Exploring the Consequences of Species Interactions Through the Assembly and Disassembly of Food Webs: A Pacific-Atlantic Comparison

Abstract

Species interactions are best seen and understood through perturbations of the species themselves. These perturbations can be accomplished on small scales through controlled manipulations, but they sometimes happen on larger scales through more fortuitous contrasts over space or time. We used similarities and differences between the North Atlantic and Pacific oceans in an effort to improve understanding of the key dynamic processes that regulate the abundance and distribution of species. The North Atlantic biota is impoverished; many of its recent elements are derived from the North Pacific through transarctic biotic interchange. Sea otters, absent from the North Atlantic, drive a trophic cascade in the North Pacific by limiting herbivorous sea urchins, in turn allowing kelp and other macroalgae to flourish. Otariid pinnipeds are also absent from the North Atlantic. We suggest that the absence of sea otters as competitors and of otariid pinnipeds as predators favored the evolution of large body size in Atlantic cod. Cod therefore assumed the ecological role of sea otters in the North Atlantic, creating a functionally analogous ecosystem in which the collapse of cod has led to a kelp-to-urchin phase shift. The decline of cod and other finfish has also been instrumental in driving neritic/oceanic habitats in the North Atlantic from domination by fishes to domination by shrimps and other crustaceans. We hypothesize that the recent collapse of North Pacific pinnipeds helped drive that ecosystem in the opposite direction-from domination by shrimp and other crustaceans before the collapse to domination by fish today.