Download Lecture 17: Biogeography

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
C
parent pop’n
xyz
x
xyz
z
x2
x1yz1
z2
Phylogeny reflects relationship to
source of population
A
y
A
x1
B
x2
A
z1
C
z2
Vicariance Model
x
x1
x1
A x
2
y
y1, y2 B
z
C
Phylogeny reflects sequence of
separation
C
z
B
y1
B
y2
A
x1
A
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…