How Introduced and Invasive Species Alter Ecological Balance

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  • 1:12 Introduced Species
  • 2:22 Alteration of…
  • 4:00 Local Extinction
  • 5:33 Invasive Species
  • 7:19 Lesson Summary
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Lesson Transcript
Instructor: Joshua Anderson
What happens to your block when a new neighbor moves in? Something changes, right? Now think about that on an ecological scale: what happens to an environment when a new SPECIES moves in?


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 also has to 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
Organisms Adapt to Live With Other Organisms

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 introduced species 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 ecosystem. The introduced species has now proven that it can survive in the environment in the presence of all of the native species, 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 ecological balance, 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.

The effects of an introduced species on other organisms can have a domino effect
Effects of Introduced Species Have Chain Reaction

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 locally extinct, 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 invasive species 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.

The mongoose is an invasive species putting native Hawaiian bird populations at risk
Mongoose Invasive Species

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