Population Ecology

Chapter 53

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

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.

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

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

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

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

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

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

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

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.

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

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

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

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

Global Carrying Capacity• Estimates for carrying capacity vary widely, but

average in the 10-15 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