Exotic species often reduce the abundance or diversity of species in marine ecosystems, but some exotics may benefit native species, such as when habitat is enhanced. In Chesapeake Bay, the exotic macroalga Gracilaria vermiculophylla (Rhodophyta) has flourished and dispersed widely, yet the consequences for native species diversity and abundance are not well known. We experimentally examined the capacity of the structurally complex G. vermiculophylla to provide nursery habitat for the blue crab Callinectes sapidus in Chesapeake Bay, where native eelgrass nursery habitat has dwindled. We also examined ontogenetic shifts in survival across alternative nursery habitats. In field surveys, juvenile density was similar in macroalgae and eelgrass, but lower in unvegetated mud habitat. In field experiments, juvenile survival was positively related to crab size in mud but negatively in eelgrass, confirming the paradigm of a predationinduced ontogenetic shift from seagrass to unvegetated habitat. In contrast, irrespective of crab size, survival was higher in macroalgae than in either native habitat. Thus, exotic habitat-forming macroalgae can compensate for severe declines in seagrass nurseries, and facilitate the emergence of a novel ecosystem.

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The Asian tiger mosquito, Aedes albopictus, appears to have been extirpated from Palmyra Atoll following rat eradication. Anecdotal biting reports, collection records, and regular captures in black-light traps showed the species was present before rat eradication. Since then, there have been no biting reports and no captures over 2 years of extensive trapping (black-light and scent traps). By contrast, the southern house mosquito, Culex quinquefasciatus, was abundant before and after rat eradication. We hypothesize that mammals were a substantial and preferred blood meal for Aedes, whereas Culex feeds mostly on seabirds. Therefore, after rat eradication, humans and seabirds alone could not support positive population growth or maintenance of Aedes. This seems to be the first documented accidental secondary extinction of a mosquito. Furthermore, it suggests that preferred host abundance can limit mosquito populations, opening new directions for controlling important disease vectors that depend on introduced species like rats.