General Ecology: Lecture 8
Intraspecific Competition (Part 1)
Reading (this lecture): Ch. 12, pp. 195-199 (skip sections on plants)
I.
II.
III.
Intraspecific competition: The basics
A.
Review of logistic growth curve [Fig. 11.5, 11.9]
1.
Population growth slows at approximately K/2. Why?
B.
What is intraspecific competition?
1.
Refers to a struggle with members of one’s own species to gain needed resources
when those resources are limited either in abundance or by access.
2.
Affects the birth, death and growth of individuals, and thus of the population as a
whole.
3.
Intraspecific competition is a key element of the process of natural selection.
C.
Scramble/exploitative competition: Each individual has approximately equal access to
the limited resource, resulting in a reduction of fitness that affects individual population
members approximately equally.
1.
Scramble competition: competition is so severe that none of the equal competitors
get enough and all die prior to reproduction. Blow fly example: [Fig. 12.1]
(NOTE: Details are in text!)
a)
Decline of the population (starting at high point)

What causes this decline?
b)
Why doesn’t the population die off completely?
c)
What causes the population to increase again?
2.
Exploitative competition: all individuals have approximately equivalent decreases
in fitness, but may still survive/reproduce.
a)
Similar to, but less severe than, scramble competition
D.
Contest/interference competition: Unequal access to a resource results in only fraction
of the population suffering deleterious effects, rather than all individuals in the population.
1.
Individuals with particular characteristics may be favored for growth and
reproduction, leading to natural selection of those traits
a)
Example: Competition among male elephant seals for beachmaster status
and thus access to females.
Effects of intraspecific competition on growth and fecundity (animals)
A.
Intraspecific competition can decrease growth rates of vertebrates
1.
Example 1: Effects of population density on frog (Rana tigrina) growth rates [Fig.
12.2]
a)
Compare growth curves of populations reared at different densities
b)
High density also reduces chances of successful metamorphosis.
2.
Example 2: Effects of population density on harp seal growth [Fig. 12.3]
a)
Minimum age of sexual maturity increases with population size

Note that time actually goes backwards on the graph.
B.
Intraspecific competition can also decrease fecundity in vertebrates.
1.
Example 1: Number of seal births as a function of pop. density [Fig. 12.4]
a)
Note time built-in time-lag for x-axis of Fig. 12.4
2.
Example 2: Similar fecundity pattern for elk in Yellowstone [Fig. 12.7]
a)
Again, note time lag
3.
One hypothesis (Fowler, 1981): large mammals with long lifespans and low
reproductive rates will maintain a high population growth rate beyond K/2, and
almost to K, and then will overcompensate.
a)
Example 1: Bison. Note rapid drop ~400 adults [Fig. 12.6]
b)
Example 2: St. Paul reindeer herd population crash [Fig. 11.4]
Role of stress in mediating density-dependent responses
A.
Stress hormones secretion (especially adrenocorticoid hormones) may increase at high
densities, affecting many body systems (gonads, immune systems, etc...)
B.
Pheromones from older, mature members of a population may suppress reproduction in
younger members
1.
Studies in wild house mice
a)
Transfer of urine (via filter paper) from mature females a high density
population to juvenile females in low density conditions  delayed puberty
of juvenile females (control urine from mature females held at low density
did not)

What form of competition is this?
b)
Transfer of dominant adult male urine to the low-density population
accelerated the onset of puberty in females

Following this logic, what might you expect it to do in juvenile
males?
Study questions
1.
At what point on the logistic growth curve do population growth rates (dN/dt) begin to decrease?

Indicate this point visually on the logistic curve that plots N against time,

What is the value of N where this begins to occur?

Why does the decrease occur at this point? Provide a generalized reason, and then list
several specific causal factors related to this more general reason. (NOTE: Be sure you are
clear on the difference between the population’s size, as indicated by N and growth rate, as
indicated by dN/dt!)
2.
Define intraspecific competition, being sure to include in your definition the key condition
necessary for it to occur.
3.
Explain the population cycle created by scramble competition in blowflies, being sure to include:

What caused the population, when high, to decline?

Why didn’t the population completely die off?

What caused the population to increase again?
4.
Compare “scramble/exploitative” competition with “contest/interference” competition, in terms of:

The behavior of the individuals involved

The effects on different subgroups within the population
5.
Describe the key negative effects of intraspecific competition/high population densities on animals,
using examples to back up your answer. Also, be familiar with/understand all examples (i.e.
figures 12.2-12.7.
6.
Explain the potential interrelationships among population density, stress and growth rates/fecundity
in animals.
7.
Using the example of urine-transfer experiments in mice, explain the potential role of pheromones
in contest/interference competition. Be sure you understand the control treatment!