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The Decline and Demise of Species
At some point in their history, species face a deteriorating environment across their range. Such deterioration may place a species on a trajectory toward rarity and ultimately extinction. Schonewald-Cox and Buechner (1991) proposed four possible forms this trajectory may follow:
1. The number of individuals declines but the range size remains constant.
2. The number of individuals declines in concert with range size such that the density of individuals remains constant.
3. The number of individuals and range size decline in concert, but the number of individuals declines at a faster rate than range size, leading to a reduction in the density of individuals over time.
4. The number of individuals and range size decline in concert, but the number of individuals declines at a slower rate than range size, leading to an increase in the density of individuals over time.
Trajectory (1) seems unlikely because the collapse in total population size does not impact range size. Trajectory (2) may be common when species face continuous habitat loss, but where persisting habitat patches retain their original character. Trajectory (3) may reflect range erosion due to habitat loss coupled with the degradation of the remaining habitat. Trajectory (4) may result when those areas in which a species achieves higher densities are differentially preserved. Given the feasibility of trajectories 2 to 4, the one a given species is likely to take is impossible to predict, unless we have very specific information on the long-term causes of decline and their distribution in space. The better we understand the environmental conditions correlated with the presence of a species, the more informed our speculations will be on the consequences of environmental change. Whether we have such understanding can be tested on contemporary species distributions as Margules and Austin (1995) have done on Eucalyptus radiata in New South Wales, Australia. They calculated the probability of the species occurrence at all combinations of rainfall, temperature, and rock types in their study area. They generated a geographical map of the probability of species occurrence for each 1/100-degree grid cell. There was a good correspondence between the actual and predicted local distribution.
Kinds of Rarity
Contemporary rare species would seem to be a fertile field in which to interpret trajectories of species decline. However, rarity does not necessarily signal a species in decline (Kruckeberg and Rabinowitz, 1985; Fiedler and Ahouse, 1992). Some species may be recent endemics (e.g., Clarkia biloba, Limnanthes bakeri); others may be ancient endemics (e.g., Torreya californica, Ginkgo biloba). Unfortunately, we know little about the abundance and distributions of rare species over long periods of time. Rabinowitz (1981) described alternate forms of rarity on the basis of the geographical distribution (wide, narrow), habitat specificity (broad and narrow), and local population size (everywhere large and everywhere small). All combinations of distribution, habitat specificity, and population size (except narrow range, small populations, and broad habitat specificity) are feasible, thus yielding seven forms of rarity.
Fiedler and Ahouse (1992) categorize rarity within a temporal as well as spatial context. Temporal persistence may be recognized as long or short and spatial distribution as wide or narrow. Rare species in the short/wide category never are locally abundant. Many are herbaceous neoendemics occurring in species-rich environments (e.g., the north American orchid Isotria medeoloides). Species in the long/wide category are not locally abundant. Most are trees or shrubs of species-rich environments. Some are paleoendemics (e.g., the California nutmeg, Torreya californicd). Many species in the short/narrow category are locally abundant (e.g., the California vernal species Limnanthes bakeri). Species in the long/narrow category vary in local abundance. Typically they are trees or shrubs and are members of a pre-Pleistocene flora (e.g., the Macnab cypress, Cupressus macnabiand).