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flow regime (Q = 178 m3 s–1) with theriver flow velocity (u) of 0.49 m s–1.The overall attenuation times (toverall,days) from the combined effect ofvolatilization and degradation (Figure1a), as well as the ratio between theattenuation times from volatilization(tvol) over the attenuation times fromdegradation (tdeg) (Figure 1b), areshown. Attenuation times by vola-tilization are longer (degradation willbe the faster removal mechanism) forchemicals with low to mediumvolatility (the air–water [aw] partitioncoefficient –4 < log Kaw < –2) andwith low hydrophobicity (theoctanol–water [ow] partition coeffi-cient 0 < log Kow < 4.5). Conversely,attenuation times from volatilizationare shorter (the compound remainsfor less time in water) for compoundswith 0.5 < log Kaw< 2 and 0 < log Kow
< 8. However, channelization, tiledrains, or flooding will increase streamflow, thereby decreasing the retentiontime and potential to attenuate pollu-tants through biodegradation.
Natural attenuation processes likevolatilization and degradation candecrease the concentration of pollu-tants considerably (tens of kilometersaway from their source of input) andserve to moderate organic pollutantoutputs to seas and oceans, accordingto RIOPOP. For example, dissolvedpolychlorinated biphenyl concentra-tions decreased from upstream todownstream in the Delaware Riverfrom 1200 to 420 picograms per liter(Rowe et al. 2007) and in the EbroRiver from 43–108 nanograms perliter (ng L–1; Fernández et al. 1999) to8.9 ng L–1 (Gómez-Gutiérrez et al.2006), which is consistent withvolatilization losses. Alkylphenolconcentrations in the Hudson Riveralso decreased from upstream todownstream by well-documenteddegradation processes (Van Ry et al.2000). On the other hand, volatilizedchemicals can be deposited to nearbyor remote watersheds (Berglund2003), thus limiting the effectivenessof management strategies at thewatershed level and pointing to one“global watershed” for semi-volatileorganic pollutants.
The ubiquitous and complex natureof these attenuation processes hasmade it difficult to estimate the poten-tial of natural attenuation in mitigat-ing organic chemical contaminationclose to and downstream from pollu-tion sources. The characterization ofriverine attenuation processes such asbiodegradation and biotransformationcan provide insights for possible biore-mediation strategies. Through mutat-ing or acquiring degradative genes,these bacteria can adapt and prolifer-ate in the environment as a result ofthe selection pressures created by thepollutants (Van der Meer 2006). Thestudy of pollutant cycling and bacter-ial response to organic pollutants willimprove current riverine pollutionremediation strategies and manage-ment tools.Rosalinda Gioia* and Jordi DachsDepartment of EnvironmentalChemistry, IDAEA-CSIC, Barcelona,Spain *([email protected])
Berglund O. 2003. Periphyton density influ-ences organochlorine accumulation inrivers. Limnol Oceanogr 48: 2106–16.
Da Silva BFD, Jelic A, López-Serna R, et al.2011. Occurrence and distribution ofpharmaceuticals in surface water, sus-pended solids and sediments of theEbro River basin, Spain. Chemosphere85: 1331–39.
Fernández MA, Alonso C, Gonzalez MJ,and Hernandez LM. 1999. Occurrenceof organochlorine insecticides, PCBsand PCB congeners in waters and sedi-ments of the Ebro River (Spain).Chemosphere 38: 33–43.
Gómez-Gutiérrez AI, Jover E, Bodineau L, etal. 2006. Organic contaminant loadsinto the western Mediterranean Sea:estimate of Ebro River inputs. Chemo-sphere 65: 224–36.
Gurr CJ and Reinhard M. 2006. Harnessingnatural attenuation of pharmaceuticalsand hormones in rivers. Environ SciTechnol 40: 2872–76.
Möller A, Ahrens L, Surm R, et al. 2010.Distribution and sources of polyfluo-roalkyl substances (PFAS) in the RiverRhine watershed. Environ Pollut 158:3243–50.
Muir DCG and Howard P. 2006. Are thereother persistent organic pollutants? Achallenge for environmental chemists.Environ Sci Technol 40: 7157–66.
Rowe AA, Totten LA, Xie M, et al. 2007.Air–water exchange of polychlorinatedbiphenyls in the Delaware River.Environ Sci Technol 41: 1152–58.
Van der Meer JR. 2006. Environmentalpollution promotes selection of micro-bial degradation pathways. Front EcolEnviron 4: 35–42.
Van Ry DA, Dachs J, Gigliotti C, et al.2000. Atmospheric seasonal trends andenvironmental fate of alkylphenols inthe lower Hudson River estuary.Environ Sci Technol 34: 2410–17.
