Download Evaluating the role of the dingo as a trophic

Austral Ecology (2007) 32, 492–501
doi:10.1111/j.1442-9993.2007.01721.x
Evaluating the role of the dingo as a trophic regulator in
Australian ecosystems
A. S. GLEN,1* C. R. DICKMAN,2 M. E. SOULÉ3 AND B. G. MACKEY4
Department of Environment and Conservation and Invasive Animals CRC, Dwellingup Research
Centre, Banksiadale Rd, Dwellingup,WA 6213, Australia (Email: [email protected]), 2Institute of
Wildlife Research, School of Biological Sciences A08, University of Sydney, New South Wales, 4School of
Resources, Environment and Society, College of Science, The Australian National University, Canberra,
ACT, Australia; and 3Professor Emeritus, University of California, Santa Cruz, USA
1
Abstract The importance of strongly interactive predators has been demonstrated in many ecosystems, and the
maintenance or restoration of species interactions is a major priority in the global conservation of biodiversity. By
limiting populations of prey and/or competitors, apex predators can increase the diversity of systems, often exerting
influences that cascade through several trophic levels. In Australia, emerging evidence points increasingly towards
the dingo (Canis lupus dingo) as a strongly interactive species that has profound effects on ecosystem function.
Through predatory and competitive effects, dingoes can alter the abundance and function of mesopredators
including the introduced red fox (Vulpes vulpes) and feral cat (Felis catus), and herbivores including the European
rabbit (Oryctolagus cuniculus). These effects often benefit populations of native prey, and diversity and biomass of
vegetation, but may not occur under all circumstances. For example, the social structure of dingoes is of great
importance; a pack subject to minimal human interference regulates its own numbers, and such packs appear to
have fewer undesirable impacts such as predation on livestock. Despite abundant observational evidence that the
dingo is a strong interactor, there have been few attempts to test its ecological role experimentally. Given the
well-recognized importance of species interactions to ecosystem function, it is imperative that such experiments be
carried out. To do this, we propose three broad questions: (i) do dingoes limit the abundance of other predators or
prey? (ii) do dingoes affect the ecological relationships of other predators or prey (e.g. by altering their spatial or
temporal activity patterns)? and (iii) does the removal or reintroduction of dingoes entrain ecological cascades?
Finally, we discuss the design of appropriate experiments, using principles that may also be applied to investigate
species interactions on other continents. Research might seek to clarify not only the impacts of dingoes at all trophic
levels, but also the mechanisms by which these impacts occur.
Key words: dingo, interaction, keystone, mesopredator release, trophic cascade.
INTRODUCTION
The importance of top-order predators in maintaining
ecosystem function has been demonstrated in many
marine and terrestrial systems (e.g. Paine 1966, 1969;
Estes & Palmisano 1974; Soulé et al. 1988; McLaren &
Peterson 1994; Henke & Bryant 1999). By limiting
populations of their prey and/or subordinate competitors, top-order predators can modulate the diversity of
a system, and may ultimately increase plant biomass
via a series of trophic links (Paine 1980; Pace et al.
1999; Polis et al. 2000; Schmitz et al. 2000; Ripple &
Beschta 2003). Species that perform this function are
known as keystone predators (Paine 1966, 1969). The
identification and conservation of such species, and
*Corresponding author.
Accepted for publication October 2006.
© 2007 Ecological Society of Australia
maintenance of their interactions, is of vital importance in arresting the global loss of biodiversity (Soulé
et al. 2003, 2005).
Where top-order predators are completely or virtually removed from a system, the effects on species
richness and abundance at lower trophic levels can be
profound. Previously subordinate predators may
increase unchecked, potentially decimating prey
populations (Soulé et al. 1988; Crooks & Soulé 1999).
Herbivores can become over-abundant, leading to
overgrazing on plant populations (McLaren &
Peterson 1994). Competitive relationships between
prey species may be altered (Paine 1966). In some
cases, these effects lead ultimately to community-level
trophic cascades (Polis et al. 2000) in which plant
biomass is redistributed throughout the system.
