POPULATION ECOLOGY
Campbell & Reece Chapter 53
Population Ecology
the study of populations in relation to their environment
Dynamic Biologic Processes that Influence Population Density
Population: is a group of individuals of a single species living in same generral area
Members of a population rely on same resources & are influenced by same environmental factors
They are likely to interact & breed with each other
Populations
often described by their boundaries & #s boundaries may be natural ones or
ecologists may arbitrarily define them
Population Density
# of individuals per unit area or volume
Dispersion
the pattern of spacing among individuals w/in boundaries of the population
Mark-Recapture Method
way to determine population size: ecologists cannot count all individuals in
a population if organisms move too quickly or are hidden from view
Technique: capture a random sample of individuals & “mark” & then release them.
Some species can be identified w/out physically capturing them: dolphin, whale
Mark-Recapture Method
Population Dynamics
Population density is not a static property:
Births Deaths Immigration Emmigration
Patterns of Dispersion
Patterns of Dispersion: Clumped
*most common plants & fungi clumped where soil
conditions & other environmental factors favor germination & growth
animal clumping may have to do with being successful in some way:
Mayflies swarm in great #s which increases their chances of mating (only have ~2 days)
Wolf pack more likely to kill a moose or deer than a single wolf
Mayfly Swarm
Patterns of Dispersion: Uniform
may result from direct interactions between individuals in a population
some plants secrete chemicals that inhibit germination & & growth of nearby individuals that could compete for resources
animals that show territoriality are spaced apart: often as result of antagonistic interactions
Patterns of Dispersion: Random
unpredictable spacing 1 individual’s position is unrelated to
other individuals
Demographics
study of the vital statistics of a population & how it changes over time
especially important are birth rates & death rates
Life Table
age-specific summaries of the survival pattern of a population
construct one by following the fate of a cohort: a group of individuals of the same age, from birth until all of them are dead
Life Table of Belding’s Ground Squirrels
Survivorship Curves
a graphic method of representing some of the data in a life table
plots proportion or #s in a cohort still alive at each age
Idealized Survivorship Curves
Type I Curve
flat @ first reflecting low death rate during early & middle life
then steep drop as death rate increases with increasing age
typical curve for large mammals that produce few young but take good care of them
Type III Curve
drops sharply at start reflecting high death rate among the young
flattens out as death rate for those individuals that make it out of early life decreases
typical for species that produce many offspring but provide little or no care for them:
fishes, invertebrates
Type II Curve
intermediate have a constant death rate typical of rodents, some invertebrates,
some lizards, & some annuals
Survivorship Curves
not all species fall into 1 of the 3 types some invertebrates have a “stair-step”
pattern: more vulnerable during periods when molting, less vulnerable when not molting
Reproductive Rates
Demographers tend to just look @ #s of females & how many female offspring they have
simplest way to describe the reproductive pattern of a population is to ask how reproductive output varies with the age of females
Reproductive Table
aka a fertility schedule an age-specific summary of the
reproductive rates in a population constructed by measuring reproductive
output of a cohort from birth to death
Natural Selection
favors traits that improve an organism’s chances of survival & reproductive success
every species has trade-offs between survival & traits such as frequency of reproduction, # of offspring
traits that affect an organism’s schedule of reproduction & survival make up its life history
Life Histories
are very divers but exhibit patterns in the variability
have 3 basic variables:1. when reproduction begins2. how often the organism reproduces3. how many offspring produced during
each reproductive episode
Big-Bang Reproduction
1 individual reproduces large # of offspring then die: called semelparity
Iteroparity
produce only a few offspring during repeated reproductive episodes
Semelparity or Iteroparity?
Which is better? Critical factor is survival of offspring: *when survival of offspring is low as in
highly variable or unpredictable environments, semelparity is favored
*dependable environments where competition of resources is fierce favors iteroparity
Limited Resources means Trade-Offs
sometimes see trade-offs between survival & reproduction when resources are limited
example: red deer females have higher mortality in winters following summers in which they reproduce
Species whose Young have High Mortality Rates
often produce large #s of relatively small offspring
example: plants that colonize disturbed environments usually produce many small seeds only a few reach suitable habitat smaller seeds allow them to be carried
farther
Parental Investment Increases Survival of Offspring
example: oak, walnut, & coconut trees have large seeds with large store of nrg & nutrients to help seedlings become established
example: primates provide an extended period of parental care
Population Growth
All populations have a tremendous capacity for growth
Unlimited increase does not occur indefinitely for any species, in lab or in nature
Exponential Model of Population Growth
unlimited growth does not occur for long in nature but it is assumed to be true in this model
Δ population = (# births + # immigrants ) - (# deaths + # emigrants)
Or, ignoring immigration & emigration: Δ N/Δt = B - D
Exponential Model
now use average #s of births & deaths per individual during a specified period of time
If there are 34 births per year in a population of 1,000 then the annual per capita birth rate is 34/1,000 = 0.034 = b
Exponential Growth Equation ΔN/dt = TMax N: represents a population’s
potential growth in an unlimited environment where TMax is the maximum per capita rate of increase & N is the # of individuals in the population
Exponential Growth
Exponential Model
http://www.slideshare.net/MrDPMWest/population-growth-apbio
Logistic Model
in nature, as any population density increases: each individual has access to fewer resources
eventually, there is a limit to the # of individuals that can occupy a habitat
