populations outline: properties of populations population growth intraspecific population...
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Populations
Outline:
• Properties of populations
• Population growth
• Intraspecific population
• Metapopulation
Readings: Ch. 9, 10, 11, 12
Definition
• Population is a group of individuals of the same species that inhabit a given area
Unitary organisms
Modular organisms
genet ramet
Distribution of a population
Distribution of a populationRed maple
Distribution of a populationMoss (Tetraphis pellucida)
Abundance
versus
Population density
Patterns of dispersion
Effect of scale on pattern of dispersion
Populations have age structure
Populations have age structure
Determining age
wild turkey quail
grey squirel bat
Determining age
Dispersal
• Movement of individuals in space
• Moving out of subpopulation = emigration
• Moving into a subpopulation = immigration
• Moving and returning= migration
Yellow-poplar
Ring-necked duck
Gray whale
Gypsy-moth
POPULATION GROWTH
Darwin’s 1st observation:
All species have such great potential fertility that their population size would increase exponentially if all individuals that are born reproduce successfully.
Example of exponential growth:the ring-necked pheasant, Phasianus colchicus
• Native to Eurasia• 1937: Eight birds introduced
to Protection Island (Washington state)
• 1942: Population had increased to 1,325 birds (a 166-fold increase!)
N/t = (b - d) Nt
Population Growth Models
• Assume no immigration or emigration
• Let N = population size
• Let N/ t = change in population size/unit time= total # births - total # deaths
• Let mean birth rate per individual = b= # births / individual / unit time
• Let mean death rate per individual = d= probability of death for an individual / unit time
N/ t = bN - dN
• Let r = b-d
Population Growth Models
• r = instantaneous rate of increase a.k.a. per capita rate of increase
• Calculus notation is commonly used; N/t = dN/dt
• If r > 0, population will increase exponentially at rate, dN/dt, = rN
• For an exponentially growing population, the number of individuals at time t, Nt = N0e(rt) where No = initial population size and e = base of natural logarithms
Exponential growth model: Nt = N0 e(rt)
St. Paul reindeer
Life tables
cohort - all individuals born within a periodcohort life table – survivorship of a cohort over time
lx = represents the probability at birth of surviving to any given age
Life tables
dx = represents the age-specific mortality
Life tables
qx = represents the age-specific mortality rate
Life tables
Mortality curves
sedum
Mortality curves
Survivorship curves - plot of lx vs. time
Red deer
Theoretical survivorship curves
What happened to population in 1940s?
Human population growth
Darwin’s 2nd observation:
Populations tend to remain stable in size, except for seasonal fluctuations
Darwin’s 3rd observation:
Environmental resources are limited
• In real world, populations don’t increase exponentially for very long
--> run out of resources
• An N increases, b decreases and/or d increases
Population limiting factors
Density-dependent: effect intensifies as N increases. E.g.:1. Intraspecific competition
– Between members of same species
2. Toxic waste accumulation – E.g. yeast cells: produce ethanol as by-
product of fermentation (see next slide)
3. Disease– Spreads more easily in crowded
environments
Effect of crowding on birth rate
Effect of crowding on survivorship
Intraspecific population regulation
Carrying capacity, K
= maximum number of individuals that a particular environment can support
• Take into account by the Logistic Growth Equation,
dN/dt = rN (1-N/K)
Logistic model
Logistic model
Exponential vs. logistic model
Gray squirrel
How good is the logistic model?
• Describes growth of simple organisms well, e.g. Paramecium in a lab
• Water fleas (Daphnia spp.): population initially overshoots K until individuals use up stored lipids --> crash down to K
• Song sparrows: populations crash frequently due to harsh winter conditions– N never have time to reach K– Population growth not well described by the
logistic model
Life History Strategies
• When N is usually << K, natural selection favors adaptations that increase r
--> lots of offspring
= r selection– E.g. species that colonize short-lived environments
• When N is usually close to K, better to produce fewer, “better quality” (i.e. more competitive) offspring
= K selection• E.g species that live in stable, crowded environments
Density dependence
Density dependencewith Allee effect
American ginseng
Density dependencewith Allee effect
Types of competition
• Competition: individuals use a common resource that is in short supply relative to the number seeking it
• Intraspecific vs. interspecific• Scramble vs. contest• Exploitation vs. interference
Density effect on growth
Density effect on growth
Density effect on growth
Horseweed
Density effect on growthSelf thinning
Density effect on reproduction
Territoriality
Grasshopper sparrowAmmodramus savannarum
Banding study in California: 24% of current territory holders had been floaters for 2-5 yrs. before acquiring a territory.
White-crowned sparrow, Zonotrichia leucophrys
Uniform distribution of plants occurs due to the development of resource depletion zones around each individual
Population limiting factors
Density-independent: effect does not depend on N. – E.g. weather / climate– Thrips insects:
• Feed on Australian crops (pest)• Population growth very rapid in early
summer• Drops in late summer due to heat,
dryness--> N never has time to get close to K
Density-independent factors
DRYTurbid
WETClear
Density-independent factors
e.g. Dungeness crabs
• Density-dependent factors: competition; cannibalism
• Density-independent factors: water temperature
Metapopulationsa population of populations
Chapter 12
Metapopulation: A group of moderately isolated populations linked by dispersal
Criteria for a metapopulation
1. Habitat occurs in discrete patches
2. Patches are not so isolated as to prevent dispersal
3. Individual populations have a chance of going extinct
4. The dynamics of populations in different patches are not synchronized
– i.e., they do not fluctuate or cycle in synchrony
Metapopulation dynamics:spatial scales
1. Local (within-patch)2. Metapopulation (regional)
Shifting mosaic of occupied and unoccupied patches
Checkerspot butterfly
Levin’s model of metapopulation dynamics
• E - subpopulation extinction rate = eP• e – probability of a patch going extinct/unit time• P – proportion of occupied patches
• C – colonization rate = mP (1-P)• m – dispersal rate• (1-P) – unoccupied habitats
E = C equilibrium point,Where0 = [mP(1-P)] - eP
If C>E, P increases; If C<E, P decreasesPequilibrium= 1-e/m
Bush cricket
Larger patches have larger populations (and therefore lower risk of extinction)
Skipper butterfly
Effect of habitat heterogeneity
Mainland-island population structure: one large population (low extinction risk) provides colonists for many small populations (high risk)
Rescue effect: island recolonized from “mainland”• High quality / permanent population = source population• Temporary patches = sink populations
Checker-spot butterfly
Skipper butterfly
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