First evidence of aggressive interactions by native rakali towards invasive black rats

Introduction

Invasive alien species pose a major threat to biodiversity globally (Doherty et al. 2016; IPBES 2019). Native fauna, however, can reduce the establishment of invasive species (cf. biotic resistance and Darwin (1859) naturalisation hypothesis). Even successful invaders can be challenged by native fauna, which have the potential to limit the abundance, distribution and hence impact of alien invasives (Mack et al. 2000; Cabrera-Guzmán et al. 2015). However, the mechanisms driving biotic resistance are poorly understood and native species are often assumed to be inferior to successful invasive alien species (e.g. see Gaschk and Clemente 2022).

Interspecific competition is an important factor structuring many animal communities and is thought to play a key role in biotic resistance (Thuiller et al. 2010). Exploitation competition is the most obvious means by which native species may hold an advantage over invaders (because of adaptation to the local environment). However, interference competition, characterised by a direct antagonistic interaction, is common among mammalian carnivores, and can reduce an inferior competitor’s ability to exploit a common resource (Case and Gilpin 1974; Palomares and Caro 1999). In extreme forms, interference competition may manifest as intraguild killing or predation (Polis et al. 1989). In Australia, studies on interference competition between native and invasive species have largely focused on the limiting effects of dingoes (Canis lupus dingo) on the introduced red fox (Vulpes vulpes) (e.g. Mitchell and Banks 2005; Johnson and VanDerWal 2009; Moseby et al. 2012). In order to understand the role of interference competition in biotic resistance, there is a need for more evidence of potential competitive interactions via interference mechanisms.

Rakali (Hydromys chrysogaster) also known as water rats, are Australia’s largest native rodent, weighing up to 1.3 kg. They are nocturnal, semi-aquatic mammals, often described as Australia’s otter (Williams 2019). Rakali are carnivorous, and while their diet mainly consists of aquatic prey, they are highly opportunistic predators and will also consume small mammals, birds and lizards (Watts and Aslin 1981b). Rakali predominantly forage in water and along the shoreline, but in winter spend more time hunting on land for larger vertebrate prey (Fanning and Dawson 1980). Their elusive nature means there is limited knowledge about their competitive interactions with either native or introduced species, or even their distribution and status in most parts of their range (Williams 2019; Sanders et al. 2024a). Rakali have been reported killing invasive cane toads (Rhinella marina) (Parrott et al. 2020), and invasive fish species can sometimes comprise large portions of their diet (Sanders et al. 2024b). Their interactions with invasive competitors, however, are not known.

Black rats (Rattus rattus; average weight 180 g) arrived in Australia with European settlement and are now an established invasive species with a wide distribution throughout Australia’s coastal areas (Watts and Aslin 1981a; Banks and Hughes 2012; Banks et al. 2023). They are amongst the world’s worst 100 alien species (Lowe et al. 2000) and can have severe impacts on native wildlife, especially in systems without native rodents (Towns et al. 2006). Yet Australia has a diverse native rodent fauna and there is evidence that native bush rats (Rattus fuscipes), for example, can provide biotic resistance against black rats in undisturbed forests (Stokes et al. 2009a). Both rodents are about the same size and are symmetrical competitors, however when given a territorial advantage, bush rats are behaviourally dominant over black rats (Stokes et al. 2012), and exclude them at small (Stokes et al. 2009a) and large scales (Stokes et al. 2009b). Recently, Gagnon and Bateman (2024) reported camera trap evidence of black rats as aggressors, attacking native quenda (Isoodon fusciventer) in a Perth backyard at a food patch, although quenda appeared to fend off the attack. It is possible then, that resident rakali, being carnivorous and more than five times heavier, would be behaviourally dominant and aggressive towards black rats.

Here, we report evidence of aggressive interactions between a rakali and a black rat in natural environments of Sydney Harbour. Such interactions are consistent with interference competition, which can effect demography (Berger and Gese 2007), shift habitat or dietary niches (Bonesi et al. 2004), alter spatial, temporal and foraging behaviours (Foster 2010), and have fitness effects on life-history traits (Eccard and Ylönen 2003) of subordinate competitors. Our evidence of direct aggressive encounters suggests that rakali may provide native biotic resistance to invasive black rats, a potential ecological role that warrants further investigation.

Materials and methods

The observation took place in bushland on the foreshore of Sydney Harbour (near Collins Beach, North Head, Manly Local Government Area (LGA)), New South Wales, Australia, where there are populations of both rakali and black rats (22 rakali and 11,567 black rat records for Manly LGA in the Atlas of Living Australia). An infra-red motion sensing camera (ScoutGuard model 550v3) mounted on a wooden stake was set up on 29 June 2011 as part of a pilot study to understand spatial relationships between activity of rakali and black rats. The camera was set to record 60-s video clips, with a 5-min interval between possible triggers, from dusk until dawn. The camera was aimed at a food attractant (sardines plus cardboard soaked in sardine oil) staked to the ground with a tent peg. This was one of three cameras set near Collins Beach and cameras were collected after three nights.

