Analysis of the Role of Diversity and Biogeochemistry

8.1 Direct Tests

To this point we have looked to the past to understand the changing relations between biodiversity and biogeochemisry. Recently, there has been considerable research into the role of species richness in ecosystem functioning, including biogeochemistry. There have been explicit experiments on this issue as well as observations on the impact of addition and deletion of species on natural ecosystems.

The experimental studies, although the most direct attack on this issue, have been somewhat controversial. Tilman et al, (1996) have demonstrated through very elaborate experiments that species richness per se can influence such fundamental properties of ecosystems as nitrate retention as well as ecosystem resilience (Tilman and Downing, 1994). Through experiments that controlled not only plant species diversity but also trophic web diversity, Naeem ct al (1994) have demonstrated that richness can influence the carbon flux of ecosystems. Others have shown that functional types, another measure of diversity, are more important than richness per se in determining impacts on biogeochemistry (Hooper and Vitousek, 1997). The properties measured in these sorts of experiments are subject to great variability and hence subtle differences between species performance may be intrinsically more difficult to measure than among functional types.

8.2 Field Inferences

Another approach to investigating the role of species in ecosystem functioning and biogeochemistry is to view the impacts of the additions and, in some cases, the deletions of individual species on ecosystems. These studies have been quite revealing. Invasive species are altering the biotic structure of the earth's ecosystems. The breakdown of biogeographic barriers through international commerce has resulted in large numbers of species extending their natural ranges, often over many different continents. There are a great number of examples of species introductions totally altering ecosystem properties, including alterations of hydrology, nutrient cycling, and physiographic development. Additionally, invasive species can cause massive changes in ecosystem structure and such processes as fire cycles. All of these modifications directly influence biogeochemistry. The question is, at what scale? There is ample evidence of many invasive species having profound effects locally. There are also examples of invasives having large-scale regional effects on processes influencing biogeochemistry (see Drake et al., 1989). The latter include the total conversion of the intermountain west of North America by the succession of Bro-mus tectorum and the conversion of the perennial grasslands of California by mediterranean annual grasses. These conversions have respectively altered regional fire and water cycles. A striking example of an impact of an invasive species at the continental scale has been described for the effects of rinderpest on the megafauna (and human social structures) in Africa (Sinclair, 1979). The relatively large-scale plantation forestry practiced in many parts of the world, and of course agricultural conversions, have altered water, carbon, and nutrient balances of these regions, although in the latter case there has been a conversion of biotic material, as well as the resource base itself, through irrigation and fertilization. Introduction of invasive species caused major alterations of the operation of ecosystem dynamics in large water bodies, as has been evidenced by the effects of the zebra mussel on the trophic structure and water quality of the Great Lakes (Ludyan-skiy et al, 1993) and similar effects of the Asian clam in the San Francisco Bay (Carlton, et al, 1990).

Invasive species have also inhabited areas that were formally unoccupied, as in the case of Spartina, which is now invading the mud flats of the Pacific Northwest (Daehler and Strong, 1996), altering the physiographic development in these regions. The regional impacts of the removal of individual species, and of whole functional groups, have been described for the effects of recent as well as past megafaunal extinctions in many parts of the world (Owen-Smith, 1989). These removals have totally altered ecosystem structure and dynamics.

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