1. Population Ecology
Chapter 53

2. Population Ecology
The study of populations in relation to their environment
Members of a population rely on the same resources, are influenced by similar environmental factors, and are likely to interact and breed with one another
3 fundamental characteristics
Density
Dispersion
Demographics

3. Density The number of individuals per unit or volume
The number of oak trees per square km
Counting all individuals is impractical so there are a variety of surveying methods to make a viable estimate
Sample plots
Use of an indicator (nests, burrows, tracks)
Mark-recapture method
This is dynamic and is under constant change via birth, emigration, immigration, death, etc.

4. Dispersion
The pattern of spacing among individuals within the boundaries of the population
Clumped – most common pattern; organisms group together based on needs (i.e. food, mate, etc.)
Uniform – evenly spaced pattern
Random – the position of each individual is independent of other individuals

5. Demographics
The study of the vital statistics of populations and how they change over time
Birth and death rates are particularly important
Life tables look at the survival patterns of a given population based on cohort groups (age groups)
Data in a life table can then be represented graphically using a survivorship curve
Classified into 3 types: Type I, II and III

6. Life History
Traits that affect an organism’s schedule of reproduction and survival (from birth to death)
When reproduction begins
How often the organism reproduces
How many offspring are produced during each reproductive episode
Several kinds of life histories
Big bang reproduction (semelparity)
Repeated reproduction (iteroparity)
There is a trade off between reproduction and survival
Reproduction can lower the chance of survival for the parent

7. Exponential Model Change in population size
(ΔN/Δt) = rN
ΔN = the change in population size
Δt = the time interval
r = per capita rate of increase (annual per capita birth rates – annual per capita death rates)
Under ideal conditions, population will increase exponentially
The resulting curve is J shaped

8. Logistic Model
Actual populations have limitations to population growth known as the carrying capacity (K)
Limited by a variety of resources including energy, shelter, predators, nutrient availability, water, etc.
The resulting curve is an S
Not uncommon for populations to overshoot K and then fall back

9. Population Density
Density independent – a birth rate or death rate does not change with population density
Dune fescue grass mortality is due to physical factors such as drought, rather than density
Density dependent - death rate will rise as population density rises; birth rate will fall as density increases

10. Density Dependent
Density dependent is a type of negative feedback loop that works to halt population growth as density increases
Competition for resources
Territoriality
Disease
Predation
Toxic Wastes
Intrinsic factors
Natural drop in reproduction via aggression, hormonal changes, delay in maturation, immune system depression

11. Population Dynamics
Focuses on the complex interactions between biotic and abiotic factors that cause variation in the size of populations.
Populations are not generally stable, they fluctuate for a variety of reasons
Harsh weather can weaken the organism and diminish food supply causing a higher death rate. The fewer number of animals allows the food source to rebound and become plentiful causing a higher birth rate.

12. Population Cycles
Some organisms undergo boom-and-bust cycles with average time lengths
Voles and lemmings 3-4 yr cycles
Ruffed grouse and ptarmigans 9-11 yr cycles
Ex: Lynx and snowshoe hare populations are connected because of their predator prey relationship; they rise and fall together
This cycle is thought to be related to predator-prey interactions and sunspot activity, which effects the quality of the hare’s food

13. Human Population
In 1650, human population was 500 million, 1850 reached 1 billion, today almost 7 billion.
The human population was experiencing exponential growth, but in the 1960s the growth rate began to decline
We are still growing, but at a slower rate (currently ~1.15%)
This is largely due to new diseases and population controls

14. Population Dynamics Populations vary widely from region to region
Many areas are in the process of moving from high birth rate, high death rate to low birth rate, low death rate
This is demographic transition
This is related to healthcare and education, especially for women

15. Population Dynamics
Age structure is very important in determining growth trends
Populations with many more children than adults will find population increasing
Understanding age structure and its potential effects can help countries plan accordingly
Infant mortality and life expectancy play a role in planning at the family level

16. Global Carrying Capacity
Estimates for carrying capacity vary widely, but average in the billion range
Estimate can be based on a variety of information such as extrapolation of current data or limiting factors such as the potential of available food based on diet
Must be realistic about our needs/wants
Scientists have also come up with the ecological footprint which accounts for the amount of land and water area required by each person to produce the resources consumed