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A response-surface examination of competition and facilitation between native and invasive fishes.


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Ecological theory has long recognised the importance of positive and negative species interactions as drivers of food web structure, yet many studies have only focused on competition. Because competitive and facilitative mechanisms operate simultaneously, but through different food web pathways, the balance of their combined effects can produce complex and variable responses. We used a response-surface experimental design to assess the roles of negative (e.g. intra-, interspecific competition) and positive (e.g. facilitation) interactions between native and invasive juvenile fishes. We tested whether these interactions alter the densities of planktonic and benthic invertebrates to evaluate the magnitude and mechanism(s) influencing the acceptance or resistance of biological invaders. Interactions between bighead carp (Hypophthalmichthys nobilis) and bluegill (Lepomis macrochirus) or common carp (Cyprinus carpio) were evaluated in mesocosms. Intraspecific interactions were 1.5-2.4 times stronger than interspecific interactions between carp species. The only instance of interspecific competition resulted in bighead carp reducing the daily growth of bluegill, whereas the reciprocal interaction resulted in facilitation. Facilitation occurred when bluegill increased the daily growth of low density bighead carp treatments, despite increased numbers of fishes. Bighead carp also increased densities of benthic Chironomidae larvae, which were subsequently consumed by bluegill, but did not result in enhanced bluegill growth. These suites of interactions were not observed between common and bighead carp. Our response-surface design proved useful for comparing the relative magnitude of intra- vs. interspecific competition, identifying facilitation among species, and tracing attendant effects on invertebrate communities. By accounting for the directionality of interactions within our experimental framework and tracking responses of prey at lower trophic levels, we provide a clearer understanding of how competitive effects and stressed consumers alter prey communities and influence facilitation. plain language summary is available for this article.
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Functional Ecology. 2017;1–10.  
© 2017 The Authors. Functional Ecology
© 2017 British Ecological Society
DOI: 10.1111/1365-2435.12922
A response- surface examination of competition and facilitation
between native and invasive juvenile fishes
Kirsten A. Nelson1,2| Scott F. Collins1| Greg G. Sass2| David H. Wahl1,2
1Illinois Natural History Survey, Kaskaskia
Biological Station, Sullivan, IL, USA
2Illinois Natural History Survey, University of
Scott F. Collins
Present address
Natural Resources, Boulder Junction, WI, USA
1. Ecologicaltheoryhaslongrecognisedtheimportanceofpositiveandnegativespe-
cies interactions as drivers of food web structure, yet many studies have only fo-
cused on competition. Because competitive and facilitative mechanisms operate
simultaneously, but through different food web pathways, the balance of their
2. Weusedaresponse-surfaceexperimentaldesigntoassesstherolesofnegative
tween native and invasive juvenile fishes. We tested whether these interactions
alterthedensitiesof planktonicandbenthicinvertebratesto evaluatethe magni-
ers. Interactions between bighead carp (Hypophthalmichthys nobilis) and bluegill
(Lepomis macrochirus) or common carp (Cyprinus carpio) were evaluated in
3. Intraspecific interactions were 1.5–2.4 times stronger than interspecific interac-
action resulted in facilitation. Facilitation occurred when bluegill increased the daily
growth of low density bighead carp treatments, despite increased numbers of
were subsequently consumed by bluegill, but did not result in enhanced bluegill
4. Ourresponse-surface designprovedusefulfor comparingtherelative magnitude
of intra- vs. interspecific competition, identifying facilitation among species, and
tracing attendant effects on invertebrate communities. By accounting for the direc-
tive effects and stressed consumers alter prey communities and influence
Functional Ecology
mental in describing why certain organisms thrive while others falter.
