1. Population Ecology

2. Objectives
5.3.1 – Outline how population size is affected by natality, immigration, mortality, and emigration.
5.3.2 – Draw and label a graph showing a sigmoid (S- shaped) population growth curve.
G.1.3 – Describe one method of random sampling based on quadrat methods that is used to compare the population size of two plant or two animal species.
G.1.4 – Outline the use of a transect to correlate the distribution of plant or animal species with an abiotic variable.

3. Characteristics of Populations
A population is a group of individuals of a single species that simultaneously occupy the same general area. Characteristics of populations are shaped by the interactions between individuals and their environment.
Populations have size and
geographic boundaries.
Density is the number of
individuals per unit area.
Dispersion is the pattern
of spacing among
individuals within
Darker color = greater density

4. Patterns of dispersion
Dispersion patterns depend on variable population densities. Dispersion and density both depend on resource distribution.
Random dispersion, individual positions are independent.
Uniform dispersion: individuals are evenly spaced.
Clumped dispersion: individuals aggregate in patches.
Random Uniform Clumped

5. Random sampling
Measuring population density is a difficult task. Do you …
Count individuals - often impractical if individuals do not want to cooperate, or
Use sampling techniques to estimate numbers.
The mark-recapture method: Individuals are trapped, marked with a tag, recorded, then released and captured a second time. The percentage recaptured gives an estimate of the true population.
Quadrat methods: count all individuals within a pre- cisely outlined area, then extrapolate to the larger environment. ⇒ an estimate.

6. Quadrat sampling techniques
A quadrat is a square of a convenient size, perhaps 1 m x 1 m, in which species of interest are counted.
Later relate totals to the area of interest – perhaps an entire forest.
Counting rule: Omit objects
on the left and bottom edges.

7. Quadrat sampling techniques

8. Demography
Demography is the study of factors that affect the growth and decline of populations.
Additions occur through
birth (natality) and im-
migration; subtractions
occur through death
(mortality) & emigration.
Population Growth by County
April, 2000, to July, 2003

9. Demography
Represent a life table data with a survivorship curve – which plots the number of indi viduals in a cohort still alive at each age.
Type I curve shows a low death rate early in life (humans).
Type II curve shows constant mortality (squirrels).
Type III curve shows high death rate early in life (oyster).

10. Population Growth (ideal conditions)
Exponential growth increases faster and faster un-checked, like compound interest, because resources are unlimited. Growth becomes faster and faster.
Change in population size = Births - Deaths
This model is best for single-celled life (bacteria).
a J-shaped curve

11. Population Growth (ideal conditions)
No population can in crease forever! Why?
It eventually runs out of resources:
too little food,
too little space,
too many waste products,
too many predators,
too much disease.

12. Population Growth (typical conditions)
The logistic growth model shows a leveling-off because of environmental feedback. The carrying capacity (K) is the maximum population size that can be supported by available resources.
Typically, unlimited resources are rare, so this graph is S-shaped.

13. Population Growth Stages of growth:
a) Lag phase: growth is slow as 1→2, then 2→4, 4→8, etc.
b) Exponential phase: eventually growth speeds up as 1 million → 2 million, etc.
c) Transitional phase: at some point the food begins to run out, and the growth slows down.
d) Plateau phase: carrying capacity has been reached, and births equal deaths.

14. Population Growth
The logistic growth model does not fit all situations!
Resource availability depends on the situation.
Natural selection can vary with density & environment.
There are lag times that alter the S-curve. Populations can grow large, then many may starve (birds, below).
Microbes in the lab
fit the model well.
Higher organisms in nature are influenced by
many factors.

15. Population-limiting Factors
Why do all populations eventually stop growing?
What environmental factors stop a population from growing?
Examine effects of increased population density.

16. Population-limiting Factors
Density-independent factors act the same regard-less of population size.
Natural disasters such as hurricanes, tsunamis, forest fires destroy all but the lucky individuals in large or small populations.

17. Population-limiting Factors
Density-dependent factors (negative feedback) increase their effect on populations as population density increases.
Negative feedback prevents unlim ited population growth.
Intra-specific competition for food, shelter, or mates can cause density-dependent behavior of populations.
Resource limitation in crowded populations can stop growth population by reducing reproduction.

18. Population-limiting Factors
Negative feedback prevents unlimited population growth.
Territoriality (defense of a space) may set a limit on density.
Predation may cause mortality of prey species.
Waste accumulation can regulate population size.
Ex: yeast make alco hol during ferment- ation of grape juice to wine, but yeast begin to die as alcohol content approaches 13%.
Disease spreads much
more rapidly in dense
populations.

19. Population-limiting factors
Some populations have regular boom-and-bust cycles.
Lynx & hare populations in Alaska cycle on a 10-year basis.
More hares allow more baby lynxes to survive & reproduce
More lynxes eat more hares, then lynx population crashes.
Fewer lynxes allow more hares to survive - cycle repeats.

20. Over-population
The human population has been growing almost exponen-tially for three centuries but cannot do so indefinitely.
What might put a stop to this growth?

21. An apocalypse?
Conquest, War, Famine, Disease