Soay Sheep on Hirta Island

• Provided an ideal opportunity to study how an isolated population of animals changes in size when food is plentiful and predators are absent 

Population Ecology

The study of populations in relation to their environment.

Population ecology explores how biotic and abiotic factors influence the density,distribution, size, and age structure of populations.

Population

• is a group of individuals of a single species living in the same general area. 
• members of a population rely on tje same resources, influenced bu similar environmental factors, and are likely to interact with one another. 
• ecologists define boundaries appropriate to the organism under study: could be naturalor arbitrarily defined 

Density of a Population

Is the number of individuals per unit area or volume

EX: the number of oak trees per square killometer in Minnesota

Density: A dynamic perspective

Ways of finding density

• May count the number of oak trees in several randomly located 100X100 m plots, calculate the average density in the plots and extend estimate to population size
• may estimate density from an indicator of populaion size such as nests, burrows, track, or fecal droppings
• mark and recapture method

Mark and recapture method

1. Scientists capture a random sample of individuals in a population
2. tag or mark them and release 
3. wait for marked specimens to mix back into environment
4. capture a second sample set 
5. number of marked animals captured in the second sampling(x) divided by the total number of animals captured in the second sampling (n) should equal the number of individuals marked and released in the first sampling (s) divided by the estimaed population size (N)

Immigration/Emmigration 

Im: the influx of new animals from other areas

Em: the movement of individuals out of a population and into other locations

Population Density

Dispersion: is the pattern of spacing among individuals within the boundaries of the population

within a population's geographic range, local densities may differ substantially, creating contrasting patterns of dispersion 

provide insight into the environmental associations and social interactions of individual population 

Clumped:

1. aggregated in patches.
2. may be clumped where environmental factors favor germination and growth 
3. mating habits
4. where food is readily available 
5. increase effectivness of predation or defense 

Uniform Dispersion:

1. may result from direct interactions bwtween individuals in the population
2. Territoriality: the defense of a bounded physical space against encroachment by other individuals

Random dispersion:

1. the position of each individual in a population is independent of other individuals 
2. occurs when there is an absence of strong attraction or repulsion among individuals or where key physical or chemical factors are relatively constant across the study area 
3. random patterns are not common. usually show at least a tendency towards clumped 

Demography 

1. the study of the vital statistics of popilation and how they change over time 
2. Ecologists have particular interest in birth and death rate 

1. Life table 
2. cohort
3. To build a life table:

1. age specific summaries of the survival pattern of a population
2. a group of individuals of the same age from birth until all of the individuals are dead
3.  determine number of individuals that die in each age group and calculate proportion of the cohort surviving one age class to the next 

• Survivorship Curve:
• a graphic method of representing some of the data in a life table
• a plot of the proportion or numbers in a cohort still alive at each age 

Survivorship Curve: Type 1

• flat at the start, reflecting low death rates during early and middle life, and then drops steeply as death rates increase among polder age groups
• many large mammals that produce few offspring but provide them with good care exhibit this kind of curve

Survivorship curve: Type 2

1. intermediate, wit a constant death rate over he organism's life span
2. occurs in belding's ground squirrel 

Survivorship curve: Type 3

• Drops sharply at the start, reflecting very high death rates for the young, but flattens out as death rates decline for those few individuals that survive the early period of die off
• associated with organism's that produce very large numbers of ffspring but provide little or no care 

Reproductive Rates 

A reproductive table

am age-specific summary of the reproductive rates in a population. 

constructed by measuring the reproductve output of a cohort from birth until death 

Change in population size=

births+immigrants entering pop-death-emmigrans leaving

ΔN/Δt=B-D

Change in population size(N) and t represents time 

B=number of births

D=number od deaths

(use this because we took out immigration and emmigration for simplicity sake)

Per capita birth rate 

per capita death rate

• per capita birth/death rate is the number of offspring produced/number of deaths per unit time by an average member of the population 
• annual per capita birth rate: b
• annual per capita death rate: m
• B=bN to calculate the expected number of births per year in a population of any size 
• D=mN
• ΔN/ Δt=bN-mN
  

Per capita rate of increase

BANANAS

only joking

ΔN/Δt=rN

dN/dt=r(inst)N

r(inst) the instantaneous per capita rate of increase

Exponential growth

1. Population increase under abundant food and free to reproduce at their physiological capacity is called exponential growth
2. dN/dt=r(max)N
3. the size of a population that is growing exponentially increases at a constant rate resulting in a J-shaped growth curve-->more new individuals per unit time 
4. assumes that resources are unlimited which is a rare case in the real world

