Here is a key sentence from Marx' Das Kapital (vol. 1): “Der Mensch tritt dem Naturstoff selbst als eine Naturmacht gegenüber."
Nature is not a thing, but a process. It is constituted by forces, ‘Naturmächte.’ Nature is a complex network of productive forces. The river produces erosion in rock, the fusion of hydrogen into helium produces what we experience as the sun, when a drop of water that hangs on a tap grows too big for the surface tension of the water, then the the drop falls … Next are the plants and animals. Trees reproduce themselves and parts of their environment, like oxygon for example.
Then there is man. He too reproduces himself; he procreates. And he has an impact on his environment, produces and reproduces it. Mankind is a productive force embedded within the rich segmented variety of productive forces which we call nature or the natural world.
The notion of production is central to marxism. It is not typical for capitalist society; it characterizes all societies. It not only defines man, but natural life and even nature in the broadest sense: the material world. As Leo Apostel clearly saw: the concept of production defines the concept of causality.