Geoscience Reference
In-Depth Information
9.2 General Overview
A situation is envisioned for which the occurrence of a target species is of
interest within some predefined region. This region consists of a number
of units at which the species may be either present or absent (or otherwise
categorized). What constitutes a unit will depend on the target species and
objectives of the study or monitoring program. The units may be defined in
terms of natural features such as ponds or patches of habitat or arbitrarily
defined as grid cells, stream segments, transects, and so on. The collection of
all units within the region is considered the statistical population of interest.
These issues are discussed in further detail later in this chapter.
The status of the species at each unit can be one of multiple categories at a
particular point in time. Examples of such categorizations include presence/
absence; used/unused; presence with breeding/presence without breeding/
absence. The unit's status may change over time through certain dynamic
processes. It is presumed that there is a period of time when the status of the
species can be regarded as stable or static. Here, that time period is referred
to as a
season
, which may or may not correspond to a biologically relevant
season such as a climatic or breeding season.
Surveys are conducted to determine the current status of the species at the
units. It is not necessary that all units within the region of interest be sur-
veyed as a sampling scheme can be used to select a subset of units to survey,
with the results generalized to the entire population of units. The total num-
ber of units within the region of interest is denoted here as
S
, and the size of
the sample from this population is
s
.
A practical consideration is that often the field methods used to survey
for the species in the units will be imperfect. That is, the true status of the
species at the unit will not always be observed, and there is the potential for
misclassifying the unit. It is assumed that any potential misclassification
will be one way; that is, there are some types of observations for which the
species status can be definitively determined, while other observations are
associated with potential ambiguity. For example, detection of the species
confirms the species is present, but nondetection does not confirm the spe-
cies is absent.
Because of imperfect detection, multiple surveys for the species should be
conducted within each season. The exact nature of the repeat surveys can
vary with different applications, but options include discrete visits to each
sampling unit or multiple surveys conducted within a single visit. Further
suggestions are made in the section on study design.
