Pleistocene and Holocene Epochs (Ice Age)

• Period of marked change in species despite short duration

• Recent event relates to current species

• Traceable change through tree rings, animal and human middens, pollen, marine indicator species

• Also important because event did not obliterate record of past events

Glaciation

Minor glaciation

Glaciation

Glaciation

Causes of Glaciation

• Earlier glaciations caused by contiental drift

• Continents 2 mya near/in current positions

• Once thought Pleistocene glaciation caused by changes in solar output

• Relatively stable solar output for last 590 million years (Gates 1993)

• Been linked to Milankovich cycles + albedo

ObliquityEccentricity

Periodicity of 100,000 yr

Periodicity of 41,000 yr

Periodicity of 22,000 yr

Precession

Extent of Glaciation

• Most of Pleistocene and Holocene were glacial with short inter-glacial periods

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Extent of Glaciation

• 80% of glacial ice in Northern Hemisphere– North America, Europe, Atlas Mtn. (NW

Africa)

• Southern Hemisphere– Chile and Argentina– Australia – limited to Victorian Alps, Central

Plateau of Tasmania– New Zealand Alps

Climatic Effects

• Sheer Size of glaciers – area covered and height (2 – 3 km) changed wind and current patterns

Lake levels rose in SW US

Climatic Effects

• Less fluctuation in temperature near glaciers

Climatic Effects

• Temperatures lower away from equator. Tropic drier

How did tropical species maintain and even increase diversity?

Sea Level Fluctuations

• Rapid glacial and interglacial fluctuations

• Sea level dropped 100 – 160 m during glacial periods

• Created land bridges

Retreat of Wisconsin glacier caused rapid rise in sea level (plus compression of crust, causing sea water to enter part of Great Lakes

Result – some Atlantic species found in Great Lakes, including several species of coastal plants

Range of seaside spurge (Ammophlia brevigulata) – note disjunct range

SHEER MASS - Weight of glaciers compressed crust!!

Biogeographic Responses to Glaciation

• Biogeographic dynamics of Pleistocene triggered by:

• Changes in location, extent, and configuration of a species prime habitat

• Changes in the nature of climatic and environmental zones

• Formation and closing of dispersal routes

Biota’s Response to Glaciation

• Species were adapted to long-term conditions of relative stable climates, reponses were:

• Able to “float” with their optimal habitat as it shifted

• Remained in in same location and adapted to new conditions

• Range reduction and extinction• See Box 9.1

Biogeographic Responses to Glaciation

• Some vegetative and marine zones increased they areal coverage

• Steppes, savannahs open-canopied ecosystems (generally drier climate)

• Closed-canopy ecosystems generally decreased (especially tropical rain forests

• Changes greatest in mid-latiturdes (35 to 55°)

Elevational change in Andes.Rise and compression

Elevational change in SW US mountains

Change in upper elevational limit of forests – note timing of responses

Drier Climate – recurring theme

Variation in relative abundance of vegetative communities since last glacial maximum. Note variability over time and rapid change.

Barriers and Corridors

• Changes in biota distribution not uniform latitudinally

• North America – many corridors– Mississippi River– Rocky and Appalachian Mountains

Barriers and Corridors

• Changes in biota distribution not uniform latitudinally

• Eurasia – corridors– Ural, Carpathian, and Atlay mountains– Rocky and Appalachian mountains

• Eurasia – barriers– Mediterranean Sea– Caucasus, Alps, and Pyrenees mountains

Corridors and Dispersal

• Lowering of elevation of montane vegetative zones as mechanisms of dispersal – cross to other mountains and mesic lowlands

• Oceanic zonal patterns also changed (Fig. 9.12) even though open ocean temperature change smaller (2 – 3°C)

• Stenothermal species had potential to move to opposite poles (Fig. 9.25)

Aquatic Ecosystems

• Glaciers are major lake builders

• Seen as aftermath of glaciation

• Kettle lakes, moraine lakes, paternoster….

• Glacial lakes– Meltwater retained by ice dams– When dams break large mass freshwater

into shallow seas, carve out river valleys

• Lake Agassiz• Released 163,000 km3

in Tyrrel Sea (Hudson Bay), Atlantic Ocean

• Also down Glacial River Warren (now Minnesota and Mississippi Rivers)

“Wet Aridlands” – Pluvial Lakes

• Formed in what are now deserts

• Large freshwater or saline lakes

• Caused by low evaporation + high precipitation

• Typically formed in broad basins between mountain ranges

• Lake Bonneville – remnants are saline lakes (Great Salt Lake)

Refugia

• “Safe” zones or habitats, offered areas where ice did not cover, even in the area of the ice sheet

• Haffer’s Pleistocene refugium hypothesis– Fragmentation of Amazonian rainforest by

precipitation levels

– Lead to isolation and divergence of species and subspecies

• New model – inundation of basin by 100 m rise in sea level and Amazon islands

Emphasis is on distribution of subspecies and number of endemic species

Nunatuks

Refugia and Endemics

Glaciation and Extinctions

• Plants – most extinctions at the onset of glacial events

• Species persistence by– Disperse with climatic zones– Refugia and dispersal– Adaptation to new conditions

Glaciation and Extinctions

• Marine Invertebrates – also most extinctions at the onset of glacial events

• Causes– Stenothermal species– Limited ability to disperse (non-planktonic

larvae)

Glaciation and Extinctions

• Terrestrial vertebrates – pattern less clear• Overkill hypothesis – impact of humans as

they expanded their range. Would lead to loss of large herbivores as well as their associated predators and scavengers

• Size-Space• Climate – many of extinctions of all sized-

terrestrial vertebrates was low and constant until the late Wisconsin

Australia:Black – extinct during Pleistocene/early HoloceneShaded – extinct or endangered after European colonizationWhite – Extant, non-endangered species