Bird Ecology and Conservation A Handbook of Techniques

measures are approximately known. (It is important to stress that estimates of
growth rate from stable populations, where 1, are not appropriate; the
growth rates need to be maximum potential growth rates, see 13.6.1). The result
of the calculation is a level of allowable removal, PBR, that can be used for assess-
ment (by comparison to estimates of H) or management (as a target level for H)
(Box 13.1).

13.2.3A note about other methods of assessing sustainability

When used to assess whether a population is being overharvested, the PBR for-
mula guards against concluding that exploitation is sustainable when in fact it is
not. When used to set target harvest limits, the PBR formula is designed to allow
a population to reach and stay above the maximum net productivity level (Wade
1998). A very similar method has been used fairly extensively to evaluate the sus-
tainability of harvest, both in mammals and in birds (Robinson and Redford
1991; Robinson 2000). This method calculates the maximum possible produc-
tion of a population and compares it to the actual harvest. Slade et al. (1998)
point out that this method tends to overestimate growth rate and annual

Theoretical basis for sustainable exploitation| 309

Box 13.1Case study: calculating the potential biological removal of

Bearded Guans

The Bearded Guan (Penelope barbata) is a cracid endemic to Ecuador and northern

Peru, and is considered vulnerable. Jacobs and Walker (1999) studied several

unprotected forest blocks in Ecuador. In the 400-ha Selva Alegre forest, Bearded

Guans were found at a density of 17.1 km^2 (95% CI: 10.4–27.9). Maximum

potential growth rate for P. barbatahas not been calculated, but using life-history

data for the closely related Spix’s Guan (Penelope jaquacu) of Peru (Begazo and

Bodmer 1998), we can calculate max
1.073 (based on the formula for Lin

Slade et al. [1998]). With FR
0.5 (because the species is considered vulnerable),

the potential biological removal is

or 0.76 birds per year from the entire forest. Thus, to guard against overharvest of

Bearded Guans from this forest tract, measures should be taken to ensure that no

more than 3 birds are taken over the course of four years. By contrast, Robinson

and Redford (1991) would calculate that a harvest greater than 6.7 birds per year

would clearly be unsustainable.

PBR
^12 max (^1) NminFR
^12 (0.0733) (10.4 km^2 )(0.5)
0.19 km^2