1. Chapter 51
Population Ecology

2. Ecology Basics Terms to know… Ecology Biotic factors Abiotic factors
Branch of biology
Relatively new science
Biotic factors
Abiotic factors
Environmental science
ecology + human interactions
Population
same species, same area, same time
Population ecology
numbers + changes

3. Features of populations
Properties that individuals lack:
Population density
Population dispersion
Birth/death rates
Growth rates
Survivorship rates
Age structure
Properties that communities lack:
Common gene pool
Reproductive success
Evolution
Economic importance (crops, forests, game animals…)

4. Population dynamics
Density – the number of individuals per unit of area or volume at a given period of time
Affected by:
Habitat
Season/Weather
Other populations
Limiting factors: the number of individuals in a population is controlled by the ability of the environment to support it
Density-dependent factors – the effect increases as population density increases
Examples:
Density-independent factors – affects the size of a population but is not influenced by changes in population density; typically abiotic

5. Population dynamics…
Dispersion – spacing in relation to other members of the population
Three basic varieties:
Clumped (aggregated distribution, patchiness)
Individuals are concentrated in specific areas
Reasons: distribution of resources, social behavior of animals (herds, family groups), reproduction
Advantages: reduced chance of predation
Uniform
Individuals are fairly evenly spaced
Reasons: social behavior of animals (territories), high levels of competition between individuals
Advantages: reduced competition
Random
Individual spacing is unrelated to others in the population
Does not occur often in nature

7. Changes in population size
Per capita – per individual
Natality – average per capita birth rate (b)
Mortality – average per capita death rate (d)
Immigration – individuals entering a local population (i)
Emigration – individuals leaving a local population (e)
Population growth rate (r):
r = (b + i) – (d + e)
If r = positive number  population is increasing
If r = negative number  population is decreasing
If r = zero  population is staying the same
Examples:

8. Intrinsic rate of increase
Maximum rate of increase when:
Conditions are ideal
Resources are abundant
Population density is low
rmax
Factors which influence this:
Age at which reproduction begins
The fraction of the life span devoted to reproduction
The number of reproductive cycles
The number of offspring produced each cycle
Different species have different intrinsic rates…
Small organisms have high rates (bacteria); large species have low rates (elephants)

9. Exponential population growth
Optimal conditions allow a constant per capita population growth (rmax)
The larger the population gets, the faster it grows
J shape curve:
Organisms cannot reproduce this way indefinitely because of increased:
competition, predation, disease, wastes

10. Logistic population growth
Population growth rate nears zero
Occurs near the environment’s limits to support the population
Carrying capacity (K) – the largest population that an area can maintain indefinitely, assuming no changes in the environment
S shape curve:

11. Density-dependent regulatory factors
Cause an increase in death rate as the population increases
Affect a larger proportion of the population, not just a larger number
Can also cause a decrease in death rate as the population decreases
Tend to regulate a population at a relatively constant size near the carrying capacity
An example of a negative feedback system

12. Cyclical population fluctuations

13. Intraspecific competition
Interference competition
Also called contest competition
Dominant individuals obtain resources at the expense of others (they interfere)
Can cause small drops in population as the individuals unable to compete die
Exploitation competition
Also called scramble competition
All individuals share the limited resource
Can cause wide variation in population size

14. Life history traits Reproductive strategies differ: Two extremes:
Semelparous – a single, large reproductive effort
Examples:
Iteroparous – repeated reproductive cycles over the life span
Two extremes:
r-selected – rapid population growth rate
Small size, early maturity, short life span, large broods, little or no parental care
Found in variable, temporary, or unstable environments
K-selected – keeps population at or near carrying capacity
Large size, late reproduction, long life span, small broods, parental care of young
Found in constant or fairly stable environments

15. Survivorship curves
Measures the probability of survival to a particular age
Three basic types:
Type I – young have a high chance of survival, probability of survival decreases with age
humans
Type II – the probability of survival does not change with age
squirrels
Type III – the probability of death is highest early in life
oysters

17. metapopulations Different populations within an ecosystem
Due to variations of habitats:
Source habitats – high quality habitats, plenty of resources, lots of reproductive success, greater population density
Sink habitats – lower-quality habitats, fewer resources, less reproductive success, lower population density
Humans are increasing the number of metapopulations as they fragment ecosystems

18. Human Populations
Our population continues to increase, but this is due to a decrease in the death rate rather than an increase in the birth rate.
This decrease is due to:
Greater food production
Better medical care
Improved sanitation practices
Zero population growth (r = 0)

19. Human Demographics Highly developed countries: Developing countries:
Low population growth rates
Highly industrialized
Low infant mortality rates
Example:
Developing countries:
Moderately developed
Birth rates and infant mortality rates high but declining
Medium industrialization
Examples:
Less developed
Highest birth and infant mortality rates
Lowest amounts of industrialization

20. Age Structure Diagram
The number and proportion of people at each age of a population

21. Age Structure Diagrams…
Rapid growth:
Pyramid shaped – as children mature, they become parents of the next generation and there are more of them than the previous group
Slower growth or decline in population:
More tapered bases – the number of prereproductive and reproductive ages are close to the same  slower, but still positive growth
Small bases – a higher percentages of postreproductive ages than reproductive and prereproductive ages  negative growth (decline)

22. Environmental degradation
People overpopulation….
Developing countries
Examples:
Consumption overpopulation…
Developed countries