Carrying Capacity: (K) the maximum population size that a particular environment can sustain
Carrying Capacity
varies over space & time with the abundance of limiting resources: Energy Shelter Refuge from predators Nutrients Water Suitable nesting sites
Carrying Capacity
per capita birth rate decreases if there are not enough nutrients for adults to maintain themselves or if disease or parasitism increases with density
per capita death rate increases for same reasons
either way: results in lower per capita rate of increase
Logistic Growth Model
per capita rate of increase approaches 0 as the carrying capacity is reached
Logistic Growth Equation
Logistic Model
fits few real populations perfectly, but it is useful for estimating possible growth
Life History Traits are Products of Natural Selection
traits that affect an organism’s schedule of reproduction & survival make up its:
life history: 3 main variables:1. age @ 1st reproduction2. how often the organism reproduces3. # offspring produced/ reproductive episode
Semelparous Organisms
reproduce once & die aka “big-bang”
Iteroparous Organisms
produce offspring repeatedly
Semelparous vs. Iteroparous? 2 critical factors:1. survival rate of offspring
low chance survival where environment variable or unpredictable: semelparity
2. likelihood adult will survive to reproduce again
adults less likely to survive: semelparity
“Trade-Offs” & Life Histories
trade-offs between reproduction & life histories:
Trade-Offs
selective pressures influence the trade-off between # & size of offspring Plants:
those that colonize disturbed environments have small seeds…only few reach suitable habitat
Animals: those with high predation rate have larger #s
offspring (quail, mice, sardines)
Trade-Offs
other species put large investment to insure survival of offspring trees with large seeds (walnut, brazil nut)
provide nutrients that increases offspring’s chances of survival
K-Selection/ R-Selection
selection for traits sensitive to population density & are favored @ high densities is known as K-selection = density dependent selection
selection for traits that maximize reproductive success in low density environments is called r-selection
Density-Dependent Factors
refers to any characteristic that varies with population density
birth or death rate changes based on population density often because water &/or nutrients become
scarce
Density-Independent Factors any characteristic not affected by
ppulation density birth & death rates stable no matter
what the population density is
Determining Equilibrium for Population Density
red line = density-independent death rate
blue line = density-dependent birth rate
junction = equilibrium density
Mechanisms of Density-Dependent Population Regulation
w/out some type of (-) feedback between population density & rates of birth & death: a population would never stop growing
Mechanisms of Density-Dependent Regulation
1. Competition for Resources
competing for nutrients & other resources decreases the birth rate
farmers add fertilizer to reduce competition for nutrients
Mechanisms of Density- Dependent Regulation
2. Predation as # prey increases
a predator may develop preference for that species
Mechanisms of Density- Dependent Regulation
3. Toxic Wastes Brewer’s Yeast
used to convert carbohydrates to ethanol in wine-making but anything > 13% alcohol it toxic to the yeast
Mechanisms of Density- Dependent Regulation
4. Intrinsic Factors some species experience increased
death rate * decreased birth rate when population density reaches certain point even when there is sufficient nutrients white-footed mice: immune system altered
more deaths & reproductive maturity delayed fewer births when certain density reached
Mechanisms of Density- Dependent Regulation
5. Territoriality when space becomes
resource competing for it can limit population size
maintaining a territory insures there will be enough food to live & reproduce
Mechanisms of Density- Dependent Regulation
6. Disease if transmission rate
depends on certain level of crowding then it is density-dependent
respiratory viruses spread thru air: is more easily spread in large cities than in rural areas
Mechanisms of Density- Dependent Regulation
7. Population Dynamics the normal fluctuations in population
size from yr to yr
Mechanisms of Density- Dependent Regulation
8. Stability & Fluctuation because changing environmental
conditions disrupt populations, they all experience size fluctuations
Emigration
when a population becomes crowded & resource competition increases often emigration #s increase
example: when slime mold resources become scarce some single celled individuals group together (amoeba group) a & form a slug
Metapopulation
form when a # of local populations are linked
local populations in a metapopulation can be thought of as occupying discrete patches of suitable habitat in a sea of unsuitable places
Human Population Growth
since ~ 1650 has been growing exponentially
w/in last 55 yrs rate of growth has fallen by nearly ½
predictions: rate will continue to decline until ~2050 & reach equilibrium ~2100
Human Population Growth
Demographic Transition
movement from high birth rate & high death rate low birth rate, low death rate
occurred in human population with industrial revolution
Age Structure
relative # of individuals of each age use “pyramids”
USA: #s fairly steady except for “baby boomers”: increased birth rate that followed up to 20 yrs after WWII
Age Structure: Rapid Growth bottom-heavy
pyramid skewed toward
more young individuals
ex: Afghanistan, Congo
Age Structure: Slow Growth
fairly steady growth over time
can be do to steady birth rate or falling birth rate + steady increase in immigration
ex: USA
Age-Structure: No Growth
smaller base: #s under reproductive age are under-represented
ex: Italy, Germany
Infant MortalityLife Expectancy
Infant Mortality = # of infant deaths per 1,000 live births
Life Expectancy @ birth = predicted average length of life
both vary widely among human populations
if infant mortality high parents may choose to have more children to increase odds some will reach adulthood
Comparing Industrialized & Developing Nations
Global Carrying Capacity
is uncertain has changed over time due to advances
in technology
Ecological Footprint
is the aggregate land & water area needed to produce all the resources a person or group of people consume & to absorb their wastes
it‘s 1 measure of how close we are to Earth’s carrying capacity
We‘re using many of Earth’s resources in unsustainable manner
Annual per capita Energy Use
http://www.worldometers.info/world-population/
http://www.worldometers.info/world-population/