TitleSmall but mighty: headwaters are vital to stream network biodiversity at two levels of organization
Publication TypeJournal Article
Year of Publication2011
AuthorsFinn, DS, Bonada, N, Murria, C, Hughes, JM
JournalJournal of the North American Benthological Society
Type of ArticleJournal Article

Headwaters (stream orders 1-2) traditionally have been considered depauperate compared to mid-order streams (orders 3-4)-a conclusion that arises from a perception of streams as linear systems and emphasizes change in average alpha (local) diversity along streams. We hypothesized an opposite pattern for beta (among-site) diversity and suggest that headwaters might account for a large degree of basin-scale biodiversity if considered within the more realistic framework of streams as branching networks. We assembled pre-existing biodiyersity data from across the globe to test this hypothesis broadly at the population-genetic (mitochondrial haplotype diversity within species) and community (species/taxonomic diversity) levels, with a focus on macroinvertebrates. We standardized 18 (9 headwater and 9 mid-order) population-genetic and 16 (10 headwater and 6 mid-order) community-level ecoregional data sets from 5 global ecozones for robust comparisons of beta-diversity estimates between the 2 stream-size categories. At the population-genetic level, we applied measures of among-site variation commonly used at both population-genetic (F-ST and Phi(ST)) and community (Sorensen's dissimilarity with both presence/absence and abundance data) levels and developed a novel strategy to compare expected rates of loss of gamma (regional) diversity as individual sites are eliminated sequentially from regions. At the community level, we limited analyses to Sorensen's presence/absence measures. We found that Sorensen's dissimilarity was significantly greater among headwaters than among mid-order streams at both population-genetic and community levels. We also showed that individual headwater reaches accounted for greater proportions of genetic gamma diversity than did mid-order reaches. However, neither F-ST nor (Phi(ST) was significantly different between stream-size categories. These measures, which have been used traditionally for comparisons of population-genetic variation, measure proportions of total variation rather than solely among-site variation (i.e., they also are influenced by within-site variation). In contrast, Sorensen's dissimilarity measures only among-site variation and, therefore, is presumably more useful for reflecting general beta diversity. Overall results suggest that, on average, headwaters probably contribute disproportionately to biodiversity at the network scale. This finding demands a shift in thinking about the biodiversity contributions of small headwaters and has strong conservation implications for imperiled headwater streams around the world.

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