Ecological resources are defined as species, communities or habitats which act, or are likely to act, as key structuring components of the ecosystem. Ecological resources form a critical component of the natural capital stock by providing, or potentially providing, food and fibre (eg silk fibres, coatings and adhesives), genetic resources (eg natural products with medicinal or chemical properties whose genetic structure can be used as the basis for valuable medicines), biological assimilative capacity (eg natural filtration, bioremediation and sequestration), and ecological insurance against catastrophic change. Ecological resources also have a major role in supporting the scientific, existence, landscape and recreational values of Hong Kong's natural capital stock (see Section 7 - Other Natural Capital Stock Functions).

Maintaining these important functions of ecological resources is strongly dependent on maintaining biodiversity, ie the diversity of living creatures and their habitats. Hong Kong has a surprisingly high level of biodiversity, particularly when this is considered in the context of the long history of human impact on the natural environment in this area which includes deforestation, erosion, fire, hunting/trapping, pollution and the introduction of exotic species (Dudgeon and Corlett 1994). Possible explanations for this high degree of biodiversity could include Hong Kong's position on the boundary between tropical and temperate species distributions, the wide range of physical landscapes, and/or the fact that the wide variety of habitat types are all still represented despite significant loss in many cases.

Two prime sources of information for a discussion of Hong Kong's ecological resources natural capital stock are the Hong Kong University's Biodiversity Survey and the SUSDEV 21 Terrestrial Habitat Mapping and Ranking Based on Conservation Value Baseline Survey. The HKU study's focus is on recording the presence of plants and animals in specific non-marine (ie above the high tide mark) locations across Hong Kong and identifying "hotspots" where high biodiversity is recorded. The SUSDEV 21 study has adopted a complementary approach involving delineating terrestrial and coastal habitats on the basis of satellite imagery and aerial photographs, and assigning an ecological value to those habitats through field surveys and use of existing scientific information. To the extent that findings are presently available from these studies (the HKU study is still being prepared for general use), they have been incorporated into this natural capital stock discussion.