AEC 360   Ecology

Lecture 16:  Population Regulation and Dynamics

What factors limit population growth in nature?

I.  Density-dependent population regulation 

    1.  Density Dependent factor:  a factor that has an influence on individuals within a population that is related to the degree of crowding within that population

    2.  Density independent factor:  influence on individuals within a population is not related to the degree of crowding; can regulate population growth rate but not population size

    3.  Examples of density-dependent population regulation in nature

        -  Fruit Fly (Fig 12.7) and Common Tern (Fig. 12.8) examples

        -  By manipulating density, population performance can change (deer example) 


        - Density affects plant populations (Fig. 12.9)


    4.  Gause's (1934) experiment on population limitation in Paramecium (Fig. 12.16)

    5. As density increases, per capita growth rate decreases

II.  Predicting density-dependent population regulation

    A.  Change in population growth rate with density in humans (Pearl and Reed)

        1.  The US human population growth rate, r, was decreasing over time
        2.  Predicted that population size would eventually level out
        3.  Made a slight modification to the exponential growth equation, allowing r to decrease
        4.  The carrying capacity is the population size where the growth rate is zero

    B.  In the logistic equation, the change in population size (dN/dt) is found by making the following modifications to the exponential growth model:

                --  r = ro(1-N/K)
                --  ro is just r at very small population sizes
                -- K is the carrying capacity

     C.  Implications of the logistic equation
                -- r decreases with population size
                -- change in population size (dN/dt) is highest at half of carrying capacity
                -- Population size increases and then levels off over time

III.  Positive density dependence is possible
      At low densities, increasing density can positively affect populations
        - easier to find a mate; avoid predators; etc
        - called an Allee Effect Fig. 12.13 (see 12.11, 12.12)

IV.  Population fluctuations

   A. Carrying capacity can be considered an equilibrium population size -- theoretically, populations should be at about carrying capacity most of the time

        -However, we see that many populations fluctuate through time, never reaching an equilibrium value

    B.  Density independent population fluctuations (Fig. 12.5)

    C.  Density dependent cycles (Fig. 13.9)

            1.  What causes populations to cycle? (Fig. 13.10)
                    --  Regular environmental changes (el Niņo)

                    --  Intrinsic dynamics

                        -type of fluctuation resulting from time delay will depend on population growth rate

                        -damped oscillation: population gets closer and closer to carrying capacity

                        -limit cycles: numbers alternate between high and low values

                    -- Interactions with predators or disease   Figure 13.11, 13.12    

                        - Case study with Red Grouse


            2.  What allows regular oscillations to persist is time delays (time lags) in the ability of a species to respond to changes in its environment, including in abundances of interacting species