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Digging by the woylie Bettongia penicillata (Marsupialia) and its effects upon soil and landscape characteristics in a Western Australian woodland

Garkaklis, Mark Jurids (2001) Digging by the woylie Bettongia penicillata (Marsupialia) and its effects upon soil and landscape characteristics in a Western Australian woodland. PhD thesis, Murdoch University.

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Until recently the potoroid rat kangaroo Bettongia penicillata (the woylie), once common and abundant across the southern third of Australia, was threatened with extinction, and restricted to three small populations in the southwest of Western Australia. Feral predator control and habitat management have seen a recovery in the populations of the woylie. This has provided an opportunity to study some of the functional relationships between this mycophageous marsupial and the environment it inhabits. The woylie feeds predominantly on the hypogeous faiiting bodies of ectomycorrhizal fungi, making diggings that disturb the soil surface. This study was the first to examine the impact of this digging activity (biopedturbation), on the edaphic and floristic structure of the ecosystems inhabited by woylies.

An open population of 20 - 49 woylies was studied in a 70 ha area of the Dryandra Woodland, approximately 200 km to the southeast of Perth. The total number of woylie diggings estimated in the study site ranged from 5 000 ha^-1 year^-1 in April 1995 to 16 000 ha^-1 year^-1 in April 1996. This corresponds to a digging rate of between 38 to 115 diggings woylie^-1 night^-1, and an average soil turnover of approximately 6 tonnes woylie^-1 year^-1. No seasonal pattern in woylie digging activity was apparent.

The decay process in woylie diggings was examined using simulated diggings. The average period of decay for diggings less than 60 mm in depth (mean depth = 45.0 ± 1.5 mm) was 29.5 ± 3.1 weeks. The average period of decay for diggings equal to or greater than 60 mm (mean depth 75.9 ± 3.1 mm) was 79 ± 10 weeks. There was a significant relationship between the initial depth of the digging and the period of decay (r^2 = 0.534, p < 0.001). Loose coarse organic material was found in all diggings before they were completely filled-in.

Soils in the study site were found to be water repellent, usually in the top 1 cm of the soil, and as woylies forage for hypogeous fungi they disturb this surface layer. In situ measurements showed the undisturbed woodland soil surface was severely water repellent, whereas diggings had low water repellency and acted as preferential water infiltration paths after autumn rainfall events. In simulated diggings the low water repellence of the soil disappeared after approximately 2 years and organic material accumulated in the diggings, resulting in the formation of sub-surface water repellency at the base of the filled-in digging. In 25% of simulated diggings this buried organic material was invaded by masses of fungal hyphae which contributed significantly to the sub-surface water repellence. Thus, soil water repellency in the southwest of Western Australia is spatially and temporally heterogeneous and this heterogeneity is caused by digging animals.

The distribution of soil nutrients, soil bulk density and soil particle size were also affected by woylie digging. In simulated diggings, available nitrate, ammonium and sulphur were significantly lower in decayed diggings than in undisturbed soils. Nitrate is susceptible to leaching in preferential water infiltration areas, whilst equilibrium reactions between ammonium and nitrate, and between sulphur and the mobile sulphate, can potentially result in decreases in both of these soil nutrients in water infiltration zones over time.

Soil bulk density decreased significantly in both decayed simulated diggings and also at the larger quadrat scale. Areas excluded from woylie digging had a significantly higher soil bulk density than those subjected to digging, and excluded areas had soil bulk densities similar to woodlands where woylies did not occur. Remnant woodlands where digging animals are extinct may have soil physical properties that are less amenable to plant growth and productivity than where they are present.

Soils in decayed diggings had a lower mean particle size, especially in soils with a high gravel content. The decay process favours the mobilisation of finer particles from the digging spoil which appear to wash back into the digging itself. The result is a surface soil with a heterogeneous particle size distribution. Digging by woylies results in patches of fine soil interspersed with coarse gravel piles on the surface.

Woylie exclusion did not change plant recruitment or plant growth. Established woody perennials are unlikely to be grazed by woylies, which are predominantly mycophageous. Other studies have shown that a reduction in the productivity due to herbivory of plants that have symbiotic relationships with ectomycorrhizae results in a reduction in the productivity of mycorrhizal fungi. However, although this study showed no direct herbivory by woylies upon woody perennials, they were observed digging for the bulbs of hemicryptophytes (family Haemondoraceae) and also caching fruits of Santalum trees. Thus, direct interactions between the woylie and certain woodland species do occur.

The soils of the southwest of Western Australia reflect deep in situ weathering of the laterite profile, indicative of a geologically stable environment. However, this study concludes that, at a smaller scale, southwestern Australian soils are subject to dynamic processes that create heterogenous soils, both spatially and temporally. The driving force behind these processes is the digging fauna. Digging is a significant perturbation in the environment and the changes that occur as a result of biopedturbation affect a number of important soil properties. The management of woodland and forest ecosystems where these fauna are now extinct may not reflect the true nature of these ecosystems and suggests that the maintenance of a diverse vertebrate fauna may be important to them.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Division of Science and Engineering
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Bradley, Stuart and Wooller, Ron
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