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Non-linear responses of vegetation to orbital forcing across the temperate to tropical transition
in South America
4th PAGES Open Science MeetingThe Past: A Compass for Future Earth
14th February 2013
K.D. Bennett
Geography, Archaeology and PalaeoecologyQueen's University Belfast
Northern Ireland
Introduction
How have tropical climates changed over the late Cenozoic?
How did organisms respond?
What are the implications?
'Stability' v ‘change’ as drivers of speciation
Two big questions for global biodiversity are:1. Why do we have millions of eukaryote species?2. Why are most of them at low latitudes?
J. Zachos, M. Pagani, L. Sloan, E. Thomas, and K. Billups. Trends, rhythms, and aberrations in global climate 65 Ma to present. Science, 292:686-693, 2001.
Cenozoic global temperature trends
Overall an erratic cooling, accelerating towards the present, with higher amplitude fluctuations
A. Berger. Long-term variations of caloric insolation resulting from the earth's orbital elements. Quaternary Research, 9:139-167, 1978.
Latitudinal variation in insolation 250-0ka BP
High latitude: 40-kyr cycle dominant
Low latitude: 20-kyr cycle dominant
In phase
Out of phaseShould lead us to expect complex patterns of change by latitude
P. Braconnot, B. Otto-Bliesner, S. Harrison, S. Joussaume, J.-Y. Peterchmitt, A. Abe-Ouchi, M. Crucifix, E. Driesschaert, T. Fichefet, C. D. Hewitt, M. Kageyama, A. Kitoh, A. Laîné, M.-F. Loutre, O. Marti, U. Merkel, G. Ramstein, P. Valdes, S. L. Weber, Y. Yu, and Y. Zhao. Results of PMIP2 coupled simulations of the Mid-Holocene and Last Glacial Maximum - Part 1: experiments and large-scale features. Climate of the Past, 3:261-277, 2007.
LGM versus modern climates
Annual temp Precipitation
T: differences large at high latitude; small at low latitude, as now or cooler everywhere
P: variable, some large differences at low latitude, both drier and wetter
W. Wüster, J. E. Ferguson, J. A. Quijada-Mascareñas, C. E. Pook, M. da Graça Salomão, and R. S. Thorpe. Tracing an invasion: landbridges, refugia, and the phylogeography of the Neotropical rattlesnake (Serpentes: Viperidae: Crotalus durissus). Molecular Ecology, 14:1095-1108, 2005.
Phylogenetic data: Neotropical rattlesnakes
Chronology of dispersal events in Crotalus durissus: gradual spread over 2 Myr
1.85 Ma
1.54 Ma
1.08 Ma
Present
G. Hewitt. The genetic legacy of the Quaternary ice ages. Nature, 405:907–913, 2000.
Phylogenetic data: mid-high latitude
Spread is a late Quaternary phenomenon
Age(Myr)
1
2
3
0
Alnus
Quercus
Trees Shrubs
Gradual spread of Alnus and Quercus into S America
Lower amplitude fluctuations before 2 Ma
Palaeoecological data: pollen from High plain of Bogotà
H. Hooghiemstra. Quaternary and upper-Pliocene glaciations and forest development in the tropical Andes: evidence from a long high-resolution pollen record from the sedimentary basin of Bogotá, Colombia. Palaeogeography, Palaeoclimatology, Palaeoecology, 72:11-26, 1989.
The last 16 kyr in southernmost Chile 53.6ºS
S. L. Fontana and K. D. Bennett. Postglacial vegetation dynamics of western Tierra del Fuego. The Holocene 22: 1337-1350, 2012.
10 ka
Laguna Ballena
Forest (Nothofagus)
10 ka10 ka
Shrubs and herbs
The last 16 kyr in south-eastern Brazil 29.5ºS
V. Jeske-Pieruschka and H. Behling. Palaeoenvironmental history of the São Francisco de Paula region in southern Brazil during the late Quaternary inferred from the Rinc Tao das Cabritas core. The Holocene 22: 1251-1262, 2012.
2.9 ka
Rincão das Cabritas
Forest (Nothofagus)
Herbs
Age 14C yr BP
Latit
ude
Timing of major vegetation change by latitude in South America
ca 10 ka
Quaternary response: mid- and high- latitudes
Major climatic changes (and ice-sheets): high amplitude response to orbital forcing
Pattern of expansion and contraction of forest on 40-kyr (early Quaternary) to 100-kyr timescales (late Quaternary)
Present patterns completely dominated by the last oscillation (since 100 ka), most change ca 10-14 ka
Tertiary: hot (and wet?), ‘stable’
Late Quaternary: 100-kyr oscillation superimposed from northern ice-sheets;
Early Quaternary: cooling, increasing amplitude 20-kyr oscillations
Present patterns result from a combination of these three layers: none is strong enough to dominate continuously
All periodicities: variable amplitude climate, especially precipitation, response to orbital forcing
gradual spread
diversification
biome shifts
Quaternary response: low latitudes
Chaotic behaviour of environmental change at low latitudes
Characteristics of chaotic systems:
Deterministic (‘butterfly effect’)
Sensitive to initial conditions
Self-similarity
Unpredictable
Cannot rewind
Three levels:
1. Climate system itself2. Response of ecosystems to climate change3. Organism interactions
Tropical biodiversity - a necessarily complex model
Periodicities of climate change vary over time
Amplitudes of climate change are relatively small and variable
Response of vegetation highly variable and not in proportion to the forcing climate change (‘non-linear’)
Outcomes:1. Major changes in vegetation happen unpredictably and at a wide range of times2. Lineage splitting independent of these changes
No process is strong enough to dominate
Conclusions: consequences
The higher diversity of tropical ecosystems is because of this stability, after all
What do we mean by ‘stable’ climate? Equatorial climates of the Quaternary may be as stable as climate can ever be
Biodiversity is, non-linearly:1. Globally, a function of time (since last mass extinction);2. Regionally, a function of (relative) ‘stability’;3. Everything else: the detail.
Processes of developing biodiversity are complex, only weakly connected to environmental change