Small but mighty: headwaters are vital to stream network biodiversity at two levels of organization

Abstract 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 &agr; (local) diversity along streams. We hypothesized an opposite pattern for &bgr; (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 biodiversity 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 &bgr;-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 (FST and &PHgr;ST) and community (Sørensen's dissimilarity with both presence/absence and abundance data) levels and developed a novel strategy to compare expected rates of loss of &ggr; (regional) diversity as individual sites are eliminated sequentially from regions. At the community level, we limited analyses to Sørensen's presence/absence measures. We found that Sørensen'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 &ggr; diversity than did mid-order reaches. However, neither FST nor &PHgr;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, Sørensen's dissimilarity measures only among-site variation and, therefore, is presumably more useful for reflecting general &bgr; 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.