When LORD created humans, LORD said : "Behold, I have given you every plant yielding seed which is upon the face of all the earth, and every tree with seed in its fruit; you shall have them for food." [Gen 1:29]
Therefore a vegetarian finds no difficulty in survival.
The vegetable we eat are a tiny portion of all the annuals, biennials, perennials on the earth! Many wild herbs are in fact "vegetable". (Many are poisonous, but in fact they are not. Our body need time to adapt to them. There is a snake farm which supply anti-dotes. The person in charge injected into himself venoms, very small amount in the beginning, then increase the amount over the years. Now, even if a poisonous snake bites him, he will not be affected.)
Grasshoppers, locusts, crickets are also edible. [Lev 11]
But to gather grasshoppers would be too laborious. Chinese keep chickens, and they collect the grasshoppers and lay eggs.
Chinese also keep ducks, and they collect food in the pond indirectly for them. Pond collects energy from the sun, and makes algae and water weed grow. Pond is very productive as there is plenty of water.
Chinese also keep fish in ponds, and they do the same as collectors.
Sheep and cattle also gather food indirectly for human. (We cannot eat grass, sheep and cattle convert grass into food for us.)
If we obey LORD's commandment to deal kindly with the animal, they will be our partners, and will add to our joy.
Also, Chinese farmers grow big leafy trees around the farm, and this attract birds, and the birds kill the insects. Chinese farmers also grow garlic, ginger, ... and these repel insects too.
Wu Siu Yan