Suchergebnisse

A Drifter ultralight aircraft was used as a platform for line-transect aerial
surveys of three species of kangaroo in the sheep rangelands south-east of
Blackall and north of Longreach in central-western Queensland in winter 1993
and 1994. Favourable comparisons between the results of ultralight surveys and
those made from a helicopter flying the same transects and foot surveys along
another set of transects, all within a few days of the ultralight survey,
confirmed the expectation we had that an ultralight would be a satisfactory
and much cheaper vehicle for conducting aerial surveys of kangaroos. The
comparisons are even more favourable when data for the three species surveyed
are combined, pointing to a problem in species identification and underlining
the importance of using only experienced observers for aerial survey of
kangaroos, whatever the platform. The use of an ultralight aircraft could have
particular value where a comparatively small area, such as an individual sheep
or cattle property, is under consideration. In this paper, we present the
numerical comparisons, along with an evaluation of the practicability of using
this type of aircraft. We also describe a possible future scenario in which an
accreditation process could see approved kangaroo surveyors undertaking
property assessments by ultralight, under contract to graziers or other
interested parties.

As part of a large-scale monitoring program
linked to the management of kangaroos in the South Australian pastoral zone,
the western grey kangaroo populations have been surveyed annually with
fixed-wing aircraft over the 15-years-period 1978–92. Western grey
kangaroos are restricted in their distribution to the southern regions of the
pastoral zone. During the period of the study, western grey kangaroo numbers
showed no long-term trends, but did show some marked fluctuations, principally
in association with a severe drought. Despite this, and unlike red kangaroos
in the South Australian pastoral zone, no consistent, direct association
between changes in western grey kangaroo numbers and antecedent rainfall could
be demonstrated. The postulated reason for this is that most of the regional
western grey kangaroo populations examined in this study were low-density
populations at the edge of the range of this species. Outside of drought,
these populations are likely to be limited by factors other than food, such as
climate and unmodified resources in the form of suitable habitat. Also,
because boundary populations may well only be maintained by constant loss and
recolonisation, local extinctions associated with drought may result in
extended delays in the re-establishment of populations in marginal areas. Over
the period 1978–92, these populations were harvested commercially at
annual rates of 5–25%, which were, on the whole, considered to be
below the rates suggested to be maximum and sustainable.

Most of Australia's sheep rangelands are
enclosed by a dingo-proof fence. Within these rangelands, where dingoes
(Canis lupus dingo) are rare, red kangaroos
(Macropus rufus) are considered to be food limited
because their numbers respond to fluctuations in pasture biomass that are
driven by highly variable rainfall. Outside this region, where dingoes are
common, kangaroo densities are generally substantially lower, suggesting that
dingoes are an important limiting factor. However, it is unclear whether
dingoes can regulate kangaroo populations. In this study, red kangaroo and emu
(Dromaius novaehollandiae) numbers were monitored for
varying periods during 1978–92 by aerial survey on both sides of the
dingo fence in three areas in the north of the South Australian pastoral zone.
Densities of red kangaroos and emus were lower outside the fence, although the
disparity varied between areas and over time. The similarity in the
environments on both sides of the fence and the marked step in kangaroo
density at the fence are consistent with dingoes strongly limiting these prey
populations. In the north-east of the pastoral zone, where kangaroo and emu
densities are greatest, the contrast in density across the fence was most
pronounced. Furthermore, the trends in density over time differed across the
fence. Outside the fence, red kangaroos and emus remained at low densities
following drought as dingo numbers increased. Inside the fence, red kangaroo
and emu populations showed a 'typical' post-drought recovery. The
data therefore suggest that, in some situations, dingoes may not simply limit
red kangaroo and emu populations, but also regulate them. For this to occur,
predation rate would need to be density dependent at low prey densities. The
availability of alternative prey, and the reduction in the numbers of all prey
during drought may provide the mechanism.

The use of line-transect methodology, on foot or from a helicopter, is likely
to return the most repeatable, least biased estimates of kangaroo density.
However, the associated costs make both methods impractical for broad-scale
surveys. For these, a fixed-wing aircraft remains the most cost-effective
platform. Limitations of the standard fixed-wing method (200-m strip
transects) are well known, but it continues to be used because it provides an
index of trends, because there are now long runs of data (almost 20 years in
some cases) collected in this standard form and an alternative method is
lacking. In this study, four variations of fixed-wing surveys of kangaroos
were investigated: two line-transect methods (involving different scanning
techniques), the standard 200-m strip transect and a 100-m strip transect.
Surveys using these methods were compared with helicopter line-transect
surveys along the same flight lines in three areas (5000–7500
km2) in western Queensland. Both fixed-wing
line-transect methods failed to produce consistently accurate estimates of
density for all three species surveyed: red kangaroos
(Macropus rufus), eastern grey kangaroos
(M. giganteus) and common wallaroos
(M. robustus). While generally more accurate than the
uncorrected strip-transect counts, they were no less variable. However, the
strip-transect counts still need to be corrected for bias for which this study
offers revised estimates of correction factors for eastern grey kangaroos
(3.7–10.2) and common wallaroos (3.8–4.1), and estimates for red
kangaroos (1.7–2.7) that support currently used values. An attractive
alternative is to survey in 100-m strip transects, which offer improved
visibility (correction factors of 1.0–1.8 for red kangaroos,
2.1–3.6 for eastern grey kangaroos and 1.7–2.1 for common
wallaroos) and are therefore likely to be more accurate and repeatable.
However, these advantages need to be assessed in relation to continuing long
runs of data using the standard 200-m strip transect. Correction factors for
wallaroos are conservative as helicopter-based density estimates are known to
be underestimates. Further work is needed to assess the generality of
correction factors, both spatially and temporally.