Approaches to urban vegetation management and the impacts on urban bird and bat assemblages

Highlights

• We examined three vegetation management approaches used in urban green spaces.

• Bird and bat assemblages responded positively to increases in native plant richness.

• Bird species richness increased with greater understorey vegetation volume.

• Bat species richness increased with increasing density of trees >81 cm in diameter.

• These measures can be implemented without compromising multiple uses of green space.

Abstract

To balance the needs of people and biodiversity in cities, local governments are increasingly incorporating green spaces and urban greening initiatives into urban planning frameworks. Despite this, there is little information on which vegetation features or management actions are most useful in supporting biodiversity within these green spaces. We assess the effect of three vegetation management approaches that are commonly suggested to improve outcomes for urban biodiversity, including: (1) increasing the proportion of native vegetation; (2) increasing the density of trees, and (3) increasing the volume or complexity of understorey vegetation. We use a network of 39 urban green spaces (including golf courses, public parks and residential neighbourhoods) to assess how these vegetation management approaches impact urban bird and bat communities. The richness of both birds and bats increased with an increase in the proportion of native plants, bird species richness increased with increasing volume of understorey vegetation, and increasing large tree density led to increased bat activity. Our study suggests that increasing native vegetation composition and understorey vegetation volume, and the retention of large trees are practical vegetation management approaches that improve outcomes for urban birds and bats. Our data also suggests that as urbanisation and infill development proceeds, retention of large green spaces (such as golf courses) will be critical for urban biodiversity conservation, as these areas supported a component of the bird and bat fauna which was not recorded in other green space types.

Introduction

Networks of green spaces can occupy substantial areas of land within a city (for example, ranging from 1.2–46% cover in European cities, Fuller & Gaston, 2009). However, green spaces are increasingly being lost to urban in-fill development in response to rising housing demand (Jim, 2004). Urban green spaces include both natural (remnant) or planted vegetation, on public and private land, such as residential gardens, recreation fields, public parks and reserves (Tzoulas et al., 2007). They can provide an array of ecosystem services to urban residents such as recreation, cooling, stormwater runoff reduction, noise reduction and carbon storage (Derkzen, van Teeffelen, & Verburg, 2015; Livesley, Ossola, Threllfall, Hahs, & Williams, 2015; Ossola, Hahs, & Livesley, 2015; Tzoulas et al., 2007), in addition to their enormous potential to support urban biodiversity (Bryant, 2006, Hahs et al., 2009; Sandström, Angelstam, & Mikusiński, 2006; Shwartz, Turbé, Julliard, Simon, & Prévot, 2014). Subsequently, there is global interest from city governments to manage urban green spaces so that they support people and biodiversity (Bryant, 2006, Sandstrom, 2002, Walmsley, 2006). However, despite the spread of urban greening initiatives (Walmsley, 2006), and recent growth of programs such as wildlife gardening (Goddard, Dougill, & Benton, 2010), and biodiversity-friendly park management schemes (Shwartz, Muratet, Simon, & Julliard, 2013), there is little empirical evidence to support the use of specific vegetation management regimes in urban green spaces to provide for an array of biodiversity and subsequent ecosystem services (Lin & Fuller, 2013).

Much of our understanding of the value of urban green spaces for biodiversity has come from studies of urban birds (Magle, Hunt, Vernon, & Crooks, 2012; McDonnell and Hahs, 2008, Shwartz et al., 2014). Birds are the most frequently studied urban taxa, probably because they are prevalent across many habitats, conspicuous, easy to observe and have wide public appeal (Larsen, Bladt, Balmford, & Rahbek, 2012; Shwartz et al., 2013). Birds are a significant component of the urban fauna, and 20% of global bird species can be found in cities (Aronson et al., 2014). Studies of urban birds have increased our understanding of ways to design and manage urban green spaces to conserve bird assemblages, however, it is unclear if this understanding is equally relevant to other taxa. The size of urban green spaces, and the density, size and origin of trees within them, have been consistently shown to positively influence the richness and diversity of urban bird assemblages (Chace & Walsh, 2006; Fontana, Sattler, Bontadina, & Moretti, 2011; Ikin, Knight, Lindenmayer, Fischer, & Manning, 2013; Shanahan, Miller, Possingham, & Fuller, 2011; Shwartz et al., 2013; Stagoll, Lindenmayer, Knight, Fischer, & Manning, 2012; White, Antos, Fitzsimons, & Palmer, 2005). Urban green space with a greater diversity of habitat elements, including understorey and overstorey canopy vegetation, leaf litter, logs and long grass, have also been shown to benefit bird assemblages (Evans, Newson, & Gaston, 2009; Shwartz et al., 2013; Stagoll, Manning, Knight, Fischer, & Lindenmayer, 2010). Scientists recommend that urban landscape managers can improve bird habitat by retaining large trees (Stagoll et al., 2012), increasing the proportion of native vegetation (Chace & Walsh, 2006), and improving habitat complexity or diversity (Shwartz et al., 2013). However, unless other fauna respond to green space attributes in the same way as birds, these vegetation management actions may not lead to improved outcomes for urban biodiversity more broadly.

