Humans can start wearing thick protective clothing, so your venomous spiders are useless. Guns, flamethrowers, our feet, animals are on borrowed time.
Are the animals willing to deliberately sacrifice themselves to get us? I could imagine a few flying insects could fly into noses and just crawl down into our throats and lungs and choke us.
I think 3/4 of the human population would be dead on day 1. The rest would be dead whenever they next set foot out of doors. There would be hundred or thousands of animals per person.
The only people who could survive would have to do something like grow yeast and fungi in caves.
There are enough insects in the world that they’d be able to kill every human by just swarming around everyone’s head, getting in the nose and mouth and making us suffocate or choke to death.
I can’t believe this thread is over 20 posts long, and yet I’m the first to rush in and wilfully ignore the intent of the OP by pointing out that humans ARE animals, and therefore we’d be compelled to attack each other.
Youns are slipping, guys.
We’re already attacking each other, so that’s nothing new.
But if you’re talking intelligence, then aren’t you also talking survival instinct?
And in that case, 300 million humans with greater intelligence and and greater weapons and greater mass than the majority of animals would prevail.
Yabut we’re not ALL attacking each other, is what I’m hijacking here. We couldn’t fight off all the OTHER animals collectively trying to kill humans, if we were ALSO collectively compelled to kill humans. Could we? Hah? Could we? Go ahead, admit I’m right. Confess that my hijack is technically accurate and that I am therefore superior. Say it.
I’m really, really sorry, but this pleases me greatly. 
Well now, the “Got Milk” commercial seems appropriate here.
Prior thread on point: Animals vs. Man - who wins? - In My Humble Opinion - Straight Dope Message Board
I would like to point out that there would be regionalized give-and-take between the two factions. The obvious things like fish and water-bound mammals would not be a factor if humans stayed on land. That in my mind would be a naturally observed truce. Deserts (and other environments) are not nice to some insects, animals, birds and mammals while others flourish. There would be pockets where the animals (sans humans) would have the upper hand, and other places where humans would control. Then it would be all about which side would take the risk to go to the enemies camp to attack. Humans have the capacity and knowledge to flourish virtually in any environment in the whole planet, while the animals are still constrained to their natural environment with limited abilities to survive outside of it.
Also, how are the carnivores gonna stay alive unless they start eating their own comrades…would they be able to go vegan? Doubtful. Not enough humans to feast on to keep them alive, they will probably perish first. Mosquitos also need blood from their comrades to be able to keep their offspring alive…spreading disease among their mammalian comrades as well. If we had advanced warning to gather and hunker, we could wait them out for a month in a defensive posture, and then go on a “contained scorched earth” policy. No animal other than us has the ability to use fire as a tool and a weapon, where animals cringe–or get cinged.
Humans win; but the prize seems to be an empty world.
Oh yeah, that thread…I thought I had some good ideas there although some have disagreed with me. Well again, I would not lay down and let the critters take me without a fight.
If the attack happens in Minnesota in January, I’m really not all that frightened about the bugs. Depending on food supplies for both me and my animal attackers, maybe I’m not that concerned about the animals either. In July, it’s a different story.
The ants alone could probably do us in, if they put their collective mind to it. Certainly there are enough ants to swarm over every human.
Yup. The weight of all the ants in the world is greater than the weight of all the humans in the world (multiple cites if you search). So, “overwhelmed” is understating things a bit. My rough calculations, assuming an average ant mass of 3 milligrams, indicate that on an equal weight basis, a 200-pound human could expect to face off against 30 million ants. That’s an awful lot of swatting.
In most regions of the world, ants compose 15-20% of the total biomass, up to 25% in the tropics. If they didn’t eat each other, we’d be toast already.
Well I suppose if you just stand there and let them get you.
In some places they eat chocolate covered ants and having them come right to you could be a bonus. Mmmm, chocolate covered protein.
The Saul Bass-directed movie Phase IV depicted just such a global antmageddon scenario. Of course in that film the ants had the added advantage of a suddenly enhanced collective intellect, which enabled them to strategize and improvise impressively, including a memorable reversal of the old magnifying glass trick.
Are you thinking just for food consumption, or all plants?
There’s a gentleman named Leiningen who’d like a word with you about that.
http://www.cowswithguns.com/cowmovie.html Be afraid,very afraid.