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 ABC parent pop’n xyz x xyzz x 2 x 1 yz 1 z 2

Phylogeny reflects relationship to source of population A A B A C y x 1 x 2 z 1 z 2

Vicariance Model A C B x y x1x1 z y 1, y 2 x1x2x1x2

Phylogeny reflects sequence of separation C B B A A z y 1 y 2 x 1 x 2

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 = cA z 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…