Richard Parker, the adult Bengal tiger who ended up in the lifeboat with Pi, helped keep Pi Patel alive in many ways in Yann Martel's novel Life Of Pi.
One way Richard Parker contributed to Pi's survival is by ridding the boat of another predator. The tiger is responsible for killing the hyena. The hyena is a predator who surely would have killed Pi had the tiger not gotten to him first. In point of fact, the only reason Pi is not in danger from Richard Parker at the beginning is that of the tiger's seasickness. In chapter 53, Pi describes the demise of the hyena at the hands of Richard Parker:
"Richard Parker had risen and emerged. He was not fifteen feet from me. Oh the size of him! The hyena's end had come, and mine . . . the flame-coloured carnivore emerged from beneath the tarpualin and made for the hyena. The hyena was leaning against the stern bench, behind the zebra's carcass, transfixed. It did not put up a fight. Instead, it shrank to the floor, lifting a forepaw in a futile gesture of defence. The look on its face was of terror. A massive paw landed on its shoulders. Richard Parker's jaws closed on the side of the hyena's neck."
Another way he keeps Pi alive is due to the threat of his presence. Pi understands he must master Richard Parker or he will become prey for him. This keeps him on alert and keeps his mind busy. He can't afford to make careless mistakes or to become lazy with the threat of the mighty predator on board. From chapter 57:
"It is Richard Parker who calmed me down. It is the irony of this story that the one who scared me witless to start with was the very same who brought be peace, purpose, and I daresay, even wholeness."
A third way he keeps Pi alive is by his actions on the meerkat island. Pi thought he'd arrived at a type of place that could sustain his needs inevitably. He noticed that Richard Parker returned to the lifeboat every night, which made him curious to find out why. It's Richard Parker's actions that help him put the pieces of the puzzle together and discover that the island is carnivorous.
"At night, by some chemical process unknown to me but obviously inhibited by sunlight, the predatory algae turned highly acidic and the ponds became vats of acid that digested the fish. This was why Richard Parker returned to the boat every night."
And lastly, Richard Parker helps keep Pi alive through companionship. The next step up from physiological and safety needs in Maslow's Hierarchy of Needs is the need for belonging and love. Although Richard Parker never developed love or relationship with Pi, Pi does see Richard Parker as a companion after he gets over his fear of him. This is evidenced in his lament that the tiger didn't even turn to look at him when they land in Mexico. Pi felt that they had developed an understanding and camaraderie toward each other. Even if it was one-sided, this feeling of having a type of relationship would have helped Pi combat extreme loneliness and help to give him a will to survive. From chapter 94:
"At the edge of the jungle, he stopped. I was certain he would turn my way. He would look at me. He would flatten his ears. He would growl. In some such way, he would conclude our relationship. He did nothing of the sort."