BIOL 4120: Principles of Ecology
Lecture 11: Intraspecific
Population Regulation
Dafeng Hui
Office: Harned Hall 320
Phone: 963-5777
Email: dhui@tnstate.edu
11.1 Logistic Population Growth
Exponential
growth
model can
not explain
the
population
growth here
Need other
equation
Barnacle
Birth rate and death rate change
with population size N
dN/dt=rN
r=b-d
b=b0-aN
d=d0+cN
dN/dt=[(b0-d0)(a+c)N]N
dN/dt=(b0-d0)[1(a+c)/(b0-d0)*N]N
dN/dt=r N(1-N/K)
K=(b0-d0)/(a+c)
K: carrying capacity: maximum sustainable population size under
prevailing environmental environment.
From exponential growth to logistic
growth
Logistic population growth
Logistic Population Growth
Logistic Population Growth

As resources are depleted, population
growth rate slows and eventually stops:
logistic population growth.
• Sigmoid (S-shaped) population growth
curve.
• Carrying capacity (K) is the number of
individuals of a population the environment
can support.
• (r) is per capita increase rate.
A few examples
11.2 Population regulation involves
density dependence

Environment limits population growth by
altering birth and death rates.
• Density-dependent factors



E.g., Disease, Resource competition
They influence a population in proportion to its size
Resulted in slowing the rate of increase
• Density-independent factors


E.g., Natural disasters
Influence population without regard to the # of
individuals or if the proportion of individuals affected
is the same at any density
Regulation
of
population
size by
densitydependent
factors
Regulation
of
population
size by
densitydependent
factors
11.3 Competition results when resources
are limited

Intraspecific competition: competition among
individuals of the same species
• Scramble competition: growth and
reproduction are depressed equally across
individuals in a population
 All individuals receiving insufficient
resources  local extinction
• Contest competition: some individuals claim
enough resources while denying others a share
 A fraction of the population suffers.
Successful competitors to sustain the
population.
Competition results when resources are
limited


Exploitation: indirectly influencing each
other by consuming the same resources
(eat same grass by zebras , compete for
water uptake by trees, indirectly)
Interference: direct influencing each other
by preventing others to occupy a habit or
access resources (birds, animals).
11.4 Intraspecific competition affects growth
and development


Intraspecific competition is
usually density-dependent. It
increases gradually, at first
affects growth and development,
then individual survival and
reproduction.
Relationship of body growth and
density
Tadpole and white clove
Horseweed experiment
Kyoji Yoda, plant
ecologist
Horseweed was planted
at a very high density
(100,000 seeds m-2)
Density declined in
several months
Weight per seedling
Self-thinning: progressive decline in density
increased.
and increase in biomass of remaining
individuals in a population.
11.5 Intraspecific competition can reduce
reproduction

Intraspecifc competition can
function to reduce fecundity
corn
Harp
seals
shrub
Growth and mortality in steelhead trout
(Keeley et al.)
11.6 Territoriality can function to regulate
population growth



Home range: the area that an
animal normally use during a
year
Home range varies with the
available of food resources,
model of food gathering, body
size and metabolic needs.
Territory: a defended area by
an animal.
• Territory could be part of or
the same as home range.
• Use well-defined behavioral
patterns: song and call,
intimidation displays, attack
and chase, marking with
scents.
Territories of the
grasshopper sparrow

Territory is protected
•
•
•
•
•
•
Food
Mating
Nesting site
Attraction of mates
Avoidance of suboptimal habitat
Needs energy

May not be optimal strategy
when resources are low
11.7 Density-independent factors can
influence population growth
Factors such as temperature,
precipitation and natural
disasters (fire, flood and
drought) are densityindependent factors
The End
Modes of Competition

Intraspecific:
• Competition with members of own
species.

Interspecific:
• Competition between individuals of two
species - reduces fitness of both.
Intrinsic Rates of Increase

On average, small organisms have
higher rates of per capita increase
and more variable populations than
large organisms.
Compare of logistic with
exponential growth curves
Prediction of growth
for gray squirrel
population
r=0.18, K=200,
N0=30