 These effects may be exacerbated by climate changes in river flow, and the combined effects are likely to reduce the abundance of native trees in favour of non-native herbaceous and drought-tolerant competitors, reduce the habitat quality for many native animals, and slow litter decomposition and nutrient cycling.Climate change effects on human water demand and irrigation may intensify these effects.The US Global Change Research Program (2009) assessed the literature on the impacts of climate change on agriculture in the United States, finding that many crops will benefit from increased atmospheric concentrations and low levels of warming, but that higher levels of warming will negatively affect growth and yields; that extreme weather events will likely reduce crop yields; that weeds, diseases and insect pests will benefit from warming, and will require additional pest and weed control; and that increasing concentrations will reduce the land's ability to supply adequate livestock feed, while increased heat, disease, and weather extremes will likely reduce livestock productivity.