Rodent Demographic Patterns
Rodents have been the subjects of innumerable investiga-
tions on population dynamics, and so relatively a great deal
is known about their demographic patterns. We can say
with confidence that rodents exhibit the full known range
of demographic behaviors. Effective conservation action re-
quires that we know the demographic details for each spe-
cies of concern.
Common species
Some species are indeed common. They may be widespread
as well as abundant. Such species inhabit readily available
habitats, are habitat generalists, or utilize agricultural
crops. Mostly they are herbivores, have high reproductive
rates, and short life spans. They are thus low on the food
chain and are adapted for rapid population growth. Such
common species are generally not of conservation concern,
although commonness does have its hazards. Common spe-
cies attract predators and parasites, may experience intense
intraspecific competition, and invite persecution by hu-
mans. Natural selection may push common species toward
poor dispersal capabilities, and large gene pools experience
genetic inertia. We know from our experience with the
Passenger Pigeon (Ectopistes migratorius) that even super-
abundant species can become extinct in only a few decades.
Common species are the stereotypes of the rodent world.
Examples include species of Microtus, Peromyscus, Apode-
mus, Mus,and Rattus. Because of their commonness, such
species are often major players (keystone species) in their
communities. As such, they may have significant top-down
effects on vegetation, as well as bottom-up impacts as sub-
stantial contributors to higher trophic levels. Lastly, they
can have significant nontrophic interactions with other spe-
cies that share their habitat. An example is the relation
between California voles (Microtus californicus;fig. 38.2)
and western harvest mice (Reithrodontomys megalotis) in
central coastal California (Heske et al. 1984). The harvest
mouse is favored at low vole densities, presumably because
it profits from use of vole runways (commensalism), whereas
at high vole densities harvest mice are completely excluded
from the habitat (amensalism).
Rare species
Rarity comes in various flavors, and the conservation im-
plications are not the same for all types. Rabinowitz (1981)
classifies uncommon species into seven types, based on
three criteria: geographic range large or small, habitat re-
quirements broad or narrow, and local population size. Spe-
cies most prone to extinction would have small geographic
ranges, narrow habitat requirements, and small local pop-
ulation sizes. Most resilient of the rare forms would be
those that occur at low densities, but have large geographic
distributions and can live in a broad range of habitat types.
Rodents illustrate all of these forms of rarity. For ex-
ample, the southern bog lemming (Synaptomys cooperi) is
nowhere common, but has a large geographic range and
occurs widely in grasslands across eastern North Amer-
ica. Similarly, the North American beaver (fig. 38.3) is very
widespread, but is a habitat specialist and reaches densities
of only a very few individuals per square kilometer (see also
Busher, chap. 24 this volume). More vulnerable would be
the Point Reyes jumping mouse (Zapus trinotatus orarius),Issues in Rodent Conservation 455Figure 38.2 California vole (Microtus californicus), an example of a common
and intensively studied arvicoline rodent that has contributed considerably to
our understanding of grassland communities. Richmond Field Station, Richmond,
California; 3 June, 1994; photo by author.Figure 38.3 Beaver dam and pond (Castor canadensis). This rodent is a na-
tional symbol of Canada, a mascot of Oregon State University, a model for indus-
triousness, and a major player in the European colonization of North America.
Danby, New York; 7 October, 1950; photo by author.