Demography

Population Ecology

Study of what factors affect the number of individuals

of a species in a location & over time

Population

a group of interacting individuals of a single species in a single location

influenced by biotic & abiotic factors

population descriptions

Dispersion

Size & Density

Demographics

Population Dispersion

describes the pattern of individuals in space

Random, uniform, clumped

Random dispersion

position is not impacted by other individuals

(unpredictable)

Uniform dispersion

even spacing reinforced by interactions with others

-Chemical inhibition

-Territoriality

Clumped dispersion

individuals cluster in space

-Resource distribution

-Social groups

-Reproductive behavior

population size

total number of individuals in a population across all areas

population size equation

density x area

population density

number of individuals in a defined area

varies across the distribution if organisms are not uniformly dispersed

Measuring Population techniques

Full Census

Estimations:

-Census (sample plots)

-Mark Recapture

-Population Genetics

Full Census

a count of every individual in a population

not usually feasible

Sample Plots

Aka ecological census or quadrats

study area is divided into subunits to count individuals

-Count every individual in randomly selected areas (average population density)

estimates the density extrapolate to get the population size

Most effective for evenly spaced or sedentary organism

Mark-Recapture

Individuals in an area are captured, marked, and recapture

Population size is estimated by the proportion of marked animals in recapture sampling event

estimates the population size extrapolate to get the density

Mark-Recapture assumptions

marked & unmarked individuals have equal probability of sampling

no birth, death, immigration, emigration during resampling

Population mixes thoroughly between sample

Mark recapture equation

marked(M)/population(N) = recaps(r)/new captures(n)

N=Mn/r

Mark recapture errors

Over & under-estimation occurs if assumptions are violated

If animals avoid second capture: population size inflated

If animals prefer recapture: population size underestimated

Measuring Populations: DNA

Individuals can be identified from genetic sequence

DNA samples can be used to build genetic profiles of each

individual in a population — like forensics for wildlife

can be useful if animals are hard to capture or identify as individual or to determine which individuals are reproducing

Genetic profiling

DNA fingerprints are used to identify individual

Demography

The study of populations & how they change over time

Demography descriptions

Age & Sex structure, Survivorship, Life History(reproduction)

Age structure

distribution of individuals at different ages in a population

sex structure

the ratio of males to females in a population

Age & Sex Structure

"Snapshot" of a population in time

Relative proportions of ages & sex ratios in population

impact population stability and growth

Age structure can be used to predict how likely a population is to grow or shrink

Age sex structure diagram types

Rapidly Expanding - Many young, very few old

Steadily Expanding - Many young, few old

Slowly Expanding or Stationary - Many young, many middle aged

Contracting - Few young, many middle aged

Sex ratio effect

Growth may be constrained w/ skewed sex ratio

Sex-ratio skews impact reproduction & population growth

Female biased ratios typically increase birth rates, male-biased decrease...

strong skews in either sex decrease mating opportunities & increase inbreeding

Survivorship

A cohort (age group) is tracked through time from birth to death

Life tables

summarize mortality, survival, & reproduction in a population by age-group

survivorship curve

summarize life table data: deaths over time

static or time-specific life table

records the number of individuals of each age in a

population at a single time point

survivorship downsides

It can be difficult (or impossible) to follow all of a cohort

throughout their lives

Type I survivorship curve

low death rates during early and middle life

steep increase in deaths in older age groups

Type II survivorship curve

constant death rate over entire lifespan

Type III survivorship curve

very high death rates for young

low death rates for those that survive early life

Type I Survival

High survival when young

Small number of offspring

High parental investment

Type II Survival

Equal survival across ages

Relatively few offspring

Low parental investment

Type III Survival

Low survival when young

Many offspring

Low parental care

Long-lived

Life History

Natural selection shapes traits related to reproduction & survival because resources are limited

-these traits describe a species' life history

Natural selection acts on current reproductive allocation &

future reproductive success

reflects balance between survival & reproduction over

lifetime of parent with different combinations of reproductive trait

Reproductive onset

when reproduction begins

Reproductive Frequency

how often to reproduce

Reproductive output

how many offspring each time

fecundity

an organism's reproductive capacity

determined by tradeoffs

Reproductive onset tradeoffs

Reproduction is risky

Early reproduction:

-lowers risk of dying w/out producing offspring...

-at expense of growth & health of parent + offspring

Later reproduction:

-produces more or larger offspring that are more likely to survive...

-risk of not surviving to reproduce at all!

Reproductive Frequency tradeoffs

Reproduction is energetically costly! Frequency reflects resource availability & cost to parents

semelparous species

reproduce only once in a lifetime

Many short-lived:

-might not have a 2nd chance in ephemeral or disturbed

habitats

Some long-lived:

-wait for the right conditions then devote all resources to reproductive success

iteroparous species

reproduce multiple times

balance investment in each reproductive event

Reproductive Output tradeoffs

Offspring take time & energy

Tradeoff in number offspring produced & offspring investment

Low chance of offspring survival?

-Make more offspring & wish them luck

Good chance of survival?

-Make less offspring & help them survive

r/K selection theory

Parental investment balances conflicting demands for time, energy, nutrients in habitats that vary in stability

K-selected organisms

invest more in few offspring, each has a high chance of survival

Maintain high reproductive rate at high population density

Competitive species with stable populations

Live near carrying capacity in predictable environments

Tend to be slow developing, long-lived, & large-bodied

r-selected organisms

produce many offspring each with a low chance of survival

Maximum reproductive success in low density

Populations are below carrying capacity—little competition

Common in unpredictable, disturbed, unstable environments

Tend to be short-lived, rapidly developing & small-bodied