In Australia, a wealth of observational evidence,
backed by a small but growing body of experimental
work, suggests that removal of the continent’s largest
T H E D I N G O A S A T R O P H I C R E G U L ATO R
terrestrial predator can lead to such impacts. The
dingo (Canis lupus dingo) is a large canid, which represents an early form of domesticated wolf, and was
introduced to Australia by Asian seafarers some 3500–
4000 years ago (Gollan 1984; Corbett 1995). The
arrival of the dingo is likely to have contributed to the
extinction of the thylacine (Thylacinus cynocephalus)
and the Tasmanian devil (Sarcophilus harrisii) on the
Australian mainland (Corbett 1995). However,
changes in human hunting technology around the
same time may also have been involved ( Johnson &
Wroe 2003). Since the advent of the pastoral industry,
the dingo has been viewed as an agricultural pest, and
human persecution has seen its virtual extirpation
from much of south-eastern Australia (Rolls 1969;
Glen & Short 2000; Fleming et al. 2001). In many
areas where the dingo still persists, it is reviled and
subject to continued human harassment such as poisoning, shooting and trapping (Fleming et al. 2001).
Given the dingo’s role as the top predator in Australian terrestrial ecosystems, what effects does its
removal or persecution have upon ecosystem function?
Three relatively recent arrivals in Australia, the red fox
(Vulpes vulpes), feral cat (Felis catus) and European
rabbit (Oryctolagus cuniculus), have become major
threats to biodiversity and agriculture (Rolls 1969;
Saunders et al. 1995; Williams et al. 1995; Dickman
1996). All three of these species interact with the dingo
and with each other (Pech et al. 1992; Corbett 1995;
Holden 1999; Holden & Mutze 2002; Glen &
Dickman 2005). Rabbits, in turn, are linked to plant
biomass, vegetation structure and diversity through
direct grazing effects (Auld 1992; Williams et al. 1995;
Lord 2000). Thus, persecution of the dingo may have
impacts that cascade through the trophic levels from
predator to mesopredator to herbivore and ultimately
to primary producers. In many respects, the situation
of the dingo mirrors that of the grey wolf (Canis lupus
ssp.) in North America (e.g. Bradley et al. 2005), in
that its management is governed by the conflicting
goals of protecting livestock while maintaining topdown ecosystem regulation and ecosystem integrity.
Here we investigate evidence for trophic regulation of
ecosystems by the dingo, suggest some questions for
future research, and discuss the design of experiments
to test these. We see these evidence-based steps as
critical for providing informed management of the
dingo in future.
Evidence for impacts of dingoes
Large carnivores can limit the density of smaller ones
by several means, both direct and indirect. Direct
effects include interspecific aggression or predation
(e.g. Palomares & Caro 1999), physical exclusion from
contested resources (e.g. Johnson & Franklin 1994) or
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kleptoparasitism (e.g. Gorman et al. 1998). Indirect
effects may result from exploitation competition where
prey or other resources are limiting. There is evidence
to suggest that dingoes affect feral cats and foxes by all
these means. Of course, predators also affect herbivore
populations through direct predation (e.g. Pech et al.
1992; Banks 2000), and there is evidence that a
number of native and introduced herbivores, including
rabbits, are limited by predation from dingoes. In
general, solitary dingoes kill small- to medium-sized
prey, whereas cooperative hunting by a stable pack is
required to subdue larger animals (Corbett 1995). As
discussed below, stable packs may occur only where
there is minimal human disturbance.
Direct effects
The most direct and obvious means by which dingoes
might suppress smaller predators is by killing them,
and indeed this has been documented in several
studies. For example, Marsack and Campbell (1990)
directly observed dingoes preying on foxes, and found
fox remains in 6.1% of 49 digestive tracts and 2.4% of
82 dingo scats. Cat remains were also found in one gut
sample and one scat. Consumption of cats by dingoes
has also been reported by Newsome et al. (1983),
Lundie-Jenkins et al. (1993), Thomson (1992a),
Corbett (1995) and Paltridge (2002). In addition, Pettigrew (1993) reported that an adult cat fitted with a
radio collar was killed by a dingo. Similar interactions
have been reported in the UK between domestic dogs
(Canis lupus familiaris) and red foxes. Juvenile foxes
were preyed upon by dogs, while adults were harassed,
chased and occasionally killed.These interactions limit
the distribution of foxes in Bristol (Harris 1981).
