There’s a corner up the back of my zone 4 garden with soil like the rest of our area, but worse. Sandy, rainshadowed in winter, heat-burned in summer, hydrophobic off the scale. No sign of any previous soil conditioning, though I know there must have been some – there’s a dead citrus tree in the middle which must have got to at least twenty years old before carking it, and an equally old but healthy grape vine growing over the skeleton. Even the weeds grow spindly and tough. I planted some garlic there in midwinter when the soil profile was starting to moisten a bit, and left it. This week I’ve been clearing the weeds and doing soil prep with kaolin and manure before mulching around the garlic that made it through. I stood there with a watering can sprinkling water around and watching it trickle across the surface without absorbing, pooling in any little accidental depression left from weeding, and thought of bandicoots.
The soil here is notorious for its poverty – Bassendean sands have been named by the UN as one of the worst soils in the world. But when you explore the local bushland, you wouldn’t realise that. Plants grow in abundance, thickly. My zone 1 garden is an extension of what might have been here before clearing, with banksia trees and low grass-like undergrowth packed in and around. Banksia trees are tricky in gardens – they die if you fertilise them. They have a proteoid root system that is so efficient at sucking up nutrients that they can literally eat themselves to death. It’s a shallow but intensive root system, and it doesn’t like being dug around. So when I unwittingly mulched them with a fast-breaking-down and fast-compacting mulch last year, I had a problem. That compacted ex-mulch soil needed to be dug through and blended with the sand below without compromising the root system. Pretty much the only way to do it was carefully, by hand, section by section, with a small digging fork. I have spent much time this month cursing myself as I dig.
But I’ve also spent the time thinking about banksia’s natural habitat. Which is what my tracker/survivalist friends call “low debris”. There’s not much on the ground that you’d think of as mulch, and what there is doesn’t break down fast at all. I’ve heard bushwalkers mutter “nothing but sand and prickles”, and that sums it up. The soil doesn’t dry out completely though. The top layer of sand becomes hydrophobic in the spring and summer sun, forming a protective barrier against moisture loss during the long dry. That hydrophobia’s broken down by the gentle autumn sprinkly showers, too light to make real puddles. Instead the droplets gather in any tiny depression and work their way in, breaking the hydrophobia from the side as well as the top. By the time of the real winter rains, the soil is ready to accept water again.
That makes little depressions very important. What out there in the wild world makes little holes the size and depth of my digging fork? What could make a constant tiny turnover of soil placed widely enough to not hurt the roots of the banksia woodlands that cover Perth?
Bandicoots.
Bandicoots have a long pointy nose that they use to sniff out any tasty treat they can find – insects, buried grubs, fungus, worms, roots – and they dig little holes around the place that are just the right size and shape to get that nose into.
So I went looking to see if I could find any actual research on the ways bandicoots affect their environment. And, score, I found some released within the last year from the Centre of Excellence for Climate Change and Forest and Woodland Health at Murdoch University.
Background/question/methods: Mammals that forage for food by biopedturbation can alter the biotic and abiotic characteristics of their habitat, potentially influencing ecosystem structure and function. Bandicoots, bilbies, bettongs and potoroos are the primary digging mammals in Australia, although the majority of these species have declined throughout their range. Our study examined the foraging activity of the southern brown bandicoot, a persisting digging Australian mammal. The amount of soil displaced and physical structure of foraging pits were examined from moulds of fresh foraging pits. We recorded soil water repellency and soil moisture levels along the foraging pit profile and adjacent undug soil. We also examined seedling germination in artificially dug and undug sites.
Results/conclusions: An individual southern brown bandicoot created 45 new foraging pits per night, and could displace approximately 3.4 tonnes of soil each year. Soil water repellency was highest on undug earth, and varied throughout the profile of a foraging pit. Soil moisture was greatest along the slope of the foraging pit. Seedling germination of native species was highest in artificially dug sites compared to undug controls. Southern brown bandicoots can displace substantial amounts of soil and their foraging pits create a microhabitat that facilitates higher germination of native plant species. The digging activities of this species are likely to contribute towards ecosystem processes, and the persistence of bandicoots may play an important role in maintaining the health and function of our woodlands and forests.
Whee, “biopedturbation”. Jargon for “animals dug it up with their feet”.
But seriously, note some key points here. First, my guess about the efficacy of little holes for water penetration is quite accurate – they tested it. Second, the holes are also acting as a catchment for windblown seeds and similar, so they’re a perfect little microhabitat for germination. That means natural ecosystem resilience. And third, we used to have quite a few small mammals that did this job. Most of them aren’t around as much anymore, but the bandicoots have hung on. That makes them even more important.
The researchers talk more about their work in articles in the Guardian and the ABC. The Centre has a blog covering other work they do.
So I’m not sure how I want to use this new understanding I’ve developed. I know the concept’s not unknown in permaculture – I’ve watched a video of arid-lands rehab that used a “presser” to make patterns of holes that caught water and seeds to naturally revegetate a protective swale. It seems to be an important idea for drylands soil. Forest-like soil/food systems often use chickens for soil turnover, they’re a forest bird, but that’s a higher-fertility more-constant-moisture situation than I’ve got. What I’m creating is a woodland, not a forest. Bandicoots can protect lawns from damage, but I don’t have any lawn. But knowing that the grass-like understorey of our banksia woodlands is important to the bandicoots, I think I will be planting more of it in zone 1. And making sure as always that the cats come inside at night.
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