Results

On 29 June 2011 at 22:22 AEST, the camera trap recorded a rakali next to a rock and hidden by vegetation, as a black rat approached the attractant (Fig. 1). The rakali then leapt out and chased off the black rat (Fig. 2) (see Supplementary video for camera trap video footage).

Fig. 1.

Still from video footage of a rakali waiting in ambush (top right corner; indicated by the blue arrow and circle) as a black rat approaches the attractant.

Fig. 2.

Sequential stills from video footage showing a rakali waiting in ambush (top right corners; indicated by the blue arrow and circle in (a)) as a black rat approaches the attractant (a–d). The rakali (blue circle) then leaps out and chases the black rat (purple circle) off (e, f) (29 June 2011 at 22:22 AEST).

The rakali appeared in two earlier videos on the same night taken at 21:55 and 22:07. Black rats appeared in 14 videos prior to the altercation and six videos after the altercation. The last black rat video prior the altercation was 28 min before the rakali arrived, and the first black rat video following the altercation was 20 min after the altercation. It is not clear whether the black rats in these videos were the same individual that was ambushed by the rakali. The rakali was not captured on either of the other two cameras set near Collins Beach.

Discussion

Our camera trap evidence of a rakali lying in wait and launching an aggressive attack is consistent with rakali being dominant in interference competition with black rats. Understanding behavioural interactions between competitors has heavily relied on indirect evidence, such as avoidance of cues and segregation of temporal or spatial activity (Dickman 1991; Maitz and Dickman 2001). This is partly because direct observation was, for a long time, difficult to obtain, especially for small cryptic nocturnal wildlife inhabiting dense vegetation. The rise in the use of camera traps for wildlife surveys has changed this to some extent (Caravaggi et al. 2017), but video evidence of direct encounters between small competitors (or predator and prey) in dense vegetation remains relatively uncommon (but see Gagnon and Bateman 2024). This can partly be attributed to many camera trap programs using photos rather than videos (for logistical reasons), and considerable luck is needed to capture an encounter. Thus, our evidence of a direct aggressive interaction between a rakali and a black rat suggests the aggression might be common, offering the possibility that rakali have potential as native biological control agents for invasive black rats, through interference competition.

We do not know if the observed aggressive interaction is typical behaviour, however it nevertheless suggests the potential for rakali to exert sublethal impacts on black rats, and possibly some predation capacity. The sublethal effects arising from fear can be profound and have even greater impact on potential prey than lethal effects (Preisser et al. 2005). Little is known about the terrestrial hunting style of rakali. In our video footage, the rakali was lying in wait as the black rat approached, seemingly unaware of the rakali’s presence, a rakali ambush rather than a cursorial attack. This observation fits with other accounts of the rakali’s aquatic predatory behaviour to sit-and-wait at the waters edge with parts of their vibrissae submersed (Hanke et al. 2020). Given the rakali’s carnivorous diet and much larger size compared to black rats, intraguild predation on black rats is possible. Captive rakali have been known to kill and consume Rattus sp. that have strayed into their cages (Olsen 1982).

While black rats are good climbers and likely access arboreal niches in the landscape (Smith et al. 2016) that rakali use far less, the same is also true for bush rats, which given a territorial advantage, are still able to exclude black rats (Stokes et al. 2009a). Thus, despite the semi-aquatic lifestyle of rakali, the presence of rakali may limit black rat resource use, potentially reducing the abundance and impacts of this invasive species in certain areas. While we cannot discern whether the observed interaction was aggressive interference competition or intraguild predation, the ecological consequences (limiting resource use) are the same.

Our findings suggest there is potential for rakali to play a positive role in biotic resistance against invasive black rats. Greater understanding of the complex competitive interactions between these two species could better inform the management of invaded mammal communities in Australia. Rakali persist in all major cities in Australia where black rats as human commensals are also most common (Banks and Smith 2015; ALA 2025) and both species have some overlap in natural habitat preferences (e.g. Smart et al. 2011). However, the factors influencing rakali habitat use and abundance in these urbanised environments are not well known. Further research into the conservation and promotion of rakali is thus needed to better understand its potential to impose top-down pressure on invasive black rat populations. Animal behaviour is increasingly being recognised as a potential tool in conservation biology (Greggor et al. 2016). Rakali aggression towards black rats may offer such potential.

Supplementary material

Supplementary material is available online.