Elephants and exponential Growth

• grew exponentially for about 60 years after they were first protected from hunting. 
• elephants eventually caused enough damage to vegetation in the park-->collapse in food 
• had to begin using birth control

Carrying Capacity

as population density increases, each individual has access to fewer resources

Carrying capacity (K): the maximum population size thag a particular environment can sustain 

The Logistic Population Growth Model 

• dN/dt=r(max)N[(K-N)/K]
• assumes that populations adjust instantaneously to growth and approach carrying capacity smoothly. In reality there is often a delay before the negative effects of an increasing population realized-->may cause population to overshoot  its carrying capacity temporarily

Allee effect

1. The logistic model is based on an assumption: regardless of population density, each infdividual added to a population has the same negative effect on population growth rate
2. Allee effect: individuals may have a more difficult time surviving or reproducing if the population is too small. One plant alone could be damaged more by weather than a whole bunch of plants

Life History

the traits that affect an organism's schedule of repproduction and survival

Depends on three main variables: 

1. when reproduction begins
2. how often they reproduce
3. how many offspring per reproductive episode 

Semelparity

"one shot" pattern of big bang reproduction. Reproduces once with many offspring and then dies

Example: salmon, agave "century plant" 

Iteroparity

Semelparity vs. Iteroparity

Current hypothesis: 

there are two critical factors: the survival rate of offspring and the likelihood that the adult will survive to reproduce again.

• when survival rate is low(in highly variable or unpredictable environments)                 -->semelparity 
• dependable environments where adults are more likely to survive and breed again and competition for resources may become intense-->iteroparity

Trade off between reproduction and survival

No organism could produce as many offsprng as semelparity and provision them as well as iteroparous 

• high mortality rates often produce large numbers of relatively small offsprng (dandelions, mice)
• extra investment on the part of the parent greatly increases the offspring's chance of survival (Brazil nut tree, humans)

K-Selection

• Selection traits that are sensitive to population density and are favroed at high densities
• density dependent selection 
• operates in populations living at a density near the limit imposed by their resources(the carrying capacity,K) where competition among individuals is stronger 
• mature trees in old growth forest 

r-Selection

• Selection for traits that maximize reproductive success in uncrowded environments(low densities) 
• density independent selection
• maximizes the per capita rate of increase (r) and occurs in environments in which population densities are well below carrying capacity or individuals face little competition 
• weeds

1. Density Independant
2. Density Dependent 

1. a birth or death rate that does not change with population density 
2. a death rate that rises as population density rises and a birth rate that falls with rising density

Competition for resources

• increasing population density intensifies competition for nutrients and other resources, reducing reproductive rates
• farmers add fertilizer to crops to stop competition

Predation

• Predation can be an important cause of density dependant mortality if a predator captures more food as a population density increases
• as prey population builds up, predators may prefer that prey over other prey
• cutthroat trout concentrate on a particular insect species when that insect species has a rise in population. then switch to another species when they become abundant 

Toxic Wastes

• Yeasts such as the brewers yeast are used to convert carbohydrates to ethanol in wine making. 
• the ethanol that accumulates is toxic to yeasts and attributes to density dependant regulation of yeast population 
• wine alcohol content is less than 13% because that is the maximum concentration of ethanol that mosr wine producing yeasts can tolerate 

Intrinsic Factors

• Reproductive rates of white footed mice in a fireld enclosure can drop even when food and shelter are abundant
• this drop is because at high population densities females have more aggresive interactions and hormonal changes that delay sexual maturation and depress the immune system 

Territoriality 

• can limit populaton density when space becomes the resource for which individuals comepete
• cheetahs use a chemical marker in their urine to warn other cheetahs of boundaries
• Australian gannets defend their territory when nesting by calling and pecking at one another 
• maintaining a territory increases chances that the animal will get enough food to reproduce 
• the presence of surplus, or nonbreeding, individuals is a good sign that territoriality is restricting population growth 

Disease 

If the transmission rate of a particular disease depends on a certain level of crowding in a population, then the disease's impact is density dependent

in humans, respiratory diseases (flu, TB) strike humans more whn the population is dense 

Population Density 

• All populations show a fluctuation in size from year to year or place to place.
• study of population dynamics focus on the biotic and abiotic factors that cause cariation in population sizes 

Stability and Fluctuation 

• Soay sheep (although no predators and abundance of food) had a highly fluctuating population
• most likely because of harsh weather, particular cold and wet winters, can weaken the sheep and reduce food supply
• increase in density of parasites 
• moose and wolf in Isle Royal

