Ecology: Lecture 8Ecology: Lecture 8

Intraspecific Competition

Population growth rate (Population growth rate (ddN/N/ddt) t) as a function of population size as a function of population size

(N)(N) Intraspecific competition is one of the

density-dependent factors that decreases population growth rate at higher population densities (especially >K/2)

What is intraspecific What is intraspecific competition?competition?

DEFINED Struggle with members of one’s own

species to gain needed resources when those resources are limited either in abundance or access.

IMPORTANCE: Affects the birth, death and growth of

individuals, and thus of the population as a whole.

Key element of the process of natural selection.

Scramble/exploitative Scramble/exploitative competitioncompetition

DEFINED: Each individual has approximately

equal access to the limited resource reduction of fitness is approximately equal

Scramble competition: so severe that none of the competitors get enough all die prior to reproduction Blowfly example [Fig. 12.1]

Example: blow fly Example: blow fly experiments [Fig. 12.1]experiments [Fig. 12.1]

Experimental design (key aspects)

How can scramble competition lead to oscillation of the population? What causes the severe decline? Why doesn’t the entire population die

off? What causes the rapid rise?

Scramble/exploitative Scramble/exploitative competitioncompetition

Exploitative competition: all individuals have approximately equivalent decreases in fitness, but may still survive/reproduce. Similar to, but less severe than, scramble

competition.

Contest/interference Contest/interference competitioncompetition

DEFINED: Unequal access to a resource only

fraction of the population suffers serious deleterious effects. Individuals with particular characteristics

may be favored for growth and reproduction, leading to natural selection of those traits

Example: Competition among male elephant seals for beachmaster status access to females.

Effects of intraspecific Effects of intraspecific competition on growth and competition on growth and

fecundityfecundity Example 1: Effects of population

density on frog (Rana tigrina) growth rates [Fig. 12.2]

Compare growth curves of populations reared at different densities

High density also reduces chances of successful metamorphosis.

Example 2: Effects of population density on harp seal growth [Fig. 12.3]

Minimum age of sexual maturity increases with population size Note that time actually goes backwards on the graph.

Fig. 12.1, 12.2, and 12.3 were not available as PowerPoint, but will be shown in class. Be sure you understand them!

Fecundity vs. density Fecundity vs. density (harp seals)(harp seals)

Number of seal births is a function of population density. Note the time lag (x-

axis) Has the population

increased or decreased over time?

Fecundity vs. density Fecundity vs. density (elk)(elk)

Is the relationship similar to that for the seals? Note again the built-

in time lag

Fecundity vs. densityFecundity vs. density(bison)(bison)

How does the graph for bison compare to that for seals and elk?

Fowler’s hypothesis Large mammals will

maintain a high population growth rate beyond K/2 (to near K) and then overcompensate.

Relate to long response time lag (w)

Overshoot of K followed by crash Overshoot of K followed by crash (reindeer herd on St. Paul I., Pribolof Islands)(reindeer herd on St. Paul I., Pribolof Islands)

Possibly explained by Fowler’s hypothesis/ long time lag (w)

Role of stress in mediating Role of stress in mediating density-dependent density-dependent

responsesresponses Stress hormone secretion

(especially adrenocorticoid hormones) may increase at high densities, affecting many body systems (gonads, immune systems, etc...)

Increases in spontaneous abortion in females

increased susceptibility to disease

Role of stress in mediating Role of stress in mediating density-dependent density-dependent

responsesresponses Pheromones from older, mature

members of a population may suppress reproduction in younger members

Example: Studies in wild house mice Basics of experiment with female urine (be able to

explain!) Controls? Key results

How did urine from “high-density” mature females affect the juvenile females?

What form of competition is this? Basics of experiment with male urine

How did male urine affect females in the low-density population?

What might you expect the same urine do to juvenile males?