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DEMOGRAPHY

- READINGSFREEMAN, 2005
- CHAPTER 52Pages 1192-1196

What is a Population?

- A group of individuals living in a particular

area. - Individuals that interact while seeking resources

and in producing offspring. - Members of a group that are subject to the same

local conditions of the environment. - Members of a single species.

Patterns of Dispersion

- Individuals in a population may be distributed

according to 3 basic patterns of dispersion - Random
- Uniform
- Clumped

Random

- What?
- Scattered no regularity and affinity
- Why?
- Environment uniform individuals solitary

Uniform

- What?
- About equal distance apart regular with no

affinity - Why?
- Resource competition antagonism

Clumped

- What?
- Grouped in some places, absent in others

irregular with affinity - Why?
- Resources patchy individuals aggregate

The Frog Problem

- Dr. T., an ecologist, wanted to find out how

many frogs live in a small pond. On the first

trip to the pond, 55 frogs were caught, banded,

and released. The second trip to the pond, 72

frogs were caught, of those 72 frogs, 12 were

banded. Assuming the banded frogs had thoroughly

mixed with the unbanded frogs, how many frogs

live in the pond?

CAPTURE/RECAPTURE OF FROGS

- How many frogs in the pond?
- If X number of frogs in pond, the

proportion of marked frogs on the 1st day must

equal the proportion of marked frogs on the 2nd

day. - X 72 or X 55 x 72 330 frogs
- 55 12 12

ABUNDANCE

- Most population studies begin with a statement of

abundance. - The number of individuals in a population may be

obtained by - 1. census- Counting all individuals.
- 2. sampling- Counting a known fraction to
- arrive at an estimate of total number.
- Many ecological studies require use of sampling,

such as capture/recapture or the plot method.

PLOT METHOD OF SAMPLING

- Subdivide an area into equal sized plots.
- Randomly sample a known proportion of the area.
- Calculate the average number of individuals per

plot. - Multiply this average times the number of plots

in the area.

A Maple Tree Problem

- A biology student wanted to know how many maple

trees there were in his hometown. He knew the

town covered an area of 520 blocks. To estimate

how many trees, he counted all trees in a random

sample of 10 blocks and found that there were 45

maple trees. How many trees are there in his home

town?

PLOT METHOD FOR MAPLE TREES

- How many maple trees in the town?
- Let X number of maple trees in town. If a

known proportion of equal sized plots are sampled

at random, then the number of trees in the area

is equal to the average number of trees per plot

times the number of plots. - X 45 or X 520 x 45 2340

trees - 520 10 10

ACCURACY OF ESTIMATES

- Both the capture/recapture and the plot methods

are most accurate when distribution of

individuals is either uniform or random. Clumped

distributions of individuals are highly subject

to error. - Larger sample sizes provide the more accurate

estimates.

Understanding Check

- A biologist was concerned about reports that

fewer small mouthed bass were being caught in

Joes Pond. He conducted a capture-recapture

study to estimate the size of this fish

population. He caught and marked 200 bass and

returned them to the pond. The following day, 240

bass were caught of which 60 were marked. When

the fish population of any species falls below

500, the pond should be closed to fishing. Should

the pond be closed?

POPULATION ECOLOGY

- The study of how and why the number of

individuals change over time. - Changes in abundance are made through comparison

of direct counts or estimates in numbers of

individuals. - Changes in density or numbers of individuals per

unit area or volume are often used where

population sizes are very large or difficult to

sample.

MORTALITY, NATALITY AND MOVEMENT

- New individuals are added to a population by

NATALITY (BIRTHS) or IMMIGRATION (IN MOVEMENT). - Existing individuals are removed from a

population by MORTALITY (DEATHS) or EMIGRATION

(OUT MOVEMENT).

POPULATION DYNAMICS

- If natality (births) and immigration (in

movement) exceed mortality (deaths) and

emigration (out movement), then populations

increase. - If mortality (deaths) and emigration (out

movement) exceed natality (births) and

immigration (in movement), then populations

decrease.

POPULATION DYNAMICS

- If natality (births) and immigration (in

movement) equals mortality (deaths) and

emigration (out movement), then populations are

stationary there is no increase or decrease in

number. - Stationary populations are rare, but minor

fluctuations around a mean or average population

size is common.

A POPULATION DYNAMICS PROBLEM

- A prairie is inhabited by a ground squirrel

population for which - natality is 25 per year
- mortality is 20 per year
- immigration is 5 per year
- emigration is 10 per year
- This population is
- a. increasing.
- b. decreasing.
- c. stationary.

DEMOGRAPHY

- A study of deaths, births and movements and

predictions of how these factors determine the

size and structure of populations through time. - Involves construction of life tables,

survivorship curves, fecundity tables and

calculation of reproductive output.

POPULATION PROJECTIONS

- Can be made using
- 1. Life table, maturnity tables and

reproductive outputs. - 2. Age structures.
- 3. Mathematical models that
- incorporate birth rates, death
- rates and doubling times.

BASIC DEMOGRAPHIC TOOLS

- Life tables are constructed from age specific

deaths. lx is the proportion of individuals

surviving to a given age. - Age specific fecundity. mx is the number of

female births to females of a given age. - Net reproductive output, Ro , is sum lx mx over

all age classes.

RO AND POPULATION DYNAMICS

- The following rules can be used to determine if a

population is stationary, increasing or

decreasing. The rules are - If Ro 1, then population is stationary.
- If Ro 1, then population is growing.
- If Ro

LIFE TABLE

- A device for showing mortality changes associated

with an age interval (X). - The number of deaths at a given age (DX) is

recorded. - The number of survivors at the beginning of an

age interval (SX) is determined. - The proportion of newborns that survive to a

given age interval (lX) is calculated.

A SIMPLE LIFE TABLE

A SIMPLE LIFE TABLE

A SIMPLE LIFE TABLE

A SIMPLE LIFE TABLE

AGE SPECIFIC SURVIVORSHIP (lx)

- The proportion of live births that survive to the

beginning of any age interval is defined as age

specific survivorship (lX). - The proportion of the original population alive

at age X0 is always 100 or 1.00. Thus, l0

1.00 - lX for any subsequent age interval is Sx/S0.

A SIMPLE LIFE TABLE

A SIMPLE LIFE TABLE

A SIMPLE LIFE TABLE

SURVIVORSHIP CURVE

- The proportion of individuals living to various

ages is the survivorship of a population. - A survivorship curve is constructed by plotting

age specific survivorship (lx) and age (X). - Survivorship curves indicate those ages at which

mortality is high.

TYPE I SURVIVORSHIP

- Some juvenile mortality
- Secure middle age
- High mortality at old age

TYPE II SURVIVORSHIP

- Some to substantial juvenile mortality
- Constant mortality thereafter

TYPE III SURVIVORSHIP

- Heavy juvenile mortality
- Relative security thereafter

A GRAY SQUIRREL POPULATION IN A WOOD LOT

- This squirrel population living in an Ohio

woodlot has a type II survivorship curve. - Typical of a population with accidental death.

A PALM TREE POPULATION WITH A TYPE III

SURVIVORSHIP

A POPULATION OF DRUMMONDS PHLOX

DEMOGRAPHY

- READINGSFREEMAN, 2005
- CHAPTER 52Pages 1192-1196

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