1. APES Chapter 6

2. Chapter 6 Objectives
List the levels of complexity found in the natural world
Describe common factors that affect populations
Contrast how density-dependent and density-independent factors affect population size
Explain growth models, reproductive strategies, survivorship curves and metapopulations
Describe species interactions and the roles of keystone species
Discuss the process of ecological succession
Explain how latitude, time, area, and distance affect the species richness of a community

4. What is a population? Group of individuals….. Of the same species
Living in the same area
Using the same resources
Responding to the same stimuli

5. Characteristics of Populations
Population size
Population density
Population distribution (dispersion)
Population sex ratios
Population age structures
Population survivorship curves

6. 1. Determining population size
Small populations - count them all
Large populations
Random sampling (evenly dispersed organisms)
Mark and Recapture (moving animals)
Transect sampling (measure changes in population size over time or an area)

7. Random Sampling Lay a grid out over an area
Sample random grid segments
Calculate the population of the whole area
How many plants in the field?
16 total squares
How many in squares 5 and 13?
7 and 7
Avg # per square = 7
7 X 16 = 117 total plants
More samples = better results!

9. Mark and Recapture
Biologists capture and mark animals, then release them.
They revisit the site and trap again (often multiple times)
They use the ratio of marked to unmarked animals to calculate the total population

10. Marking animals – pit tags on a sea turtle

11. Marking animals – a monk seal

12. Transect Sampling Establish a transect line through the site
Sample continuously or at intervals
Common in wetland studies

13. 2. Population Size vs. Population Density
Larger population doesn’t always mean a higher population density
Population size:
U.S. has 292 million people
Japan has175 million people
Population density:
US has 30 people/square mile
Japan has 337 people/square mile

14. White tailed deer population density
Prior to European settlement
10 deer / square mile
Threshold for habitat damage
20 deer/ square mile
Current densities in CVNP
130 deer / square mile
CVNP is addressing this with a Management Plan and Environmental Impact Statement (EIS)

15. 3. Distribution/ Dispersion
Random
Uniform
Clumped

19. Why would different types of dispersion occur?

20. 4. Population sex ratios Usually 50/50
Some populations have many more females (fig wasps have 20:1 ratio)
More females = higher population growth
Environmental factors can affect sex ratios (chemical/temperature changes)

21. 5. Population Age Structures
Groupings by age and sex
Shows if a population is growing or shrinking and the males vs. females
Which country is growing rapidly?

22. Ages Structures reflect growth rate
(Birth rate + immigration) - (death rate + emigration)
Remember …..
Immigrate In
Emigrate Exit

23. Population Growth Rates

24. Survivorship curves

25. K vs r strategists K-selected species r-selected species
Low growth rate, few or infrequent offspring
Stable populations (hover around K)
Large mammals and most birds
r-selected species
High growth rate, many offspring
Don’t hover around K, cycle between rapid growth followed by die-offs
Small, reproduce frequently, no parental care
Note that many species lie somewhere in between

26. Population Growth Models
Help ecologists understand population changes
Two basic types
Exponential growth (J curve)
Logistic growth (S curve)

27. Exponential Growth ∆N/∆t = rN
Change in population over time = Growth rate * Population size
Growth starts out slowly then increases rapidly
Growth continues indefinitely
An idealized model

29. Logistic Growth (S curve)
∆N/∆t = rN(K-N/K)
N = population size
t = time
r = growth rate
K = carrying capacity (max. population)
Limiting factors restrict the size of the population

30. More realistic…
Reaches carrying capacity -->
Exponential ---->

31. Even MORE realistic Populations tend to overshoot K
This is followed by a die-off or population crash

32. Limiting Factors Density Dependent
Effects are more significant as the population grows
Availability of food
Effect of predators
Availability of nesting sites
Infectious disease

33. Effect of food

34. Effect of predators…

35. Limiting Factors Density Independent
Affects about the same percentage of the population, no matter the population density
Adverse weather
Pesticides/herbicides
Fires
Landslides
Habitat destruction

36. Dangers of low population numbers
Low population sizes lead to low levels of genetic diversity within a population
This makes the population more vulnerable to disease and the effects of inbreeding

37. Genetic Bottleneck