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Estimating the ability of birds to sustain additional
human-caused mortalities using a simple decision
rule and allometric relationships
Peter W. Dillingham*, David Fletcher
Department of Mathematics and Statistics, University of Otago, P.O. Box 56, Dunedin, New Zealand
Many bird species are subject to human-caused mortality, either through direct harvest
Received 7 January 2008
(e.g. game birds) or through incidental mortalities (e.g. ﬁsheries-related bycatch of seabirds,
Received in revised form
impact with vehicles, wind turbines, or power lines). In order to assess the impact of addi-
20 April 2008
tional mortalities on birds, both the number of birds killed and their ability to sustain those
Accepted 21 April 2008
deaths must be estimated. Niel and Lebreton [Niel, C., Lebreton, J.-D., 2005. Using demo-
Available online 11 June 2008
graphic invariants to detect overharvested bird populations from incomplete data. Conservation Biology 19, 826–835] applied a simple decision rule [Wade, P.R., 1998. Calculating
limits to the allowable human-caused mortality of cetaceans and pinnipeds. Marine Mam-
mal Science 14, 1–37] to estimate the level of additional human-caused mortality or poten-
tial biological removal (PBR) that can be sustained for bird species given only (1) estimates
of the population size, adult survival, and age at ﬁrst breeding, and (2) the current popula-
tion status and management goals. We provide guidelines for appropriate use of the
method and case studies comparing results from this method to other approaches. Partic-
ular focus is placed on applying the method to Procellariiformes. PBR limits may then be set without a population model and when monitoring levels are minimal, and in a computationally straightforward manner. While this approach has many advantages, there are limitations. The PBR rule was initially developed for cetaceans and pinnipeds and there have been no adaptations for the unique biology of birds which may need further consideration. Additionally, because this is a simplifying method that ignores differences in life stages, it may not be appropriate for very small populations or for those listed as ‘critically endangered’, and further work is needed for situations where mortalities have large gender or age bias.
Ó 2008 Elsevier Ltd. All rights reserved.
The annual growth rate, current status, and management
objectives for a species determine the level to which it can sustain additional mortalities. Among bird species, harvests or incidental mortalities come from a variety of sources, such as indigenous harvest, recreational hunting, collision with
man-made objects (vehicles, wind turbines, power lines),
and bycatch in ﬁsheries. This work was motivated by our
work on ﬁsheries bycatch of seabird species (primarily Procellariiformes such as albatrosses, shearwaters, and petrels) in New Zealand where limited demographic information is
available. Typically, demographic information is limited to
rough estimates of the population size, adult survival, and
* Corresponding author: Tel.: +64 3 479 7765.
E-mail address: firstname.lastname@example.org (P.W. Dillingham). 0006-3207/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2008.04.022
B I O L O G I C A L C O N S E RVAT I O N
age at ﬁrst breeding (Brooke, 2004). This constraint meant that a method for estimating potential biological removal from
minimal information was needed.
In the marine mammal setting, Wade (1998) developed a
simple rule for estimating allowable...