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 space and 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/km 2

Density is particularly useful for plants and other sessile organisms Examples: barnacles sagebrush

Density can give clues about environmental quality or ecological processes Environmental quality Blue tits in southern Europe nest in both deciduous and evergreen oak forests, habitats that differ in environmental quality (what does it mean?)

Environmental quality and blue tits Parus caeruleus

Density can give clues about environmental 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,

J.A. Gervais Clair de Beauvoir Burrowing owls at Lemoore NAS live within the Air Operations area and in small easements surrounded by industrial agriculture Many owl populations are tied to the abundance of their prey

Changes in density of owl nests and voles found in pellets Nest Density Vole biomass/Pellet

Plants and Density “The Law of Final Constant Yield” Yield = total biomass of vegetation per unit area (single species) Total yield is independent of plant density 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 other individuals in population

Movement We 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 Clumped Evenly Spaced Random Result from interactions among individuals and patterns of resource occurrence

UNGRADED WRITING ASSIGNMENT 1.What is the law of constant final yield? 2. What are the three basic spacing patterns?

Self-thinning in Plants As final yield approached, some individual plants die, while the survivors get bigger

This characteristic has also been referred to as the –3/2 thinning law- However, the –4/3 ratio appears to more accurately reflect what is happening.