As well as exerting predatory effects, dingoes probably exclude smaller predators from resource-rich
patches by scent marking. Odours produced by urine,
faeces and glandular secretions are used widely by
carnivores to communicate ownership of territories
and food resources (such as rabbit warrens), social
dominance and reproductive status (Ewer 1973;
Macdonald 1979, 1987; Thomson 1992b; O’Neill
2002). Such odours can also deter subordinate
species, which may be vulnerable to aggression or predation (e.g. Dickman & Doncaster 1984). Observations in semiarid areas of Australia suggest that foxes
are excluded from the warrens of greater bilbies
(Macrotis lagotis) and nest mounds of malleefowl
(Leipoa ocellata) by the presence of dingo faeces
(O’Neill 2002). By denying foxes access to such
resources, dingoes might limit the abundance and distribution of their smaller counterparts. Perhaps more
importantly, this phenomenon could reduce the
impact of foxes on these endangered prey species, even
if fox abundance is not reduced.
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Dominant predators may also exclude subordinate
ones on a larger spatial scale. For example, in North
America, red foxes have been observed to occupy the
interstices between the home ranges of coyotes (Canis
latrans) (Harrison et al. 1989), and coyotes appear
similarly to avoid areas frequently used by wolves (Arjo
& Pletscher 1999).
In addition to killing or interfering with subordinate
predators, dingoes also exert predatory effects on
herbivores. Rabbits are a staple prey in many areas,
and native herbivores such as kangaroos and wallabies
are also commonly consumed (Robertshaw & Harden
1985, 1986; Thomson 1992a; Corbett 1995). Dingoes
may be capable of limiting or regulating populations of
herbivores, particularly in the presence of alternative
prey. Corbett (1995) reported that rabbit populations
can increase following dingo control, and suppression
of rabbits by dingoes is thought to have beneficial
effects on vegetation communities (Newsome 2001).
In addition, Caughley et al. (1980) and Pople et al.
(2000) reported that populations of red kangaroos
(Macropus rufus) are more abundant in the absence
than in the presence of dingoes, although differences in
habitat and livestock grazing practices may contribute
to this pattern (Caughley et al. 1980; Newsome et al.
2001). Similarly, Newsome (1990) observed that
eastern (Macropus giganteus) and western grey kangaroos (M. fuliginosus), euros (M. robustus), feral goats
(Capra hircus) and feral pigs (Sus scrofa) all occurred
where dingoes were excluded by fencing, but were
absent from adjacent areas where dingoes still
occurred. The predatory effects of dingoes may,
however, be environmentally modulated (Newsome
et al. 1989), so that little or no effect of dingoes is
observable when resources such as food and water are
abundant (e.g. Eldridge et al. 2002) or where there is
dense cover (vegetation).
As well as affecting the abundance of herbivores,
predators can influence the foraging behaviour of their
prey. For example, Ripple and Larsen (2000) hypothesized that grey wolves alter the habitat use of elk
(Cervus elaphus), thereby protecting some areas from
grazing and allowing recruitment of aspen (Populus
tremuloides) (see also Ripple & Beschta 2003).
Dingoes, by excluding herbivores such as rabbits or
red kangaroos from certain areas, may be capable of
exerting similar effects. Such changes in the behaviour
or distribution of a prey species may benefit other
native herbivore species.
Robertshaw and Harden (1986) produced evidence
for the influence of dingo predation upon the timing of
reproduction in a swamp wallaby population; dingo
predation induced the wallabies to reproduce yearround rather than seasonally or episodically, reducing
the potential for predator-swamping and potentially
exposing the wallabies to a greater size-range of
predators. This could be an example of a top predator
trapping a prey population in a (multi-predator) ‘predation sump’.
