THE NATURAL COMMUNITIES PROGRAM
Todd Keeler-Wolf

Since its inception, the California Natural Diversity Database (CNDDB) Natural Communities
Program has had the responsibility of maintaining up-to-date records of the state's rare natural
communities. This is a separate function from that of the CNDDB rare plant program, but is in
every way analogous. The rationale for maintaining an inventory of natural communities in
conjunction with individual species is grounded in the philosophy of the community as an
amalgamation of species interacting within a common physical environment (an ecosystem). As
such, the community responds to natural or unnatural environmental changes and can be thought
of as an indicator of the overall health of the ecosystem and its component species.

The natural communities inventory is a kind of coarse filter, which maintains surveillance on a
broader spectrum of biological diversity than can be accomplished through monitoring only the
individual rare species. Natural communities have a number of different attributes than
individual species:
They are usually more physically defined than populations of individual species, including a
given structure and discernable edges or transitions to adjacent communities.
They reflect distinct environmental conditions defined by the group of species of which they
are composed. Thus, they can be considered ecological units, rather than just assemblages of
species.
They typically cover an area of moderate size (from one to several hundred acres).
They often form physical units that are treated as habitats by animals and plants and are thus
the most tangible surrogates for ecosystems.

Such attributes enable them to be mapped using remote sensing tools such as aerial photography
or satellite imagery. Mapping the distribution of natural communities locally, regionally, and
statewide gives conservationists and resource managers a valuable tool to understand the range of
natural ecosystems, their abundance, and their relative security.

Defining Natural Communities
Although they are complex assemblages of species, communities do share certain similarities
with species. Like species, some communities are common, some are naturally rare, and some
have become rare as a result of anthropogenic influences. California's environmental variation is
extreme, from coast to mountains to deserts, from sand to serpentine outcrops to flooded alkaline
sinks. Like plants, a given natural community typically occurs in only a small portion of this
environmental spectrum, and species composition in different communities varies widely and is
often non-overlapping. The consequence of this diversity, coupled with a vast range of
detrimental impacts, is that some communities contain a wealth of rare species and are indeed
rare themselves, while other communities are rare but do not contain rare species. Although the
CNDDB maintains records for only the rare types, the classification system we use considers all
of California's naturally occurring communities.

How does one go about defining and classifying all the natural communities of California? The