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Agenda Lecture notes on Population and limiting factor Concept Maps Online Population Growth Lab Homefun: Study for your Test Next Friday 9-18 is our Unit One Test!!! Study!
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Welcome to Class! 9-14 and 9-15 Complete the following:
Pick two different biomes, discuss how are their bioticand abiotic
factors different in your journal New entry Population Grow and
Limiting Factors Check A-Day Students Foodweb and Bean Pyramids Go
to restroom? Define a vocabulary before go Tutoring hours:
Wednesdays afterschool 4-5pm Mondays, Wednesdays, Fridays morning
7:30am Agenda Lecture notes on Population and limiting factor
Concept Maps
Online Population Growth Lab Homefun: Study for your Test Next
Friday 9-18 is our Unit One Test!!! Study! How Populations Grow
Read the lesson title aloud to students. Learning Objectives
Describe how ecologists study populations.
Identify factors that affect population growth. Describe
exponential growth. Describe logistic growth. Click to reveal each
learning objective in turn. Read the objective aloud or ask a
volunteer to do so. Distribute the lesson worksheet and instruct
students to use vocabulary terms from the presentation to build a
concept map. Tell students to be sure their concept maps include
all of the following vocabulary terms: population density, age
structure, immigration, emigration, exponential growth, logistic
growth, carrying capacity. Describing Populations
Geographic range Density and distribution Growth rate Age structure
Remind students that a population is a group of organisms of a
single species that lives in a given area. Researchers study
populations geographic range, density and distribution, growth
rate, and age structure. Geographic Range Tell students that the
area inhabited by a population is called its geographic range. A
populations range can vary enormously in size, depending on the
species. A bacterial population in a rotting pumpkin, for example,
has a range smaller than a cubic meter. The population of cod in
the western Atlantic, on the other hand, covers a range that
stretches from Greenland down to North Carolina. Draw students
attention to the photo of Salvinia. Tell students that in 1998,
this floating fern was discovered in a small pond in Houston.
Salvinia had been imported as an easy-to-grow aquarium plant. But
some found their way into natural bodies of water and began to
spread. Salvinia plants can double their dry weight in less than
three days, rapidly forming floating mats that block sunlight and
crowd out native species essential to local aquatic animals and
waterfowl. Salvinia has been reported in Texas, all other Gulf
Coast states, and Arizona. Point out that Salvinias natural range
includes southeastern Brazil and northern Argentina. But humans
have carried Salvinia to tropical and subtropical areas in Africa,
India, Australia, and other South Pacific islands. It is spreading
across the southern United States largely because tiny pieces
survive attached to boats and other recreational gear. Density and
Distribution
Tell students that population density refers to the number of
individuals per unit area. Populations of different species often
have very different densities, even in the same environment. For
example, a population of ducks in a pond may have a low density,
while fish in the same community may have a higher density. Explain
that distribution refers to how individuals in a population are
spaced out across the range of the populationrandomly, uniformly,
or mostly concentrated in clumps. Have volunteers come to the board
to draw lines connecting the photos with their correct distribution
patterns. Click to reveal the lines showing the correct pairings.
Growth Rate Growth rate = 0 Population size is unchanged.
Population size is growing. Growth rate < 0 Population size is
decreasing. Tell students that a populations growth rate determines
whether the size of the population stays the same, increases, or
decreases over time. Salvinia populations in their native habitats
stay more or less the same size over time. In other words, those
Salvinia populations have a growth rate around zero. Salvinia
populations in Texas and other southern states, by contrast, have
very high growth rates, which means that those populations are
increasing rapidly in size. Populations can also have negative
growth rates, which means that they are decreasing in size. For
example, populations of cod off the coast of New England have
dropped so low due to overfishing that even using the latest
hi-tech equipment, total catch has fallen dramatically. Draw
students attention to the three statements on the screen. Read each
statement and ask students to identify for each whether the
population size is growing, decreasing, or is unchanged. Click to
reveal the correct answers in turn. Tell students that the photo
shows a population of bacteria. Ask: Imagine a few bacteria enter a
hosts body and start to reproduce. Would you expect the growth rate
to be zero, greater than zero, or less than zero? Answer: If the
bacteria are reproducing in a new host where there are plenty of
resources, the growth rate is probably greater than zero. Ask:
Suppose after a few days the host is given an antibiotic. After
several days, would you expect the growth rate to be zero, greater
than zero, or less than zero? Answer: After the antibiotic has been
working for a few days, the population of bacteria has probably
started to decrease, so the growth rate would be less than zero.
