Duckweed also threatens the local ecosystem because it shades the lake: Other plants in the lake do not get enough light, and die off. In certain conditions, the weed may concentrate heavy metals and bacteria such as "Salmonella" and "Vibrio cholerae", the bacterium that causes cholera. Even though it causes many problems, the weed may also have some use: Duckweed can be treated to feed poultry, or to make paper. 