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Date:
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Keystone and Invasive Species and their effect on Biodiversity
Read the scenarios below. Determine if the species is a keystone species or invasive species. Highlight
evidence from the text to support your conclusion. Explain how the presence of these species effected the
area’s biodiversity. Explain how the presence of the keystone species effects the ecosystem’s carrying
capacity. Explain how this relates to limiting factors (density independent and density dependent). Finally,
explain how this organism gets the nutrients it needs to survive.
Key Vocabulary
Keystone species-a species on which other species in an ecosystem largely depend, such that if it were
removed the ecosystem would change drastically.
Invasive species- an organism that is not indigenous, or native, to a particular area. Invasive species cause
great economic and environmental harm to the new area.
Carrying Capacity- the number of people, other living organisms, or crops that a region can support without
environmental degradation.
Autotroph- an organism that can produce its own food using light, water, carbon dioxide, or other
chemicals. Because autotrophs produce their own food, they are sometimes called producers.
Heterotroph- A heterotroph is an organism that eats other plants or animals for energy and nutrients. The
term stems from the Greek words hetero for “other” and trophe for “nourishment.”
Herbivore- An herbivore is an organism that feeds mostly on plants. Herbivores range in size from tiny
insects such as aphids to large, lumbering elephants.
Carnivore- A carnivore is an organism that mostly eats meat, or the flesh of animals.
Sometimes carnivores are called predators. Organisms that carnivores hunt are called prey.
Density Dependent Limiting Factors- These limiting factors tend to be b—having to do with living
organisms. Competition and predation are two important examples of density-dependent factors.
Density Independent Limiting Factors- These have an impact on the population, whether the population is
large or small, growing or shrinking. For example, a wildfire that sweeps through a dense forest in the
Everglades has a big impact on every population in the community, regardless of the density of any one
population. Wildfire is abiotic (nonliving), and most density-independent limiting factors fall in this
category.
Scenario 1: Purple Sea Star
D Density
Dependent
Limiting
Factorsfactors
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the ecosystem. Pisaster ochraceus, commonly known as purple sea stars, are a major predator of mussels and
important examples of density-dependent factors.
barnacles on Tatoosh Island. With the sea stars gone, mussels took over the area and crowded out other
species,
including
benthic algae
that supported
communities
sea snails,
limpets and
Lacking a
Density
Independent
Limiting
FactorsThese ofhave
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on bivalves.
the population,
keystone species, the tidal plain's biodiversity was cut in half within a year.
whether the population is large or small, growing or shrinking. For example, a
wildfire that sweeps through a dense forest in the Everglades has a big impact
on every population in the community, regardless of the density of any one
Keystone species or Invasive species:
How did the presence of the keystone species effect the biodiversity?
How did the presence of the keystone species effect the ecosystem’s carrying capacity?
What limiting factors did the keystone species effect?
Is the keystone species an autotroph or heterotroph?
If heterotroph, is it an herbivore or carnivore?
Scenario 2: African Elephant
Herbivores can also be keystone species. Their consumption of plants helps control the physical and biological
aspects of an ecosystem. In African savannas such as the Serengeti plains in Tanzania, elephants are a
keystone species. Elephants eat shrubs and small trees, such as acacia, that grow on the savanna. Even if an
acacia tree grows to a height of a meter or more, elephants are able to knock it over and uproot it. This
feeding behavior keeps the savanna a grassland and not a forest or woodland. With elephants to control the
tree population, grasses thrive and sustain grazing animals such as antelopes, wildebeests and zebras. Smaller
animals such as mice and shrews are able to burrow in the warm, dry soil of a savanna. Predators such as lions
and hyenas depend on the savanna for prey.
Keystone species or Invasive species:
How did the presence of the keystone species effect the biodiversity?
How did the presence of the keystone species effect the ecosystem’s carrying capacity?
What limiting factors did the keystone species effect?
Is the keystone species an autotroph or heterotroph?
If heterotroph, is it an herbivore or carnivore?
Scenario 3: Zebra Mussels
Zebra mussels are native to the Black Sea and the Caspian Sea in Central Asia. Zebra mussels arrived in
the Great Lakes of North America accidentally, stuck to large ships that traveled between the two regions.
There are now so many zebra mussels in the Great Lakes that they have threatened native species.
Keystone species or Invasive species:
How did the presence of the keystone species effect the biodiversity?
How did the presence of the keystone species effect the ecosystem’s carrying capacity?
What limiting factors did the keystone species effect?