Indirect effects
In addition to predation, competitive aggression or
exclusion, dingoes can affect cats and foxes through
competition for limiting resources. Competition is
likely to be particularly strong during times of low food
availability such as drought (Corbett 1995) and in
regions with little vegetation cover. Substantial dietary
overlap occurs between the three species, and dingoes
can monopolize large carcasses which would otherwise
serve as a source of carrion for cats and foxes (Corbett
1995; Mitchell & Banks 2005). Paradoxically, in some
situations, dingoes may benefit cats and foxes, which
scavenge from carcasses killed by their larger rivals
(e.g. Paltridge et al. 1997). Similar relationships have
been reported by Macdonald (1987), Dickman
(1992), Creel (2001) and Switalski (2003), where subordinate competitors suffer predation or kleptoparasitism from larger predators, but may also scavenge
carcasses killed by the dominant species.
If dingoes do indeed limit populations of cats and
foxes, we would expect to see inverse relationships
between these species in terms of abundance and
distribution. Although the presence of dingoes does
not preclude the occurrence of cats or foxes (e.g.
Catling & Burt 1995; Paltridge et al. 1997), it has often
been observed that foxes are scarce where dingoes are
abundant, and vice versa (Thompson 1983; Jarman
et al. 1987; Johnson et al. 1989; Smith & Quin 1996;
McRae 2004). Newsome et al. (1997) found an inverse
relationship between dingo and fox activity in
Kosciuszko National Park and Nadgee Nature Reserve
in south-eastern Australia, potentially reflecting a relationship between the densities of the two species, or
behavioural avoidance of dingoes by foxes. Similarly,
Dickman (1996), using data presented by Catling and
Burt (1994), showed that the abundance of cats in
southern New South Wales was negatively correlated
with that of dingoes. However, the most striking
example of this negative relationship comes from
Newsome et al. (2001). These authors measured the
relative abundance of foxes and feral cats in adjacent
areas on both sides of the dingo fence, which excludes
dingoes from much of south-eastern Australia. Indices
of fox abundance were between 20.6 and 7.1 times
higher in the absence than in the presence of dingoes.
Cats were at similarly low densities on both sides of the
fence (Newsome et al. 2001).
Despite the striking examples presented above,
Catling and Burt (1995) found no significant relationship between the abundances of dingoes and foxes in
forested habitats of eastern New South Wales. Structural complexity of habitat can moderate competitive
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T H E D I N G O A S A T R O P H I C R E G U L ATO R
interactions and intraguild predation (Petren & Case
1998; Creel 2001; Finke & Denno 2002). It is possible
therefore that densely vegetated areas such as those
studied by Catling and Burt (1995) more readily allow
foxes to coexist with dingoes.The availability of shelter
may facilitate avoidance or escape from the larger
predator. The same effect of habitat structure may
apply to the relationship between dingoes and feral
cats (Pettigrew 1993; Dickman 1996; Edwards et al.
2002).
Little is known of the relationships between dingoes
and smaller native predators such as quolls (Dasyurus
spp.). The western (D. geoffroii) and northern
(D. hallucatus) quoll generally consume small prey
(Soderquist & Serena 1994; Oakwood 1997) and are
therefore unlikely to suffer strong competition with
dingoes for food. The spotted-tailed quoll
(D. maculatus) preys mainly on medium-sized
mammals (Belcher 1995, 2000; Dawson 2005; Glen &
Dickman 2006), and may therefore experience some
exploitation competition from the dingo. There is
also evidence of interference competition. Oakwood
(2000) found that competitive aggression from
dingoes is a major source of mortality for the northern
quoll. Belcher (1995) suggested that dingoes steal prey
from spotted-tailed quolls, although it is also thought
that quolls may scavenge from carcasses killed by
dingoes (Edgar & Belcher 1995). In addition to these
direct effects, dingoes likely have an indirect positive
influence on quoll populations. If dingoes suppress
populations of cats or foxes, their presence may be of
overall benefit to quolls; a possible example of indirect
commensalism. Similarly, wolves in northern Europe
are thought to have benefited arctic foxes (Alopex
lagopus) by suppressing populations of red foxes
(Hersteinsson et al. 1989). In the USA’s Greater
Yellowstone Ecosystem, wolves suppress coyotes but
not red foxes, so that foxes are released from competition with coyotes (Smith et al. 1999).
