Paleoecology is the study of the environmental relationships of organisms in the geological past.
A lot of assumptions must be made given the paucity of data available in order for paleoecologists to generate ecosystems of the past. They must assume:
The ecological relationships we use today to describe system dynamics are those that held in the past. Trophic dynamics, energy flow transfers, competition & predation, parasitism and so on where common controlling determinants of ecosystem functioning. We have no real reason to doubt this at this time.
How true are these assumptions ? I remember hearing a talk by a paleoecologist who stated that this particular prehumanoid creature ate this, looked like this and so on based on a fossil femur piece 3" in length. A bit of skepticism was in order, though I certainly applauded his creative reconstruction.
II. What are some problems inherent in paleoecology reconstruction?
1. What we don't see may be as important as what we do: Not every creature was fossilized. Organisms with soft-body parts are difficult to find. At best you can get a casting or impression of what was there, but no part of it. Organisms with shells, skeletons and plants with wood or high production of pollen will be over-represented and others under represented. Portions of ecosystems which were anaerobic vs. aerobic were more likely to be preserved.
2. Fossil beds are composites of fossils - a shallow lake make be the burial ground of not only the creatures who lived there but also those washed in by rivers. Animals digging through layers of mud or soil may 'contaminate' the historical record by bringing earlier or later fossils to a strata.
3.The older the material, the more likely it was modified, or destroyed by geological events or biological intrusions.
4. At best you are sampling just a portion of what existed; what happened to be there & could be captured at the time that an volcanic explosion or great flood erupted allowing fossilization.
In spite of this pictures of earlier systems have been constructed and in some cases, population dynamics have even been construed.
II. Before we start in on the specifics on what happened in the past you should review plate tectonics . An excellent place is: http://pubs.usgs.gov/publications/text/dynamic.html from which the following diagram is taken, and reproduced just in case you can't get there right now.
Once you understand how the geological system works you can get a better understanding why the flora and fauna fossils that exist are where they are located:
Since we can't really get a grip on all the eras why don't we just concentrate on the Pleistocene and the post glacial time period.
Pleistocene extended from 2,000,000 years till post glacial recent period of 10,000 years ago. It is known as the 'Ice-age", the time of the first true humans & the mixture and thinning out of the mammalian fauna. It was a period of great climatic fluctuations throughout the globe with 4 major periods of ice sheet advance and retreat in NA. Between each glaciation period there was an interglacial period during which conditions changed from cold to temperate. Thus a successional progression occurred starting with tundra plants and boreal spruce & fir, replaced by pine & birch. Eventually the soil would build up again and temperature warm enough for the more typical oak, beech and ash
During this period there was also a spatial discontinuity in climate. Along with the continued drying and cooling due to the glaciers which led to the developments of forests dominated by evergreens ( why evergreens?) in the north there was also the spread of more arid lands and grasslands in the interior. The trend in increasing dryness was a result of the uplift of the western mountains ( remember the climate class). The rain shadow caused the change from forest to savanna ( grassland with few trees) to grassland. During these millions of years animals immigrated &emigrated changing species with the climate.
The last great ice sheet, the Laurentian reached it s maximum advance about 18,000 BP during the Wisconsin glaciation period. Canada was under ice, as you will see in the diagram below. Tundraextended across the US along its border with a bit of an extension into the Appalachians. West of Kansas there was basically just desert like areas. As the glacier finally retreated, the boreal forests disappeared to be replaced by grasslands
The following maps were taken directly from the following site: http://www.soton.ac.uk/~tjms/adams4.html
A period summary of the global situation at 18,000, 8,000 and 5,000 years ago.
Overwhelmingly, the picture of the world 18,000 radiocarbon years ago is of a much colder and more arid place. Treeless and barren landscapes were more extensive almost everywhere, even at low latitudes. The only notable exception is in the southwestern USA, where there was a greater area of woodland than at present. The precise degree to which tropical forests retreated under the glacial-age climate remains far more speculative than for the higher latitude forests. However, it seems safe to assume that there was a net reduction in forest cover in all three main tropical forest regions, relative to the pre-agricultural Holocene.
By 8,000 years ago - in the early Holocene - dense and woody vegetation had made a dramatic comeback. Forests occupied a greater area than present in many regions, for instance extending further north in the high arctic. In some areas, such as the Sahara, climate seems to have been much moister than at present, allowing savanna and grassland to exist in areas that are now barren desert. In other areas, such as western Canada, conditions were possibly somewhat drier than now. However, the overall picture is of considerable similarity to the present-day world, as compared to a very different situation at 18,000 years ago.
By around 5,000 years ago, the broad distribution of biome types in some areas seems to have been essentially identical to that existing today, although differences can certainly be found in many regions.. The km. per year advance of forest species although slow, allowed the major transition to that what we see today.
Animals of that time :
Large animals which roamed the US included the mastodons, giant beaver, musk oxen, giant tortoises, ground sloths, horses, camels, mammoths, sabre-tooth cats, dire wolf, . They disappeared rather suddenly about 10-11 thousand years ago. A few large animals remained: bison, condor and grizzly bear. Most of the species which survived were smaller - deer, antelopes, raccoon, mallards etc.
Sites to visit to get information on the fauna of the US can be found at the following address:
To get directions information on specific animals go to:
I did an inquiry on mastodons for all time in US- here's what was found: obviously the critters were common to all US at singular points in time...
1. What differences would you find between the fauna of 16000 years ago and that of 1000 years ago?
2. What theories exist to explain the loss of the large Pleistocene mammals? What counter arguments exist? can you offer an alternative explanation given that it was primarily large mammals only that were affected??
3. Choose one of the following, research it and tell the class what this animal was about...
Ground Sloths ) * Beautiful Armadillo * * Short-faced Skunk * Dire Wolf * Short-faced Bear * American Lion * Jaguar * Saber-toothed Cats * Giant Beaver * Perissodactlya (horses, rhinos, and tapirs) * Horses * Tapirs , camels, and pigs) * Peccaries (extinct) * Stag-moose (extinct) * Bison * Musk Ox (some types extinct) *American Mastodon (extinct) * Mammoths (extinct)