Stachowicz, & Bertness, 2003). However, processes such as com-
petition and predation typically receivemore attention than others
likefacilitation,althoughthis disparityis diminishing(e.g.Bertness&
Callaway,1994;Michalet& Pugnaire,2016;Simberloff &VonHolle,
1999).Because competitiveand facilitativemechanismscan operate
simultaneously through unique food webpathways, the balance of
Experimentalassessments ofspeciesinteractionshavelongused
additive and substitutive frameworks (reviewed in Connell, 1983;
Connolly,1986; Gibson, Connolly, Hartnett, & Weidenhamer, 1999;
Goldberg & Barton, 1992; Gurevitch, Morrow, Wallace, & Walsh,
1992). More recently, response-surface designs have been used,
which manipulate species across a range of differingdensities (e.g.
Inouye,2001).Aresponse-surfacedesignis ideal forevaluating spe-
ciesinteractionsbecause itallows fortheappraisal ofthemagnitude
of intra- and interspecific interactions between species (Forrester,
approachmay be particularly useful forthe study of biological inva-
sions, as the numbers of native and invasive organisms can vary greatly
Moreover, it can provide needed insight into the roles of negative
Globally, accidental or purposeful introductions of fishes into
aquatic ecosystems has greatly homogenised fish communities (Rahel
2002; Ricciardi & Kipp, 2008; Strayer & Dudgeon, 2010). Success
or failure ofa newly introduced fish (or anyorganism) may depend
onthesuite of negativeandpositive interactions an invaderexperi-
ences within a community and its associated habitat characteristics
(Rodriguez, 2006; Simberloff& VonHolle, 1999). If competition for
shared and limited resourcessufficiently stresses one or many spe-
cies, negative species interactions and attenuating effects on food
effects for some organisms and additional negative effects for others
can be difficult, and as a consequence, there is a poormechanistic
understanding of how species interactions might elicit feedbacks
withinpreycommunities thatresultinfacilitationofonefishspecies
by another.
We testedfor negative and positive interactions between juve-
nilebighead carp (Hypophthalmichthys nobilis),bluegill(Lepomis mac-
rochirus),andcommoncarp(Cyprinus carpio)intwoexperiments.Each
River Basin of North America, bluegill are a native centrarchid, whereas
andcommon carparefacultativeplanktivoresthatgenerallyfeedon
(Britton etal. 2007; Spotte,2007). Bighead carp are obligate plank-
tivoresthat efficientlyfilterzoo- and phytoplankton (Kolar& Lodge,
2002; Sampson, Chick, & Pegg, 2009; Collins& Wahl, 2017). Using
replicated mesocosmexperiments, we quantified and compared the
petition,alongwith potential facilitation among juvenilebluegilland
bigheadcarp (Experiment 1) andjuvenilecommon and bigheadcarp
structure of aquatic invertebrate communities. Because these fishes
consume similar preyresources, we predicted that strong competi-
tionwould produce a stressful environmentfor one or both species
thatintensecompetitionforprey resourcescreates a stressed envi-
ronment, with feedbacks within the food web that drive facilitation
2.1 | Study design and system
Response-surface designs have been used to examine the relative
roles of intra- and interspecific species interactions (e.g. Forrester
etal., 2006; Inouye, 1999); however, focus has largely been limited
to negative species interactions. Such studies have reasoned that
significantnegativeslopesgeneratedfrom statisticalmodelsindicate
etal., 2006). We extended this reasoning to account for positive
slopes,which we interpreted asthepresence of positive species in-
teractions, specifically facilitation within or between species. Here,
(intra-andinterspecificcompetition)andpositive(interspecific facili-
tation)effects between juvenile fishes in twoseparateexperiments.
