Carrying Capacity
Population Growth of Duckweed
Basics About Population Growth
(pages 530-533)
• The environment has a carrying capacity
for every organism based on the available
resources
• If resources are abundant and readily
available, then a population will show
exponential growth
• During exponential growth the increase in
population size varies with the number of
individuals present
• Increase is gradual at first, but rises sharply as
more individuals become part of the population
• Unlimited resources allow everyone to
reproduce!
Environmental Resistance Factors
• Populations will continue to grow
exponentially until they come up against
limiting factors or resistance factors
• Resource limitations
– Food, nutrients, water, sunlight
– Shelter, space
• Competition from other species
• Predation
• Waste accumulation
Logistic Growth
• In the real world resources are often limiting
• The first part of the growth resembles
exponential growth
• But over time, the population size levels off in
logistic growth at a point called the carrying
capacity
• The carrying Carrying
capacity of the
environment
Capacity
represents the
maximum number
of individuals of a
particular species
that an area can
support indefinitely
without degrading
Carrying
Capacity
• Natural populations seldom approach
carrying capacity smoothly
• Or stay at the carrying capacity once reached
• They usually fluctuate around the carrying
capacity
• Overshooting it or falling below depending on
resources
Duckweed: Lemna minor
• A small, floating aquatic
plant that forms a solid
cover on the surface of
freshwater ponds,
marshes, lakes and quiet
streams
• Very important in the
aquatic ecosystem as an
essential link in the food
chain.
– Eaten by fish
– birds (ducks, herons)
• Shades extensive areas of a pond,
reducing algae growth
• Good for bioremediation projects
– Ability to take out nitrogen and phosphorus
from water
• Useful as a water crop
– as a source of food for animals and poultry
Structure of Duckweed
• Leaves and stems
merge in a common
structure called a
frond or thallus
• Has one root
• Fronds have 3 veins
• Air spaces help the
plant to float
Reproduction
• Reproduction is asexual
• Vegetative reproduction
• Unlike the leaves of more ordinary plants, each
duckweed frond contains buds from which more
fronds may grow.
• Until they mature, daughter fronds remain attach
to the parent frond
• Rapidly growing plants can have 3-4 attached
fronds
How do Duckweed Survive?
• Autotrophic
– Make their own food
through
photosynthesis
• Require sunlight
• Require nutrients
– Phosphorous,
nitrogen, other
minerals
• Need space to grow
Experiment:
Population growth in Duckweed
1. Each group will set up TWO flasks
2. Each flask has 100-200 ml pond water
3. Place 2 duckweed plants in one flask
4. Place 15 duckweed plants in the other flask
5. Count all thalli (leaves) greater than 2 mm as
full grown.
6. Place labeled flasks under the grow lights in
room 201
7. Record your data twice a week for 3-4 weeks
(Create your own data table before you leave
today)
What kind of population growth do you expect
from the two different Duckweed flasks?
• Exponential growth
– J-shaped growth curve
• Logistic growth
– S-shaped or sigmoidal