Unsustainable fishing simplifies food chains and, as with aquaculture, can result in reliance on a few economically valuable species. This lack of diversity may increase risks of ecological and economic disruptions. Centuries of intense fishing have extirpated most apex predators in the Gulf of Maine (United States and Canada), effectively creating an American lobster (Homarus americanus) monoculture. Over the past 20 years, the economic diversity of marine resources harvested in Maine has declined by almost 70%. Today, over 80% of the value of Maine's fish and seafood landings is from highly abundant lobsters. Inflation-corrected income from lobsters in Maine has steadily increased by nearly 400% since 1985. Fisheries managers, policy makers, and fishers view this as a success. However, such lucrative monocultures increase the social and ecological consequences of future declines in lobsters. In southern New England, disease and stresses related to increases in ocean temperature resulted in more than a 70% decline in lobster abundance, prompting managers to propose closing that fishery. A similar collapse in Maine could fundamentally disrupt the social and economic foundation of its coast. We suggest the current success of Maine's lobster fishery is a gilded trap. Gilded traps are a type of social trap in which collective actions resulting from economically attractive opportunities outweigh concerns over associated social and ecological risks or consequences. Large financial gain creates a strong reinforcing feedback that deepens the trap. Avoiding or escaping gilded traps requires managing for increased biological and economic diversity. This is difficult to do prior to a crisis while financial incentives for maintaining the status quo are large. The long-term challenge is to shift fisheries management away from single species toward integrated social-ecological approaches that diversify local ecosystems, societies, and economies.

Rarely do ecologists have the data needed to assess the impacts of invading species on biodiversity, i.e. pre‐ and post‐invasion census information from both invaded and control sites. Using a 21‐year time series, we demonstrate that the invasion of Harp Lake, Ontario, Canada, by the Eurasian spiny water flea, Bythotrephes longimanus, a zooplanktivore, was accompanied by a rapid and long‐lasting reduction in the average species richness of crustacean zooplankton, particularly of cladoceran taxa. No such reduction was observed in seven nearby un‐invaded lakes over the same two decades. If the Harp Lake results are typical, we predict a widespread reduction in crustacean zooplankton richness on the Canadian Shield for three reasons. Shield lakes provide the invader with good habitat. Its dispersal rates and colonization success are high. Zooplankton richness in Harp Lake is now unusually low for a Shield Lake of its size and acidity.

Large benthic decapods play an increasingly important role in commercial fisheries worldwide, yet their roles in the marine ecosystem are less well understood. A synthesis of existing evidence for 4 infraorders of large benthic marine decapods, Brachyura (true crabs), Anomura (king crabs), Astacidea (clawed lobsters) and Achelata (clawless lobsters), is presented here to gain insight into their ecological roles and possible ecosystem effects of decapod fisheries. The reviewed species are prey items for a wide range of invertebrates and vertebrates. They are omnivorous but prefer molluscs and crustaceans as prey. Experimental studies have shown that decapods influence the structuring of benthic habitat, occasionally playing a keystone role by suppressing herbivores or space competitors. Indirectly, via trophic cascades, they can contribute to the maintenance of kelp forest, marsh grass, and algal turf habitats. Changes in the abundance of their predators can strongly affect decapod population trends. Commonly documented nonconsumptive interactions include interference-competition for food or shelter, as well as habitat provision for other invertebrates. Anthropogenic factors such as exploitation, the creation of protected areas, and species introductions influence these ecosystem roles by decreasing or increasing decapod densities, often with measurable effects on prey communities. Many studies have investigated particular ecosystem effects of decapods, but few species were comprehensively studied in an ecosystem context. A simplified synthetic framework for interpreting eco system roles of decapods was derived from the available evidence; however, more experimental and long-term observational studies are needed to elucidate mechanisms and shed light on the longterm consequences of decapod fisheries.

The nonindigenous predatory cladoceran Bythotrephes longimanus is spreading rapidly among Canadian Shield lakes, but only one case study of its impacts exists. In Harp Lake, the abundances of several cladoceran and one cyclopoid species fell after the invasion, and far fewer species benefited. To determine if Harp Lake provides typical results, we compared the summer crustacean zooplankton communities of 17 invaded and 13 noninvaded (reference) lakes in Ontario. The communities of the two lake groups differed. Average species richness was 30% higher in the reference (15.3 species) vs. the invaded lakes (11.8 species). Total zooplankton biomass was significantly lower in the invaded lakes, mainly because of lower abundances of all common epilimnetic cladoceran species. As these results were quite similar to those of Harp Lake, it is apparent that current summer zooplankton communities of Canadian Shield lakes with Bythotrephes differ substantially from noninvaded lakes.

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