Bacterial population Age Structure Only certain age groups can
reproduce.
Only females produce offspring. Explain that to fully understand a
plant or animal population, researchers need to know more than just
the number of individuals it contains. They also need to know the
populations age structure and the number of all individuals with
male or female reproductive organs that a population contains. Ask
students to consider why knowing how many animals of each age group
are in a population would be important. Ask them also to consider
why it would be particularly important in animal populations to
know how many females were in the population. Allow students to
share several ideas before emphasizing that knowing numbers of
individuals at different ages is important because most plants and
animals cannot reproduce until they reach a certain age. Also,
among most animals, only females can produce offspring. Click to
reveal the bullet points. Ask: For a given animal species, which
population would have a greater growth rateone in which 90 percent
of the individuals were juveniles, or one in which 90 percent of
the individuals were of reproductive age? Answer: the population in
which the majority are of reproductive age Population Growth
Immigration Births Emigration Deaths
Tell students that factors that affect population size are birth
rate, death rate, and the rate at which individuals enter
(immigrate to) or leave (emigrate from) the population. Use the
diagram to explain how these factors cause a population to increase
or decrease: Explain that populations can grow if more individuals
are born than die in any period of time. In other words, a
population can grow when its birthrate is higher than its death
rate. If the birthrate equals the death rate, the population may
stay the same size. If the death rate is greater than the
birthrate, the population is likely to shrink. Point out that birth
means different things in different species. Lions are born much
like humans are born. Codfish, however, release eggs that hatch
into new individuals. Explain that a population may grow if
individuals move into its range from elsewhere, a process called
immigration. For example, an oak grove in a forest produces a
bumper crop of acorns one year. The squirrel population in that
grove may increase as squirrels immigrate in search of food. On the
other hand, a population may decrease in size if individuals move
out of the populations range, a process called emigration. For
example, a local food shortage or lack of other limiting resource
can cause emigration. Young animals may emigrate from the area
where they were born to find mates or establish new territories.
Have volunteers come to the board to label the diagram representing
a fish population with births, deaths, immigration, and emigration.
Click to reveal the correct labels. Ask: What two factors add
individuals to the fish population? Answer: births and immigration
Ask: What two factors remove individuals from the fish population?
Answer: deaths and emigration Ask: If the fish population stays the
same size for a one-year period, what can you assume about the
number of individuals removed from the population due to death and
emigration during that time? Answer: That number is equal to the
number of individuals added to the population by birth or
immigration. Emigration Deaths Exponential Growth Under ideal
conditions with unlimited resources, a population willgrow
exponentially. Tell students that if you provide a population with
all the food and space it needs, protect it from predators and
disease, and remove its waste products, the population will grow.
It can grow because members of the population will be able to
produce offspring. Later, those offspring will produce their own
offspring. Then, the offspring of those offspring will produce
offspring. So, over time, the population will grow. In such a
scenario, the size of each generation will be larger than the
generation before it, a situation that is called exponential
growth. Click to reveal the statement about conditions for
exponential growth. Ask students to consider how bacteria and
elephants differ in terms of their population structure and the
time it takes to produce a next generation. Guide students to
realize that bacteria reproduce much more rapidly than species like
elephants. Point out that this difference has consequences for
their patterns of population growth. Lead a discussion in which
students compare the graphs: Ask: How does the shape of the line
representing the growth of the bacterial population compare to the
shape of the line representing the growth of the elephant
population? Answer: They are very similar. Ask: How do the graphs
differ? Sample answers: The x-axis of the bacteria graph is marked
in two-hour increments; the x-axis of the elephant graph is marked
in 250-year increments. The y-axis of the graph representing the
bacterial population is marked in increments of 100,000; the y-axis
of the graph representing the elephant population is marked in
increments of 5 million. Ask: Why are different time increments
used in the two graphs? Answer: Elephants reproduce at a much
slower rate than bacteria. Describe for students a hypothetical
experiment with a single bacterium that divides to produce two
cells every 20 minutes: We supply it with ideal conditionsand
