Soil disturbance by native mammals and the germination and establishment of plant species

This thesis investigates disturbances created by the digging of small native mammals and their role in the germination and establishment of native plant species in eastern Tasmania, with a particular focus on species considered to be rare or threatened. At least 15 rare and endangered plant species live in habitats where digging appears to be common. Soil disturbances, which are created by bettongs, bandicoots and echidnas, can be prolific, particularly on soils of lower fertility. Turnover rates of up to 2.9% of the ground surface per annum were recorded. Diggings by bettongs, and to a lesser degree by bandicoots, are spatially associated with trees in dry sclerophyll and grassy woodlands and forests. The understorey varies in species composition depending on the degree of tree influence, so not all understorey species are close (at the scale of several metres) to where digging disturbances are most abundant. Digging creates small patches of bare ground which, on sandy soils, can be a harsh environment with low nutrient and moisture availability. However, diggings provide better conditions for the germination of many species, particularly those with small seeds and perhaps also some species which have hard-seeded dormancy. This latter effect may be a result of the considerable soil heating imposed by direct solar radiation on bare soil. Several species had higher densities of seedlings on animal diggings, although mortality rates were generally very high on both digging disturbances and undisturbed areas. Seedlings died faster, at the same rate, or slower on diggings compared with undisturbed areas, depending on the species and the circumstances. Simulated animal disturbances proved to be effective in stimulating germination, and more substantial survival of seedlings was recorded in these experiments than on natural diggings. Many plant species in this study have seed stored in the soil at depth. Digging probably brings buried seed to the surface which would otherwise not germinate. Rare or threatened species which were found to benefit from either natural diggings or simulated disturbances included Lepidium hyssopifolium, L. peudotasman icum, L1siopetalum micranthum, Vittadinia muelleri and Velleia paradoxa. No rare species appeared to be completely dependent on animal digging for a regeneration niche, but the potential exists to use mechanical disturbance as a management tool for some species. The density of an exotic annual grass, Briza maxima, was considerably less on larger digging mounds compared with undisturbed areas, even three years after the creation of the mounds. There was evidence that native seedlings were experiencing less competition from B. maxima on these mounds and perhaps have a better opportunity for establishment