Through limitation of cats and foxes, dingoes probably also have indirect positive effects on populations
of native marsupials and rodents, including the greater
bilby (Pettigrew 1993), conilurine rodents (Smith &
Quin 1996) and the endangered parma wallaby
(Macropus parma) (Short & Smith 1994). Miller and
Harlow (unpub. 2005) have documented a similar
pattern in Grand Teton National Park in Wyoming.
The lower abundance and altered habitat use of
coyotes in an area frequented by wolves was associated
with higher rates of capture of small mammals, presumably because coyotes consume more small prey
than do wolves.
Effects of dingo removal
If the influence of dingoes upon ecosystems is as pervasive as suggested by the evidence above, we should
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expect to see substantial impacts when dingoes are
removed. Observational evidence suggests that
removal of dingoes may allow introduced predators to
invade areas where they have previously been rare or
absent. For example, Short et al. (2002) reported that
foxes invaded areas of the Tanami Desert in central
Australia following dingo control. Such new arrivals
can have devastating effects. A single immigrant fox is
believed responsible for extirpating a local population
of the endangered rufous hare-wallaby (Lagorchestes
hirsutus) (Lundie-Jenkins et al. 1993). Densities of
feral cats have also been observed to increase following
the removal of dingoes (Lundie-Jenkins et al. 1993;
Pettigrew 1993; Christensen & Burrows 1995). Similarly in North America, expansion in the distribution
of coyotes has followed the decline of the grey wolf
(Mech 1970).
Such effects may not require the complete removal
of the dingo from an area. A strongly interactive
species might become ‘effectively absent’ if its abundance is reduced below a threshold of ecological
effectiveness. The relationships within and between
trophic levels might break down, just as they would if
the strong interactor had been removed entirely (Soulé
et al. 2005). Thus, widespread lethal control could
potentially reduce dingo populations below their ecologically effective density in the region, even if populations are not entirely eradicated. Such a situation
would negate any potentially beneficial effects of
dingoes. Remnant dingo populations following widespread lethal control may also be spatially patchy.
Social disruption of dingo packs
A further danger is that control of dingo populations
may lead to undesired ecological effects by destroying
the social structure of dingo packs. Within a stable
pack, dingoes exist under a strict social system
enforced by the dominant pair. This has implications
for both the abundance of dingoes and their
behaviour. A dominance hierarchy exists, and breeding
is restricted to the dominant pair (Corbett 1995). In
this way, the birth rate remains at a fraction of its
potential maximum, and the pack limits its own
numbers. Stable packs also occupy and defend clearly
defined territories, demarcated by scent-marks
(Corbett 1995). When the stable structure of a pack is
disrupted, for example by human interference, these
strict behavioural controls are lost. If a dominant
female is killed, subordinate females may then produce
several litters in place of the usual one, so that lethal
control leads paradoxically to increased dingo abundance (O’Neill 2002).
As well as facilitating higher abundance of dingoes,
disruption of packs can alter the age structure of
the population (Allen & Gonzalez 1998). Young,
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inexperienced, leaderless individuals will be less proficient at killing large prey such as kangaroos, and
dingoes hunting alone tend to kill smaller prey
(Thomson 1992c; Corbett 1995).Thus, the breakdown
of pack structure may change the focus of predation
from large mammals (such as kangaroos and feral
herbivores) to smaller ones. In addition, relatively naïve
individuals must presumably devote a high proportion
of their activity to exploring their new territories and
establishing a dominance hierarchy. This combination
of circumstances is likely to intensify predation on
livestock and small native prey, which represent easier
pickings than large, mobile prey such as kangaroos.