Scenario 4: Wolves
The Greater Yellowstone Ecosystem (GYE) is an enormous and diverse temperate ecosystem stretching across
the boundaries of the U.S. states of Montana, Wyoming and Idaho. The GYE includes active geothermal basins,
mountains, forests, meadows and freshwater habitats. The elk, bison, rabbit and bird species in the Greater
Yellowstone Ecosystem are at least partly controlled by the presence of wolves. The feeding behavior of these
prey species, as well as where they choose to make their nests and burrows, are largely a reaction to wolf
activity. Scavenger species, such as vultures, are also controlled by the wolf activity. When the U.S.
government designated land for Yellowstone National Park in the late 19th century, hundreds of wolves
roamed the GYE, preying primarily on abundant herds of elk and bison. Fearing the wolves' impact on those
herds, as well as local livestock, governments at the local, state and federal level worked to eradicate wolves
from the GYE. The last remaining wolf pups in Yellowstone were killed in 1924. This started a top-down trophic
cascade in the Greater Yellowstone Ecosystem. A trophic cascade describes changes in an ecosystem due to
the addition or removal a predator. A top-down trophic cascade describes changes that result from the
removal of an ecosystem's top predator. (A bottomup trophic cascade describes changes that result from the
removal of a producer or primary consumer.) Lacking an apex predator, elk populations in Yellowstone
exploded. Elk herds competed for food resources, and plants such as grasses, sedges and reeds did not have
time or space to grow. Overgrazing influenced the populations of other species, such as fish, beaver and
songbirds. These animals rely on plants and their products — roots, flowers, wood, seeds — for survival. The
physical geography of the Greater Yellowstone Ecosystem was also impacted by the loss of wolves and
subsequent elk overgrazing. Stream banks eroded as wetland plants failed to anchor valuable soil and
sediments. Lake and river temperatures increased as trees and shrubs failed to provide shaded areas. Starting
in the 1990s, the U.S. government began reintroducing wolves to the Greater Yellowstone Ecosystem. The
results have been noteworthy. Elk populations have shrunk, willow heights have increased and beaver and
songbird populations have recovered.
Keystone species or Invasive species:
How did the presence of the keystone species effect the biodiversity?
How did the presence of the keystone species effect the ecosystem’s carrying capacity?
What limiting factors did the keystone species effect?
Is the keystone species an autotroph or heterotroph?
If heterotroph, is it an herbivore or carnivore?
Scenario 5: Sea Otter
One example of a trophic cascade involves the work of James Estes, an American marine biologist. He
discovered that islands without sea otters had an abundance of sea urchins, the main prey of sea otters. The
forests of kelp that once grew there had disappeared. Instead, huge urchins littered the barren seafloor,
having consumed every kelp plant in sight. By contrast, kelp flourished on nearby islands where sea otters
survived or had been reintroduced. The discovery was important. That's because kelp forests provide
nourishment for fish and other sea animals. Everywhere Estes looked he found the same picture. Islands with
sea otters had healthy kelp forests. Islands without otters had barren sea floors littered with sea urchins and
no kelp. Kelp forests play a key role in maintaining global environmental health. More and more carbon
dioxide is in the atmosphere. This is causing an increase in global temperatures. This carbon dioxide is
absorbed by the ocean, making it more acidic. This harms many species. Estes has calculated that healthy kelp
forests have the capacity to absorb billions of kilograms of carbon. Our results were eye-opening," he states.
"The difference in annual absorption of atmospheric carbon from kelp photosynthesis between a world with
and a world without sea otters is somewhere between 13 and 43 billion kg (13 and 43 teragrams) of carbon."
Keystone species or Invasive species:
How did the presence of the keystone species effect the biodiversity?
How did the presence of the keystone species effect the ecosystem’s carrying capacity?
What limiting factors did the keystone species effect?
Is the keystone species an autotroph or heterotroph?
If heterotroph, is it an herbivore or carnivore?
Scenario 6: Beaver
Beavers begin their dams at night. They start by dragging a tree across a shallow stream. They then fill it in
with rocks and mud carried with their paws, and branches dragged by using their big front teeth. The dams
keep water from flowing away.
The beaver dams help the environment in ways that humans cannot easily copy. They alter land into a sponge
that soaks up water. Their gnawing and nesting habits promote richer soil and slow down water flow,
improving endangered fish habitats. Their dams raise the level of underground water. This feeds bushes
alongside streams that help keep river banks from washing away. It also creates habitats for birds and deer,
and even the endangered California red-legged frog.
Keystone species or Invasive species:
How did the presence of the keystone species effect the biodiversity?
How did the presence of the keystone species effect the ecosystem’s carrying capacity?
What limiting factors did the keystone species effect?
Is the keystone species an autotroph or heterotroph?
If heterotroph, is it an herbivore or carnivore?
