The term habitat refers to the type of environment in which an organism or species occurs. For plants, habitat is mostly defined by its physical attributes (e.g., rainfall, temperature, topographic position, soil texture and moisture) and its chemical properties (e.g., soil acidity, concentrations of nutrients and toxins, oxidation reduction status). For terrestrial animals, the habitat is defined by the physical structure (e.g., grassland, shrub-dominated, or forested) and the plant species composition. Habitat for aquatic organisms is mostly determined by physical factors (such as running versus still water, depth, and light availability) as well as climatic and chemical components (especially nutrients). Broad ecological characteristics may also be included in the characterization of habitat, for instance forest, prairie, or tundra habitats on land, and littoral or pelagic ones in water. Within a given habitat there may be various micro-habitats, for example the hummocks and hollows in bogs, or patches of disturbance in forests.