Ecology: Lecture 8 Intraspecific Competition

Population growth rate (dN/dt) as a function of population size (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 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 competition  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 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 competition  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 competition  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 competition on growth and fecundity  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 (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 (elk)   Is the relationship similar to that for the seals?  Note again the built-in time lag

Fecundity vs. density (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 (reindeer herd on St. Paul I., Pribolof Islands)   Possibly explained by Fowler’s hypothesis/ long time lag (w)

Role of stress in mediating density-dependent responses  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 density-dependent responses  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?