Gregory Horrocks

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

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.

Landscape-scale conservation of an endangered migrant : the Swift Parrot (Lathamus discolor) in its winter range

Abstract The endangered Swift Parrot ( Lathamus discolor ) is threatened by disturbance processes in both its breeding and its over-wintering ranges. Here we report on the spatial distribution of Swift Parrots over two successive winters in the box-ironbark system of central Victoria, Australia. The parrots showed little site-fidelity overall, and varied significantly among years in their regional distributions. Moreover, in some years it appears that relatively small remnant patches become significant elements in the landscape for the over-wintering parrots. Logistic-regression analyses suggest that the occurrence of Swift Parrots may be linked to the intensity of flowering by the Golden Wattle ( Acacia pycnantha ) and also is associated with the summed density of aggressive, nectarivorous honeyeaters of the box-ironbark system. About 17% (a statistically significant amount) of the variance in the presence of Swift Parrots was explicable by these two variables. On the other hand, densities of Swift Parrots (where present) appeared to be related to densities of other nectarivorous species and of Noisy Miners ( Manorina melanocephala ). Although statistically significant, just 5% of the variance in densities of Swift Parrots was explained by these variables. There were no relationships between measures of eucalypt flowering and the occurrence or density of Swift Parrots, nor with densities of potential aerial predators. These statistical patterns were difficult to explain because Swift Parrots seem to depend upon eucalypt nectar, with few feeding observations on racemes of Golden Wattle. Moreover, the positive correlation with the honeyeaters is difficult to reconcile with the lack of correspondence with eucalypt flowering. From a conservation standpoint, inter-annual variation in use of the major areas of the box–ironbark system by Swift Parrots is pronounced so that a very broad perspective needs to be maintained for their management in over-wintering regions. Moreover, remnants as small as 10 ha are utilized in some years by Swift Parrots so that preservation of remnant patches must be encouraged.

Climate drying amplifies the effects of land-use change and interspecific interactions on birds

AbstractContext Climate change may amplify the effects of land-use change, including induced changes in interspecific interactions.ObjectivesTo investigate whether an avifauna changed over a period of severe drought, and if changes in avifaunas were related to changes in vegetation characteristics and the irruption of a despotic native species, the noisy miner Manorina melanocephala.MethodsIn the box–ironbark forests of south-eastern Australia, we resurveyed the avifaunas and remeasured vegetation characteristics in 120 forest transects in 2010–2011 that had previously been measured in 1995–1997.ResultsThe avifauna changed markedly over the prolonged drought, and changes were more marked in smaller fragments of remnant vegetation in which more pronounced vegetation change had occurred. The noisy miner increased differentially in smaller remnants adding to the declines, especially for small-bodied birds.ConclusionsLong droughts interspersed with short wet periods are projected for the region, so the imposition of climate effects on an already much-modified region has profound implications for the avifauna. The noisy miner has (and continues) to benefit from both land-use and climate change, so future sequences of drought interspersed with short wet periods are likely to lead to further changes in the avifauna as the miner extends its occupancy. Differential reductions in small nectarivores and insectivores will affect ecosystem processes, including the control of defoliating insects, seed dispersal and pollination.

Revegetation and the Significance of Timelags in Provision of Habitat Resources for Birds

In many approaches to landscape visualisation and reconstruction for biodiversity management, vegetation is represented as being either present or absent. Revegetation is assumed to be possible, and new vegetation appears ‘immediately’ in a mature state, which is likely to drastically overestimate habitat suitability in the short-term. We constructed a simple temporal model of resource provision from revegetated agricultural land to estimate habitat suitability indices for woodland birds in south-eastern Australia. We used this model to illustrate the trajectory of change in biodiversity benefits of revegetation. As vegetation matures, its suitability for a given species changes, so a time-integrated assessment of habitat value is needed. Spatial allocation strategies, such as offsets, that may provide high value habitat in the long-term but imply shorter term population bottlenecks from a paucity of key resources (e.g. tree hollows) must be avoided. Given that vegetation may not meet both foraging and breeding requirements of a given species, populations may be limited continuously — by foraging constraints at some times, and by breeding constraints at other times. Animal species differ in their resource requirements so that optimisation involves compromises among species. Temporal processes associated with revegetation and differences in resource requirements of species complicate landscape reconstruction. Nevertheless, our analyses suggest that the time-course of vegetation development must be incorporated in models for optimising landscape reconstruction and for calculating revegetation offsets.