2. Introduction to Ecology CERC Certificate Program Columbia University Session 2 – Population Ecology

3. Population Ecology Why is this field important? What is a population? Why does a population change in size? Unlimited, exponential population growth Logistic population growth Exponential vs. Logistic growth Before all that however… Goals for the day

4. An Exercise… From a study of your own design: –Work through the 11 steps of the scientific method –What did you do?

5. Scientific Method - Steps 1-5 1.Observe or suspect pattern 2.Posit cause or significance of observed difference 3.Create answerable question to explain pattern 4.Create testable hypotheses –Null (H o ) and alternate hypotheses (H a ) 5.Design experiment

6. 6.Collect data (descriptive stage) 7.Analyze data, primarily using statistics 8.Evaluate hypotheses, reject H o ? 9.Make conclusions based on data 10.Note problems in current work 11.Predict future directions for research Scientific Method - Steps 6-11

7. Population Ecology Why is this field important? What is a population? Why does a population change in size? Unlimited, exponential population growth Logistic population growth Exponential vs. Logistic growth Goals for the day

8. Important Applications of Population Ecology - Local Growth rates of introduced species Population Viability Analyses of endangered speciesPopulation Viability Analyses –What is the minimum number of individuals needed to ensure a 90% chance of survival for 100 years Population genetics of endangered species –Rosenbaum’s work with whales

9. Important Applications of Population Ecology - Regional Metapopulation analyses –Tracing the survival of all component populations –More in a bit Captive Breeding projects at zoos –Applied metapopulation analyses

10. Population Ecology Why is this field important? What is a population? Why does a population change in size? Unlimited, exponential population growth Logistic population growth Exponential vs. Logistic growth Goals for the day

11. What is a Population? Components? Definition : –One species –One area –Isolated from other areas –Able to interbreed Example: Only minimal genetic flow, at most

12. What is a Metapopulation? Components? Definition : –One species –Multiple areas –Isolated from other areas, further away –Able to interbreed Example: Only minimal genetic flow, at most

13. Characteristics of a Population What features can we measure of a population? Features: –Size –Age structure –Sex ratios –Effective population size –Birth rate –Death rate –Immigration –Emigration

14. Population Ecology Why is this field important? What is a population? Why does a population change in size? Unlimited, exponential population growth Logistic population growth Exponential vs. Logistic growth Goals for the day

15. Why Does Population Size Change? Density Independent Forces –Forces that are at work irrespective of the population density Density Dependent Forces –Forces that vacillate depending on the population density

16. Density Independent Forces Types? Examples –Climate –Topography –Latitude –Altitude –Rainfall –Sunlight In Sum: Abiotic factors –Exceptions do exist!

17. Density Dependent Forces Types? Examples –Within species Breeding spaces Food Mates Foraging spots –Between species Predation Parasitism Pollinators Competition In Sum: Biotic factors –Exceptions do exist!

18. Indeterminate Factors Most influences are pretty constant and Deterministic Opposite of deterministic factors is Stochastic forces Examples –Environmental: Droughts, floods, asteroids, volcanoes, fires, etc. –Demographic: Crash in effective population size, series of single sex born, etc.

19. Small Populations Usually at great risk Why? ­Small population size ­Small genetic diversity ­Highly susceptible to stochastic forces ­Poor competitors with resident biota Severely limited adaptability

20. Population Ecology Why is this field important? What is a population? Why does a population change in size? Unlimited, exponential population growth Logistic population growth Exponential vs. Logistic growth Goals for the day

21. Types of Population Growth Exponential –Unlimited, rapid growth –Often called Malthusian –Growth without bounds Logistic –Growth within natural limits –What sets that limit? –What is the limit? –More in a moment…

22. Exponential Population Growth Examples of this? –Think close to home Often an unnatural occurrence Conditions under which this occurs naturally –Introduced species –Nutritionally enriched environments –Cultural innovations?

23. Exponential Population Growth Equation Derivation Which measured population growth components can change? They are: –Birth –Death –Immigration –Emigration Relationship between these? N o + B + I - D – E

24. Exponential Population Growth Equation Derivation The equation for population change over a unit t (time)  N /  t = N o + B + I - D – E Simplify the equation –Assume a closed population –Eliminate migration (I, E)  N /  t = N o + B - D –Create a growth rate (r) = (B-D)/t  N /  t = (r)(N o ) –This is the basic exponential growth equation

25. Exponential Population Growth Equation - Implications  N /  t = (r)(N o ) What can be experimentally changed here and how does our close-to-home example apply? Only r can change –r in humans has been continually increasing with technology When r = 0, the population growth has stopped –What is this timepoint called?

26. Population Ecology Why is this field important? What is a population? Why does a population change in size? Unlimited, exponential population growth Logistic population growth Exponential vs. Logistic growth Goals for the day

27. Carrying Capacity – Unique to Logistic Growth Definition? A summary of all factors regulating population sizes –Density dependent –Density independent –Determinate –Stochastic Site and species specific value

28. Logistic Population Growth What is added in this form of population growth? The Carrying Capacity is added –What is it and what determines it? –Typically summarized as K How would we modify the exponential population growth equation to reflect this?

29. Logistic Population Growth Equation Derivation Add the Carrying Capacity (K) – how?  N /  t = (r)(N o ) –Base Expon. Equation  N /  t = (r)(N o )(1-(N/K)) –Base Logistic equation –(1-(N/K)) is the unoccupied portion of the carrying capacity

30. Logistic Population Growth Equation - Implications  N /  t = (r)(N o )(1-(N/K)) –Base Logistic equation Implications: –As N ~ K, population increase stops –Logistic is a special case of Exponential, when K = infinity

31. Growth Matters! How many humans can we expect? –May be unlimited? –What about implications of Ecological Footprint exercise? –Currently 6 billion people –Hotly contested

32. Growth Matters! r-selected species –Why most weeds are weedy –Edge species are typically r-selected –Invasive species are often r- selected

33. Growth Matters! K-selected species –Why we don’t get many species of oaks in most young forests? –Climax communities –Susceptible to habitat fragmentation

34. Assignment for Next week: Population Growth Models –Learn more about the consequences of these models Instructions are all online, and available hereavailable here Turn in at beginning of class next week –We will discuss it then

35. Proximate Ecological Fields - Revisited Trends down pyramid: –Increase in geographic scale –From single species to multiple species –Increasing number of ecological factors that may be influential –Decreasing certainty in results Population Community Ecosystem

36. Next Week: The Tour of Ecology Continues Population ecology Community ecology –Next week’s emphasis Ecosystem ecology Conservation Issues