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The impact of feral camels (Camelus dromedarius) on woody vegetation in arid Australia

Authors:
  • NT Government
  • Northern Territory Department of Environment & Natural Resources, Alice Springs, Australia
  • University of Wisconsin-Madison

Abstract and Figures

Data on the extent of feral camel damage on trees and shrubs in inland Australia are scarce, and there is currently no universally accepted theoretical framework for predicting the impact of a novel large mammal browser on arid vegetation. In other (mainly mesic) grassy systems, large mammal browsers can strongly suppress woody biomass across landscapes by limiting the transition of saplings to adulthood and by significantly thinning adult tree canopies. The recent Australian Feral Camel Management Project provided an opportunity to assess the impacts of camel browsing on woody vegetation in inland Australia. We examined browsing intensity and severity (stunting and canopy loss) in 22 species of woody plants in camel-affected regions across inland Australia prior to camel removal operations. The severity of plant damage increased with camel density as both trees and shrub growth were strongly suppressed where camel densities exceeded 0.25km-2. In most tree and shrub species tested, camel browsing significantly stunted plants, suggesting that camel browsing has long-term impacts on plant populations. Browsing also reduced canopy volume in several species, including the structurally important Acacia aneura F.Muell. ex Benth. Thus, in this dryland ecosystem, camels can curtail the regeneration and growth of woody species enough to threaten ecosystem health. To avoid adverse impacts on woody plant populations, camel densities should be maintained at 0.25 camels km-2 or less over as much of inland Australia as possible.
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The impact of feral camels (Camelus dromedarius) on woody
vegetation in arid Australia
Jayne Brim Box
A,G
, Catherine E. M. Nano
A
, Glenis McBurnie
A
, Donald M. Waller
B
,
Kathy McConnell
A
, Chris Brock
C
, Rachel Paltridge
D
, Alison McGilvray
E
, Andrew Bubb
F
and Glenn P. Edwards
A
A
Flora and Fauna Division, Department of Land Resource Management, PO Box 1120, Alice Springs,
NT 0871, Australia.
B
Department of Botany, University of Wisconsin, Madison, WI 53706, USA.
C
Brock Environmental, PO Box 411, Yarra Junction, Vic. 3797, Australia.
D
Desert Wildlife Services, Alice Springs, NT 0871, Australia.
E
Department of Parks and Wildlife, Frankland District, Walpole, WA 6398, Australia.
F
c/- Ninti One Limited, Alice Springs, NT 0871, Australia.
G
Corresponding author. Email: Jayne.Brimbox@nt.gov.au
Abstract. Data on the extent of feral camel damage on trees and shrubs in inland Australia are scarce, and there is currently
no universally accepted theoretical framework for predicting the impact of a novel large mammal browser on arid vegetation.
In other (mainly mesic) grassy systems, large mammal browsers can strongly suppress woody biomass across landscapes by
limiting the transition of saplings to adulthood and by signicantly thinning adult tree canopies. The recent Australian Feral
5Camel Management Project provided an opportunity to assess the impacts of camel browsing on woody vegetation in inland
Australia. We examined browsing intensity and severity (stunting and canopy loss) in 22 species of woody plants in camel-
affected regions across inland Australia prior to camel removal operations. The severity of plant damage increased with
camel density as both trees and shrub growth were strongly suppressed where camel densities exceeded 0.25 km
2
. In most
tree and shrub species tested, camel browsing signicantly stunted plants, suggesting that camel browsing has long-term
10 impacts on plant populations. Browsing also reduced canopy volume in several species, including the structurally important
Acacia aneura F.Muell. ex Benth. Thus, in this dryland ecosystem, camels can curtail the regeneration and growth of woody
species enough to threaten ecosystem health. To avoid adverse impacts on woody plant populations, camel densities should
be maintained at 0.25 camels km
2
or less over as much of inland Australia as possible.
Additional keywords: arid vegetation, browsing impacts, shrubs, trees.
Received 3 August 2015, accepted 4 March 2016, published online dd mmm yyyy
Introduction
Dromedary camels (Camelus dromedarius) were rst brought
to Australia in 1840 as working animals on exploratory
expeditions. Today Australia supports the only large feral
5dromedary camel (hereafter referred to as camel) population
in the world. These animals likely arose from domesticated
animals released into the wild in the 1920s and 1930s (McKnight
1969). Although camels are widespread across inland Australia,
population densities vary considerably among regions. Just
10 prior to the commencement of the Australian Feral Camel
Management Project (AFCMP) in 2009, the camel population of
inland Australia was estimated at approximately one million
animals, with regional densities ranging from <0.1 to >2 camels
km
2
(Saalfeld and Edwards 2010). Despite the long time that
15 camels have been naturalised in inland Australia, little is known
about their impacts on native vegetation, reecting the lack of
any systematic program to assess and monitor landscape-scale
impacts before the AFCMP.
Camels are known to travel large distances in Australia
(Lethbridge et al.2010; Secomb 2013), sometimes ranging
5thousands of km within a single year (Edwards et al.2001). Their
movements vary with seasonal conditions, rainfall and other
factors, including patterns of re and drought. Feral camel
densities within a particular region or site can change rapidly.
When conditions favour camel congregations, camels exert
10signicant pressure on local food and water resources.
Quantifying these impacts broadly across inland Australia
presents a complex set of challenges. Environments vary
considerably over such distances, making it hard to assess how
climate (viz. prolonged drought interspersed with sporadic
15ooding rainfall), episodic res in the regions highly ammable
grasslands, and camel movements combine and interact to
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Journal compilation Australian Rangeland Society 2016 www.publish.csiro.au/journals/trj
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http://dx.doi.org/10.1071/RJ15073
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affect impacts on local vegetation. Although we see progress in
understanding the relative importance of climate, re and
edaphic factors for vegetation patterning in arid Australia (e.g.
Russell-Smith et al.2007; Bradstock 2010), a universally
5accepted framework for ecosystem structure and function still
eludes us. Thus, the introduction of this large mammal browser
adds further uncertainty to our understanding of the relative
importance of bottom-up (e.g. low rainfall, nutrient-poor soils)
versus top-down (e.g. browsing, re) controls of vegetation
10 patterns in inland Australia.
Given that camel densities vary greatly across time and space,
it is logical to expect that vegetation damage will be most severe
when and where camel densities are highest. This provides an
opportunity to use regional variation in camel densities as a
15 natural experimentto study camel impacts (sensu Diamond
1983). The densitydamage relationship between woody plants
and large herbivores has been established for numerous
browsers in a range of ecosystems (Augustine and McNaughton
1998; Vázquez and Simberloff 2004; Wigley et al.2014). It is
20 less clear how this relationship varies by plant species and form
(i.e. tree or shrub). Given that all ungulates are selective in their
choice of food plants, we expect camel impacts to vary by species
due to differences in palatability, nutrient levels, and defensive
traits, including plant form (i.e. tree or shrub).
25 Browsing, like re, strongly favours grasses over trees (Levick
and Rogers 2008). Although both browsing and re can kill
woody species (Levick and Rogers 2008; Staver et al.2009,
2012) they act more commonly to: (a) prevent sapling growth
and maturation and, (b) reduce adult canopy biomass. Modelling
30 shows that even small top-downeffects on tree growth can
curtail population growth and thus reduce woody cover at the
landscape scale (Sankaran et al. 2004; Hartnett et al. 2012).
