Geoscience Reference
In-Depth Information
IMPROVING INDEX SURVEYS
The few studies attempting to validate indices suggest that population indices
and absolute abundances are rarely related via a simple positive, linear rela-
tionship with slope constant across habitats and over time. Thus animal ecol-
ogists would do well to proceed cautiously when designing and implementing
index surveys. In particular, index validation should be considered a necessary
precursor to implementing index surveys. Some guidance on the relationship
of the index to abundance may be found in the literature, but index validation
studies are rare. Lacking such information, conducting a pilot study using the
index in areas where abundance is known or can be estimated is useful. Such a
validation study would need to be replicated across multiple sites that exhibit
variation in population size or density, or over time at a site where abundance
varies over time. Making multiple estimates of the index:abundance ratio at
each site and time period is also useful so that the contribution of sampling
error to the overall noise in the index-abundance relationship among sites can
be estimated. Validation studies also may be advisable throughout a monitor-
ing program's life span because the index may need to be periodically cali-
brated or updated (Conroy 1996).
Ecologists should also be aware that developing indices that have a 1:1 rela-
tionship with abundance will most reliably reflect changes in abundance. If the
slope describing the index-abundance relationship is low, then large changes in
abundance are reflected in small changes in the index. Such small changes in
the index are more likely to be obscured by variation in the index-abundance
relationship than if the slope of the index-abundance relationship were higher.
Methods of reducing index variability and increasing the precision of the
index-abundance relationship include adjusting the index by accounting for
auxiliary variables such as weather and observers. In practice, these factors may
be overlooked if many years of data are gathered because the short-term bias
they introduce typically is converted simply to error in long-term data sets. In
an ideal situation, each index would be validated, adjusted for sampling error
by accounting for external variables, and corrected to linearize the index and
make it comparable across habitats and over years. However, this is rarely an
option for regional-scale surveys conducted across multiple habitats over many
years by many people and involving multiple species, although it may be pos-
sible for local monitoring programs focused on single species.
The following advice may be useful to animal ecologists for improving index
surveys. First, the basic relationship between the index and abundance should
be ascertained to determine whether the index might yield misleading results
and therefore should not be implemented. Second, any results from trend analy-
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