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
