Macroalgal and urchin barren communities are alternately stable and persist in the Aleutians due to sea otter presence and absence. In the early 1990s a rapid otter population decline released urchins from predation and caused a shift to the urchin-dominated state. Despite increases in urchin abundance, otter numbers continued to decline. Although debated, prey quality changes have been implicated in current otter population status. This study examined otter prey abundance, size, biomass, and potential energy density in remnant kelp forest and urchin-dominated communities to determine if alternate stable states affect prey quality. Findings suggest that although urchin barrens provide more abundant urchin prey, individual urchins are smaller and provide lower biomass and potential energy density compared to kelp forests. Shifts to urchin barrens do affect prey quality but changes are likely compensated by increased prey densities and are insufficient in explaining current otter population status in the Aleutians. 1. Introduction Natural communities can exist at multiple stable points in time or space [1]. Stable points are characterized by a specific structural and functional species assemblage recognizably different from other assemblages that can occur under the same set of environmental conditions. Such states are nontransitory, persist over ecologically relevant timescales, and are therefore considered domains of stable equilibrium [2, 3]. Although multiple stable states can exist simultaneously, communities typically alternate from one stable state to another, a shift often conveyed by a large perturbation applied directly to the state variables (e.g., population densities; [4]). Significant changes in the abundance of key species are widely cited as evidence of phase shifts ([5, 6] but see [7]) and have been documented both experimentally [8] and empirically [9, 10] in coastal marine ecosystems. In general, predator removal causes prey community shifts enabling one or few algal or invertebrate competitive dominants to proliferate. In ecological studies in the Aleutian Islands, the presence and absence of dense sea otter populations can instigate state shifts between two alternately stable nearshore communities, one dominated by kelp and the other by sea urchins [11–13]. With sea otters present, sea urchins are reduced to sparse populations enabling kelps to flourish. With sea otters absent, dense sea urchin populations overgraze and exclude foliose macroalgae. In the early 1990s, a rapid sea otter population decline (ca. 25% per year) caused a
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