How common are rare taxa in long-term benthic macroinvertebrate surveys?

The term rare has spatial and temporal, as well as conservation and management, connotations when applied to ecological surveys. We examined the frequency of temporal occurrence of benthic macroinvertebrates in 19- to 20-y survey collections from 4 sites and 7- to 8-y survey collections from 6 sites in northern California streams. We found that a large proportion of taxa (17–33%) were rare (occurred in only 1 y) in all of the sites examined, regardless of season, and that density increased with increasing temporal commonness. Taxa that could be identified only to higher taxonomic levels, i.e., unresolved taxa, were an additional component (0–7%) of the rare species encountered. The biological traits of the rare and common taxa generally provided explanations for the cause of rarity or commonness, e.g., common taxa tended to have short life cycles with multiple generations per year and rare taxa had low dispersal capabilities. The results of our study clearly demonstrate that temporally rare taxa are common in long-term benthic macroinvertebrate surveys; this prevalence may require re-examination of how long-term surveys are interpreted in both ecological studies and biomonitoring.