Quantifying the ecological values of brigalow regrowth for woodland bird communities: a hierarchical landscape approach
PhD project by Michiala Bowen
The conversion of native forests to pastures and crops is one of the most extensive causes of deforestation worldwide. Concomitant with agricultural landscape modification are the processes of habitat loss and fragmentation, which are major causes of species’ extinctions, population declines and altered ecosystem functions. However, in many tropical, sub-tropical and temperate regions, abandoned agricultural lands are reverting to regrowth or secondary forest, which represents an important opportunity for passive landscape restoration. Regrowth may be particularly important in highly modified landscapes, where the area of remnant habitat may be insufficient to support viable plant and animal communities without some form of restoration. Some studies of fauna populations in regrowth habitat have found recovery of species richness within several decades, although recovery of species composition may take at least 100 years and some species may be permanently lost. While these findings are encouraging, they generally fail to account for the landscape context in which regrowth occurs and focus mainly on tropical forests.
The aim of this thesis was to advance the understanding of fauna recovery in regrowth forests on abandoned agricultural land by: i) comparing woodland bird communities in a replicated chronosequence of semi-arid sub-tropical regrowth forests; and ii) quantifying how the ecological values of regrowth habitat vary among stand-, patch- and landscape-levels of ecological organisation. A review of 68 studies of fauna recovery in regrowth forests, revealed that current knowledge is limited by the predominance of studies conducted: in tropical rainforests; with minimal replication of sites; in landscapes within proximity of large tracts of relatively undisturbed mature forests; and with limited consideration of the influence of the spatial context on fauna recovery in regrowth habitat.
This study makes a significant contribution to understanding fauna recovery in regrowth forests by quantifying the recovery of estimated bird species richness to levels similar to remnant habitat, within a period of 30-50 years, in highly modified semi-arid agricultural landscapes in sub-tropical Australia, within a period of 30 – 50 years. This is important for the recovery of brigalow ecosystems, an endangered ecological community where regrowth is currently given minimal protection from further clearing.
Comparisons of the importance of habitat attributes using model averaging and hierarchical partitioning of generalised linear models of the species richness of woodland birds showed that bird species richness was positively associated with patch age, and that stand-level factors such as grazing pressure and the abundance of mistletoes (Amyema spp.) were also important. The spatial context of habitat patches (size, shape and isolation) was equally important for bird species richness, with more species of woodland dependent, nectar/frugivores and non-ground foraging insectivores occurring in less modified landscape contexts, and the converse for generalist species, ground foraging insectivores and granivores. Importantly, ordination of counts of individual species against patch age and patch context highlighted that many of the species considered sensitive to landscape modification in southern Australia, also demonstrated sensitivity to patch context and a preference for remnant habitat in the Brigalow Belt. However, several species (e.g., eastern yellow robin) also occupied regrowth habitats, suggesting that these species may respond positively to landscape restoration through targeted retention of brigalow regrowth. The landscape-level amount of habitat varied in importance among regrowth age classes and bird groups. In general, the amount of remnant vegetation and number of remnant patches in the landscape were of lower importance than local attributes. However, the amount of remnant and old (> 30 years) regrowth in the landscape did have an important positive influence on the number of woodland bird species and species’ abundance; suggesting that regrowth is making an important contribution to landscape recovery in the study area.
Mistletoe abundance was strongly dependent on particular species of frugivores for seed dispersal (e.g., mistletoebird, spiny-cheeked honeyeater and painted honeyeater), and varied considerably among three sub-regions of the study area. In general, mistletoe abundance increased in linear patches and more highly modified landscapes but was also dependent on the abundance of seed dispersers and brigalow stand condition. These findings suggest that narrow linear patches in brigalow landscapes can have important conservation values for woodland birds.
The study outcomes have important implications for research and management of regrowth habitat, both within Australia and internationally. From an international perspective, the study highlights the need for greater consideration of the importance of regrowth habitat in a landscape context for conserving and restoring fauna communities. From an Australian perspective, the study provides important baseline information for the conservation and management of woodland bird habitat in fragmented brigalow landscapes. Prior to this research, very little was known on the spatial ecology of woodland birds in the region. The study highlights the important conservation values of small and often linear remnant brigalow stands for woodland birds and the considerable potential for restoration of habitat for a diverse range of species through the retention of regrowth habitats. In particular, the research outcomes suggest that targeting the retention of regrowth towards increasing the size and reducing the isolation of remnant brigalow will produce the greatest biodiversity benefits. Brigalow regrowth stands will need to be retained for at least 50 years and probably longer to maintain viable woodland bird communities. For this to happen on a regional-scale, brigalow regrowth needs to be given greater recognition for potential biodiversity benefits either within a legislative framework or by incentive schemes to promote the long term persistence of regrowth habitat within the landscape.
Cultural landscapes : a history and analysis of the drivers of landscape change in the southern brigalow belt, Queensland, 1840-2004
PhD project by Leonie Seabrook
Human-induced modifications to the Earth's surface are important in driving landscape change over wide areas. Since 1950, vegetation clearing in the Southern Brigalow Belt bioregion of Queensland has occurred at rates comparable with the Amazon Basin, threatening landscape health and biodiversity. This project aimed to develop a historical perspective of landscape change in the Southern Brigalow Belt since 1840, focusing on the clearance of native vegetation for agriculture, and the cultural and environmental factors driving this change over a range of spatial and temporal scales.
At the regional level (1,000,000s ha), historical documents and statistics relating to agricultural production and population were integrated with pre-clearing and remnant regional ecosystem mapping to identify the drivers and trends of vegetation loss in each settlement phase. At the sub-regional level (100,000s ha), a similar methodology was combined with mapping of vegetation loss since 1945 using aerial photography to quantify in more detail the temporal aspects of vegetation change in two Southern Brigalow Belt shires, Tara and Taroom.The effect of regional drivers of deforestation were tested against present day patterns of vegetation at a property level using regression trees and generalised linear models. I used a predictive modelling approach, combining and generalised linear models, to analyse data from structured questionnaires and explain the relationship between the economic, demographic and cultural characteristics of landholders and retention of woody vegetation at a property level (100s ha). Finally, I used semi-structured interviews to gain an understanding of vegetation management issues faced by landholders in the Southern Brigalow Belt in the light of Queensland's recently introduced vegetation management regulations.
I found that the drivers of landscape change operated in synergy with each other, and although one could assume more importance at a particular time, they were, in general, inter-related both within and across the scales of human society. In the Southern Brigalow Belt, environmental constraints significantly slowed agricultural development until technology, policy and economic drivers combined in the mid-20th century, leading to extremely rapid landscape change. Temporal trends of deforestation followed a sigmoidal curve, with initial slow change accelerating though the middle phase then slowing in recent times, with intensification rather than new development. Although regional drivers, including land tenure, roads and agricultural intensification were important instigators of deforestation, soil infertility had the most significant influence on the proportion of native vegetation retained at a property scale, followed by property size. Landholders generally retained more native vegetation if they valued it for aesthetic reasons or if it provided production benefits, with higher education and training having some additional influence. Landholders highlighted a range of issues facing them as a result of Queensland's new vegetation management policies. These included: loss of income from future development; lower land values for land covered by native bushland; and the need for compensation or cost-sharing when landholders are providing a public benefit through vegetation management, by halting the loss of biodiversity and reducing greenhouse gas emissions. I identified future policy directions to address these issues and encourage vegetation recovery in the region towards recommended minimum ecological thresholds of 30% vegetation cover.
