Biodiversity, hands down. I was formally introduced to biodiversity concepts in junior college, but I then realized that my mother has always practiced biodiversity in her gardening practices. She chooses each plant individually to grow harmoniously with the other varieties of her garden. She has a small orchard of varieties we had to order from Seed-Savers Exchange, and Trees of Antiquity, LLC., (Apples - brown sweet, black twig, crimson gold; Pear - Comice, Ornamental (beneficial for honey bees); Black Walnut (15 years old); Italian Prune Tree (50 years old); and a Rainier Cherry). We also cultivated 7 varieties of rhodedendron, 12 varieties of roses, sword fern, northwest blackberry, 4 camilias, two rows each of raspberry, loganberry, salal berry and tundra blueberry plants, maintain nine old-growth trees on our family property including douglas fir, red ceder, mountain ash/rowan, madrone, and one 100+ year old Rhododendron. In terms of the wildlife we support, we have 4 hives of local honey bees, as well as regular attendance by mason bees, wild honey bees (the ginger colored ones), bumble bees, dragonflies, 24 varieties of small birds annually, and a small community of ducks, one blue heron, 3 band-tail pigeons, a recorded dove in 2019, and two sets of medium size raccoons.
To me the lessons of biodiversity come down to this "it's important to be different, because we all provide something new to the world around us" or by being different we effect our ecosystem in unique ways that other species cannot always imitate. If we can accept that everything has a valuable contribution to the health of our planet, and also understand the amount of change and harm our own species has inflicted, how could biodiversity not be a huge priority?