Download The Population Dynamics of Red and Western Grey Kangaroos in

The Population Dynamics of Red and Western Grey Kangaroos in Arid New South
Wales, Australia. I. Population Trends and Rainfall
P. Bayliss
The Journal of Animal Ecology, Vol. 54, No. 1. (Feb., 1985), pp. 111-125.
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Thu Mar 6 23:32:29 2008
Journal ofAnilnal Ecolog.19(1985), 54, 11 1-125
THE POPULATION DYNAMICS O F RED AND WESTERN GREY KANGAROOS IN ARID NEW SOUTH WALES, AUSTRALIA. I. POPULATION TRENDS AND RAINFALL BY P. BAYLISS*
New South Wales National Parks and Wildlife Service, C/- Division of Wildlife and Rangelands Research, CSZRO, P.O. Box 84, Lyneham, A.C.T., Australia 2602* SUMMARY
(1) Red and western grey kangaroo populations were monitored on Kinchega National
Park and some adjoining sheep stations on a quarterly basis between 1973 and 1981.
Kangaroos inside the Park were constrained whilst those outside were not.
(2) The years 1973-1976 experienced exceptionally high rainfall providing kangaroos
with a superabundance of food. The preceding years were marked by the late 1960's
drought and low kangaroo densities, and the following years by average rainfall and
high kangaroo densities.
(3) The kangaroo populations exhibited two general trends in relation to the exceptional
and average rainfall years: a rapid rise and an asymptote. The average exponential rates of
increase (?) between 1973 and 1977 were 0.36 p.a. and 0.27 p.a. respectively for red and
western grey kangaroos inside the Park; and 0.44 p.a. for both species outside. However,
these rates were not significantly different. Since 1978 all populations had a zero average
rate of increase and the high densities were maintained.
(4) There was no significant difference between the population dynamics of the
kangaroos inside the Park and on the adjoining sheep stations, regardless of densities being
2.5 times higher on the Park. It is hypothesized that competition for food by sheep
suppressed kangaroo densities outside the Park.
(5) The exponential rate of increase (r p.a.) of all populations was positively correlated
with lagged rainfall.
(6) Rainfall and rate of increase of both species inside the Park exhibited regular trends
on time. The regularity was strong for the western grey kangaroo but weak for the
red kangaroo.
INTRODUCTION
This study investigates the relationship between the populations dynamics of red kangaroos
Macropus rufus (Desmarest) and western grey kangaroos M. fuliginosus (Desmarest)
and their food supply. The relationship is called the numerical response (Holling 1959)
and here it is concerned only with changes in density due to fluctuations in food supply
resulting from a variable rainfall.
The dynamics of the kangaroo populations in this study were measured by the
exponential rate of increase on a yearly basis: it was a direct and concise summary of the
differential between births and deaths inside the Park, and in addition incorporated
* Present address: Conservation Commission of the Northern Territory. P.O. Box 38496, Winnellie, N.T.,
Australia 5789.
112
Kangaroo population d-vnamics I
possible dispersal effects for free-ranging kangaroos outside the Park. This paper is in
two parts: The first part describes the study area and outlines the methods of study;
the second part examines general trends in kangaroo numbers via the average yearly
rate of increase for the periods 1973-1977 and 1978-1981, representing exceptionally
high and average rainfall years, respectively. The population dynamics of kangaroos
inside the Park are compared to those outside. A more detailed analysis is presented
where changes in the rates of increase are examined in relation to changes in the amount
of rainfall.
STUDY AREA A N D METHODS
The study area
The location of the Park and sheep stations is described in Fig. 1. The Park was
dedicated in 1967 and domestic sheep removed. A kangaroo-proof fence was erected
around the north, south and western boundaries in 1972. As the Darling River cuts a
deep channel along the eastern boundary, the kangaroos inside the Park were effectively
constrained. This is in contrast to the unbounded kangaroos on the sheep stations. Hence
the study area provides a large-scale experiment whereby the populations outside the Park
form the control, and those inside the treatment. Outside they are harvested, have no
restrictions on large-scale dispersal and share their food supply with sheep. Inside they
have restricted powers of large-scale dispersal, and lack harvesting and possible competition for food from sheep. The eastern grey kangaroo M. giganteus Shaw and euro,
M , robustus (Shaw) occur in the study area but in very low densities.
