Environmental Engineering Reference
In-Depth Information
between reproductive events is largely a consequence of mothers providing
extended care for their offspring. Pandas can breed up to about sixteen years
of age and they appear to display a Type II survivorship curve during the
course of that lifetime.
In theory, species with extended birth intervals tend to have a slower ca-
pacity to recover from disturbances than species with less protracted birth
intervals. In light of threats due to habitat destruction, the question facing
managers is this: Can the Panda population sustain itself under current habi-
tat conditions and demographic rates in the population? Zhou and Pan
(1997) showed that the population could indeed continue to produce suffi-
cient numbers of individuals to support positive population growth.
This estimate of the population's propensity to sustain itself can be
viewed as a baseline reference.We can now enlist the machinery of struc-
tured population modeling to ask other questions that also aid in manage-
ment planning. For instance, suppose that we wanted to know how sensitive
the population might be to further habitat destruction. Let's suppose that
habitat destruction meant that a female could no longer provision both her-
self and her offspring as well as under conditions of intact habitat.As a con-
sequence, she and her offspring will suffer increased risk of age-specific
mortality.The question then is:To which mortality rate is the population
more sensitive, a mother's or the offspring's?
This question can be answered by systematically changing one parame-
ter value at a time and then projecting how the population will grow after
the parameter value has been changed. For example, suppose we estimated
that a certain level of habitat destruction would lead to a decrease in cub
survivorship from natural levels of 83 percent down to 70 percent.We can
then reduce the parameter value p associated with cub survival from 0.83
down to 0.7, leave all other parameters the same as before, and then pro-
ject what the future population would look like. In this case, we see that
the population will persist, but over a two-hundred-year time span it will
have two hundred fewer individuals than observed under higher survivor-
ship (figure 5.4). If we change the survival of young, soon-to-breed females
by a similar amount then we will see a much larger change in population
size over time. Indeed, we will see a loss of 118 0 individuals.This implies
that the population is far more
sensitive
to changes in the survival of females
that are reaching reproductive age than it is to cub survival.That is, conser-
vation efforts should be devoted to protecting young females more than
cubs.