Goheen et al.(2010) predicted that the impact of browsing on
plant demography should be especially strong in ecosystems
35 where herbivores are large, abundant, and have high energetic
demands. In inland Australia, camel impacts on ecologically
important canopy tree species like Acacia aneura F.Muell. ex
Benth. and other mulga type speciesmay have cascading
effects, as these species determine the structure and function of
40 wooded patches within the broader grassy matrix.
Information on the impacts of camel browsing on vegetation
in arid Australia is quite limited. Camels are known to
preferentially feed on Australian plants on the basis of their
palatability (Dörges and Heucke 1995,2003), but available data
45 were derived from a paddock-scale study where camel
movements and density were controlled. In reviewing camel
impacts on vegetation, Edwards et al.(2008) cited no quantitative
information on damagedensity relationships for feral camels
browsing on wild vegetation. Quantitative information on the
50 relationship between vegetation damage and camel density could
inform decisions on camel control and management in arid
Australia, similar to what Edwards et al.(2010) presented on the
relationship between camel density and infrastructure (e.g.
fencing) damage on pastoral properties.
55 The AFCMP project provided an incentive and opportunity to
study relationships between browsing impacts and camel
densities at the landscape-scale in inland Australia. Here, we
present a summary of camel impacts on 22 woody species
across 116 sites in seven camel-affected regions of inland
Australia. We sampled these sites just before camel removal
operations commenced within each region. We quantify the
direct effects of camel browsing on woody populations by
focusing on the demographic processes of woody plant growth
5and mortality. We test two hypotheses: that local browsing
damage primarily reects camel density, and that browsing
impacts vary in response to plant form (tree or shrub). Thus, we
expect all woody species to be strongly reduced in growth and
survival in areas where camel densities are highest, as foragers
10generally become less selective as local resources become scarce
(sensu MacArthur and Pianka 1966). We also expect browsing
damage to be more pronounced in shrubs and trees whose
biomass is primarily in the browse zone (i.e. below a height of
3.5 m), implying that shrubs, unable to grow beyond the browse
15zone, should experience greater reductions in growth and
survival than trees.
Methods
Study site locations
We measured camel impacts on 22 tree or shrub species at 116
20sites across three Australian jurisdictions: Northern Territory
(NT), South Australia (SA) and Western Australia (WA)
(Table 1and Fig. 1). We chose sites based, in part, on their being
key environmental asset areas within the AFCMP (Ninti One
Limited 2013). At most sites other large browsers, such as cattle,
25were absent. For the NT, we monitored 42 sites allocated across
three areas: the western portion of the Simpson Desert (12 sites),
the Katiti and Petermann Aboriginal land trusts (19 sites), and the
West MacDonnell and Watarrka national parks (11 sites). The
two national parks served as control sites as camels are largely
30excluded from these areas through fencing and other measures.
These sites allowed us to obtain morphometric data from plants
Table 1. General location of study area, number of sites in each area,
and the estimated camel densities in each area, the latter based on density
estimates provided in Saalfeld and Edwards (2010)
State/
Territory
General location Sites
(#)
Estimated camel
densities
(camels km
2
)
NT controlsites in West
MacDonnell and Watarrka
national parks
11 0
Katiti-Petermann Aboriginal
Land trusts
3 0.50
8 1.00
8 2.00
Western Simpson Desert 5 0.25
7 0.50
SA Anangu Pitjantatjara
Yakunytjatjara lands
1 0.10
11 0.25
6 0.50
9 1.00
Purni Bore area 22 0.50
WA Karlamilyi National Park 1 0.10
13 0.25
Martu Determination Area 6 0.10
5 0.25
BThe Rangeland Journal J. Brim Box et al.
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that had incurred minimal damage from re or browsing
and whose biomass thereby reected background resource
availability. Two study areas were located in SA: Anangu
Pitjantatjara Yakunytjatjara lands (27 sites) and the Purni Bore
5area (22 sites), which is on the eastern edge of Witjira National
Park. The 25 sites in WA were distributed between Karlamilyi
National Park and the surrounding Martu Determination Area.
At the start of the project, camel densities across inland Australia
were estimated to range from <0.1 to >2 camels km
2
, based
10 on aerial survey data collected across all three jurisdictions
(Saalfeld and Edwards 2010). We used the data presented in
Saalfeld and Edwards (2010) to assign each study site to one of
six camel density categories: 0, 0.1, 0.25, 0.5, 1 or 2 camels km
2
(Table 1). Details of how camel densities varied spatially
15 across environmental asset areas, as well as pre- and post-
AFCMP, are presented in Hart and Edwards (2016).
Plant form
We sampled 22 woody species, including both shrubs and trees.
Trees can potentially grow to heights above 3.5 m, beyond the
20 browse zone, whereas shrubs cannot. Because our aim was to
examine camel browsing impacts on plant species known to be
browsed by camels, the majority of plant species selected
(~80%) were considered preferredor extremely preferredby
camels (Dörges and Heucke 2003). We also included one
25 species, hop bush (Dodonaea viscosa Jacq.), which is considered
unpalatableto camels or only eaten when nothing else is
availableto serve as another type of control. Likewise, we
included the quandong (Santalum acuminatum (R.Br.) A.DC.),
considered very highly palatable to camels or endangered due
to camels browsingby Dörges and Heucke (2003) to see if
effects could be detected despite its rarity in camel-dense areas.
Measuring browsing impacts
We collected vegetation data over 3 years (201012), sampling
5each population on three occasions at roughly 12-month
intervals. Here, we present data from the rst monitoring event.
In a companion study we focus on the multiple effects of
temporally- and spatially variable browsing pressure, re and
climate on woody species over the 3-year time frame, a synopsis
10of which is presented in Brim-Box et al.(2013). Within each
target population we assessed at least 40 individual plants
across all size classes present at a site. Pilot studies revealed this
to be an appropriate number given that many species occurred as
small and often disjunct populations across the study region.
15The location of each plant was recorded with a GPS so that
individuals could be tracked through time. We assessed over
4000 individuals of the 22 species among the 116 study sites.
To explore which metrics were most useful and reliable, we
used several techniques to assess camel impacts on woody
20vegetation (as recommended by Rooney and Waller 2003; and
Waller et al.2009). At each site, we measured 12 attributes for
each individual tree or shrub: plant height; height to browse line;
stem diameter at ground height; canopy width, length and cover;
percentage defoliation and broken branches; number of stems;
25whether the plant was owering or fruiting; and overall plant
health. This broad approach allowed us to: (a) increase the
probability of detecting some browsing impacts, (b) discover
which metrics showed the strongest relationships, and (c) assess
which metrics could best accommodate the highly variable
Watarrka & West MacDonnell National Parks
Petermann ALT
Simpson Desert
Karlamilyi National Park
Purni Bore
APY Lands
Fig. 1. Generalised map of asset areas where vegetation sites were established.
Camel browsing impacts on woody vegetation The Rangeland Journal C
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nature of browsing impacts. The use of a wide range of
browsing impact measures is particularly appropriate in this study
given the dearth of information on camel impacts on Australian
vegetation.
