote speciation(will get to this on Wednesday)

Ecological time








N S

max

min

mean

G&B,01

4. Variation. The tropics are more "stable"

This is Bruce's PhD thesis

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Sunday et al., 2010

only slight variation across continents, oceans

4. Vague theory: Specialization is more likely in the tropics

Pielou* (1969) noted that "northern" species are morewidespread 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 largeraway from the equator.

* Elizabeth C. Pielou, perhaps Canada's second or third-most famous ecologist ever...

Area: Rapoport’s Rule does hold for birds:

actual size relative to availableland

(N and S v. different)

Southern Mexico

G&B,01

--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 latitudesconsidered (Owens et al., 1999)

--specialist/generalist debate is ongoing...

But the generalist/specialist evidence is not that clear:

-- 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)








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), andPop 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?

latitude

# individualsper species

0 90

prediction

temperate +tropical

Tropics vs. temperate abundances: wildfowl

Temperate species aremore locally abundant

1996

Breeding Bird Survey

Currie et al., 2005Ecology Letters

ln(in

divi

dual

s pe

r spe

cies

)

Prediction: tropical species are more locally abundant?

plots of trees

So, according to David Currie et al. (2005), the "more individuals" hypothesis is not supported by the data.

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Predicting species richness for vertebrates using historical areaand a measure of "energy" or productivity

Jetz and Fine, 2012

(note: delta AIC is a measure of how well the model fits thedata, using maximum likelihood (remember that?)) Smalleris better (higher log-likelihood)

(Rohde, K., 1992) suggested faster generation time at higher T and so more adaptation per unit time...

5. Energy hypothesis (b) - the “faster evolution” theory

Cited >280 times

More specific drivers for the faster evolution theory:

1. more UV, more mutations2. faster generation time (physiology faster)3. biotic interactions are ‘stronger’ in the tropics

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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

"energy" and substitution rate are correlated in ectotherms...

E.g. 94 sister-pairs of amphibianslower 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)

Wright et al., GEB 2010

N, or montane

S, or lowland

Sister clade with morespecies (x) usually has highersubstitution rate (branch length)

And substitution rate and diversification may be correlatedin some groups

Mutation rate is linked to diversification in birdsLanfear et al., PNAS 2010

Spencer Myrtle (BISC440, 2013)testing this now in my lab...

(slides by Carolyn Duckham, ‘04)

Environmental energy and evolutionary rates in flowering plants

T. Jonathan Davies, Vincent Savolainen, Mark W. Chase, Justin Moatand Timothy G. Barraclough

Proc. R. Soc. Lond. B (2004) 271, 2195–2200

We can put them together...

Major radiation of recent geological times (100 mya) Are the dominant primary producers of terrestrial environments Completely dependent on solar energy input Significant correlation between molecular evolution rates and species richness

Francis & Currie 2003

Angiosperms

Environmental Energy

Direct Measures: UV, Temperature, AETIndirect Measures: Area, Elevation, Latitude

Faster Evolution Theory

Species Richness RateEnvironmental Energy RateMolecular Rate

86 sister-family comparisons of angiosperms

A B C D

Soltis 1999

SR MR SR

EE EE MR

A sr BSpecies Richness measurelog (# of species in A) – log (# of species in B)

Molecular Rate measureMolecular Branch Lengthrepresentative taxon per family (node density effect)rbcL, atpB, 18S rDNA Maximum Likelihood branch length leading to each species

Xa – Xb / mean of branch lengths

Focused on rbcL and atpB third position sitesThat leave amino acid sequence unaffected when a substitution occurs. (Neutral Theory)

A mr B

Environment Energy measureXa-Xb

A ee 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)

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 2– molecular rate (or substitution rate) as response variable and environmental variables as explanatory variables.

Results: Temperature alone: r2=0.19 of the variation in species richness (!)

Results: indirect measures Latitude r2= 0.15 UV seems the most important underlying EE variable.

SR rate

EE rate

MR rate

EE rate

Result : r2 = 0.08, p= 0.004

Test 3– Species Richness as the response variable with Molecular rate as the Explanatory variable.

SR rate

MR rate

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 SRIF both important: need further investigation to determine relative importance of direct and indirect factors.

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Environmental Energy

SpeciesRichness

Does rate of molecular evolution explain species richness after controlling for EE? (Ie. look at residualson this plot) 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.

T.J Davies et al, 2004

No......

Is speciation rate higher in tropics? Jason Weir (2007)birds mammals

sister species ages

oldest haplotype

oldest phylotype

Sister speciesare older in the tropics

Species containOLDER genes the tropics

Species containOLDER subclades the tropics

speciation

extinction

estimated rates from sister-species ages

so more bird species in south because they don’t go extinct;but not due to larger populations...so we’re still stuck.

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These sister species divergence datescome from a molecular clock and mtDNA

Do you see a problem?

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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 diversificationRicklefs et al., JEB 2007

Latitude alone does not predict one measure of diversification rate in birds (shorter edge lengths): Jetz et al., 2012

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Bottom 25%

Jetz et al., 201240

Median diversification rate of all species in assemblageon a 110*100 km grid

Jetz et al., 2012

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 diversificationRicklefs 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 diversificationnearer the equator: Soria-Carrasco and Castresa, 2012

If looking at the whole tree, evidence for faster speciation in tropical mammals: Rolland et al., 2014

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Doesn't hold for all Orders, e.g. rabbits and pikas (which are steppe-associated), nor Carnivores

speciation extinction net diversification

Why are there more species in the tropics?

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

*perhaps via lower extinction?

45Averaged across 13 taxa (so not raw numbers)

like most sharks!

Marine latitudinal gradients Tittensor et al., Nature 2010

12000 spp, all taxa (inverts, plants, verts)

Documents

The tropics are species-rich and: 4. More stable. More ... · is better (higher log-likelihood) ... (slides by Carolyn Duckham, ‘04) ... Least Squares regressions through the origin