Population growth and life histories Flashcards
change in population size=
(births + immigration)-(deaths +emigration)
What factors affect these processes
intrinsic and extrinsic factors
Intrinsic
built into genetic basis of each species eg age of first reproduction
Extrinsic
Outside the individual
eg predation
Extrinsic factors that effect pop size can be divided as
Density dependent and density independent factors
Density dependent factors
- pop growth rate decreases as pop size and crowding incr
- degree to which a factor affects growth depends on density
- eg effects of food supply
Density independent factors
Influence populations independently from population size
Influence of a factor is the same, effect does not depend on density
eg snow fall at wrong time of year
How do populations grow
exponentially
change in pop size over time=
∆N/∆t = B-D
But more useful to calculate b per capita birth rate and d per capita death rate, so B=bN and D=dN
hence = (b-d)N
If r= intrinsic growth rate= b-d , then ∆N/∆t=rN
bigger r
pop growing faster exponentially
r=tangent to the curve in graph of pop size (N) plotted against time (t)
r<0
declining
r=0
stationary
r>0
increasing
what prevents populations growing indefinitely
limited resource
Growth rate decr as number of individuals approaches
carry capacity (K)
exponential growth model vs logistic growth model
expon doesn’t consider limited resources, whereas logistic growth model takes into account K and the fact that there are limited resources
carrying capacity
The carrying capacity of a biological species in an environment is the maximum population size of the species that the environment can sustain indefinitely, given the food, habitat, water, and other necessities available in the environment.
Logistic growth model
dN/dt= rN(K-N)/K
When N<
K/K = 1, so growth is defined by r
When N approx = K
0/K = 0
this halts growth
Problems with logistic model
Approximates real pop growth well, but other factors besides K are important eg lagtime in response to change in conditions
What is a common trade off
Life length vs reproduction
Life history traits incl
how long to develop how fast a pop grows Age and size at reproduction number of reproductive seasons Life span Number and size of offspring
semelparous
many reproductive season
iterparous
few reproductive seasons
Large r
some animals live short time, but reproduce profusely
small r
live for long time, produce few offspring
life history
pattern of allocation to growth, development, reproduction, survival etc
the ‘perfect’ life history
mature at birth
Continuously produce high quality offspring in large numbers
Live forever
Why doesn’t ‘perfect’ exist
Amount of resources available is finite all resources can be invested into reproduction because wouldn’t survive that long with predators etc
Finite resources generate trade offs
commonly studied trade offs
life span vs reproductive output
Size vs number of offspring
Life span is a result of
genetics and environment
life span and sensescence
senescence is how long an organism could live, and life span would be determined by senescence if organisms lived in a protected environment
Why don’t organisms reproduce the moment they are born?
insufficient resource so produce poor quality offspring, low chance of survival
But if you wait too long, death comes before reproduction
Combination leads to intermediate age as optimum time for reproduction
Can life history vary with enivironment
yes
Trade off between size and number of offspring
There are limited resources to make many large offspring
For small offspring many can be made be little point in the mother investing in something so small, and if chance of survival is small
As clutch size incr, probability for survival of each offspring decr
All offspring equally attended to so low chance of survival
r-selected pop
reproduce rapidly and use resources wastefully
Success relies on finding new resources elsewhere
K selected pop
K selected pops breed slower and use resources more efficiently
r selected characteristics
small organisms short life large progenies high dispersal unlimited environment low comp high predation
K selected characteristics
larger individuals slow to mature few offspring high parental care intense comp for limited resources Crowded More resources invested in aggression etc
PArity
number of episodes of reproduction
fecundity
fertility
