As I said, with this colony I've been the one giving them the seeds. However, I've witnessed other colonies in my yard showing up to damaged Trillium seedpods and making off with them too, so it does happen naturally. For the purpose of taking photos though it's so much easier to just shoving the seed in the ant's mouth and saying "Hold This!" so I can get a shot or two of it.
While Myrmecochory is a natural process, it is not a requirement for seeds to be successful. Gardeners can just as well plant the seeds themselves as they would a packet of tomatoes. They take two years to germinate, and the better half of a decade to flower after that, but the planting them on your own is just that easy. In nature though it's to the plant's benefit that these seeds germinate far away because their rhizome (root) is going to expand and send up new shoots to form a clump in a few years, and it's no good having tons of seedlings trying to grow in their way. Actually it's even easier for the gardener to dig up the rhizome either while they're flowering or a few weeks before (Yes, while they're flowering!) and shake or cut the rhizomes apart that way. Each individual rhizomes wants to grow into its own clump anyhow so it's worth it to break them up every few years.
An issue though is it's not an ideal place for Trilliums to grow. It's just too dry in that part of the garden so even if they do flower, I'll have to work hard to keep them watered or they'll go dormant before setting seed. This is likely why I haven't seen any seedlings growing until now. Another thought is the soil might not have been right. We were always in the habit of removing leaf litter from the gardens, but in recent years I've started leaving the leaf litter where it falls, sometimes even raking it off the paths and right into the garden for extra winter coverage. In theory this should help build the soil as the leaves break down, and the new leaves added add insulation for the plants below.
