1. Population Ecology

2. Populations
Population ecology is the study of populations in relation to the environment
A population is a group of individuals of the same species that live in the same area

3. Population Ecology: Vocabulary
To describe populations, we have to talk about how they are situated
Density:
The number of individuals per unit area/volume
Example: 47 elephants/km2
Dispersion:
The pattern of spacing among individuals in a population
Clumped
Uniform
Random

5. Uniform Environmental conditions are uniform
Causes COMPETITION or antagonism between organisms (territoriality)

6. Clumping Most common Reproductive patterns favor clumping
Social behaviors lead to clumping
Optimal density is usually intermediate (medium)

7. Random No competition No tendency to group/clump
Conditions are uniform
Rarely happens!

8. Estimating Population Size
Rarely, it is possible to count the number of species in a population
Usually, a statistical method is needed to determine population size
The mark-recapture method can be used to estimate the size of a population
Capture, mark, release
Recapture and count
Equation:
N = Number marked x Total catch 2nd time
Number of marked recaptures

9. Factors That Influence Population Size
There are 3 major factors that influence population size:
1. the number of births
2. the number of deaths
3. the number of individuals that enter or
leave a population
- immigration:
individuals entering an existing population
- emigration:
individuals leaving an existing population

10. DEMOGRAPHICS
In looking at populations, biologists must also look at demographics (the vital statistics of a population and how they change over time)
Birth rates
Death rates
Life tables
Survivorship curves
Reproduction rates
Growth rates
Age structures

11. DEMOGRAPHICS
One tool in demographics is to estimate the life span of organisms
To do this they often create a life table, age summary of a population
A cohort (group of individuals of the same age) are studied to see what percentage of the population dies
This can determine life expectancy, survivorship at each age category, or male vs. female survivorship

12. Survivorship Curves
A way to represent a life table is a survivorship curve, a plot of the proportion of individuals that survive at each age group
When discussing survivorship curves, there are 3 general categories

13. Survivorship Curves Type I- live to old age & die (most large mammals)
Type II- constant mortality rate (rodents, lizards, hydra)
Type III- high mortality at young age, but if they survive they live a long life (sea turtles).

14. DEMOGRAPHICS
Another important aspect of demographics is reproductive rates
For obvious reasons, biologists who study reproductive rates only focus on females
Age of fertility
Number of offspring for each age group
Time of year
Spawning cycles
Reproductive tables summarize this data

15. Life Histories
Life history are traits that affect an organism’s schedule of reproduction and survival
Clutch size:
Number of offspring produced at each reproductive episode
Semelparity
A life history in which an organism spends most of its energy in growth and development, expend their energy in one large reproductive effort, and then die
Many insects, annual plants, salmon, etc.

16. Life Histories Iteroparity
A life history pattern in which organisms produce fewer offspring at a time over a span of many seasons
Example: humans, panda bears, etc.

17. Life Histories (Semelparity vs. Iteroparity)
Many factors contribute to the life history of an organism
Finite resources
Reproduction vs. survival
Number and size of offspring
Paternal investment in offspring

18. POPULATION GROWTH MODELS
Different models of how populations grow

19. Formulas off your Cheat Sheet
Rate: dY/dt
Generic expression for change of some variable (Y) over time (t)
Population Growth: dN/dt = (B – D)
The change in population size (N) over time (t) is the same as the birth rate (B) minus the death rate (D)
Exponential Growth: dN/dt =rmax •N
The change in population size over time is equal to the growth rate (rmax) multiplied with the current population size

20. Formulas off your Cheat Sheet
Logistic Growth:
dN/dt =rmax •N • [(K-N)/K]
The growth rate over time is the same as exponential growth adjusted for carrying capacity (K)

21. Types of Population Growth (start here)
One of the biological imperatives is to reproduce and pass on genetic material to succeeding generations.
Yet population growth is controlled by the environment and limited resources
This causes different patterns of population growth

22. Patterns of Population Growth
Exponential Growth:
Occurs in ideal conditions with unlimited resources
J shaped curve
Example:
1 bacterium (reproducing every 20 minutes) could produce enough bacteria to form a 1-foot layer over the entire surface of the Earth in 36 hours

23. Exponential Growth
Exponential growth is a useful model when studying populations that are introduced into a new, unfilled, environment
Recovery after a catastrophe
Exponential Growth:
dN/dt =rmax •N
rmax is the maximum rate of population growth for the species

24. Patterns of Population Growth
Exponential growth cannot continue indefinitely
It is characteristic of populations who are entering a new environment OR those whose numbers are rebounding from a catastrophic events

