Increasing energy use, climate change, and carbon dioxide (CO2) emissions from fossil fuels make switching to low-carbon fuels a high priority. Biofuels are a potential low-carbon energy source, but whether biofuels offer carbon savings depends on how they are produced. Converting rainforests, peatlands, savannas, or grasslands to produce food crop-based biofuels in Brazil, Southeast Asia, and the United States creates a "biofuel carbon debt" by releasing 17 to 420 times more CO2 than the annual greenhouse gas (GHG) reductions that these biofuels would provide by displacing fossil fuels. In contrast, biofuels made from waste biomass or from biomass grown on degraded and abandoned agricultural lands planted with perennials incur little or no carbon debt and can offer immediate and sustained GHG advantages.

Biodiversity lies at the core of ecosystem processes fueling our planet's vital life-support systems; its degradation--by us--is threatening our own well-being and will disproportionately impact the poor.

Complementarity and sampling effects may both contribute to increased invasion resistance at higher diversity. We measured plant invader biomass across a long-term experimental plant diversity gradient. Invader speciesÕ biomass was inhibited in more diverse plots, largely because of the presence of strongly competitive C 4 bunchgrasses, consistent with a sampling effect. Invader biomass was negatively correlated with resident root biomass, and positively correlated with soil nitrate concentrations, suggesting that competition for nitrogen limited invader success. Resident root biomass increased and soil nitrate concentrations decreased with the presence of C 4 grasses and also across the diversity gradient, suggesting that diverse plots are more competitive because of the presence of C 4 grasses. In addition to this evidence for a sampling effect, we also found evidence for a complementarity effect. Specifically, the percentage of plots that had lower invader biomass than did the best resident monoculture (i.e. that had invader ÔunderyieldingÕ) increased across the species richness gradient. This pattern cannot be explained by a sampling effect and is a unique signature of complementarity effects. Our results demonstrate the importance of multiple mechanisms by which diversity can increase invasion resistance.

The ecological impact of biofuels is mediated through their effects on land, air, and water. In 2008, about 33.3 million ha were used to produce foodbased biofuels and their coproducts. Biofuel production from food crops is expected to increase 170% by 2020. Economic model estimates for landuse change (LUC) associated with food-based biofuels are 67-365 ha 10 −6 l −1 , leading to increased greenhouse gas emissions for decades compared to business as usual. Biodiversity is reduced by about 60% in U.S. corn and soybean fields and by about 85% in Southeast Asian oil palm plantations compared to unconverted habitat. Consequently, the largest ecological impact of biofuel production may well come from market-mediated LUC. Mitigating this impact requires targeting biofuel production to degraded and abandoned cropland and rangeland; increasing crop yields and livestock production efficiency; use of wastes, residues, and wildlife-friendly crops; and compensatory offsite mitigation for residual direct and indirect impacts.