The Tomorrow People1973

Tim:
They are known as Bolboids, and they're a kind of parasite.

Andrew:
What, a creature that lives on other creatures?

John:
Yes, Andrew.

Tim:
They latch onto the native species, wrap themselves around them, and slowly digest them.

Mike:
And that's what they're doing to us. Ew.

Tim:
It is a process which takes several months. They always start with whichever is the most advanced form of life on the planet, then they work their way down the scale, latching onto different life forms until they have stripped the planet bare.