Allen and Gonzalez (1998) divided three pastoral properties into treatment areas, where dingoes were subjected to poison baiting and experimental controls,
where no baiting occurred. Calf losses were consistently
higher where dingoes were baited than where they were
left undisturbed.Their results indicate that baiting does
not always reduce dingo numbers and, more importantly, that livestock losses are not necessarily related to
the abundance of dingoes (Allen & Gonzalez 1998).
Similarly, Fleming et al. (2006) report that losses
incurred by livestock producers are independent of the
density of wild dogs. O’Neill (2002) suggested that
immigrant dingoes yet to establish their territories are
also ineffective at excluding foxes and feral cats.
difficult and expensive. This is not to say that such
experiments should not be undertaken; they are the
only means by which a robust, convincing case for
competition and other interactions may be obtained.
However, the time and resources needed to carry out
such experiments will be more easily justified if they
are accompanied by other, smaller-scale experiments.
These might provide preliminary evidence to justify
the expenditure of greater resources on large-scale,
more definitive experiments. There are two ways of
approaching this question: first by comparing many
areas with and without dingoes at a single point in
time, and second by conducting experiments at a more
limited number of sites, spanning a longer time period.
Do dingoes affect the ecological relationships of other
predators or prey?
Regardless of whether a strongly interactive predator
directly affects the abundance of other species, it may
exert a strong influence on ecosystems if it alters the
behaviour of other species (Schmitz et al. 1997). For
example, subordinate species may be forced to alter
their use of habitat, or reduce their activity at certain
times of day, in order to avoid aggressive encounters
with the dominant. In turn, this may alter the effects of
subordinate predators on prey populations.
Future research
The dingo, based on the evidence reviewed here, is a
strong candidate for consideration as a strongly interactive species, and therefore, for protection (Power
et al. 1996; Soulé et al. 2003, 2005). It is imperative
that the dingo’s ecological role be further investigated
using rigorous scientific testing. We propose three
broad questions that can provide the focus for future
experiments: (i) do dingoes limit the abundance of
other predators or prey? (ii) do dingoes affect the
ecological relationships of other predators or prey (e.g.
by altering their spatial or temporal activity patterns)?
and (iii) does the removal or reintroduction of dingoes
entrain ecological cascades?
Do dingoes limit the abundance of other predators
or prey?
Despite its conceptual simplicity, a number of logistical considerations render this question a challenging
one to test experimentally. Ideally, putative interactions should be tested with controlled, replicated
experiments that manipulate the densities of predators
(Dickman 1996; Robley et al. 2004; Glen & Dickman
2005). However, the necessary scale of such experiments, both in terms of area and time, makes them
Does the removal or reintroduction of dingoes entrain
ecological cascades?
Assuming that dingoes directly influence their competitors or prey, do these effects lead to greater abundance or diversity of species at lower trophic levels? If
dingoes can limit populations of cats and foxes, this
might lead to increased abundance or expanded distribution of some prey species. Similarly, if dingoes
limit rabbits or other herbivores, this might lead to a
greater diversity and/or biomass of vegetation.To illustrate this, consider a hypothetical example of a trophic
cascade (Fig. 1), which might occur following the
re-establishment of functional dingo packs in an arid
area of Australia. In the absence of dingoes, foxes and
feral cats are abundant, suppressing or extinguishing
populations of small and medium-sized native
mammals such as rufous hare-wallabies. Large native
and introduced herbivores – virtually free of predators
– are abundant, leading to overgrazing, soil erosion
and compaction, thereby reducing plant biomass and
structural habitat complexity. Addition of dingoes to
this degraded system could be expected to have myriad
effects. First, foxes and feral cats would likely be
reduced in abundance, allowing endangered species
like the rufous hare-wallaby to escape predator regulation and recover.
