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Growing up in the U.S., I was warned at various times of the dire consequences of a variety of introduced pests (usually insects).

Japanese beetles, gypsy moths, and most recently the brown marmorated stink bug are all introduced pests that, at various times, were described as serious threats to our ecology.

These threats aren't confined to arthropods, either. The giant African land snail is causing a stir in Florida (indeed, Florida seems to suffer from an excessive variety of introduced species.

"Lack of native predators" is frequently cited as the primary reason many invasive species are considered such a risk to the ecology.

I understand that these introduced species can place tremendous pressure on native species that fill similar ecological niches, and may even push these species out of the region due to competition for food and habitat. However, do the overall ecologies that these species are introduced to adjust over long periods of time?

The numbers of Japanese beetles and gypsy moths don't seem anywhere as high as when I was a child. Has the ecosystem adjusted, or has the overpopulation self-corrected as the species ran low on food through over-consumption? Or are the populations still just as problematic now as they were 30 years ago, and I just am not seeing the bigger picture?

What is the long-term impact that we've seen from invasive, introduced species? Is there a significant difference on the long-term impact between introduced flora, arthropods, or mammals?

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You may also be interested in the classic example of rabbits in Australia. –  terdon Aug 30 '13 at 16:40
    
Good question, and I'm excited to read some good answers. I'd guess just as an idea that introduced species have fundamental effects on ecosystems when the native species that they detrimentally affect are community structuring species, eg. Emerald Ash Borer in an Ash woodland. –  Oreotrephes Aug 31 '13 at 0:20
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Oh, and Homo sapiens are a pretty seriously destructive / transformative invasive species, I suppose. –  Oreotrephes Aug 31 '13 at 0:46
    
This is extremely broad and a little vague. It may help to break it up, or at least specificy the exact issues you are interested in. What do you mean by "damage"? That they drive out any other species? That they reduce productivity? That they reduce species richness? That they reduce the aesthetic value of the landscape? –  adam.r Dec 1 '13 at 19:32
    
It might also help to define what you meant by "ecosystem". If we consider the intestinal tract of a human as a microbial ecosystem, it's rather straightforward to show that invasive species cause long-term damage, by killing the host. –  Fomite Dec 3 '13 at 22:17

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The answer really depends on how you think of invasive. One extreme answer is to say that all things are relative, and that the concepts of local and invasive are all relative. This matters to a certain extent because we (ecologists) draw a fuzzy line between invasive and naturalized (http://onlinelibrary.wiley.com/doi/10.1046/j.1472-4642.2000.00083.x/abstract). You could start with some basic species that we all think of as either good, local, or neutral. Take the earthworm. Most people think of it as a common native species, but it's actually an invasive species that has radically changed much of North America that came over with the Europeans (http://www.dnr.state.mn.us/invasives/terrestrialanimals/earthworms/index.html). Similarly, brown trout are also invasive, coming to the US in the 1800's (http://www.dfg.ca.gov/fish/resources/WildTrout/WT_BrownDesc.asp).

As far as why invasive species succeed, it's still an open question. What you reference is the enemy release hypothesis, but there are others such as disturbance, diversity of the new community, and just the traits. Here's a good review: http://onlinelibrary.wiley.com/doi/10.1002/ece3.431/abstract http://onlinelibrary.wiley.com/doi/10.1111/j.1461-0248.2009.01418.x/full

To answer your question about adapting, I wouldn't trust your memory. It's important to keep in mind that most likely your memory is flawed, and you are only observing a very limited number samples with an incomplete sampling methodology. For instance gypsy moths undergo wild population fluctuations, and have for over 100 years. (http://link.springer.com/article/10.1007/PL00012004). How well do ecosystems bounce back from invasions depends on lots of factors. Often time the ecosystem can survive, but the some species won't. An easy example would be Norther Hardwood forests. Chestnuts (extirpated by an invasive fungus)are mostly gone from the wild, but the forest persists, as well as many of the species that rely on them. So in part the answer to your question depends on scale. The forest survived, but an individual species was nearly destroyed.

Ecological economists have tried to put a value on the destruction that invasive species have caused too (http://www.sciencedirect.com/science/article/pii/S0921800904003027).

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