Vörösmarty C, McIntyre PB, Gessner MO,et al. 2010. Global threats to humanwater security and river biodiversity.Nature 467: 555–61.
doi:10.1890/12.WB.017
Non-native species promotetrophic dispersion of foodwebsPeer-reviewed letterEstes et al. (2011) reported that theloss of large apex consumers has dras-tically altered ecosystem functioningworldwide, through the mechanismof “trophic downgrading”, andurgently appealed for interdiscipli-nary research to forecast the effectsof this phenomenon on ecosystemprocess, function, and resilience.Although we agree with the authors’premise that the “loss of apex con-sumers is arguably humankind’s mostpervasive influence on the naturalworld”, this study and others con-tinue to account for only one side ofthe biodiversity ledger – by failing torecognize that humans often selectfor and introduce large-bodied non-native species, which frequentlyreplace lost native predators (Eby etal. 2006; Byrnes et al. 2007; Blanchetet al. 2010). For instance, the world-wide introductions of predatorymammals on islands represent newupper-trophic-level species. Thereality is that non-native speciesnow represent a substantial fractionof local and regional diversity (Sax etal. 2002; Leprieur et al. 2008) andhave led to both the extirpation ofnative species (Clavero and García-Berthou 2005) and the replacementor addition of new apex consumers(Griffiths et al. 2010; Schlaepfer et al.2011). Although trophic downgrad-ing of native communities hasundoubtedly occurred, the potentialfunctional compensation repre-
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sented by the introduction of non-native species has yet to be fullyappreciated (Wardle et al. 2011).
Here, we tested whether the addi-tion of non-native species can com-pensate for native species loss andresultant trophic downgrading offood webs. We used freshwater fishesas a model system because non-native introductions of such organ-isms are widespread, yet variable,and have modified the trophic struc-ture of countless ecosystems world-wide (Leprieur et al. 2008; Cucher-ousset and Olden 2011). To do so,we collected information on thepresence of native and non-nativefish species from 13 watersheds.Watersheds were selected to bewidely distributed across the globeand have comprehensive fish species’lists (additional information avail-able in WebTable 1). For eachspecies, we recorded its trophic posi-tion based principally on diet studies(www.fishbase.org). Although weacknowledge that local environmen-tal conditions and time since intro-duction could affect these estimatedtrophic positions, the selected vari-ables provide an opportunity to per-form large-scale comparisons oftrophic structure. We then charac-terized the trophic structure in thehistorical period before species intro-ductions (ie current native speciesonly, assuming no extinction withinthe watershed) and in the contem-porary period after species introduc-tions (ie current native and non-native species) using an equalweighting of the trophic position foreach species in each watershed.
Nine out of 13 watersheds showedevidence of a slight increase in meantrophic position in response to non-native species introductions, althoughoverall the change in the contempo-rary time period was modest and notstatistically significant (paired t test,t12 = 1.38, P = 0.19; Figure 1a). Thisreflects the fact that the meantrophic position of native versusnon-native species did not differ sig-nificantly (paired t test, t12 = 0.34, P= 0.74). An interesting, but some-what unexpected, finding was that
the introduction of non-nativespecies significantly increased thevariability of the trophic position (ie“trophic dispersion”): a consistentand significant pattern across allwatersheds (paired t test, t12 = 3.68,P < 0.01; Figure 1b). This pattern isdriven by the fact that non-nativefish species in both upper and lowertrophic positions have been intro-duced to freshwater ecosystems (Ebyet al. 2006; Gido and Franssen 2007;Cucherousset and Olden 2011), withall food webs decreasing (or showingno change) in their minimumtrophic position (Figure 1c) andincreasing (or showing no change)in their maximum trophic position(Figure 1d).
Our investigation of freshwaterfishes across the world suggests thatthe introduction of non-native speciesmight, as predicted, increase thelength of food chains through theintroduction of top predators and,unexpectedly, modify the basal struc-
ture of food webs through the intro-duction of herbivorous fish species.Blanchet et al. (2010) found thatintroduced fish species had signifi-cantly larger body size than nativespecies as a consequence of humanselection for aquaculture and angling(Eby et al. 2006; Gozlan 2008).Apparently this subset is composed ofboth large-bodied predators (hightrophic position, eg salmonids, cen-trarchids) and large-bodied herbivo-rous species (low trophic position, egcyprinids, cichlids), leading to theobserved trophic dispersion of foodwebs. A similar pattern occurs for ter-restrial mammals on islands with theintroduction of large predatory andherbivorous species worldwide (egCourchamp et al. 2003; Maron et al.2006). Biological invasions interactsynergistically, antagonistically, and/oradditively with multiple human-in-duced impacts on ecosystems (Strayer2010), and their effects on food-webstructure could be idiosyncratic. How-
Figure 1. (a) Mean, (b) coefficient of variation (CV), (c) minimum, and (d)maximum trophic position of freshwater fish in the historical period before speciesintroductions (ie current native species only, x-axis) and in the contemporary periodafter species introductions (ie current native and non-native species, y-axis) in the 13studied watersheds. When displayed, the values in the panels indicate the number ofoverlapping points.
