11.19

Population Dynamics

• Populations can overshoot carrying capacity and subsequently crash

• Carrying capacity can also change through time as well. Following an “overshot” of the population, K often declines
  

• Predator-prey cycles: lynx and hare. Hare increases in population, and lynx follows but delayed trends

• Bottom-up hypothesis (about lacks on food chain): when their populations reach high density, hares use up all their food and starve; in response, lynx also starve

• Top-down hypothesis (about mega carnivores and predators): lynx populations reach high density in response to increases in hare density. At high density, lynx eat so many hares that the prey population crashes.

• Krebs and colleagues studied this: different plots with different environments
  3 control plots, 1 plot with no lynx, 2 plots with extra food for hares, and 1 plot with no lynx and extra food for the hares

• Conclusion: Hare populations are limited by both predation and food availability. When predation and food limitation occur together, they have a greater effect than either factor does independently.

Plot 3 would support the bottom up hypothesis
Plot 2 would suppose the top down hypothesis

Case Study: Human Population Growth

• Stable fecundity = available resources

• Pyramid shaped fecundity = scarce resources

• Human carrying capacity: has it changed? When will our population stop growing?

Species Interactions and Community Structure

• Commensalism — least studied and least supported type of interaction. Could be fake lol. (+/0)

• Competition — extremely common interaction. Lowers the fitness of all individuals involved. Intraspecific or interspecific. Negative for both. Most species avoid competing with other species.
  

• Niche: the range of resources that the species is able to use, or the range of conditions it can tolerate

• Experiments by GF Gause in the 30’s used Paramecium to study competition

• Separately, two paramecium species showed logistic growth

• Together, with the same food supply, only one species showed logistic growth (but with reduced carrying capacity). In contrast, the second species was driven to extinction