This population of salmon was able to withstand very warm river water, because Hells Canyon gets hot. No other species of salmon is known that has this tolerance for warm water. Then a dam was built that blocked the spawning migration of the Hells Canyon salmon, and the warm water tolerance was lost. If global warming occurs or trees are cut along a river bank, any river might warm up. At that time, salmon that aren't able to tolerate the warmer water will die.
Diversity also means that there is a greater chance of new special abilities developing.
And diversity is also important because all those differences are just plain interesting. Other people sometimes think Darwin meant that only those who can kill and use and dominate others will survive. But that kind of thinking reduces the diversity that we just saw was so important. Furthermore, people who think they can kill and dominate nature often destroy people as well. So, deep ecologists believe in diversity.