Tropical Rainforests in the Pleistocene

Tropical Rainforests
in the Pleistocene
Tropical Rainforests in the Pleistocene
• tropics stable during
Pleistocene?
• 1° C temperature drop based on
1976 CLIMAP study of warm vs.
cold loving forams (vs. 10° C in
North Atlantic)
Tropical Rainforests in the Pleistocene
Tropical Rainforests in the Pleistocene
Paleothermometers have since
pointed to colder and drier
tropics
• vegetation zones in mountains
• pollen analysis points to 4-6°
• well characterized events in
C cooling in Pleistocene
(Hawaii, Bolivia)
temperate montane systems
shifted down and up during
glacial - interglacial cycles
• snow line altitudes in Andean
and Hawaiian mountains show
lowering during Pleistocene
Pollen analysis of Mauna Loa cores over
the last 40,000 years (Hotchkiss 2001)
• various chemical signatures
(CaCO3, noble gases in water)
• re-analysis of 1976 CLIMAP
data indicates 3-4° C drop
Grand Canyon elevation shifts
North Great Basin elevation shift
Tropical Rainforests in the Pleistocene
Tropical Rainforests in the Pleistocene
• palynological data for Central
and South America mountains
indicate similar shifts
(repeatedly?) during Pleistocene
and Holocene
• repeated shifts probably
presented montane species with
cycles of sympatry in glacials
and allopatry in interglacials —
speciation pump
• how zones shifted is still
debated
Lisianthius species complex
Central Panama
Tropical Rainforests in the Pleistocene
Tropical Rainforests in the Pleistocene
Rainfall levels
Evidence is pointing to cooler
tropics during glacials
— the Refugia Debate
Jurgen Haffer, geologist and “birder,”
proposed “Refugia” hypothesis in 1969
(Science, July 11)
But what impact on the
biogeography of tropical lowland
plants and animals?
• observed that present distribution of
rainfall over South America gives rise
to both forested and non-forested areas
Wettest areas
• the limits of the non-forested habitats
closely coincided with annual ppt of
1500-2500 mm
• and that areas > 1500 mm rainfall
linked with present day centers of
diversity and biogeographical patterns
of distribution
Tropical Rainforests in the Pleistocene
Rainfall levels
Tropical Rainforests in the Pleistocene
— the Refugia Debate
— the Refugia Debate
• proposed that climatic fluctuations in
Pleistocene forced repeated contraction
into refugia in glacials followed by
expansion of forest in interglacials
Jurgen Haffer, geologist and “birder,”
proposed “Refugia” hypothesis in 1969
(Science, July 11)
Wettest areas
“During several dry climatic periods of the Pleistocene
and post-Pleistocene, the Amazonian forest was divided
into a number of smaller forests which were isolated
from each other by tracts of open, nonforest vegetation.
The remaining forests served as "refuge areas" for
numerous populations of forest animals, which deviated
from one another during periods of geographic isolation.
The isolated forests were again united during humid
climatic periods when the intervening open country
became once more forest-covered, permitting the refugearea populations to extend their ranges. This rupturing
and rejoining of the various forests in Amazonia
probably was repeated several times during the
Quaternary and led to a rapid differentiation of the
Amazonian forest fauna in geologically very recent
times.”
Tropical Rainforests in the Pleistocene
— the Refugia Debate
• recent speciation of birds in the Amazon
fits his model
• species centered over these proposed
“tropical forest refugia”
Toucan “sibling species”
in Amazon basin
• allopatric speciation would lead to
“species pump”
Refugia
Tropical Rainforests in the Pleistocene
— the Refugia Debate
• pictured-winged Heliconid
butterflies suggested same pattern
of correlation of sibling species and
tropical forest refugia
Tropical Rainforests in the Pleistocene
Tropical Rainforests in the Pleistocene
— the Refugia Debate
— the Refugia Debate
• Sir Ghillean Prance provided
• Sir Ghillean Prance provided
many examples of Amazonian plant
taxa that supported the “tropical
forest refugia” hypothesis
many examples of Amazonian plant
taxa that supported the “tropical
forest refugia” hypothesis
• Prance proposed 26 refugia based
on plants alone
Plant “tropical forest refugia” based
on centers of plant diversity are
correlated with areas of wet
conditions during dry periods
Distribution of Chrysobalanaceae
in wet tropical forests
Tropical Rainforests in the Pleistocene
Tropical Rainforests in the Pleistocene
Tabebuia ochracea
(Bignoniaceae)
Distribution of this savanna tree
• “Reverse refugia” were also found for
savanna trees like Tabebuia
• Savanna refugia were in areas <1500
mm rainfall
The elegance of the model was the congruent refugial maps for butterflies,
frogs, lizards, and families of plants
Tropical Rainforests in the Pleistocene
Tropical Rainforests in the Pleistocene
Opposition to the Refugia Model
• Pollen evidence for the
replacement of rain forests
with savannas has been
found
• collecting data
biased for
refugia areas –
“museum
model”
• Hypsithermal
aridity?
