The three principal natural terrestrial habitats in the Philippines occur along elevational gradients, with lowland forest, montane forest, and mossy forest as the primary elements, arranged from the bottom to the top of each mountain. The elevational range of the habitats on any specific mountain varies based on the elevation of the mountain's peak, local topography, and average annual rainfall. The habitats occur lower on mountains that are near the coast and/or in areas with high rainfall, and higher on mountains that have high peaks, are far away from the shore, and/or are relatively dry. On a given mountain range, the elevation of these types of forest may overlap, and there is always fairly gradual transition between them.
In addition, there are several natural habitats that occur over more limited areas but are locally very important. Included among them are forest over limestone (known as karst), forest over ultrabasic (or ultramafic) soils that are produced by geological features called ophiolites, and seasonal (or "deciduous") forest.
We use "primary forest" and “old growth forest” to refer to forest that has not been disturbed significantly by major human activities. "Secondary forests" have been disturbed, often heavily, by logging, burning, mining, or erosion; they range from well-regenerated second growth of natural forests, to planted tree farms of exotic species, to scrubby areas with scattered trees, and grassland. Pine forest is principally the product of regular burning at mid to high elevations on Luzon and Mindoro, often on steep slopes. We use "agricultural areas" to refer to a range of habitats, from coconut palm plantations to mixed pasture, fields, and orchards, and varying densities of human habitation.
The following sections refer briefly to each of these types of habitats, including those that are natural and those that are caused by human activity.