1. Population Ecology

2. Population Size
The overall health of a population can often be monitored by tracking how its size changes.
Population size – the number of individual organisms present in a given population at a given time
It may increase, decrease, undergo cyclic change, or remain the same over time

3. Determining Population Size
The most direct way to determine population size is to directly count…however, this is rarely possible!
Sampling is the most common method of determining population size
Ecologists count the number of individuals in a smaller, sample area
That number is then extrapolated to estimate the entire population
EX – if there are 100 oak trees in 1 km2 – it can be assumed there are about 1000 oak trees in 10 km2

4. Determining Population Size, con’t
Sometimes it is not possible to count even a small sample of the organisms of the population
Rare organisms, or those that prefer to remain hidden can be particularly difficult to count
In these cases, scientists often use signs of the organisms – tracks, droppings or other signs can be used to estimate population size

5. Population Density
Population density – the number of individuals within a population per unit area
Ex – 1500 toads/4 km2 or 300 toads/km2
Essentially, population density is a measure of how crowded the population is
In general, larger organisms have lower population densities – as they require larger habitats to meet their needs

6. Different Densities
High population density can make it easier for organisms to group together and find mates – but it can also make for limited resources
Overcrowded populations may also be more vulnerable to predators and disease
Populations must find the right balance

7. Population Dispersion
Population dispersion– how organisms are arranged within their population area
Also called population distribution
Population dispersion can be random, uniform, or clumped

8. Random Dispersion
There is no discernable pattern to how individual organisms arrange themselves
This occurs when the resources needed are readily available in all parts of the area
Another factor is the lack of a strong influence from other organisms

9. Uniform Dispersion
Individual organisms are evenly distributed throughout the area
May occur because of competition for territory or space

10. Clumped Dispersion
Individual organisms arrange themselves according to the availability of needed resources
This is the most common pattern in nature

11. Factors That Determine Population Growth
Populations increase in size when more organisms enter than leave
Entering a population occurs primarily through immigration and birth
Populations decrease in size when more organisms leave than enter
Leaving a population occurs primarily through emigration and death

12. Birth and Death Rates
The rate at which individuals is born is called natality
The rate at which individuals die is called mortality
Both natality and mortality are generally expressed per 1000 individuals over a given time
Ex: 25 deaths/1000 in the year 2013
If all other factors are equal, if birth rate is higher than death rate, the population size will increase
If death rate is higher than birth rate, the population size will decrease

13. Survivorship
Survivorship refers to the likelihood that an individual will survive to a certain age
Individuals of different ages have different probabilities of dying
A survivorship curve is used to show how the likelihood of death varies with age within a population

14. Migration
Sometimes organisms move around – staying in certain locations for specific times/seasons
This seasonal movement into and out of areas is called migration
Depending on the season can either increase or decrease population size

15. Immigration & Emigration
Immigration – arrival of organisms to an area
Increases the population size
Emigration – organisms leaving an area
Decreases the population size

16. Calculating Population Growth
A population’s overall growth rate can therefore be expressed as:
(Individuals added) – (Individuals subtracted)

17. Calculating Population Growth
A sample population has an annual birth rate of 18/1000; their death rate is 10/1000. Annual immigration is 5/1000 and the emigration rate is 7/1000.
What is the overall growth rate of this population? Is this population growing?

18. How Populations Grow
Populations increase in size according to 2 patterns
Exponential Growth
Logistical Growth
Growth rates tend to change according to the resources available to the organisms in the population

19. Exponential Growth
When populations increase by a fixed percentage each year – they are said to be undergoing exponential growth
Normally only occurs in nature when the starting population is small and environmental conditions are ideal
Can not continue indefinitely!

20. Logistic Growth In reality – exponential growth rarely lasts long
Limiting factors come into play and limit the population growth
Water, space, food, predators, disease
Carrying capacity – the largest population size a given environment can sustainably support
The regulation of an initial exponential growth curve by limiting factors is called logistic growth

21. Logistic Growth

22. Population Growth in Nature
The logistic curve is a simplified model – and real populations can behave differently
Some populations fluctuate or cycle
Others rise quickly, only to overshoot and then crash
Carrying capacity is not fixed – as environments change, so do their capacities to support populations

23. Population Growth in Nature

24. Limiting Factors
Limiting factors regulate population growth in a variety of ways
Some limiting factors are more effective in dense populations
Density-dependent factors
Some limiting factors effect all populations the same
Density-independent factors

25. Density-Dependent Factors
Density-dependent factors include competition, predation, and disease
Each of these factors has a greater effect on a dense population
A less-dense population would be better able to adjust to these factors and prevent a change in growth

26. Density-Independent Factors
Density-independent factors are generally catastrophic events that change an environment dramatically
Fires, floods, landslides, volcanic eruptions, etc.
These events effect all populations equally – rapid reduction in population size

27. Reproductive Potential
Reproductive potential indicates the maximum ability of an organism to produce offspring in ideal conditions
This is a characteristic of the organism, NOT the environment
Many factors can affect reproductive potential
Gestation time – how long it takes for an embryo to develop and be born
Gestation number – how many babies are born at a time
Generation time – span from an organism’s birth to when it can have its own offspring

28. Reproductive Potential
Reproductive potential can be affected by environmental change and can determine how well a population will rebound from a decline
Reproductive potential is measured under ideal conditions – so less than ideal situations will result in lower actual numbers
A population with a high reproductive potential will rebound more quickly from an environmentally-triggered decline