Sammendrag
Since the 1990s, the density of the invasive red king crab Paralithodes camtschaticus
has increased dramatically in coastal areas in northern Norway. We investigated its direct and
indirect effects on food web structure and ecosystem properties (e.g. species biomasses and production)
in the Porsanger Fjord in the study period 2009−2011 using 5 subarea Ecopath food web
models. The 5 baseline models with different red king crab densities were compared and the food
web effects of crab removal were explored through simulations in Ecosim. King crabs were important
as benthic predators and exerted strong top-down effects on long-living invertebrates such as
predatory gastropods, asteroids, detritivorous echinoderms and herbivorous sea urchins. The crab
experienced little predation from fish or other predators at higher trophic levels, thus food web
effects of the red king crab generally stayed within the benthic compartment. Red king crab
removal decreased system omnivory and resulted in higher food web biomass−low turnover systems,
with relatively lower production:biomass ratios of benthic invertebrates. Other ecosystem
properties (e.g. total production, consumption, ascendancy and overhead) were little affected by
crab abundance and suggest stable systems. Effects of crab removal were less significant in baseline
models with low initial crab biomass, and high benthic production by detritivores in the inner
fjord may buffer future predation in this area. Indirect effects of crab predation included a positive
cascade effect on macroalgae due to predation on herbivorous sea urchins and a negative effect
on benthic-feeding birds, indicating competition for invertebrate prey.
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