- What's a
the dynamics of population size.
Tables: accounting for differences in age.
- r and K
selected life histories.
Population Growth and Regulation
- It's not conservation, preservation or recycling!
- It is:
- The scientific study of the relationships between organisms and their
- An attempt to understand the distribution and abundance of organisms.
- It varies, ecologists use the word "population" in several ways; typically
a "somewhat isolated collection of individuals."
- Properties of Populations:
- Size or Density.
Ecologists characterize the abundance of organisms by measuring their
density; individuals per square meter (or per meter cubed).
Density can be measured by counting, by transect or quadrat sampling, and
by mark-release recapture.
- Distribution (or range).
The geographical limits of the population.
The clumpedness of the population; a population may over-dispersed
(regularly spaced) or under-dispersed (more clumped than expected).
There are two "basic models" of population growth. Exponential growth, and
- In exponential growth, the population's growth rate increases with
- The models indicate growth to infinity if r>0, and decline to zero if
- r is called the "intrinsic rate of growth", and is a basic parameter of
many ecological models and processes. What are the units of r?
- In logistic growth the population growth is "checked" by resource
limitations. As the the population approaches its "carrying capacity" K its
growth rate slows down.
In logistic growth there are "density-dependent" changes in growth rate
that REGULATE population growth.
Ecologists see "density dependent" regulation as a general feature of
population that stabilizes the natural world.
- BUT, for some species density-independent processes are important too.
- The exponential and logistic models treat all individuals in a population
the same; but we know that baby's don't reproduce but adults do!
- Ecologists take account of this by constructing "life tables" for
populations that show
Life tables show that species vary enormously in their "life
- the survivorship of different ages &
- the fecundity of different ages
Some show lots of mortality early in life, but little when they are older.
Others show the reverse pattern.
Some reproduce throughout their lives; other reproduce in one enormous bout
- Many ecologists believe that these difference in life history reflect the
action of natural selection, &
they believe that organisms that frequently find themselves in the "r"
dominated part of the logistic growth curve have different life histories than
those that find themselves in the K dominated part of the growth curve.
- r-selected organisms live in ephermal (weedy) habitats, they are constant
exploitly new habitats, and their population are typically growing fast. This
emphasizes "reproduction now". So r-selected species are:
- Small as adults.
- Mature and reproduce early
- Prone to non-repeated reproduction.
- Produce large numbers of offspring.
Classical r-selected species are weeds, shrimp in ephermal ponds, etc.
- K-selected organisms live in stable persistent habitats, their populations
are typical very stable. This emphasizes delayed reproduction, and adult
competitiveness. So K-selected species are:
- Large as adults
- Mature and reproduce late.
- Reproduction repeatedly.
- Produce small numbers of larger offspring.
Classical K-selected species are trees, elephants, etc.
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