Stochasticity and stage structure Flashcards
Define exponential growth.
Growth that becomes ever-more rapid in response to increasing population size.
What size populations does environmental stochasticity affect?
All sizes.
What affect does increasing environmental variation have on exponential growth?
Lowers exponential growth rate.
Why does environmental variation decrease growth rate?
As individuals adapt to changing conditions the least fit ones will die.
Which mean, the arithmetic or geometric, does environmental stochasticity consider?
The geometric mean.
What kind of factors does environmental stochasticity affect. Give 3 examples.
- Fecundity
- Mortality
- Competitive ability
What size populations does demographic stochasticity affect and why?
Small populations. When there are fewer individuals in a population chance events have a larger effect.
Give an example of a chance event that creates demographic stochasticity.
Death before mating.
The simplest model of exponential growth makes 3 assumptions about organisms. What are they?
They are the same
- age
- sex
- size
The simplest model of exponential growth takes into account disparity between organisms. True or false?
False - it assumes they are all the same.
Give a case study of where demographic stochasticity had an affect on a population.
Kakapos in New Zealand - in a small population females were preferentially predated.
In the Kakapos case study, sex determination in the offspring correlated to food abundance. Which food ratio produced which gender?
Low food = females
High food = males
Define a cohort.
A group with a shared character, e.g. age.
Define survivorship.
The proportion of a population surviving to a given age.
How do you calculate survivorship?
Proportion of survivors/initial population size
There are 3 types of survivorship curve. Explain Type I.
There is a low mortality rate early on That increases with time. These animals have few offpspring that are heavily invested in by the parents so are less likely to die young. Because there are fewer of them if they die when they reach maturity it has a greater effect on the population.
Give an example of a group of animals that fit Type I survivorship curves.
Mammals.
There are 3 types of survivorship curve. Explain Type II.
The mortality rate is relatively constant throughout an organism’s lifetime.
Give examples of animal groups that fit Type II survivorship curves.
Birds or reptiles.
There are 3 types of survivorship curve. Explain Type III.
Mortality rate decreases with increasing age. This is because these organisms have huge volumes of offspring so the mortality rate is very high early on.
Give examples of animal groups that fit Type III survivorship curves.
Fish.
Define the mortality rate.
Proportion of individuals that die in a year.
How do you calculate mortality rate?
Number of deaths in a year/number of original individuals
How do you calculate survival rate as a factor?
1 - mortality rate.
Define the fecundity rate.
The number of offspring a female is expected to have in her lifetime (if she survives).
How do you calculate the fecundity rate?
Survival rate x birth rate
What is R(0)?
The net reproduction rate
Define the net reproduction rate.
The average number of offspring females in a population are expected to have in their lifetime.
How do you calculate the net reproduction rate?
The sum of all the fecundity values (in a study of multiple females in a population).
If R(0) = 1, what does this mean?
There is no population growth, the female only replaces herself.
When R(0) > 1, what does this mean?
There is population growth.
When R(0) < 1, what does this mean?
There is population decline.
Which other term is R(0) similar to?
λ
How do you calculate survival rate as a number?
Total population size x the survivorship value.
When R(0) or λ are constant, a single growth model can be applied to the population. Why?
We are only interested in the total number of individuals within the population, not the details of the population.
What does the model N(t+1) = λN(t) signify?
The proportion of individuals at each life stage stays constant over time.
When a population becomes too large why is N(t+1) = λN(t) no longer an appropriate model?
Other factors come into play, like competition.
Why are stage-structured models important in conservation?
They can indicate which cohort needs to be culled to manage a large population.
What is a life stage?
A cohort of the same age.
Generally the larger the population, the lower the fecundity rate. True or false?
False - fecundity rate increases with population size.
Which kind of variation has a greater impact on exponential growth?
Environmental stochasticity.
