Short term, mankind.
Long term, the evolution of the Sun.
Yes
Humans and human activity are the biggest threat to humans.
Jellyfish are eaten by squid. I believe that this is the biggest threat that they face.
the market economy was the single biggest threat
Churchill thought that Hitler was the biggest threat in WW2
Biggest threat is a landowner with a 22 rifle or 12ga shotgun.
you are
Itself
a lion.
Dragonflies biggest threat is birds, they some times eat dragonflies.
The biggest threat to the red crabs on Christmas Island is the influx of asylum seekers moving to the island. The second biggest threat comes from ants. They spray poison in the eyes of the crabs and then eat them.
The biggest threat to coral reefs, and tropical rainforests is poachers and distruction.