• Last interglacial –
ca. 110kya?
Summary pollen diagram
from Rondônia, Brazil
Collecting intensity of plant taxa used in the construction of refugial arguments
Tropical Rainforests in the Pleistocene
Tropical Rainforests in the Pleistocene
Opposition to the Refugia Model
Opposition to the Refugia Model
• ecological reasons
for concentration of
species in putative
“refugia”
• ecological reasons
for concentration of
species in putative
“refugia”
•
soil
•
•
local climate
•
local climate
•
vegetation type
•
vegetation type
•
rainfall
•
rainfall
•
geology
•
geology
•
river boundaries
•
river boundaries
Coincidence of range of major zones of habitat uniformity and
postulated refugia
soil
Coincidence of range of major zones of habitat uniformity and
postulated refugia
Tropical Rainforests in the Pleistocene
Opposition to the Refugia Model
Tropical Rainforests in the Pleistocene
Opposition to the Refugia Model
• Measured variations with depth of the
carbon isotope composition (_13C) of soil
organic matter (SOM)
• Sediment cores
from the Amazon fan
provide an insight on
the past vegetation of
Amazonia at the scale
of that landscape and
do not support the
assumption that large
areas of savanna
replaced forest
• Detects vegetation changes between
communities with varying proportions of
C3 vs. C4 photosynthesis OR of forest vs.
savanna species
The impact of climate changes during the
Holocene on vegetation in northern French
Guiana
Pollen data from modern Amazonian river muds (upper panel)
and from ice age deposits of the Amazon Fan
Freycon et al. (2010) Quaternary Research 73:220–225
Tropical Rainforests in the Pleistocene
Opposition to the Refugia Model
Opposition to the Refugia Model
• Measured variations with depth of the
carbon isotope composition (_13C) of soil
organic matter (SOM)
Savanna
Tropical Rainforests in the Pleistocene
• Detects vegetation changes between
communities with varying proportions of
C3 vs. C4 photosynthesis OR of forest vs.
savanna species
• Distribution of intra-specific
variation does not fit model of
refugia and expected patterns of
spatial proximity
• No support for closed forest sites being
savanna in the past – although one
savanna site was more mesic in the past
Forest refugia
The impact of climate changes during the
Holocene on vegetation in northern French
Guiana
Freycon et al. (2010) Quaternary Research 73:220–225
Licania apetala
Chrysobalanaceae
Tropical Rainforests in the Pleistocene
Opposition to the Refugia Model
Tropical Rainforests in the Pleistocene
Opposition to the Refugia Model
• Phylogeographic analysis of
Amazonian birds based on DNA
does not support Refugia model
based on either proximity of
refugia in area cladogram nor
recency of speciation
• Pre-Pleistocene events!
Postulated African refugia
Contact points of passerine bird species
• Assuming refugia, allopatric speciation, and equal rates of expansion from a
refugia, present day contact zones of birds are not situated “in-between” refugia as
expected
Tropical Rainforests in the Pleistocene
Opposition to the Refugia Model — the winner?
Paul Colinvaux
Amazon Expeditions: My Quest for the Ice-Age Equator
Yale Univ Press, 2008
Tropical Rainforests in the Pleistocene
Gracious losers?
Tropical Rainforests in the Pleistocene
Good read!
[see lecture webpage for pdf]
Tropical Rainforests in the Pleistocene
Tropical Rainforests in the Pleistocene
• But the debate does NOT go away – especially in African tropics!
Tropical Rainforests in the Pleistocene
• sampling across the region
including 4 postulated upland
forest refugia
Tropical Rainforests in the Pleistocene
• Bayesian clustering using
10 microsatellite loci identify
4 genetic groups
Tropical Rainforests in the Pleistocene
• Bayesian clustering using
10 microsatellite loci identify
4 genetic groups
• Their distributions match
that of forest refugia
postulated from patterns of
species richness and
endemism
• Results cannot be explained
on the basis of present-day
ecological conditions
Tropical Rainforests in the Pleistocene
• Similar study done on a wind
pollinated and bat dispersed
timber tree widespread in
Africa
Tropical Rainforests in the Pleistocene
• Similar study done on a wind
pollinated and bat dispersed
timber tree widespread in
Africa
• Despite widespread
distribution and capability of
long distance gene flow,
marked geographical genetic
structuring
• Isolated in separate
geographical regions probably
for several glacial cycles of
the Pleistocene
Tropical Rainforests in the Pleistocene
Tropical Rainforests in the Pleistocene
• Strong patterns of regional
differentiation that are
consistent with refugial
hypotheses for central Africa
• Coalescent theory suggests
east-west divergence at
53,000 – 120,000 years
• Similar study done
with mtDNA and
nuclear DNA on 4
subspecies of
African gorillas
Geographic distribution of major mitochondrial
haplogroups across central Africa