Tree canopy defoliation impacts avifauna
Introduction
For many wildlife species, tree canopies provide breeding sites, shelter from harsh weather and predators, and resources such as food and nesting materials (Nakamura et al., 2017). Canopies are a major determinant of sub-canopy layers; a reduction in canopy often results in an increase in sub-canopy vegetation (Stone and Wolfe, 2004, Feldmann et al., 2018). Leaf-fall from canopies also provides additional habitat and resources for other flora and fauna (Antos and Bennett, 2006, Davis et al., 2016).
Birds do not generally occur uniformly through forests, but select specific elements of habitat which provide the resources they require. Influences on the probability of occurrence of birds manifest themselves at multiple scales, including the scale of individual trees (Moore et al., 2013; Muiruri et al., 2016). Many terrestrial bird communities are influenced by habitat complexity and structure (Collard et al., 2009, Beskardes et al., 2018, Duren et al., 2017). Habitat characteristics in conjunction with life history traits related to foraging and breeding guilds (Lindenmayer et al., 2002) often predict species’ presence in a habitat (Rotenberry, 1985, Villard et al., 1999). Tree canopies provide foraging opportunities for specialist woodland bird species to exploit floristic components (Nadkarni, 1994, Pavey and Nano, 2009, Nakamura et al., 2017) and invertebrate resources (Basset, 2001, Catterall et al., 1998, Ellwood and Foster, 2004). Due to the narrow range of resources that specialist bird species require, a change in foraging resources due to canopy defoliation can be detrimental to their survival (Catterall et al., 1998, Ortega and Capen, 2002, Bueno et al., 2018). When there is a decline in canopy condition due to defoliation or habitat disturbance, ‘edge’ areas and gaps increase (Barrette et al., 2017). This often results in an influx of opportunistic species that generates greater competition between sometimes aggressive generalist-edge species and specialist-interior species (Collard et al., 2009). Limited ecological plasticity of specialist species makes it more difficult for them to adjust to ecological disturbance such as canopy defoliation (Hansen and Urban, 1992, Dunford and Freemark, 2004). In addition to the direct effects of canopy change on avian communities, the change in canopy also may have an indirect effect by influencing sub-canopy vegetation and ground cover (Belsky et al., 1993), here referred to as ’indirect effects’. For example, an increase in sub-canopy vegetation may limit manoeuvrability and impede the capacity of some bird species to use such habitat (Antos and Bennett, 2006).
Tree canopy defoliation may result from a variety of processes. In Australia, these include disease, drought, fire, and herbivory by invertebrates or arboreal mammals (koalas Phascolarctos cinereus and possums), processes which may act synergistically (Specht and Morgan, 1981, Landsberg and Cork, 1997, McAlpine et al., 2015, Moore et al., 2013). When koalas are implicated as the cause of defoliation, there often is much debate regarding whether management to reduce their browsing pressure is required and acceptable (McAlpine et al., 2015). In such cases, any management usually focuses on reducing the welfare impacts of defoliation on the koalas rather than the effects of defoliation on other fauna (e.g. Todd et al., 2008, Whisson et al., 2012).
In southern Australia, the koala has been implicated as a primary cause of defoliation of manna gum (Eucalyptus viminalis) (Martin, 1985, Natural Resource Management Ministerial Council, 2009, McAlpine et al., 2015). Manna gum is a preferred food tree of the koala such that where high-density koala populations occur, over-browsing can lead to widespread tree defoliation and the subsequent starvation of koalas (Menkhorst, 2008; Whisson et al., 2016). At Cape Otway, Victoria, manna gum defoliation due to a high density koala population is conspicuous (Whisson et al., 2016). Although a management program has been implemented to address concerns for koala welfare, it does not consider the consequences for other canopy fauna (Department of Environment, Land, Water and Planning, 2015).
Our study aimed to determine the relationship between manna gum canopy defoliation and use of individual trees and their sub-canopy vegetation by woodland birds. Specifically, we predict reduced usage of more defoliated trees. However, defoliation may be associated with more complex understorey structure, and so host more understorey birds. This information is critical for understanding the potential effects of koala-induced tree defoliation for avian species.
Section snippets
Ethics statement
This study was approved by the Deakin University Animal Ethics Committee (A06-2010) and conducted under permit (10005322) by the Victorian Department of Environment, Land, Water and Planning. Permission to access private property was obtained from landholders: A. Evans, L. Corke, S. Neal, F. and K. Fotinas, C. and P. Marriner, A. and J. Marriner and G. Woodcock.
Study area
Our study was conducted in long unburned manna gum woodland on private land and in the Great Otway National Park at Cape Otway,
Site characteristics
Focal trees were of similar canopy size, height and DBH (S1 Table). Percentage canopy cover was negatively correlated with canopy defoliation class (rs = −0.906, p < 0.001; Table S1). The number of dead limbs increased with defoliation class (Table S1).
Neighbouring Eucalyptus trees were either manna gum (92.7%, 51/55) or messmate stringybark (E. obliqua; 7.3%, 4/55). The distance to the nearest neighbouring tree did not differ between defoliation classes (Table S1). Highly defoliated trees were
Discussion
Worldwide, loss or degradation of forest canopies is recognised as having considerable consequences for ecosystem function and biodiversity (Nakamura et al., 2017). However, at local scales there may be limited understanding of the ecological importance of tree canopies, which may lead to inadequate management of threatening processes. In our study, declines in bird species richness and the complexity of bird species’ assemblage at individual trees were associated with increasing levels of
Conclusions
Single-species conservation efforts have been criticised as, among other things, being too focused and thus neglecting broader ecological contexts (Simberloff, 1998). Some single species recovery efforts have conflicted with those of other threatened species (Roemer and Wayne, 2003) but rarely has a recovery effort been so successful that, in certain parts of the range, a species negatively affects co-occurring biodiversity. We demonstrate such a negative effect. Tree defoliation is not a
Acknowledgements
Sue Guinness, Kelly Leigh, Renee Mead, Bron Richardson and Brett Whitfield assisted with data collection. Funding was provided by the Australian Geographic Society and the Department of Environment, Land, Water and Planning. Manuscript preparation was supported by the Beach Ecology And Conservation Hub (Venus Bay).
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These authors contributed equally to this work.