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.