Name:_________________________________________________________________________________
How Introduced and Invasive Species Alter Ecological Balance
Adaptation
For this lesson, I'm going to start with a question: 'are organisms adapted to their environment, or are they
adapted to the organisms around them?' The answer to this question is, quite simply, both. You can't even really
say which is the more important factor to adapt to; it's just a fact of nature that every species has to be adapted
to its environment, and it has to also be adapted to live with all of the other species that are found in its
environment. If an organism can't adapt to its environment, then obviously it will not survive; but the same
holds true for other organisms that share its habitat. No matter how well adapted a particular organism is to its
environment, if there is another species present that it just cannot find a way to survive with, then it still has a
problem and won't survive. As a result, every organism in every habitat has already found a way to survive, not
only in its environment, but also with all of the other organisms it shares its environment with.
Organisms adapt to both their environments and the other organisms around them
Introduced Species
So, given that all organisms are adapted to live in the presence of all of the other organisms in their
environment, what would happen if people suddenly added a new species to the environment? First of all, the
new species added to the environment would
be considered an 1) ______________________________________________________ because it is a species
that is living outside of its native environment and was brought there by human activity. Now, if the
species isn't adapted to the environment, it will die. Also, if the species cannot survive in the presence of one or
more of the other organisms already in the environment - whether it's because of predation or competition or
some other reason - then again, the species will die. However, if that new species can find a way to survive the
environment and all of the other organisms around it and begins to breed, then it will start to affect other
organisms in the 2)________________________. The introduced species has now proven that it can survive in
the environment in the presence of all of the 3)________________________________________________or
species that are naturally found in an area and were not introduced by humans.
Alteration of Ecological Balance
Now the question becomes: what will happen to the 4)_____________________________________________,
or the condition of equilibrium among the different species in an ecosystem? Now that the introduced
species is a factor in the ecology of the ecosystem, it will now probably compete with some native species for
resources, change grazing patterns in the ecosystem, or change the predator/prey dynamics. Who knows, it
could do all three! If it's an herbivore, grazing patterns are likely to be changed. The introduced species may
specifically target a certain type of plant that before was not heavily grazed upon, or maybe it simply
outcompetes a similar herbivore that is the only prey item for a particular predator. If the introduced species is a
plant, it will compete with other plants for water, sunlight, and nutrients. What will happen then, if only certain
types of herbivores can eat the introduced species and the native plants that the other herbivores can eat have
their numbers reduced because the introduced species is outcompeting them for sunlight or water? Or what if
the introduced species is a large predator and it's introduced into an ecosystem where there are no native large
predators? In all of these cases, several native species populations can be impacted by just one new species.
As you can see, there can be a domino
effect if the introduced species affects
native species A, which then in turn
affects native species B, which has an
impact on native species C, and so on
and so forth. These types of scenarios
lead us to ask the question: which
native species can survive in an
ecosystem with the introduced
species? If there are some native species which cannot survive alongside the invasive species, then they
will go, 5)__________________________________________ which is when a species ceases to exist in a local area.
Local extinction of one or more native species is a likely outcome when an introduced species becomes
established.
History is full of examples of introduced species causing the extinction or near extinction of native species. The
introduction of the mongoose in Hawaii was meant to control rats in sugar cane plantations, but the mongoose
has preyed on native birds so heavily that many species are endangered or even extinct. Tilapia and snakehead
fish have been introduced to countless streams, lakes and rivers throughout the Indonesian Islands and other
locations around the world, where these predatory fish almost always eat any native fish species to extinction. If
you take a drive down any highway in California and see grass-covered hills, almost all of the grasses you see
are introduced species - unless you happen to be in Yosemite or some other rare nature preserve where the
native bunchgrasses still exist. However, if you're driving in some areas of Australia, you may see hills that are
orange because they're covered by a native Californian plant, the California poppy. The species I've mentioned
here are all considered to be 6) ________________________________________or non-native species whose
introduction into an area has caused economic or ecological harm. This definition isn't all that specific and
leaves a lot of room for interpretation based on how someone views ecological harm, but if the species threatens
or causes the extinction of native species or dominates a landscape, it's usually considered invasive.
Why Are Invasive Species so Successful?
Now you may be wondering, how is it that invasive species can cause so much harm in a non-native
environment, and yet not be invasive in their native habitat? For instance, the mongoose is an invasive species
in Hawaii and other areas, yet it's threatened in parts of its native habitat. The reason why this happens is that
native species that didn't evolve with an introduced species aren't equipped with adaptations to the presence of
the introduced species. For instance, birds that are native to parts of India, where mongooses live, may have
developed methods to evade or defend themselves against mongooses, so their populations aren't threatened. In
other cases, the introduced species is subject to predation in its native habitat but not in the new habitat, and
without a predator to keep its population down, it reproduces unchecked. You may remember that co-evolution
can occur between a predator and its prey with each species evolving alongside the other which has a balancing
effect on the relationship, but when one species is moved to a new area without the other, that balance no longer
exists, and the resulting situation is almost impossible to predict.
Lesson Summary
Organisms are adapted both to their environment and the other organisms that live around them. If people bring
a species to a new environment, it is considered an 7)_________________________________________
because it is a species that is living outside of its native environment and was brought there by human activity.
Now, if the species isn't adapted to the environment or it cannot survive in the presence of one or more of the
other organisms already in the environment, the species will die. However, if that new species can find a way to
survive the environment and all of the other organisms around it, it begins to breed, and then it will start to
affect other organisms in the ecosystem.
Introduced species often change the 8)______________________________________, or the condition of
equilibrium among the different species in an ecosystem, and one way that the ecological balance is often
changed is through local extinction, which is when a species ceases to exist in a local area. Species that cause
local extinctions are often referred to as 9)___________________________________________ or non-native
species whose introduction into an area has caused economic or ecological harm. Introduced species often
become invasive because other organisms that share their native environment and have evolved with them to
achieve ecological balance usually aren't present in the new habitat, so the introduced species is no longer
subject to a natural check that evolved in response to its presence.
10) What is the difference between an introduced species and an invasive species?
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