SO, we have an idea of what a population is…. HOW DO WE DESCRIBE IT? ividuals of a single species living wit a given area”
Dec 19, 2015
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 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?)
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
1997.5 1998.0 1998.5 1999.0 1999.5 2000.0 2000.5
2
3
4
5
6
7
8
9
1996.5 1997.0 1997.5 1998.0 1998.5 1999.0 1999.5 2000.0 2000.5
30
40
50
60
70
80
90
19991998 20001997
Nes
t D
ensi
ty
Vo le b
ioma s s/ P
el le t
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
Spacing of individuals
Patterns of individuals within a population give clues to the processes affecting that population
General spacing patterns: Random
Positions not influenced by positions of other
individuals in population
Movement
We are primarily concerned with movementsamong 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 Spaced
Random
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