25. Patterns of Population Growth
Logistic Growth:
Pattern of population growth which takes into account the effect of population density on population growth
Occurs when resources become more scarce
Characterized by an S-shaped curve

26. Patterns of Population Growth
dN/dt =rmax •N • [(K-N)/K]
Carrying capacity (K):
The maximum number of individuals that a particular environment can support over a long period of time
Determined by such limiting factors as crowding and food resources
Graph levels off at carrying capacity
K-selected populations (equilibrial populations) live near or at the carrying capacity

27. Carrying Capacity

28. K-strategists (Life history)
Density stays near carrying capacity.
Large, slow growing organisms
Small population sizes
Long life span; slow maturation
Few young/small clutch size
Reproduce late in life
Parental care
Most large mammals; endangered species

29. r- strategists (Life history)
Grow exponentially when environmental conditions allow; when conditions worsen, population size plummets.
Short life span
Reproduce early in life
Many offspring/large clutch size
Usually small in size
Little or no parental care
Bacteria, some plants, insects

30. Environmental Factors
Abiotic and biotic influences on population size

31. Limiting Factors
There are a number of factors that limit the size of populations:
Density-dependent limiting factors (depends on the size of the population)
Density-independent limiting factors (does not matter the size of the population)

32. Density-Dependent Limiting Factors
The effect of density-dependent limiting factors intensifies as the population increases
Intraspecific competition
Food, space, etc.
Territoriality
Predation
Waste build up
Disease (if caused by pathogen/contagious)

33. Density-Independent Limiting Factors
The occurrence and severity of density-independent limiting factors are unrelated to population size
Climate
Disease (if not caused by pathogen/not contagious)
Pollution

34. The Interaction of Limiting Factors
Density-dependent and density-independent limiting factors often work together to regulate the size of a population
Deer in snowy winter
Starve from lack of food (density-dependent)
Severity of winter/depth of snow determines access to food (density-independent)

35. Population Dynamics
Population dynamics is the study of the environmental factors that cause variations in the population size
Looks at fluctuations in population over time to examine stability
Immigration and emigration also affect population
Metapopulations are when you have several interconnected populations

36. Boom-and-bust cycle
Another phenomena that affects population are predator-prey relationships.
Each population is interdependent and causes a boom-and-bust cycle
The prey population increases which causes the predator population to increase
The prey are over hunted and their population crashes
This causes the predator population to crash
Now, with fewer predators, the prey population can again increase (recovery gives a geometric increase)

37. Boom-and-Bust Cycles

38. Human Population Growth

39. Increase in Human Population
Agricultural Revolution - Major period of population growth began when humans started to cultivate crops and domesticate animals
Industrial Revolution – Improved food production and distribution
Health Care – germ theory lead to improved hygiene, better waste removal and water treatment

40. Decrease in Human Population
Plague – disease that greatly reduces the size of population (Black Plague in 1300’s reduced the population in England by 50%)
Famine –a severe food shortage causing starvation and death (Potato Famine of 1840’s/China )
War – death by combat, disease, cut off from food supply (Germany /WWI/WWII)

41. Human Population Growth
The human population is unlike any other organism
Since about 1650, we have remained in an exponential population increase
Population increases by about 201,000 people/day worldwide

42. Human Population Growth

43. Human Population Growth
Even though there is a tremendous increase in human population, it is not evenly distributed around the globe
Regional areas have different population trends
Some regions have stable regional human populations (birth rate is the same as death rate)
Other regions show incredible growth rates

44. Industrialized Nations
An emerging nation usually has a very high birth rate, but also a high death rate (disease, lack of modern medical treatment, famine)
An industrialized nation usually has a low death rate, but also a low death rate
Moving from an emerging nation to an industrialized nation is known as a demographic transition

45. Human Population Growth
In the 1950s, mortality rates began to rapidly drop (advances in medicine and sanitation)
Yet, the birth rates have not always dropped
Has caused a huge increase in population in some nations
About 80% of the world’s population lives in emerging nations

46. Age Structure
One way to determine human population growth is to look at the nations’ age structure, relative number of individuals at each age
By looking at the age structure of a population, you can determine the population growth

47. Age-Structure Diagrams

48. Human Population Growth
Implications of exponential human population growth:
Lack of food supplies
Lack of space
Lack of natural resources (metals, fossil fuels, etc)
Lack of sites for waste disposal
Ecologists cannot agree on a carrying capacity for Earth (2 – 40 billion)
Are we going to reach carrying capacity through individual choices and/or government programs? OR
Is Earth’s population going to “level off” as a result of mass deaths?