BIOL215 Lecture 14 Notes
The rate of increase in a population that we are obtaining is the "instantaneous rate of increase" - at
one specific point in time, how the population is increasing. The equation is:
In the case of an annual species that becomes the annual rate of increase (reproductive rate for an
annual species is equal to the rate of increase per year of that population; is the simplest case)
In the simplest case, the annual species occurs when G=1 (one generation time)
The instantaneous rate of increase is calculated by:
We can use the rate of increase to build another measure of population growth or to develop an
equation that will allow us to predict a population over time. We call this the geometric growth,
calculated by:
where r is intrinsic capacity for increase in a given environment
Under a geometric growth:
• Any population subject to fixed schedule of natality and mortality grows in geometric way
• Geometric growth dictate fixed and unchanging age distribution, the stable age distribution
If the rate of increase is less than zero, the population will decrease
If the rate of increase is zero, the population will stay stable (same)
If the rate of increase is one, the population will double
If the rate of increase is greater than zero, the population will increase
In geographic model, the population on the log scale increase always of the same amount per year or
per time interval because r is fixed (therefore, linear curve)
On an arithmetic scale, you have an exponential growth (a population that will increase and as you
have more and more individuals, the increments will increase faster and faster) We look at population growth rate because it serves a great purpose in conservative ecology known
as 'Minimum Viable Populations'
Minimum viable populations are what you want to achieve if you are a conservation ecology and your
purpose is to protect the species. You want to ensure the population size will persist for a specified
time (100 years)
• Demographic stochasticity (stochasticity means phenomenon that comes by chance)
• Example: species that would reproduce and have many more males instead of balanced sex
• Is a factor that can explain population reducing in size
• Genetic stochasticity
• Example: female biased population will cause genetically similar offspring due to lack of gene
pool, thus potential in-breeding
• Environmental stochasticity and natural catastrophes
Are destructions of the environment that come in an unforeseen way. Less fit individuals will
•
have a harder time surviving
• Vortex of extinction (good example comes from Heath hen extinction)
The minimum viable population size is the size of the population to avoid starting this vortex that will
lead the population to extinction
In general for bird species, you need to have at least a population of 10,000 birds to avoid extinction.
The MVP that we should try to maintain and protect are very high
Deborah Rabinowitz's helps explain '7 forms of rarity' (rarity: the characteristics of being rare) based
on 3 characteristics:
• Local population size,
• Habitat specificity (i.e. live only on coast)
• Geographic range (i.e. located only in a certain region)
Prairie chicken example shows how genetic stochasticity caused a declining population due to low
percentage of hacked eggs because of limited gene pool, thus

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