Experiment 1 was conducted in August 2011 and used eight treat-
ments (1–8; n=5replicatespertreatment;seeFigure1)totestfor
bluegill.WeconductedExperiment2duringAugust 2012 and used
Both experimentswere conducted in mesocosms (1,325-L poly-
Lake, Kinmundy, IL, USA. Water was filtered through a 64 μm mesh net
were collected and introduced to the mesocosms after two weeks and
During these four-week periods during 2011 and 2012, conditions
Functional Ecology
source populations. We obtained age-0 bighead carp from Osage
Catfisheries,Inc., Missouri,USA.Bluegillandcommon carpdensities
high- density treatments. Initial lengths (±SD)of bluegill and bighead
ofbighead carpand common carpwere 56±6mmand50±16mm
respectively. During the firstweek of each experiment, mortalities
2.2 | Data collection and analyses
2.2.1 | Fish interactions
Fish lengths and weights (nearest mm, 0.1g) were recorded at the
start and end of each experiment and used to estimate mean daily
growth (length, mm/day; weight, g/day).Weusedlinearfixedmodels
to evaluate the net effect (positive or negative) of interactions be-
densities(Experiment 1) and common carp and bighead carp densi-
ties(Experiment 2) asfixedeffects (SAS 9.2;PROCMIXED). Due to
limited space, mesocosms were setup at two locations at the Sam
ParrBiologicalStation.Wedetected noeffectofmesocosmlocation
gill, F = 1.94, p=.28;bighead carp, F = 0.29, p=.60)or Experiment
2 (block; common carp, F = 0.11, p=.73; bighead carp, F = 1.64,
p=.21).Additionally,no interaction between block and densitywas
detectedforeither Experiment 1 (bluegill, F = 1.19, p = .29; bighead
carp, F = 0.24, p=.62) or Experiment 2 (common carp, F = 0.22,
p=.64; bighead carp, F = 0.38, p=.54).Slope coefficients (β) were
estimatedandusedtodescribethepercapitaeffects ofconspecifics
andheterospecificsonfishgrowthrates. Separatemodelswere con-
(Brownand Forsythe’s Testfor Homogeneity ofVariance).We used
a log10transformationwhenresidualsfailedtomeettheassumptions
of ANOVA. The sign of β coefficients (+, −) and the significance of
conspecifics,heterospecifics, and their interactionswereused to in-
termfrom the linear fixed effectmodelsindicates that the effect of
addingone individual ofeitherspecies depends uponthedensity of
the other species. When coefficients indicated a negative effect of
conspecificsand heterospecifics,andno statisticalinteractionswere
detected,comparisons ofthe relativemagnitudeof interspecificand
intraspecificinfluences on fishgrowthwere assessed astheratio of
2.2.2 | Limnological and invertebrate sampling
Within each mesocosm, water temperature (°C), dissolved oxygen
(mg/L), total phosphorus, chlorophyll-a, turbidity (nephelometric
units,NTU), phytoplankton,and zooplanktonwerecollected priorto
fish introduction and then weekly until the end of the experiment.
Total phosphorus concentrations in the water column were col-
lected (2×45ml subsamples per mesocosm) and determined using
thecolorimetric molybdenum blueascorbicacid method withaper-
sulfate digestion. Chlorophyll-a was obtained by filtering 100 ml of
water through glass fibre filters (0.7 μmporesize[Millipore,Billerica,
Massachusetts, USA]), extracting chlorophyll-a in 90% acetone for
24hr,and then measuring fluorescence using afluorometer(Turner
Design, model TD700). Zooplankton were sampled with a vertical
tubesampler (70mm diameter×0.4m long; 1.5L)andpreservedin
a10%buffered formalinandrose Bengalsolution.Oneachsampling
the mesocosm, combined, and filtered through a 20 μm mesh net
55 and 20 μm mesh nets. Tests for changes in limnological condi-
tionsand inzooplanktonand rotiferdensitieswere conductedusing
repeated-measuresANOVA(SAS 9.2,PROCMIXED; Treatmentand
changesthroughtime,andfor potentiallagged effectsoftreatments
manifesting through time. Densities of benthic macroinvertebrates
werequantifiedbyplacingtwowhitetiles(116.6cm2 pertile) atthe
bottomof eachmesocosm.At theendof theexperiment, tiles were
removed from each mesocosm and macroinvertebrates were stored
in ethanol with rose Bengal, then identified and counted. Given the
FIGURE1 Experimentaldesignoftreatmentsfortheassessment
bluegill (Lepomis macrochirus)andbigheadcarp(Hypophthalmichthys
commoncarp(Cyprinus carpio)andbigheadcarp(Treatments1–9).
