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Lecture 24
November 20, 2014
Population Ecology 53.2-53.5
I)
Introduction
a) Population = group of potentially interbreeding organisms of same species
i) living in same area/ same time
ii) share common gene pool
iii) share same resources
iv) influenced by same environmental factors
b) Ecology = study of interactions between living organisms and physical environment
c) Population Ecology:
i) Considers number of individuals of a species in an area & pop dynamics
d) Populations Dynamics
i) Study of change in population
ii) How and why pop size increases or decreases over time
iii) determine processes common to all populations
iv) Different features of population studied
II) Changes in Population Size: Pop Growth
a) Evaluation of Change in population
i) All populations have potential to change in size over time
(1) Change in population size = Births + Immigrants entering population - Death - Emigrants
leaving populations
III) Change in Population Size Over Time
a) Let's consider just births (B) and deaths (D) and no immigration or emigration
Δ𝑁
b) Population Growth Rate can be expressed mathematically as: Δ 𝑡 = 𝐵 − 𝐷
i) ΔN = change in population size
ii) Δ t = time interval
iii) B = number of births
iv) D = number of deaths
IV) Per Capita (Per individual0 Growth Rate
a) Now express B as average birth (bN) per capita (per individual) per year →
i) b= per capita birth rate → number offspring produced per year by average member of
population → calculate
ii) divide total number birth by total number of individuals
(1) Ex: 75 births per year in population of 1,000 individuals
(a) b= 75/1000 = 0.075 births per individuals
iii) B=bN = expected number of births per year in population of any size
(1) Ex: In a population of 200 individuals
(2) B (number of births) = 0.075 births per individual x 200 individuals = 15 births
b) now express D as average death (mN) per capita (per individual) per year →
i) m= per capita death rate → (= mortality) expected number deaths per year → calculate
(1) divide total number of death by total number of individuals
(a) Ex: 20 deaths per year in population in 1,000 individuals
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(i) m = 20/1000 = 0.020 mortality
ii) D = mN = expected number of deaths per year in population of any size
(1) Ex: In a population of 200 individuals
(2) D ( number of deaths) = 0.020 deaths per individuals x 200 individuals = 4 deaths per
year
Δ𝑁
c) The population growth equation can be revised Δ 𝑡 = 𝐵 − 𝐷 = 𝑏𝑁 − 𝑚𝑁 = 𝑁(𝑏 − 𝑚)
d) Population ecologists → most interested in difference between per capita birth rate & per capita
death rate
e) Per capita rate of increase = r = b - m
i)
Δ𝑁
Δ𝑇
= 𝑟𝑁
ii) if r < 0 → population declining
iii) if r > 0 → population increasing
iv) if r = 0 → zero population growth (ZPG)
V) Instantaneous Growth Rate
a) Ecologists prefer to use calculus to express population growth instantaneously rather than over
long time periods
b) Instantaneous growth rate can be expressed as
dN
dt
= 𝑟instN
c) Where rinst is the instantaneous per capita rate of increase
VI) Exponential Growth
a) Exponential population growth = population increase under idealize conditions (lot of food +
free max breeding)
b) → rate of increase at maximum = rmax > 0 and is constant
c) The equation of exponential population growth is
dN
dt
= 𝑟instN
VII) Models of Population Growth
a) Exponential Growth model
i) members of population has abundant food + free to reproduce at physiological capacity:
growing at its rmas
ii) Exponential population growth → a J-shaped curve
iii) Equation:
dN
dt
= 𝑟instN
VIII) Exponential Growth Model Limitation
a) Exponential growth model is not very good because doesn't really happen in nature → not
realistic
i) Can't continue indefinitely because of environmental resistance → limits set by
environment
ii) would run out of food resources, living space, & would build up toxic wastes
b) Certain populations have exponential growth for short time but over longer periods, growth
rate →slow down and decrease to nearly zero → next model
IX) The Logistic Growth Model
a) Takes into account environment resistance
b) Carrying capacity (K) = maximum population size the environment can support
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c) Show population grows more slowly near its carrying capacity → more realistic
d) Sigmold or S-shaped curve: first part is exponential growth then leveling out near carrying
capacity
X) The Logistic Growth Model Equation
a) first part of equation same as exponential growth
b)
K−N
K
dN
dt
= 𝑟instN (
K−N
)
K
reflects decline in growth as population reaches K
c) If N is small (not close to K) → term (K-N)/K will be close to 1 & rate of population growth high
