Environmental Engineering Reference
In-Depth Information
might include age, sex, size or location caught. Potential covariates of this kind
should be recorded in the field whenever possible. However, even after controlling
for observable covariates, unexplained differences in catchability between individ-
uals often remain. To deal with this, methods are available that model individual
heterogeneity in capture probability without the need for covariates, but these
cannot be relied upon to deliver unbiased estimates if data are sparse.
Changes in capture probability over time
are most likely to arise as a result of
variation in capture effort, although it is also possible that changes in catchability
occur across the population during the survey. If such variation exists without
being controlled for, abundance will be overestimated. It is therefore important to
test whether significant variation exists by modelling changes in capture probabil-
ity over time. However, this reduces precision, and it is therefore best to standard-
ise effort to reduce the chance of variation if possible.
Animals may respond to capture, either by becoming more wary, and therefore
less liable to capture in future, or, if baiting is used, by seeking out the bait, thus
increasing their chances of capture. Where traps are used, these
behavioural
responses
are known respectively as trap shyness and trap happiness. Trap shyness
might be minimised by using the least invasive form of capture available, while
trap happiness might be reduced by setting out bait for a period prior to capture
in an attempt to get all individuals to the trap happy stage. If such field methods
are inappropriate or ineffective, an analytical model can be used that controls
for behavioural responses, although the greater model complexity again reduces
precision.
2.3.4.2 Population closure
The assumption of population closure is violated if, during the survey, individ-
uals enter the population through
immigration or birth
, or leave it through
emigration or death
. If the rate of capture is fast enough to accumulate an
adequate sample within a short space of time relative to the rate of population
turnover, the assumption will be fully or approximately satisfied. However, if the
survey takes place during a period of rapid change due to migration, or is so
extended that significant numbers of births or deaths are likely to take place, it is
necessary to estimate rates of flow into or out of the population in order to control
for them and so provide unbiased abundance estimates. This can be done using the
same form of data, but using open population models that control for turnover
(Sections 2.4.2.2 and 2.4.3.2).
An important consideration in relation to population closure is the impact that
capture and marking may have on demographic rates. An example of such an effect
is the clipping of multiple toes in amphibians in order to identify them on recapture,
which has been shown to substantially reduce survival rates in some species
(McCarthy and Parris 2004). As well as being ethically questionable, such impacts of
research on the study species are self-defeating, because they prevent objective infer-
ence about the population in its natural state. Every possible effort must therefore be
made to ensure that capture and marking do not impact on the study subjects.
