Fire's growing impacts on ecosystems: Fire has played a prominent role in the evolution of biodiversity and is a natural factor shaping many ecological communities. However, the incidence of fire has been exacerbated by human activity, and this is now affecting ecosystems and habitats that have never been fire prone or fire adapted. Kelly et al. review how such changes are already threatening species with extinction and transforming terrestrial ecosystems and discuss the trends causing changes in fire regimes. They also consider actions that could be taken by conservationists and policy-makers to help sustain biodiversity in a time of changing fire activity. Science , this issue p. eabb0355

Summary: 1. Managing fire to achieve hazard reduction while providing for biodiversity conservation is complex in fire‐prone regions. This challenge is exacerbated by limited understanding of post‐fire changes in habitat and fuel attributes over time‐scales commensurate with their development, and a paucity of empirical research integrating the effects of fire on these attributes.2. We used a 110‐year post‐fire chronosequence to investigate temporal development in habitat resources used by fauna, and fuels for fire in semi‐arid Mallee vegetation, south‐eastern Australia. Fire‐history mapping previously limited investigation to 35 years post‐fire. The patterns of temporal change over 110 years for 13 variables, representing key attributes of habitat and fuel, were explored using nonlinear mixed models and data from 549 sites.3. Most habitat and fuel attributes exhibited changes in abundance and rate of development over extended periods, emphasizing the importance of documenting post‐fire dynamics over long time‐frames. Further, developmental patterns were mostly nonlinear, indicating that a shorter temporal perspective (e.g. 20–30 years post‐fire) may obscure, or provide an inaccurate understanding of, long‐term changes.4. There were striking differences in the post‐fire dynamics of some habitat and fuel attributes. Leaf litter and spinifex grass Triodia scariosa, which function as both habitat and fuel, increased rapidly after fire followed by a plateau or slow decline after 20–30 years. In contrast, live tree stems were not predicted to develop hollows until 40 years, after which time the density of live hollow‐bearing stems, an important habitat feature, increased steadily.5. Synthesis and applications. Fire affects the development and abundance of resources over substantially longer periods than can be examined using fire‐mapping based on satellite imagery. Our results demonstrate that post‐fire changes in mallee vegetation influence fire hazard and faunal habitat in different ways. Critically, the cover/abundance of most primary fuel sources did not increase substantially beyond around 30 years post‐fire; whereas important habitat attributes changed in ways that affect faunal occurrence for over a century. Fire management must explicitly acknowledge the potential for fire to affect fauna and fuel differently, and for these effects to operate over time‐frames that may extend well beyond current understanding.

Biodiversity can benefit from fires tailored to suit particular ecosystems and species

Background: ‘Megafire’ is an emerging concept commonly used to describe fires that are extreme in terms of size, behaviour, and/or impacts, but the term’s meaning remains ambiguous.Approach: We sought to resolve ambiguity surrounding the meaning of ‘megafire’ by conducting a structured review of the use and definition of the term in several languages in the peer‐reviewed scientific literature. We collated definitions and descriptions of megafire and identified criteria frequently invoked to define megafire. We recorded the size and location of megafires and mapped them to reveal global variation in the size of fires described as megafires.Results: We identified 109 studies that define the term ‘megafire’ or identify a megafire, with the term first appearing in the peer‐reviewed literature in 2005. Seventy‐one (~65%) of these studies attempted to describe or define the term. There was considerable variability in the criteria used to define megafire, although definitions of megafire based on fire size were most common. Megafire size thresholds varied geographically from > 100–100,000 ha, with fires > 10,000 ha the most common size threshold (41%, 18/44 studies). Definitions of megafire were most common from studies led by authors from North America (52%, 37/71). We recorded 137 instances from 84 studies where fires were reported as megafires, the vast majority (94%, 129/137) of which exceed 10,000 ha in size. Megafires occurred in a range of biomes, but were most frequently described in forested biomes (112/137, 82%), and usually described single ignition fires (59% 81/137).Conclusion: As Earth’s climate and ecosystems change, it is important that scientists can communicate trends in the occurrence of larger and more extreme fires with clarity. To overcome ambiguity, we suggest a definition of megafire as fires > 10,000 ha arising from single or multiple related ignition events. We introduce two additional terms – gigafire (> 100,000 ha) and terafire (> 1,000,000 ha) – for fires of an even larger scale than megafires.