watch. After 20 minutes, the bacterium divides to produce two
bacteria. After another 20 minutes, those two bacteria divide to
produce four cells. At the end of the first hour, those four
bacteria divide to produce eight cells. After three 20-minute
periods, we have 2 2 2, or 8 cells. Another way to say this is to
use an exponent: 23 cells. In another hour (six 20-minute periods
total), there will be 26, or 64 bacteria. In just one more hour,
there will be 29, or 512. In one day, this bacterial population
will grow to an astounding 4,720,000,000,000,000,000,000
individuals. If this growth continued without slowing down, in a
few days this bacterial population would cover the planet. Draw
students attention to the graph for bacterial growth: Explain that
if you plot the size of this population on a graph over time, you
get a J-shaped curve that rises slowly at first and then rises
faster and faster. Click to reveal the circle around the area where
the population is rapidly increasing and the label. Explain that if
nothing interferes with this kind of growth, the population will
become larger and larger, faster and faster, until it approaches an
infinitely large size. Point out that, unlike bacteria, a female
elephant can produce a single offspring only every two to four
years. Newborn elephants take about ten years to mature. But as can
be seen in the elephant graph, if exponential growth continued, the
result would be impossible. In the unlikely event that all
descendants of a single elephant pair survived and reproduced,
after 750 years there would be nearly 20 million elephants. Click
to reveal the circle showing the data point for 20 million
elephants. Population is rapidly increasing. Logistic Growth When a
populations growth slows and then stops, following aperiod of
exponential growth Phase II Point out that the planet is not
covered with either elephants or bacteria. So, exponential growth
cannot explain population growth fully. Draw students attention to
the S-shaped curve in the graph. Tell them that this curve
represents what is called logistic growth. Logistic growth occurs
when a populations growth slows and then stops, following a period
of exponential growth. Many familiar plant and animal populations
follow a logistic growth curve. Click to reveal the definition of
logistic growth. Explain that something has to happen to keep a
population from growing exponentially forever. Point out that
population growth in real-world populations is logistic growth.
This kind of growth shows three distinct phases. Describe the three
phases of population growth. After describing each phase, ask for a
volunteer to go to the board and draw an arrow connecting the phase
with the region on the graph that represents it. Click to reveal
the correct arrows. Phase 1: Exponential growth: After a short
time, the population begins to grow exponentially. During this
phase, resources are unlimited, so individuals grow and reproduce
rapidly. Few individuals die, and many offspring are produced, so
both the population size and the rate of growth increase more and
more rapidly. Phase 2: Growth slows down: In real-world
populations, exponential growth does not continue for long. At some
point, the rate of population growth begins to slow down. This does
not mean that the population size decreases. The population still
grows, but the rate of growth slows down, so the population size
increases more slowly. Phase 3: Growth stops: At some point, the
rate of population growth drops to zero. This means that the size
of the population levels off. Under some conditions, the population
will remain at or near this size indefinitely. Ask: During which
phase does the population grow most rapidly? Answer: Phase I Ask:
During which phase does the population size stabilize? Answer:
Phase III Remind students that a population grows when more
organisms are born (or added to it) than die (or leave it). Thus,
population growth may slow for several reasons. Growth may slow
because the birthrate decreases. Growth may also slow if the death
rate increasesor if births fall and deaths rise. Population growth
may also slow if the rate of immigration decreases, the rate of
emigration increases, or both. Misconception Alert: Several common
misconceptions about population growth are revealed when students
are asked to graph population growth. Two common errors in students
graphs are: (1) the omission of Phase II, in which growth
slowsstudents show rapid growth followed by abrupt change to no
growth; and (2) the placement of the initial point representing the
starting population at the origin, or (0, 0), thus implying the
impossiblea population growing from zero. Use the graph to help
address these common misconceptions. Point out Phase II in the
graph, and explain that this phase represents a population that is
still growing, but more slowly than during Phase I. Also point out
the initial point on the graph, and have students note that it is
not at the origin. Remind them that all populations must start with
at least one individual (in the case of asexually reproducing
organisms) or one pair of organisms. Phase III Phase I Carrying
Capacity The maximum number of individuals of a particular species
that aparticular environment can support Population stabilizes at
carrying capacity. Ask: What happens when the birthrate and the
death rate are the same, and when immigration equals emigration?
