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A complete population count is just what the name says - an enumeration of all the individuals that make up a population. Obviously, for this method to be tractable, there must be few enough individuals to count. As such, this method is typically only used for very rare plants or early invaders.
If there are, indeed, few enough of a species that complete population counts are a possibility, then this is the preferred method because there is no sampling error and no estimatation because the entire population is being censused. Likewise, any observed change in the population is a real change, and not influenced by samples taken from the population. The only question is whether or not the change is ecologically meaningful to the species.
In order to ensure meaningful results from this method, a consistent census unit must be established. In some cases, this will be the individual. It could also be at the patch level (i.e., spatially isolated, contiguous area where the species is found). Patch-level censusing might make sense for clonal species like aspen where it is difficult to determine one individual from another and where the number of individuals may vary by year but not be a reliable indicator of the overall condition or trend of the species.
As stated above, the biggest advantage of this method is that it is a complete census of a population and therefore has no statistical sampling error associated with it (assuming of course, that all the individuals were in fact censused).
In reality, it can often be difficult to achieve a complete population count. Accuracy of this method suffers when individuals are missed. In this case, apparent changes over time can be due to detecting (or not) individuals that were missed during the previous census. Errors like this are exceedingly difficult to correct even if they are known to occur.
Other factors that contribute to difficulty in implementing complete population counts are: large census area, large population size, dense vegetation surrounding the species being censused, presence of similar species, and cryptic species or life stages.
The ability to reliably conduct complete population counts should be assessed before relying on this method.
For estimating total population size, the Estimate of Population Size and Presence/Absence methods are similar to complete population counts (although they are samples of the population and not absolute counts). Number of individuals per unit area (i.e., density) can be sampled using methods like Density Quadrats or Nested Frequency plots. Some form of Boundary Mapping is often applied in conjunction with Complete Population Counts.
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