Apologies in advance, this entry isn’t as tightly edited as I prefer but I’m still swamped.
The American Chestnut. Once three billion chestnuts grew in the Eastern US. Reading the wikipedia page today the discovery of individual trees that have survived long enough to flower is worthy of report. The culprit, a bark fungus introduced from Asia. Asian Chestnuts can survive the infection, but American ones had never had the evolutionary pressure to develop ways to survive. Conceptually it’s the same as when people who’ve lived for many generations without exposure are exposed to smallpox, (although if it came down to it, I’d choose losing three billion trees in a second over the millions of people killed by smallpox).