Rodent Societies: An Ecological & Evolutionary Perspective

stable equilibrium called the minimum threshold density
(fig. 38.4). Densities falling below this threshold are headed
for extinction. Factors that act in this negative way may be
genetic, demographic, or social. In dioecious species like
rodents, at least one male and one female with reproduc-
tive capacities are minimally required for demic survival.
Beyond this, a more substantial group may be needed to
maintain sufficient genetic variation for avoiding inbreed-
ing depression, or for preserving an essential polymorphic
complex such as is needed for effective immune responses.
More than a few individuals may be needed to provide ad-
equate care of young, achieve a viable age structure, or to
survive an onslaught of predation. The latter may involve
predator swamping,the synchronization of breeding so that
vulnerable young are available to predators in large num-
bers but only for a brief period, assuring that an adequate
number survive (Darling 1938). Predation may also be an-
tiregulating in multiannual cycling populations in which
prey declines precede that of their predators. This gener-
ates a situation in which the predator/prey ratio increases
as prey numbers drop, thus accelerating the intensity of pre-
dation as prey decline (Hansson 1984; Pearson 1985; Li-
dicker 1988, 1991, 1994). Finally, social behavior may play
a major role in influencing the minimum threshold. Spe-
cies that live in social groups presumably profit from their
group behavior. Therefore, at least up to some point, larger
numbers means more benefits. This is the classic Allee Ef-
fect (Allee 1931, 1951). Conversely, as numbers drop there
is a group size that is too small for the social benefits to
be maintained. Antiregulating effects at low densities are
appropriately called the Darling Effect (Darling 1938), al-
though they are sometimes labeled “the Allee Effect” (Cour-
champ et al. 1999; Lidicker 2002). Whatever it is called, the
more complex a species’ social system is the higher will be

its minimum threshold density, and therefore the greater

its risk of local extinction when numbers are small. For

rodents, this implies that highly social species, such as

naked mole-rats (Sherman et al. 1991; Faulkes and Bennett,

chap. 36 this volume), prairie dogs (Cynomys;Hoogland,

chaps. 37 and 39 this volume), various ground squirrels

(Spermophilus;Murie and Michener 1984, Hare and Mu-

rie, chap. 29 this volume; Van Horne, chap. 39 this volume),

beaver (Busher, chap. 24 this volume), and flying squirrels

(Glaucomys;Koprowski, chap. 7 this volume) will carry

this added risk factor. Even species with much simpler so-

cial behaviors overall, but which require group behavior at

certain times (e.g., thermoregulatory advantage for over-

wintering, such as in Microtus xanthognathus;Wolff and

Lidicker 1981), will have high minimum thresholds.

Social activities engaged in by rodents that could be lost

at low densities include construction of burrows and dams,

reproductive stimulation, cooperative breeding, predator

swamping when vulnerable young are present, alarm call-

ing, food storage, group thermoregulation, and manage-

ment of appropriate vegetation height. Lastly, we need to

be reminded that deterministic influences at low densities

can interact synergistically or additively with various sto-

chastic factors.

Community and Landscape Contexts

It has become increasingly obvious over the past several

decades that the welfare of target species of interest cannot

be fully understood unless it is placed in the context of

the community and landscape in which they live (Lidicker

1995; Barrett and Peles 1999). All species interact (coact)

with other species in their communities. Coactions can be

trophic (generally exploitative, / ), competitive (/ ),

cooperative (mutualistic, / ), commensalistic (/ 0), or

amensalistic (/ 0), and often they are dynamic in nature

(Lidicker 1979b). As has been abundantly documented, it is

difficult to predict the effect on species when exotic species

become new members of their community. The fact that

such exotic introductions are widely acknowledged as sec-

ond only to habitat destruction in causing species extinc-

tions emphasizes that often the impacts of introductions are

negative for resident species (Holdaway 1999). Disruption

also occurs when community members are removed. The

magnitude of change occurring when this happens directly

measures the dominance relations of the removed species.

When species with high dominance are removed, mas-

sive changes in the community occur. Imagine the impact if

redwood trees (Sequoia sempervirens) were removed from

a redwood-dominated forest. Very few species of that forest

community would persist on the site. Plotnick and McKin-

Issues in Rodent Conservation 459

Figure 38.4 Social benefits may generate anti-regulating (inverse density de-
pendent) forces that will contribute to a minimum threshold density (mtd); K 1
and K 2 are equilibrium densities. A. Deterministic changes in population size (N)
over time when regulating forces only are operating and when anti-regulating
influences are added. B. Population growth rate as a function of population
size, with and without anti-regulating forces.