THINKING LAB
Life History — Putting It
All Together
An equilibrial life history is the result of selective forces acting
in a relatively stable environment, just as an opportunistic
life history is the result of the forces typical of unpredictable
environments. Because these forces act simultaneously on
more than one of an organism’s traits, each of these
idealized life history categories is characterized by a whole
set of features, most of which have been discussed in this
chapter. This Thinking Lab enables you to summarize your
understanding of the features typical of r- and K-selected
life histories, which are explained on pages 496–498.
You Try It
1.Construct a table like the one shown here. Fill it in by
answering the following questions. Your answers should
describe characteristics of the environments in which
r- versus K-selection occurs, as well as some of the
life history features that result from these two types
of selection.
(a)Is the climate or environment relatively stable or is it
unpredictable?
(b)Is the population regulated mainly by density-
independent or density-dependent factors?
(c)What is the form of the survivorship curve: Type I
(low death rate for young individuals) or Type III (high
mortality early in life)?
(d)Does the population vary in size or does it tend to
remain stable near the carrying capacity of the
environment?
(e)Is the level of competition in the population high
or low?
(f )Do organisms grow quickly and reach sexual
maturity at a young age, or do they grow slowly and
delay reproduction?
(g)Does each organism in the population typically
reproduce only once or several times over its lifetime?(h)Do the organisms usually produce many small
offspring or a few larger ones?
(i)How extensive is parental care?
( j)What is the relative body size of the organisms?
(k)Do the organisms have relatively long or short lives?
2.Study Table 14.2. Construct two life tables, one for an
extremely r-selected species (one at the “r” end of the
r–Kselection continuum), and the other for an extremely
K-selected species. Your tables should reflect what
you know about the differences between the fecundity
and survivorship of organisms with these two types of
life histories.r-selection
(opportunistic
life history)K-selection
(equilibrial life
history)
climate/environment
type of factors regulating the
population
survivorship
population size/density
level of competition
growth rate/age of sexual
maturity
number of reproductive
events during lifetime
number and size of offspring
parental care
body size (small/large)
life spanComparison of opportunistic and equilibrial life historiesChapter 14 Population Ecology • MHR 497when conditions are good, but may suddenly crash
(perhaps due to a storm, flood, or drought).
Individuals making up such populations are
selected to take advantage of whatever opportunity
they can to reproduce, since conditions may
become unfavourable at any moment and their
chance of surviving to reproduce again is small.
An organism like this, which typically produces
many young in a single burst of reproduction,
displays what is often referred to as an opportunistic
life history. Because populations made up of such
organisms have a high rvalue (a high growth
rate), this type of life history is also referred to as
r-selected. Natural selection that produces such a
life history is referred to as r-selection.
In contrast, populations living in relatively stable
environments generally fluctuate much less in size.
In fact, they usually remain near an equilibrium
point set by the carrying capacity (K) of the
environment. Because they live in high-density
situations, individuals in such populations must
compete with one another for limited resources.
The winners of these interactions are the individuals
whose adaptations give them a competitive
advantage. These adaptations tend to make them
highly specialized for survival and reproduction in