The course of my career, especially my thinking about food web dynamics, were deeply influenced by a fortuitous meeting with R.T. Paine at Amchitka Island in 1971. Paine was a member of fellow graduate student J.F. Palmisano’s thesis committee and went to Amchitka for a field site visit. Palmisano introduced us. My interest was in sea otters and kelp forests and my goal at the time was to understand how an ecosystem could support so many of these large, metabolically active predators. Paine convinced me that my question was both uninteresting and would be difficult to answer, urging me to instead consider the influences of sea otters on kelp forests. I did that by traveling to nearby islands where otters had been hunted to extinction in the Pacific maritime fur trade, discovering through the contrast of islands with and without sea otters that these predators limited sea urchins, in turn enhancing kelp populations. That discovery came roughly 5 years after publication of the first of Paine’s classic studies of the influence of sea star predation in rocky intertidal communities, thus providing early supporting evidence for the ideas of keystone species and trophic cascades (although the latter term was not coined until 1980).
Results/Conclusions
Further comparisons of islands with and without sea otters in the Aleutian archipelago demonstrated that the otter-urchin-kelp trophic cascade influenced numerous other species and ecological processes, largely via altered NPP, changes in biogenic habitat, and altered flow. Subsequent contrasts with Australasian kelp forests further suggested that the otter-urchin-kelp trophic cascade blocked the co-evolution of defense and resistance in marine algae and their herbivores, thus explaining why North Pacific kelps were chemically poorly defended and why North Pacific kelp forests were so commonly beset by radical phase shifts between kelp-dominated and overgrazed states. Additional studies in the Aleutian archipelago involving both killer whales and introduced arctic foxes further established the importance of linkages across ecosystems (oceanic and nearshore realms in the case of killer whales; sea and land in the case of foxes).
Paine urged me to challenge the tacit assumption of population regulation through production and transfer efficiency across trophic levels. The influence of that challenge convinced me that while bottom-up processes and pathways are frequently important for setting limits of abundance, the realized abundance of species and populations are commonly set by other more serpentine pathways and more complex processes within food webs.