Nature features heavily in these structures. Many towers incorporate parks and sky gardens, helping to increase the overall biodiversity of a city, with numerous bird and small animal species finding homes and nesting places. Careful environmental controls ensure that these creatures are protected while not becoming a nuisance for human residents. The outside of buildings are often covered with vegetation, or special membranes, designed to filter pollutants and capture CO2.* Government regulations now require a large percentage of buildings to be fitted in this way, making it a dominant style of architecture today. The artificial parts of this outer layer can also adjust to wind conditions, temperatures, moisture levels and sunlight in order to produce optimal thermal comfort for the human and animal occupants. Algae bio-fuel cells adorning the facade can also absorb CO2 while acting as an additional source of electricity