Wild Bee Diversity Increases with Local Fire Severity in a Fire‐Prone Landscape

Utah State University DigitalCommons@USU All PIRU Publications Pollinating Insects Research Unit 4-3-2019 Wild Bee Diversity Increases With Local Fire Severity in a Fire‐Prone Landscape Sara M. Galbraith Oregon State University James H. Cane Utah State University Andrew R. Moldenke Oregon State University James W. Rivers Oregon State University Follow this and additional works at: https://digitalcommons.usu.edu/piru_pubs Part of the Biology Commons Recommended Citation Galbraith, S. M., Cane, J. H., Moldenke, A. R., and Rivers, J. W.. 2019. Wild bee diversity increases with local fire severity in a fire‐prone landscape. Ecosphere 10(4):e02668. 10.1002/ecs2.2668 This Article is brought to you for free and open access by the Pollinating Insects Research Unit at DigitalCommons@USU. It has been accepted for inclusion in All PIRU Publications by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. Wild bee diversity increases with local fire severity in a fire-prone landscape 1, 2 3 1 SARA M. GALBRAITH , JAMES H. CANE, ANDREW R. MOLDENKE, AND JAMES W. RIVERS 1Department of Forest Ecosystems and Society, 321 Richardson Hall, Oregon State University, Corvallis, Oregon 97331 USA 2USDA-ARS Pollinating Insects Research Unit, BNR 257 Old Main Hill, Utah State University, Logan, Utah 84322 USA 3Department of Botany and Plant Pathology, 2082 Cordley Hall, Oregon State University, Corvallis, Oregon 97331 USA Citation: Galbraith, S. M., J. H. Cane, A. R. Moldenke, and J. W. Rivers. 2019. Wild bee diversity increases with local fire severity in a fire-prone landscape. Ecosphere 10(4):e02668. 10.1002/ecs2.2668 Abstract. As wildfire activity increases in many regions of the world, it is imperative that we understand how key components of fire-prone ecosystems respond to spatial variation in fire characteristics. Pollina- tors provide a foundation for ecological communities by assisting in the reproduction of native plants, yet our understanding of how pollinators such as wild bees respond to variation in fire severity is limited, particularly for forest ecosystems. Here, we took advantage of a natural experiment created by a large- scale, mixed-severity wildfire to provide the first assessment of how wild bee communities are shaped by fire severity in mixed-conifer forest. We sampled bees in the Douglas Fire Complex, a 19,000-ha fire in southern Oregon, USA, to evaluate how bee communities responded to local-scale fire severity. We found that fire severity served a strong driver of bee diversity: 20 times more individuals and 11 times more species were captured in areas that experienced high fire severity relative to areas with the lowest fire severity. In addition, we found pronounced seasonality in the local bee community, with more individuals and more species captured during late summer, especially in severely burned regions of the landscape. Two critical habitat components for maintaining bee populations—flowering plants and boring insect exit holes used by cavity-nesting bees—also increased with fire severity. Although we detected shifts in the relative abundance of several bee and plant genera along the fire severity gradient, the two most abundant bee genera (Bombus and Halictus) responded positively to high fire severity despite differences in their typical foraging ranges. Our study demonstrates that within a large wildfire mosaic, severely burned forest contained the most diverse wild bee communities. This finding has particularly important implications for biodiversity in fire-prone areas given the expected expansion of wildfires in the coming decades. Key words: Apoidea; bees; fire severity; managed forest; mixed-conifer forest; pollinators; relative differenced normalized burn ratio; wildfire. Received 21 December 2018; revised 21 February 2019; accepted 25 February 2019. Corresponding Editor: T’ai Roulston. Copyright: © 2019 The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. E-mail: [email protected] INTRODUCTION (Biesmeijer et al. 2006, Brosi and Briggs 2013), with implications for flowering plant diversity as Animal pollinators play a critical role in main- well as the broader ecological communities that taining biodiversity in natural and managed sys- depend on flowering plants as critical habitat fea- tems, with most of the world’s flowering plants tures. In addition to promoting and maintaining either requiring or benefitting from pollination biodiversity, an estimated 70% of global crops services for reproduction (Ollerton et al. 2011). used for human consumption are enhanced by Changes in animal pollinator populations can pollination services (Klein et al. 2007), valued at drive changes in wild plant persistence €153 billion annually (Gallai et al. 2009). ❖ www.esajournals.org 1 April 2019 ❖ Volume 10(4) ❖ Article e02668 GALBRAITH ET AL. Concerns remain high regarding the economic canopy, and exposes mineral soil; in turn, this is and ecological consequences of long-term popula- likely to shift communities