A decline in species-specific habitat quality that leads to reduced survival and/or reproductive success in a population e.g. related to changes in food availability cover or climate. (Source: IUCN)
Loss of suitable habitat for a given species as such that the particular species no longer occurs in that area. (Source: D.B. Lindenmayer, J. Fischer: Habitat Fragmentation and Landscape Change, Island Press 2013) This concept also includes habitat fragmentation, caused by geological processes or human activities.