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5 Biodiversity (continued) Species importance Some species clearly play very important roles in ecosystems. In some cases, the addition or deletion of a single species can lead to dramatic changes in ecosystem functions, including pro- ductivity or nutrient uptake. For example, the introduction of a nitrogen-fixing tree species to Hawaii resulted in substantially altered productivity and nutrient dynamics in submontane forest ecosystems. Species that exert such strong control over ecosystems are termed keystone species. It is not at all clear that most species in an ecosystem have such important effects. In other words, it may be possible to lose a number of species from an ecosystem and yet observe little overall impact on ecosystem function. This could be the case if several species that perform approximately the same function are present in the original ecosystem. The situation in which multiple species play a similar role has been termed species redundancy. If species redundancy is a common phenomenon, ecosystem function should be largely independent of species diversity as long as major functional types are represented. Thus, when one species is lost from an ecosystem, some other species with a similar function may become abundant and compensate for the lost species, leaving the ecosystem as a whole relatively unaffected. Indeed, ecosystem processes often do remain stable despite large fluctuations in the abundance of the various species involved. In addition, the relationship between ecosystem function and biodiversity is often observed to be nonlinear [panel (b) of Fig. 3], suggesting that, at least initially, the loss of species would have little overall effect. The term species redundancy may seem to imply that all spe- cies are not necessary for an ecosystem to function properly. However, species redundancy may be an essential feature for the long-term health of ecosystems. Just as engineers include multiple structures with redundant functions to increase overall reliability of the final structure, ecosystems with sets of functionally redundant species may have a built-in safety net that is lacking in species-poor ecosystems. Biodiversity is nature's insurance policy against disasters. Rare species (those that occur in low abundance) may also ap- pear to contribute relatively little to overall ecosystem func- tioning. However, during dramatic environmental changes (for example, acidification of a lake), rare species can become very abundant, thereby compensating for reductions in other spe- cies. Even species that appear relatively unimportant because they are rare and functionally redundant with others may in fact be important in stabilizing ecosystem function during periods of rare, but intense, stress. The overwhelming variety of life has captivated the human imagination for centuries, so it is surprising how much scien- tific uncertainty currently surrounds the role of biodiversity. Ignorance probably reflects the fact that biodiversity has been taken for granted; only over the last few decades, as biodiver- sity's staggering decline became more apparent, did ecologists start investigating what exactly is being lost. Most experiments provide compelling evidence that at some point the erosion of biodiversity will impair ecosystem function and stabil- ity. However, these same experiments also show that a great deal of biodiversity can typically be lost with minimal effects. The value of biodiversity may well be revealed only on huge timescales that incorporate extremely infrequent, but dramatic, environmental challenges. If this is the case, standard short- term experiments will not be able to document the value of many species. Clearly, any failure of short-duration, small-scale experiments to identify a function for total biodiversity or for every species should not be used as a disingenuous argument to excuse human-caused extinctions. + ward ' s science 5100 West Henrietta Road • PO Box 92912 • Rochester, New York 14692-9012 • p: 800 962-2660 • wardsci.com This article was originally published by McGraw Hill's AccessScience. Click here to view and find more articles like this.