EMMA DEBANY AND DERRIAN DURYEA

Chapter 6: Population Biology

What’s Population?

Population: all members of a single species living in specific area at same time.

How Can We Express Population Growth?

exponential growth: theoretical unrestricted increase in populations, no limiting factors.

Exponential growth can be expressed as an equation!

dN

dtrN

The Equation

dN: change in # of individualsdt: change in timer: rate of growth—a fraction representing the average

individual contribution to population growth. If r is POSITIVE, population increasing. r NEGATIVE population decreasing. r is ZERO, no change, and dN/dt = 0

N: number of individuals in population

dN

dtrN

The Equation Continued

Previous equation also known as Biotic potential:

the potential of a population to grow if nothing is limiting its expansion.

How Many Years Will A Population Take to Double?

If population is growing exponentially (without limits), this equation finds how many years a population will take to double:

divide 70 by annual percentage growth = approximate doubling time in years

in other words:70 / x% = doubling time (years)

(for instance, a population growing at 35% doubles in 2 years)

What About When There are Limits to Growth?

Carrying capacity: max. population of any species that can be supported by a particular ecosystem

Overshoots: when a population goes over the carrying capacity of its environment--death rates begin to rise past birth rates

Population crash: after an overshoot, population decreases as fast OR faster than it grew

Growth and Decline of Populations

Populations normally cycle through growth and decline

Regular Cycles: depend on simply factors (like seasonal algae blooms that depend on light and temperature)

Irregular Cycles: depend on complex environmental relationships (like outbreaks of migratory locusts in desert)

Irruptive growth: long periods of low population size then a sudden population growth

Stable Populations

Remember exponential growth?

There’s also logistic growth!

logistic growth: species grow exponentially when resources unlimited BUT pop. growth slows when carrying capacity of environment is approached

(Population will decrease if it exceeds carrying capacity)

Formula For Logistic Growth!

dN/dt: change in numbers over timer: exponential growth rateN: population sizeK: carrying capacity

dN

dtrN(1

N

K)

Formula For Logistic Growth!

(1-N/K): represents relationship between N (pop size) at any given time step and K (# of individuals the environment can support)

If N is less than K, 1-N/K will be positive, and means the population is growing (smaller numbers greater than 0 is slow growth, larger numbers faster growth)

If N is more than K, 1-N/K will be negative and the population will be decreasing.

dN

dtrN(1

N

K)

Logistic vs Exponential Growth

Exponential: also known as J CurveLogistic: also known as S curve

J curve: theoretical growth without restraint toward biotic potential

S curve: stabilization in response to environmental resistance

Factors that Limit Populations

Populations regulated by internal and external factors

internal: maturity, body size, hormonal status

external: habitat, food availability, interactions with other organisms

External Limits: Density Dependent vs Independent

density-dependent: limits dependent on population density food and water disease, stress, exposure to predators or

parasites

density-independent: limits not involved with population/density of animals Example: drought / early frost Habitat destruction: floods, fires, etc

What’s Environmental Resistance?

Environmental resistance: environmental factors that tend to reduce population growth rates.

Resistance is larger and rate of logistic growth smaller as population approaches carrying capacity

K-Adapted? R-Adapted? Whaaaat?

R adapted: species that persist by depending on high rate of reproduction and growth rapid reproduction High mortality of offspring Will vershoot carrying capacity and die back

K adapted species: reproduce more slowly as they approach the carrying capacity of the environment

R-Adapted Species

R Adapted Species grow exponentially

Move quickly into disturbed environmentsgrow rapidlymature quicklyproduce many offspringdo little to care for offspringdepend on sheer numbers and dispersal

techniques to ensure some survive

R –Adapted Graph

K-Adapted Species

LargerLive longerMature slowerProduce fewer offspring in each

generationFewer natural predators

Factors that Increase/Decrease Populations

Natality: production of new individuals by birth, hatching, germination, cloning

(sensitive to environmental conditions)

Successful reproduction tied to: nutritional levels, climate, soil, water conditions, social interactions

Fecundity vs Fertility

Fecundity: physical ability to reproduce

Fertility: measure of the actual number of offspring produced

(Because of lack of opportunity to mate, fecund individuals may not contribute to pop growth)

Immigration Additions to Populations

Methods of immigration:

• Wind (seeds, spores, small animals carried distances

• Fur/feathers/intestines• Water• Self-transportation (birds fly, fish swim,

wolves walk)

Immigration Continued

Some ecosystems can be maintained by constant influx of immigrants

Mortality/Death Rate

How to calculate mortality/death rate:

divide the # of organisms that die in a certain time period by the # alive at the beginning of the period

X1 / X2

But What Is Mortality?

