Can you explain why ecosystems are both strong and fragile?

The ecosystems of our planet have undergone countless changes and adaptations since the beginning of life. They have prevailed and thereby give the impression that they are 'strong,' or maybe better said, 'resilient.' But even though ecosystems have a tremendous capacity to recover, they are not infallible. Clearly with global warming we are seeing that our ecosystems are, indeed, fragile and if we don't start acting out of the long-term health for our planet, we will cause irreparable damage.
I'm optimistic and very concerned at the same time. In my parents' home there is a picture of my dad on the Matterhorn (in Switzerland) in 1973 and one of my brother on the same mountain in 2006. Both pictures were taken during the summertime. The one of my dad shows substantial snow and greenery all over the mountain. There is nothing of the kind in my brother's picture; just a big, brown hill. In just over thirty years, the change of that ecosystem is dramatic. Sure, it's surviving, but clearly fragile. We need to get all of our energy behind saving the environment, or else my children won't have mountains to climb!