5Modied browsing index
To evaluate browsing impacts on woody plants, we used a
modied version of the browsing index developed by Morellet
et al.(2001,2003). This index has many advantages, including:
(1) its statistical properties are well-characterised, (2) it is
10 sensitive to changes in browser population densities, (3) it allows
changes over time to be interpreted, and (4) it can be calibrated
against known herbivore densities. The index was originally
developed for roe deer in temperate forests in France and has not
previously been used to assess camel browsing impacts. Morellet
15 et al.(2001,2003) used a grid network to assess browsing
impacts in a large set of 1-m
2
plots. Although this design suits
European forest ecosystems, it could not be directly transferred
to the present study given the patchy distribution of woody
plants at our sites. We therefore applied it opportunistically
20 to the populations sampled. Our modied browsing index is
dened as:
Browsing Index ¼ð1þnbÞ=ð2þntÞ
where nb is the number of trees in a stand with any evidence of
25 browsing present and nt is the total number of trees measured. In
its original form, the Morellet index omitted all information
concerning which woody species are browsed, making it is easy
for land managers to apply. Nevertheless, Morellet et al.(2001,
2003) further suggested that species-specic indexes could also
30 be used to track browsing pressure. Given the variability in
browsing impacts over species and the lack of baseline data for
the browsing tolerance of Australian species, we applied this
index to individual plant species. We also wanted to avoid
lumping data across species as this could mask changes in the
35 browsing index as more browse-tolerant or resistant species
replaced susceptible species.
Mortality, pruning and defoliation severity
To supplement the browsing index, we also estimated the
mean percentage of trees or shrubs at each site with broken
40 branches (pruning) and/or defoliation due to camel browsing. We
also assessed camel browsing-induced mortality in 14 tree
or shrub species at the NT sites. We tested for evidence of
browsing-induced stunting, as evinced by above-average stem
diameters for a given height of plant (see Nano et al.2012). We
45 computed ratios of basal stem diameter to height to obtain an
inverse measure of stunting. We also noted evidence of browse
linesin trees and taller shrubs by examining canopy loss in
a vertical plane (top to crown base). Finally, we examined
differences in estimated total canopy volume (m
3
) between
50 browsed and non-browsed plants.
Statistical analyses
We used one- and two-way ANOVA to test for differences in
browsing intensity, pruning and defoliation among the six camel
density categories, and to examine interactions between camel
55 density and plant form. We checked assumptions regarding the
normality of residuals and their homogeneity of variances
and applied lack of ttests to check our models. Lack of t tests
compute an estimate of pure error, based on a sum of squares,
using the measurements taken from each shrub or tree
5(Schlotzhauer 2007). When the results of an ANOVA showed
signicant differences among the camel density treatments, we
used Tukeys HSD post-hoc test to examine those differences.
For comparisons between two groups, we used Studentst-test.
We used analysis of covariance (ANCOVA) to examine
10relationships between trunk thickness, canopy volume and
height to canopy base (dependent variables) and height (the
covariate) comparing browsed and unbrowsed trees/shrubs
(the factor). We log-transformed dependent variables where this
improved linearity and the homogeneity of residual variances. As
15with the ANOVA, we applied lack of tnesstests and checked
to ensure that the models met assumptions of linearity with
normal and homogeneous variances. We tested for homogeneity
between slopes by including an interaction term between the
covariate and the factor in the original model. If the interaction
20term was not signicant (P>0.05), we re-ran the model dropping
the interaction term (Engqvist 2005). If the interaction was
signicant, we used the JohnsonNeyman procedure (Johnson
and Neyman 1936) to determine the range of covariate values
within which there were no signicant differences between
25browsed versus non-browsed trees or shrubs (White 2003).
All statistical analyses used JMP 11 (SAS Institute, Cary,
NC, USA).
Results
Camel densitybrowsing severity relationships
30As expected, the proportion of shrubs and trees browsed in each
population increased as camel densities increased (Table 2).
Results of the two-way ANOVA suggested that although camel
density signicantly affected the browsing index (F
5,122
= 20.27,
P<0.0001), plant form did not (F
1,122
= 0.0014, P= 0.97). In
35addition, the effects of camel density and plant form were
independent (interaction F
5,122
= 0.52, P= 0.76). Thus, trees and
shrubs were browsed at similar intensities across camel densities.
As camel densities increased, the proportion of individuals
browsed in each stand increased (Fig. 2), as expected, an effect
40that was highly signicant (F
5,128
= 21.41, P<0.0001). Browsing
was signicantly higher at camel densities of 2, 1 or 0.5 camels
km
2
than at lower camel densities (Tukeys post-hoc test,
a= 0.05, Q = 2.89). At sites with the highest camel densities, over
90% of the trees and shrubs present were browsed.
45Pruning and defoliation
Pruning was higher for trees in the browse zone than for trees that
had grown above the browse zone, as expected (F
1,2862
= 3.86,
P= 0.0497). This effect was not signicant in shrubs, probably
reecting the fact that >97% of the shrubs measured occurred
50in the browse zone. Pruning increased in areas of higher camel
density in both trees and shrubs (F
5,122
= 11.49, P<0.0001)
(Fig. 3). This pruning effect did not depend on plant form
(F
1,122
= 0.0009, P= 0.97), and there was no signicant
interaction between camel density and plant form (F
5,122
= 0.81,
55P= 0.54). Thus, camels prune trees and shrubs similarly.
This pruning effect signicantly increased at densities of 1 and
DThe Rangeland Journal J. Brim Box et al.
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2 camels km
2
relative to lower camel densities (Tukeys post-hoc
test, a= 0.05, Q = 2.89). At sites with the highest camel density,
the mean number of broken branches per plant was >60%.
Average amounts of defoliation at a site also increased with
camel density, but these effects were somewhat smaller than
those observed for browsing intensity or pruning (F
5,119
= 4.03,
Table 2. Browsing index scores for individual plant species across camel density categories
Palatability scores are based on Dörges and Heucke (2003). Plant number represents all plants measured across all sample sites. Values are
means standard error
Form Species Palatability Plants (#) Camel density (camels km
2
)
0 0.10 0.25 0.50 1.00 2.00
Shrub Acacia ligulata 6 178 0.02 ± 0.41 ––0.57 ± 0.17 0.86 ± 0.38
Shrub Acacia ramulosa 3 160 –––0.52 ± 0.25 ––
Shrub Acacia tetragonophylla 649 ––0.02 ± 0.12 0.63 ± 0.12 0.58 ± 0.08
Shrub Acacia victoriae 6 456 0.02 ± 0.26 0.65 ± 0.15 0.85 ± 0.12 0.95 ± 0.13 0.98 ± 0.26
Shrub Atriplex nummularia 640 –––0.36 ± 0.00 ––
Shrub Atriplex vesicaria 648 –––0.06 ± 0.00 ––
Shrub Chenopodium auricomum 540 ––0.43 ± 0.00 –––
Shrub Chenopodium nitrariaceum 440 –––––0.98 ± 0.00
Shrub Dodonaea viscosa 1 160 –––0.47 ± 0.23 ––
Shrub Eremophila longifolia 6 782 0.03 ± 0.15 0.10 ± 0.21 0.35 ± 0.12 0.59 ± 0.11 0.58 ± 0.14
Shrub Rhagodia spinescens 6 160 –––0.47± 0.15 ––
Shrub Santalum lanceolatum 6 751 0.03 ± 0.13 0.02 ± 0.30 0.25 ± 0.10 0.88 ± 0.30 0.82 ± 0.21 0.84 ± 0.21
all shrub species combined 2864 0.03 ± 0.10 0.05 ± 0.13 0.34 ± 0.07 0.58 ± 0.05 0.74 ± 0.08 0.91 ± 0.15
Tree Acacia aneura 5 61 0.02 ± 0.00 ––0.67± 0.00 0.93 ± 0.00
Tree Acacia estrophiolata 566 –––0.67 ± 0.17 0.98 ± 0.24
Tree Acacia oswaldii 7 157 ––0.36 ± 0.26 0.71 ± 0.37 0.97 ± 0.37
Tree Acacia paraneura 540 ––0.02 ± 0.00 –––
Tree Acacia pruinocarpa 5 96 0.02 ± 0.00 ––––0.98± 0.00
Tree Atalaya hemiglauca 6 225 0.02 ± 0.07 0.95 ± 0.07 0.75 ± 0.07 ––
Tree Brachychiton gregorii 5 120 –––0.95 ± 0.06 0.86 ± 0.04
Tree Codonocarpus cotinifolius 6 269 0.29 ± 0.15 0.26 ± 0.10 –––
Tree Pittosporum angustifolium 6 208 0.02 ± 0.26 0.40 ± 0.19 0.77 ± 0.13 0.72 ± 0.15
Tree Santalum acuminatum 7 82 0.02 ± 0.02 0.91 ± 0.02 0.93 ± 0.02 ––
All tree species combined 1324 0.02 ± 0.09 0.28 ± 0.17 0.47 ± 0.07 0.77 ± 0.07 0.79 ± 0.09 0.96 ± 0.12
0.8
0.6
0.4
0.2
0
0 0.10 0.25
Camel density (camels km–2)
Browsing index
0.50 1.00 2.00
Fig. 2. Mean and standard error of the browsing index across camel density
categories for trees and shrubs combined (plant form, and the interaction
between plant form and camel density, had no signicant effects on the
browsing index). Densities of 0.5, 1, and 2 camels km
2
were signicantly
higher than at lower camel densities or the control sites. At sites with the
highest camel densities, over 90% of plants showed evidence of browsing.
