Why are the Tropics sobiodiverse?

We "know" there are more species “in the tropics”

1. Oldest known ecological pattern(Humboldt, 1807)2. Well-known by Darwin and Wallace3. Driven much ecological research

And the tropics are special:

-most species live there!

4

Bird Species richness per 110x110 km grid square(so, strictly speaking, alpha diversity)

Rosenzweig, 1995 G&S, 01

135/191 studies of taxa (to the end of 2002) showed a latitudinal gradient (71%)

-at all spatial scales

-aquatic, terrestria; plants and animals

-both alpha diversity (so communities are richer) and gamma diversity (entire landscape is richer)

major exceptions: aquatic plants, parasite communities,parasitoid wasps, marine birds and marine mammals

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): Monday's workshop

5. Richer. More energy = more species (environmental mean)

a. productivity (so interprovincial SAR again!) b. actual rate of evolution is higher

The tropics are species-rich and:

1. Null model:

Random ranges thrown down on 2-D map give rise to latitudinal patterns, due to ‘hard barriers’ at the edges:this is known as the “mid domain effect” (or MDE)

General Explanations

(b) uniform range sizes on a 'domain’ (e.g. continent). x is a latitude, and dotted line crosses the species to give spp richness(c) resulting latitudinal gradient. (b) produces small ranges towards the edges, which is not whatwe see on a global scale. Also, larger ranges produce strongerlatitudinal curves (three curves in c)]

Colwell & Lees, TREE 2000

MDE

max

imum

rang

e si

ze

11

But Mid-domain effect can operate at other scales.

12

reef size

richnesscontour

Ecol. Lett. 2005

MDE + Area is bestmodel

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): Monday's workshop

5. Richer. More energy = more species (environmental mean)

a. productivity (so interprovincial SAR again!) b. actual rate of evolution is higher

The tropics are species-rich and:

14

Are the "tropics" bigger?

Most of the world is tropical...

gradient we see isdue to autocorrelation

Another look at the same pattern...

1. Null model - hard borders at the extremes(not perfect fit, sort of ignored)

2. More area: leads to higher net diversification rates(for some reason)

General Explanations

1. Pacific has more species than Atlantic2. Africa has more savannah species than Neotropics3. Neotropics has more tropical tree species than Africa

Observational evidence from Rosenzweig 1995:

Species richness, Area and Latitude

World’s Birds (Gaston & Blackburn, Macroecology,01)

r2 = 0.71

19

Madagascar

Greenland

India

AusEurope

Neotropics

Africa

NA

Area predicts richness

Log1

0.sp

p

Log10.area 20

Madagascar

Greenland

India

AusEurope

Neotropics

AfricaNA

latitude predicts richness too

Log1

0.sp

p

21

Here, area and latitude are not correlated

Log1

0.ar

ea

22

Madagascar Greenland

India

AusEurope

Neotropics

AfricaNA

take the residuals of richnesson latitude

Log1

0.sp

p

23

area predict richness after controllingfor latitude

Which dot representsMadagascar?

Log10.area 24

Which dot representsMadagascar?

Log10.area

area predict richness after controllingfor latitude

25

Madagascar

Greenland

India

AusEurope

Neotropics

AfricaNA

take the residuals of richness on area

Log1

0.sp

p

Log10.area 26

Latitude predicts richnessafter controlling for area

Which dot representsGreenland? Africa?

27

Latitude predicts richnessafter controlling for area

Which dot representsGreenland? Africa?

2. So for birds, both area and latitude affect species numbersindependently. So Rosenzweig doesn't have total story...