Since writing my first article for the Yerba Buena Chapter newsletter in 1994, I’ve focused primarily on the individual taxa – extant, extirpated, or extinct – that make up San Francisco’s natural heritage. Aside from occasional departures to discuss communities or particular locales, the focus has always been on the species, subspecies, and varieties themselves. It dawned on me that I’ve never actually discussed the concept of rarity. I think it’s about time.

Biologically, the term “rare” typically refers to a species’ limited geographic distribution, limited population size, or limited numbers. Taxa (a taxon is a distinct entity such as a species, variety, or subspecies) can be considered rare if they occur as infrequent individuals but scattered over a wide geographic area, as abundant individuals with a very narrow geographic range, or as infrequent individuals with a narrow geographic range. The term endemic is also frequently used to refer to organisms restricted to a particular locality. Plants that only or mostly occur on a particular soil type, such as serpentinite, are said to be edaphic endemics. The presumed origin or evolution is thought to play an important role in rarity or endemism. Paleoendemics, for example, are taxa that were once more widespread but have become isolated due to climate changes over geologic time. Neoendemics are taxa that were never common, but are believed to have diverged from a more widespread ancestor (i.e., speciation).

The concept of rarity is much less clear-cut than one might think. This is primarily due to the fact that the species concept, and especially the subspecies concept, is debatable. The American evolutionist Ernst Mayr defined a species as “a group of actually or potentially interbreeding natural populations that is reproductively isolated from other such groups.” In other words, hybrids between species occur rarely in nature. Truth is, taxonomists have historically had little or no experimental evidence on which to base their taxonomic decisions. Species were typically differentiated based solely on morphological features (form and structure), phenology (growth and reproduction), and assumptions regarding their evolutionary relationships. Species are therefore groups of organisms that differ in one or more characteristics and do not intergrade extensively when occurring together in nature.

Populations of organisms within the same species that are more or less distinct from one another but still intergrade may be classified as subspecies or varieties. A large number of the taxa in California that are considered to be rare are subspecies of more common species. This is where the taxonomists fall into one of two categories – lumpers and splitters. Lumpers tend to be conservative in formally distinguishing a population or race as a subspecies. Splitters tend to be willing to do so based on very subtle differences between populations. The decision to lump or split populations can have great conservation implications from a legal standpoint.

A species might occur from Oregon to Baja California, but consist of large, isolated populations along a continuum from north to south. As a species, the organism would not normally be regarded as rare. However, if the populations are distinct enough to be legitimately classified as subspecies, they could be regarded as rare. If such populations are under imminent threat of extinction, the populations could even warrant protection under the state or federal Endangered Species Acts.

The term “rare” can mean many things to many people. It can mean infrequent, uncommon, or unusual. But it also implies highly valued or special, two very subjective and emotionally charged terms. In my consulting business, we don’t like to use the word “rare” for this very reason. Instead, we refer to taxa as having “special-status.” Special-status species are those that are state- or federally-listed as Endangered or Threatened, are on the California Department of Fish and Game’s lists of special plants and animals, are on the California Native Plant Society’s 1A, 1B, or 2 lists, meet the criteria listed in the California Environmental Quality Act (CEQA) guidelines, or are otherwise protected under such legislation as the Migratory Bird Treaty Act or specific sections of the California Fish and Game Code. This is a large list.

Are all special-status species rare? By no means. Do they represent valuable biological resources? Certainly. In any case, pursuant to CEQA, impacts to special-status species must at least be addressed when assessing potential effects of proposed human activities on the environment.