Beginner Gardening: Are these Aphids?, 1 by Morph

Morph wrote:
I didn't really know where else to put this question, so I thought I'd ask here. I trimmed back some growth from the bottom of my birch trees. I laid the brush on the deck and yesterday my DH notices these green tiny bugs all over the trimmings!

I'm attaching the photo and I hope you can see these critters. If they are aphids, I'm probably in big trouble. How could there possibly be so many? This is just ONE small piece of a huge pile of trimmings!