The nature of past environments, and changes in these, can be determined from direct and indirect sources of evidence. Under the assumption that present distribution patterns reflect both past and present-day environmental conditions, indirect evidence is available in the form of present distribution patterns of species and genetic diversity. Levels of diversity, or differences in the number of organisms between areas, can be used to indicate the nature of past environments where high diversity and endemism are facilitated by relative environmental stability (Fjeldså and Lovett, 1997). The corollary of this is that intervening areas of relatively low species diversity and endemism have been impacted much more severely by past environmental change. However, one of the problems with this kind of evidence, assuming present-day distribution patterns do carry an imprint of past conditions, is determining when in the past environmental change actually took place. The only way to provide this is by accessing direct sources of information on the nature of past environments.

One of the mainstays of past environmental reconstruction is from pollen data: past vegetation composition and distribution, and changes in this, can be determined by remnants of the vegetation (pollen) preserved within accumulating sedimentary basins. Each individual plant species is enclosed within an environmental envelope within which temperature, moisture, seasonality and soil type (amongst other factors) are able to support plant growth and allow flowering and reproduction. By overlapping the individual environmental envelopes ecosystems form, if the environment changes, then the associated composition of the ecosystem will reflect this change. By accessing lake or bog sediments, that preserve pollen as sediments accumulate, it is possible to reconstruct plant communities of the past. When these ‘snap-shot’ reconstructions are placed within a time frame provided by radiocarbon dating, how the vegetation at a single site has changed over time can be determined. Given that the present environmental envelopes that encompass these ecosystems today are known, the past environmental conditions can be determined. Due to the complexities in understanding environmental changes and site-specific influences on the sedimentary process (Jolly et al., 1997), it is necessary to have data from multiple sites to construct robust records of ecosystem change and environmental consequence.

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References

Fjeldså, J., Lovett J.C., 1997. Biodiversity and environmental stability. Biodiversity and Conservation 6, 315-323.

Jolly, D., Taylor, D.M., Marchant, R.A., Hamilton, A.C., Bonnefille, R., Buchet, G., Riolett, G. 1997. Vegetation dynamics in central Africa since 18,000 yr BP: pollen records from the interlacustrine highlands of Burundi, Rwanda and western Uganda. Journal of Biogeography 24, 495-512.