Environmental Engineering Reference
In-Depth Information
2. A cross-sectional study in which we collect data on the number of
individuals of different ages in a population and their birth history.
For humans, these data can usually be obtained from historic town
records or from headstones on graves.With wildlife species, there
are various techniques that have been developed to age individuals
and obtain reproductive values (Schemnitz 1980). For example,
hunter check stations run by State Wildlife agencies routinely are
used to collect this kind of information from carcasses of hunted
animals.These values are used to estimate age- or stage-specific sur-
vival and reproductive data. If we plot the values of population size
against time (in this case, age is a surrogate for time) we can gener-
ate what is known as a
survivorship curve
(e.g., see Box 5.1).
For the data presented in Box 5.1, we find that the population size de-
clines with a downward curve.That is, it declines rapidly at first and then
slows its rate of decline with increasing age.This is not, however the only
form of survivorship curve. Indeed, there are three general forms (Pianka
1988) known as Type I, II, and III survivorship curves (figure 5.3).Type I
curves are characteristic of long-lived, large bodied species such as large
mammals. Birds and some mammals display Type II curves. Reptiles, am-
phibians, and insects tend to display Type III curves (Pianka 1988).The ex-
ample in Box 5.1 approximates a Type III survivorship curve.
Survivorship is environmentally determined. But, it is also a species char-
acteristic that has been molded by evolution. For example, a Type III curve
is often associated with species that cannot provide extended care for their
offspring. Such species produce many more offspring than one would ex-
pect to survive and the newborn individuals must survive the mortality
gauntlet on their own (e.g., sea turtles and predation).Alternatively, species
that are able to provide extended care
tend to produce fewer offspring and
they invest more heavily in each of
them to ensure that they survive the
juvenile phase. This is important to
keep in mind. Whenever we use
management to change the survival
probabilities of individuals in a spe-
cies we are effectively imposing new
selection pressures on these species
Whenever we use management to
change the survival probabilities of
individuals in a species we are
effectively imposing new selection
pressures on these species and
hence are potentially playing with a
strategy that has been molded by
the species' evolutionary history.
