The hierarchical spatial structure of the river network supports high biodiversity in headwater streams

Background/Question/Methods

Headwater streams have not received much attention from researchers or conservationists because they are assumed to support small populations and low α-diversity. However, because of the dendritic form of stream networks, the contribution of headwater streams to network γ-diversity may high if there is significant β-diversity among tributaries.

To test this, we investigated the diversity of the benthic macroinvertebrates at nine sites in each of three 1st to 3rd order streams within a stream network on a tropical mountain: the Mount Kinabalu, Borneo. At each site, we calculated sample level α-diversity, species turnover among microhabitats (riffles, pools and litter accumulations) within sites, and site-level α-diversity, and compared them among stream orders. Second, we compared species composition among sites of the same orders, and calculated β-diversities among sites of same stream order and the overall diversity of each stream order in the network. Finally, we compared the species present in each stream order and evaluated the contribution of the each order to the γ-diversity of the stream network. 

Results/Conclusions

Species composition differed among stream orders, and site-level α-diversity was higher in 3rd-order streams than 1st- and 2nd-order streams. In contrast, β-diversity among sites of same order was higher in 1st- and 2nd-order streams than 3rd-order streams. Species totals combined across sites were similar in 1st- (121 species), 2nd- (117 species) and 3rd-order (115 species) streams, and together and highly contributed to the γ-diversity of the stream network (totally 168 species). Previous research at Kinabalu and elsewhere has shown that most adult aquatic insects remain within 15m of the stream back, and rarely fly over ridges suggesting that high β-diversities among tributaries reflects limited dispersal among them. Our results demonstrate the high contribution of headwater streams to the overall biodiversity of stream networks, and highlight their conservation importance.