The narrator does a great job of telling us the protagonist's strengths early on:
Leiningen had always known how to grapple with life. Even here, in this Brazilian wilderness, his brain had triumphed over every difficulty...First he had vanquished primal forces by cunning and organization, then he had enlisted the resources of modern science.
These past successes have given Leiningen an unshakable self assurance that is rare in the average person.
We see the plantation owner prepare for the approaching ant colony with multiple levels of defense (from flooded ditches to petrol sprinklers to flaming motes) using man power and the technology available to him. Leiningen is also very skilled in handling his workers. Early on, simply his air of calm wisdom causes them to follow his lead, when their instincts would normally tell them to run. When the ants continue to encroach upon the house and outbuildings, so that the workers appear ready to abandon him, Leiningen cleverly uses reverse psychology to keep them there:
"Well, lads," he began, "we've lost the first round. But we'll smash the beggars yet, don't you worry. Anyone who thinks otherwise can draw his pay here and now and push off. There are rafts enough...and plenty of time still to reach them." Not a man stirred.
He knows none of them would dare lose face by quitting in front of the rest. And so Leiningen convinces his workers to continue risking their lives, even though none of them owns a penny of what they defend.
However, before we judge him for this, consider the ultimate strength with which he saves his ranch. When all else fails and the ranch is overcome with man-eating ants, Leiningen's stubborn perseverance will not let him lose. With nothing more than petrol-soak rags covering himself, he personally runs the considerable distance to the weir in order to open the river's flood gate, flooding his entire ranch and sweeping the ants away. At one point he is down, being eaten nearly to the bone, but he reasons that "He couldn't die like that! And something outside him seemed to drag him to his feet." True to his stubborn nature, he succeeds in saving the ranch and his workers, then recovers from near death to rebuild his property.