Population Dynamics 1 Flashcards
Are populations dynamic and what does this mean?
Yes- populations numbers don’t remain static
What does population variability depend on and describe an example of this?
time-scale
e.g. Australia- plague of mice affecting grain crops- population of mice exploded when grain stores were discovered = impacting food supply
What is this graph showing?
human population size plotted on log scale
- Population has undergone massive increase but has not always been consistent growth- so not exponential
What is this graph showing?
per capita human birth rate
- If wiggly line is above dashed line (at 0) =every 1 person is giving rise to more than just themselves
- Dip in population caused by black death
- Human population growth rate is slowing
What can population fluctuations represent?
dynamics with multiple organisms = interactions
What is this graph showing?
White line = density of rabbits from 1940s-1970s
Green line = density of stoats from 1940s-1970s
Both show similar patterns = high population density at first and then this drops off
= Predator-prey interaction
What are the 2 conclusions that can be drawn from this graph?
= 2 conclusions:
- Dynamics of prey are being driven by predator = top-down effect
- Dynamics of predator being driven by prey = bottom-up effect
Which conclusion was suggested and how was this discovered?
If add in myxomatosis (= disease that affects rabbits) helps us to understand what is going on
= rabbit population drops after introduction of disease and predator population falls after this = most likely rabbit that is driving population density of stout = bottom up effect
What is this graph showing?
repeating patterns that can be regular or irregular that cant be predicted
e.g. Hornbeam seeds = important food source for voles
Population dynamics of seeds and voles in Poland:
White = mast years = trees produce seeds = big abundance of seeds only in certain years
- Following this = bursts on no. of voles = more food resources so more reproduction
Name the 4 key demographic processes
- Birth rate
- Death rate
- Emigration
- Immigration
What does Nt+1 represent?
number of organisms at certain time point in future
What does Nt represent?
number of organisms currently
What is the fundamental equation used to understand what is driving population dynamics?
usually written as rates (lower case instead of upper case letters)
In closed populations what 2 demographic processes are not included?
immigration
Emigration
What is Lambda?
Finite rate of increase = specific measure of the potential for exponential growth
What is the equation for lambda?
When do populations increase in terms of lambda?
Populations increase when λ > 1
When do populations increase?
- Typically when birth rate > death rate in closed populations
- Or when I is large = sink populations e.g. Germany- population growth is balanced despite low birth + death rate, but immigration is high
What must be understood in order to understand population change?
life cycles
Define fecundity
= production of eggs/offspring- number of eggs per female = fecundity rate
What are the 2 types of life history and what are the differences?
Age specific- life stages determined by time
- sexual vertebrates do this
- Survival + fecundity rates are age specific
Stage specific- life stages determined by stage of growth
- lots of plants do this
- Probability of transitioning to next stage + probability of surviving/dying at each stage based on development
- Fecundity rates = varies with stage
Other than life cycles, what is variation due to?
Body plant
Modes of growth
what is the difference between modular and unitary modes of growth?
- Modular = add more modules/units to get bigger e.g. stems, branches
- Unitary = fixed body form where new cells are added
What are the 2 types of unitary growth?
indeterminate = do not have a size in which growth stops
determinate = mature body size in which growth stops
Name some different modes of reproduction
- Separate sexes
- Dioecious plants
- Hermaphrodites
- Sequential hermaphrodites
- Asexual
- Alternation of generations
what is a Dioecious plants + example?
plants that have males and females e.g. Holly with berries = female
What are hermaphrodites?
can be male and female at the same time- still do meet and reproduce sexually
What are Sequential hermaphrodites?
male and female but not at same time e.g. certain fish begin as males and develop into females later on
What are asexuals?
Apomixis / parthenogenesis = ability to reproduce without a partner e.g. whiptail lizard
What is meant by Alternation of generations in terms of reproduction?
have different reproductive modes at different parts of life cycle e.g. aphids- depends on time of year can be clonal or sexually reproduce
What does animal ecology focus on?
determinate growth and separate sexes with sexual reproduction
in animal ecology what 2 key areas of variation are examined?
- Overlapping = can have babies at same time as other generations e.g. humans- not case for many animals versus non-overlapping generations e.g. insects reproduce at certain time of year and then they die and leave eggs
- Age structure = cant assume fecundity rate is flat throughout life cycle
What is this non-overlapping generation showing
e.g. Isolated population of grasshoppers
2.5 females 2.8 males per 10² of grassland
- each female adult lays on average 7.3 egg pods- 11 eggs per pod = 7.3 x 2.5 = 18.3 egg pods
- 18.3 x 11 = 201 eggs per 10m²
- Some eggs die- 8% survive to 1st Instar = 16 eggs survive
- More survive until next stage 2nd Instar = 11.4
- ¾ survive through each instar and become more like adult
- When adulthood is reached = 5 adults per 10m² = end with the same = equilibrium = stable
What is an intermediate life cycle?
some overlapping generations but not complete overlapping
What is this intermediate population showing?