70
60
50
40
30
20
10
0
–10
0 0.10 0.25
Camel density (camels km–2)
0.50 1.00 2.00
Broken branches (%)
Fig. 3. Mean and standard error of the percentage of broken branches
(i.e. pruning) in all woody plants across camel density categories. Plant
form and its interaction with density were not signicant. Pruning was
signicantly higher at densities of 1 and 2 camels km
2
than at lower camel
densities. At the highest camel density, over 60% of the branches per plant
were broken.
Camel browsing impacts on woody vegetation The Rangeland Journal E
PROOF ONLY
P<0.002). Again, plant form did not have any independent
effect (F
1,119
= 0.0278, P= 0.27). Woody plants at sites where
camels existed at the two highest densities were defoliated
signicantly more than those at control sites (Tukeys post-hoc
5test, a= 0.05, Q = 2.89).
Plant mortality
Camels rarely killed adult trees. Mortality across the 14 focal
NT species averaged <2%, and we documented no mortality from
camel browsing in nine species. However, plumbush (Santalum
10 lanceolatum R.Br.), a clonal shrub, suffered signicant mortality
(7%). We found no discernable trend towards higher mortality
at sites with higher camel densities, even with plumbush. Thus,
mortality provides a poor index of browsing.
Stunting due to camel browsing
15 Ratios of height to basal stem diameter (i.e. trunk thickness)
emerged as a sensitive indicator of camel browsing for both
shrubs and trees. The ratio of height to basal stem diameter was
signicantly higher for unbrowsed (57.7) versus browsed (40.9)
shrubs (F
1,2466
= 274.44, P<0.0001) and signicantly higher for
20 unbrowsed (47.6) versus browsed (36.8) trees (F
1,1302
= 58.69,
P<0.0001) (Fig. 4). Thus, camel browsing stunts the growth of
both trees and shrubs (e.g. whitewood) (Fig. 5). We also examined
these ratios in unbrowsed versus browsed individuals in 16
species (we lacked control data for the remaining six). Camel
25 browsing stunted growth in 10 of these, including ve shrubs
(umbrella bush Acacia ligulata A.Cunn. ex Benth., horse mulga
Acacia ramulosa W.Fitzg., hop bush, long-leaf emubush
Eremophila longifolia (R.Br.) F.Muell., and plumbush) and ve
trees (umbrella wattle Acacia oswaldii F.Muell., dead nish
Acacia tetragonophylla F.Muell., whitewood Atalaya
hemiglauca (F.Muell.) F.Muell. ex Benth., desert poplar
Codonocarpus cotinifolius (Desf.) F.Muell., and native apricot
5Pittosporum angustifolium Lodd., G.Lodd. and W.Lodd.).
Trends in the other six species were not signicant.
Canopy loss
For trees with canopies in the browse zone (80% of all
individuals sampled), height to crown base was signicantly
10higher in browsed versus unbrowsed trees of the same height
(one-way ANCOVA, F
3,838
= 150.9, P<0.0001) (Fig. 6a).
Because the interaction term between height and browsing
was signicant (F
1,840
= 25.80, P<0.0001), we applied the
JohnsonNeyman procedure to identify the region of no
15signicance. No signicant differences were found between
browsed and non-browsed trees less than 1.2 m in height. We did
not nd any such browse line effect in shrubs, presumably
reecting their shorter stature, which only rarely supported
a canopy above the browse line.
20Canopy loss was most apparent in the ecologically important
mulga-typetrees, Acacia aneura,A. paraneura Randell and
A. ramulosa (Fig. 6b). In these species, browsed individuals
suffered reduced canopy volume (hence biomass) relative to
unbrowsed trees (one-way ANCOVA, F
3,190
= 320.98,
25P<0.0001). The interaction term between height and browsing
was signicant (F
1,192
= 27.80, P<0.0001), and no signicant
differences were found between browsed and non-browsed
trees less than 2.2 m in height. Conversely, canopy loss was
especially pronounced in browsed trees over 2 m in height. For
58
Form
Shrub Tree
56
54
52
50
48
46
44
42
Ratio of height to trunk thickness
40
38
36
Browsed Non-browsed Browsed Non-browsed
Fig. 4. The ratio of height to basal stem diameter (trunk thickness) fell considerably in browsed shrubs and
trees relative to unbrowsed plants. This ratio emerged as a sensitive indicator of camel browse for both plant
forms. Stunted plants often lose the ability to escape severe browsing by growing above the browse zone.
FThe Rangeland Journal J. Brim Box et al.
PROOF ONLY
example, trees 3 m in height had only about half the canopy
of unbrowsed trees of the same height.
Discussion
Two key ndings emerged from this study. First, although it had
5previously been observed that camel damage to trees and shrubs
could be severe in inland Australia, few data existed to judge
overall levels of browsing, woody plant mortality, and thus the
potential for widespread changes in vegetative cover. Despite the
severe browsing effects we observed in areas of high camel
density, overall tree mortality from camel impacts remains, for
the moment, modest across the region. Specically, averaged
over species, death from browsing was only about 2%. A species
that showed notably higher levels of mortality was the palatable
5shrub, plumbush, which suffered 7% mortality from camel
browsing. Its congener, the quandong tree, is also considered to
be highly palatable to camels and susceptible to browsing. Camels
may have contributed to its recent population declines and
threatened status in the NT (Woinarski et al.2007). However, we
10do not yet have any scientic studies that have quantied
population declines from camel damage in any plant species
outside of controlled paddocks, even including species thought
to be most vulnerable to camel impacts such as quandong,
umbrella wattle and plumbush.
15In mesic African savanna, large mammal-browsers like
elephants and giraffes drive woody-grassy ratios, and hence
vegetation structure, by killing and suppressing adult trees
(Goheen et al. 2010). The low mortality found in this study might
lead one to infer that camel browsing is not yet strongly affecting
20woody species density or site oristic composition. However, in
inland Australia most of the drought-adapted woody species
grow slowly, tolerate most stresses (pursuing a stress-tolerant
strategy sensu Grime 1979), and suffer low rates of mortality.
