Change in dominance determines herbivore effects on plant biodiversity.

Herbivores alter plant biodiversity (species richness) in many of the world's ecosystems, but the magnitude and the direction of herbivore effects on biodiversity vary widely within and among ecosystems. One current theory predicts that herbivores enhance plant biodiversity at high productivity but have the opposite effect at low productivity. Yet, empirical support for the importance of site productivity as a mediator of these herbivore impacts is equivocal. Here, we synthesize data from 252 large-herbivore exclusion studies, spanning a 20-fold range in site productivity, to test an alternative hypothesis-that herbivore-induced changes in the competitive environment determine the response of plant biodiversity to herbivory irrespective of productivity. Under this hypothesis, when herbivores reduce the abundance (biomass, cover) of dominant species (for example, because the dominant plant is palatable), additional resources become available to support new species, thereby increasing biodiversity. By contrast, if herbivores promote high dominance by increasing the abundance of herbivory-resistant, unpalatable species, then resource availability for other species decreases reducing biodiversity. We show that herbivore-induced change in dominance, independent of site productivity or precipitation (a proxy for productivity), is the best predictor of herbivore effects on biodiversity in grassland and savannah sites. Given that most herbaceous ecosystems are dominated by one or a few species, altering the competitive environment via herbivores or by other means may be an effective strategy for conserving biodiversity in grasslands and savannahs globally.

Potential environmental benefits from increased use of bioenergy in China.

Because of its large population and rapidly growing economy, China is confronting a serious energy shortage and daunting environmental problems. An increased use of fuels derived from biomass could relieve some demand for nonrenewable sources of energy while providing environmental benefits in terms of cleaner air and reduced emissions of greenhouse gases. In 2003, China generated about 25.9 x 10(8) metric tons of industrial waste (liquid + solid), 14.7 x 10(8) metric tons/year (t/y) of manure (livestock + human), 7.1 x 10(8) t/y of crop residues and food-processing byproducts, 2 x 10(8) t/y of fuelwood and wood manufacturing residues, and 1.5 x 10(8) t/y of municipal waste. Biofuels derived from these materials could potentially displace the use of about 4.12 x 10(8) t/y of coal and 3.75 x 10(6) t/y of petroleum. An increased bioenergy use of this magnitude would help to reduce the emissions of key air pollutants: SO(2 )by 11.6 x 10(6) t/y, NO(X) by 1.48 x 10(6) t/y, CO2 by 1.07 x 10(9) t/y, and CH4 by 50 x 10(6) t/y. The reduced SO(2) emissions would be equivalent to 54% of the national emissions in 2003, whereas those for CO2 are 30%. It is important to recognize, however, that large increases in the use of biomass fuels also could result in socioeconomic and environmental problems such as less production of food and damage caused to natural habitats.