1 adult per 4m² of sand dune- produce 5040 seeds which lots of them get eaten but some reach surface of sand and form a seed bank
- Unknown proportion too deep in sand to germinate = dormancy for potential germination
- Potential invading seeds blown in so not a closed system = immigration which keeps population from extinction
- 7% of seeds present on adult make it to seed bank
- 11% make it through winter to germinate and form next group of plants
- End up with 0.17 adults for every 1 adult that you started with
- = declining population- eventually will go extinct
What is this overlapping population showing?
e.g. Great Tits- Oxford
1. 5 males + 5 females per hectare
2. Females have 10 eggs each = 50 eggs per hectare per year
3. Eggs cared for = 84% hatch into nestlings = 42 hatch
4. 71% of these survive (19.8) to fledge
5. = 3 x fledglings to adults
6. Fledglings can’t look after themselves and only 10% make it to reproductive adults = but still end up with 4 males + 4 females
7. Extra adults- many experienced adults make it through the winter and contribute more eggs to next generation
8. 50% fecundity for existing adults
10% fecundity for new adults but have more eggs
What is a key point in overlapping generations?
age structure in population affecting both fecundity and survival + all individuals are not equal
what are the 2 types of life tables?
cohort
static
What can population data be put into?
life table
What do life tables determine?
if population is increasing or decreasing or in equilibrium
Name + define the different columns in a life table
x = life stage
nx = number of individuals at each stage in any given area
lx = age specific survival rate
qx = age specific mortality rate
Log nx = natural log of population size at each stage
kx = k factor- killing power
bx = fecundity = fecundity
Name the life stages (x) in this table
egg
5 larval stages (L1 to L5)
pupal stage
adult
how do you calculate the Ix?
the proportion of individuals alive at each stage- nx of target life stage / nx of 1st life stage (total number of offspring / eggs)
How do you calculate the age specific mortality rate (qx)?
difference between proportion surviving in one stage and proportion of survival in previous stage
e.g. from table:
4160 - 2326 = 1834
1834 / 4160 = 0.44 = qx of egg stage
how do you work out the k factor?
Took difference in natural log between one stage and the next
What does this life table suggest about the population and why?
Suggests population is in stasis as 92 adults in the end with 1/2 being female so 92 x (90/2) = 4140 = close to what we started with
What is the difference between the 2 types of life table?
- Cohort life tables are most informative but tricky/impossible to obtain – take a group of individuals and follow throughout life cycle and work out their fecundity and survival at each stage
- More common to use static life tables = snapshot of population (=several cohorts)- look at survival and mortality at each different life stage that is present at that time
- insects almost impossible to do as they do not have overlapping generations
- Easier to do with longer lived organisms e.g. elephants
Why are life tables studied?
Various benefits e.g. build up picture how species demographics varies fundamentally in different ways- so can group organisms based on their schedule of survival and fecundity across their life cycle
Instead of plotting a life table what can be plotted instead?
can plot simpler graphs- life table lx (survival) and bx (fecundity) curves across time
Describe what these lx and bx curves are showing for red deer
- Clear linear decline in survival as deer age
- Fecundity jumps as they become reproductively mature and then rapidly declines with age
Why do these survival and fecundity curves vary?
(particularly survival) vary lots between species depending on fundamental biology of organisms
Describe the shape of each type of survival curve and what organisms they represent
Type I = humans, zoo animals = mortality in childhood is rare, most mortality happens with increasing age
Type II = large mammals/birds, seed banks = linear decline through time
Type III = birds, fish, small mammals, insects = opposite to type 1 etc
However in reality there is a mixture of the shapes of these curves
Describe the shape of a common fecundity curve for an iteroparous organism
terms of fecundity, most organisms follow pattern seen in Red Deer =
1. short period in beginning where there’s no reproduction
2. a peak in reproduction
3. gradual decline
= classic for an iteroparous organism
What is the difference between an iteroparous organism and semelparous organism?
iteroparous organism = organism that can keep reproducing through their life
Opposite = semelparous = have lots of babies and then die afterwards e.g. mayfly
Other than lambda, what is another way to express population change?
Similar to lambda = R0 = life time reproductive success = reproductive output of average individual
How do you calculate R0?
To get that value take each life stage and multiply the proportion of original survivors by the rate at which individuals can have new offsprings- then add them all up
Describe what different values of R0 mean
If R0 is greater than 1 = population growing because each individual on average will be having more than 1 offsprings
If R0 less than 1 = population declining
R0 = 1 = population is static
Describe the method of a natural experiment conducted on colobus monkeys
- Population of colobus monkeys within range of chimps that hunt them
- 2nd population of colobus monkeys outside of range of predatory chimps
Use data across life cycles of non-hunted and hunted populations and plotted life table data onto graph to compare viability of populations
Describe what this graph is showing (colobus monkeys and chimps)
Survival declines overtime in both populations- but early decline in non-hunted monkeys (yellow) is less steep comparted to hunted monkeys.
= more juvenile mortality in hunted
For conservation reasons- what is the impact of hunting from the chimps of the viability of the populations?- so need to know fecundity = dotted green line provides an estimate for average adult fecundity
- Due to limitations could not determine fecundity at each age class
Compare the population viability analysis of each population of colobus monkeys
R0 calculated for each population
- unhunted population = 1.476 = above 1 so population should be growing
- hunted population = 0.654 babies per individual = population in decline
Explain for the results for the colobus monkey experiment
= predator-prey interaction that has a consequence that completely changes the likely long-term outcome of population dynamics from increasing to decreasing to potential extinction
But ignored other factors like immigration and emigration
