bsci160 - population ecology 1

population

a group of individuals of the same species living in the same general area

• at 1 point in time

• can change over time

ecology

the study of the interactions of organisms with their environment and with each other

population ecology

the study of populations in relation to their environment

• including influences on population size, growth, age structure & more

  • range of tolerance

• measure

  • abundance (#)

  • density

  • dispersion

  • population size

abundance (N)

number

• important for assessing potential for increase in absolute #

• crucial in conservation

density

N / area or volume

• ecological factors influenced by density of pop

  • not absolute #

• competitive interactions

dispersion

patterns of dispersion

• clumped → most common

  • patchy aggregates

• uniform → even

• random → unpredictable

population size

# of individuals in population at one point in time

• take a sample

ecological sample

a portion, piece, or segment that is representative of a while

• b/c impossible to measure whole

• be careful of bias

• sample based on needs, prior info, & specifics of site

sample assumptions

• sample is representative of whole

  • needed to take enough samples so accurate representation is obtained

sampling bias

tend to favor or look for a specific outcome

• opposite is true too

sampling types

• random

• stratified

• systematic

• cluster

random sampling

every element has equal probability to be selected

stratified sampling

have previous knowledge of environment & need equal samples in each condition

• 3 diff environments → 5 samples in each

systematic sampling

sampling by creating a grid and collected in even increments

• spatial autocorrelation - samples closer together will be more similar

  • don’t want

cluster sampling

randomly choosing 4 areas of a site & randomly sample within those areas

• cost prohibitive effect - limiting factor (cost, resource, etc)

example: cholla cacti population in Joshua tree National Park

Lincoln Index

capture-mark-recapture

• animals captured, counted, tagged & released

• capture again after time pass

  • previously tagged animals counted & unmarked organisms are marked

  • abundance calculated using formula

• small pop. → recapture most marked ind.

• large pop. → recapture few marked ind.

  • multiply 2 big # & divide by small #

    • (A x B) / c

Imagine that you tag and release 15 birds, and when you collect 15 birds later that year, only one of them is tagged. What is your estimate of the total population size?

population size other methods

• sometimes able to recognize ind.

• counting nests

• satellite images

population dynamics

△ in pop size → ↑ interest > total pop size

• declining pop → extinction risk

population growth

measured by census populations over diff years

population size equation

(births - deaths) + (immigrants - emigrants)

immigration

influx of new individuals from other areas

• “in”

emmigration

movement of individuals out of a population

• “exit”

birth death model

• B_t - total # births in pop @ 1 time point (time=t)

• D_t - total # deaths in pop @ time t

• N_t - pop size @ time t

• N_t+1 = Population size at time t+1

per capita model

per capita - “per individual” birth & death rates

• (b) → avg. # of offspring produced by single individual

  • b = # births / ind. in past year

• (d) → probability of dying

  • d = # deaths / ind. in past year

per capita growth rate (r)

r = b - d

• intrinsic growth rate & rate of increase

  • r > 0 → pop growing

  • r < 0 → pop shrinking

  • r = 0 → pop stable

True/False: A population cannot persist in an environment where per capita growth rate is negative

TRUE

• population will decline to 0 unless somethings changes r

growth rate comparison (r)

r = △N / N_t =

• △N → pop size from t to t+1

• N_t → pop size at t

r example