Figure 12.1. Remaining primary forest in Costa Rica.tats (Rabinowitz 1981). Each type of rarity has differ-
ent causes and implications for conservation manage-
ment. Some species have always been rare in one or
more of these respects. For example, they may be rep-
resented solely as a small population that only re-
cently became isolated and diverged from a more
widespread species. The loss of such species has less
of an impact on global biodiversity than, for example,
the extinction of an ancient, evolutionary distinctive
lineage. Alternatively, some aspect of a species' bi-
ology may make it constitutionally uncommon—its
position as a top carnivore in a food web, or its geo-
graphical restriction to a mountaintop or small island.
To the extent that rarity is a fundamental feature of
the biology of such species, they may be less prone
to extinction than they appear. By studying the dy-
namics of naturally rare species, conservation biolo-
gists may be able to draw lessons about populations
that have been driven to rarity by human activities.
Of particular concern are once-abundant species
that have experienced sudden population declines.
These species may have been driven to rarity due
to predators (including excessive harvesting by hu-
mans), disease organisms and other parasites, com-
petitors for limited resources, or amensals (species
that have a negative impact on other species but are
not themselves harmed by the interaction). For spe-
cies that depend on other species (e.g., plants that
have obligate mycorrhizal associations or plants that
rely on specialized insects for pollination), the loss
of their mutualists can cause their own rarity. Short-
term physical perturbations such as fire, storms, or
drought can lead to rarity, as can longer term distur-
bances such as climate change (for specific examples,
see references in Soule and Wilcox 1980, Meffe and
Carroll 1997). Alone, each of these factors can nega-
tively affect population growth; in combination, ago-
nistic species interactions plus physical perturbations421 Conservation Biology