No, eclipses aren't harmful to animals or plants (unless the animal gets scared and runs off a cliff!). It's only as dark as twilight - not very dark really. Just dark enough to see the brightest stars and planets with a "sunset" all around you. I have been to eight total eclipses, and have seen animals react in several of them. The most interesting reaction was when we were on a ship in the Caribbean. The dolphins came up and looked around. You could almost hear them thinking "What on earth is happening?". Other times birds start to roost and sing their evening songs. The darkest part of the eclipse (totality) is just a few minutes, not long enough for anything bad to happen to a plant. Just enjoy the view!!