Ecological Applications, 26(1), 2016, pp. 115–127 © 2016 by the Ecological Society of America
Bird functional diversity decreases with time since disturbance: Does patchy prescribed fire enhance ecosystem function? Holly Sitters1, Julian Di Stefano, Fiona Christie, Matthew Swan, and Alan York Fire Ecology and Biodiversity Group, School of Ecosystem and Forest Sciences, University of Melbourne, 4 Water Street, Creswick, VIC 3363, Australia
Abstract. Animal species diversity is often associated with time since disturbance, but the effects of disturbances such as fire on functional diversity are unknown. Functional diversity measures the range, abundance, and distribution of trait values in a community, and links changes in species composition with the consequences for ecosystem function. Improved understanding of the relationship between time since fire (TSF) and functional diversity is critical given that the frequency of both prescribed fire and wildfire is expected to increase. To address this knowledge gap, we examined responses of avian functional diversity to TSF and two direct measures of environmental heterogeneity, plant diversity, and structural heterogeneity. We surveyed birds across a 70-year chronosequence spanning four vegetation types in southeast Australia. Six bird functional traits were used to derive four functional diversity indices (richness, evenness, divergence, and dispersion) and the effects of TSF, plant diversity and structural heterogeneity on species richness and the functional diversity indices were examined using mixed models. We used a regression tree method to identify traits associated with species more common in young vegetation. Functional richness and dispersion were negatively associated with TSF in all vegetation types, suggesting that recent prescribed fire generates heterogeneous vegetation and provides greater opportunities for resource partitioning. Species richness was not significantly associated with TSF, and is probably an unreliable surrogate for functional diversity in fire- prone systems. A positive relationship between functional evenness and structural heterogeneity was common to all vegetation types, suggesting that fine-scale (tens of meters) structural variation can enhance ecosystem function. Species more common in young vegetation were primarily linked by their specialist diets, indicating that ecosystem services such as seed dispersal and insect control are enhanced in more recently burnt vegetation. We suggest that patchy prescribed fire sustains functional diversity, and that controlled use of patchy fire to break up large expanses of mature vegetation will enhance ecosystem function. Key words: southeast Australia; avifauna; biodiversity; disturbance; functional redundancy; Great Otway National Park and Forest Park; niche theory; species composition; vegetation structure
on functional diversity (Luck et al. 2013, Barbaro et al. 2014), which measures the range, abundance, and distribution of functional trait values in a community, providing a link between species diversity and ecosystem function (Diaz and Cabido 2001, Mason et al. 2005). Better understanding of the impacts of disturbance on functional diversity is crucial because functional diversity is likely to be linked to ecosystem resilience (Walker 1995), which is the capacity of a system to absorb shocks, reorganize, and retain the same structure and function (Holling 1973, Walker et al. 2004). In theory, ecosystems are more resilient when they comprise several functionally equivalent species that differ in their responses to disturbance; if a species
Introduction Relationships between faunal species diversity and time since disturbance have been identified in a range of terrestrial ecosystems (Haney et al. 2008, Pons and Clavero 2010, Horn et al. 2012, Levin et al. 2012), but the impacts of disturbances such as fire on ecosystem function are poorly understood. Ecosystem function is associated with functional trait diversity, rather than simply the number of species in a community (Hooper et al. 2005). Consequently, there is growing emphasis on the impacts of landscape change Manuscript received 15 August 2014; revised 7 April 2015; accepted 21 April 2015. Corresponding Editor: T. R. Simons. 1E-mail:
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population declines following disturbance, compensatory behavior in equivalent species ensures that the function of the group is retained (Walker 1995). A key assumption of functional diversity measurement is that traits under consideration are those that drive ecosystem function (Walker et al. 1999). Birds, for example, exhibit diverse traits and perform a wide array of ecosystem functions (Sekercioglu 2006). Seed dispersal, pollination, and pest regulation are perhaps the most influential avian ecological functions (Cordeiro and Howe 2003, Wenny et al. 2011); other important functions include nutrient cycling and ecosystem engineering through the construction of burrows and cavity nests (DeVault et al. 2003, Casas-Criville and Valera 2005). Quantification of functional trait diversity uses continuous measures to capture the richness, evenness, divergence, and dispersion of the traits that drive ecosystem function (Mouchet et al. 2010). Measurement of the major aspects of functionality should provide insight into how trait diversity, and ultimately ecosystem function, changes along a disturbance gradient. Disturbances such as fire can influence functional diversity directly; for example, by causing mortality to less mobile species, and promoting traits associated with movement and dispersal (Brotons et al. 2005), or indirectly through modification of environmental heterogeneity. Heterogeneous environments are conducive to niche diversification because they provide multiple opportunities for resource partitioning, and are expected to support species with a greater diversity of traits (Schoener 1974). Two major aspects of environmental heterogeneity at the local (patch) scale are vegetation structural heterogeneity and plant species diversity, both of which influence bird species richness and composition (MacArthur and MacArthur 1961, Fleishman et al. 2003). Reduced avian functional diversity relative to natural systems has been identified in agricultural landscapes (Flynn et al. 2009), which typically harbor low levels of both structural heterogeneity and plant species diversity. Disturbances such as high-severity wildfire may have a similarly homogenizing impact on environmental heterogeneity in the short term, and may also yield a reduction in functional diversity (Hidasi-Neto et al. 2012). Fire severity refers to spatial patterns of vegetation damage, and results from interplay between fire weather, fuel, and terrain (Bradstock et al. 2010). Unlike high- severity wildfire, prescribed fire of low severity might generate environmental heterogeneity in the short term because it can create a mosaic of burnt and unburnt vegetation (Penman et al. 2007); it is therefore plausible that both environmental heterogeneity and functional diversity increase following prescribed fire. The rate and extent to which functional diversity returns to prefire levels are likely to be primarily a function of changes in environmental heterogeneity.
