Walcot finds a "brutish necessity" in nature that he compares to man's inhumanity to other men. Walcot writes "The gorilla wrestles with the superman." In other words, man's most primitive instincts, his inner "gorilla," is driven by the same natural bestiality as his seemingly sophisticated instincts, such as the striving to be a Nietzschean superman: both lead to death and destruction.
The savagery that Walcot finds in nature in, for instance, the "bloodstreams of the veldt" and the "violence of beast on beast" play out in humankind as well in the bloody colonial situation Walcot has experienced in Africa but also in the persecution of the Jews or the conflict of Spanish Civil War. We humans call our bloodlust "courage" and try to sanctify it as seeking "divinity," but it is merely the same violence that drives the beasts in nature.
This is a very different view of nature and humankind than, say, the Romantic poets believed in. Nature is not a gentle, healing force that renews the soul, and humans are not innately good at heart. In this poem, nature has a Hobbesian savagery and man, as simply another animal, is just as savage as the nature from which he springs.