Thus, the considerable reductions in biomass, growing branch
25tips, and height documented here could curtail growth and
eventually limit reproduction and regeneration, as has occurred
elsewhere in Australia (Crisp 1978). In inland Australia,
species that incur mortality from browsing or grazing may be
susceptible to serious losses via the combined/interactive
30effects of browsing, re and drought (Watson et al.1997). Species
such as quandong, plumbush and umbrella wattle warrant
further research to better understand the extent to which camel
browsing may limit reproduction and regeneration across arid
Australia.
35Our second major nding was that camel browsing, although
resulting in minimal mortality, strongly limits plant growth.
Browsing impacts varied greatly across the landscape in direct
response to camel density. Once density exceeded 1 camel km
2
,
14
12
10
8
6
Basal stem diameter (cm)
Hei
g
ht (cm)
4
2
0
50 100 150 200 250 300 350
Fig. 5. Relationship between height and trunk thickness in browsed
(crosses, black regression line) and unbrowsed (circles, grey regression line)
whitewoods (Atalaya hemiglauca). Unbrowsed whitewoods had signicantly
smaller trunks than browsed trees of the same height (one-way ANCOVA,
F
2,197
= 59.21, P<0.001). Thus, browsed trees are stunted. As the regression
slopes did not differ signicantly from each other (interaction F
1,197
= 0.29,
P= 0.59) this term was dropped from the model.
Height to crown base (m)
Hei
g
ht (m)
Canopy volume (m3)
region of non-significance
non-browsed
browsed
non-browsed
browsed
region of
non-significance
3.0
(a)(b)
40
30
20
10
0
2.5
2.0
1.5
1.0
0.5
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0
Fig. 6. Two examples of canopy loss: (a) For all tree species combined, the height to crown base in browsed trees (crosses,
black regression line) taller than 1.2 m was signicantly higher than in non-browsed trees (circles, grey regression line), (b) For
mulga-type species, a signicant loss of canopy was evident in browsed trees greater than 2.2 m in height.
Camel browsing impacts on woody vegetation The Rangeland Journal G
PROOF ONLY
most trees and shrubs were damaged, often severely. At the
highest density, over 90% of trees or shrubs per site were browsed,
and 60% of each plants branches were broken. In contrast, at
lower camel densities (<0.25 camels km
2
), fewer than 40% of the
5trees and shrubs present at a site were browsed and, on average,
fewer than 20% of their branches were broken. Thus, camels can,
and do, have severe impacts on woody plants when they occur at
densities >1 camel km
2
.
Of the 16 species we assessed, 10 showed stunting in
10 response to browsing. Stunted plants are in turn susceptible to
repeated browsing, allowing them little opportunity to reach
their full potential in terms of canopy height, volume, and
reproductive success. Our data suggest that camels at higher
densities can browse shrubs and low trees (<3.5 m) to the extent
15 that they become stuck in a browse trap, analogous to the re
trap, wherein saplings are prevented from maturing into adults
(Staver et al. 2009; Staver and Bond 2014). As a secondary
impact, tall browsers can directly suppress adult trees by reducing
their canopy biomass, potentially reducing tree cover and
20 fecundity (e.g. see Levick and Rogers 2008; Staver et al. 2009;
Hartnett et al.2012). We found stunted height growth and
signicant canopy loss in several woody species, including the
structurally important tree species mulga and the widespread
species whitewood. We thus have evidence that in this dryland
25 ecosystem, the population dynamics of woody species may be
strongly affected by camel browsing at both pre-adult and adult
life-stages.
Camel densities that result in suppressed tree growth and
reproduction in aridland Australia seem likely to have detrimental
30 impacts on ecosystem function. In dryland systems, tree cover
is already strongly limited by low rainfall and soil nutrient
availability. Yet these trees play critical roles in providing
ecosystem structure that supports many processes and wildlife
species. Canopy damage in wooded patches can have many, and
35 varied, cascading effects. Browser-affected woody patches with
thinned canopies have altered microhabitat condition, making
them more permeable to wind, and water and energy transfers via
increased evapotranspiration (see Levick and Rogers 2008).
Signicant branch damage in aridland trees and shrubs, and the
40 subsequent loss of canopy volume, results in less intercepted
rainfall, causing declines in inltration and increased surface
runoff (Slatyer 1965; Pressland 1976). In addition, declines in
canopy biomass can reduce habitat availability for birds
(Fuller 2012) and the ability of aridland trees to act as carbon
45 sinks (Keith et al.2009). All these ow-on effects apply
especially to the widespread structural dominant of inland
Australia, mulga (and its various taxonomic allies), given this
speciesreliance on stem ow(Whitford 2002), its role in the
redistribution of surface water (Tongway and Ludwig 1990), and
50 its importance as primary habitat for a suite of mulga-dependent
birds (Cody 1994; Pavey and Nano 2009). Further, it has been
shown that browsed wooded patches are more susceptible to
species invasions (Levick and Rogers 2008). In sum, in inland
Australia, where tree growth is already constrained by periodic
55 intense wildre and drought, we must manage camel densities
if we are to avoid loss of woody biomass and its accompanying
decline in ecosystem function and native plant and animal
biodiversity.
Conclusion
As the density of camels increased at a site, browsing damage to
woody species increased, greatly reducing biomass, productivity,
and regenerative capacity, potentially affecting other species and
5ecosystem processes. Although overall mortality from browsing
appeared low over the short course of this study, camel browsing
alters woody population demography and canopy cover in ways
that may eventually limit regeneration and persistence in the
plant species most affected in this study. Our results suggest
10that camel densities >0.25 camels km
2
cause undesirable levels
of vegetation damage. This estimate is similar to the density
suggested by Edwards et al.(2010) to avoid damage to
infrastructure, and broadly accords with target camel densities
recommended for key asset areas by the AFCMP (Ninti One
15Limited 2013). In contrast, our estimates are lower than many
of the species-specic estimates made by Dörges and Heucke
(2003) for camel stocking rateson pastoral properties. To avoid
damage to woody plants across inland Australia, we recommend
that managers seek to maintain densities at <0.25 camels km
2
20over as much of the landscape as possible.
Acknowledgements
The authors thank and greatly appreciate the guidance, discussions with, and
participation of the following Traditional Owners and rangers during this
project; Conrad Abbott, Ronnie Allen, Ezekiel Andrew, Bernard Bell, Selwyn
25
Burke, Veronica Purrurle Dobson, Allen Drover, Mary Gibson, Michael
Hayes, Dennis Hunt, Ross Jackatee, Elaine James, Raymond James, Ruby
James, Rene Kulitja, Rodney Kunoth, Pantjiti McKenzie, Jacob Nelson, Tony
Paddy, Christobell Protty, Brett Stockman, Reggie Uluru, Sandy Willy and
Roy Yaltjanki. We thank Donna Digby and Pat Hodgens of the DLRM, Kym
30
Schwartzkopff and Jason Britten of the NTPWC, and Andrew Schubert of
Desert Wildlife Services for their participation in eld collections. We
thank Kanyirninpa Jukurrpa, Anangu Pitjantjatjara Yankunytjatjara Land
Management, and the Central Land Council for facilitating the participation of
Traditional Owners and ranger groups in all aspects of the project, and in
35
particular we thank Martin Campbell, Tracey Guest, Chris McGrath, Jude
Pritchard and Sam Rando of the Central Land Council for their support and
guidance. We are especially grateful and indebted to Peter Latz, who provided
invaluable insights and suggestions on both sampling design and central
Australian plants in general. Quentin Hart, Wal Whalley and two anonymous
40
reviewers provided very helpful comments on earlier drafts of this manuscript.