FIG. 1. M a p of study area showing the location of the air survey transects on Kinchega National
Park and the abutting sheep stations west of the Darling River. (+). Air survey transects:
(D). Kinchega National Park:
lakes: (4.
road.
(a.
FIG.2. ( a ) Monthly rainfall (mm) between 1973 and 1981 for Menindee. and (b) the percentage
deviation of annual rainfall from the long term annual average (235 mm) between 1973
and 1981.
The annual average rainfall is 235 mm and long term records indicate that it is highly
variable, with a standard deviation of 30%, but with a slight winter dominance of 10%
(Beadle 1948; Lawrie 1979). Mean annual rainfall is 10% of the mean annual pan
evaporation. The study area is therefore characteristically arid. Figure 2 shows the
extreme monthly and yearly variation in rainfall (mm) between 1973 and 1981 for the
nearby town of Menindee. In 1974 the study area received nearly four times its annual
average rainfall (880 mm). Temperature and solar radiation are relatively constant
throughout the year, hence rainfall is the limiting factor to plant growth. Because rainfall
is discontinuous, water inputs are described as short term driving 'pulses' in arid systems
(Noy-Meir 1973).
The basic landforms are flood plains, and sand plains and dune fields, respectively
forming 66% and 44% of the area. River red gum Euca(~ytuscamaldulensis Dehnh., and
black box E. larglflorens F. Muell trees dominate the open woodland on the river and
creek flood plains. The sand plains are dominated by black bluebush Maireanapyramidata
(Benth.) P.G. Wilson, a perennial chenopod (Chenopodiaceae) shrub. whilst scattered on
the dune fields are stands of rosewood Heterodendron oleifolium Desf. and belah
Casuarina cristata Miq. trees. Annual forbs, grasses and chenopods dominate the
herbaceous flora in both landforms. and the occurrence of these plants depend largely on
the effecti~eness of rainfall.
Survey techniques
Kangaroos were monitored by standardized aerial surveys on a quarterly basis,
providing almost 9 years of consistent survey data.
Surcey cariables
Surveys were flown at 90 m above ground at a speed of 167 km hk'. Both species
were counted on transects with a width of 100 m per observer. except on the first two
surveys when a 200 m per observer width of transect was used. Bayliss (1980)
corrected these two surveys down to a 100 m per observer count using Eberhardt's
(1968) model as qualified by Caughley, Sinclair & Scott-Kemmis (1976). The majority of
114
Kangaroo populatiorl dj~namicsI
the surveys were flown in a Cessna 206 and the boundaries of the transects were
demarcated by streamers attached to each wing strut. On each survey two observers
counted on opposite sides of the plane. Bayliss (1980) found no significant difference in
observer efficiency between species and over time.
Survey design
Kinchega and the adjoining sheep stations west of the Darling River were sampled by
systematic transects (see Fig. 1). Both surveys sampled approximately 39 km2 of country
and the sampling intensity on Kinchega was 9 % of its dryland area. Each survey took
2 h, beginning soon after first light when kangaroos were most active and visible. Two
survey replicates were flown on the Park on consecutive days. On the third day the
sheep stations were surveyed once.
The index of abundarlce
The numbers of red and western grey kangaroos that were counted by both observers on
all transects on a survey were added to provide an index of abundance for each species in
both locations. The survey replicates for the Park were averaged.
Caughley, Sinclair & Scott-Kemmis (1976) formulated correction factors to convert
relative indices of kangaroo density to absolute estimates. However, this study examines
relative changes in abundance rather than absolute changes, hence the degree of visibility
bias in unimportant because it is held constant by standardized survey procedures.