Scenario 7: Kudzu
The Asian plant kudzu (Pueraria montana var. lobata) was introduced to North America during the 1876
World's Fair in Philadelphia, Pennsylvania. In the 1930s, the government encouraged people in the Southern
United States to plant the fast-growing vine. They hoped this would prevent soil erosion. But the kudzu quickly
grew out of control, earning it the moniker "the vine that ate the South. "How did the presence of the
keystone species increase biodiversity?
Keystone species or Invasive Species:
How did the presence of the keystone species effect the biodiversity?
How did the presence of the keystone species effect the ecosystem’s carrying capacity?
What limiting factors did the keystone species effect?
Scenario 8: Prairie Dog
Residing in the grasslands of central and western North America, prairie dogs support some 130-plus other
species. In addition to serving as a food source for coyotes, eagles, the endangered black-footed ferret, and
other animals, they are ecosystem engineers, maintaining the health of arid grasslands by churning, aerating,
and fertilizing soil as they create vast and intricate underground colonies. Their digging allows an array of
vegetation to thrive, which in turn supports a greater number of elk, bison, and other grazers. And their
burrows provide shelter for animals like rattlesnakes, burrowing owls, and jackrabbits. When prairie dogs
disappear from their native grasslands habitat, woody plants can take over, fundamentally altering the prairie
ecosystem.
Keystone species or Invasive Species:
How did the presence of the keystone species effect the biodiversity?
How did the presence of the keystone species effect the ecosystem’s carrying capacity?
What limiting factors did the keystone species effect? Population of rattlesnakes, owls, jackrabbits, soil
Is the keystone species an autotroph or heterotroph? heterotroph
If heterotroph, is it an herbivore or carnivore? herbivore
Scenario 9: Bees
With the aid of other pollinators (like hummingbirds), bees support the reproduction of as much as 90
percent of the world’s flowering plants. Not only do they pollinate fruits, vegetables, and other crops that
provide humans with everything from food to clothing to fuel, but they also help produce the seeds, nuts,
berries, and fruit that countless other species in ecosystems around the world survive on. Without bees, there
would be a bottom-up cascade of consequences throughout the food chain. Despite this, bees like the
endangered rusty patched bumblebee have failed to receive crucial protections in the United States.
Keystone species or Invasive species:
How did the presence of the keystone species effect the biodiversity?
How did the presence of the keystone species effect the ecosystem’s carrying capacity?
What limiting factors did the keystone species effect?
Is the keystone species an autotroph or heterotroph?
If heterotroph, is it an herbivore or carnivore?
Scenario 10: Brown Tree Snake
The brown tree snake (Boiga irregularis), native to Australia and Indonesia, has caused the extinction of
endemic bird species in Guam. It has also caused widespread power outages by climbing electrical lines. It is
thought to have arrived there in the 1950s by hitching a ride in military aircraft and cargo. In Hawaii, officials
are on high alert to stop the brown tree snake from spreading to the islands so it doesn't cause damage.
Keystone species or Invasive Species:
How did the presence of the keystone species effect the biodiversity?
How did the presence of the keystone species effect the ecosystem’s carrying capacity?
What limiting factors did the keystone species effect?
Is the keystone species an autotroph or heterotroph?
If heterotroph, is it an herbivore or carnivore?
Scenario 11: Saguaro Cactus
Native to the Sonoran Desert of the southwestern United States and northwestern Mexico, the saguaro cactus
provides critical nesting spots for birds like red-tailed hawks and woodpeckers (the latter of which peck new
nest holes each year, leaving old holes for other birds). These cacti serve as an important source of
sustenance, too. Their blooming flowers feed bats, birds, and bees, while their fruit, which ripens when the
desert is at its driest, is often the sole wet food source for myriad mammals, insects, and other species.
Keystone species or Invasive species:
How did the presence of the keystone species effect the biodiversity?
How did the presence of the keystone species effect the ecosystem’s carrying capacity?
What limiting factors did the keystone species effect?
Is the keystone species an autotroph or heterotroph?
If heterotroph, is it an herbivore or carnivore?
Scenario 12: Silver Carp
Silver carp is a species of fish that escaped from fish farms in the 1990s and is now common in the Missouri
River of North America. These fish feed on plankton, tiny organisms floating in the water. Many native fish
species, such as paddlefish, also feed on plankton. The feeding cycle of the paddlefish is slower than that of
the carp. There are now so many carp in the lower Missouri River that paddlefish do not have enough food.
Keystone species or Invasive species:
How did the presence of the keystone species effect the biodiversity?
How did the presence of the keystone species effect the ecosystem’s carrying capacity?
What limiting factors did the keystone species effect?
Is the keystone species an autotroph or heterotroph?
If heterotroph, is it an herbivore or carnivore?