Article

Strong top-down control in Southern California kelp forest ecosystems

National Center for Ecological Analysis and Synthesis, 735 State Street, Santa Barbara, CA 93101, USA.
Science (Impact Factor: 33.61). 06/2006; 312(5777):1230-2. DOI: 10.1126/science.1128613
Source: PubMed

ABSTRACT

Global-scale changes in anthropogenic nutrient input into marine ecosystems via terrestrial runoff, coupled with widespread
predator removal via fishing, have created greater urgency for understanding the relative role of top-down versus bottom-up
control of food web dynamics. Yet recent large-scale studies of community regulation in marine ecosystems have shown dramatically
different results that leave this issue largely unresolved. We combined a multiyear, large-scale data set of species abundances
for 46 species in kelp forests from the California Channel Islands with satellite-derived primary production and found that
top-down control explains 7- to 10-fold more of the variance in abundance of bottom and mid-trophic levels than does bottom-up
control. This top-down control was propagated via a variety of species-level direct and indirect responses to predator abundance.
Management of top-down influences such as fishing may be more important in coastal marine ecosystems, particularly in kelp
forest systems, than is commonly thought.

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    • "Both types of factors are influential in the marine environment, yet top-down effects appear very strong in some ecosystems where the removal of apex predators has led to a major restructuring of the community and foodweb (e.g. Frank et al., 2005; Halpern et al., 2006). Nevertheless, debate about the relative importance of top-down vs. bottom-up controls in marine ecosystems continues. "
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    ABSTRACT: Many snow crab fisheries have fluctuated widely over time in a quasi-cyclic way due to highly variable recruitment. The causes of this variability are still debated. Bottom-up processes related to climate variability may strongly affect growth and survival during early life, whereas top-down predator effects may be a major source of juvenile mortality. Moreover, intrinsic density-dependent processes, which have received much less attention, are hypothetically responsible for the cycles in recruitment. This study explored how climate, larval production, intercohort cannibalism and groundfish predation may have affected recruitment of early juvenile snow crab in the northwest Gulf of St Lawrence (eastern Canada) over a period of 23 years. Abundance of early juvenile snow crabs (2.5-22.9 mm in carapace width), representing the first 3 years of benthic life, came from an annual trawl survey and was used to determine cohort strength. Analyses revealed a cyclic pattern in abundance of 0+ crabs that may arise from cohort resonant effects. This pattern consisted of three recruitment pulses but was reduced to two pulses by age 2+, while the interannual variability of cohort strength was dampened. This reconfiguration of the earliest recruitment pattern was dictated primarily by bottom water temperature and cannibalism, which progressively overruled the pre-settlement factors of larval production and surface water temperature that best explained abundance of 0+ crabs. The results strongly suggest that bottom-up and density-dependent processes prevail over top-down control in setting the long-term trends and higher-frequency oscillations of snow crab early recruitment patterns. © 2015 International Council for the Exploration of the Sea 2015. All rights reserved. For Permissions, please email: [email protected] /* */
    Full-text · Article · Jun 2015 · ICES Journal of Marine Science
    • "Both types of factors are influential in the marine environment, yet top-down effects appear very strong in some ecosystems where the removal of apex predators has led to a major restructuring of the community and foodweb (e.g. Frank et al., 2005; Halpern et al., 2006). Nevertheless, debate about the relative importance of top-down vs. bottom-up controls in marine ecosystems continues. "
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    ABSTRACT: Snow crab landings in the Gulf of St. Lawrence (eastern Canada) have fluctuated greatly since the beginning of the fishery due to highly variable recruitment. This variability is established early in life and may result from bottom-up processes related to climate variation, top-down effects and/or population-intrinsic processes. We explored how climate, larval production, intercohort cannibalism and groundfish predation may have affected recruitment of snow crab in the northwest Gulf of St. Lawrence over the period 1989‒2012. Abundance of early juvenile snow crabs (3‒25 mm carapace width), representing the first three years of benthic life, came from an annual trawl survey and was used to determine year-class strength (YCS). Larval production and surface water temperature during the planktonic phase were the two main factors determining YCS up to about 8‒9 months after settlement time. The population structure set by larval supply to the benthos was progressively modified by intercohort cannibalism and bottom water temperature effects. Cannibalism was the most important factor affecting YCS after three years of life and appears to be a strong driver of snow crab recruitment variability. The findings suggest that climatic and population-intrinsic processes are more important than top-down effects in determining snow crab recruitment.
    No preview · Conference Paper · Aug 2014
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    • "In recent decades, Kellet's whelk has expanded its range north of Point Conception up the central California coast (Herrlinger, 1981) (Fig. 1) during periods of both seawater warming and variation in ocean circulation (Zacherl et al., 2003). The whelk is an important predator and scavenger that may exert topdown effects on algae, herbivores and planktivores in kelp forests (Halpern et al., 2006). The whelk also supports a rapidly growing commercial fishery (Sweetnam, 2006). "
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    ABSTRACT: AimTo improve our understanding of how parasitism interacts with geographical range expansions by quantifying diversity and abundance of parasites in 25 populations of a large marine snail, Kellet's whelk (Kelletia kelletii), throughout its historical and recently expanded range, which are separated by a well-known biogeographical boundary.LocationCalifornia coast (western North America).Methods Parasitological examinations were conducted on 199 whelks from 25 subtidal reefs throughout its expanded and historical ranges. We calculated infection risk, parasite intensity, and parasite species diversity. Abiotic (temperature, latitude, distance from range limit) and biotic (host density) variables were analysed as potential drivers of differential parasitism between expanded- and historical-range populations.ResultsCompared with historical-range whelks, expanded-range whelks were 20% as likely to be infected by parasites, and those that were infected had 6% the number of individual parasites. On average, expanded-range whelks had 14% the number of parasite species than the historical-range whelks. The marked decrease in species richness of parasites infecting expanded-range whelks was only partly explained by the low numbers of parasites. The reduced parasite abundance and diversity in the expanded-range whelks was not explained by the examined abiotic factors or by whelk density.Main conclusionsExpanded-range populations of Kellet's whelk experience substantially lower parasite abundance and diversity than the historical-range populations, despite relatively poor demographic performance. The reduced parasitism observed resembles the enemy escape typically characterizing invasive species. A possible explanation for the observed ‘parasite escape’ is that the biogeographical boundary limits the movements or drives the low abundance of other host species (elasmobranchs) required to complete the life cycles of the ‘missing’ parasites. We suggest that parasite escape may generally characterize range-margin expansions and be important in permitting expansions into what may otherwise be marginal habitats. This parasite escape may directly counter the spread of infectious diseases associated with global warming-induced range shifts.
    Full-text · Article · May 2014 · Journal of Biogeography
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