INTRODUCTION
Predation is one of the most pervasive and powerful forces acting on
populations. Not only does predation directly impact a population’s
demography (Schoener & Spiller 1996), it also imposes natural selection
(Abrams 2000). The pressure that predators impose on populations will
vary through time and space for many reasons, including tightly coupled
predator-prey dynamics, predator movement, prey switching, or stochastic
processes (Lima & Dill 1990; Sih 1992). The fact that predation is not
constant, and that antipredator defences may be costly, suggests that
flexible responses to predation pressure will often be favoured (Sihet al. 2000; Berger et al. 2001). There is, in fact, a
great deal of empirical evidence that flexible responses to predation
are common and vary with the perceived risk of predation (e.g. Relyea
2003; Brown et al. 2013; Cunningham et al. 2019).
Predators also play a powerful role in structuring communities (Esteset al. 2011). Some of our best evidence for this comes from the
introduction of novel predators. Invasive predators can cause
extinctions (Medina et al. 2011; Woinarski et al. 2015;
Doherty et al. 2016), and alter trophic structures and ecosystem
function within recipient communities (Courchamp et al. 2003;
Simberloff et al. 2013). These cascading outcomes are often
treated as purely numeric effects: predators depress the size of prey
populations, and the altered numbers of prey can cause cascading
numerical changes down trophic levels (Ripple et al. 2001). These
numerical effects are undeniably important, but the fact that predators
can also elicit phenotypic change in prey populations—through
phenotypic plasticity and natural selection—means that subtler
ecological effects may also manifest. Prey species living alongside
predators may forage at different times, or in different places compared
with the same species in a predator-free environment (Laundre et
al. 2010). Such behavioural shifts can alter downstream species
interactions in potentially complex ways (Fortin et al. 2005;
Suraci et al. 2016).
Because predator invasions are rarely intentional or anticipated, there
is a scarcity of controlled empirical work on the effects of novel
predators on recipient communities and the mechanisms via which these
effects play out (but see Lapiedra et al. 2018; Pringle et
al. 2019). Such tests are needed, however, if we are to predict
invasive species impacts, and improve both conservation management (Sihet al. 2010a) and our understanding of how communities are
structured (Sax et al. 2007).
Northern quolls (Dasyurus hallucatus ) were, until recently, a
common predator across northern Australia. They have declined over the
last several decades, following the general decline in northern
Australian mammals (Woinarski et al. 2015)(Braithwaite &
Griffiths 1994). More recently, the invasion of toxic invasive prey
(cane toads, Rhinella marina ) has resulted in dramatic,
range-wide population declines in northern quolls (Shine 2010; Oakwoodet al. 2016)(Moore et al. 2019). For their conservation,
northern quolls have recently been introduced to a number of offshore
islands where they have never previously existed.
In 2017, a population of 54 northern quolls were introduced to a
25km2 island off the coast of north-western Northern
Territory, Australia (Kelly 2019). Prior to this introduction, Indian
Island (Kabarl) lacked mammalian predators, and large native reptilian
predators had recently been reduced to near extinction by the invasion
of cane toads. We take advantage of the introduction of northern quolls
to a new island to directly test the effects of quolls as a novel
predator on an island ecosystem and observe how native prey populations
adjust to mitigate the impacts of their arrival. Since quolls are an
ecologically novel predator on this island, we predict that this
introduction may result in demographic effects (reduced survival and
abundance) in invaded prey populations. If behavioural adjustments are
able to reduce the demographic effects of a novel predator, we predict
rapid behavioural changes in quoll-exposed melomys populations, such as
changes in personality composition, foraging behaviour and responses to
predator-scent, may manifest through time.