The loss of biodiversity has many consequences that we understand, and many that we do not. It is apparent that mankind is willing to sustain a great deal of biodiversity loss if there are concomitant benefits to society; we hope they are net benefits. In many cases, the benefits seem to accrue to a few individuals only, with net societal loss. However, as noted below, it is extremely difficult to estimate the future costs of losses in biodiversity, or of environmental damage. As stated by Tilman (2000), “The Earth will retain its most striking feature, its biodiversity, only if humans have the prescience to do so. This will occur, it seems, only if we realize the extent to which we use biodiversity.”
Losses in biodiversity in rainforests cause significant changes in ecosystem functioning. About ecosystem functioning in tropical rain forests we know very little, but we do know that ecosystems are affected by changes in the number and kinds of species which they contain, an idea originally conceived by Charles Darwin and Alfred Russel Wallace. Intact ecosystems function best, since the organisms composing them are specialized to function in that ecosystem to capture, transfer, utilize and, ultimately, lose both energy and nutrients. The particular species making up an ecosystem determine its productivity, they affect nutrient cycles and soil contents, and they influence environmental conditions such as water cycles, weather patterns, climate and other no-biotic aspects.
What regulates the normal functioning of an ecosystem? There are many interrelated, complex (and little-known) factors which ensure this function. There appear to be certain keystone species which are critical in the functioning of any ecosystem, as well as other species which are of more marginal importance, the loss of which would not be catastrophic. In some ecosystems it is possible that many species can disappear without serious degradation of most of the functional aspects of the ecosystem (Grime, 1997). Many ecologists nevertheless feel that the total number of species has a great effect on ecosytem functioning, and there is some evidence that it does (Loreau and Hector, 2001; Loreau, et al., 2001, for example). However, it cannot be simply the number of species, but also the fact that different species utilize different resources in a particular environment (“niche complementarity”) that affects function. Species, existing in proximity to each other, nevertheless utilize different microhabitats and nutrient combinations, and so are capable of exploiting all aspects of the environment. Conversely, reducing biodiversity may lead to a diminishment of productivity because of the loss of some of these niche roles; that is, a loss of functional diversity. However, how many species can be lost without serious alterations of an ecosystem; which species can be lost without serious consequences; and under what circumstances species loss will cause the destruction of the ecosystem, we do not know, for the most part.
One important fact that is often forgotten is that ecosystems are, as it were, living structures. They are constantly changing and subject to evolution, so that at various times they will be composed of different organisms. According to pollen data and information gathered from fossil bones, few modern ecosystems are the same as they were 10,000 years ago. They originate bit by bit as the environment changes, and as species become extinct or shift their geographical range or their anatomy or behavior.
As we continue to lose species at a rapid rate, we must discover which losses will have the most deleterious consequences on ecosystems. At present, we know little, and what we do have is information on short-term, small-scale experimental plots. We need to know more, much more. It is vital to realize, then, that biodiversity does not mean simply the number and kinds of living organisms present. Biodiversity depends upon the habitats and ecosystems which support them. As John Muir said, “When we try to pick out anything by itself, we find it hitched to everything else in the universe.”
a. Species richness: Most research has been done on the effects of species richness (that is, the number of species) on the functioning of ecosystems. Most of the research into the interrelationships between biodiversity and ecosystem functions has been done on plants, which affect soil processes, decomposition, water retention and many other ecosystem functions. Higher plant diversity does not appear to have any great effect on soil processes such as decomposition rates, but does affect productivity and enhances the stability of ecosystem processes. For example, in some, but not all, experimental systems, increasing plant diversity alone, or increasing the diversity of plants, herbivores, and decomposing organisms augments productivity (NPP). It has generally been found, at least in temperate grasslands, that areas with greater biodiversity have higher productivity. Plots with greater numbers of species had a greater above- and below-ground plant biomass, higher rates of nitrogen fixation, and retained nutrients better than plots with fewer species (Tilman, et al., 2001a; Reich, et al., 2001). On average, for a 50% reduction in biodiversity, there will be a 10% – 20% loss of productivity. The productivity of a monocrop field is less than half that of a highly diverse field (Tilman, 2000). In part this may be due to a greater loss of nutrients from fields with low diversity than from more diversified plots. A rich array of species of mycorrhizae also seems to have a positive effect on plant productivity. There are suggestions that loss of species richness may affect many ecosystem processes (nutrient cycling, increased uptake of carbon, and others) in addition to productivity. However, we have little evidence for this as yet, and there seems to be no straightforward relationship between species diversity and many other ecosystem functions. (For a review, see Chapin, et al., 1998.)
b. Species composition: The array of species in an ecosystem (species composition) must also be important to its function. Certain species will have a greater influence than others, particularly if they are among those groups which capture and transfer energy or nutrients, or which affect environmental conditions regulating these processes. Hawaiian forests have been disrupted by the introduction of a nitrogen-fixing tree, Myrica faya, which has led to a great increase in nitrogen supply and altered greatly the properties of these forests. In another case in Hawaii, nonnative grasses were introduced to improve cattle grazing, but since these grasses are flammable, they have caused a 300-fold increase in fires in the forests into which they spread. Most woody plants are damaged or destroyed by fires, while grasses generally are not, since their deep root systems are maintained even when the superficial portions of the plant are lost. This in turn reduces evapotranspiration and rainfall. If tropical forest trees are removed and their place is taken by savannah grasses, the evapotranspiration which is so conspicuous a feature of tropical rainforests would be severely curtailed, decreasing rainfall (and eliminating the possibility of forest regeneration or even survival of remnants). There is some support for the idea that many species are “redundant”; that is, several species play equivalent roles in an ecosystem. Thus, one or more of these equivalent species could be lost without irretrievably damaging the ecosystem. But, in general, each loss of species will lead to impoverishment of the system. Davidson (2000) uses the metaphor of threads being pulled from a tapestry, until finally it becomes threadbare, and the grand design is lost.
c. Species interactions: Species interactions are perhaps the most important aspect of ecosystem functioning. Species are not just “there,” they are interacting at some level with all the other organisms in the system, forming highly complex interlocking systems. They compete, they parasitize, they cooperate, they prey, they provide food or shelter. In these interactions they also modify the nonbiological aspects of the ecosystem: the availability of nutrients, energy sources (such as sunlight), water, nitrogen (nitrogen-fixing organisms) and the like.
i) Mutualistic interactions: These interactions are essential in ecosystems. One example is the mycorrhizal associations between fungi and the roots of plants, or the decomposition of organic material in the soil by microorganisms, each species of which may contribute different enzymes to the decay process. The organic compounds thus released are taken up by forest plants, which provide the organic matter for the next cycle.
ii) Trophic interactions: Ecosystem functions depend greatly upon trophic interactions among species within that system. For instance, if carnivores are removed, prey species populations may grow tremendously, leading to a series of changes in the system.
d. Ecosystem stability: the”diversity-stability hypothesis”: The idea behind this hypothesis is that biodiversity acts as a stabilizing factor in ecosystems, and that therefore highly diverse ecosystems can act to reduce the impact of changes in the environment. Since humans are now altering so many environmental variables – atmospheric gases, surface temperatures, water quality – it behooves us to maintain as many areas with high biodiversity as possible. The more species, the more likely that at least some will remain after environmental changes occur. Some species may also be capable of mitigating the effects of the changes.