Population growth & regulation
One foundational idea in ecology is that, when given plentiful
resources and an absence of controls, populations grow
rapidly. They go through what is called exponential
population growth. This is true of basically all populationshumans, and bees, and skunk cabbage, etc…
Population growth is mathematically modeled- check out the book if
you want the calculus. The main idea is that r is the rate of population
increase. When there are more births than deaths in a population, r is
positive. When births and deaths are balance- it is 0, when more
deaths than births, it goes negative. R is sometimes called the intrinsic
rate of population increase, and is associated with exponentially
increasing populations… thus “r-selected” is really referring to a
species adapted to rapid population growth.
reindeer or caribou
Latin name = (__________ ________)
reindeer or caribou
Latin name = (Rangifer tarandus).
This is an example of “exponential growth” the population has encountered a
situation where resources are plentiful and there are no predators to regulate
growth. Note that this is the situation with white tailed deer in Ohio! If you
introduce a predator this curve does not do this. Take home point, births strongly
outweigh deaths, population is skyrocketing.
whooping cranewas driven nearly to extinction. Down
to 21 birds total! Their has been a
massive conservation effort- and the
population is responding like you
would expect for a species being
protected from population regulation.
http://www.youtube.com/watch?v=XiikddBUOQs&NR=1
http://www.youtube.com/watch?v=fQ7-nSpZMDE&NR=1
Population Ecology is about to impact
your life- whether you like it or not.
Based on a 2005 Lexington, Kentucky,
street tree survey- it is estimated that there
are more than 10,000 ash trees in the
Urban Service Area (LFUCG 2007).
The ash component of Louisville
Kentucky's tree population is 17%.
http://www.emeraldashborer.info/
buprestid
Exponential population growth is a concept that explains a condition
that normally does not occur in nature.
Resources are limited!
Concept of “carrying capacity.”
Concept of “carrying capacity.”
• As a population grows,
competition for (limited) resources
increases.
• As the population demands on
resources approach the ability of
the habitat to provide resources
(K- carrying capacity), the growth
rate levels off
• This is the logistic (rather than
exponential) model of population
growth.
• Populations can exceed K at
times, but usually crash afterward
(reindeer example).
• But even the logistic model, is
based on single species
interactions in a constant
environment….
Exponential and logistic growth
models have been incredibly
useful in ecology… but do not
apply in most natural systems
In most populations in (relatively)
undisturbed systems, the population
growth rate is dynamic, through time
and space.
Stochastic processes intervene in
population growth. (think- rockslide)
Populations are influenced by limited
resources. (density dependent
regulation- logistic growth)
Populations are influenced by other
species (interspecific competition)
Populations are influenced by presence
and density off: predators (& prey),
mutualists, parasites, pathogens, etc.
Table 9.1 in book
The mortality rate (qx from life table)
is simply the number of individuals
who have died divided by the total
number of individuals at the start of
the time period measures changes
through time.
(e.g., in year 1, 79/159 = 0.5.
In other words, half of the individuals
died during that time period. If you
plot qx through time you get a good
idea of the mortality pressures over
the life of the study organism.
A life table is an accounting
method used to track
population numbers though
assessment of survivorship
year to year. You start out
with a particular number of
individuals (eg, 530, in the
table). Then you track how
many die each year to
calculate a mortality rate.
The mortality rate (qx from life table)
is very often life-stage dependant.
Individuals are more vulnerable at
some stages than others. This is
useful, especially for managers. For
instance, if your goal as a natural
areas manager is to restore a
particular plant that has been lost.
You have to decide whether to plants
seeds, or seedlings, and if seedlingshow old should they be? In this
example, you might choose to plant
rosettes.
Mortality
Survivorship
The “transpose” of the mortality curve is a survivorship curve.
This gives an idea of how many individuals make it each yearthe mortality pressures on that organism. The shape of the
survivorship curve matters. Different shapes are associated
with different life history patterns. Organisms that have
relatively uniform survivorship across their life would have a
curve like this one for squirrel. There is year to year variation,
but overall relatively smooth,
Mortality
Survivorship
In contrast, an organism like this sedum
does terribly through the germination and
establishment phase! From July to
December almost everyone dies. From
then on, though, the curve is pretty flat.
The organism has a maximum life span, so
there is mortality, but once you get past the
establishment phase, survivorship is strong
Red deer survivorship curves.
This organism has very strong
survivorship the first few years
of live- then- especially for
males, the survivorship falls off
rapidly.
This is evidence of maternal
investment in offspring.
Once the deer heads out on its
own, mortality rates go up.
Survivorship curves have
been categorized into
three broad shapes.
• Type I curve, early survivorship, mortality increases when the organisms
reaches old-age. (humans, large mammals)
• Type II curve, constant mortality/survivorship. (rodents, birds, some plants)
•Type III curve, early mortality, if you make it to maturity then survivorship is
high. (trees, fish, invertebrates)
Although we generally think population growth is
related to resources, competition, predation, etc…
there are some factors- unrelated directly to resource
issues that have a significant influence on long-term
population growth. We have assumed thus far a
deterministic process for population growth. The
numbers react to the resources and to life history
limitations. But, there are other forces in play…
Demographic stochasticity- is caused by variation
in birth and death rates. In reality, mating,
reproduction, and death, is not completely orderly
and predictable. Fluctuations in these parameters
factors can intervene, driving the population numbers
in ways that cannot easily be predicted.
Environmental stochasticity- is caused by random
variation in birth and death rates due to changes in
environmental conditions. Drought, etc.
Demographic unpredictability is especially
important in small populations. These small
populations are vulnerable to extinction and loss,
partly because there are just smaller numbers- but
also because of some specific ways these
populations behave. You cannot assume that you
can go from a small population to a big one, just
based on resource availability.
Allee effects- are population limiting factors
related to birth and death rates specifically
associated with small populations. For Panex as
the population numbers go down, so do the
number of fruits produced per plant.
WHY?
They should have MORE resources if there are
fewer plants. Death rates can also be influencedif organisms use gregarious behavior to avoid
predators, for instance. Numbers could be below
a particular threshold. Also- smaller numbers =
less genetic diversity and the possibility of
imbreeding depression.
Extinction Vortex