Answer: Population growth stops. Point out that the population may
still rise and fall somewhat, but the ups and downs average out
around a certain population size. Draw students attention to the
broken horizontal line through the region of the graph where
population growth levels off. Click to reveal a black box
highlighting this portion of the graph. Tell students the point at
which that line intersects the y-axis represents what ecologists
call the carrying capacity. Carrying capacity is the maximum number
of individuals of a particular species that a particular
environment can support. Once a population reaches the carrying
capacity of its environment, a variety of both biotic and abiotic
external factors can affect the population in ways that stabilize
it at that size. Click to reveal the definition of carrying
capacity and the label specifying that population stabilizes at
carrying capacity. Overview: How Populations Grow
1. Ten individual per square hectare is a description of population
2, When resources are limited, a population will grow . 3. When
growth rate is , the population isgrowing. density logistically
greater than zero Have volunteers fill in the blanks with the terms
that correctly complete the statements. Click to reveal the correct
answers. Student Worksheet Answers
Exact layout of the concept map and specific intervening text will
vary from student to student, but student concept maps should
include the following terms with relevant links between them:
population density, age structure, immigration, emigration,
exponential growth, logistic growth, carrying capacity. Limits to
Growth Read the lesson title aloud to students. Learning Objectives
Identify factors that determine carrying capacity.
Identify the limiting factors that depend on populationdensity.
Identify the limiting factors that do not depend onpopulation
density. Click to reveal each learning objective in turn. Read the
objectives aloud or ask a volunteer to do so. Limiting Factors
Limiting factors determine the carrying capacity of an
environmentfor a species. Point out that scientists classify
limiting factors into two groups: density-dependent factors and
density-independent factors. Click to reveal circles and labels
showing how the factors are grouped. Tell students that they will
learn more about these groups of factors in the slides that follow.
Ask: How might each of these factors increase the death rate in a
population? Answer: Competition: Organisms may not have enough
resources to survive; Predation: Organisms die when they are eaten;
Parasitism and disease: Organisms are killed; Natural disaster and
unusual weather: Organisms are killed or resources are diminished.
Distribute the lesson worksheet and instruct students to create a
Venn diagram comparing the two categories of limiting factors,
density dependent and density independent, which they will learn
about in the slides that follow. Density dependent Density
independent Density-Dependent Factors
Density-dependent limiting factors operate strongly whenpopulation
density reaches a certain level. Tell students that
density-dependent limiting factors operate strongly when population
densitythe number of organisms per unit areareaches a certain
level. Explain that these factors do not strongly affect small,
scattered populations as much. Density-dependent limiting factors
include competition, predation, herbivory, parasitism, disease, and
stress from overcrowding. Note that some of these involve abiotic
external factors and others involve biotic external factors.
Competition More individuals use up resources sooner.
Individuals may competefor food, water, space,sunlight, shelter,
mates,territories. Tell students that when populations become
crowded, individuals compete for food, water, space, sunlight, and
other resources that are limited. Some individuals obtain enough to
survive and reproduce. Others may obtain enough to live but not
enough to raise offspring. Still others may starve or die from lack
of shelter. Thus, competition for changing resource bases that are
limited can lower birthrates, increase death rates, or both. Lead a
short discussion guiding students to make their own conclusions
about how competition can affect population growth. Remind students
that four general factors affect population growth. Ask: What four
factors affect population growth? Answer: birthrate, immigration,
death rate, emigration Then, guide students to tie these factors to
competition. Ask: How can competition affect the birthrate of a
population? Answer: If competition results in individuals not
obtaining enough resources to reproduce, the birthrate of the
population may decrease. Ask: How can competition affect the death
rate of a population? Answer: If individuals cannot obtain enough
resources to survive, the death rate may increase. Ask: How can
competition affect the rates of immigration and emigration? Answer:
If there is not much competition for the resources in an ecosystem,
individuals from other ecosystems may move in, increasing
immigration rate. If competition for resources is severe, the rate
of emigration may increase as individuals seek other ecosystems in
which to live. Click to reveal the bullet points onscreen. Close
the discussion by reiterating the following: Competition is a
density-dependent limiting factor, because the more individuals in
an area, the sooner they use up resources. Often, space and food
are related. Many grazing animals compete for territories in which
to breed and raise offspring. Individuals that cant establish and
defend a territory cannot breed. Competition can also occur among
members of different species that attempt to use similar or
overlapping resources that are limited. This type of competition is
a major force behind evolutionary change. PredatorPrey
Relationships
Tell students that the effects of predators on prey and the effects
of herbivores on plants are important density-dependent population
controls. One classic study focuses on the relationship between
wolves, moose, and plants on Isle Royale, an island in Lake
Superior. The graph shows that populations of wolves and moose
fluctuate over time. Make sure students understand that two
separate sets of data are plotted on the graph. Point out the left
and right vertical axes, which are numbered in different
increments. Explain that the left vertical axis and the blue line
represent the wolf population; the right vertical axis and the red
line represent the moose population. Ask: What general trends are
shown in this graph? Answer: An increase in the wolf population is
usually accompanied by a decrease in the moose population. A
decrease in the wolf population is usually accompanied by an
increase in the moose population. Use the graph to emphasize this
cyclical nature of the predator-prey relationship: Explain that
sometimes, the moose population on Isle Royale grows large enough
that moose become easy prey for wolves. When wolves have plenty to
eat, their population grows. As the wolf population grows, wolves
begin to kill more moose than are born. This causes the moose death
rate to rise higher than its birthrate, so the moose population
falls. As the moose population drops, wolves begin to starve.
