I've also got major ants nests at the allotment, they nest near the edge of my beds and make me do a silly dance when they get dugup (so they don't crawl into my wellies). Apart from that they seem to churn up the soil pretty well; I did blame them for eating crops for a while but watching them more closely it appears they are more interested in live creatures below the soil. I do expose their eggs if I hit a nest, otherwise I let them be.
