(iv) Changes associated with glaciation

Major changes to the earth’s habitats during glaciation advances include:

(1) Elimination of terrestrial habitat. Huge areas where covered by ice, clearly eliminating habitat for animals and plants. At the height of the last glaciation, the area of the ice sheets was estimated to cover about 33% of the earth's surface. On the other hand, over 2.8 million sq. km of ocean floor were exposed as new terrestrial habitat as sea levels dropped (see below). Canada today covers about 10 million sq. km.

(2) Temperature. Global average temperature drops of 4-5 degrees C (from current levels). Average sea temperatures were less extreme owing to the greater heat holding capacity of water (average of 2-3 degrees C).

(3) Shifts in climate regimes. There were large scale changes in global climate regimes (temperature and precipitation) that drove changes in biome distribution on the earth. In general, there were shifts towards the equator during glacial periods and shifts toward the poles during interglacials. Global changes were greatly influenced on a local scale by factors such as local topography and proximity to moderating influence of oceans.

For instance, the massive size of glaciers themselves reduced the extent of Arctic wind flow to non-glaciated areas. Also, as cool air descended down the faces of glaciers (2-3 km drop), the air warmed as air pressure increased and decreased the distance among individual air molecules (known as adiabatic heating). These processes tended to make environments near the glacial margins fairly moderate in terms of temperature. Descending warming air also tended to result in relatively dry environments at glacial margins.

A notable exception to the above includes the current SW United States much of which is desert. At glacial maximum, the shear size of the North American ice sheets tended to cause a southward deflection of the "jet stream" (horizontally oriented winds that flow between the Hadley Cells) blowing from the west. These winds picked up plenty of moisture over the Pacific Ocean resulting in elevated rainfall in the American Southwest. This area is rich in low flat basins separated by isolated mountain ranges. These basins filled with water owing to heavy rainfall and resulted in the formation of large pluvial lakes. The largest such lake was Lake Bonneville, a lake some 50,000 sq. km in area and over 330 m deep, in Utah and parts of Nevada and Idaho. A second was Lake Lahontan in eastern Nevada. These lakes shrunk dramatically as the glaciers retreated and the present day Great Salt Lake occupies only a tiny portion of the former Lake Bonneville (less than 25%). Lake Lahontan has been reduced to a small (about 5) number of very tiny, isolated lakes (e.g., Pyramid and Walker lakes - see figure below). The formation and subsequent decline in size of these lakes had some important evolutionary consequences for fish (see later notes on Speciation) in these areas (provided opportunities for dispersal and vicariant speciation). One of these was the evolution of a distinctive subspecies of cutthroat trout, the Lahontan cutthroat trout Oncorhynchus clarkii henshawi.

The Lahontan cutthroat trout (above) evolved in these huge pluvial lakes given the large habitat size and the abundance of an endemic forage fish known as tui chub (Gila bicolor). Lahontan cutthroat trout are well known for their large size and physiological tolerance to high pH (caused whrn the pluvial lakes evaporated to their current size). These amazing trout migrated upstream in the Truckee River to Lake Tahoe to spawn in its tributaries and were (are) the subject of a famous recreational fishery that such Hollywood glitterati as Clarke Gable participated in. That inlet to Pyramid Lake (one of the remnants of Lahontan Lake) was subsequently, and regrettably, damned and the population is largely sustained by hatchery production. Click HERE for more information on conservation of this fish and the tui chub.

(4) Changes in sea level. There were large global and local changes in sea level during the Pleistocene glacial cycles. Global changes were driven by the tremendous take-up of moisture as the glaciers grew and the release of water as they melted. AT the last glacial maximum (LGM) the total volume of ice was about 84 million cubic km! Currently, the ice caps comprise about 32 million cubic km, so that is a big difference in water locked up as ice at the LGM. Global changes that occur in all oceans at the same time are known as eustatic sea level changes. The average change was up to 130-135 m at the height of the last glaciation.

Changes that occur on a more local basis are usually caused by the depression of land from the weight of glaciers. This so-called "downwarping" of the lithosphere could be up to 300 m in coastal areas. When ice sheets retreat, the "rebound" of the land typically takes much longer than the retreat of the glaciers. This often results in a massive inflow of marine waters that form wide expanses of shallow seas. As the land uplifts over time ("rebounds") local sea level drops. Such local changes in sea level are called isostatic sea level changes.

In our area, eustatic sea level changes resulted in the current coastline being up to 30 km offshore from its present location with an average drop of sea level of about 100 m. When the glaciers retreated some 10,000 years ago, the coastline of B.C. was flooded with shallow seas to the extent that areas at elevations of 100 m above current sea level were flooded by marine waters.