Starvation raises the wolves death rate and lowers their birthrate,
so the wolf population also falls. When only a few predators are
left, the moose death rate drops, and the cycle may repeat. Click
to reveal the black circle around the point representing the CPV
outbreak on the graph. Explain that the population at this time was
affected by an outbreak of canine parvovirus (CPV). Ask: Based on
the graph, what effect did the canine virus outbreak have on the
moose population? Sample answer: The large decrease in wolf
population is probably due to the virus. With a smaller wolf
population, the moose death rate dropped, leading to a much higher
population after several years. Click to reveal the circle around
the point showing wolf population growth around the year 2000. Ask:
What might explain this spike in the wolf population? Sample
answer: The large spike in moose population a few years before
increased the amount of prey available, possibly increasing birth
rate and decreasing death rate in the wolf population. Tie the
concept of predatorprey relationships to humans: Explain that in
some situations, human activity limits populations. For example,
humans are major predators of codfish in New England. Fishing
fleets, by catching more and more fish every year, have raised cod
death rates so high that birthrates cannot keep up. As a result,
the cod population has been dropping. The cod population can
recover if we scale back fishing to lower the death rate
sufficiently. Biologists are studying birthrates and the age
structure of the cod population to determine how many fish can be
taken without threatening the survival of the population. Herbivore
Effects Populations of herbivores and plants cycle up and down
likepopulations of predators and prey. Tell students that herbivory
can also contribute to changes in population size. From a plants
perspective, herbivores are predators. So it isnt surprising that
populations of herbivores and plants cycle up and down, just like
populations of predators and prey. On parts of Isle Royale, large,
dense moose populations can eat so much balsam fir that the
population of these favorite food plants drops. When this happens,
moose may suffer from lack of food. Click to reveal the statement
about herbivore and plant populations. Guide students to make
inferences about the connections between herbivory and wolf
populations on Isle Royale. Ask: If moose populations become very
large, what will happen to wolf populations? What will happen to
plant populations? Answer: Moose populations will, after several
years, increase. Populations of plants that the moose feed on will
decrease. Ask: If the plant populations decrease due to large moose
populations, what will ultimately happen to the wolf
population?Answer: Too many moose means too few plants. Moose
populations will die off through increased predations and through
starvation. With a smaller moose population, the wolf population
will eventually decrease. Parasitism and Disease
Parasites and diseases can spread quickly through densehost
populations. Stress from overcrowding can lead to lower birth
rates,higher death rates, and higher emigration rates. Tell
students that parasites and disease-causing organisms feed at the
expense of their hosts, weakening the hosts and causing stress or
death. The ticks on the hedgehog in the photo, for example, feed on
their hosts blood and carry diseases. Parasitism and disease are
density-dependent effects because the denser the host population,
the more easily parasites can spread from one host to another.
Click to reveal the first bullet point stating why disease is
density dependent. Remind students of the a dramatic drop in the
wolf population around 1980 due to an outbreak of CPV. Explain that
at that time, a virus accidentally introduced to the island killed
all but 13 wolvesand all but three females. This drop in the wolf
population enabled moose populations to skyrocket to 2,400. Those
densely packed moose then became infested with winter ticks that
caused hair loss and weakness. Tell students that overcrowding can
also lead to increased stress within a population. Explain that
some species fight among themselves if overcrowded. Too much
fighting can cause stress, which weakens the bodys ability to
resist disease. In some species, overcrowding stress can cause
females to neglect, kill, or even eat their own offspring. Thus,
overcrowding can lower birthrates, raise death rates, or both.
