Relationships between terrestrial vertebrate diversity, abundance and availability of coarse woody debris on south-eastern Australian floodplains

Abstract

Coarse woody debris (fallen wood, CWD) has been largely stripped from both rivers and their floodplains in the southern Murray-Darling basin of south-eastern Australia. Some of our work suggests that as little as 20 t/ha (on average) remains on floodplains where once the figure may have been closer to 90–125 t/ha. Here we examine the consequences of this depletion of a potentially significant habitat-structural element on the terrestrial vertebrates of the floodplain forests. Three major forests were studied: Gunbower Island, Barmah Forest and the Ovens River floodplain (all in northern Victoria, Australia). In each forest, seven graded (by loads of CWD) sites were investigated over 2 years. Our results show that the only native terrestrial mammal (yellow-footed antechinus Antechinus flavipes) occupies sites in significantly higher densities when wood loads exceed 45 t/ha. Ground- and CWD-using birds are more prevalent, and in richer diversity, in the vicinities of accumulations of woody debris. Overall, fallen-wood loads do not appear to relate significantly to avian patterns apart from at the local scale (i.e. near wood accumulations). Neither frogs nor reptiles appear to be influenced by fallen-wood loads. These results suggest that restoration targets might reasonably be set at about 40–50 t/ha, but it seems that birds would be aided by the imposition of a high variance in CWD-load densities rather than an even distribution. Numbers of reptiles are very low, which may reflect the very broad-scale depletion of fallen timber from these habitats; similar impacts may have been expressed in wood-depleted box-ironbark forests immediately to the south of the floodplain forests.