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 70% is passing

by Phyllis Griffard, Education and Outreach

You might be surprised, but none of us in ANPP, not even The Nature Dude himself, has a garden that is 100% native. On our 7 acres in Sunset, Pete and I have planted Ruellia, Ligularia, bottlebrush, Lantana, tropical milkweed (just a little) and Crocosmia in specific beds for quick fill when we moved in and as showy supplements for pollinators. We also have plenty of rambunctious natives (poison ivy, dewberry, water oak) and invasive non-natives (Chinese privet and tallow, Japanese honeysuckle and climbing fern) that we try to keep under control, especially close to the house. Yes, the more native and diverse, the better. But how native does your garden have to be to function optimally as habitat? Narango, Tallamy and Marra have done the experiments and number crunching that help to answer that question: 70%, easy to remember as a passing grade in a college class. But unlike an exam grade, life actually does depend on passing this test.Let's break that down. It's not 70% of the plants or 70% of the species, but the amount of biomass in an area that is native. Biomass means just what it sounds like, the amount of plant material in weight. Of course you can't dig up all the trees, grasses and forbs and weigh it on a scale. So scientists have developed methods for estimating biomass in a given area.  Dr. Narango spoke recently at Baton Rouge Audubon Society's meeting, where she explained how one might estimate biomass. Watch her talk here. Check out their graph below, which is about chickadees as a representative bird species. Notice that any yard with only half the biomass in native plants was not able to keep the chickadee population constant over time. Half isn't good enough to fend off decline. Loss of insects due to insufficient native plants is probably why we have seen such dramatic reductions in bird populations in just a few decades. There just isn't enough for their insect prey to eat in yards that are less than 70% native in biomass.

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