Survivorship: percentage of a cohort that survives to a certain age

Life expectancy: the probable number of years of survival for an individual

Life Expectancies in US

Rose during 20th Century1900: 47.3 years expectany2003: 77.4 years expectancyDifferences between sexes, races, economic class

Life Span

Life span: longest period of life reached by a given type of organism

Most organisms don’t live anywhere near the maximum life span for their species

Major factors in early mortality:

Predation Parasitism Disease Accidents Fighting Environmental influence (climate,

nutrition)

Emigration

Emigration: the movement of the members out of a population size.

When a group/individual immigrates to a new area, they emigrate out of an old area

Same techniques used for immigration are used for emigration

Can help protect a species if area is overpopulated

Population Growth Factors

What are the factors that regulate population growth?

These factors primarily affect natality and mortality

Types of Factors:

Intrinsic: operating within individual organisms or between organisms in the same species

Extrinsic: imposed from outside the population

More Types of Factors

Biotic: caused by living organisms (tend to be density-dependent)

Abiotic: caused by nonliving components of the environment (tend to be density-independent)

Which Factor is More Important in Regulating Population Dynamics?

Has been much debate

In general, depends on: the particular species involved that species’ tolerance levels The stage of growth and development of organisms

involved Ecosystem where the organisms live The way combination of factors interact

Abiotic Generally Density-Independent

Weather or climate are most important factors

Extreme cold, high heat, drought, excess rain, severe storms also important

Factors don’t always diminish population: After a rainstorm (an abiotic factor) the desert will

flourish Some forests need fires to bloom

Density-Dependent Factors

Density-dependent factors reduce population size

Decrease natalityIncrease mortality

Result of not only interactions between populations of a community, but also interactions within a population

Interspecific Interactions

These interactions occur between speciesPredator vs preyPrey species can also benefit:

Moose are killed by wolves Old/sick moose are killed off This strengthens herd of moose as a whole Also benefits wolves

Intraspecific Interactions

These occur within species

Animals in species compete for resources

Population density is low = resources plentiful

Pop. Density high = resources low

Stress and Crowding

Stress related diseases: when pop. density is high, organisms have symptoms of this

Too much competition/too close proximity to other organisms

Can affect reproduction, thus lowering population density once more—population fixes itself

Case Study 2: THE LOCUSTS

Locust plagues have been tragically destructive throughout history

Ever few decades rain comes to the desert and locusts flourish

This high population density for some reason causes them to stop reproducing, grow longer wings, and swarm desert

The swarms devour hundreds of thousands of plants and all die within a few weeks.

Case Study 2: THE LOCUSTING

Locust swarms can affect livelihood of 1/10 of the earth’s population with all the plants/crops they destroy

In 2004 heavy rains in the Sahara cause locust swarm

28 countries in Africa and Mediterranean were affected

Crop losses reached 100% in some placesThis study illustrates power of exponential

growth and danger of boom and bust life cycles

Conservation Biology

Small, isolated populations can undergo declines due to environmental change, genetic problems, or random events

Island Biogeography

Island Biogeography: a theory that MacArthur and Wilson came up with in 1967 to explain why islands have fewer species than the mainland

Theory explains that diversity in isolated habitats = balance between colonization and extinction rates

Biogeography Continued

Islands have low colonization rates because islands are hard to reach

Limited habitat forces population to be small

Therefore larger islands closer to mainland are more populated and more diverse

Genetics = important in survival/extinction of small, isolated populations

Hardy-Weinberg equilibrium: in large populations. If mating is random, no mutations occur, the distribution of gene types will preserve genetic diversity

Genetic Drift

Genetic Drift: the gradual changes in gene frequencies (occurs in small populations due to fewer individuals with slight genetic variation being involved in mating)

Founder effect/demographic bottleneck: a few members of the species survive a disaster, or colonize a new habitat isolated from other members of species

Results in loss of genetic diversity

Seals and Cheetahs

Elephant seals were nearly hunted to extinction, but their numbers are now normal again. They are also now all almost nearly identical genetically

All male cheetahs are nearly identical genetically, suggesting they all came from one common male ancestor This lack of diversity responsible for a low fertility

rate an low survival rate of offspring