The state's urban environments are partly wooded but also bear a heavy load of built structures and human environments that are not hospitable to many other species. At the same time, pollutants in waterways, mainly from urban sources, can be toxic to many species or may support algae and bacteria that lead to hypoxia and the death of aquatic animals. However, Greater Boston boasts extensive parklands, and efforts have been made in Massachusetts to reduce environmental pollution in both urban and rural parts of the state.