Chapter 5: Stochasticity in Growth Rates Flashcards
discrete (geometric) growth rate λ
the ratio of abundance in year t+1 (Nt+1) to abundance in year t (Nt)
λ =
Nt+1 / Nt
NT =
N0 * λ^T
continuous (exponential) growth rate
the instantaneous per capita growth rate
discrete (geometric) growth rate equations
λ = Nt+1 / Nt
NT = N0 * λ^T
r =
ln(λ)
λ =
e^r
continuous (exponential) growth rate equations
r = ln(λ)
λ = e^r
when use an exponential growth model for wild populations?
- in newly established populations
- populations recovering from catastrophic declines
- invasive, pest outbreaks
population growth is often variable
stochastic
2 population growth variances
- sample variance
- process variance
which variance is visualized with error bars?
sample variance
what causes stochasticity in growth rate over time?
- sample variance
- process variance
stochasticity
variation or bounce
sample variance
- aka observational error
- nature isn’t varying, but our estimation error makes it seem like it is
process variance definition
- the one actually affecting changes in abundance and the one we care about
process variance: internal drivers
- age structure
- sex ratios
- density dependence
- connectivity
process variance: changes due to interacting species
- predation
- competition
- parasitism
- human harvest
process variance: stochastic factors
- environmental stochasticity
- demographic stochasticity
process variance factors
- internal drivers
- changes due to interacting species
- stochastic (random) factors
environmental stochasticity
due to extrinsic factors that cause mean vital rates to change randomly over time
example of an extrinsic factor
weather
forms of environmental stochasticity
- drought
- early springs
- warmer winters
- summers
- hurricanes
- forest fires
are the effects of environmental stochasticity more or less dependent on population size?
less
what causes demographic stochasticity?
due to random deviations from mean birth and death rates
demographic stochasticity population is made up of…
an integer number of individuals
what size population is important in demographic stochasticity?
small populations
does demographic stochasticity cause deviations?
yes, deviations from the 50:50 sex ratio
how does demographic stochasticity arise?
arises strictly from population size and not from environmental variability
demographic stochasticity causes…
unexpected declines in small populations
how do you estimate exponential trends for wild populations?
regress log abundance sizes against time, the slope of the regression is r
state space models to estimate growth rate can estimate:
- population trend in the presence of both process and sample variance
- how much of the total variance comes from each
state space models to estimate growth rate is …
math and data heavy
