Red-Tailed Black Cockatoo

1.3 Decline

As Figure 2 revealed, no comprehensive surveys were carried out until recently, making direct and accurate estimation of population decline impossible. Despite this limitation, a rough estimate can still be made for the purposes of this exercise by extrapolating current population density to past, inferred range. This can be done by:

Noting the limits of previous sightings and specimens on Figure 2 (plus Red Rock, 30 km north-west of Melbourne, not shown).
Noting that the birds seem to be dependent upon Brown/Desert Stringybark (Eucalyptus baxteri/arenacea) for food and roosting for much of the year (72% feeding records, Joseph 1982).
Combining the historical records with the map of distribution of Brown/Desert Stringybark (Figure 3) and making two assumptions: that past range*2 approximated to the area of stringybark, and that birds did not occur west of The Murray River in S.A. nor east of Melbourne, Vic. Record this area in km2. (For a detailed map of past habitat, see www.redtail.com.au)

Figure 3. Past Distribution RTBC and Stringybark

map

This figure can be cautiously compared with those for the present day by:

Recording approximate present population, as per Q. 1.
“ “ “ range*2 by examining Figure 3 and estimating the % of range *1 covered by native vegetation, which we will call range*2. (To enable calculation we will identify all native vegetation on Figure 3 within range *1 as Brown/Desert Stringybark forest).
Dividing population by range*2 to get simple population density per km2. Record this figure for use in question 5.

Note: *1 and 2 There are two measures of range: 1.Extent of Occurrence (as mentioned in Q.3), and 2. Area of Occupancy (A.O.), which is the area within the E.O. that is occupied by a species - e.g. the area utilised for feeding etc - and does not count unutilised zones, such as cleared or urban land.
Joseph (1982) stated that he thought that the “birds seemed to be more or less evenly distributed”.

Figure 4. Present Distribution RTBC and Stringybark

map

Q.4 Estimate past range area by counting the squares and percentage of squares on Figure3 occupied by stringybark between The Murray River and Melbourne.

Q.5 Multiply the figure calculated in question 4 by the population density determined previously, to arrive at a past population figure. By what % may the population have declined?

Q.6 The environment of south-east S.A. and western Victoria began to be altered by settlers from around the 1830s onward. If we make 1830 the base year, how much may population have declined/year to the present day? (assuming constant rate of decline). How much (%) would this be in 10 or 20 years?

Q.7 We have made numerous assumptions and strung assumptions together to reach a figure for rate of decline: criticise the process.