Bighead carp density
Density of bluegill
or common carp
1 4 5
2 6 8
3 7 9
Functional Ecology
groupswereinvestigatedusingCONTRASTstatements.Log10 trans-
formations were applied to response variables when necessary to
satisfythe assumptionsofANOVA. Forall statisticalanalyses(linear
fixedeffect models, repeated measures ANOVA,one-wayANOVA),
significance was determined at p < .05.
3.1 | Experiment 1: Test of competition and
facilitation between juvenile native bluegill and
invasive bighead carp
3.1.1 | Fish interactions
Juvenile bluegill growth decreased with increasing conspecific and
within a mesocosm was a major determinant of growth. The nega-
tive slopes and overlapping confidence intervals describing bluegill
Bigheadcarpgrowthwasunaffectedby densitiesof theirconspecif-
ics(Table1).However,lowdensitybigheadcarp treatments experi-
enced increased growth as densities of bluegill increased, indicating
3.1.2 | Invertebrates
Pronounced changes in total macrozooplankton density were ob-
served between treatments, through time, and with treatment effects
manifesting through time (Time, F4,123 = 80.48, p<.001; Treatment,
F7,31 = 8.83; p<.001; Treatment×Time, F28,23 = 5.06, p < .001;
Figure3a). The presence of the filter-feeding bighead carp greatly
reduced macrozooplankton densities below those observed in fish-
less controls (BHC vs. Control, F1,30 = 14.36, p<.001;Mixvs.Control,
F1,30 = 27.52, p<.001;Figure3a).Yet,densitiesofmacrozooplankton
(i.e.bluegill andbighead carp)treatments(BHC vs.MIX, F1,30 = 2.40,
p=.13), indicating bighead carp were the key driver of changes to
planktonicprey. Filter-feedingbybigheadcarpreduced totalmacro-
zooplanktonbelow treatments withthevisual-foraging bluegill (BLG
vs. BHC, F1,30 = 15.54, p<.001;BLG vs.Mix,F1,30 = 34.35, p<.001).
Within the macrozooplankton community, four of five zooplankton
taxawere greatly reduced inthepresenceof predatory fishes, each
contributing to the overall reduction in planktonic invertebrates
FIGURE2 Meandailygrowth(mm/day,
±1 SE)of(a)bluegill(Lepomis macrochirus)
carp(Cyprinus carpio)and(d)bigheadcarp
“low” and “high” refers to a fish density
Bighead carp density
Daily growth (mm/day)
Bluegill density
(a) (b)
Common carp density
Bighead carp density
(c) (d)
Experiment 1: Native vs. invasive
Experiment 2: Invasive vs. invasive
Low Bluegill
High Bluegill
Low Bighead carp
High Bighead carp
Low Common carp
High Common carp
Low Bighead carp
High Bighead carp
Functional Ecology
Small-bodied rotifersexhibited divergent responsestopredation
by bluegill and bighead carp (Treatment, F7,33 = 2.84, p=.02; Time,
F4,64 = 1.98, p=.11;Treatment×Time,F28,89 = 1.06, p=.40).Rotifers
in bluegill only treatments were similar to fishless controls indicating
limitedpredatoryeffects ofthesevisualforagersonsmall bodiedin-
vertebrates (BLG vs. Control, F1,30 = 1.65, p=.21). Densities of roti-
those observed in fishless controls (BHC vs. Control, F1,30 = 15.16,
p<.001)andbluegilltreatments(BLGvs.BHC,F1,30 = 16.44, p<.001).
Rotifer densities were similar between treatments containing bighead
carp(BHC vs.MIX,F1,30 = 0.11, p=.75), indicatingtheirpresenceas
the driver of rotifer reductions.
mon sessile taxa sampled from tiles at the end of the experiment.
