I should premise that I use this term in a large and metaphorical sense, including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny.

Two canine animals, in a time of dearth, may be truly said to struggle with each other which shall get food and live.

But a plant on the edge of a desert is said to struggle for life against the drought, though more properly it should be said to be dependent on the moisture.