We human beings control all the other animals and all the plants. Whether the future of the world is bright depends on us human beings. Then could we say 'We human beings are decisive for the future of the world'?
'decisive' means producing a definite result as in, "England had a decisive win over Botswanaland in the World Cup, 10-1."
• (of a person) having or showing the ability to make decisions quickly and effectively.
What is meant in your sentence is that our actions now will eventually decide the continued existence of species of animals and plants, and even whether this planet is fit to live on.
You can* use the expression 'decisive for', but it usually applies to an action or a fact rather than a person. And some users might feel that the word 'decisive' should only be used when a decision is involved: We don't know whether the Earth will continue to support carbon-based life forms for 10 years or another 10,000, but we do know that the actions of mankind will be decisive for that survival.
*PS I thought I was the first respondent; this 'can' is not directed at you, David.
Last edited by BobK; 20-Mar-2008 at 14:46.
Reason: PS added