The ravages of ice: How glaciation affects biotas

Imagine a sheet of ice creeping inexorably across the landscape over millions of years, moving downwards from the poles. Regions once mild in climate become part of an ever expanding frozen wasteland. This terrible, slow process of advancing ice is called glaciation. In the past, glaciation events have shaped our planet. Individual glaciers can carve through rock and flatten landscapes. The raw power of ice has shaped the very nature of life on Earth.

The general cause of glaciation events is a change in Earth’s mean temperature and an increase in snowfall at higher latitudes. The underlying causes of such temperature shifts however vary. Factors which can cause the Earth’s mean temperature to drop by a few degrees Celsius include a large-scale increase in volcanism and meteor collisions. In both scenarios, less solar energy reaches Earth’s surface due to more dust in the atmosphere, and the temperature drops as a result. Glaciation is not a randomly occurring process however and the commonly accepted  theory to explain patterns of glaciation is that of Milankovitch cycles.  Milankovitch was a Serbian scientist who observed that not only does the angle of Earth's tilt change, but that our planet 'wobbles' on its axis. He also noted that the Earth's distance from the Sun varies as the orbital path our planet follows changes in shape. The force behind the global temperature changes during glaciation is thus once more a change in how solar radiation reaches Earth. As a result of such fluctuations, climatic conditions on Earth are dissimilar during glacial and inter-glacial periods.

Figure 1: Variations in Earth's orbit associated with periods of glaciation.

Naturally, such changes affect Earth’s biota. Regions once suitable for particular species become less so and the range of species may contract or the species may suffer extinction. Other species may have the opportunity to expand their ranges. Species ranges may also simply shift in latitudinal terms. Entire biomes may move polewards or towards the equator. Another possibility is that a species will be able to adapt to the new conditions within its current distribution. The nature of the response by individual species depends on the characteristics of those species. In order to survive a period of glaciation, a species must both adapt to changes in its physical circumstance such as altered conditions of climate and geography and be intrinsically suited for the new world in which it finds itself or able to disperse to a suitable area.  Over time, natural selection may result in a change in the morphology of a species. Bigger individuals for example may better survive an icy world due to their superior heat retention and these genes may be passed along preferentially.

Besides changing the distribution of species, glaciation may cause entirely new species to form. One of the driving forces of speciation is geographic isolation. When a glaciation event occurs, species may retreat to refugia – isolated pockets of suitable habitat – and within these, entirely new species may form over tens of thousands to millions of years. When the ice retreats, these species can once more spread, expanding their ranges into new suitable territory.

Figure 2: Species may escape unfavourable climatic conditions by retreating into refugia

Due to the extinction of some species and the adaptation or formation of others, periods of glaciation change the structure of communities and eventually the biotas of entire regions. Further changes take place when the ice recedes. New species spreading into new territory may compete as they expand their ranges and further extinctions may occur. Though some communities may survive unchanged during glaciation, entirely novel ones will also form. Thus as the tide of ice expands and retracts over geological time, it leaves in its wake a changed world.

Works Cited:

BROWN, J.H. & LOMOLINO, M.V. 1998. Biogeography. 2nd Edition. Sinauer Associates Inc., Massachussets.

PARMESAN, C., ROOT, T.L. & WILLIG, M.R. 2000. Impacts of Extreme Weather and Climate on Terrestrial Biotas. Bulletin of the American Meteorological Society 81(3): 443-449. Available online at: http://journals.ametsoc.org/doi/pdf/10.1175/1520-0477(2000)081%3C0443%3AIOEWAC%3E2.3.CO%3B2 (Accessed 11/04/2013).

RAND, A.L. 1948. Glaciation, an Isolating Factor in Speciation. Evolution 2(4): 314-321. Available online at: http://www.jstor.org/stable/2405522 (Accessed 11/04/2013).

RITTER, M. E. The Physical Environment: An Introduction to Physical Geography. 2006. Available online at: http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/title_page.html

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SCHIEBER, J. Milankovitch Cycles and Glaciation.  http://www.indiana.edu/~geol105/images/gaia_chapter_4/milankovitch.htm 

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