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Ecology: Lecture 8

Ecology: Lecture 8. Intraspecific Competition. Population growth rate ( d N/ d t) 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).

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Ecology: Lecture 8

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  1. Ecology: Lecture 8 Intraspecific Competition

  2. 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)

  3. 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.

  4. 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]

  5. 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?

  6. 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.

  7. 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.

  8. 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.

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

  10. 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?

  11. Is the relationship similar to that for the seals? Note again the built-in time lag Fecundity vs. density (elk)

  12. 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) Fecundity vs. density(bison)

  13. Possibly explained by Fowler’s hypothesis/ long time lag (w) Overshoot of K followed by crash (reindeer herd on St. Paul I., Pribolof Islands)

  14. 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

  15. 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?

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