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Chapter 10

Chapter 10

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Biological Sciences
Marc Cadotte

Lecture 8, Chapter 10 Four major types of population growth patterns: 1) Exponential Growth  Can occur when conditions are favorable in which, for a limited time, pop. increases/decreases by constant proportion at each point in time  Rapid growth affected by “regulatory” density dependent factors  Cannot grow indefinitely  Can be represented by two equations i. Reproduction occurs at discrete time periods ii. Reproduction occurs continuously  Periods of exponential growth can occur when: i. Within established range of a species ii. Species finds new area (look at cattle egret example) 2) Logistic Growth  Population increases exponentially at first but then approaches equilibrium  “plateau” is the maximum population size (carrying capacity)  Pop. growth does not precisely match predictions of LG  For the carrying capacity (K) to be constant, birth and death rates at a particular density must not change from one year to the next o Not realistic in nature  Instead, the birth or death rate may take on range of value  environmental conditions changes over time  pop size fluctuates at an average value of K o Much more realistic in nature 3) Population Fluctuations  seen in all populations  size rises + falls over time  occurs as erratic increases/decreases in abundance from overall mean value  can also occur as deviations from a population growth pattern  Can be small or explosive (population outbreak)  affected by density-dependent & density-independent factors 4) Population Cycles  alternating periods of high & low abundance occur after constant intervals of time  caused by internal factors (hormonal, behavioral changes) & external factors (weather, predators)  but no universal cause  example: lemming abundance rise and fall every 4 years, population cycle driven by predators (stoats) Delayed Density Dependence  Definition: delays in the effect that density has on population size  Individuals born in given time period are influenced by pop. densities several time periods age  Contributes to population fluctuations o Cause growth, survival or reproduction of individuals to vary over time  pop growth rate vary from one time period to the next o ex. delay in response of predators to prey density  predator numbers fluctuating over time  dN/dt = rN[1 – (N (t-T)K]  dN/dt is rate of change in pop size at time t  r is pop growth rate under ideal conditions  N is pop size at time t  K is carrying capacity  N (t-T)ndicates pop growth rate is reduced by pop size at t – T in the past  Fluctuations depend on r and T values (Robert May)  01.57  regular cycle  fluctuates about K  stable limit cycle  Overall, population fluctuations become more pronounced as rT increases  Nicholson’s Blowflies  Conducted two separate experiments (fig 10.11)  In the first experiment, Nicholson provided adult blowflies with unlimited food but restricted maggots to 50g of food per day o Lots of foods for adults  many adults  many produced eggs o Hatched eggs + low food  lots of maggots died before adulthood  produced few adults  adult pop declined after reaching peak o Low levels in
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