Foster_et_al_2006

Foster_et_al_2006 - COMMENTARY Surveying nature Ecological...

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www.sciencemag.org SCIENCE VOL 313 22 SEPTEMBER 2006 1737 Top-Down Vs. Bottom-Up Effects in Kelp Forests IN THEIR REPORT “STRONG TOP-DOWN CON- trol in southern California kelp forest ecosystems” (26 May, p. 1230), B. S. Hal- pern et al. conclude that these forests show strong top-down (consumer-driven) control and that bottom-up (resource-driven) con- trol in such systems may often be over- estimated. These conclusions run counter to most of the extensive literature ( 1–4 ) on the ecology and natural history of kelp forests in south- ern California. There are numerous exam- ples of the importance of storms and low nutrients over large spatial and temporal scales, especially during El Niños ( 3 , 5–7 ) but also from decadal climate shifts ( 8 ). Halpern et al . used a short-term data set that did not include El Niño–Southern Osci- llation or decadal climate shifts. Moreover, they used satellite-derived offshore chloro- phyll a concentration data as a measure of “resources” without establishing that these data were a good proxy for nutrients or pri- mary production in nearshore kelp forests and despite evidence to the contrary [e.g., ( 9 , 10 )]. The primary evidence for top-down effects was correlations interpreted by Halpern et al. as showing that spiny lobsters and Kellet’s whelks were “significantly important species, likely due to their strong impacts on key grazers of kelp (urchins) and algae (limpets and snails).” There is indirect evidence that lobsters may affect urchins ( 11 , 12 ), but Kellet’s whelk is primarily a scavenger ( 13 ) whose abundance has been negatively corre- lated with kelp forests ( 14 ). Neither animal has been shown to have “strong” impacts on their prey species in California kelp forests. Halpern et al. could think of no mechanism by which the two other significant species “con- trol” algae. The diets of these fish indicate no such mechanism; the correlations likely result from habitat preferences ( 15 ). The lack of sig- nificant correlation between kelp and urchins is counter to their hypothesis but was not dis- cussed. The analytical results may be generally misleading due to weak trophic links [e.g., LETTERS I BOOKS I POLICY FORUM I EDUCATION FORUM I PERSPECTIVES 1741 Ecological risks from biofuels New evidence for meteorite origins 1742 1743 Surveying nature LETTERS edited by Etta Kavanagh Debating the Cause of a Neurological Disorder IN HIS ARTICLE “GUAM’S DEADLY STALKER: ON THE LOOSE WORLDWIDE?” (NEWS FOCUS, 28 JULY, p. 428), G. Miller presents an objective review of Cox’s reformulated cycad hypothesis ( 1 ). This hypothesis suggests that ALS-PDC, a neurological disorder once common in the Chamorro people of Guam, is caused by eating fruit bats, who have a toxin, β -methylamino- L -alanine (BMAA), in their bodies from ingesting cycad seeds. However, the case is even less compelling than Miller suggests. 1) BMAA is present in Guamanian cycad seeds (
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This note was uploaded on 08/06/2008 for the course ESM 260 taught by Professor Lenihan during the Spring '08 term at UCSB.

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Foster_et_al_2006 - COMMENTARY Surveying nature Ecological...

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