The papers in this special edition are based on work undertaken through the
Australian Feral Camel Management Project (20092013) that was supported
by the Australian Government and other project partners and coordinated
by Ninti One. The preparation and publishing of this special issue was
45
supported by Ninti One.
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... This is likely because the latter have very soft wood and branches up to 1 cm in diameter can be consumed by camels (Appendix S5). Stunting has been reported before in Dhofar (Miller et al., 1988), and as a result of camel and cattle browsing in other arid environments (Pour et al., 2012;Box et al., 2016). ...
... At 28 of the 30 sites, stocking rates (TLU) prevailing at the time of data collection (mean = 104 TLU/km 2 ) exceeded the sustainable limit of 20 TLU/km 2 for arid rangelands (Jahnke, 1982). Furthermore, at all sites, prevailing camel densities (mean = 53/km 2 ) far exceeded the limit imposed in arid Australia (0.25/km 2 ) to avoid undesirable levels of vegetation damage (Box et al., 2016). The ecological impacts of pastoralism in Dhofar must be reduced to limit further degradation of the Anogeissus cloud forests, which are globally unique, have a high conservation value, and provide important ecosystem services. ...
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Questions It is frequently reported that overstocking of camels, cattle and goats is degrading the Anogeissus cloud forest, which is endemic to a 200 km stretch of coastal mountains in southern Arabia. However, livestock impacts on the vegetation have not been assessed. Furthermore, we have a limited understanding of the impacts of large‐bodied browsing livestock, such as camels, in woodland and forest rangelands. Therefore, in this study, we examine the effects of livestock browsing on the species composition, density, and phytomorphology of woody vegetation in the Anogeissus cloud forests in the Dhofar Mountains of Oman. Location Data was collected at 30 sites in the Jabal Qamar mountain range in western Dhofar, Oman. Methods The point‐centered quarter method was used to sample the composition, density and structure of woody vegetation. Constrained correspondence analysis was used to quantify the effects of livestock browsing on woody plant species composition, whilst effects on plant density were analysed using mixed effects models. Standardised major axis regression was used to examine differences in height‐diameter allometry (stunting) under different stocking rates. Results Fog density, topographic position and long‐term stocking rates were found to be important factors affecting woody species composition. We found lower species diversity and plant density, and higher frequencies of unpalatable species, under higher stocking rates. Juveniles showed a stronger response to stocking rates than adults, and several common species exhibited stunted morphology under high stocking rates. Conclusions Browsing by large‐bodied livestock, such as camels and cattle, can substantially alter the species composition, structure, and phytomorphology of woody vegetation in semi‐arid woodlands and forests. Juveniles are particularly susceptible to browsing which alters woody vegetation demography and inhibits regeneration potential. Our results support previous suggestions of overstocking in Dhofar and highlight the importance of swift measures to reduce livestock browsing pressure in the Anogeissus cloud forests.
... The capability of the camels to "open up" dense bush by browsing on shrubs and trees has been noted before. This article tries to expand on this concept utilizing more literature (NET 9;Box et al. 2016). ...
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... Large feral animals further impact water sites by affecting vegetation. They graze selectively on vegetation, causing stunting of woody shrub and trees (Brim Box et al. 2016b), suppressing plant recruitment (Edwards et al. 2010) and contributing to weed dispersal (Dobbie, Berman, and Braysher 1993;Ansong and Pickering 2013). As camels can eat 80% of the common plants in central Australia, many with important cultural uses (Edwards et al. 2010), their impact on sacred water places can be profound. ...
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... In Australia many such species threaten the continent's unique biodiversity (McLeod 2004). Moreover, because feral camels occupy relatively low rainfall areas, including fragile desert regions, their management in Australia needs to be primarily focussed on reducing their impacts, particularly during dry periods (Edwards et al. 2010;McBurnie et al. 2015;Brim Box et al. 2016). Between 2009 and 2013, the Australian Feral Camel Management Program reported that 126 291 camels were removed in key areas to protect biological and cultural assets (Hart and Edwards 2016;Lethbridge et al. 2016). ...
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A better understanding of the movement of feral dromedary camels (Camelus dromedarius) in Australia would be useful for planning removal operations (harvest or culling), because the pattern and scale of camel movement relates to the period they reside in a given area, and thus the search effort, timing and frequency of removal operations. From our results, we suspect that the dune direction influences how camels move across central Australia; particularly effects like the north–south longitudinal dune systems in the Simpson Desert, which appeared to elongate camel movement in the same direction as the dunes. We called this movement anisotropy. Research suggests camel movement in Australia is not migratory but partially cyclic, with two distinctive movement patterns. Our study investigated this further by using satellite tracking data from 54 camels in central Australia, recorded between 2007 and 2016. The mean tracking period for each animal was 363.9 days (s.e.m.=44.1 days). We used a method labelled multi-scale partitioning to test for changes in movement behaviour and partitioned more localised intensive movements within utilisation areas, from larger-scale movement, called ranging. This involved analysing the proximity of movement trajectories to other nearby trajectories of the same animal over time. We also used Dynamic Brownian Bridges Movement Models, which consider the relationship of consecutive locations to determine the areas of utilisation. The mean utilisation area and duration of a camel (n=658 areas) was found to be 342.6km2 (s.e.m.=33.2km2) over 23.5 days (s.e.m.=1.6 days), and the mean ranging distance (n=611 ranging paths) was a 45.1km (s.e.m.=2.0km) path over 3.1 days (s.e.m.=0.1 days).
... They also reduce wildlife visitation to waterholes (Brim Box et al., 2019). Sixteen plant species, including one vulnerable species (Santalum acuminatum), are considered highly vulnerable to local extinction from camel grazing in central Australia (Edwards, Allan, et al., 2008;Edwards, Zeng, et al., 2008, Table 1), although there is no quantitative evidence of increased adult mortality and declines in palatable species (Brim Box, Nano, et al., 2016). ...
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Changed fire regimes and the introduction of rabbits, cats, foxes, and large exotic herbivores have driven widespread ecological catastrophe in Australian arid and semi‐arid zones, which encompass over two‐thirds of the continent. These threats have caused the highest global mammal extinction rates in the last 200 years, as well as significantly undermining social, economic, and cultural practices of Aboriginal peoples of this region. However, a new and potentially more serious threat is emerging. Buffel grass (Cenchrus ciliaris L.) is a globally significant invader now widespread across central Australia, but the threat this ecological transformer species poses to biodiversity, ecosystem function, and culture has received relatively little attention. Our analyses suggest threats from buffel grass in arid and semi‐arid areas of Australia are at least equivalent in magnitude to those posed by invasive animals and possibly higher, because unlike these more recognized threats, buffel has yet to occupy its potential distribution. Buffel infestation also increases the intensity and frequency of wildfires that affect biodiversity, cultural pursuits, and productivity. We compare the logistical and financial challenges of creating and maintaining areas free of buffel for the protection of biodiversity and cultural values, with the creation and maintenance of refuges from introduced mammals or from large‐scale fire in natural habitats. The scale and expense of projected buffel management costs highlight the urgent policy, research, and financing initiatives essential to safeguard threatened species, ecosystems, and cultural values of Aboriginal people in central Australia.
