SYMP 4-6 - Trophic cascades in rivers

Tuesday, August 8, 2017: 10:40 AM
Portland Blrm 251, Oregon Convention Center
Mary E. Power, Department of Integrative Biology, University of California Berkeley, Berkeley, CA

I arrived at the University of Washington interested in living in a clear-flowing tropical river for a few years, and in finding out how animals got enough to eat in such rivers. I soon was captivated by Bob Paine’s unforgettable graduate lectures, brief but memorable lunch-line conversations, and in Paine Bull sessions, his tales of vivid natural history (“the sea slug Navanax is a ‘marine boa constrictor’”) mixed with cosmic ecological insights. Thanks to the Smithsonian Tropical Research Institute, I got my three years of living in the Rio Frijoles of central Panama with a guild of charismatic armored catfish that grazed benthic algae. Back in Seattle, as I tried to write up my dissertation, I found myself overwhelmed by details and data, with no clear overview of how to begin. Bob, in a crucial (for me) meeting, told me “You’ve figured out how those catfish deal with a fluctuating, heterogeneous environment”. It was a head-slapping moment. “Oh, that’s what I’ve done!” and it catalyzed papers on grazer return times and impacts on sediment, algal biomass and productivity; predator-induced resource avoidance along depth gradients; and the ideal free distribution that catfish achieve by tracking light-limited algae over kilometer scales.


Subsequent opportunities to work in prairie, Ozark, and coastal California rivers have convinced me that Bob’s world view applies as powerfully in clear-flowing rivers as in intertidal landscapes. In both arenas, ecologists can tower (or swim) over miniaturized worlds, looking for patterns and underlying ecological interactions among relatively small organisms. These worlds, when “green”, are thickly carpeted with attached macroalgae. When “barren”, both benthic worlds can nevertheless sustain considerable consumer biomass because of the high productivity, food quality, and persistence of benthic microalgae. Heavy grazing in rivers obliterates benthic algal biomass within weeks; when released from grazing by physical factors, predators, or experiments, algal biomass recovers within weeks or months. Their rapid dynamics make both intertidal and clear rivers tractable arenas for probing controls in food webs, although studies in rivers demand snorkeling, and some tolerance of floods. In an interesting historical twist, the first reports of cascading impacts of predators may be Hrbacek’s (1961) studies of ponds dug into the floodplain of the River Elbe in what is now the Czech Republic—work that may have helped provoke Fred Smith’s mind-blowing question to Bob Paine’s graduate class at the University of Michigan—“Why is that tree green?”