When absolute estimates of kangaroo density were required the Caughley, Sinclair & ScottKemmis (1976) correction factor equation for red kangaroos in open habitats was used for
both species. Recent experiments indicate that grey kangaroos may require a higher
correction factor than red kangaroos and hence their densities may be underestimated
(unpublished data). All grey kangaroos counted were assumed to be western grey
kangaroos.
Statistics
The correlation coefficient in all analyses is represented by the upper case R to avoid
confusion with the lower case r used here to represent exponential rate of increase.
RESULTS
Trends in kangaroo numbers
Acerage rates of increase (i)
A polynomial regression model was used to test if the average rate of increase of the
populations slowed with time. The regression of logged abundance index on time (measured
in months since the start of the surveys) was tested for deviations from linearity using the
method outlined in Zar (1974). The results are summarized in Table 1 and show that
the average rates of increase slowed with time (coefficient T2was significant and negative).
The lack of a strong significance for western grey kangaroos inside the Park is attributed
to a large variability in the data. For convenience the general time trend is not viewed as
a smooth parabola of events, but as two distinct linear trends truncating in 1977: that is,
a rapid rise between 1973 and 1977 and an asymptote between 1978 and 1981.
TABLE1. Summary of the F-ratio test for the added quadratic term T2 to the
linear regression of logged adundance indices on time (T)
F*
Significance T'
Coefficient T?
Park
Reds
W. greys
7.46
2.03
P < 0.05
P < 0.10
-0.00018
-0~00013
Outside
Reds
W. greys
11.96
4.58
P < 0.01
P < 0.05
-0~00031
-0.00025
* F ratio test for the addition of the T2term to the linear regression. n = 31.
.
7 ? ( a ) Red kangaroos Kinchega
/-:.
=
-
*.
*_
.,
. - *. . .. . . .
6 - (b) Western grey kangaroos K n c h e g a
*.
***
/*:5
5-
r (c)
5ed karqaroor
..
4
*.
.
r
i d ) Grey kangaroos outside
FIG. 3. The quarterly logged abundance Indices (In Y) on time in months (T) for (a) red
kangaroos. (b) western grey kangaroos inside the Park. and (c) red kangaroos. (d) western
grey kangaroos outs~dethe Park. All populations show an initial rapid rise and an asymptote.
Figure 3 show the logged indices of abundance for both species in both locations for
each survey between 1973 and 1981. All populations show marked fluctuations in density
about the average time trends. These fluctuations reflect biases, sampling error and real
changes due to boom or bust juvenile recruitment patterns (Bayliss 1980). The two time
trends are analysed in detail below.
The rise
The average rate of increase (7) between 1973 and 1977 was calculated for each
population by regressing the logged abundance indices on time in months since the start
of the surveys (n = 13). The slope of this line gave F month-' and this was converted to
F p.a. The regression equations and calculated i values are summarized in Table 2(a).
The regression slopes and intercepts were compared by covariance analysis, the
results indicating that within the variability of the data the average rates of increase were
not significantly different (slopes: F = 0.61, d.f. = 3/44). The regression intercepts
were significantly different (intercepts: F = 63.92, d.f. = 3/47, P < 0.001), indicating that
the populations may have had different starting densities.
Kangaroo population d~warnicsI
116
TABLE2. (a) Summary of regressions between logged abundance index ( Y ) and
time (7') between 1973 and 1977. and the calculated average rate of increase
( 3 + S.E. (b) Summary of the regressions between logged abundance index ( Y )
and time (T) between 1978 and 198 1 (included in a mean index)
Species
(a) 1973-1977
Red
W. grey
Red
W. grey
Location
Park
Park
Outside
Outside
Equation
In
In
In
In
Y = 5.15 + 0 . 0 3 T
Y = 4.22 + 0.02T
Y = 3.95 + 0.04T
Y = 2.72 + 0.04T
R'
Significance
( r ) & S.E.