... Feral nonnative animals abound in the region and include camels, goats, rabbits, donkeys, and cats. Large herbivores such as camels put extensive pressure on vegetation where they gather, which leads to soil erosion, especially around water points (either natural springs or artificial water points), whist browsers such as goats damage vegetation across a range of vegetation types (Brim Box et al., 2016). Feral cats (domestic cats, Felis catus, which have gone wild) are widespread in these deserts and are a major problem given their prey is mostly native animals, ranging from small mammals, reptiles, birds to invertebrates (Doherty et al., 2015), whilst another introduced predator, the red fox (Vulpes vulpes) occurs at relatively low densities throughout these deserts (Burrows et al., 2003). ...
Chapter
The Great Sandy Desert (394,900 km2), Gibson Desert (156,300 km2), and Little Sandy Desert (110,900 km2) are three important desert regions within the much larger Australian Arid Zone which covers ~70% of the continent. The three adjoining deserts are characterized by very hot temperatures and low rainfall (averaging 250–350 mm per annum) which is highly variable from year to year, and strongly seasonal (summer-dominant). Sand-dune systems, sandstone mesas and rocky plains dominant the landscapes; the vast majority of these (92% by area) being covered by hummock grasslands (Triodia spp.) with scattered eucalypt trees (Eucalyptus and Corymbia spp.) and Acacia shrubs. Plant diversity is relatively high for deserts (over 2000 plant taxa known), with only two threatened plant species recognized to date. Diversity of reptiles and birds is also relatively high, but many small- to medium-sized mammals have been lost from these deserts or are near extinction. Despite the almost complete lack of vegetation clearing across these deserts, and the mostly good to excellent condition of the vegetation, there are still insidious threats to be managed, including widespread wildfires, feral animals and uncontrolled grazing, and weeds. Levels of protected land are well above-average globally (covering some 43% of the area), but are relatively low in terms of formal conservation estate (for instance only 7% for the Great Sandy Desert). Most (85%) of the protected lands across the three deserts comprise Indigenous Protected Areas which provides a means to effectively manage the threats and safeguard natural assets over large expanses of these deserts provided there is adequate and ongoing support given to the Aboriginal custodians.
... In Australia, however, large populations of non-domestic herbivores, both native and exotic, have persisted in rangelands used primarily for livestock production despite control efforts and a limited degree of commercialisation. The non-domestic herbivores found almost ubiquitously throughout the southern rangelands include kangaroos, unmanaged goats 1 and rabbits, and in some areas feral pigs (strictly, omnivores), donkeys and dromedary camels the latter representing the only large population of feral dromedary camels in the world (Brim Box et al. 2016). Harrington et al. (1984) (citing Harrington 1983) noted that 'Australia is the only continent in the world where large native herbivores have actually been advantaged by the incursion of pastoral man', that advantage flowing from the provision of artificial water sources and the suppression of dingos. ...
Article
In Australia, particularly in the southern rangelands, large populations of native and feral herbivores (including kangaroos, goats, rabbits, pigs, donkeys and camels, depending on the location) co-exist with domestic livestock. In recent decades the concept of ‘total grazing pressure’ has been developed, and widely accepted, to denote the total forage demand of all vertebrate herbivores relative to the forage supply. This concept provides a framework within which both domestic and non-domestic species can be managed to allow commercially viable livestock production, landscape maintenance or restoration and species conservation. The concept should have relevance wherever pest animal control programs, biodiversity conservation, or commercialisation of wildlife are conducted in conjunction with extensive livestock production. The rationale for the compilation of the Special Issue is outlined.
... Feral nonnative animals abound in the region and include camels, goats, rabbits, donkeys, and cats. Large herbivores such as camels put extensive pressure on vegetation where they gather, which leads to soil erosion, especially around water points (either natural springs or artificial water points), whist browsers such as goats damage vegetation across a range of vegetation types (Brim Box et al., 2016). Feral cats (domestic cats, Felis catus, which have gone wild) are widespread in these deserts and are a major problem given their prey is mostly native animals, ranging from small mammals, reptiles, birds to invertebrates (Doherty et al., 2015), whilst another introduced predator, the red fox (Vulpes vulpes) occurs at relatively low densities throughout these deserts (Burrows et al., 2003). ...
Chapter
Full-text available
The Great Sandy Desert (394,900 km2), Gibson Desert (156,300 km2), and Little Sandy Desert (110,900 km2) are three important desert regions within the much larger Australian Arid Zone which covers ~70% of the continent. The three adjoining deserts are characterized by very hot temperatures and low rainfall (averaging 250–350 mm per annum) which is highly variable from year to year, and strongly seasonal (summer-dominant). Sand-dune systems, sandstone mesas and rocky plains dominant the landscapes; the vast majority of these (92% by area) being covered by hummock grasslands (Triodia spp.) with scattered eucalypt trees (Eucalyptus and Corymbia spp.) and Acacia shrubs. Plant diversity is relatively high for deserts (over 2000 plant taxa known), with only two threatened plant species recognized to date. Diversity of reptiles and birds is also relatively high, but many small- to medium-sized mammals have been lost from these deserts or are near extinction. Despite the almost complete lack of vegetation clearing across these deserts, and the mostly good to excellent condition of the vegetation, there are still insidious threats to be managed, including widespread wildfires, feral animals and uncontrolled grazing, and weeds. Levels of protected land are well above-average globally (covering some 43% of the area), but are relatively low in terms of formal conservation estate (for instance only 7% for the Great Sandy Desert). Most (85%) of the protected lands across the three deserts comprise Indigenous Protected Areas which provides a means to effectively manage the threats and safeguard natural assets over large expanses of these deserts provided there is adequate and ongoing support given to the Aboriginal custodians.
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Invasive species can impact their new environments in different ways, including through exploitation competition and interference competition. For the past decade we have documented the severe degradation of central Australian waterholes by feral camels. Not surprisingly, large feral herbivores can have profound negative impacts on aquatic biodiversity. Less understood was the extent that feral camels impact on native terrestrial wildlife for access to water. From 2011 to 2013 we used camera traps at six waterholes in central Australia to document the co‐occurrence of feral camels and some native wildlife. We used circular statistics and univariate analyses to evaluate activity budgets, visitation frequency and species co‐occurrence for camels, dingoes and bird species that require daily or regular access to water. When camels were present, birds and dingoes visited waterholes less frequently than on days camels were absent. The daily activity budget of birds shifted when camels and dingoes were present, and dingo activity shifted when camels were present. Although the temporal overlap of camels and birds was low, it was not less than expected by chance. Our data suggest that feral camels are the superior resource exploiter at these arid waterholes and reduce wildlife visitation and alter activity budgets. It remains to be tested whether this translates to longer‐term impacts on native birds and dingoes. Feral camels occur throughout inland Australia. We examined whether camels impacted on native terrestrial wildlife for access to water, a limited resource in the arid zone. Camels were the superior resource exploiter at arid waterholes and reduced wildlife visitation and altered activity budgets.
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`Lost from our landscape: threatened species of the Northern Territory' provides comprehensive information on the Northern Territory's threatened plant and animals species. For each of the 203 species that have been included, a full dossier has been provided - the result of extensive work by researchers from the Northern Territory Department of Natural Resources, Environment and The Arts. Each dossier provides a description of the species, its conservation status (in the Territory and Australia), its distribution, ecology, the threatening processes and conservation objectives. This book provides an insight into the Northern Territory's threatened species and offers hope for the protection of the Territory's natural heritage.
