Let’s explore what we can learn from “indicator” plant species… the first one I would love to discuss is yarrow, Achillea millefolium. Common yarrow is growing in abundance in my yard - a full sun exposure site near a wetland. The area is horribly disturbed, full of invasive species, and the soil is full of gravel which was presumably added for the purpose of stabilizing a building site in a spot that never should have built upon. What can yarrow teach us about the needs and current condition of land and plant communities? 
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Yeah! Yarrow seems to grow in areas that have been damaged, where the body of the Earth needs general healing much like this plant ally heals the bodies of human animals (and likely other animals as well but I’ve never heard much about this.) Your yard sounds like one such place, a site that is sunny with less than optimal soil conditions. In the Cascadian cities I’ve lived in it does seem to like lots of sunlight, and areas that have been disturbed like construction sites or the edges of streets where grass meets the forest.
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I tend to think of indicator species in the context of indicator species analysis (ISA). ISA is a fairly simple statistical technique to determine which species tend to occur in which habitat types. One step in the process of determining “who lives with whom”. I would think yarrow would end up being a generalist in grassland conditions. If I were to come up with a hypothesis about why yarrow is growing where it is, I would look first at soil nutrients. Perhaps the disturbance on your site has opened up a niche in which shade-intolerant species can thrive, but the often N-limited invasive species can’t compete due to low soil fertility. It would be interesting to take soil samples at various points and make a note about the species composition at each point.
Just a random guess!
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