0.51
0.31
0.65
0.57
P < 0,001
P < 0.01
P < 0.001
P < 0.01
0.36 i 0.07
0.27 & 0.10
0.44 & 0.10
0.44 i 0.12
In mean abundance index and sample slze ( N )
(b) 1978-1981 Red
Park
W. gre)
Park
Red
Outside
W. gre)
Outside
6.46
5.30
5.55
4.40
(18)
(17)*
(18)
(18) N.S.
N.S.
N.S.
N.S.
0
0
0
0
n = sample size. n for period (a) = 13 in all cases. N.S. = not significant.
S.E. = standard error.
* Bad count excluded.
The asjwlptote
The asymptotic density between 1978 and 1981 for each population was calculated
a s the mean of the logged abundance indices ( n = 18). Regression analysis indicated
that the F values for each population were not significantly different from zero and the
results are summarized in Table 2(b). Single-factor analysis of variance indicated that the
differences in starting density between each population were maintained: that is, the ratio
of reds and western greys was 3 to 1. respectively. and the ratio of kangaroos inside and
outside the Park was 2 . 5 to 1. respectively.
The effects of sheep, harcesting and large-scale dispersal on kangaroo densities outside
the Park
Despite a substantial difference in kangaroo density between location, average rates of
increase did not differ significantly. Predation and disease were almost non-existent in
the study area (Arundel. Beveridge & Presidente 1979) and other potential limiting factors
such as cover and water were similar between locations (Bayliss 1980). Hence similar
average rates of increase suggest
(i) similar food resources per head of population in both locations, implicating
competition for food by sheep. o r
(ii) more food per head at the lower densities with some external factor suppressing
rate of increase, implicating kangaroo harvesting and/or large-scale dispersal. These hypotheses are examined below. Sheep
The recommended sheep stocking rate (N.S.W. Soil Conservation Service) of sixteen
sheep km-* was added t o the total density of kangaroos outside the Park between 1979
and 198 1. Table 3 compares the total kangaroo density inside the Park to the total large
mammal herbivore density outside. The assumptions are that one sheep is equivalent
TABLE3. A comparison of the total large mammal herbivore density (km-2) inside
the Park and outside between 1979 and 1981
Year
Kinchega
Kangaroos
1979
1980
1981
Mean
44.5
41.3
43.3
43.0
Kangaroos
Outside
Sheep*
23.5
20.2
17.9
25.6
25.6
25.6
Total
49.1
45.8
43.5
46.1
* Sheep are converted to kangaroo equivalents (1 sheep = 1.6 kangaroos).
TABLE4. Harvesting rate and percentage offtake of kangaroos on the properties
surveyed outside the Park between 1979 and 1981
Year
Harvest rate (km-')
% offtake
1979
1980
198 1
Mean
0.58
0.69
1.65
0.97
3.5
5.1
8.2
5.60
to 1.6 kangaroos (McIntosh 1966; Forbes & Tribe 1970; Hume 1974), rabbit and
invertebrate grazing pressure are similar in both locations, and that the average stocking
rates hold for all properties in the survey area (graziers in fact usually stock above the
recommended rates, Young & Miles 1982). The results suggest that large mammal
biomass and hence grazing pressure in both locations may be similar.
Haruesting
Annual harvest rates and yearly percentage offtake estimates of kangaroos between
1979 and 1981 due to licensed commercial shooting are presented in Table 4. Illegal
shooting cannot be assessed, but was assumed to be insignificant. Commercial harvests
in 1978 took less than 4 % of the population and in previous years less than 1%
(Bayliss 1980).
Dispersal
Similar average rates of increase between locations argues also against large-scale
dispersal modifying densities differentially outside the Park. The larger fluctuations in
density of both species outside the Park are attributed to larger sampling errors (see Fig. 3).
