Pyric Herbivory and the Nexus Between Forage, Fire and Native and Introduced Large Grazing Herbivores in Australian Tropical Savannas

Abstract

Earth’s tropical savannas typically support high biomass of diverse grazing herbivores that depend on a highly fluctuating resource: high-quality forage. An annual wet–dry cycle, fire and herbivory combine to influence forage quality and availability throughout the year. In the savannas of northern Australia, a depauperate suite of large native (marsupial) herbivores (wallaroos [Osphranter spp.] and the agile wallaby [Notamacropus agilis]) compete for resources with non-native large herbivores introduced in the late nineteenth century, particularly bovines (feral and managed cattle [Bos spp.] and feral water buffalo [Bubalus bubalis]) that now dominate the landscape. Anecdotal reports of recent population declines of large macropods and negative impacts of bovines highlight the need to better understand the complex relationship between forage, fire and abundance of native and introduced large herbivores. The pyric herbivory conceptual model, which posits complex feedbacks between fire and herbivory and was developed outside Australia, predicts that native and introduced large herbivores will both respond positively to post-fire forage production in Australian savannas where they co-occur. We used grazing exclosures, forage biomass and nutrient analyses and motion-sensor camera-trapping to evaluate the overall robustness of the pyric herbivory model in the Australian context, specifically whether forage quantity and quality are impacted by herbivory, season and fire activity, and which forage attributes most influence large grazing herbivore abundance. Forage quantity, as measured by live, dead and total herbaceous biomass and proportion of biomass alive, was higher inside herbivore exclosures, even at relatively low densities of herbivores. Forage quality, as measured by fibre content, was not affected by herbivory, however, crude protein content of live herbaceous biomass was greater outside herbivore exclosures. Recent fire was an important predictor of all measures of forage quantity and quality. Recent fire occurrence decreased overall quantity (biomass) but increased quality (decreased fibre content and increased crude protein content); late dry season fires resulted in forage with the highest crude protein content. The predictions of the pyric herbivory conceptual model are consistent with observations of the feeding behaviour of introduced bovines and some large macropods in northern Australian savannas, lending support to the global generality of pyric herbivory in fire-prone grassy biomes.

Pyric herbivory in a temperate European wood‐pasture system

“…The combination of fire and herbivory, that is, pyric herbivory, has received much attention in North America, Africa and Australia (Bond, 2005; Bowman et al., 2011; Kirkpatrick et al., 2011; Reid et al., 2023). Despite some studies on pyric herbivory in the southern, Mediterranean, parts of Europe (e.g., see San Emeterio et al., 2023), we are not aware of similar studies in the northern temperate parts of Europe.…”

“…Since then, the term has been used frequently, mostly in studies from North America (e.g. see Allred et al, 2011;Lautenbach et al, 2021;Leverkus et al, 2018;Starns et al, 2020), Australia (Reid et al, 2023) and Africa (Archibald, 2008;Archibald & Hempson, 2016;Donaldson et al, 2018;Eby et al, 2014;Krook et al, 2007). These studies suggest that pyric herbivory creates heterogeneity on much broader scales than when fire and herbivory act as two separate forces (see also Fuhlendorf et al, 2009).…”

“…The process of pyric herbivory is driven by the preference of herbivores to graze in recently burned areas (Allred et al., 2011; Archibald et al., 2005; Archibald & Bond, 2004; Donaldson et al., 2018; McGranahan et al., 2014). Recently burned areas attract herbivores because grass that re‐sprouts after a fire has an increased nutritional value, leaf‐to‐stem ratio, live‐to‐dead‐tissue ratio and digestibility of dry matter relative to before the area was burned (Allen et al., 1976; Blair, 1997; Eby et al., 2014; McGinty et al., 1983; Reid et al., 2023; Sittler et al., 2019; Thapa et al., 2022). The attraction of herbivores is often strong in the growing season following a fire event and can remain several years after the fire (Ranglack & Du Toit, 2015).…”

“…American bison Bison bison prefer to graze in recently burned areas in prairies in the USA (Ranglack & Du Toit, 2015; Raynor et al., 2015; Winter et al., 2015), and both cattle and American bison can spend up to 70% of their time in recently burned patches (Fuhlendorf & Engle, 2004; West et al., 2016). However, the influence of fire on herbivory differs between herbivores of different size, digestive physiology and feeding type (Nieman et al., 2022; Reid et al., 2023). In Africa, studies on herbivore behaviour have found that larger herbivores tend to be less attracted to recently burned areas than smaller herbivores (Eby et al., 2014) and ruminants are more attracted than non‐ruminants (Nieman et al., 2021).…”

Direct and indirect effects of fire on parasites in an African savanna

“…Fires in savannas and grasslands facilitate mammalian herbivore grazing by removing dead and moribund vegetation and initiating high‐quality regrowth (Archibald & Bond, 2004; Van de Vijver et al, 1999). Fire‐driven changes to the vegetation layer concentrate herbivores on recently burned patches in a magnet effect termed ‘pyric herbivory’ (Fuhlendorf et al, 2009), a phenomenon that has been documented in Australia (Reid et al, 2022), North America (Allred et al, 2011), Asia (Thapa et al, 2022) and Africa (Archibald & Bond, 2004) in both livestock and wildlife. Fire and subsequent herbivory can drive changes in grass community composition (Koerner & Collins, 2014), create heterogeneity in vegetation structure (Donaldson et al, 2018), impact tree–grass interactions (Donaldson et al, 2022) and drive long‐term changes in animal distributions (Burkepile et al, 2016).…”