SO, we have an idea of what a population is….

HOW DO WE DESCRIBE IT?

“Individuals of a single species living within a given area”

Some Useful Information:

• How many individuals?• How old are they? How long do they live?• How many young do they have?• What is the genetic makeup of the group?• What is their distribution?

Note that these can vary through space and time…

Population structure

More Useful Information:

• Density• Spacing patterns among individuals

– Territory size– Distance between conspecific plants

• Movement patterns

These too may vary through spaceand time…

Boundaries are often hard to determine….

If boundaries unclear, or if population is very large, density may be the descriptive tool we most need

10 individuals/km2

Density is particularly useful for plants and other sessile organisms

Examples:barnaclessagebrush

Density can give clues aboutenvironmental quality or ecological processes

Environmental quality

Blue tits in southern Europe nest in both deciduous and evergreen oak forests,habitats that differ in environmentalquality

(what does it mean?)

Environmental quality and blue tits

Parus caeruleus

Density can give clues aboutenvironmental quality or ecological

processes

Ecological processes

High densities of an intertidal algae,Enteromorpha, occur in tide pools where they are sheltered from grazing by the snail Littorina littorea.

Lubchenco 1978

Changes in density assumed to reflect changing local

conditions

Example: Burrowing owls in California,1996-2001

J.A. Gervais

Clair de Beauvoir

Burrowing owls at LemooreNAS live within the AirOperations area and insmall easements surroundedby industrial agriculture

Many owl populationsare tied to theabundance of their prey

Changes in density of owl nests and voles found in pellets

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19991998 20001997

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Plants and Density

“The Law of Final Constant Yield”

Yield = total biomass of vegetationper unit area (single species)

Total yield is independent of plantdensity above a certain threshold

Yield Density Relationships:Examples

Spacing of individuals

Patterns of individuals within a population give clues to the processes affecting that population

General spacing patterns: Clumped

•Social interactions•Resource availability•Dispersal patterns

Gray wolves (social)

Burrowing owls(resources)

General spacing patterns: Evenly Spaced

•Social antagonism•Competition for resources in plants

Chinstrap penguins

General spacing patterns: Random

Positions not influenced by positions of otherindividuals in population

MovementWe are primarily concerned with movements

among populations (not within them)

This type of movement is called dispersal

We will discuss this further in the next lecture

Measuring and modeling dispersal is a major challenge in population biology

SUMMARY• Populations can be described by a

number of characteristics:– Size– Density– How long individuals live– Reproductive rate– Distribution of individuals– Distribution of populations

Density is especially useful

• Can indicate environmental quality and ecological processes

• Changes in density can indicate changes in environmental conditions

• Most sensible way of describing populations for:– Sessile organisms– Populations without clear boundaries– Very large populations

Plants and density:

• The law of final constant yield– Increasing density eventually does not

increase the yield, or plant biomass

Spacing patterns

ClumpedEvenly SpacedRandom

Result from interactions among individuals and patterns of resource occurrence

UNGRADED WRITINGASSIGNMENT

1.What is the law of constantfinal yield?

2. What are the three basicspacing patterns?

Self-thinning in Plants

As final yield approached, some individualplants die, while the survivors get bigger

This characteristic has also beenreferred to as the –3/2 thinning

law-

However, the –4/3 ratio appearsto more accurately reflect what

is happening.