1. Chapter 6
Population Ecology

2. Core Case Study: Southern Sea Otters: Are They Back from the Brink of Extinction?
They were over-hunted to the brink of extinction by the early 1900’s and are now making a comeback.
Figure 8-1

3. Population Trends from 1990 - 2012
17,000 in 1800’s
Less than 100 in 1914

5. Case Study: CA Condor Large pre-historic population; ice age
In 1987, only 22 condors were left.
Captured all wild birds; captive breeding program
Decline due to:
Pesticides (DDT)
Poisoning (lead)
Starvation
Accidents

7. Levels of Complexity

8. Population
Population – composed of all individuals that belong to the same species and live in a particular area at a given time.
Unit of evolution
Cause of decrease and increase in population
Boundaries can be arbitrary
Dynamic (always changing)

9. Changes in Population Size: Inputs & Outputs
Populations increase through births and immigration
Populations decrease through deaths and emigration

10. Communities & Ecosystems
Community - incorporates all of the populations of organisms within a given area.
Boundaries can also be arbitrary
Scientists interested in interaction between species
Ex: Maple-Beech-hemlock community
Communities that experience similar temperature & rainfall are grouped into biomes
However, species of trees varies from community to community. Ex: Eucalyptus vs. oak (Aust. vs N. America/Europe)
Ecosystems – consists of all abiotic & biotic components in a particular location.
Focused on the flow of energy & matter; cycling of nutrients through a system

11. Population Ecology
births + immigration > deaths + emigration = population growth
births + immigration < deaths + emigration = population decrease
Population Ecology is the study of the factors that cause populations to change.
Knowing factors that affect population are useful for controlling it
CA Condor (extinction) or Argentine ants (invasive)

12. Population Characteristics
Population Size (N) – total number of individuals within a defined a given time.
Population Density – is the number of individuals per unit area (or volume in case of aquatic organisms).
Help estimate if species is rare, abundant, or overpopulated
Help aid in wildlife management

13. Population Characteristic
Population Distribution – description of how individuals are distributed with respect to one another.
How a population occupies space
Three types:
Random – Trees in a forest
Uniform – territorial nesting birds, tree farm
Clumped – schooling fish, flocking birds, & herding animals

14. Population Distribution
Most populations live in clumps although other patterns occur based on resource distribution.
Figure 8-2

15. Population Characteristics
Population Sex Ratio – the ratio of males to females
Usually close to 50-50; fig wasps (20:1 females/males)
Knowing ratio helps scientists estimate number of offspring in next generation
Population Age Structure – description of how many individuals fit into a particular age category.
Helps predict how rapidly a population can grow
Large proportion too young or too old to reproduce will help understand the growth or decline of a population

16. Age Structure: Young Populations Can Grow Fast
How fast a population grows or declines depends on its age structure.
Prereproductive age: not mature enough to reproduce.
Reproductive age: those capable of reproduction.
Postreproductive age: those too old to reproduce.

17. Age Structure Diagram

18. Factors That Influence Population Size
Density Dependent Factors: influence an individual’s probability of survival & reproduction that depends upon the size of population.
Amount of available food (greater affect on large populations); predation, disease, competition, parasitism
Gause Experiments – shows how food supply controls population growth
Rapid growth early, then population plateaued later
Double the amount of food, populations also doubled
Confirmed limiting resource = food
Limit to how many individuals the food supply can sustain is Carrying Capacity (K) without destroying the habitat.
Predict how much individuals an environment can sustain

19. Factors That Influence Population Size
Density-Independent Factors: have same effect on an individual’s survival & reproduction at ANY population size.
Examples: tornado, hurricanes, floods, fires, volcanic eruptions, and other climatic events
Mortality (death) increases in such an event regardless of population size
Bird populations are regulated by these factors

20. Gause’s Experiments

21. Population Growth Models
Important tools in explaining population fluctuations.
Used to predict population size at any moment in time.
Growth Rate – number of offspring an individual can produce in a given time period, minus the deaths.
Maximum potential for growth is called the intrinsic growth rate (r). Under ideal conditions.

22. Population Growth Models
Exponential Growth Model:
N0 = starting population
r = intrinsic growth rate
N = future size of population
t = time which population grows
r = 0 if population is constant; r > 0 if population is increasing; r < 0 if population is decreasing

23. Exponential Growth Model
Populations not limited by resources, growth is very rapid.
Rate is fixed
More births occur with each step in time.
This model produces a J-Shaped curve .
Exponential growth is density independent.
Gause’s experiment showed that no population can grow indefinitely

24. The Logistic Growth Model
Modified model to incorporate environmental limits on exponential growth.
Logistic growth model – a population whose growth is initially exponential but slows as population reaches carrying capacity (K).
Produces an S-shaped curve

25. Variations on the Logistic Growth Model
Overshoot – producing more offspring than environment can hold, leading to a die-off
Seasonal: mate in winter but population is larger than spring carrying capacity
Reindeer are a good example (see graph)
Overshoot & die-off can be a recurring cycle as populations will oscillate around the carrying capacity.

26. Types of Population Change Curves in Nature
Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically.
Stable: fluctuates slightly above and below carrying capacity.
Irruptive: populations explode and then crash to a more stable level.
Cyclic: populations fluctuate and regular cyclic or boom-and-bust cycles.
Irregular: erratic changes possibly due to chaos or drastic change.

27. Natural Population Curves
Fig. 9-7 p. 194

28. Population grew exponentially then crashed to only 8 individuals
Population grew exponentially then crashed to only 8 individuals Most likely due to lack of food

29. The Role of Predation in Controlling Population Size
Top-down control: predator controls the cyclic fluctuations of the prey
Bottom-up control: herbivore-plant interaction influences the cyclic fluctuations of the predator

32. Reproductive Strategies & Survivorship Curves:
r-Selected
High intrinsic growth rate
Reproduce often w/ lots of offspring
Large fluctuation around carrying capacity
Ex: insects, rodents, small fish
K-Selected
Low intrinsic growth rate
Abundance of species is determined by carrying capacity.
Small fluctuations
Ex: Large mammals, most birds
K selected crow (generalist) elegant tern (specialist)
K selected are less likely to exceed their carrying capacity where r are more likely to grossly exceed carrying capacity and there is a shorter recovery if they do.

34. Survivorship Curves: Short to Long Lives
The way to represent the age structure of a population is with a survivorship curve.
Late loss population (Type I) high survival rate; live to an old age.
Elephants, rhinoceros, humans, whales, large reptiles
Constant loss population (Type II) constant; die at all ages.
Corals, Jellyfish, songbirds, rodents
Early loss population (Type III) low survival rate; die at young ages.
Annual plants
Insects