To give further credence to this argument, the hypothesis that the variance around the
curvilinear regression of logged abundance on time is inversely proportional to the
sampling intensity was examined. Twice the area was sampled inside the Park as outside
(two survey replicates compared to one), and with twice the sampling intensity the
variance of the estimate outside is expected to be twice that inside. If it is greater then
this would argue in favour of significant large-scale dispersal of kangaroos outside the
Park. However, the variance ratios for both species were considerably less than 2 . 0
(1.4 each), arguing against large-scale dispersal although this hypothesis cannot be tested
statistically because the transects were not random and independent.
The relationship between rainfall and kangaroo food supply
Standing herbaceous biomass (kg ha-' O.D.W.) was measured on Kinchega every
quarter between 1980 and 1981 (G. Robertson, unpublished data). These estimates
118
Kangaroo population d ~ ~ n a m i 1c s
were best correlated with the previous 6 months of rainfall ( n = 9, R = 0.84, P < 0.01).
Unexplained variance (29%) in this relationship is attributed to sampling error and
the reduction of standing crop due to total grazing offtake (mammals and invertebrates).
Herbaceous biomass was also correlated with a soil moisture index and an index of
plant growth (Fitzpatrick & Nix 1970). However, rainfall remained the best predictor,
and this is not surprising since it contributed almost totally to the values of the other
two indices.
Ellis et al. (1977) showed that red kangaroos and sheep shifted their diets to different
plant groups with seasonal changes in pasture composition and food availability. Red
kangaroos ate mainly grasses but switched to annual forbs when the composition of
the pasture changed in that direction. Forbs are short-lived plants providing kangaroos
with food high in energy and nitrogen. Grey kangaroos also eat mainly grasses (Kirkpatrick
1965) and probably select forbs when they are available.
Rainfall is therefore a reasonable index of the quantity and quality of kangaroo food
supply: after good rainfall herbaceous biomass builds up rapidly, the pasture then being
primarily composed of ephemerals and annuals; during prolonged periods of low rainfall
the pasture is dominated by perennial shrubs, ephemerals and annuals contributing little.
The relationship between changes in the rate of increase (r) and changes in rainfall
The average rate of increase (9 summarized changes in abundance over a number of
years, and carried no assumptions about constant rate of increase, constant supply of
food or stable age distribution (Caughley 1977). However, there are marked fluctuations in
kangaroo numbers about the average time trends (see Fig. 3) and marked changes in
rainfall suggests rapid and marked changes in food supply (see Fig. 2). The fluctuations in
kangaroo numbers and food supply allow a more detailed examination of the dynamic
relationship between the two.
Rate of increase (r)
Detailed changes in the rates of increase were measured by taking the logged abundance
index in one season and subtracting from this the logged abundance index in the same
season of the previous year. This procedure allowed an exponential rate of increase
( r ) to be measured almost every 3 months between 1973 and 1981. These are yearly rates
and are instantaneous between the two indices of abundance. They include sample error
and bias, and are not independent because they overlap with previous and following rates
of increase. Bayliss (1980) showed that aerial survey estimates of kangaroos are subject to
seasonal visibility bias. However, by measuring the rates between the same season in
consecutive years much of this bias is held constant.
Intervals of rainfall and time lags
Although the herbaceous biomass was best correlated with the previous 6 months of
rainfall (hence with a 3 month time lag), this may not be the best index of kangaroo
food supply in relation to the yearly rate of increase. Marked changes in the rate of
increase during non-drought periods would largely reflect average juvenile survival over
that year (average breeding response and adult mortality would contribute little). Hence
an appropriate rainfall-index of food supply in relation to juvenile survival is one that
accounts for pasture growth and persistence during the interval of advanced lactation.
The most appropriate interval of rainfall in relation to recruitment could be anywhere
r
( 0 ) Red kangaroo, P o r k
0'6/ ( c ) Red k a n g a r o o ; outside
0'6[ ( d ) Grey kangaroo;
Time l a g ( m o n t h s )
FIG. 4. Correlation between rate of increase ( r ) and rainfall, at various time lags and intervals
of rainfall, for (a) red and (b) western grey kangaroos inside the Park, and for (c) red and
(d) western grey kangaroos outside the Park (A-A).
