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|Monitoring and Measuring Communities of Animals|
While most monitoring programs are created to measure changes in populations it is also possible to measure changes in groups of species or communities. Here the objective is to assess changes in the number of species present rather than changes in the number of individual animals of one species. In addition to analyzing changes in the number of species over time it is also possible to estimate the number of extinctions, the number of colonizations, and the species turnover that have occurred.
It would be unusual for a monitoring program to have as its primary objective the measurement of the number of species present. Most conservation and management activities target level information with information about community richness and turnover of secondary importance. Fortunately, when groups of species are being monitored, measurements of community change can usually be appropriately extracted from the same dataset.
The process of evaluating any measurement of community change and integrating it within a monitoring program is done in the exact same way as that of a measure of population change. All the factors and considerations that have been mentioned in this website with references to population monitoring also pertain to measuring communities: variance, bias, your objectives for precision, and the time periods over which you want to measure trend all must be considered.
As with population measures, information on species richness can be analyzed using indices or estimates. Species richness indices are simply the lists of species found on a plot or collection of plots. The most important assumption implicit here is that while not every species in the area may have been detected, the proportion of species detected is similar across all time periods. As with an index to population size, the ratio of species found to species missed will vary from year to year. This will be true at the plot level as well across a collection of plots. Note that differences in the skills of the observers who collect these data are likely to be the primary source of bias when using methods that rely on the observational skill of technicians to find (detect) the species (for example, bird point counts, area searches for herps, butterfly counts).
Rather than using potentially biased indices, estimators can be employed to partially correct for such bias (again, assuming the proper models were chosen and the assumptions weren’t violated). Estimators estimate the fraction of the species total that were missed. Note, that it is, of course, impossible to put species names on the species estimated to have been present but not seen, thus these techniques do not provide a complete inventory of the species present, but do inform you as to how many more are likely to be present but unfound. Also note that the estimated number of species present includes only those species that have some reasonable probability of being detected by the counting technique that is being employed.
In addition to the basic community parameters mentioned above many other community indices have been created. The most commonly used can be broadly classified as diversity indices and a class of indices that have been named the Index of Biotic Integrity. Both measures take counts or estimates of individual species populations and combine them mathematically into a score that is meant to be some integrated measure of a region or site’ complexity or health. These measures are again subject to the same underlying biases and variability of their component population counts as well as the added interpretational difficulties of the at times complex mathematical combinations of these counts. Because of that they have to be considered carefully, as errors may be compounded rather than relieved when counts are combined in a complex way. Recent efforts have shown that such indices can demonstrate strong relationships with known environmental gradients such as from degraded to healthy, but caution is still required as they carry the burden of both measurement and interpretational uncertainties.
In most situations measurements of community level parameters will be a secondary objective of a monitoring program.
Note: Links to Species Richness Estimators, Diversity and IBI sites need to be added.
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