Environmental Engineering Reference
In-Depth Information
should be applied if there is any indication that plots cannot easily be counted with
complete certainty. For species that are extremely difficult to observe directly,
mark-recapture methods are the most common choice. However, it is often diffi-
cult to meet the assumptions of these methods, particularly regarding lack of het-
erogeneity in capture probabilities, and great care is needed to ensure that field
methods are designed to minimise bias. Offtake-based methods might appear to be
ideal for exploited species, and may be the only practical option in some cases,
although they are the most prone to bias of any of the methods discussed, and
should be used with more than usual caution. It will usually be far preferable to
estimate abundance independently of harvest if possible.
The methods discussed in this section represent the core of the analytical toolbox,
but this is an active area of research, and new methods are constantly being devel-
oped.
New technologies
can be applied innovatively to extend the utility of exist-
ing census methods. For example, individual animals can be identified from
genetic analysis of faecal, feather or hair samples (Rudnick
et al
. 2005; Petit and
Valiere 2006), or from camera trap images of species with naturally unique markings
(Karanth and Nichols 1998), so allowing mark-recapture methods to be applied.
Also,
combinations of analytical methods
are increasingly being developed in order
to alleviate some of the existing constraints. For example, Borchers
et al
. (1998)
provide a means of applying mark-recapture theory to data from multiple observers
to estimate the detectability of individuals on line transects when detection on the
line is not certain, while Efford (2004) provides a distance-based method to esti-
mate the effectively sampled area in mark-recapture studies.
Trapping webs
offer
the possibility of applying distance-based analytical techniques to trapping data
(Jett and Nichols 1987; Buckland
et al
. 2004; Lukacs
et al
. 2005), while
traps or
lures
can be used in conjunction with behavioural studies to increase detectability
for distance sampling of difficult-to-observe species (Buckland
et al
. 2006).
Developments of this kind will no doubt proliferate in the future.
Demographic (or vital) rates are the processes that lead to change in population
size; births, deaths, immigration and emigration. While population size is a key
indicator of the state of the system, a full understanding of biological sustainabil-
ity also requires an estimate of the population's productive potential. For example,
many of the sustainability assessment and prediction techniques introduced in
Chapters 4 and 5 require us to know the intrinsic rate of increase for the logistic
growth model (Section 1.3.1.1), or rates of productivity and survival, which
together determine population growth rate. In this section, we introduce methods
for estimating population growth rate, followed by methods for estimating sur-
vival (including harvest mortality) and productivity rates. We then briefly discuss