back to the early tion declines of pollinators (Pollinator Health Task successional pre-forest stage. In contrast, low- Force 2015, IPBES 2016), yet we lack a founda- severity fires only burn the most flammable fuels tional understanding of wild pollinator population and often have little influence on tree mortality dynamics in many natural and human-dominated (Perry et al. 2011). In fire-adapted forests, many ecosystems (Bartomeus et al. 2018). understory plant species have adaptive strategies Understanding how ecological communities that allow them to compete after canopy and/or respond to large-scale disturbances is a critical duff cover is reduced by severe fire (Bond and topic for successful pollinator conservation, espe- van Wilgen 1996). Although fire severity influ- cially as many disturbance regimes shift world- ences the composition of understory plant com- wide (Turner 2010). Wildfire is a natural munities by selectively influencing recruitment disturbance agent that plays a key role in shap- (Bond and van Wilgen 1996), severe wildfires are ing many terrestrial ecosystems (DeBano et al. followed by a temporary increase in plant species 1998, Pyne et al. 1996, Bowman et al. 2009), and richness and forb cover in fire-adapted regions it can enhance biodiversity via increases in habi- due to reduced light competition (Keeley 1987, tat heterogeneity and reductions in species domi- Keeley et al. 2003, Huisinga et al. 2005). The dif- nance (Bond and van Wilgen 1996, Pyne et al. ferences in post-wildfire habitats caused by vari- 1996, DeBano et al. 1998). Historical wildfire ation in fire severity, in turn, are expected to have regimes have shifted markedly due to extensive important consequences for local pollinator com- suppression efforts of the past (Bowman et al. munities. 2009), and global climate change has led to alter- Nevertheless, nearly all studies examining ations in the frequency, severity, and extent of post-fire responses of pollinators in forest ecosys- wildfire (Westerling et al. 2006, Miller et al. 2009, tems to date have either used limited-severity Flannigan et al. 2013, Jolly et al. 2015). In the prescribed fire (Campbell et al. 2007, Rubene western United States, the average wildfire sea- et al. 2015, Rodrıguez and Kouki 2017) or have son length increased 64% between the periods of focused on small fires (Potts et al. 2003, Bogusch 1970–1986 and 1986–2003 (Westerling et al. et al. 2014, Lazarina et al. 2017), both of which 2006), and total burn area has increased at a rate are unrepresentative of the mosaic of fire severity of 355 km2 per year (Dennison et al. 2014). Fire that is typical of contemporary wildfires in many activity is projected to continue to change rapidly regions of the world (Strauss et al. 1989, Stocks in the coming decades, potentially faster than et al. 2002, Lentile et al. 2005, Halofsky et al. many terrestrial species’ ability to accommodate 2011). The lone exception is a recent study that (Krawchuk et al. 2009). Given these ongoing investigated the influence of the diversity of fire changes, a stronger understanding of how wild- histories (pyrodiversity) on pollinators after a fire characteristics influence biodiversity is criti- mixed-severity fire in forest/scrub habitat (Poni- cal, and this is particularly true for pollinators sio et al. 2016), and researchers found positive inhabiting fire-prone landscapes. effects of pyrodiversity on pollinators only Despite the important and changing role of within plots that burned at low to moderate fire wildfire in most biomes, we have a poor under- severity (Ponisio et al. 2016). Given that high- standing of how characteristics of fire influence severity wildfires are expected to have stronger pollinator diversity within forest ecosystems effects on pollinator habitat (i.e., increased flow- (Koltz et al. 2018, Rivers et al. 2018a). Fire sever- ering plant abundance) than low-severity wild- ity is an important characteristic of wildfire that fires as outlined above, additional research is is measured by the amount of change in organic needed to understand the response of pollinator matter due to burning (Keeley 2009), and wild- communities and their habitat to local fire sever- fires can have markedly different effects on eco- ity itself. Furthermore, because pollinators play a logical communities depending on the degree of fundamental role in maintaining plant popula- fire severity. For example, high-severity fire typi- tions that serve as the basis for terrestrial food cally acts as a stand-replacing event that kills webs (Kearns and Inouye 1997), understanding dominant overstory trees, removes the forest pollinator response to wildfire will help us ❖ www.esajournals.org 2 April 2019 ❖ Volume 10(4) ❖ Article e02668 GALBRAITH ET AL. predict broader ecological shifts as wildfire activ- estimated 14.5 yr pre-settlement, lengthening to ity continues to increase in many regions of the 21.8 yr during a century of active fire suppres- world (Jolly et al.

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