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
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
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
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.
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 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 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 ecological balance, 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 invasive species, 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.
After watching this lesson, you should be able to:
- Define introduced species and native species
- Explain how introduced species may affect an ecosystem
- Understand when an introduced species is considered an invasive species, and why invasive species are successful