Ch52: Population Ecology (con’d)
“fecundity” is also a key part of a life table; it’s defined as # of females produced by
An organism’s “life history” consists of how it allocates its resources to growth,
repro, and activity related to survival. SURVIVAL VS REPRODUCTION.
Ex) In France, 50% of 1-yr-old female L. Vivipara lizards reproduce, but they don’t
all survive. Fecundity is high but survivorship is low in France. Even within the
species, there’s variation in life-history traits.
Life-history traits form a continuum. Orgs w/ low fecundity have high survivorship
n tend to grow slowly, and invest time in traits that help them survive.
The mustard plant Araidopsis thaliana grows to sexual maturity fast, but only live a
few months. However, life-history traits can change if conditions change.
A population’s growth rate is change in # of Indivs in popu (N) per unit time (t).
If NO immigration OR emigration is occurig: growth rate=N.r. R is the “per-capita
rate of increase” and is the diff bet (r=b-d) birth rate n death rate.
If birth rates r as high as possible and death rate r as low, then r reaches maximum
called “intrinsic rate of increase”, rMax. Each species w/ certain life history has
characteristic Rmax that does NOT change. But @ any time, the “per-capita rate of
increase” of each popu is likely to be much LOWER THAN RMAX.
A population’s r is also likely to be diff from r values of other populations of SAME
species and to CHANGE over time
10. “Exponential popu growth” occurs when r doesn’t change over tim
e. It doesn’t
depend on # of indivs in population. When increases in size of popu do not affect r,
growth is “density independent”
Exponential growth is observed
when ppl found
new habitats, but can’t continue. Exponential growth is indep of population size
11. However, when “popu density”--# of indivs per unit area—gets very high, popu’s
per-capita birth rate shud decrease n death rate increases, so r declines. So in the
natural world, it’s mostly density DEPENDENT.
12. Carrying capacity, K, can change depending on conditions. If population is below
the Carrying capacity, the popu shud continue to grow. It shud be proportional to
(K-N)/K, which is the proportion of “unused resources n space”.When N is small,
it’s close to 1, and growth rate is high.
This is called “logistic growth equation” and
describes “logistic population growth”—change in growth rate that occurs as
function of popu size.
13. Density-dependent growth for popu has 3 sections: exponential growth, then begins
to decline(N increases, density dependent), growth rate reaches 0 @ carrying
capacity (graph is FLAT)
14. populations change as a result of BOTH density-Independent n dependent factors.
Density-“indep” is usually ABIOTIC (weather patterns); they change birth and