WMSU-ISMP-GU-003.00
Effective Date: 7-DEC-2016
ACTIVITY 6
POPULATION REGULATION: RESOURCE AVAILABILITY
I.
Introduction
It has long been apparent, even before the time of Darwin (middle 1800s), that
(1) populations established in a new environment tend to increase in size, but do so
at various rates, and (2) no population continues to grow indefinitely. Therefore, the
growth of populations must be limited by external or internal forces, which
presumably do not act identically on every species. The question of how populations
grow and how that growth is regulated has been a central problem of ecology since
the field was born.
Initially, if a small population is established in a benign environment without
competitors, it will increase in numbers relatively rapidly. Sooner or later, however,
some aspect of the environment will act to dampen the rate of population growth by
preventing the organisms from growing and reproducing at their physiological
maximum. Features of the environment such as temperature are referred to as
limiting factors when they act to slow population growth; features such as
water, minerals or food sources that are directly consumed by the organisms are
limiting resources. For all intents and purposes, limiting factors and limiting
resources are the same thing because places in the environment where external
factors are not limiting (a habitat with the right temperature, for example) essentially
act as a limiting resource that is “consumed” by the organisms occupying that habitat.
II. Objectives:
At the end of this activity, students should be able to:
1. Determine the different factors regulating population growth
2. Discuss limiting factors from limiting resources
3. Describe how population grow with different treatment used
III. Concept/s Explanation
Ecologists have long been fascinated with the various ways that different
species have adapted to survive and reproduce in a wide variety of habitats. Studies
on the factors controlling growth of populations can be difficult because many
organisms, especially larger plants and animals, have long life-cycles that do not
allow us to collect information over the entire life-cycle in a reasonable time. Imagine
trying to study population growth of oak trees, which can live for up to 800 years! As
a substitute, ecologists often choose small, fast-growing species that complete
their life cycles relatively quickly, and therefore allow us to examine the
environmental factors controlling population growth in studies of comfortable
duration. This is a compromise of course, because we must be careful that any
generalizations we arrive at from work on fast growing species apply to oak trees too.
Duckweeds are attractive as test subjects for studies on population growth
because they are easy to maintain in the laboratory and they reproduce asexually at
a phenomenal rate. Therefore, duckweeds have been the subject of a great deal of
research in ecology, toxicology and physiology.
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The small duckweed, Lemna minor (Fig. 1a) is a common plant found floating
on the surface of still, enriched waters such as swamps, ditches and farm ponds. The
plant is extremely simple; there are no stems, and flowers are very rare. Each
duckweed plant, called a thallus (plural thalli) consists of a tiny floating leaf with a
single rootlet hanging a few centimetres into the water below. New thalli develop in
pockets on the edge of the parent plant and frequently remain attached, so that
clumps of two, three or more thalli are usually observed.(Fig 1b)
Fig 1a. Lemna minor
Fig 1b. Thalli
If environmental conditions (chiefly light, nutrients and temperature) are not
limiting, duckweed populations tend to increase geometrically, as each thallus
“doubles” to produce a second thallus. This allows the plant to rapidly colonize the
water surface during a short growing sea son. Under optimal conditions, duckweed
populations double every 2-3 days. In the laboratory, doubling in seven days or less
is commonly observed.
V.
Work/Practice Exercise
1- Secure 10 of 100 ml beakers. Label 5 beaker with low- density and 5 beaker
with high-density and replicate each treatment 3 times.
2- Add 10 ml of pond sediment and 90 ml of growth medium to each beaker.
3- Mark the level of the medium on the side of each beaker and drop a penny
into each one.
4- Trace quantities of copper released from the pennies effectively control
cyanobacteria, which otherwise compete with the duckweed and foul the
beakers. The pond sediment enhances growth of the duckweed, probably by
providing organic nutrients or by regulating the dose of copper
5- Transfer 10 duckweed thalli to each of the low-density beakers, and 50 thalli
to each of the high-density beakers.
6- Place the beakers in the mesh baskets (Fig 2) and place the baskets beneath
one of the high-intensity lamps at the side of the room.
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WMSU-ISMP-GU-003.00
Effective Date: 7-DEC-2016
Fig. 2-Mesh Basket
7- Inspect the beakers every day, at roughly the same time each day for seven
days
8- On each visit, remove the entire basket from under the light, then count the
number of thalli in each beaker
9- Try to be consistent about what constitutes a new thallus. Do not remove the
plants from the water. Note any changes in color or size of the duckweed
thalli, or any growth of cyanobacteria, which appear as a dark scum around
the edges of the plants.
10- Gently stir the medium to bring fresh nutrients into contact with the roots.
11- Top up the beaker with distilled water if needed; evaporation is rapid under
the lights.
12- Replace the basket of beakers under the light at a different location than
before. Randomization of beaker locations is important because light intensity
beneath single lamps declines steeply with distance.
Note: You should divide up the work of making daily counts and replacing
lost water among the members of your group
13- Accomplish Table 1and Table 2 in your worksheet
14- Compute for Mean and Standard Deviation
15- Plot the results of your experiment on a graph as total number of thalli against
time in days (Figure 3) both for low and high density treatment. Make
separate lines for the low-density and high-density treatments on the same
graph. Use the daily means of all replicates for each treatment also in your
worksheet.
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WMSU-ISMP-GU-003.00
Effective Date: 7-DEC-2016
ACTIVITY 6
POPULATION REGULATION: RESOURCE AVAILABILITY
WORKSHEET
Name_______________________________
Section_____________________________
Date submitted________________
Class Schedule________________
Table 1. Number of Thalli for the entire experimental period for low-density treatment
Replicate
Day 0
1
10
2
10
3
10
Mean
10
Standard
Deviation
0
Day 1
Day 2
Day 3
Day 4
Day 5
Day 6
Day 7
Table 2. Number of Thalli for the entire experimental period for high-density treatment
Replicate
Day 0
1
50
2
50
3
50
Mean
50
Standard
Deviation
0
Day 1
Day 2
Day 3
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Day 4
Day 5
Day 6
Day 7
WMSU-ISMP-GU-003.00
Effective Date: 7-DEC-2016
Figure 3. Typical growth curve of Lemna minor
Guide questions:
1. Explain the factors that resulted to your data in Table 1 and 2.
2. What could be the possible reason/s of the growth curve in Figure 3?
Conclusion:
References:
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