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
a lion.
Itself
Dragonflies biggest threat is birds, they some times eat dragonflies.
Fire.
Humans