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HOW INVADED IS INVADED?
Citation:
Lee II, H AND B. Thompson. HOW INVADED IS INVADED? Presented at Third International Conference on Marine Bioinvasions, La Jolla, CA, March 16-19, 2003.
Description:
One thrust of invasion biology has been to compare the extent of invasion among communities and biogeographic regions. A problem with such comparisons has been the plethora of metrics used and the lack of standardization as to what data are incorporated into the metrics. One source of variation is whether the cryptogenic species, the species of unknown origin, are included in the counts of non-native species. Another source of variation is the indeterminate taxa, the taxa not identified with sufficient taxonomic resolution to classify them as native or nonindigenous. Indeterminate species can constitute a sizable proportion of the individuals and taxa in benthic studies, and thus impact metrics based on percentages. Yet another source of variation is the spatial scales over which the metrics are calculated, which can range from measures on individual samples to entire biogeographic regions. To explore the variation these different approaches introduce, we calculated a suite of metrics for both highly invaded soft-bottom communities (San Francisco Estuary) and for low-to-moderately invaded systems (EMAP survey of the small Pacific coast estuaries). These results suggest that differences among the approaches can equal or exceed differences expected among estuaries. For example, the percentage abundance of non-native species in the EMAP survey ranged from 6.5% to 28.7% depending on how the value was calculated. The effect of spatial scale on species richness is demonstrated by the decrease in the percentage of the species composed of nonindigenous species from 42% at the grab scale to 11% at the estuary scale in the San Francisco Estuary. Because of these sources of variation, it is critical for researchers to report exactly how their estimates of the extent of invasion were calculated and to compare only metrics calculated in the same manner and over the same spatial scale.