Demography

Population

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

population ecology

What factors affect the number of individuals of a species in a location & over time

Population descriptors

• Dispersion

• Size & Density

• Demographics

Population Dispersion

describes the pattern of individuals in space

Random dispersion

position is not impacted by other individuals (unpredictable and rare)

Uniform dispersion

even spacing reinforced by avoidant 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 Density

number of individuals in a defined area; varies across the distribution if organisms are not uniformly dispersed

Ecological Census (sample plots)

study area is divided into subunits (quadrats) to count individuals

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

• Extrapolate to get total population size

Most effective for evenly spaced or sedentary organism

aerial surveys

divide land into sections, count animals in each area

total count

all animals in an area counted; sometimes impractical

sample counts

only sections of each section are counted; remaining estimated with algorithm

transect lines

perpendicular to major physical barriers i.e. waterways and mountains

Mark-Recapture

Individuals in an area are captured, marked, and recapture

Mark-Recapture Assumptions

• marked & unmarked individuals have equal probability of sampling

• no birth, death, immigration, emigration during resampling

• Population mixes thoroughly between sample

eMark-Recapture equation

M/N = r/n

marked/population = recaps/new captures

N = Mn/r

over-estimation of mark-recap error

animals avoid second capture

under-estimation of mark-recap error

animals prefer recapture (‘trap-happy’)

Population Genetics

Individuals can be identified from genetic sequence; DNA samples can be used to build genetic profiles of each individual in a population; forensics for wildlife

useful if animals are hard to capture or identify as individuals; used to determine which individuals are reproducing

Demography

study of populations & how they change over time

demography descriptors

• Age & Sex structure

• Survivorship

• Life History (reproduction)

Age structure

distribution of individuals at different ages in a population; may constrain growth if ratio is skewed

Sex structure

ratio of males to females in a population; may constrain growth if ratio is skewed (Female biased ratios typically increase birth rates, male-biased decrease with limits)

population pyramid

compares age and sex structures

cohort

age group

life tables

summarize mortality, survival, & reproduction in a population by age-group; also tracks disease outbreaks & interventions

static / time-specific

records information about a population at a single time point

Survivorship curves

summarize life table data: deaths over time

Type I Survival

low death rates during early & middle life; steep increase in deaths in older age groups

Type II Survival

constant death rate over entire lifespan

Type III Survival

very high death rates for young; low death rates for those that survive early life

Reproductive onset

when reproduction begins

Early reproduction lowers risk of dying without producing offspring… but at expense of growth & health

Later reproduction produces more or larger offspring that are more likely to survive… but there’s a risk of not surviving to reproduce at all

Reproductive frequency

how often to reproduce; semelparous vs. iteroparous

Reproductive output

how many offspring each time; reflects parental investment

Good chance of survival? Make less offspring & help them survive

Low chance of offspring survival? Make more offspring & wish them luck!

fecundity

organism’s reproductive capacity; determined by tradeoffs and natural selection

semelparity

“fuck and die”; one reproductive event in life history followed by death

iteroparity

multiple reproductive events (possible) before death

K-selected species

tend to live longer, be larger, have feeer offspring at a time; spend more time raising children, longer childhood

population flatlines at carrying capacity

r-selected species

tend to live shorter, be small, have more offspring at a time; spend less time raising children, short childhood (if at all)

population grows exponentially dependent on biotic potential

biotic potential (r)

how quickly a species can reproduce and what percentage of that offspring makes it to maturity in a given environment

carrying capacity

maximum population size of a biological species that can be sustained by that specific environment