Paleoecology is the study of the relationships between fossil organisms and their environment. Paleoecology combines a knowledge of both the biology and the ecology of living organisms--which indicates how ancient animals and plants may have looked and lived--and of geological processes--which account for why their remains are found where they are.
Fossils of both land and oceanic organisms document climatic changes associated with the advances and retreats of glaciers in Pleistocene time (ice ages). Fossil spores and pollen from sediments of bogs and lakes in northern Europe and in the American Southwest have been extensively studied (pollen stratigraphy). Knowledge of the present-day climatic preference of the plants represented by these fossils allows scientists to map the past distribution of vegetation and to show that glacial advances and retreats were associated with changes not only in temperature (paleotemperature) but also in the amount of rainfall. Cores of Pleistocene sediment taken from the seafloor similarly record changes in the distribution and abundance of the hard coverings of microscopic plants and animals (micropaleontology) and thus the shifting positions of oceanic currents and warm-water and cold-water masses.
Most paleoecologists study remains of benthic (bottom-dwelling) organisms from sediments that were deposited in shallow continental-shelf, or epicontinental, seas, because these are the sedimentary environments best represented in the geological record. Benthic marine organisms are usually rooted in the seafloor, attached to some other organism, or capable of only slow movement (for example, clams and snails). Thus they are usually buried by incoming sediment at or very near to the place where they lived. Both the kind of sediment in which a fossil assemblage is found and the nature of the fossils themselves indicate to paleoecologists the distribution of ancient seaways and the depths of the seas. Coarser sediments are generally deposited nearer to shore and finer ones farther offshore. Sedimentary beds with fine, parallel laminae, or thin unit layers, indicate quiet waters; those with undulating, rippled surfaces represent nearshore wave activity. As for the fossils themselves, coral reefs, for example, indicate very shallow, warm, well-lighted waters. Large, stout shells indicate organisms adapted to turbulent water conditions, whereas thin, fragile shells suggest that the organisms lived in calm waters. Many organisms leave traces in the sediment as they move about to feed; soft-bodied organisms are usually recorded only by such fossils.
The fossil record vividly demonstrates that life on Earth has changed greatly during geologic time. Paleoecologists ask, however, whether any regular patterns in the distribution of organisms are maintained even though individual species evolve and become extinct. One such pattern is suggested by the observation that different kinds of environments support different numbers of species. Many more species, for example, exist in the tropics than in polar regions. Similar diversity gradients exist among benthic marine species of both ancient and modern seas. Generally, very few species (but large numbers of individuals) live in environments--for example, beaches, bays, and lagoons--that are subjected to stressful conditions, such as high wave activity or large variations in temperature or salinity. In offshore environments, where physical conditions are more stable, species diversities are higher, but numbers of individuals per species are fewer.
The most satisfactory explanation of this pattern appears to be as follows. For a species to avoid extinction in stressful environments, individuals of that species must be adapted to reproduce rapidly and to disperse over wide geographic areas. The species thus makes up for the vulnerability of local populations to unfavorable conditions. Where environmental conditions are more nearly constant, species need not be adapted for broad dispersal and rapid reproduction. The resulting smaller population sizes of species permit a given habitat to contain individuals of a greater number of different species.
Sara S. Bretsky