what is the meaning of the marked sentence

I carry guide books with me when I hike, to identify flora and fauna that catch my eye. Once identified, their names escape from me in an instant, like the names of strangers at a crowded party. Over the last year, I have learned the dogtoothed violet, the serviceberry bush, and the false morel mushroom — only a tiny percentage of all the specimens that I have looked up. Although I refer to a conifer guide when I'm cross-country skiing, I am still not trustworthy on the difference between a spruce and a fir. (Now I remember — a fir has short, flat needles, and a spruce has short, pointy needles that aren't as flat. I think.) But let the smallest piece of commercial-packaging trash appear along the trail, and I can give you the species, genus, and phylum every time. That fan of reflected light, for example, flickering stroboscopically in the rippling current of the creek, comes from a flattened part of a beer can on the creek bottom, a beer can that even at this distance I can identify as belonging to the genus Budweiser and the species Bud Dry.

That's the hard part: living with the realization that we have junk-filled brains. <Much of the litter we bring with us into the wilderness is of the mental variety; past a certain point, our minds really cannot grasp places that are completely trash-free.> The Fanta Grape soda can drawing bees in the middle of a supposedly pristine wilderness campsite provokes our outrage and disgust, of course. But underneath those feelings, and less comfortable to admit, is a small amount of recognition and even relief. The Fanta can is us, after all.