12 months; (-0).
6 months;
(@-@I.
3 months.
between 3 and 12 months. More important, kangaroo numbers do not show an immediate
response to changes in rainfall because of the time between plant growth, breeding and/or
juvenile survival, and measurable recruitment. The purpose of the following analysis is to
determine the most appropriate interval of rainfall and average time lag in relation to
rate of increase.
For each population the rates of increase were correlated with 3, 6 and 12 month
intervals of rainfall at increasing time lags. The analysis started at a zero time lag,
increased by increments of 3 months, and finished at a time lag of 27 months. The
S.P.S.S. computer package (Kim & Kohout 1975) was used to produce scattergrams
of all combinations to determine if transformation of data was necessary. However, the
linear model was considered appropriate for this analysis.
The results are illustrated in Fig. 4 for both species in both locations. Rate of
increase of both species inside the Park was best correlated to all rainfall intervals with
a 6 month time lag. The time lag possibly reflected the combined effects of
120
Kangaroo population dvnamics I
plant growth after rain, juvenile survival during advanced lactation, and the detection of
juveniles by aerial survey after they leave the pouch. The results outside the Park are
more difficult to interpret. Reds were best correlated to the 3 and 6 month interval of
rainfall with a zero and 2 1 month time lag. The 12 month interval of rainfall gave a good
correlation at the 3 month time lag. Western greys followed a similar pattern except
that there was no relationship with rainfall in the distant past. The rapidity of the response
to rainfall of both species suggests dispersal effects. Previous analysis suggests that
large-scale dispersal is trivial, hence these results probably reflect localized movements
from wooded to open habitats after rainfall, increasing the visibility of kangaroos. The
lack of any significant correlation at a 6 month time lag suggests that the data outside
the Park may be too variable to detect recruitment bursts due to juvenile survival, except
that for red kangaroos after a long time lag. For this reason, and the possibility that
localized movements may introduce a complex tangle of time lags, further analysis of
the population dynamics of kangaroos outside the Park in relation to rainfall is excluded
from this paper.
Titne trends in rainfall and kangaroo recruitment
Changes in animal density over time are examined for random or periodic trends by
serial or auto-correlation with subsequent plotting of a correlogram (Moran 1952, 1953;
Poole 1974). However, kangaroo densities in this study show an increasing time trend
between 1973 and 1977. It is therefore inappropriate to analyse density for any regularity.
The appropriate measure is the exponential rate of increase, the instantaneous measure of
changes in density.
Any regularity in the pattern of juvenile recruitment is likely to be reflected in the
degree of regularity of rainfall. Hence rainfall is examined for regularity first. Winter and
spring (W-Sp) and summer and autumn (S-A) were used as two time periods per year
in this analysis because rainfall showed no seasonality. In arid environments these time
periods may have more meaning in relation to plant growth than seasons. They respectively represent the periods when rainfall would be most and least effective in terms of
plant growth. The serial correlation coefficients (r,) between the rainfall periods at t = 0
and at regular intervals of time (1 unit = 6 months) in the future t = 1, 2, . . ., 6 were
calculated. The serial correlations were plotted against the serial time (t) to produce a
correlogram and this is illustrated in Fig. 5a. The correlogram troughs deeply at time 2
and peaks at time 4 . 5 with little evidence of damping (although stochastic fluctuations
can prevent this). Hence there was a weak rainfall periodicity of 15 months. A crude
interpretation is that a good W-Sp or S-A fall in 1 year was usually followed by a bad
fall in W-Sp or S-A the following year.
To determine if recruitment for both species had a corresponding regularity their rates
of increase were averaged between W-Sp and S-A periods between 1973 and 1981
and subjected to serial correlation. The correlograms are illustrated in Fig. 5b & c for
western grey and red kangaroos respectively. The correlogram for the western grey
kangaroo peaks and troughs deeply with little evidence of damping: the 15 month
periodicity is the same as that for rainfall. Western grey kangaroos therefore show a
strong regularity in recruitment, possibly related to the regularity of rainfall. In contrast
the correlogram for red kangaroos has shallow peaks and troughs and shows some
evidence of damping quickly. The regularity of recruitment was very weak with a rough
periodicity of 18 months.
