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BIOL 302

LECTURE 18: POPULATION STRUCTURE

INTRODUCTION

BIDE are very critical in determining population structure

And all the previous patterns discussed

OUTLINE

What survival, age and sex ratios tell us about a populations past and future

Patterns of survival: life tables and curves

Age distributions and population dynamics

Operational sex ratios: why rarely 1:1?

Dispersal: causes and consequences for population density

POPULATION STRUCTURE

Can be defined by a number of factors

o Ex: patterns of mortality, age distributions, sex ratios, dispersal

Dall sheep populations (Adolph Murie)

o Showed that mortality due to wolf predators mostly occurred in young and very

old individuals

o Prevented legislation that would have wiped out wolves

PATTERNSOFSURVIVALAND MORTALITY

Pattern of survival and mortality among individuals in population is a fundamentalparameter of population structure

Work on humans (insurance companies) has helped ecologists to understand the patterns

and concepts in animal populations

Survivorship curve: summarizes pattern of survival in a population

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Life tables: bookkeeping device to track births, survivorship and deaths in populations

3 WAYSTO ESTIMATE SURVIVAL

Cohort life table

o Pearl took information from insurance companies and told people that they

should be doing this in order to understand populations

Static life table

Measure age distribution

o Gives you a static life table often

COHORTLIFE TABLE

Identify individuals all born at the same time and keep records from birth to death

o Most complete picture of survivorship in a population

Easy to interpret but often difficult or impossible to collect these data

o Ex: The cohort may be huge (dandelions), highly mobile (Arctic terns) or

dangerous (alligators)

Dont have to measure at birth but NEED to be same age

o Ex: In sparrows mortality rate is 78% in first year, can be highly variable

Example: Song sparrows from Mandarte Island, just off Vancouver BC

o Tiny island and has a very small number of song sparrows but they all stay there

o All birds hatched in 1976 had died 6 years later

o 115 birds hatched but only 25 birds were seen alive the next year (~21% =

proportion surviving, 78% mortality rate)

Mortality rate is 78% in first year of life

Mortality rates can be high in the first year of life and increase

again when they get really old

o Symbols replace words in the table heading

X = age in years, weeks, or days (time intervals as precise as possible

depending on breeding)

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Nx = number seen alive

Ix = proportion surviving

Dx = number dying

Qx = mortality

This is the hardest to do, but ideal

STATICLIFETABLE

Snapshot

Record age at death of large number of individuals over narrow window in time

o Worried about environmental variation

Requires accurate estimation of age at death

Called static because assumes the population parameters are stationary (not changing

with time or environment)

o Almost never true (rarely met in real world situations)

DALL SHEEP STATIC LIFE TABLE (FIG 11.3)

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Using: age, number of survivors at beginning of year, and number of deaths during year

All the sheep in this sample have a maximum life span of 14-15 years

Per capita is a common metric

Dall sheep have a high survival of young

o Very high survival until about the age 9

o Sheep ten years old and older are easier prey for wolves and die at a high rate

To create survivorship curve from life table:

- Age goes on the x-axis and

- Log (number of survivors) goes on the y-axis

AGE DISTRIBUTION METHOD

Measure (estimate) how many individuals of each age class are in the population

Calculate the difference in proportion of individuals in each age class, assume this =

survival

o Assumes differences in numbers from one age class to next due to mortality, not

emigration

o Assumes population stable in size

A common method (practical) but less accurate

Note: also produces a static life table

LECTURE 19: POPULATION STRUCTURE CONTINUED

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LIFE TABLES: ARECAP

Static life tables (compiled from snapshots of

survival OR measuring age distributions) can

produce same data as cohort life tables, as

long as:

1) Populations are in equilibrium (not

growing or shrinking, and no significant

immigation or emigration)

2) Environment is not changing over time

In reality, these conditions almost never hold,

so static life tables are used

o Static life tables are never as good as

cohort, often use

Ie: Insurance companies. Dont

know when people are going to die.

Survival: Survivorship Curves

Logarithmic y scales

Ex: Age vs. Number of Survivors (11.4)

o These generate a constant rate of mortality (or fertility, or other population

demographic measure) is seen as a straight line

Log scale enable quick between-population comparisons

SURVIVORSHIP: THREE MAIN PATTERNS:

Type I: Patterns of Survival: High Survival Among Young (11.5)

Patterns can be quite similar regardless of large taxonomic differences or their

Ex: Flox. Survivorship is relatively high until reach flowering, and then survival

plummets. This is because these plants live for

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Ex: Rotifer. Complete pattern is survival by 11 days.

Ex: Humans

Type II: Patterns of Survival: Constant Rates (11.6)

Lines are fairly linear

Ex: Sparrow. Approximately constant rate.

