Agricultural landscape modification increases the abundance of an important food resource: Mistletoes, birds and brigalow
Introduction
The spatial patterning of habitat within landscapes is an important influence on the distribution and abundance of species. Human landuse frequently alters landscape pattern through the concomitant processes of habitat loss and habitat fragmentation, with important implications for biodiversity (Lindenmayer and Fischer, 2006). Vegetation clearance for agriculture reduces habitat area and patch size, increases number of patches, edge lengths, and the distance between patches (Fahrig, 2003). The direct effects of landscape change are well documented, particularly with regards to declines in population size and species diversity (Wilcox and Murphy, 1985, Andrén, 1994). These effects are not always negative and habitat fragmentation may benefit some native species (e.g., Laurance et al., 2001, Burgess et al., 2006). However, an advantage to one native species from habitat fragmentation may have further indirect and more pervasive effects on plant and animal communities if the dynamics of inter-specific interactions like trophic relationships are altered (Ewers and Didham, 2006, Valladares et al., 2006, Banks et al., 2007).
Hemi-parasitic mistletoes provide an interesting example of the dynamics of altered species’ abundance in response to human-induced changes to landscape structure (e.g., Rodríguez-Cabal et al., 2007). Mistletoe populations are sensitive to human landuse and can substantially increase in abundance in commercial production forests and agricultural landscapes, where management activities are often targeted at the control or removal of these perceived pest species (see references within Calder and Bernhardt, 1983, Shaw et al., 2004). However, mistletoes are increasingly being recognised as important ecological resources (e.g., Watson, 2001). Mistletoe abundance is intricately linked to the functioning of ecosystems through multiple trophic pathways with animal pollinators, seed dispersers, folivores; and through their potential to influence plant biomass, microbial populations and forest productivity by increasing litterfall, particularly in nutrient poor environments (see references within Calder and Bernhardt, 1983, March and Watson, 2007). At a local level, spatial patterns in mistletoe abundance are influenced by their dependence on animal seed dispersers to provide them with appropriate establishment sites and on healthy host trees which support them to maturity (Norton et al., 1995, Lavorel et al., 1999, López de Buen et al., 2002, Ward, 2005). In fragmented agricultural landscapes, altered mistletoe abundance may occur as a consequence of complex interactive influences of modified landscape pattern on both mistletoes and their host trees and animal vectors (Reid and Yan, 2000). However, few studies have explicitly examined the role of landscape pattern on mistletoe abundance.
Mistletoes are widespread in Australia’s forests and woodlands and altered abundances are often reported from temperate agricultural landscapes (Reid, 1995, Reid and Yan, 2000). In semi-arid regions, mistletoes are generally regarded as important food resources for animals and may have altered abundances as a consequence of agricultural landuse (Reid and Lange, 1988, Reid, 1990). The grey mistletoe (Amyema quandang) is a widespread semi-arid species which provides an almost year-round supply of food for nectarivores and frugivores in Acacia woodlands where few other sources of fruit and nectar are available (Reid, 1990). This mistletoe is a particularly important food resource for the nationally threatened painted honeyeater (Grantiella picta), whose breeding cycle coincides with peak A. quandang fruit availability (Barea and Watson, 2007). Brigalow (Acacia harpophylla) is a common host tree of A. quandang (Stanley and Ross, 1995, Downey, 1998) and characterises Acacia woodlands of the Brigalow Belt bioregion, which extends through semi-arid tropical, subtropical and temperate eastern Australia (Fig. 1a) (Sattler and Williams, 1999). Since the late 1800s, these forests and woodlands have undergone extensive and rapid conversion to highly fragmented agricultural landscapes predominantly for sheep and cattle grazing and broad-acre cereal cropping (Seabrook et al., 2006). The most extensive period of clearing commenced after World War II with the advent of broad-scale mechanical clearing (Johnson, 1964). High clearing rates continued until recently, with approximately two million hectares cleared in Queensland during the last 20 years (Natural Resources and Water, 2007). Over 90% of brigalow dominated communities were cleared and remnants are now protected as endangered ecological communities (EPBC Act 1999: DEH, 2001). Populations of A. quandang may be of critical importance for the maintenance of the biodiversity values of fragmented brigalow landscapes, particularly for animals dependent on nectar and fruits, like the painted honeyeater which is a breeding visitor to these forests.
In this study, we adopt a spatially explicit hierarchical landscape approach to examine the influence of human-modified landscape pattern on the abundance of A. quandang in brigalow dominated remnants occurring in fragmented agricultural landscapes of southern Queensland, Australia. Specifically, we ask: how does A. quandang abundance respond to the interactive influences of landscape pattern, and local bird disperser abundance and host condition? We apply an information theoretic approach (Burnham and Anderson, 2002) to test and rank alternative generalised linear models of landscape connectivity, edge contrast, patch shape, brigalow condition and bird disperser abundance to explain mistletoe abundance. We use hierarchical partitioning (Mac Nally, 2000, Mac Nally, 2002) to evaluate the independent effect of each explanatory variable and model averaging to rank the importance of these variables (Burnham and Anderson, 2002).
Section snippets
Study area description
The study was conducted in the Tara Downs subregion of the brigalow Belt South Bioregion, (27°16′S, 149°40′E to 27°52′S, 150°13′E) in southern Queensland (Fig. 1). The region has a sub-humid to semi-arid climate (Lloyd, 1984) with mean daily temperature ranging from 18 °C to 33 °C in summer and 4 °C to 21 °C in winter (Bureau of Meteorology, 2007). Peak rainfall normally occurs during summer, with an average annual rainfall of 650 mm (Bureau of Meteorology, 2007).
Three study areas were selected with
Results
A. quandang occurred at 37 of the 50 survey sites (Fig. 1c–e). Mistletoe count was highly variable, with between 1 and 70 individual plants recorded per site (mean = 11.48 ± 16.46 sd). The species was largely absent or in low abundance at sites within the Erringibba study area (mean = 2.32 ± 3.45 sd). High abundances were recorded in The Gums study area (mean = 25.47 ± 20.61 sd) and mixed abundances were recorded at the Southwood study area (mean = 6.93 ± 8.96 sd). Mistletoe numbers varied within the two large
Discussion
In this study, we demonstrate that agricultural landscape modification has increased the abundance of an ecologically important species, the hemi-parasitic mistletoe, A. quandang, in an area of semi-arid brigalow woodland of eastern Australia. In particular, narrow linear patches appear to promote the proliferation of mistletoe plants. However, spatial patterns in habitat alone did not completely explain the abundance of this mistletoe species, which also responded strongly to the local
Conclusion
Consideration of inter-specific relationships can help to better inform interpretations of the response of species to altered landscape patterns. In this study, the abundance of A. quandang was best explained by a combination of landscape pattern, bird disperser abundance and host condition. While our research design precludes determination of causal relationships, the findings suggest that this mistletoe species benefits from some aspects of landscape modification but this is moderated by
Acknowledgements
The authors would like to thank Jonathan Rhodes for providing statistical advice; Sean Hough for GIS data extraction assistance; Marion Firns for field assistance; and all the landholders who provided access to study sites. Satellite imagery was supplied by Queensland Murray Darling Committee and remnant vegetation maps incorporated data from Queensland Herbarium (regional ecosystem) and Queensland Department of Natural Resources and Water (Statewide Landcover and Trees Study). This research
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