AEC 360  Ecology

Lecture 14:  Population demography

I.  Using demographic population models in conservation biology: sea turtle example

    A.  all early conservation efforts for sea turtles focused on eggs and hatchlings

    B.  What’s the problem?
            1.  Juveniles and adults spend time off-shore
            2.  Tasty things are off-shore too

            3.  Drowning while caught in fishing gear is largest source of mortality of subadult and adult sea turtles

     C.  Crouse, Crowder, and Caswell used life tables to show that even a small decrease in subadult and adult mortality would have much greater influence on population growth
            1.  Protecting 100% of eggs would not cause populations to grow

    D.  Conservation actions that came directly from analysis of demography
            1.  in areas where turtles are common, fishermen have limits on how long they can soak their nets
            2.  shrimp fishermen must use a turtle-excluder device (TED) in their trawl nets

II.  Age, size or stage structured populations
    A.  For many populations, we know more about the growth rate than what can be estimated from the entire population as a whole
        1.  Survival and reproduction contribute to fitness...  and to population growth

        2.  Many species have different rates of survival and reproduction at different life stages

            -- See Fig. 12.19 for survivorship curves 

        3.  These may relate to size, age, or life form

        4.  Review definitions:
            Survivorship is the probability of living to a certain age, size or stage
            Fecundity is the number of offspring produced at a given age, size or stage

    B.  Life Tables

        1.  A summary by age, stage, or size of the survivorship and fecundity of individuals in a population

        2.  From information in a life table, you can estimate population growth rate  

        3.  Also can observe how changes to a population at a specific age, stage, or class will affect growth rate

        4.  Usually track only females

x = age
lx = survivorship to age x
bx = birth rate at age x

    C.  Calculating rates from life tables

        1.  We can determine the average number of offspring produced by each female, which is just the sum of the birth rate per age class:

                Ro = net reproductive rate =   (sum of, represented by the Greek symbol sigma)  lx bx

        2.  Now we can approximate the population growth rate by also calculating the mean generation time:

            T =   (sum of) xlx bx /   (sum of)lx bx

        3.  Finally, we can approximate the growth rate

            ra = ln(Ro)/T