Ex: Northern water snake. Has slightly higher mortality rates as individuals age. By age

of four most are dead.

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Type III: Patterns of Survival: High Mortality of Young (11.7)

Ex: Cleome. Start with roughly 1

million seeds, only 39 plants survive to

1 year of age. Can be a long lived

plant, but number of progeny produced

relative to survival is huge.

Very common r-selected species. A lot

of small offspring.

3 TYPESOFSURVIVORSHIP CURVES (PEARL, 1928)

I: Humans. Low per captita rate of

mortality through most of life, then

increases drastically.

II: Songbirds. Constant per capita rate of mortality

III: Oysters. Many marine organisms (in addition to plants) show these survivorship

curves.

Note: Ix. Graph is parameterized.

SURVIVORSHIPCURVESYIELDMORTALITYRATEPLOTS (KREBS, 2001)

Death rate per capita

Telling same message, just different means of representation.

AGEDISTRIBUTION: WHYIMPORTANT?

Standard models of population growth assume birth and death rates constant

But many natural populations have age structure that affects these rates

Ex:

o Tadpole populations will not grow until they become grogs.

o A population of geriatric, post-reproductive ages will become extinct

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So we need to know how many individuals of reproductive age + rate of reproduction

AGEDISTRIBUTIONS: A POPULATIONTHATISPERSISTING

Ex: White oak in mature oak history forest in Illinois (11.9)

Abundance of young trees means

sufficient reproduction to replace oldest

individuals as they die.

Found age profile.

The population of oaks is dominated by

young individuals.

This age structure shows that older trees

are being replaced by younger trees

Population is likely to be successful

Age distributions: A decliningpopulation

Ex: Rio Grande cottonwoods in central

New Mexico.

o Population dominated by older individuals (40-50 years old)

o Very little recruitment of young trees - no successful reproduction at this site for

over 10 years

o Older trees are not being replaced by young trees.

o WHYISTHISPOPULATIONINTROUBLE?

Usually has regular flood cycle, which creates fertile, wet environment for seedlings.

This has been eliminated because the damming (for irrigation) has wiped out the regular

flood cycles.

Studies important for forecasting

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COMPARING 2 POPULATIONSOFTHESAMESPECIES (KREBS, 2001)

Stable Age Distribution (top)

o Many young, fewer middle, less old

o Stable: will grow exponentially, healthy rate of recruitment

o Fertility Rate > Mortality Rate

o Used to describe situation where there is a constant age structure, mortality and

birth rates are fixed, and populations grow exponentially

o In text, use word stable to say population that is persisting, dont discuss this

Stationary Age Distribution (bottom)

o Fertility is declining at a fixed rate

o The Fertility rate = Mortality rate

o The population will not change with time, constant

o Rare

AGE STRUCTURE PYRAMIDSIN HUMANS (KREBS, 2001)

Measures proportion of individuals in each of age categories.

3 different age pyramids

Type Description Example

Zero growth The offspring coming from the boomers is fewer

in numbers

Looming bulges are senior people

Fewer young people to support older

Most of Western

Europe (Ex: Italy),

close to Canada

Slow growth Still have boomer bulge

Recruitment of young is better

USA

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Slightly more healthy structure

Rapid growth Large populations of young people

Small proportion of middle aged and older people

surviving

Kenya

AGEDISTRIBUTIONSREVEALPASTEVENTS

Ex: Large Cactus Finches on Isla Genova, Galapagos

o 1983:

Fairly even distribution among age classes 1-5

Massive gap in year 6. 1977 breeding year- correlated to El Nino eventwhich lead to massive drought.

o 1987:

There have been more droughts (1984, 1985) that knock out age 2 and 3

classes

Can still see remnants of 1977 droughts, with no 10 year old birds

LECTURE 20: POPULATION STRUCTURE CONTINUED

SEXRATIOS

Population sex ratios may vary because of relative fitness of different sexes in a

population

Most primary sex ratios at birth are close to 1:1

o Why? (why so many males?)

Dont need as many females as males to reproduce successfully

Statistics of getting XY vs. XX- what that mechanism works

Primary Sex Ratio vs. Operational Sex Ratio (OSR)

o Ratio at birth is slightly male-biased in humans.

o Ratio of individuals that are capable of breeding is 1:1 at early adulthood, then

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steadily more female based

Fisher (1930) argued that selection favours parents investing equal allocation in sons and

daughters

o Frequency dependent selection: Relative fitness of producing males or females

not inherent property of trait itself, but instead depends on relative frequency of

phenotypes in population

o Have to allocate equal energy to have equal numbers

o NB: If sons and daughters cost different amounts to rear, then a skewed sex

ratio will result, even though parents are investing equally in both

If it costs 2x more energy to produce male than female (requires more

energy to provide energy in utero and milk)- would expect sex ratio to be

skewed.