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Dingo
-
-
Fox
Cat
+
-
Rufous harewallaby
Large
herbivores
+
-
+
Small vertebrates
Plant
biomass /
understorey
complexity
+
Fig. 1. A hypothetical example of a trophic cascade. Establishment of functional dingo packs in this previously degraded
system leads to reduced abundance of foxes and feral cats, increased abundance of endangered rufous hare-wallabies, reduced
abundance of large native and feral herbivores, increased plant biomass, increased diversity and structural complexity of the
understorey, and increased abundance and diversity of small vertebrates. Solid arrows show direct effects, broken arrows show
indirect effects, and signs (+, –) indicate the overall effect on the species or category. For simplicity, only selected interactions are
shown.
Second, dingoes could be expected to limit the
density of large herbivores, reducing grazing pressure
and allowing plant biomass to increase. The resultant
increase in habitat complexity would probably have
further benefits for smaller prey species, augmenting
the benefits of release from predation by cats and
foxes. Thus, in terms of measurable outcomes, the
introduction of dingoes to this hypothetical system
could be expected to cause: (i) reduced densities of
foxes and feral cats; (ii) increased density of rufous
hare-wallabies; (iii) reduced densities of large herbivores; (iv) increased plant biomass; (v) increased diversity and structural complexity in the understorey; and
(vi) increased abundance and diversity of small
vertebrates.
Design of predator manipulation experiments
MacNally (1983) outlined three steps for establishing
the importance of interspecific competition within a
guild: (i) measure overlap of resource use between
species; (ii) demonstrate that the presence of one
species alters the resource use of another; and (iii)
demonstrate that the interaction between species has
impacts upon populations (e.g. reduced density) or
individuals (e.g. reduced fecundity). These last two
steps require manipulative experiments in which one
or more species is removed or introduced. Removal
experiments may be undesirable if, as in the case of the
© 2007 Ecological Society of Australia
dingo, the species involved is believed important to
ecosystem function. In such cases, reintroduction
experiments are more desirable, both ecologically and
ethically (Soulé et al. 2005).
As outlined by Dickman (1996), predator removal
or introduction experiments should follow a BACI
(before-after, control-impact) design. Following an
initial monitoring period, predator numbers are
manipulated at a number of sites, while the remaining
sites are left undisturbed, acting as experimental
controls. Replication is essential, and the power of the
experiment increases with more treatment and control
sites (Dickman 1996).The spatial scale of such experiments must be proportional to the home ranges of the
study animals. Each study site must be large enough to
permit the occurrence and detection of population
growth. In addition, the study sites must be separated
by sufficient distance to ensure that they are relatively
independent of each other (Dickman 1996). Given the
highly mobile nature of dingoes, study sites would
need to be of the order of tens or hundreds of square
kilometres, and separated by distances of at least
50 km (unless fenced). In order to avoid pseudoreplication, treatment and control sites should either
be interspersed (if there are few study sites) or
assigned randomly (if there are many sites). To avoid
other confounding factors, treatment methods should
also be simulated in the non-treatment areas. For
example, if predators are removed by spotlighting and
shooting, animals at the non-removal sites should be
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subjected to spotlighting and the firing of blank
ammunition (Dickman 1996).
As well as an appropriate spatial scale, perturbation
experiments must run over a sufficiently long time so
that treatment effects may be differentiated from the
stochastic variations typical of Australian landscapes,
especially those due to the year-to-year variability in
rainfall and associated primary productivity (Dickman
1996; Robley et al. 2004; Davey et al. 2006). Experiments must also span sufficient time to allow for a
response to the perturbation.The importance of this is
well illustrated by Moseby (2002), who monitored
populations of small mammals within and outside a
predator exclosure in arid South Australia.Three years
after the exclosure was built, there was no significant
difference in small mammal captures inside and
outside the fence. However, in the subsequent 2 years,
a significant difference became apparent, with a fourfold increase inside the exclosure. Thus, even dramatic
and ecologically relevant changes in abundance may
be overlooked when the duration of experiments fails
to reflect the time scale of major environmental
perturbations.
Having established areas and a multi-year time
frame, careful thought must also be given to precisely
what to measure. Which taxa are most likely to show
a response to the predator manipulation? How can
their abundance and distribution best be measured?
