Buds_gardens2012,
Ants could well be the problem but they can't cut through the skin so something else such as birds would have had to cut the skin first but as soon as its open anything could munch at it (ants). Straw is agood idea to raise the bed and it stops slugs and snails (they might be the problem). Also to stop birds you could put a net down with small holes (so the birds feet don't get trapped in the net as well as it stop birds actually still getting in the net). For the ants you could apply ant powder down and refresh every now and then.
I hope you've found this useful!