Download Limits on Populations.

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POPULATIONS
A population is a group of organisms of the same species living together in an area.
Population density is the number of organisms of a particular species living in
an area
with a specific size. A population is said to be dense when there are a large
numbe
organisms living in a relatively small area, and sparse when there are fewer organi r of
sms
living in the area (or in a larger area).
Limits on Populations.
Field mice may have litters with six or more pups, and they can reproduce
every six weeks, It takes only six weeks for a mouse to become sexually
mature. In 6 months, a population of 20 mice could become a population
of 5120 mice. Mice have been around for millions of years, so why is it
that when you look out the window you see grass and trees, birds and
squirrels, and not a vast carpet of millions and millions of mice?
The reason is that there are limits on all populations, including those
of the prolific mouse.
Population Growth Patterns
Changes in population size in a community occur when individuals
are
added to or removed from a population (Table I).
If natality (the birth rate) increases while other factors remain
constant, the population will increase. The same holds true if
immigration increases. The reverse effect occurs if there is an increase
in
mortality (the death rate) or emigration. In populations in natural
ecosystems, all four factors interact with natality and mortality
generally
having the greatest, effect. The four factors involved in population growth
can be expressed mathematically by the formula
Population growth = (births + immigration) (deaths + emigration)
Biotic Potential
One of those limits on population is imposed by the species on itself.
Species vary in their capacity to reproduce. iotic potential is the
maximum number of offspring that a species could produce, if resources
were unlimited. You have seen how quickly field mice reproduce, but many
animals have a much lower biotic potential. For example, mature female
black bears give birth to one or two cubs after a gestation period of 7.5
months. Generally, bears take at least two years to mature, during which
time their mother will not give birth again. Biotic potential isregulared by
four important factors, shown in Figure 1.
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In mature ecosystems, where resources tend to be constant or
available in predictable patterns, populations remain relatively
stable over
the long term (population growth 0). Increases in nasality are
balanced
by increases in mortality or increased emigration. This balance is referred
to as dynamic equilibrium, or a steady state.
Open and Closed Populations
Ecologists dassi’ populations as either open or closed, In most natural
ecosystems all four factors (nataliry, mortality, immigration, and
emigration) are acting on the population of each organism. These
populations are said to be open populations. However, immigration
and
emigration do not happen in laboratory settings and in some
game reserves,
so populations of organisms in
these situations are considered
closed populations only
narality and mortality affect their
population size.
—
The
global
population
of
humans or any other type of
organism is also considered a
dosed population. People do not
emigrate to other planets, and as
far as we know none are
immigrating either, Changes in
the size of a global population
result only from natslity and mortality
inttt.
Factors That Affect
Population Size
Faster
nasality
Descriptiso
births)
mortality (deaths)
Immigration
emigration
the camber of
offspring of a
species born in sne
year
the number of
individuals of a
species that die in
one yesr.
the number sf
isdividualt ste
species moving
into an existing
population.
the samber sI
individuals of a
species moving out
of on existing
population.
iclty tot
The ricrher of
ng that roach
reprodcnve age
The female sea turtie sys
many eggs butonty stew of
her sff$ptisl even reacl the
sea, antifewerstil reach
rntan.rity,’
C
Factsts that determine biotic potential
tbdrol
ILr.
.
c,k mate anlycnce per year
cfr’the faL
Factac tcegth et
reprodactive Ill.
Deseriptiso: “e age ct
seaus ‘aLrt arc tm
rr’ber s’ yea’s the
ndivitia. ca :eptnce.
Afnica eeparrs reach
seaus maturty at abOt :1
years stage tt flay
rep•’ncuce art.. they are tD.
Table I.
Factors That Limit Populations
cast;. enpsylatIpn
Facts$ they cans. e.pnpulaIle
to hierakac
fctore that
16 decraae
favosrable ‘girt
favourable remperatarn
‘avourabe chencal envirsrrment
too much or too little girt
too cold or iso wamr
on!avnLrable chemical environment
sifficant food
sw number or ow effectiveness of
insufficient toed
high number or hgh effectiveness of
pndatnrs
predators
Ins or weak diseases and parasites
abrit to compete for resources
many or strnrrg diseases and parasites
inability to s’sccassfully rsorpete for resources
For example, a fern plant produces more than 50 000 spores in a single
year (Figure 2). If all fern spores germinated, fern plants would cover all of
North America within two generations of the first plant. This doesn’t happen
because of the limiting biotic and abiotic factors. If the weather is wetter than
usual, the soil is moist, and many fern spores will germinate, so the fern
population will increase. A return to drier weather will not only prevent
spores from germinating, but will also kill plants in exposed areas, so the
population declines. The presence of many grazing animals will reduce
the population of ferrss, and if there are few grazers the population will grow
(Figure 2). Fluctuations like these, caused by one factor, can occur in natural
ecosystems; however, most populations are affected by more than one factor
at a time.
