Biology Reference
In-Depth Information
from 1978 to 1984. Populations were assessed by aerial surveys, con-
ducted within 3-4 hours of sunrise or sunset. A high-winged aircraft with
a speed of 185 km/h was used. It flew along transect lines 76m above
ground. Two observers sitting on opposite sides of the plane scanned a
200m wide strip. Counts were corrected for ''sightability.'' Average
population densities in the kangaroo management zones were shown to
range from 0.2 km
2
to 11.3 km
2
. Between 1978 and 1981, kangaroo
numbers increased, although rates of increase in the various management
zones differed. In some zones, population size almost doubled between
1980 and 1981, coincident with very high rainfalls which were 20-50 %
above average. During periods of low rainfall, population numbers
declined. In order to determine more accurately the effects of rainfall,
the calculated annual exponential rates of population increase were cor-
related with rainfall for intervals of 3, 6, and 12 months, with increasing
time-lags ranging, in steps of 3 months, from 3 to 24 months. Because
there must be a limit to the rate of increase, the form of the numerical
response was expected to be asymptotic. Hence, a model representing
such asymptotic relationships, i.e., the Mitscherlich equation was selected
(for details see Cairns and Grigg
1993
, further references therein). Figure
7.2
shows that increases of reproductive rates level off, i.e., become asymp-
totic, when rainfall becomes very high. The most consistent and strong-
est positive correlations between rates of population increase and
rainfall were obtained for the shortest time-lag. Cairns and Grigg (
1993
)
discuss the possible mechanisms causing population fluctuations. They
consider it likely that broad-scale shortage of food in a drought may
directly lead to the death of more vulnerable individuals. There may
also be effects on effective natality: females may enter anoestrus, and
pouch young may not survive. Generally, drought may result in heavy
juvenile mortality and, to a lesser extent, heavy mortality among the very
old kangaroos. Also, more males than females die (as shown in other
surveys).
Rainfall is the predominant factor determining population densities of
for a system for which certain assumptions concerning the numerical
responses of kangaroos to pasture biomass (which in turn is determined by
annual rainfall) have been made (Caughley
1987b
). Different assumptions
may modify the exact shape of the curve representing kangaroo densities,
but the general shape remains the same. It is important to note that the
annual changes in pasture biomass in this model are a product of inter-
actions between rainfall over the year and herbivore density. Herbivore
