population ecology. populations population ecology is the study of populations in relation to the...

Population Ecology

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

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

Uniform

Environmental conditions are uniform

Causes COMPETITION or antagonism between organisms (territoriality)

Clumping

Most common Reproductive

patterns favor clumping

Social behaviors lead to clumping

Optimal density is usually intermediate (medium)

Random

No competition No tendency to

group/clump Conditions are

uniform Rarely happens!

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

Factors That Influence Population Size

There are 3 major factors that influence population size:1. the number of births

2. the number of deaths3. the number of individuals that enter or leave a population

- immigration: individuals entering an existing population

- emigration: individuals leaving an existing population

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

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 populationA 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

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

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).

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

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.

Life Histories

IteroparityA life history pattern in which

organisms produce fewer offspring at a time over a span of many seasons

Example: humans, panda bears, etc.

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

POPULATION GROWTH MODELS

Different models of how populations grow

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

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)

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

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

Exponential Growth

Exponential growth is a useful model when studying populations that are introduced into a new, unfilled, environmentRecovery after a catastropheExponential Growth:

dN/dt =rmax •Nrmax is the maximum rate of

population growth for the species

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

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

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 timeDetermined by such limiting factors as crowding and food resourcesGraph levels off at carrying capacityK-selected populations (equilibrial populations) live near or at the carrying capacity

Carrying Capacity

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

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

Environmental Factors

Abiotic and biotic influences on population size

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)

Density-Dependent Limiting Factors

The effect of density-dependent limiting factors intensifies as the population increases Intraspecific competition

Food, space, etc.TerritorialityPredationWaste build upDisease (if caused by

pathogen/contagious)

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

The Interaction of Limiting Factors

Density-dependent and density-independent limiting factors often work together to regulate the size of a populationDeer in snowy winter

Starve from lack of food (density-dependent)

Severity of winter/depth of snow determines access to food (density-independent)

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

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)

Boom-and-Bust Cycles

Human Population Growth

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

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 1870-1890)

War – death by combat, disease, cut off from food supply (Germany 1618-1648/WWI/WWII)

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

Human Population Growth

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 trendsSome regions have stable regional

human populations (birth rate is the same as death rate)

Other regions show incredible growth rates

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

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

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

Age-Structure Diagrams

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?