Multi-century periods since fire in an intact woodland landscape favour bird species declining in an adjacent agricultural region

Abstract

Habitat modification by fire and habitat loss via anthropogenic vegetation clearance and fragmentation both impact animal populations. Yet, there has been limited investigation as to whether animals that decline under one of these types of habitat change also decline under the other, and how their cumulative impacts affect the status of species and communities. Using a ~400-year chronosequence in the world's largest extant temperate woodland in south-western Australia, we examine how time since fire affects bird community richness, reporting rates and composition, and whether taxa grouped on the basis of responses to vegetation clearance and fragmentation in an adjoining agricultural landscape are associated with either recently-burnt or long-unburnt woodlands. Consistent with substantial changes in vegetation composition and structure after fire in obligate-seeder eucalypt woodlands, woodland bird communities were strongly affected by fire. Species richness and total reporting rates increased with time since fire, and community composition changed across the entire multi-century span of the chronosequence. Woodland birds most negatively impacted by vegetation clearance and fragmentation were strongly associated with long-unburnt woodlands. In a regional south-western Australian context, where extensive vegetation clearance has substantially reduced the range and populations of many woodland bird species, the ability of remaining unfragmented woodlands to support populations of these species will be strongly contingent on appropriate fire management. Specifically, as stand-replacement fires have affected 25–30% of extant woodland over recent decades, management to limit the extent of fire in remaining long-unburnt woodlands would appear a priority for conservation of woodland bird diversity.

Introduction

Fire is a recurrent disturbance in seasonally dry biomes worldwide (Archibald et al., 2013) and has played a role in shaping biome distribution, function and composition for millions of years (Pausas and Keeley, 2009). An understanding of how biota respond to fire is crucial for managing fire for biodiversity conservation (Driscoll et al., 2010), as recent alterations to fire regimes via changing land management (Pausas and Keeley, 2009) have contributed to many fauna declines. These include fauna species declining due to fire suppression (e.g. in pine-oak forests, Rose and Simons, 2016) or, perhaps more commonly, due to high incidence of fire and associated lack of time for development of hollows or other habitat resources (Woinarski and Recher, 1997; Clarke, 2008; Croft et al., 2016; e.g. in fynbos, Chalmandrier et al., 2013; mallee woodlands, Taylor et al., 2012). While the immediate effects of fires on fauna can be significant, it is often fire-associated changes to vegetation that ultimately drives the response of animals (Fox, 1982). Anthropogenic changes to habitats, particularly vegetation clearance and fragmentation, have also widely contributed to fauna declines (Andrén, 1994; Saunders, 1989). The extent to which biota is jointly susceptible to habitat modification through inappropriate fire regimes, and habitat loss via vegetation clearance and fragmentation, is poorly understood.

Temperate woodlands are one of the most imperilled biomes both globally and within Australia (Yates and Hobbs, 1997; Hoekstra et al., 2005; Prober et al., 2017). The temperate eucalypt woodlands across much of the intermediate rainfall zone (250–1200 mm) of southern Australia have been extensively cleared and modified for agricultural activities since European colonisation (Prober et al., 2017). In many districts native vegetation cover is <10%, with remaining woodland remnants subsequently exposed to a range of associated processes that can lead to further vegetation degradation (Yates and Hobbs, 1997; Prober and Smith, 2009; Prober et al., 2017). Mirroring the changes to vegetation, many Australian temperate woodland bird species have become threatened through substantial and ongoing population declines, range contractions or local extinctions (Robinson and Traill, 1996; Saunders, 1989; Saunders and Ingram, 1995; Ford et al., 2001; Ford, 2011). Postulated causes of temperate woodland bird declines are many and interacting, but primarily are thought to be loss of habitat, the isolation and edge effects of fragmentation, and fragmentation-linked habitat degradation and its interaction with predation, competitive interactions, and food resources, all modulated through climatic cycles such as droughts (Ford et al., 2001; Ford, 2011; Stevens and Watson, 2013). Having avoided the extensive land transformation affecting other Australian temperate eucalypt woodlands, the Great Western Woodlands (GWW; the world's largest extant temperate woodland; Watson et al., 2008) of south-western Australia is considered to have a largely intact temperate woodland bird community. Specifically, the GWW supports populations of a range of species that have declined in woodlands of the adjoining south-western Australian wheatbelt and in eastern Australia, and is sufficiently large and intact enough to allow mobile bird species to track resources at a landscape scale (Recher, 2008; Fox et al., 2016). However, multiple, large (>100,000 ha) wildfires have collectively burnt a substantial proportion of total woodland area in the GWW in recent decades (McCaw et al., 2014; Gosper et al., 2018), hence any fire regime-related decrease in the capacity of the GWW to support woodland birds is likely to have regional and national implications for woodland bird conservation.

