Why is acid rain harmful?
It changes the ph of both terrestrial and aquatic ecosystems, thereby putting them at risk of long-term survival.
Animals and plants evolve and adapt to certain ecological niches, including factors like; temperatures, moisture, chemicals and ph or acidic/basic conditions. When these environment conditions are unchanging, the organism thrive and do well.
However, when one or more of these variable start to change quickly (like say ph from acid rain or global climate change), the organisms struggle to stay alive, reproduce and pass on their genes to the next generation. Their numbers will probably drop and they may face extinction if they can't adapt quickly enough.
So, for example, a deer in a forest that is undergoing acidification might be able to move to a more pristine forest not affected by acid rain. But for a fish in a small lake, or worse, a pine tree that has no legs to move, both of these organisms have limited ability to adapt to acid rain. Death is the most likely outcome.