Chironomidae densities were greatest in the sole presence of big-
headcarp(Treatment,F7,32 = 7.39, p=.021;Figure3c)and aboutten
TABLE1 Summarystatisticsfromregressionmodelstestingforeffectsofconspecificandheterospecificdensity,andtheirinteraction,on
changes in length and biomass of juvenile bluegill (Lepomis macrochirus),bigheadcarp(Hypophthalmichthys nobilis),andcommoncarp(Cyprinus
carpio).DisplayedaremodelR2 values, regression coefficients (β),andp- values for an associated significance test (H0: β=0)foreachtermin
the model
Experiment 1 Bluegill Bighead carp Interaction
Bluegill(g/day) .73 0.050 −0.006 <.001 −0.009 <.001 0.001 .001
Bluegill(mm/day) .56 0.150 −0.017 .002 −0.025 .002 0.003 .03
Bigheadcarp(g/day) .45 −0.008 0.006 .04 −0.001 .64 −0.001 .14
Bigheadcarp(mm/day) .45 0.014 0.024 .03 −0.003 .71 −0.003 .09
Experiment 2 Common carp Bighead carp Interaction
Commoncarp(g/day) .36 0.031 −0.003 <.01 −0.002 .26 0.0001 .43
Commoncarp(mm/day) .50 0.254 −0.019 <.01 −0.008 .24 0.001 .51
Bigheadcarp(g/day) .27 0.027 −0.001 .30 −0.002 .05 0.0001 .51
Bigheadcarp(mm/day) .30 0.236 −0.011 .33 −0.018 .07 0.0006 .66
FIGURE3 Responseof
Chironomidae invertebrates to the
1 tested for the effects of bluegill
(Lepomis macrochirus)andbigheadcarp
(Hypophthalmichthys nobilis)(Treatments
commoncarp(Cyprinus carpio)andbighead
mean and letters (w, x, y, z)represent
treatment differences within each
Bighead carp density
Bluegill densityCommon carp density
100 200 400Scale
(a) (b)
(e) (f)
0.08 0.42 0.85Scale
panels: a, b, d, e
panel: c
# L–1 # cm–2
yz yz
yz yz
xy xyx
Macrozooplankton Rotifers Benthic Chironomidae
Functional Ecology
Chironomidae densities solely within bluegill treatments were similar
3.1.3 | Limnological conditions
Limnological conditions (e.g. temperature, dissolved oxygen) were
similar between treatments (Table2). Although these metrics did
vary through time, no treatment effects manifested through time,
sitiesof zooplanktonand elevatedin mesocosmswithfishes atthe
conclusionoftheexperiment(Table2).Chlorophyll-a concentrations
weresimilaramongtreatments(Treatment;F7,32 = 1.14; p=.36),but
decreased through time (Time; F4,28 = 15.07; p<.001), with con-
centrations 3 to 5+ times greater in treatments with fishes relative
to fishless controls (Treatment×Time; F28,57 = 2.01; p=.01). Total
phosphorus concentrations varied over time (Time; F4,123 = 4.83;
p<.001); however, concentrations did not vary between treat-
ments(Treatment; F7,36 = 1.05; p=.41) and no specific treatments
F28,122 = 0.86; p=.67).
