Engineering fire: the drivers of low fire intensity over gopher tortoise mounds in a Florida sandhill

Although large grazers are well known to alter fire regimes, small herbivore effects on fire have received comparatively little attention. The gopher tortoise (Gopherus polyphemus) is a herbivorous reptile that acts as an ecosystem engineer in upland, fire-dependent ecosystems of the southeastern U.S. Many animals rely on their deep burrows for refuge from extreme temperatures and fire. At the same time, gopher tortoise's burrowing and foraging activities may decrease fire intensity and severity by reducing plant biomass and/or by altering the flammability of the adjacent plant community. We examined the spatial scale and evidence for each mechanism underlying potential fire effects in sandhill at Archbold Biological Station in south-central Florida. We selected 30 existing burrows varying in activity status (active, inactive, abandoned) as well as non-mound control points in relatively open microsites. We characterized plant biomass and community composition within 15 m of mounds and control points and quantified 11 fire-related traits for 23 common plant species. Our analysis of pre- and post-fire drone imagery from an earlier fire in our study area found a localized reduction in fire severity within about 2 m of tortoise burrows. Our analysis of contemporary burrows showed that mounds of both active and inactive tortoise burrows had lower plant and litter cover than abandoned mounds and the vegetation matrix beyond the mound itself. Tortoise effects on community-level flammability were minor and unlikely to modify fire intensity. Overall, the highly localized soil disturbance associated with burrowing is likely the primary means by which gopher tortoises may decrease fire severity surrounding their burrows. Critically, our study highlights how small animals can potentially shape fire behavior via direct reduction of fuel loads.