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Ecology: Lecture 8 Intraspecific Competition
Population growth rate (d. N/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 Power. Point, 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 (xaxis) § 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?