TABLE2 Resultsfromrepeated-measuresANOVAtestingforchangesinbiotic(phytoplankton,invertebrates)andabiotic(temperature,
dissolvedoxygen,turbidity,totalphosphorus)variablesinresponsetothepresenceofbigheadcarp(Hypophthalmichthys nobilis)andbluegill
(Lepomis macrochirus;Experiment1)andbigheadcarpandcommoncarp(Cyprinus carpio;Experiment2)
Response Effect
Experiment 1 Experiment 2
df F p df F p
Biotic Chlorophyll-a Treatment 7 1.14 .36 8 1.38 .24
Time 4 15.07 <.0001 4 50.6 <.0001
Treatment×Time 28 2.01 .01 32 2.42 <.0001
Cyclopoid Treatment 7 4.12 .003 8 1.72 .13
Time 4 8.84 <.0001 4 54.54 <.0001
Treatment×Time 28 3.5 <.0001 32 1.35 .12
Nauplii Treatment 7 8.6 <.0001 8 11.57 <.0001
Time 4 103.76 <.0001 4 77.71 <.0001
Treatment×Time 28 6.42 <.0001 32 2.6 <.0001
Bosminidae Treatment 7 2.93 .02 8 1.14 .36
Time 4 2.61 .04 4 17.63 <.0001
Treatment×Time 28 1.12 .33 32 0.6 .95
Ceriodaphnia Treatment 7 5.19 <.001 8 10.19 <.0001
Time 4 32.77 <.0001 4 77.78 <.0001
Treatment×Time 28 2.14 .002 32 1.71 .02
Chydoridae Treatment 7 1.69 .15 8 2.02 .07
Time 4 25.98 <.0001 4 17.63 <.0001
Treatment×Time 28 1.19 .25 32 1.27 .18
Rotifera Treatment 7 2.84 .02 8 2.46 .02
Time 4 1.98 .11 4 35.55 <.0001
Treatment×Time 28 1.06 .4 32 2.23 .002
Abiotic Temperature Treatment 7 0.85 .55 8 0.86 .56
Time 4 2,364.52 <.0001 4 1,567.13 <.0001
Treatment×Time 28 1.02 .45 32 0.54 .98
Dissolvedoxygen Treatment 7 0.86 .54 8 2.39 .03
Time 4 49.47 <.0001 4 70.54 <.0001
Treatment×Time 28 0.75 .76 32 0.53 .98
Turbidity Treatment 7 0.74 .63 8 0.99 .45
Time 4 7.8 <.001 4 27.66 <.0001
Treatment×Time 28 1.79 .04 32 0.48 .99
Phosphorus Treatment 7 1.05 .41 8 0.76 .64
Time 4 4.83 <.001 4 0.89 .47
Treatment×Time 28 0.86 .67 32 0.77 .8
Functional Ecology
3.2 | Experiment 2: Test of competition and
facilitation between juvenile invasive common
carp and bighead carp
3.2.1 | Fish interactions
cifics, but declined with increasing conspecifics (Table1; Figure2c),
density reduced common carp growth 1.5 (weight) and 2.4 (length)
timesmorethan similar density increases by bighead carp (Table1).
duced bighead carp growth 1.6 (length) and 2 (weight) times more
3.2.2 | Invertebrates
Total macrozooplankton density varied between treat-
ments (Treatment, F8,35 = 12.55, p<.001), through time (Time,
F4,142 = 109.68, p<.001), and responses manifested by treatments
through time (Treatment×Time, F32,142 = 3.44, p<.001; Table2;
Figure3d). Densities of macrozooplankton were similar between
bighead and common carp treatments (CCP vs. BHC, F1,34 = 0.02,
p=.89), indicating filter-feeding by each species had similar effects
on prey. Combined predatory effects of common and bighead carp
(mixed)further reduced macrozooplankton densities relative to big-
head carp-only treatments (MIX vs. BHC, F1,35 = 29.16, p<.001).
Rotifer densities differed by treatment (Treatment, F8,53 = 2.46,
p=.02)andthroughtime(Time,F4,64 = 35.55, p<.001),with rotifers
increasingwithin select treatments through time (Treatment×Time,
F32,90 = 2.23, p=.002; Figure3e).Bighead carp only treatments had
lower rotifer densities than common carp only treatments(BHC vs.
CCP, F1,53 = 12.57, p=.001) and mixed species treatments (BHC
vs. MIX, F1,53 = 4.01, p=.05), but were similar to controls(BHC vs.
Control, F1,53 = 0.04, p=.83).Mixedspeciestreatmentswerealsosim-
ilartocontrols(MIXvs.Control,F1,53 = 1.76, p=.19).Incontrast,roti-
thancontrols(CCPvs.Control,F1,53 = 7.21, p=.01).
