Population Ecology

Study of Populations in

relation to their environment.

Environmental influences on

Pop. DensityDistributionAge structuresize

Population: Individuals of a single species that occupy the same general area.

Important Characteristics

1. Density

2. Dispersion

Density

Number of individuals per unit area or volume.Ex: Diatoms - 5 million/m3

Trees - 5,000/km2

Deer - 4/km2

Dispersion

The pattern of spacing among individuals within the boundary of the population.

How do you count a population?

1. actual count

2. Random plots, then extrapolate

3. Indirect indicators: number of nests; burrows; tracks; droppings

4. MARK & RECAPTURE METHOD

Mark and Recapture Method N= Population estimate Trap, capture, mark, release. Later-recapture, determine proportion of those recaptured

that were marked.

N= # marked in first catch X Total # in 2nd catch Number of marked recaptures

Assumptions: 1. the proportion of marked animals in the 2nd trapping is equivalent to the proportion of marked

animals in the total population2. Those marked mix uniformly within the pop.

Population densities not static

Birth

Death

Immigration

Emmigration

Dispersal patterns

3 general patterns

Know these!!

1. Clumped PatternMost common patternResults form patchy environmental conditions-food, nutrient availability. Ex: Mushrooms under rotting log.

Groups enhance predationSafety. May increase chances for survival.Ex: Schooling behavior Flocks of birds humans

2. Uniform Dispersion

Individuals are evenly spaced

Results from direct antagonistic interactions

Plants secrete chemicals-inhibit nearby germination & competition (alleopathy)

territoriality

3. Random Dispersion

Spacing varies unpredictably

Absence of strong attractions or repulsions between individuals.

RARE pattern in nature

Example: windblown seeds land randomly & germinate.

Demography

Study of vital statistics of a population & how they change over time.

Add

Lose

2 most important featuresAge structure-relative numbers of

individuals of each age group in a population

Sex ratios

1. Life Tables

How long, on average, are individuals of a given age expected to live.Age-specific summary of survival patterns.

Follow the fate of a cohort (indiv. Of same age) from birth to death.

Do males or females have a higher death rate? Who lives longer?

2. Survivorship CurvesGraphic representation of data in Life Tables.

Plot of the proportion or numbers in a cohort still alive at each age.

3 general Curve Types: ( be able to recognize and describe traits; members exhibiting each; example organism)

Type I—humans, elephantsType II-squirrelType III-oysters, clam

Draw and label in your notes

Type I

Low early deaths.Steep decline in death rates among those older Produce few offspring, but provide good care.

Ex: HumansOther large mammals

Type IIIntermediate

Constant death rate over life span.Ex: Annual plants small mammals ( grey squirrel),

lizards

Type III High death rates for the young. ( sharp dip in curve initially)

Few live to adulthoodAssociated with:Produce large numbers of offspring-but provide

little or no care.

Ex: Oysters

VariationsCurve type may change between young and adults.

Ex: Nestlings - Type III Adult Birds- Type II

Stair-step curveInvertebrates—crabs; high mortality

during molt, followed by low mortality

Life-History Traits

Life History: Timing of reproduction and death.Highly diverse, but do show patterns

Determines how populations grow.Results from Natural Selection.

Darwinian “fitness”

Darwinian “fitness”

Survive AND Reproduce

Reproduction and survival—Life History Traits

1. # of reproductive episodes

# offspring per reproductive episode

Age at first reproduction

2 Common Reproductive Patterns

1. Semelparity: “big-bang” reproduction1 reproduction event

Salmon, agave ( desert plants) , annual plants

2. IteroparityFewer offspring at a timeOver many reproductive seasons

Population Studies & Reproductive Rates

Focusing on females and female offspring.

2 Kinds of Reproductive Strategies identifiedRK

Life History Selection

1. "r" Selected species

2. "k" Selected species

“r" – Selected Species (density independent selection)

Increase fitness by producing as many offspring as possible.

Do this by one of these strategies:Early maturationMany reproductive eventsMany offspring in one reproductive event

“Big bang” reproduction (semelparity) (pink salmon, agaves) is one time reproduction.

r-Selected Result

Maximize reproduction so that at least a few offspring survive to the next generation.

Most offspring die (Type III curve).

“K" – Selected Species (density dependent

selection)Increase fitness by having most offspring survive.

Maintain populations at or near “K”

Do this by:High parental careLate maturationFew reproduction eventsFew offspring.

K- Selected –Results..

Maximize survivorship of each offspring

.

Few offspring, but most survive (Type I curve).

What is the strategyFor a weed?

For Garden Pests?

POULATION GOWTH

Zero population growth:

Per capita birth + Per capita death=0

Exponential Growth

Produces a “J-shaped” growth curve.

Ideal conditions

unlimited resources.

Example: Introduce into new or

unfilled environment Rebounding population

Logistic Growth-

Population growth related to Life History Traits

S-shaped” growth curve.Characteristic of “k" species.

Common when resources are limited.

Populations are limited by space, food. That limit is called the CARRYING CAPACITY

The graph shows a logistic population curve.

At what level do the deer reach their CARRYING CAPACITY?

What Limits Population Size?

Density-dependent factors: limited resources- space, food, water, air related to population size

Density-independent factors: random occurrences that can limit population - earthquake, bad weather. not affected by population size

Is disease density dependent, or density independent?

Additional Comments

Populations often overshoot “K” ( go beyond) , then drop back to or below “K”.

Regular Population Cycles

Cyclic changes in N over time.

Often seen in predator/prey cycles.

Ex: “Boom & Bust Cycle” of Snowshoe

Hare -& Lynx

Predators kill and consume other organisms. Carnivores prey on animals, herbivores consume plants. Predators usually limit the prey population, although in extreme cases they can drive the prey to extinction.

Why predators rarely kill and eat all the prey: 1.Prey species often evolve protective mechanisms such as

camouflage, poisons, spines, or large size to deter predation. 2.Prey species often have refuges where the predators cannot reach

them. 3.Switch its prey as the prey species becomes lower in abundance:

Know how to read a graph for population growth, where K is; where the

overshoot “K” is

Age Structure Diagrams

Show the percent of a population in different age categories .

Method to get data similar to a Life Table, but at one point in time.

HUMAN POPULATION GROWTH

Grown since history

1A.D. 300 million

Since 1650 ( 500 million), growing exponentionally.

Over 6 billion today

Exponential Growth

Produces age structures that are a triangle or pyramid shape.

Logistic Growth

Produces age structures that have even sizes between most age categories.

Declining Populations

Produce age structures with a narrow base and wider middles.

Reveal current and future growth trends.

Ecological Footprint

Model to predict “K”

Relative to The Available Ecologic capacity

Land usage requirements to meet our varied human needs.

SummaryKnow density and dispersion patterns.

Know Life Tables and survivorship curves ( and representative organism for the 3 types).

Know what “K” is

Be able to contrast and compare “r” and “k” strategies ( Suggest making a chart-include Logistic Curve, Exponential growth curve, characteristics of populations).

When/how density independent, and dependent growth factors affect pop size.

Know how (match) Age-structure pyramids relate to either Rapid, Slow, or Zero Pop. Growth; their shapes & what the shape means.

Summary

Know exponential and logistic growth curves.Relate them to PGR