''Or ( c )
Red kangaroo
FIG. 5. Correlogram (a) showing the weak periodicity of winter-spring and summer-autumn
rainfall at Menindee between 1973 and 1981: (b) the strong periodicity of recruitment for
western grey kangaroos: (c) the weak periodicity of recruitment for red kangaroos on Kinchega
( 1 time unit = 6 months).
DISCUSSION
Trends in kangaroo numbers
Average rates of increase (i)
In 1973, following the late 1960's drought, there were 9.1 reds kmp2 and 3.2 greys km-2
estimated inside the Park, and 2.4 reds km-2 and 0 . 7 greys kmp2 outside. Between 1978
and 1981 kangaroo densities inside the Park fluctuated around a mean estimate of
30 reds kmp2 and 9.4 greys kmp2. A similar trend occurred outside the Park with a
mean density estimate since 1978 of 12.1 reds km-2 and 3.8 greys kmp2. Between 1973
and 1977 all populations in the study averaged high yearly rates of increase and
increased in density three-five fold. Since 1977 all populations had a zero average yearly
rate of increase and hence these high densities were maintained. The dramatic shift from
low to high densities, and the persistence of these high densities, reflected the atypically
high rainfalls between 1973 and 1976, followed by adequate rainfalls thereafter.
Newsome (1977) showed that the highly unbalanced age distribution of red kangaroos
in Central Australia during the early 1960's was dominated by cohorts produced by
good rainfall conditions (and hence pasture growth) in the late 1940's, 1953-55, and 1958.
He also found that the sex ratio of red kangaroos during a drought was grossly imbalanced
in favour of females. particularly in animals of more than 3 years of age (1 :5). It is
therefore likely that the post-drought kangaroo populations in this study had both
122
Kangaroo population dynamics I
unstable age distributions and imbalanced sex ratios in favour of females. Johnson &
Bayliss (1981) showed that even under average rainfall conditions in 1979 and 1980, red
kangaroos in the study area had an imbalanced sex ratio in favour of females ( 0 . 7 : 1).
The combined effects of an unstable age distribution and imbalanced sex ratio in favour of
females resulted in an initial rapid rate of increase in the first year after drought break
rains in 1973. However, after juvenile non-breeders were pumped into the population this
rate probably ameliorated very quickly. Because rainfall conditions remained favourable
the age distribution of each population probably converged to some semblance of stability.
Hence the calculated average yearly rates of increase between 1973 and 1977 were
probably their maximum rates under the conditions imposed by convergence towards a
more stable age structure. These average yearly rates of increase are therefore termed
here the 'average maximum rates of increase' (?,,) in line with the convention of
Andrewartha & Birch (1954).
Species differences
Although red kangaroos are more demographically efficient than western grey
kangaroos in variable arid environments (Frith & Sharman 1964; Newsome 1965, 1966;
Poole 1973), there was no significant difference in their average rates of increase in this
study. Therefore any real differences in survival and fecundity between species was
likely t o be trivial overall. The results suggest that a continuous birth rate, a higher
turnover of pouch young, and a marginally more rapid breeding response to rainfall
did not favour red kangaroos over western grey kangaroos during the conditions of the
study. Additionally, the advantages of aseasonal and seasonal breeding strategies were
probably equally weighted in this environment, where 60% of the rainfall occurred in
winter and spring, and 40% in summer and autumn. Any significant differences in
recruitment would accrue mainly from differences in juvenile survival. Hence juvenile
survival was probably similar for both species in this study. The differences in reproductive strategies between the two species would be more important in environments
that favoured one species more than the other, especialy after a prolonged drought.
Grey kangaroos favour woodland habitats and 44% of the study is woodland floodplain.