Stress can also increase rates of emigration. Click to reveal the
summary statement about effects of stress. Density-Independent
Factors
Density-independent limiting factors affect all
populationsregardless of population size and density. Tell students
that density-independent limiting factors affect all populations
regardless of population size and density. Environmental change,
including unusual weather such as hurricanes, droughts, or floods,
and natural disasters such as wildfires, can act as
density-independent limiting factors. In response to such factors,
a population may crash. After the crash, the population may build
up again quickly, or it may stay low for some time.
Density-Independent Factors
Examples: hurricanes,droughts, floods, wildfires
Density-independent factorsmay actually vary withpopulation
density. Explain that events such as storms can nearly extinguish
local populations of some species. For example, thrips, aphids, and
other leaf-eating insects can be washed out by a heavy rainstorm.
Waves whipped up by hurricanes can devastate shallow coral reefs.
Extremes of cold or hot weather also can take their toll, no matter
how sparse or dense a population is. More prolonged environmental
changes, such as severe drought, can devastate populations. These
kinds of environmental changes can thus affect ecosystem stability.
Tell students that the photo shows dead fish rotting on a receding
shoreline due to drought conditions at Canyon Lake, Texas. Ask: Why
is drought a density-independent factor? Answer: It can affect
populations no matter how large or small they are. Ask students to
make inferences about the impact of the drought on a variety of
populations in this ecosystem. For example, a population of water
plants might become overcrowded as a result of a decrease in the
water level of the river. Or, plants along the riverbank might dry
out and die, limiting nesting places for some birds. Have
volunteers discuss their inferences with the class. Point out that
sometimes, the effects of so-called density-independent factors can
vary with population density. On Isle Royale, for example, the
moose population grew exponentially for a time after the wolf
population crashed. Then, a bitterly cold winter with very heavy
snowfall covered the plants on which moose feed, making it
difficult for all those moose to move around to find food. Because
emigration wasnt possible for this island population, many moose
died from starvation. The effect of bad weather on this large,
dense population were greater than it would have been on a small
population. In a smaller population, there would have been less
competition, so individual moose would have had more food
available. This situation shows that it is sometimes difficult to
say that a limiting factor acts only in a density-independent way.
Click to reveal the bullet point about the effects of density on
density-independent factors. Ask: Reconsider the drought scenario.
Is it possible for drought to act as a density-dependent factor?
Sample answer: A large population might be affected by a drought
more than a much smaller population due to competition for any
water available. Canyon Lake, TX Controlling Invasive Species
Density-independent measures? Herbicides, mechanical removal
Density-dependent measures? Predation Expensive,temporary Another
invasive species? Remind students about Salvinia, a major invasive
species along the Gulf Coast. Ask students what kinds of limiting
factors might help control Salvinia populations. Encourage students
to share their ideas. Click to reveal the bullet points as you
discuss some of the ways that researchers have been looking for
ways to apply limiting factors to control Salvinia and the
potential problems of these measures. Explain that artificial
density-independent control measuressuch as herbicides and
mechanical removaloffer only temporary solutions and are very
expensive. So ecologists tried to identify density-dependent
limiting factors that control Salvinia in its natural habitats.
Studies in South America uncovered several insects that feed on
Salvinia. Further studies showed that one particular species of
weevil feeds only on Salvinia. Thats important, because some
efforts to use one exotic organism to control another ended up
introducing another destructive invasive species. When no Salvinia
are available, these weevils starve to death before they feed on
another plant. In some parts of East Texas, weevils raised in
nurseries and released quickly became established and spread
naturally. In several places where Salvinia had caused problems,
the weevils significantly reduced Salvinia populations. Salvinia
Overview: Limits to Growth
Flood waters cover a field of wildflowers. Density dependent
Non-native snakes released into a wetland prey on native rodents.
Density independent Flu virus spreads quickly in schools. Have
volunteers come to the board to draw lines matching examples with
the correct category of limiting factors. Click to reveal correct
pairings. Wildfires spread through a grassland.