1. 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”

2. 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

3. More Useful Information: Density Spacing patterns among individuals –Territory size –Distance between conspecific plants Movement patterns These too may vary through space and time…

4. 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

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

6. 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?)

7. Environmental quality and blue tits Parus caeruleus

8. 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

9. Changes in density assumed to reflect changing local conditions Example: Burrowing owls in California, 1996-2001

10. 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

11. Changes in density of owl nests and voles found in pellets 1997.51998.01998.51999.01999.52000.02000.5 2 3 4 5 6 7 8 9 1999199820001997 Nest Density Vole biomass/Pellet

12. 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

13. Yield Density Relationships: Examples

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

15. General spacing patterns: Clumped Social interactions Resource availability Dispersal patterns

16. Gray wolves (social) Burrowing owls (resources)

17. General spacing patterns: Evenly Spaced Social antagonism Competition for resources in plants

18. Chinstrap penguins

19. General spacing patterns: Random Positions not influenced by positions of other individuals in population

20. 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

21. SUMMARY Populations can be described by a number of characteristics: –Size –Density –How long individuals live –Reproductive rate –Distribution of individuals –Distribution of populations

22. 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

23. Plants and density: The law of final constant yield –Increasing density eventually does not increase the yield, or plant biomass

24. Spacing patterns Clumped Evenly Spaced Random Result from interactions among individuals and patterns of resource occurrence

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

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

27. 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.