Can detailed demographic data (e.g. fecundity) be
obtained? Can the movements and habitat use of interacting species be monitored? Clearly, monitoring
efforts must focus on species that we know or suspect
will be affected by predator manipulation (Dickman
1996). This includes species that may be affected
directly (e.g. foxes released from competition by the
removal of dingoes), or indirectly (e.g. small prey
released from fox predation due to the introduction of
dingoes). Monitoring must also include structural and
floristic surveys of vegetation, and changes in primary
productivity so that community-level trophic cascades
(sensu Polis et al. 2000) may be detected.
Because structurally complex habitat may moderate
interactions among predators, initial experiments
should be conducted in open areas (e.g. arid central
Australia), where effects are likely to be detected most
readily. Subsequent experiments should be able to
build upon the findings of earlier ones to investigate
more subtle interactions in complex habitat (Glen &
Dickman 2005).
CONCLUSIONS
The evidence reviewed here suggests that the dingo
is an important trophic regulator in Australian
ecosystems. A growing body of evidence suggests that
dingoes can limit both mesopredators and prey, often
with significant flow-on effects (e.g. Lundie-Jenkins
et al. 1993). Compelling as this evidence appears, it is
mostly observational. Few experimental studies have
sought to clarify the impacts of dingoes. Given the
potential significance of dingoes for ecosystem function, it is imperative that carefully controlled, replicated experiments are conducted. These must seek to
clarify not only the impacts of dingoes at all trophic
levels within a system, but also the mechanisms by
which these impacts occur (sensu Schoener 1974;
Tilman 1987).
The findings of such experiments will likely
have significance for wildlife management and
conservation. For example, by ceasing to control
dingoes in some areas, or by reintroducing them where
they have been eradicated, it may be possible to
achieve benefits for biodiversity through the suppression of cats, foxes and rabbits ( Johnson et al. 1989;
Newsome 1990; Lundie-Jenkins et al. 1993; Short &
Smith 1994). Thus, although the initial arrival of the
dingo may have had a negative impact on biodiversity
by contributing to the extinction on the mainland
of the thylacine and Tasmanian devil, we posit that in
the absence of these native carnivores the dingo now
fills an important role as a beneficial top-order
predator.
It is not our intention to suggest that reestablishment of dingoes across all areas will be an
ecological panacea. A great many anthropogenic
changes have occurred since the arrival of Europeans
in Australia, not the least of which is the modification
and fragmentation of habitats on a vast scale (Recher
2002). The situation is also complicated by hybridization between dingoes and domestic or feral dogs, the
behavioural and ecological effects of which are largely
unknown. Reintroduction of dingoes to areas greatly
modified by human disturbance may not restore
systems to their former state. Indeed, some endangered prey populations may be so depleted that they
are no longer able to sustain predation from any
source, including dingoes. However, many areas might
benefit from the restoration of top-down regulation. As
emphasized by Boyce and Anderson (1999), population dynamics are not driven solely by either top-down
or bottom-up effects, but by the interaction of the two.
For this interaction to occur, however, top–down interactions should be restored where feasible and desirable
for biodiversity. In many cases this might require
retaining undisturbed dingo populations. We contend
that clarification of the dingo’s role is a priority for the
maintenance of ecosystem function in Australia.
Similar principles apply in other parts of the world
where conflict with humans has decimated populations of apex predators. The experimental approach
described here will also serve to clarify the role
of putative highly interactive predators on other
continents. We invite comments.
© 2007 Ecological Society of Australia
T H E D I N G O A S A T R O P H I C R E G U L ATO R
ACKNOWLEDGEMENTS
This review was carried out under a Linkage Grant
(LP0455163) to B. Mackey and M. Soulé from the
Australian Research Council. We are grateful to L.
Corbett, P. de Tores, N. Marlow, D. Algar and M.
Calver for discussions that were helpful in formulating
the scope of the review. Sincere thanks also to the
participants in the National Dingo Trophic Regulation
Workshop, October 2005 and in particular to C.
Holden, who coordinated the workshop. Comments
from two anonymous referees helped greatly to
improve the manuscript.
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