Does urbanization have the potential to create an ecological trap for powerful owls (Ninox strenua)?
Introduction
The human proclivity for reshaping natural environments, altering their structure and function, has been associated with the decline and homogenization of native faunal communities worldwide (Blair, 2001, Devictor et al., 2007, Robertson et al., 2013). While it is evident that urbanization is causing the restructure of faunal assemblages, the effect of urbanization on individual species is more complex than initially perceived.
Often the tolerance of a species to urban environments is linked to its flexibility in habitat use, diet and nesting/denning requirements (Garden et al., 2006). For this reason, apex predators are often perceived as intolerant to urbanization because they generally have large spatial requirements, specialist diets and specific habitat (Noss et al., 1996, Woodroffe and Ginsberg, 1998, Randa and Yunger, 2006). Recently research has established that a predator’s tolerance to urbanization is a function of their ecological flexibility (Hager, 2009, Bateman and Fleming, 2012). Urban environments, however, may not contain the full complement of resources required by a species and/or contain additional risks associated with wildlife/human interactions (e.g. electrocution, collision, persecution, poisoning) (Battin, 2004, Chace and Walsh, 2006, Tozer et al., 2012).
The powerful owl, a species of conservation importance and Australia’s largest owl, was originally perceived as a forest dependent raptor (Mansergh et al., 1989, Debus and Chafer, 1994, McNabb, 1996, Garnett and Crowley, 2000). This species has, however, been identified as inhabiting urban environments in close proximity to Melbourne (Cooke et al., 2002a), Sydney (Kavanagh, 2004) and Brisbane (Pavey, 1993). Along the east coast of Australia this species has been linked to certain ecological requirements, including structurally complex vegetation communities for suitable roosting and nesting sites, adequate water sources and a suitable prey base (Tilley, 1982, Kavanagh and Peake, 1993, Pavey, 1993, Loyn et al., 2001, Cooke et al., 2002b, Kavanagh, 2004, Isaac et al., 2008, Isaac et al., 2013).
The powerful owl primarily depredates medium sized arboreal marsupials, however, this species will also consume birds, flying foxes and insects across its distribution (Seebeck, 1976, Lavazanian et al., 1994, Pavey, 1995, Kavanagh, 2002, Fitzsimons and Rose, 2010). As an opportunistic predator, dietary composition has been shown to vary both spatially and temporally (Tilley, 1982, Pavey et al., 1994, Kavanagh, 2002, Bilney et al., 2006, Cooke et al., 2006). Although large tree cavities are required for breeding, the powerful owl can and does inhabit areas which only contain suitable roosting and prey sources (Webster et al., 1999, Cooke et al., 2002b, Hogan and Cooke, 2010). There is, therefore, likely to be a disparity between the cues influencing settlement for a dispersing individual and those facilitating reproduction later in life. Based on this we propose several questions. Is the powerful owl responding to cues such as prey availability that are facilitating this species’ settlement in urbanizing environments but are ultimately maladaptive cues for future reproduction? Secondly, what is the differential between areas the powerful owl can potentially settle versus those in which reproduction can occur? Finally, is this differential between settlement potential and reproductive potential amplified with increases in the level of urbanization to the point of potentially forming an ecological trap?
Species distribution models are often used to establish habitat suitability for species based on the relationship between the occurrence of a species and ecological variables such as rivers, tree cover and so forth. Habitat predictions derived solely by these metrics provide predictions of everywhere an animal could possibly inhabit without any constraints. Ecological systems, by their very nature are complex with diverse factors such as predation risk, competition and resource availability influencing actual occurrence (Guisan and Zimmermann, 2000, McNabb and McNabb, 2011, Bilney, 2013a). Multi-criteria Decision Analysis (MCDA) was used in this research to determine whether increasing urbanization has the potential to form an ecological trap for the powerful owl. This was completed by incorporating landscape metrics such as patch size and species specific resource metrics into the analysis. Identifying the response of the powerful owl to urbanization gradients will highlight the potential impact urbanization is having on this species.
Section snippets
Study site
The modeling and MCDA study site represents a complete urban to forest gradient in southern Victoria, Australia and spans approximately 372,136 ha. We established urban to urban-fringe and urban-fringe to forest boundaries using a land cover layer we derived from SPOT 5 imagery (Systèm Pour l’Observation de la Terre). This technique segregated the urban to forest gradient into three zones, hereafter referred to as urban, urban-fringe and forest zones (Fig. 1). Spatially closer to Melbourne, the
Habitat suitability models
We produced a total of 60 models. AUCtest of the models ranged from 0.76 to 0.91. The most parsimonious models for each species/group as defined by the highest AUC and lowest AICc all had regularization beta-multipliers ranging from 0.5 to 3 and did not include the bias layer (Table 2). Several of the EGVs were highly correlated (R2 ⩾ 0.75), and thus in each case the most ecologically applicable variable was retained.
We established that the suite of EGVs contributing the most to model
Discussion
Natural areas are complex ecological systems that provide fauna with the requirements to sustain viable populations. Urbanization of the landscape however has the ability to impact on the persistence of fauna, through the restriction of potential habitat and the limitation of critical resources (Parrish and Hepinstall-Cymerman, 2012, Njoroge et al., 2014, Robertson et al., 2013). In this research we aimed to investigate how habitat availability for a top-order predator, the powerful owl, is
Acknowledgements
Funding for this project was supplied by the Holsworth Wildlife Research Endowment and the Parks Victoria Research Partners Scheme. This project could not have been completed without the provision of atlas data from the Department of Environment and Primary Industries (DEPI) and BA.
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