Here is a link to a neat movie taken at Bamfield on Vancouver Island's west coast (Brady's Beach). It shows the "flooding" of a tidepool by the sea surge. Imagine a scale thousands of times bigger and the tidepool represents a large coastal "lake" and where massive glacial meltwater raises the level of the sea which results in flooding of coastal lakes before isostatic land rebound. You can see how lake elevation might play a role on determining to what extent lakes were flooded by the sea (and its consequences for colonization of lakes by organisms that can tolerate a wide range of salinities).

In eastern North America, when the ice sheets retreated a huge influx of marine waters inundated the present St. Lawrence River Valley such that large "inland sea" (The Champlain Sea) extended from the Atlantic coast to Lake Ontario. Such inland seas disappeared when the land rebounded and formed the current St. Lawrence River Valley draining east to the Gulf of St. Lawrence. These inland seas provided dispersal routes inland for marine, estuarine, and salt marsh species such as the "seaside" spurge (right panel of figure below). Note that the distribution of the spurge is "disjunct". Why is it not all along the St. Lawrence Valley?

Much of current Fennoscandinavia was also inundated by an inland sea when the ice sheets left that area after the so-called "Wurm" Glaciation (analagous to the Wisconsinan). Large changes in sea levels also occurred unrelated to glaciations at time. For instance, large-scale marine incursions occurred in northeastern South America (with some interesting biogeographic consequences) during the Miocene owing to tectonic changes (see Lovejoy et al. 1998, Nature 396: 421-422).

Of course, drops in sea level caused the formation of land bridges between formally isolated (by the sea) land masses. One of the best known land bridges occurred between the far northeast of Asia and extreme northwestern North America (see A below) in the current Bering Sea. This land bridge and adjacent areas are known as "Beringia" resulted in intermittent connections between the faunas of Asia and North America with profound consequences for the distributions of many plants and animals (including humans!).

To see a simulation of landbridge formation/flooding in Beringia click HERE (it takes a bit to load).

The multiple formations of the Bering Land Bridge of course also represented periods of isolation for marine faunas of the North Pacific, Arctic Ocean, and the Atlantic Ocean. Another classic example involves the various land bridges between mainland Southeast Asia, the islands of Indonesia, and Australia (see B below). The land bridges in this area tended to produce two faunal groups (with respect to terrestrial and freshwater faunas). The "Oriental" fauna (mainland SE Asia and adjacent islands) were interconnected by the land bridges along the Sunda Shelf whereas the "Australian" fauna was interconnected by the land bridges among Australia, Tazmania, and New Guinea. The current position of "Wallace’s Line" marks a profound and coincident break in such faunas in diverse groups of organisms and attests to the powerful "barrier-effect" of even fairly narrow marine waters to many organisms.

Top: Beringian land bridge.

Bottom: Land bridges in southeast Asia and Wallace's Line.

References:

Avise, J.C., D. Walker, and G.C. Johns. 1998. Speciation durations and Pleistocene effects on vertebrate phylogeography. Proc. Royal Soc. Lond. B 265:1707-1712. (Compare to the paper by Klicka and Zink, below).

Clark, P.U. and A.C. Mix. Ice sheets by volume. Nature 406: 689-690.

Haffer, J. 1969. Speciation in Amazonia forest birds. Science 165: 131-137.

Hewitt, G.M. 1996. Some genetic consequences of ice ages, and their role in divergence and speciation. Biol. J. Linn. Soc. 58: 247-276.

Klicka, J. and R.M. Zink. 1997. The importance of recent ice ages in speciation: a failed paradigm. Science 277: 1666-1669.

Lindsey, C.C. and J.D. McPhail. 1986. Zoogeography of fishes of the Yukon and Mackenzie basins. Chap. 17 In: Hocutt, C.H. and E.O. Wiley (eds.) Zoogeography of North American Freshwater Fishes. John Wiley and Sons, Inc. (In Woodward Reserve section).

McPhail, J.D. and C.C. Lindsey. 1970. Freshwater fishes of northwestern Canada and Alaska. Fish. Res. Board of Canada Bull. 173. (pages 7-26).

McPhail, J.D. and C.C. Lindsey. 1986. Zoogeography of freshwater fishes of Cascadia (the Columbia system and rivers north to the Stikine). Chap. 16, In: Hocutt, C.H. and E.O. Wiley (eds.) Zoogeography of North American Freshwater Fishes. John Wiley and Sons, Inc. (In Woodward Reserve section).

Taylor, E.B., S. Pollard, and D. Louie. 1999. Mitochondrial DNA variation in bull trout (Salvelinus confluentus) from northwestern North America: implications for zoogeography and conservation. Mol. Ecol. 8: 1155-1170.

Vermeij, G.J. 1991. When biotas meet: understanding biotic interchange. Science 253: 1099-1104.