Post on 21-Jan-2016
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…