Why do space aliens come to the earth?

It's a great place for tourists! 

Finally, they say, "All the earthmen!  Be an ecologist!  Solve the global
environmental problems!  Manage the entire global environment!  Accomplish
the symbiosis with the human beings and the earth!  And then, let's
associate with us!"

We should think about the environmental problems of the earth individually,
solve the familiar environmental problems at first, and aim at the symbiosis
with the human beings and the earth in order to establish the new space
scientific era when we can keep company equally to the space aliens!

Question: Which earth species is the most symbiotic, that is, has
symbiotic relations with the greatest number of other species? 

Hint: It's found on all continents, in all climes from desert to rain
forest, it walks around on two legs, and is very intelligent! 

MT