Seedlings grow bigger in the spoils of quenda-dug soils
By Leonie Valentine, Katinka Ruthrof & Richard Hobbs*
Australia has many digging mammals, such as bettongs and bandicoots, who substantially disrupt and modify the soil and litter layer by creating foraging pits when searching for subterranean food. As they dig they create shallow pits with an associated spoil heap of the ejected soil (Figure 1). The combination of digging and discarding soil disrupts the microhabitat layer by exposing soil at the digging site, and burying organic matter and litter under the spoil heap. Although these actions might seem small at a local scale, they may be surprisingly important for broader-scale, landscape processes, influencing soil turnover, water infiltration and nutrient cycling.
The quenda or southern brown bandicoot (Isoodon fusciventer) is a medium-sized marsupial endemic to south-western Australia. It forages for underground invertebrates, fungi and tubers. Each night they create around 45 foraging pits as they dig for food. It’s been estimated an individual bandicoot is capable of turning over nearly 4 tonnes of soil each year. The digging actions of these ecosystem engineers have been shown to change soil moisture and hydrophobicity, alter the surface litter composition and influence seedling recruitment at a local scale (Valentine et al, 2017). Quenda have also been shown to transfer fungi via their scats to seedling roots. Given this, the foraging activity of quenda could play a significant role in changes to the chemical and biological characteristics of soils, and we wanted to investigate how these changes might influence seedling growth. That is, do quenda influence the growth of seedlings, and, if so, how?
In our recent study (Valentine et al, 2018), we collected soil from the base of 20 recently dug foraging pits, the associated spoil heaps and adjacent undisturbed (undug) soil, and analysed the soils for nutrients (phosphorus, potassium, sulphur, organic carbon and conductivity) and microbial activity. Soil cores were collected from the same locations, transferred to pots and seeds of the local dominant canopy species, tuart (Eucalyptus gomphocephala), were added to the soil under glasshouse conditions. Seedling growth (including height and maximum growth) was measured over a four‐month period, with other measurements (stem width, shoot and root biomass) examined upon harvesting. Fungal colonisation rates of seedling roots (by arbuscular mycorrhizae, noted here as AM fungi) were also examined (native plants often depend on these colonising fungi for survival).
Results from our study showed that soil from the spoil heaps had the greatest levels of electrical conductivity and potassium. Both the spoil and undisturbed soil had greater amounts of microbial activity and organic carbon. In contrast, the pits had lowest levels of nutrients and microbial activity. Seedlings grown in spoil soil grew quicker than other seedlings and upon harvesting were taller, heavier, with thicker stems and a bigger root biomass than seedlings in the pit or undug soil. Colonisation with AM fungi was greatest for seedlings grown in pit soil. The best predictors of seedling growth were greater amounts of potassium, electrical conductivity and microbial activity. The best predictor of higher colonisation rates of AM fungi was less phosphorus.
Our research showed that bioturbation by ecosystem engineers, such as quenda, can alter soil nutrients and microbial activity, and subsequently facilitate seedling growth.
Why is seedling growth better? We believe that foraging by quenda creates an environment conducive for litter decomposition in the spoil heap. This subsequently returns nutrients to the soil. These extra nutrients facilitate seedling growth. While foraging for food, digging mammals such as the quenda, manipulate and alter many soil properties and these microscale disturbances may be incredibly important for ecosystem functioning.
The majority of Australian digging mammals are threatened, with many suffering substantial population declines and range contraction due to predation by foxes and cats, and clearing of habitat. The widespread decline of Australian digging marsupials is likely to be linked to a reduction in key ecosystem processes.
We believe the persistence of our native digging mammals in landscapes plays an important role in maintaining the health and functioning of ecosystems. And the evidence dug up in this investigation underscores how important that role can be.
Will any digger do?
When giving presentations on our native diggers we are often asked: What about rabbits? They dig. Surely they provide the same range of ecosystem services (and they’re not going extinct)? It’s a good question and in their native range rabbits (and hares) are considered ecosystem engineers. But rabbits are not native to Australia and the little research that has been done on this suggests they don’t replace the function of native digging marsupials.
Part of the reason is that rabbits are herbivorous as well as diggers. A lot of our bandicoots and bettongs are omnivorous, so aren’t always eating all the greenery whereas rabbits tend to eat everything. We also suspect that rabbits dig differently to bandicoots – they are likely to dig less and the pits are often visibly shallower. And native digging marsupials don’t just dig, they also move fungi about (which rabbits don’t we suspect).
So a dig is not just a dig, and any old digger isn’t necessarily enough, it’s got to be an Aussie digger.
Having said that, not much research has been done on this question so there’s much we have to dig up before we can give a definitive answer.
More info: Leonie Valentine firstname.lastname@example.org
*Leonie Valentine and Richard Hobbs are from the University of Western Australia. Katinka Ruthrof is from Kings Park Science, WA Department of Biodiversity, Conservation and Attractions.
Valentine LE, M Bretz, KX Ruthrof, G Hardy & PA Fleming PA (2017). Scratching beneath the surface: bandicoot bioturbation contributes to ecosystem processes. Austral Ecology 42: 265-276.
Valentine LE, KX Ruthrof, R Fisher, G Hardy, RJ Hobbs & PA Fleming (2018). Bioturbation by bandicoots facilitates seedling growth by altering soil properties. Functional Ecology Doi:10.1111/1365-2435.13179.