Lecture 17: Biogeography Cont’d

Historical Biogeography:

Fossil Record:

e.g. How assess disjunct distribution?

a) Once widespread, now relictual…

e.g. Tapirs - fossil record shows local extinction

b) Breakup of Gondwanaland...

e.g. Marsupials - S. Am., Aust. Antarctica

Systematics very important when fossil record incomplete (always!)

Errors are common…e.g. Age/Area Hypothesis:

Centre of Origin = Maximum Diversity

But, Adaptive Radiations (colonizing spp. adapt to fill niches)Results in lots of diversity in relatively new

arrival

Darwin’s Finches• from S. Am. mainland

• colonized Galapagos (>600 miles away)

• occupied an ecological niche with little competition

Dispersal Model

A B Cparent pop’n

xyz

x xyz z

x2 x1yz1 z2

Phylogeny reflects relationship to source of population

A A B A C

y x1 x2 z1 z2

Vicariance Model

A

C

B

x

yx1

z

y1, y2x1

x2

Phylogeny reflects sequence of separation

C B B A A

z y1 y2 x1 x2

Vicariance

• Dist’ns of monophyletic groups over areas are explained by the reconstruction of area cladograms

• Congruence of area cladograms of different taxa strengthens argument

• Lack of congruence suggests that dispersal & local extinctions important

Taxonomic composition of regional biota

• Reflects ancient & recent history & ecology

Let’s look at what explains the fauna of …

South America

1) some elements of Mesozoic Gondwanaland

- shared with other southern continents

e.g. pipid frogs, lungfish

2) Autochthonous (indigenous) groups

• arose & diversified after isolation

e.g. antbirds, edentates

3) Diversification after mid-Tertiary

• Dispersal followed by diversification e.g. cricetid rodents & primates from Africa

4) Quarternary forms

e.g. mountain lion

5) Recently arrived forms (Holocene)

e.g. cattle egret

Glacial Refugia

• Pleistocene glaciations

• Many taxa survived in refugia & speciated

• e.g. western & eastern diamondback rattler

Why are some species absent?

• Limited dispersal ability

• Extinction

• Exclusion (competition, predation)

Equilibrium of communities

In given env’t, max # spp in a community:

S = cAz

• immigration balanced by extinction

• interactions → extinctions

• leads to speciation, improved adaptations

• slows extinction rate

“ evolutionary species equilibrium”

Are communities saturated?

Probably not if:

1) some spp: little competition

2) ranges still expanding from refugia

3) niches “under-utilized”

However, invaders are rarely successful….

Why are Tropics so Diverse?

Ecological: tropics aseasonal (?)

niches narrower?

high primary productivity

more spp. can maintain viable pop’n size

Historical: temperate not recovered from glaciation

not enough time to cold-adapt

adaptive requirements limit types of taxa

Two Views of Tropics

• Cradle : major groups arise here & diversify

• Museum : survivors of groups originating elsewhere

• But…we’ve found some places with similar diversity…