Fire and ant interactions mediated by honeydew and extrafloral nectar in an australian tropical savanna

Abstract

Fire is a major disturbance affecting ecosystems globally, but its impact on mutualisms has received minimal attention. Here, we use a long-term field experiment to investigate the impact of different fire regimes on globally important ant-honeydew and ant-extrafloral nectar (EFN) mutualistic interactions in an Australian tropical savanna. These interactions provide ants with a key energy source, while their plant and hemipteran hosts receive protection services. We examined ant interactions on species of Eucalyptus (lacking EFNs) and Acacia (with EFNs) in three replicate plots each of burning every 2 and 3 years early in the dry season, burning late in the dry season every 2 years, and unburnt for > 25 years. The proportions of plants with ant-honeydew interactions in Acacia (44.6%) and Eucalyptus (36.3%) were double those of Acacia plants with ant-EFN interactions (18.9%). The most common ants, representing 85% of all interactions, were behaviourally dominant species of Oecophylla, Iridomyrmex and Papyrius. Fire promoted the incidence of ant interactions, especially those involving EFNs on Acacia, which occurred on only 3% of plants in unburnt plots compared with 24% in frequently burnt plots. Fire also promoted the relative incidence of behaviourally dominant ants, which are considered the highest quality mutualists. Contrary to expectations, frequent fire did not result in a switching of behaviourally dominant ant partners from forest-adapted Oecophylla to arid-adapted Iridomyrmex. Our findings that frequent fire increases ant interactions mediated by honeydew and extrafloral nectar, and promotes the quality of ant mutualists, have important implications for protective services provided by ants in highly fire-prone ecosystems.

Keywords: Disturbance; Dominant ants; Experimental burning; Hemipteran interactions; Mutualism.

Fig. 1

Examples of the different fire treatments at the Territory Wildlife Park near Darwin in Australia’s Northern Territory showing a an experimental fire during June 2020 at plot E2-B (burnt every 2 years since 2004), and b plot U-B (unburnt for > 30 yrs). The tallest trees are approximately 15 m in height. [Photo credits: A. Andersen]

Fig. 2

Images showing ant–honeydew and ant–extrafloral nectar mutualistic interactions: a an extrafloral nectar secretion on Acacia holosericea,, b Polyrhachis inconspicua interacting with extrafloral nectary on Acacia oncinocarpa, c Papyrius sp. 1 interacting with Sextius virescens (family Membracidae) on Acacia sp., d Oecophylla smaragdina interacting with Acizzia nymphs (family Psyllidae) on Acacia dimidiata, e Papyrius sp. 1 interacting with Glycaspis lerps (family Aphalaridae) on Eucalyptus miniata, and f Oecophylla smaragdina interacting with Sextius virescens on Acacia sp. at the Territory Wildlife Park near Darwin in Australia’s Northern Territory. [Photo credits: F.M.P. Oliveira]

Fig. 3

Mean incidence of ant interactions with eucalypt honeydew, acacia honeydew and acacia EFN across the 12 experimental burn plots at Territory Wildlife Park, Darwin, Australia. Comparisons of individual burning treatments (a) and frequently burnt (E2 + L2 + E3) vs unburnt plots (b). Codes represent plots with different fire frequency and time-since-fire: burning every 2 years late in the dry season (L2), burning every 2 years early in the dry season (E2), burning every 3 years early in the dry season (E3) and remaining unburnt (U). Different letters represent significant differences between fire regimes

Fig. 4

Mean incidence of ant interactions mediated by dominant, subdominant and submissive ants visiting honeydew and EFN resources (combined across Eucalyptus and Acacia) across the 12 experimental burn plots at Territory Wildlife Park, Darwin, Australia. Comparisons of within the individual burning treatments (a) and frequently burnt (E2 + L2 + E3) vs unburnt plots (b). Codes represent plots with different fire frequency and time-since-fire: burning every 2 years late in the dry season (L2), burning every 2 years early in the dry season (E2), burning every 3 years early in the dry season (E3) and remaining unburnt (U). Different letters represent significant differences between the fire regimes