Different burning intensities affect cavity utilization patterns by arboreal ants in a tropical savanna canopy

Highlights

• Fire had a significant effect on experimental cavity use by arboreal ants.

• There were more ant species, more colonized trees, and higher cavity occupation under fire treatments.

• The effect of fire on cavity occuaption patterns was dependent on its intensity.

• Some ant species were consistently found either exclusively or at higher rates on distinct fire treatments.

Abstract

Fire is a natural disturbance in many ecosystems worldwide, including tropical savannas, where it drives the assembly of animal and plant communities. The effect of fire is often indirect, impacting the availability of essential resources like food and shelter. The Brazilian Cerrado is the largest tropical savanna in the world, and ants comprise a very high proportion of its animal biomass. About one-third of the Cerrado ant fauna actively forages in the canopy. These ants use trees not just in search of food but also as their main shelter resource, as most arboreal ants nest mainly in cavities produced by wood-boring beetles. While it seems clear that fire can have important direct and indirect effects on arboreal ants, the number of studies using an experimental approach focusing on such impacts is relatively few. Here, we aimed to understand the impacts of fire on arboreal ants’ usage of shelter resources. Moreover, we also evaluated the potential influence of different fire intensities on these resources’ usage. For this, we assessed the colonization rates of experimental cavities under four different burning intensities: ‘control’, ‘low-intensity’, ‘intermediate-intensity’, and ‘high-intensity’. We hypothesized that fire would have a detrimental effect on cavity occupation. This effect would be stronger in higher intensity burns, once they can dramatically decrease the availability of natural cavities and, thus, the abundance and richness of arboreal ants. We also predicted that some ant species would be indicative of different fire intensities, due to differences in heat tolerance and colony size. Our data showed that fire had a major positive effect on experimental cavity use by arboreal ants, with more ant species, more colonized trees, and higher cavity occupation under fire treatments. This effect was higher with the increase of fire intensity. Some ant species were found more commonly on distinct fire treatments, which was mainly due to their different nesting site requirements and colony sizes. Jointly, these results show that fire affects arboreal ant cavity colonization patterns, and that fire intensity is extremely critical in shaping these patterns. Altogether, our work offers important insights for a better understanding of the effects of ecological disturbances on resource use patterns of a highly abundant and diverse organism.

Introduction

Fire is an important natural disturbance in many ecosystems of the world, and some systems, such as tropical savannas, are a product of frequent fire events (Bowman et al., 2009, Pyne, 1997, Simon et al., 2009). In these biomes, fire influences patterns of biodiversity and drives population dynamics and community assembly of animals and plants (He et al., 2019, Koltz et al., 2018, Pausas and Keeley, 2019, Pausas and Parr, 2018). The effects of fire disturbance on species assembly patterns can operate at different levels. While it can affect population size by direct mortality, it can also have an indirect effect on population dynamics by impacting the availability of essential resources, and especially food and shelter resources (Andersen, 2018, Fagundes et al., 2015, Frizzo et al., 2012). In many cases, both direct and indirect effects can be acting at the same time, and identifying the main cause of changes in species assemblies after fire events is notoriously hard to track.

Some insects, like ants and termites, are responsible for a large portion of the animal biomass of tropical savannas (Dawes-Gromadzki, 2008) and can be used as biological indicators (Andersen et al., 2012, Cunha, 2006, Hoffmann and Andersen, 2003, Majer, 1983, Tiede et al., 2017) since these highly diverse organisms show fast responses to ecological disturbances (Andersen and Müller, 2000, Eggleton et al., 2002, Hoffmann and Andersen, 2003, Majer et al., 2007). Although fire can have crucial effects on the communities of these insects, the main effects are not through direct mortality, but rather through changes in the habitat, and consequently on resource availability (Costa et al., 2018, Fagundes et al., 2015, Kim and Holt, 2012, Swengel, 2001). Thus, it is expected that those animals that rely directly on arboreal resources may be especially sensitive to fire (Frizzo et al., 2012), and their responses may be idiosyncratic, depending on fire intensity, duration, and frequency (Vasconcelos et al., 2017).

The Cerrado is the largest South American savanna and is also the most diverse and threatened of all grassy biomes, and is considered a hotspot of biodiversity (Myers et al., 2000). Ants comprise a high proportion of the animal biomass in this ecosystem, and about one-third of the ant fauna of Cerrado actively forage on trees (Camacho and Vasconcelos, 2015). Cerrado arboreal ants feed on other arthropods and the sugar-rich liquid food resources produced by sap-sucking hemipterans and extrafloral-nectaries (Ribas et al., 2010, Rico-Gray and Oliveira, 2007). Arboreal ants also use trees as shelter, nesting almost exclusively in abandoned feeding tunnels of wood-boring beetles (Novais et al., 2017, Philpott and Foster, 2005, Powell et al., 2011). Indeed, recent research shows that shelter resources are very limited for arboreal ants, and may be much more important in determining arboreal ant species assembly patterns than previously thought (Debout et al., 2007, Jiménez-Soto and Philpott, 2015, McGlynn, 2006, Peeters and Molet, 2009, Philpott and Foster, 2005, Powell et al., 2011). Thus, Cerrado arboreal ants are ideal organisms to study the impacts of fire events on resource use and, ultimately, on arboreal ant species assembly patterns.

Here, we aimed to understand the impacts of fire on arboreal ant usage of essential and limited shelter resources. Moreover, we also assessed the potential influence of different fire intensities on arboreal ant shelter use. For this, we assessed the colonization rates of experimental cavities under four different burning intensities: ‘control,’ ‘low-intensity,’ ‘intermediate-intensity,’ and ‘high-intensity.’ We hypothesized that fire would have a detrimental effect on experimental cavity occupation by arboreal ants, with less occupied nests per tree and also a lower number of trees with occupied nests in the fire treatments. Importantly, this effect would be dependent on fire intensity, being stronger in higher intensity burns, once they can dramatically decrease the availability of natural cavities and, thus, the abundance and richness of arboreal ants. We also predicted that some ant species would be indicative of different fire intensities, due to differences in heat tolerance and colony size.