**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