Denstty4;depeedeet
tacesro
• flood
• fire
• spraying with pesticides
• change in climate or
temperature
• deffructisn of habitat
• droaght
Densitpdepandeet factors
• food shortage
• csmpstition for mates, breeding
ornas (habitat)
• diseasu caassd bya
oricrosrganism or parasite
• introductiss stan asssic species
• increased predation
• competitiso fur water and athe,
resnarces
Populations
by answering the following
Read the accompanying pages. Prepare your own notes
e part of the question in the
(includ
le
possib
er
S
questions in FULL SENTENCE wherev
answer to make complete, detailed notes):
1. a) Define population, population density.
two? Which do you think is
b) What seems to be the main difference between these
more useful to know, and why?
2. a) Define the following terms:
3) immigration
1) natality
4) emigration
2) mortality
.
b) Give the mathematical formula for population growth
tems seem to be like. What
3. Describe what populations In mature (established) ecosys
are the reasons for this?
tions.
4. a) Distinguish clearly between open and closed popula
each?
in
size
tion
popula
the
ine
determ
to
tend
factors
b) Which
5. a) What is the blotic potential of a population?
al of a population.
b) Describe the 4 factors that determine the biotic potenti
biotic potential of a population?
the
it
affect
does
how
and
factor”
“limiting
is
a
What
6. a)
s ablotic and biotic factors can
b) Recreate Table 1 in your notes to show how variou
limit a population.
7. a) Define carrying capacity of a population.
tion?
b) What determines the carrying capacity of a popula
what happens when a
explain
tion,
popula
mouse
field
the
of
le
examp
c) Using the
tem.
ecosys
the
ty
of
capaci
g
carryin
population exceeds the
factors and density-dependent
8. a) Distinguish ciearly between density-independent
le to illustrate each.
examp
e
an
Includ
tions.
popula
affect
can
that
factors
of these types of factors.
each
of
les
examp
list
to
notes
In
your
b) Recreate Table 2
ITh
Abintic and bistir factors limit tha nsmber of
fems in an ecosystem.
Carrying Capacity
Populations fluctuate regularly due to an interaction of the many biotic
and abiotic limiting factors. However, communities do tend toward
stability. Stability is achieved when an ecosystem is in equilibrium, when
none of the populations exceeds the carrying capacity of the ecosystem.
The carrying capacity is the maximum number of individuals of a
species that can be supported indefinitely by an ecosystem. The carrying
capacity for any species is determined by the availability of resources,
such as food and water.
A population can exceed the carrying capacity of the ecosystem, but
not for long. Consider the field mouse again. Imagine that the population
of predators is lower than usual. Suddenly, the mouse population can
grow. However, the extra mice will eat all the available food. Hungry
rodents aoon become sickly making them easy prey for the hawks, owls,
and foxes that are present. The mouse population will decline again, to or
below the carrying capacity. Ecosystems soon re-establish equilibrium.
—
c.
a;t
ering
The number of organisms in an ecosystem is important when consid
the effects of some abiotic and biotic factors. A population is said to be
dense when there is a large number of organisms in a small area.
Density-independent factors affect members of a population regardless
of population density. Fire and flood are two naturally occurring events that
are density-independent. They will affect a population regardless of its size.
When the density of a population increases, other factors may limit
further growth or reduce population numbers. Density-dependent factors
affect a population because of the density of the population. Food supply,
water quality sunlight, disease, and territory are density-dependent
factors. For example, when a tree in a dense forest becomes infected with a
fungsl blight, the infection will spread more quickly than it would in a
forest where trees are separated by larger distances.
Similarly, individuals in more densely populated areas are more prone
to starvation, as food is in lower supply. Competition for food may leave
animals weak and more susceptible to predation. The density-dependent
factors in Table 2 will cause higher ynortality rates, lowering the
population density When the population density is reduced, the effects of
the density-dependent factors are also reduced.
ng
The environment provides factors that prevent populations from attaini
their biotic potential. Any resource that is in short supply is a limiting
factor on a population. Food, water, territory, and the presence of
pollutants and other toxic chemicals are all limiting factors, as shown in
Siatto
Factors that Cause
Chances in Populations
Density Dependent and
Independent Factors
Limiting Factors
Ab)ntlc
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