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ever non-native species, through thesimultaneous addition of species inhigh and low trophic positions intorecipient ecosystems, may be promot-ing greater trophic variability and miti-gating trophic downgrading caused bynative species loss. Therefore, theprocess of trophic downgrading high-lighted by Estes et al. (2011) may notbe a general rule but is likely the excep-tion in a world increasingly beingthreatened by non-native species.Julien Cucherousset1,2*, SimonBlanchet1,3, and Julian D Olden4
1CNRS, UPS, ENFA, UMR 5174EDB (Laboratoire Évolution et DiversitéBiologique), Toulouse, France*([email protected]);2Université de Toulouse, UPS,UMR 5174 EDB, Toulouse, France;3Station d’Ecologie Expérimentale duCNRS à Moulis, Moulis, France;4School of Aquatic and Fishery Sciences,University of Washington, Seattle, WA
The research leading to these resultshas received funding from the EuropeanUnion Seventh Framework Programme(FP7/2007-2013) under grant agreementnumber PERG08-GA-2010-276969.
Blanchet S, Grenouillet G, Beauchard O,et al. 2010. Non-native species disruptthe worldwide patterns of freshwaterfish body size: implications forBergmann’s rule. Ecol Lett 13: 421–31.
Byrnes JE, Reynolds PL, and Stachowicz JJ.2007. Invasions and extinctionsreshape coastal marine food webs.
PLoS ONE 2: e295.Clavero M and García-Berthou E. 2005.
Invasive species are a leading cause ofanimal extinctions. Trends Ecol Evol20: 110.
Courchamp F, Chapuis JL, and Pascal M.2003. Mammal invaders on islands:impact, control and control impact.Biol Rev 78: 347–83.
Cucherousset J and Olden JD. 2011.Ecological impacts of non-native fresh-water fishes. Fisheries 36: 215–30.
Eby LA, Roach WJ, Crowder LB, andStanford JA. 2006. Effects of stocking-up freshwater food webs. Trends EcolEvol 21: 576–84.
Estes JA, Terborgh J, Brashares JS, et al.2011. Trophic downgrading of planetEarth. Science 333: 301–06.
Gido KB and Franssen NR. 2007. Invasionof stream fishes into low trophic posi-tions. Ecol Freshw Fish 16: 457–64.
Gozlan RE. 2008. Introduction of non-native freshwater fish: is it all bad? FishFish 9: 106–15.
Griffiths CJ, Jones CG, Hansen DM, et al.2010. The use of extant non-indige-nous tortoises as a restoration tool toreplace extinct ecosystem engineers.Restor Ecol 18: 1–7.
Leprieur F, Beauchard O, Blanchet S, et al.2008. Fish invasion in the world’s riversystems: when natural processes areblurred by human activity. PLoS Biol 6:e28.
Maron JL, Estes JA, Croll DA, et al. 2006. Anintroduced predator alters AleutianIsland plant communities by thwartingnutrient subsidies. Ecol Monogr 76: 3–24.
Sax DF, Gaines SD, and Brown JH. 2002.Species invasions exceed extinctions onislands worldwide: a comparative study ofplants and birds. Am Nat 160: 766–83.
Schlaepfer MA, Sax DF, and Olden JD.
2011. The potential conservationvalue of non-native species. ConservBiol 25: 428–37.
Strayer DL. 2010. Alien species in freshwaters: ecological effects, interactionswith other stressors, and prospects forthe future. Freshwater Biol 55: 152–74.
Wardle DA, Bardgett RD, Callaway RM, andVan der Putten WH. 2011. Terrestrialecosystem responses to species gains andlosses. Science 332: 1273–77.
doi:10.1890/12.WB.018
Reply to Cucherousset et al.We agree that non-native predatorshave strongly influenced the structureand function of nature in many places.We are skeptical, however, of theauthors’ implication that these non-native predators are functional equiva-lents of those that have been lost. Thefundamental problem is that meantrophic level and food chain length donot reflect the complex ways in whichthe influences of predators spreadthrough food webs, nutrient cycles,and other ecosystem processes.James A Estes1*, John Terborgh2,Mary E Power3, andStephen R Carpenter4
1University of California, Santa Cruz,CA *([email protected]); 2DukeUniversity, Durham, NC; 3Universityof California, Berkeley, CA;4University of Wisconsin, Madison, WI
doi:10.1890/12.WB.019
Figure 2. Maps of CMAQv4.7.1 estimates of annual (a) sulfur, (b) inorganic nitrogen, and (c) acidic deposition for 2002 for a12-km grid over the continental US, where wet deposition is adjusted by the ratio of observed to modeled precipitation and thenregionally corrected for wet deposition bias, and where observed precipitation is from the Parameter-elevations Regressions onIndependent Slopes Model.
(a) Total sulfur deposition (kg S ha–1) (b) Total nitrogen deposition (kg N ha–1) (c) Total acidic deposition (S+N) (meq m–2)
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ErratumIn Greaver et al. (2012; 10[7]: 365–72), panel headings and selected y-axis labels in Figure 2 on page 367 wereincorrectly matched with their respective panels. A corrected version of the figure appears below.