Insectivorous bats are one such group that are commonly found in urban environments (Russo & Ancillotto, 2015), and are highly mobile. Previous studies examining urban bat assemblages have largely focussed on landscape factors, demonstrating that bat assemblages are more diverse in landscapes with larger patches of green space (Avila-Flores and Fenton, 2005, Gehrt and Chelsvig, 2003), and respond positively to increases in the cover of vegetation and density of trees at the landscape scale (Basham, Law, & Banks, 2011; Hale, Fairbrass, Matthews, & Sadler, 2012; Luck, Smallbone, Threlfall, & Law, 2013; Threlfall, Law, & Banks, 2012). Site level factors suggested to be beneficial to bat assemblages include the presence of large trees (Basham et al., 2011), tree hollows (Basham et al., 2011), and an abundant supply of nocturnal invertebrates (Avila-Flores & Fenton, 2005). However, few studies have investigated ways to design and manage urban green spaces to specifically promote bat assemblages (Smith & Gehrt, 2010). This issue is particularly pressing in Australia, where cities have been found to be disproportionately important for the conservation of threatened species in comparison to non-urban regions (Ives et al., 2016). In Australia, several threatened species of birds, bats and other vertebrate fauna, have been found to rely heavily on resources provided by urban landscapes (Ives et al., 2016, Threlfall et al., 2012).

To improve our understanding of the ways in which urban green spaces can be managed to support an array of taxa, we investigated the relationship between bird and bat assemblages and vegetation attributes throughout a network of large and small urban green spaces in Melbourne, Australia. Taking a multi-taxa approach can reveal important differences in the way biodiversity responds to urban environments (Beninde, Veith, & Hochkirch, 2015; MacGregor-Fors et al., 2015), enabling the development of more sophisticated approaches to green space management in both public and private spaces. We selected bats as a comparative taxa to birds due to their ubiquity in Australian cities (Caryl et al., 2014; Hourigan, Catterall, Jones, & Rhodes, 2010; Luck et al., 2013, Scanlon and Petit, 2008, Threlfall et al., 2012), and because their mobility and their insectivorous diet make them potentially highly responsive to the way vegetation is managed in urban areas, particularly as invertebrates are known to be influenced by various aspects of urban vegetation (Beninde et al., 2015). Additionally, bats have been proposed as potential bio-indicators in urban environments (Russo & Ancillotto, 2015), making it critical to compare their response to other more commonly studied taxa.

The aim of our study was to investigate how local scale vegetation management in urban green spaces influences bird and bat assemblages. To do this we examined the effect of three local scale vegetation management approaches: (1) increasing the proportion of native vegetation; (2) increasing the density of trees, and (3) increasing the volume or complexity of understorey vegetation. The ecological responses we examined were measures of the bird and bat assemblage relevant to biodiversity-friendly management goals, including species richness of bird and bat assemblages, bird breeding activity, and the activity and richness of bat species previously classified as being urban sensitive (Threlfall et al., 2012). We expected that birds would respond strongly to differences in native and exotic vegetation in different green spaces, as found by previous Australian studies (White et al., 2005). In contrast, we expected that bats would respond more strongly to structural aspects of urban vegetation, such as tree density and size, rather than the composition of vegetation per se (Kalcounis, Hobson, Brigham, & Hecker, 1999).