In Body Harvest, what's the difference between Hero and Easy

According to The Cutting Room Floor, the European version's easier mode is called "Zero" and only allows you to complete the first 4 levels.

As for the mechanical differences, it's not entirely clear. The only detail I could find is in the submission comments of the 2004-07-30 speedrun of the game by 'marshmallow', found here as of the time of this answer, which remark,

I first attempted a BH run in "Hero" mode but soon abandoned the idea as the challenge in Java became too great - I had to slow down significantly just to stay alive. Also, the rate of harvesting seemed faster and I found myself facing mutants much earlier than in normal (which the game calls easy - how do you have hard and easy without a normal?).