Riverine biodiversity in space and time: Effects of network position and hydrologic connectivity

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Travis M. Apgar and Albert Ruhi, Environmental Science, Policy, and Management, University of California-Berkeley, Berkeley, CA, Michael Bogan and Brian A. Gill, School of Natural Resources and the Environment, University of Arizona, Tucson, AZ, Shang Gao, Hydrometeorology and Remote Sensing, University of Oklahoma, Norman, OK, Dave Herbst, Sierra Nevada Aquatic Research Laboratory, University of California Santa Barbara, Mammoth Lakes, CA

Background/Question/Methods
Flow regime is often regarded as a ‘master variable’ in river ecosystems: it shapes the local physical habitat, and controls the dynamics of biological communities. Regionally, stream ecosystems are structured along dendritic networks: directional flow aids dispersal of matter and organisms, carrying propagules from isolated headwaters to well-connected mainstems. Although the effects of hydrology and network position likely interact, these factors have largely been studied separately. Here we leverage an intermittent river stream network with strong spatial and temporal gradients in hydrology to try to disentangle the effects of hydrologic variation and network position on macroinvertebrate community composition and structure. We sampled whole reaches in 16 stream sites in Chalone Creek (Pinnacles National Park, California), spanning several Strahler orders over a period of strong fluctuations in hydroclimate (2015-2021). We used advances in beta diversity-partitioning methods to evaluate changes in biodiversity over space and time, and to identify “keystone” sites, or locations that contribute disproportionately to network-wide biodiversity. Partitioning beta diversity into its richness and replacement components allowed us to better understand the mechanisms that play a more important role in driving source-sink (metacommunity) dynamics. We also placed sensors to track hydrologic conditions (wet vs dry), and used a distributed hydrologic model to simulate daily streamflow at 10-meter resolution. Results showed that three locations within the network are perennial, while the rest range in intermittency (~20-90% of the year flowing).
Results/Conclusions
The studied macroinvertebrate community was highly variable over space and time, with beta diversity being more strongly driven by species replacement than by species richness gradients (overall: 67% replacement, 33% richness). Local (site-level) contributions to beta diversity ranged from 1.43-3.47%, with both perennial and headwater sites showing statistically significant contribution values. Temporal variation in composition was also high, and contributions fluctuated with interannual variation in hydrology--for instance, a particular intermittent site acted as a ‘keystone’ during the strongest drought conditions. Taken together, these observations suggest that sites with different network position and flow conditions host different sets of taxa, with perennial sites hosting higher diversity and headwater sites more unique taxa. Despite being widespread, species in the families Simuliidae (Diptera), Chironomidae (Diptera), and Capniidae (Plecoptera) contributed most to network-wide beta diversity (4.03-6.79%). We contend that a better understanding of spatiotemporal variability in community composition and ‘keystone’ sites may lead to more robust prioritization of restoration and conservation efforts in dynamic riverine environments.