Fire is regarded as a relatively minor contributor to declines in Australian temperate woodland birds, and is typically referred to in general terms regarding possible impacts of specific fire events or “inappropriate fire regimes” on the persistence of populations isolated by fragmentation (Ford et al., 2001; Ford, 2011; Watson, 2011). Yet fire modifies the availability of habitat resources used by woodland birds, such as vegetation structure and composition (Gosper et al., 2013a, Gosper et al., 2013b; Croft et al., 2016), and this modification applies irrespective of the level of fragmentation. Furthermore, the apparent low threat of fire to temperate woodland bird communities contrasts with the postulated impact of fire on some individual woodland bird species (e.g. Luck, 2002), and bird abundance and composition in other Australian temperate and semi-arid vegetation (Acacia shrublands, Davis et al., 2016; mallee eucalypt shrublands, Taylor et al., 2012), and in temperate woodlands, forests and shrublands globally (Herrando and Brotons, 2002; Chalmandrier et al., 2013).

There has been limited research on the response of Australian temperate woodland birds to fire (Burbidge, 2003). Turner (1992) found relatively rapid (8 years) post-fire recovery of a woodland bird community, while Recher and Davis (2013) found substantially lower richness and abundance and distinct composition of birds using woodlands recently-burnt (2–5 year old) compared to woodlands burnt longer ago. These contrasting responses occurred in woodlands where, respectively, the dominant eucalypts either resprouted or were killed (obligate-seeders) by fires. Recent syntheses of eucalypt woodland vegetation dynamics after disturbance (Prober et al., 2017; Gosper et al., 2018) have recognised that woodlands dominated by resprouter eucalypts are functionally distinct from those dominated by obligate-seeders, indicating that a more nuanced assessment of the effect of fire on woodland bird communities may be informative. One plausible mechanism which could account for differences in the response of birds to fire between resprouter-dominated and obligate-seeder-dominated eucalypt woodlands is the typically more rapid recovery of vegetation biomass after fire in the former (Bellingham and Sparrow, 2000).

Studying the response of biota to fire is inherently challenging where fires are rare and fire-return intervals are long. Some forests, woodlands and shrublands have fire intervals regularly exceeding a century (Clarke et al., 2010; Lowe et al., 2012), including the eucalypt woodlands of the GWW (O'Donnell et al., 2011; Gosper et al., 2013c). In these situations, longitudinal studies after fire events are not feasible for informing current fire management. Chronosequence studies present an opportunity to observe responses over long time-frames, but require robust means to estimate the time since fire of long-unburnt vegetation in the absence of long-term fire records (Driscoll et al., 2010). In recent years, innovative methods for ageing long-unburnt vegetation have opened up the possibility of investigating relatively long-term fire responses (Clarke et al., 2010; Gosper et al., 2013c), however, few studies have examined changes over a time period of >100 years (but see Lowe et al., 2012).

Using the obligate-seeder Eucalyptus salubris (gimlet) woodland chronosequence of Gosper et al., 2013a, Gosper et al., 2013b in the GWW, we examined the response of birds to time since fire. This chronosequence is unique in terms of its length, conservatively spanning a ~400 year period post-fire. We asked the following questions, as a basis for assessing the implications for woodland bird conservation of the extent and spatial pattern of recent fires in the GWW:

• Do bird community richness, reporting rates, abundance and composition change with time since fire and, if so, over what time frame?

• How do groups of species, defined by their response to habitat loss and fragmentation in the adjoining south-western Australian wheatbelt, respond to time since fire? Of particular interest from a conservation perspective is whether birds that have declined in the wheatbelt are strongly associated with either recently-burnt or long-unburnt woodlands.