The effects of sheep, harvesting, and large-scale dispersal
The d a t a suggests that kangaroo harvesting was trivial and large-scale dispersal
probably non-existent. Therefore these two factors cannot account for all of the 2.5
density difference that exists between locations. Although a more rigorous experimental
approach to harvesting would clarify this effect (e.g. a controlled sustained-yield for a
longer period of time), an examination of the recommended sheep stocking rate revealed
a more convincing proposition: that the large mammal grazing pressure in both locations
was similar. Hence a reasonable hypothesis is that kangaroos and sheep shared the same
food supply, and that over time this suppressed kangaroo densities outside the Park
through their numerical response. Sheep caused this imposition because they d o not,
numerically, respond t o their food supply. their densities being artificially enforced.
The data in this study d o not provide a direct test of the hypothesis, however. it does
indicate that it is a viable one to test by experimentation. In favour of such a hypothesis
are the many dietary studies of kangaroos and sheep which show more similarity in the
plants they eat than differences, especially after rain (Griffiths & Barker 1966; Storr 1968:
Ellis et al. 1977).
The results argue also against any 'fence-effect' such as that described for a population
of voles by Krebs (1971). As the kangaroos inside the Park had similar population
dynamics to the unfenced control populations outside, it is possible that during the
conditions of this study kangaroos did not regulate their numbers intrinsically through
large-scale dispersal or other forms of social behaviour. However, intrinsic factors should
not be quickly dismissed. Voles have a potential r, p.a. approximately 10 times that for
kangaroos (4.0 cf. 0.40) and hence required only three generations to show a significant
fence-effect. Kangaroos may need more time spanning average rainfall years before any
measurable fence-effect is reflected in its rate of increase.
The relationship between rainfall and rate of increase
The results suggest that there is a strong association between changes in kangaroo
numbers and food supply. Rates of increase were positively correlated with lagged rainfall,
an index of food supply. The strength of this relationship was further emphasized by
examining time trends in recruitment and rainfall. Western grey kangaroos showed a
strong regularity in recruitment with the same periodicity as rainfall events. Western greys
are strong seasonal breeders and hence juvenile survival is probably strongly related to
the timing of rainfall. In contrast red kangaroos showed a weak regularity in recruitment,
possibly because as they are aseasonal breeders the regularity of rainfall is unimportant
to juvenile survival. Hence although previous analysis suggests that both species may
have similar average yearly rates of juvenile survival, the patterns of survival throughout
the year may differ. These patterns of rainfall and recruitment over time were perhaps
only coincidental to the study period and cannot be extrapolated beyond this. However,
it would be interesting to determine if this rainfall pattern held for the last 100 years.
CONCLUSION
Many studies show that food supply is an important determinant of the population
dynamics of large herbivorous mammals (Klein 1968 on reindeer on St Mathew Island,
Caughley on thar in New Zealand, Sinclair 1977 on African buffalo), and in particular
kangaroos (Frith 1964; Frith & Sharman 1964; Kirkpatrick & McEvoy 1966; Ealey &
Main 1967; Bailey, Martensz & Barker 1971). This quantitative study is another
contribution in that general direction.
Although this paper demonstrates that changes in kangaroo numbers are clearly
associated with changes in rainfall-induced food supply, it is emphasized that this
study describes a relatively simple system: there were no known pathological diseases and
parasites, no serious predation, and no evidence of any direct effects of social behaviour
affecting future densities. These results are only applicable to kangaroos experiencing
the above conditions.
ACKNOWLEDGMENTS
I am grateful to the N.S.W. National Parks and Wildslife Service for supporting this
research. The CSIRO Division of Wildlife and Rangelands Rsearch provided facilities and
assistance during writing up. Dr Graeme Caughley provided valuable assistance
throughout. Criticisms by Neil Gilbert, Charles Krebs and Jeff Short improved this
paper. Janice Rudd typed the manuscripts and Frank Knight illustrated the Figures.
124
Kangaroo population dvnamics I
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