Boom and Bust cycle

some small herbivorous mammals (volesm lemmings) tend to have 3-4 year cycles

some birds (ruffled grouse and ptarmigans) have 9 to 11 year cycles 

Snow Shoe hare and Lynx Population Cycle

1. lynx numbers rise and fall with the number of hares becasue lynx depend on hares for food 

Immigration, emigration, and Metapopulations 

• Metapopulation: a number of local populations are linked. Occupying a patch of suitable habitat in a sea of unsuitable habitats
• Immigration and emmigration are particularly important because they link metapopulations 
• helps ecologists understand population dynamic and gene flow in patchy habitats, providing a framework for the conservation of species living in a network of habitat fragments and reserves 

The Global Human population 

• Human population growth is most likely a singular case. no other animal has sustaiend so much growth for so long
• currently 6.8 billion and increasing 79 million per year, 200,000 per day
• predict a populatuion of 7.8-10.8 by 2050 
• the rate of growth began to slow in the 1960's. reached it's peak 2.2% in 1962 and by 2009 is 1.2%
• The reduction over the past four decades shows that human population has departed from true exponential growth 

Zero populaton growth 

In a stable regional human population, birth rates equal death rates. Two possible configurations for a stable populationg are

Zero Pop growth=High birth rate-High Death rate

Zero Pop Growth=Low birth rate-low death rate

Demographic Transition 

• Movement from high birth and death rates toward low birth and death rates, which tend to accompany industrialization and improved living conditions. 
• most of the cuttrny global population growth (1.2% per year) is concentrated in less industrialized countries, where about 80% of the population lives 
• reduced family size is the key to dempgraphic transition 

Age Structure

The relative number of individuals of each age in a population 

age structure is usually graphed as "pyramids" 

Afghanistan: pyramid is bottom heavy, skewed toward young individuals who will grow up and perhaps sustain the explosive growth

U.S.: relatively even until the older, postreproductive rates, except for a bulge that corresponds to the "baby boom." population is projected to steadily grow (even though lower birth rate) because of immigration

Italy: he pyramid has a small bas, indicating that individuals younger than reproductive age are relatively underrepresented in the population 

Infant Mortality and Life Expectancy 

Infant Mortality: the number of infant deaths per 1000 live births

life expectancy at birth: the preduced average length of life at birth 

• vary widely depending on region 
• reflect the quality of life faced by children at birth and influence the reproductive choices parents make 
• infant mortality high-->more babies to ensure so survive

Carrying Capacity of Humans

Ecological footprint 

The Carrying capacity of Humans is uncertain

 EF: aggregate land and water area required by each person, city, or nation to produce all the resources it consums and to absorb all the waste it generates 

we are very close to the carrying capacity of earth. With a world population of more than 6.8 billion we are already using many more resources in an unsustainable manner 

Find the population size for these two studies

1) 80 mice on the first capture, later caught 70 mice of which 14 were marker

2) 45 mice on first capture, later caught 70 mice of which 7 were marked 

if actual population size was 800, why was their such a difference between actual and predicted?

1) 400

2) 450

Because there was a positve trap bias-->peanut butter

Name that pattern of dispersion! 

1)King Penguins nest in high densities, with each bird fiercly defending it's space

2) Giant puffballs make millions of microscopic airborne spores

3) Hyenas hunting at night can bring down large wildebeast 

1) uniform

2)random

3) clumped

True/False:

1. In wheat fields resource competition can be eased by adding fertilizer 
2. cutthroat trout feed on the rarest prey
3. yeasts would produce wine with over 13% alcohol if brewers let them
4. in mice, birth rates go up and up as long as food and shelter are abundant
5. gannets and cheetahs use chemicals in their urine to makr territories 

1. true
2. false
3. false
4. false
5. false 

True/False:

1. the human population has been growing at the rate of about 200,000 per year
2. the human population can still grow, even if the growth rate declines
3. the demographic transition takes place at about the same time in all countries
4. most of the current population growth is now occuring in less industrialized countries, where about 80% of the worlds people live 

1. false
2. true
3. false
4. true 

Choose one of these factors that might affect carrying capacity of humans, and write a few sentences about how that factor might work in determining N for H. Sapians 

1. Land Area/space
2. energy use
3. food
4. nonrenewable resources 

1. humans keep expanding...eventually not enough room for all of our junk and us...if we use all of the space we'll find our carrying capacity because there will be disease and famine
2. we all use nonrenewable energy sources...if keep using them at this rate we will irreversibly change our climate
3. malnutrition and famine are common in some regions...result from unequal distribution of food 
4. could run out of nonrenewable resources