Selection has favoured equal investment of sons and daughters

SEXDETERMININGMECHANISMSAREHIGHLYVARIABLE

Genetic

Environmental

o

Depends on experiences during early development

o Ex: Alligator egg, snapping turtles.

Ex: Whip-tail lizards. A species with no males. Partenogenetic egg development caused

by a female behaving in a male-like way during breeding season.

Females may skew sex ratios in response to paternal quality

o Mechanisms unclear

Differential fertilization success by sperm.

Differential implantation

Embryo resorption

Infanticide

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HOWDO BIASED SEXRATIOSARISE?

Male and females offspring may differ in:

o Differential mortality

May cause sex ratios to vary among age classes

o Cost of production

Ex: birth weight, lactation demands

o Probability of mating

Skewed OSR due to intrasexual competition

o Dispersal Behaviour

Generalization: In many mammals, sons disperse; in many birds,

daughters disperse

DISPERSAL

Ecological process affecting population distribution, and genetic process affecting

population differentiation

Theory is relatively undeveloped related to other mechanisms; difficult to study

Most population models assume stationary populations, with animals staying within

boundaries

Dispersal influences: competition, density, and gene flow

o Gene flow important for maintenance of metapopulations

Two major types:

NATALDISPERSAL

Movement of individual after born or hatched, to first breeding site

Natal philopatry: animal that disperses short distance. Individuals stay near birth place.

ADULTDISPERSAL

After reaching adulthood and having bred (or attempted to breed) in one place, go

elsewhere for later breeding attempts.

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Do not have adult philopatry (disperses, breeds, then stays in same site for rest of life)

Can influence population structure

o Mature individuals (contributing to Ne) take experience and genes with them

This is not a commonly observed pattern

o Typically animals remain in first breeding site

There are some triggers that are associated with an adult choosing to leave a site:

o Negative breeding experience at one site (breeding failure)

o Mate changes: adult loses mate as a result of divorce (unknown whether this is

cause or effect). Males and females can choose to lose the pair bond, but many

divorces are initiated by females if the first male is not successful.

CAUSESFOR DISPERSAL:

Introduced Species

o Some of most dramatic examples come from these

o Cases where normal distribution may have been elsewhere, and through human-

induced changes animals have been introduced to new environment

o Ex: Cane toads in Australia, Zebra Mussels, Mountain Pine Beetle from W N.A.

o Ex: Honeybees

Crossbreeding of Brazil + African Bees in attempt to improve honey

production (1957)

African honeybees are aggressive, highly evolved defense mechanisms.

Have a tendency to swarm (much more developed than our bees).

These behavioural traits were represented in hybrid bees

Began to disperse and form new colonies, outcompeting European

counterparts in Brazil. Moving at 300-500km/year- within 30 years,

occupied most of S and Central America.

Block of non-occupancy at 34N/S latitudes: prediction that these bees

will not make it farther N than these areas because of winter

temperatures.

Recolonization as a result of protection

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o Ex: California Sea Otter

By early 1900s population had shrunk to 30 individuals because of

hunting for pelts. Received protection in 1911.

Recolonization happened towards south (quicker) and north, populationis now thriving

HOWFASTCANDISPERSALHAPPEN?

Massive variation across different types of organisms- different rates of population

growth and ability to take advantage of opportunities.

Differs by 3 orders of magnitude

Example:

o African Honeybees: 300-500km/year

o Rates are much smaller for larger mammals: 10-15km/year

DISTRIBUTION RANGES: CHANGING FOOD SUPPLY

Ex: Kestrels and owls response to vole density.

o Mark and recapture study to determine bird density

o Vole density changes in cyclic nature. Not always a regular cycle, but periods of

plummeting and rising (around 20 years)

o Bird and vole densities are closely mapped

o Means that in some years they may be no animals, in others there will be many.

Can be difficult to predict when this happens.

o Relates to dispersal because adults have to make decision to go elsewhere to get

food.

o Can see this pattern on Wolfe and Amherst Island

DISPERSALTHROUGH RIVERSAND STREAMS

Study by Muller from 60 years ago. He mapped the distribution of many organisms

(mainly aquatic) to see population numbers.

Things that push animals down:

o Currents

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o Flash foods

o Continuous rainfall

Things that push animals back up (what maintains presence of various populations?):

o Active behavioural changes by organisms

Stick to bottom of substrate

Fusiform body shapes so can withstand water flow

Adult stage floats down, larval stage goes back up

Colonization cycle is mediated by behaviour

Central America: Have put screens up in streams, measure what gets stuck to them.

Screen that faces uphill has most of invertebrate life- testament to active movement oforganisms.