Do Species Losses Matter for Biogeochemical Cycling

2.1 From First Principles—No

As in any comparison, one can find similarities among sets or find differences. First, I consider briefly the similarity arguments, that is, the position that species differences are not large enough to affect the kinds of processes that occur at the global level. From a functional point of view one can focus on similarities among species, as Eigen and Schuster (1977) have noted, "... millions of species, plants and animals, exist, while there is only one basic machinery of the cell; one universal genetic code and unique chi-ralities of the macromolecules." This particular observation supports the viewpoint that species differences are only frills upon the basic plan of organisms. For example, in terms of carbon fixation by plants, with the exception of very specialized organisms that use chemical energy rather than light energy, there has essentially been only one type of photosynthetic pathway, the Calvin cycle. The Hatch-Slack pathway is a relatively recent evolutionary innovation, as well as discovery by scientists (Hatch and Slack, 1966).

This similarity among species, in rather fundamental functional properties, has no doubt led to the development and use of earth system models that have little diversity content, but rather use the global BIOGEOCHEMICAL cycles in the climate system

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color of the land surface only, such as the green scum production models and the Daisy World model of Lovelock (1994). There certainly is a rationale for the use of very simple approaches in viewing the functional diversity of the world in global productivity models. The finding of Monteith (1977) that the productivity of plants is dependent on the energy absorbed rather than on species per se is an important finding that has been utilized extensively in global modeling. The CASA biogeochemistry model builds on this principle (Potter etal., 1993) with corrections for the effects of environmental stress on the basic light production relationship.

Similarity can be structural as well as functional when viewed at a regional level. One of the fundamental principles of biogeog-raphy is the concept of convergent evolution. This is based on the observation that in similar climates, even though separated geographically, comparable vegetation structures will be present. It has also been shown that similarity in structure also reflects similarity in function (Mooney, 1977). It is a fact that the convergent vegetation types of the world have comparable productivities. Thus much can be said about the earth's biogeochemistry without knowing much about the millions of species that constitute the earth's biota.

2.2 From First Principles — Yes

Since no two species can coexist on the same limiting resource (Cause, 1934) there is an inexorable evolutionary drive for biotic diversification. Natural selection favors innovation in acquiring new resources in the environment be it a new way, for a given habitat, to capture light, water, or nutrients. This means that through time for a given site, a single species is not able to acquire all of the available resources to derive maximum productivity, for example. By this reasoning, removing species should impact production and hence biogeochemistry. By the same token adding species to mixtures, as is done in agroforestry, is an attempt to optimize total system productivity.

In the following I examine the apparent contradiction between these viewpoints, looking at the various kinds of diversity that are found among organisms and its significance in terms of biogeo-chemical functioning with the main focus, again, being on the question of whether we should be concerned about species losses (or additions).

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