Well strategically ..it depends how an invader would show up...
It still may be costly and impractical to send an armada over long distances...to end up at earth...better to send something which could grow, replicate, spawn or spore. Then it would be about inaccessibility..for us..
..the need for it to have some time to be formidable enough to be a threat.
If it is truly an alien invasive species..think of what so many have done when hitching a ride to a new continent here...probably wouldn't be much different ...There is no such thing as a benign invasive species...Here in Canada we have been fighting so many invaders for a while now...Scotch Broom....Purple Loosestrife...the Pine beetle...4 varieties of Asian carp...and I'm just naming a few foreign invasive species...have done real horrible damage and helped push many indigenous species to the brink of extinction...changing the landscape as they become dominant...or the only mono-culture... that is the devastation invasive species cause.
It would be finding what "medium" ...what perfect conditions it needs initially....what is best suited to the invader...hunting it down and routing it out.. and literally repelling or exterminating it off this planet like an immune system protecting the body. Otherwise..the planet and all it's diverse life may not survive such a truly alien (not just to an ecosystem ..but worse...alien to the entire planetary organism) invader
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