impact for the well-being of the tribe.

At the beginning of Chapter Two in Lord of the Flies, Ralph calls a tribal meeting with all of the boys to discuss their new-found knowledge that they are stranded on "an uninhabited island with no other people on it" (32).  

Jack relishes the idea and immediately introduces the idea of "an army--for hunting" (32).  When Simon, Ralph, and Jack explored the island together earlier, they ran across a trail in the jungle made by pigs, and then Jack caught a small piglet, but was unable to cut its throat.  Refusing to let himself by deterred by his original failure, Jack vows to have more success next time; interestingly enough, Jack characterizes his failure with the piglet to the other boys as an accident, blaming the fact that the pig wiggled away and escaped on the fact that it was squealing so much.  Jack does not reveal the true reason, which was his initial aversion to the thought of blood, because he does not wish to appear weak to the other boys.

Throughout the novel, Jack is portrayed as an aggressive individual, who is also a successful hunter and continually challenges Ralph's leadership. Unlike Ralph, Jack values hunting and swimming, as opposed to maintaining the signal fire and building shelters. Jack represents humanity's primitive nature and rejects civility by painting his face and acting like a savage.

The majority of the boys are attracted to Jack's uninhibited lifestyle and enjoy hunting with him. As the novel progresses, they begin to view Jack as their leader and reject Ralph's authority. Jack's overwhelming bravado and masculinity provide a feeling of security for his followers. During hunting expeditions, the boys wear face paint and work as a group to kill pigs. They also follow Jack as he performs various rituals, which excite his hunters and develop their sense of loyalty towards him.

In chapter 8, Jack argues with Ralph and attempts to usurp power by holding an assembly meeting. Jack views himself as a better leader and gives the boys various reasons to vote him chief by telling them,

"[Ralph's] not a hunter. He'd never have got us meat. He isn't a prefect and we don't know anything about him. He just gives orders and expects people to obey for nothing." (Golding, 99)

Although the boys do not initially vote Jack chief, they do follow him and join his tribe while they are out collecting wood for another signal fire.

In chapter 9, Jack and his hunters kill a pig and hold a feast. While everyone is eating, Jack looks towards Piggy, Ralph, and Samneric and asks,

"Who's going to join my tribe? . . . I gave you food . . . and my hunters will protect you from the beast. Who will join my tribe?" (116)

Jack's comments reveal why the boys look towards him for protection. They view him as an experienced, fearless hunter, who can provide for them and protect each of them from the beast. The feeling of unity and protection the savages experience when they join Jack's tribe increases each member's confidence until they no longer feel threatened. Jack also manipulates his followers by performing rituals like leaving the pig's head for the beast. 

I think Jack's key contribution to the survival of the boys is to provide food, though you could maybe also argue that he is the one who best understands the need for some sort of protection. Here's Golding's panoramic shot of Jack as chief:

A fire burned on the rock and fat dripped from the roasting pigmeat into the invisible flames. All the boys of the island, except Piggy, Ralph, Simon, and the two tending the pig, were grouped on the turf. They were laughing, singing, lying, squatting, or standing on the grass, holding food in their hands. But to judge by the greasy faces, the meat eating was almost done; and some held coconut shells in their hands and were drinking from them. Before the party had started a great log had been dragged into the center of the lawn and Jack, painted and garlanded, sat there like an idol. There were piles of meat on green leaves near him, and fruit, and coconut shells full of drink.

The constant diet of fruit gives the littluns diarrhoea, and stomach aches. Jack is the one responsible for giving the meat and protein (which undoubtedly is part of the reason the boys don't get seriously ill on the island!). Jack provides food.

Yet also, Jack's hunting and hunters, his emphasis on a violent "army" under his control, makes the boys feel less vulnerable to an attack from the (anyway imaginary) beast. Jack is much better than Ralph at keeping security and a creating threat of violence.

Jack is a boy of action who wants to do things:  "We hunt!"  He and his group are baser than Piggy and Ralph; they are not the thinkers of the group because they are concerned only with primal needs, they do not consider maintaining the fire in the abstract hope of being rescued.  After he has regressed to the savage level, Jack does not seem to be concerned about being rescued.  Instead, his energies are put into asserting his power, becoming cruel and destructive as he crushes Piggy's glasses and shouts at Ralph, "Bullocks to the rules!"

As he becomes more powerful, Jack embraces the "beast" within him, paints his body, and regresses further.  He is more concerned about the control he wields by manipulating the boys' fear than he is in providing for the boys' survival as his hunters kill Simon and Piggy. (Of course, he does not want the boys under his control to die as he must use them. 

When the officer arrives on the island, Jack is thus depicted:

A little boy who wore the remains of an extraordinary black cap on his red hair and who carried the remains of a pair of spectacles at his waist, started forward, then changed his mind and stood still

For Jack there probably is no rescue; he has degenerated too far.