d) If N close to K → term (K-N)/K will approach 0 & rate of population growth will decline
e) When N= K (K-N)/K = 0 population stops growing
XI) The Logistic Model and Real Population
a) The logistic model fits few real population → better than exponential model
i) Ex: Growth of lab population of paramecia fits an S - shaped curve → grown in a constant
environment no predators/ competitors
XII) The Logistic Model Limitation
a) But curves in nature usually not perfectly logistic
i) Populations tend to fluctuate around K (carrying capacity)
ii) Environment → never constant
iii) K itself may change
b) Most populations tend to fluctuate in size around some central mean
XIII) Life History Traits: How Organisms Maximize Reproduction While Ensure Survival (Why)
a) Organism's life history → trade-offs between survival and reproductive sucess
b) Life history traits are evolutionary outcomes
c) 3 main variables:
i) Age at first reproduction
ii) How often organisms reproduces
iii) How many offspring produced per reproductive episode
XIV)
The Different Strategies in Life History
a) Semelparity = big=bang reproduction → species reproduce once and die
i) many insects, plants, fish
ii) Strategy favored in species where
(1) survival rate of offspring in low
(2) living in highly variable/unpredictable environment
(3) Adults less likely to survive so producing lots of offspring → ensures some will survive
b) Iteroparity = repeated reproduction → species reproduce many times
i) most vertebrates
ii) Strategy favored in species where:
(1) environment is less variable
(2) adults more likely to survive to reproduce again
(3) competition for resources may be intense
c) Timing of reproduction in iteroparous involves tradeoffs
i) Reproduce early in life
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(1) May reduce likelihood of survival
(2) Expending E toward reproduction instead toward own growth
ii) Reproduce later in life
(1) expending E toward own growth, but reduces time for later reproduction
XV) Two extremes of Life History Strategies in Most Vertebrates
a) Which traits are favored at different population density?
b) Population density = number of individuals of a species per unit of area or volume at a given
time
i) Ex: number of dandelions in front yard → 200 plants/ 200 square feet
c) Life history strategies: 2 extremes selections:
i) r-Selected Species = density-independent selection
(1) Favored at low density
ii) K-Selected Species = density-dependent selection
(1) Favored at high density
d) r-selected species: Favored at low density
i) Have life history traits that maximize reproduction:
(1) small body size
(2) early maturity (early production of offspring)
(3) many babies, same time
(4) little/no parental care
(5) short life span
ii) Found in variable, temporary, or unpredictable environment
iii) Focus: rapidly producing many offspring (high growth rate)
e) K-Selected Species: Favored at high density
i) Selects for life history traits sensitive to population size
(1) long life span with slow development
(2) large body size
(3) high competitive ability
(4) more defenses against predators
(5) low reproductive rate
(6) parental care of young
(7) older age reprodction
ii) Population size close to K most of time
iii) Usually in relatively stable environment
XVI)
Factors Influencing Population Size
a) What are the factors that regulate population growth? 2 basic categories:
i) Density -Independent Factors
ii) Density- Dependent Factors
XVII) Density-Independent Factors
a) Environment factors → operate without relation to density of population
b) Tend to be abiotic factors → elements of nonliving world
i) Ex: volcano wipes out 1/2 population of palm trees
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ii) Ex: weather drought → pond dries up → all fish die
c) → birth or death rate does not change with population density
d) → r-selection
XVIII) Density-Dependent Factors
a) Environment factors whose impact on population is affected by population density
i) change in population density → factor affect population
b) In general, act as negative feedback system
i) as population density increases:
(1) density-dependent factors slow population growth
(2) Either ↘ birth rate and/ or ↗ death rate
ii) as population density decreases
(1) density-dependent factors increase population growth
(2) Either ↗ birth rate and/or ↘ death ate
XIX) Density-Dependent Factors
a) Tend to cause population to maintain itself at relative constant size near K
b) Tend to be biotic factors → elements of the living world that affect organism
c) Ex: of density-dependent factors
i) Intraspecific competition for limited resources (food or territory)
ii) Infectious disease
iii) Predation
iv) Accumulation of toxic wastes
v) Parasites Territoriality
XX)