Ecological Applications Vol. 26, No. 1
Enhanced understanding of the impact of fire on functional diversity is crucial in light of increasing use of prescribed fire, and forecasted increases in the prevalence of wildfire under climate change (Stephens et al. 2012, Attiwill and Adams 2013, McCaw 2013). Most studies of species richness, functional diversity, and ecosystem function have been small-scale experimental analyses, and large- scale field studies have focused on the influence of land use types on functional diversity (Flynn et al. 2009, Luck et al. 2013). To date, most avian studies of relationships between fire and function have focused on the influence of fire on the occurrence of different feeding guilds (e.g., Leavesley et al. 2010). We are aware of only one study that has examined the influence of fire on a continuous functional diversity metric; in a study of understorey birds in Amazonian forests, Hidasi-Neto et al. (2012) found that functional diversity was unrelated to three fire frequency categories, but responded to attributes of vegetation structure. To our knowledge, ours is the first study to relate a gradient in time since disturbance to faunal functional diversity. We sought to examine responses of avian functional diversity to TSF, a putative surrogate for environmental heterogeneity, and two direct measures of heterogeneity. We made two predictions; first, we hypothesized a negative relationship between functional diversity and TSF because recent fire in our study area was patchy, low-severity prescribed fire (Department of Environment and Primary Industries 2013), which generates heterogeneity in the short term. For example, birds that feed on open ground in forest ecosystems are likely to be more common in recently burnt areas than in older vegetation (Loyn 1997, Pons and Bas 2005). Second, we anticipated a positive response of functional diversity to two direct measures of environmental heterogeneity: structural heterogeneity, and plant species diversity. We used a 70-year chronosequence in TSF spanning four vegetation types, from heathland to tall wet forest, which permitted us to determine whether functional responses were consistent across landscapes of inherently different vegetation structure and plant species composition. Methods Study area The study was undertaken in a 59 000- ha region of the Otway Ranges (the Great Otway National Park and Forest Park) in southeast Australia. The climate is generally mild (mean annual minimum and maximum temperatures are 10.5°C and 18.2°C, respectively) and features a rainfall gradient from the dry northeast (mean annual rainfall 661 mm) to the wet southwest (mean annual rainfall 1259 mm; Bureau of Meteorology 2014). In the northeast of the study
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area (30–270 m above sea level [a.s.l.]), undulating heathland of low, dense shrubs transitions to heathy woodlands of messmate (Eucalyptus obliqua), brown stringybark (E. baxteri), and red stringybark (E. macrorhyncha). Further southwest, complex topography at higher altitudes (200–650 a.s.l.) supports tall open forest dominated by mountain grey gum (E. cypellocarpa), Tasmanian blue gum (E. globulus), narrow- leaved peppermint (E. radiata), and manna gum (E. viminalis) (Department of Sustainability and Environment 2012). Vegetation and fire history maps of the study area were derived from spatial data sets comprising Landsat 5 images and fire history converted to vector layers suitable for ArcMap 10 (ESRI 2011). A vegetation layer was used to classify four broad vegetation types; heathland, dry forest, foothills forest, and wet forest (Cheal 2010). Two TSF layers corresponded to each survey year (2010 and 2011), and were derived from individual fire history layers for every year since 1939. Large wildfires affected the study area in 1939 and 1983, and prescribed fire has been applied in the region since 1982. Since 2008, prescribed burns have been implemented increasingly frequently in spring or autumn; they normally cover areas
