everything on earth survives by feeding off of something else in some way.
I expect aliens to be no different. Odds are any aliens that came here would want our resources or us in some bad way. And the fact that they have the tech to get here shows they have the means to take whatever they wanted with little effort.
and remember, when they get here they will eat the fat ones first!
NASA should to us a favor, stop enticing something to come here that we have no way to defend against.