From what I understand, in the wild they forage quite a bit and pick up whatever small bugs, plants, flowers, etc that they can find. They can go for quite a while without food if needed and can conserve energy by sitting and waiting for bugs to come near them. The variety of different bugs and what those bugs gutload on provides them with the vitamins and nutrients they need. The common feeders we offer in captivity don't come close to that which is why supplements are needed. The best thing you can do in captivity in my opinion is to provide as much variety in terms of both bugs and salads as you can. Make them work for the food a little bit and change up the offerings to keep them on their toes a bit. Adding some interactivity such as displayed in this video (and similar things you can think of) may be helpful for trying to simulate feeding in the wild and be good for overall mental and physical health https://www.facebook.com/bloodbankdragons/videos/1133664499992638/
Interestingly, termites are also associated with changing the soil composition at their mounds, raising nutrient levels including calcium.
"Lee and Wood (1971) in their investigation in Australia found that exchangeable Ca, Mg, K were generally high in termite modified soils than unmodified samples." http://scialert.net/fulltext/?doi=ijss.2007.1.14
I have another one somewhere about coastal bearded dragons.... I think they had some vertebrates as well, can't find the book mark now though I suspect they eat most things they can get in their mouth.
I don't have any other links, but my friend who lives in Cairnes used to go nearby to a park where there were a lot of wild barbatas (eastern or coastals) & they ate grubs quite often. I have a few pictures stored of him holding one while eating a large grub.
Grubs also live in the ground to where they may have higher calcium & other minerals in them.