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1 The demographies of two long-lived arid zone shrubs, Eremophila maitlandii. and Eremophila forrestii, were characterized using data from a grazing trial (1983-93) in arid Western Australia. Two sites were used, one dominated by annual pasture, the other by shrubs. Recruitment, mortality and size change were described from 11 annual samplings, encompassing conditions of prolonged drought and 1 year of unusually high rainfall. 2 The dynamics of these two species can be described by a combination of event-driven and continuous processes. Both recruitment and mortality were observed in all years for both species. Highest rates of recruitment were observed during the wet year and in the two subsequent years. Highest rates of mortality were observed in the 2 years following the year of lowest rainfall. 3 Stocking rate had no effect on the mortality rate of the unpalatable E. forrestii but the mortality rate of the palatable E. maitlandii was greater under high stocking. The effect of high stocking and low rainfall was additive, rather than excessive stocking being especially damaging in drought years. 4 Higher mortality rates were observed in the shorter and younger stages of both species. Tall individuals had very low mortality rates. Shrubs decreased in height in the year(s) before death. 5 Individuals of both species increased in height during the wet period and decreased during the drought. Net change in height over the trial period was small. High stocking led to higher rates of size decline during the drought. There was large height variation within all cohorts and many plants present at the start of the trial remained short. Taken together, these results suggest that caution must be exercised if size frequency distributions are used to infer population dynamics.
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The Australian Feral Camel Management Project achieved its feral camel density targets at nominated environmental sites, with feral camel density being used as a de facto measure of feral camel impact. The project recognised that it was only the first step in a more concerted effort to bring feral camel impacts under control and therefore had a major focus on building capacity for future feral camel management. Although it had a management focus, the project provided a valuable opportunity to improve our knowledge of feral camel damage and management with an extensive monitoring and evaluation process. The final report of the project provides 24 recommendations that should be considered by all stakeholders in undertaking ongoing feral camel management.
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Longevity, recruitment, survivorship and change in population size of three dominant shrubs were investigated at Koonamore, South Australia. Particular attention was paid to the effects of grazing by sheep and rabbits. Atriplex vesicaria Heward ex Benth. has negative exponential survivorship with a half-life of ca 11 yr. Its populations decline under heavy grazing but increase at the expense of Maireana sedifolia (F. Muell.) P. G. Wilson under intermittent grazing. They also increase after release from grazing. M. sedifolia has negative exponential survivorship with a half-life of ≥ 150 yr. Its populations decline under grazing, and even after 50 yr release from grazing continue to decline. Acacia aneura F. Muell. ex Benth. lives to ca 250 yr. Grazing prevents recruitment, causing its populations to decline. Recruitment resumes after release from grazing, but requires exceptional climatic sequences. These trends if continued will cause drastic change in the dominance of the important low (chenopod) shrubland and low woodland communities of inland Australia. /// Исследовали продолжительность жизни, возобновление, выживаемость и из-менения величины популяций 3-x доминируюших видов кустарника в Коонамо-ре, Южн. Австралия. Осбое внимание уделено влиянию выласа овец и кро-ликов. У Atriplex vesicaria Heward ex Benth. обнаружена отрицательная экспонента выживаемости при продолжительности жизни примерно 11 лет. Их популяции уменьшаются при сильном выласе и увеличиваются в размерах при умеренном выласе за счет Maireana sedifolia (F. Muell.). Они увели-чиваются также при прекрашении выпаса. M. sedifolia имеет отрицатель-ную экспоненту выживаемости при продолжительности жизни 150 лет. Ее популяции уменьшаются в размерах при выласе и даже через 50 лет после его прекращения продолжают уменьшаться. Acacia aneura F. Muell. ex Benth живет примерно 250 лет. Выпас препятствует возобновлению, что влияет на снижение величины популяции. Возобновление начинается после прекра-щения выпаса, но при исклюичительно благоприятных условиях. Эти тенден-ции очевидно приведут к сильыьм изменениям в структуре доминирования важных низкорослых кустарниковых и древесных ассоциаций внутренней Австралии.
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1.Despite widespread acknowledgement that large mammal herbivory can strongly affect vegetation structure in savanna, we still lack a theoretical and practical understanding of savanna dynamics in response to herbivory. 2.Like fire, browsing may impose height-structured recruitment limitations on trees (i.e. a ‘browse trap’), but the demographics of herbivore effects have rarely been considered explicitly. Evidence that that cohorts of trees in savannas may establish during herbivore population crashes and persist long term in savanna landscapes is anecdotal. 3.Here we use an experimental approach in Hluhluwe iMfolozi Park in South Africa, examining the response of grass biomass and tree populations to 10 years of graduated herbivore exclusion, and their subsequent response when exclosures were removed. 4.We found that grazer exclusion increased grass biomass and that, despite presumable increases in fire intensity and grass competition, herbivore – especially mesoherbivore, including impala and nyala – exclusion resulted in increases in tree size. After herbivore reintroduction, grazers reduced grass biomass over short time scales, but tree release from browsing persisted, regardless of tree size. 5.Synthesis. This work provides the first experimental evidence that release from browsing trumps grazer-grass-fire interactions to result in increases in tree size that persist even after browser reintroduction. Escape from the ‘browse trap’ may be incremental and not strictly episodic, but, over longer time scales, reductions in browsing pressure may lead to tree establishment events in savanna that persist even during periods of intense browsing. Explicitly considering the temporal demographic effects of browsing will be key for a much-needed evaluation of the potential global extent of herbivore impacts in savanna. This article is protected by copyright. All rights reserved.
Article
Feral camels have significant negative impacts on the environment and the social/cultural values of Aboriginal people. These impacts include damage to vegetation through feeding behaviour and trampling; suppression of recruitment in some plant species; damage to wetlands through fouling, trampling, and sedimentation; competition with native animals for food, water and shelter; damage to sites such as waterholes, that have cultural significance to Aboriginal people; destruction of bushfood resources; reduction in Aboriginal people's enjoyment of natural areas; creation of dangerous driving conditions; damage to people and vehicles due to collisions, and being a general nuisance in remote settlements. Negative economic impacts of feral camels mainly include direct control and management costs, impacts on livestock production through camels competing with stock for food and other resources and damage to production-related infrastructure. The annual net impact cost of feral camels was estimated to be -$10.67 million for those elements that could be evaluated according to market values. We established a positive density/damage relationship for camels and infrastructure on pastoral properties, which is likely to hold true for environmental variables and cultural/social variables as well. Therefore, irrespective of climate change, the magnitude of the negative impacts of feral camels will undoubtedly increase if the population is allowed to continue to increase. Furthermore, the likelihood that camels would be epidemiologically involved in the spread of exotic diseases like bluetongue and surra (were there to be outbreaks of these diseases in Australia) is also very likely to increase with population density. On the basis of our present understanding, we recommend that feral camels be managed to a long-term target density of 0.1-0.2 camels/km(2) at property to regional scales (areas in the order of 10 000-100 000 km(2)) in order to mitigate broad-scale negative impacts on the environmental, social/cultural and production assets of the Australian rangelands.
Article
In this paper we utilised a range of data sources to estimate the extent, density distribution and population size of the feral camel in Australia in 2008. Camels currently occupy 3.3 million km(2) and are spread across much of arid Western Australia, South Australia, the Northern Territory and far western Queensland. Up to 50% of Australia's rangelands are reported as having camels present. The research reported here supports a current minimum population estimate for the feral camel in Australia of similar to 1 million animals at an overall density of 0.29 camels/km(2). Densities vary, and the modelling of available data indicates that two substantial areas of high density are present, one centred on the Simpson Desert and the other on the Great Sandy Desert. The high density area covering the eastern part of the Great Sandy Desert has predicted densities in the range of 0.5 to >2 animals/km(2) whereas that on the Simpson Desert is in the range 0.5-1.0 animals/km(2).