Striving for ant-human mutualism

By Lauren Lewis

The concept of ecosystem services gained recognition and common use among ecologists and policy makers only within the last 20 years or so. Ecosystems services are the benefits that humans collectively receive from the functioning of ecosystems. It’s the natural processes happening around us that we don’t control but that make the world as we find it. There are many examples (trees respiring oxygen, bees pollinating crops, etc), but specifically in an urban environment, the insects around us arguably provide some of the most crucial ecosystem services. They’re barely noticeable, but hugely present.

By numbers, ants are a massive presence in cities and they provide the crucial service of eating our trash. One study estimated that there are 8000 ants for every rat in New York City and 2000 ants for every human. The research team for this estimate is led by Amy Savage, a PhD ecologist who is a leader in the effort to understand non-human urban species and their interactions with each other and with us. Her work in NYC showed that arthropods, mostly ants, carry away and digest as much discarded food as all vertebrates combined (that’s rats, squirrels, etc). Ants are active cleaners of our cities.

In the garden, we’ve long recognized that ants are among the myriad creatures necessary for healthy soil; ants aerate soil by digging and they help break plant material down into smaller pieces for digestion by worms. But a recent study illuminated another fascinating benefit of ants for soil: the “aggregate mulch” churned up by burrowing carpenter ants can reduce evaporation from the soil surface. The burrowing action of these ants leaves soil clumps with a particular texture behind them. Greater thickness of this aggregate mulch layer was associated with lower soil evaporation rate, particularly in the hours right around noon, when evaporation is highest. At Small Spot Gardens we are big advocates of mulch for its ability to reduce water needs, so now we know that supporting ant populations could be right in line with that goal.

Various ant species behave in widely divergent ways, so supporting ant populations can mean a lot of different things, but overall, there’s a consistent theme of leaving some wildness for ants to use. For epigeic ants, those who inhabit only the top layer of soil rather than burrowing and nest in existing cavities, presence of appropriate nesting materials is crucial. One study of epigeic ants and their nesting preferences found that ants showed a preference for certain cavity types (“large” 6mm wide hollow twigs) and that all nest types were more abundant in forest sites than in vacant lots or gardens. The findings suggest that efforts to mimic a forest environment (with leaf litter on the ground!) can support ant life.

The Bay Area is home to over 100 species of ants, but unfortunately the introduced Argentine Ant has driven a lot of them away by outcompeting them here and across California. The threatening presence of the Argentine Ant is all the more reason to see your garden space as a potential haven for other ant species. The plant litter you decide to leave behind instead of transferring to the green bin might mean home for a native ant colony.

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