Chironomidae larvae were the most abundant sessile inverte-
ties of Chironomidae were about 8 times lower than those observed in
sity did not differ among treatments (F8,34 = 0.93; p=.50;Figure3f).
3.2.3 | Limnological conditions
There were no significant differences in limnological parameters
among treatments, but conditions did vary through time (Table2).
Chlorophyll-a was similar among treatments, declined over the course
oftheexperiment,andhadconcentrations in fish treatments 1.5–2
timesgreater than fishless controls(Treatment×Time;F32,57 = 2.42;
p<.001).No effectsweredetected forphosphorous concentrations
Ourresponse-surface designproveduseful inidentifyingthe netef-
fect of interactions between juvenile native and invasive fishes, for
comparingthe relativemagnitudeof intra- vs. interspecificcompeti-
tion,identifying facilitationamongspecies, andtracing attendantef-
fectson invertebratecommunities.Strong exploitationof planktonic
resourcesby invasivebighead carpcreated interspecificcompetition
with bluegill, as evidenced by their reduced growth. The recipro-
cal effect of bluegill on bighead carp actually increased the growth
ofbighead carpatlow, butnothigh densities.Interestingly,the lack
ofenhancedbigheadcarpgrowthathigh densitiessuggests thatthe
mechanism(s) promoting enhanced growth at low densities have a
diminished effect as fish numbers increase and density-dependent
processes limit growth. Similar effects were not observed between
commoncarpand bighead carp perhaps because of differences be-
tween bluegill and common carp in terms of niche partitioning, be-
cause intraspecific competition was dominant for both taxa, and/or
because cyprinids are generally more tolerant of stressful environ-
ments than centrarchids.
4.1 | Magnitude of intra- and interspecific
competition among juvenile native and invasive fishes
Experimentshave long been usedtoquantify the effectsofcompe-
tition on organism growth or survival. However, many assessments
have largely focused on either intra- or interspecific interactions
assessmentsof both occurring less frequently(Inouye,1999,2001).
affected growth 1.5–2.4 times greater than interactions with hetero-
similar, though slightly less, than the competitive interactions ob-
served between coral reef fishes (intra > inter by 2–3 times; Forrester
etal., 2006). In general, density-dependent effects of conspecifics,
and the associated high degree of niche overlap, had the greatest
Resource-partitioningand nichedifferentiationcanreducethein-
tensityof interspecific competition during times of resource scarcity,
upon zooplanktonic resources; however, differenceswere observed
gillrakers, whichallowsthem to filterandremove smallparticleslike
ton, bighead carp possess the functional traits required to consume
smaller and more productive (i.e. higher rates of biomass-turnover)
Functional Ecology
food items such as rotifers and phytoplankton.By partitioning food
resources,interspecificcompetition betweencarpspecieswasdamp-
ened.In turn, intraspecificcompetition wasenhancedas conspecifics
juvenile stages, theircombined predatory effects maybe particularly
troubling in environmentswhere preyproductivity is low, or at loca-
ity, such as in the Laurentian Great Lakes of North America.
Bighead carp reduced the growthof bluegill, providing the only
instancewhere interspecificexceeded intraspecificcompetition.The
suppression of large zooplankton by bighead carp was consistent
across our experiments, othercontrolled experiments (e.g. Schrank,
theremovaloftheirpreferredzooplanktonprey,coupledwithan in-
abilityto exploit rotifersorphytoplankton, created a foodwebwith
few prey choices for bluegill. Although bluegillwere negatively af-
densities. Model coefficients (i.e. slopes representing growth) were
similar,yet heterospecifics did produce a slightlysteeper slope. The
ofeffect, but did indicatenonlinearper capita effectswerepresent.
Butler,&Wahl,2017;Sass etal. 2010).These riversystems harbour
4.2 | Facilitation among fishes
There is critical need in the study of biological invasions to better
understand the circumstances that promote the facilitation of one
species by another (Bascompte, 2010; Griffen, Guy, & Buck, 2008;
Rodriguez,2006;Simberloff &VonHolle,1999).Predicting howone
predator might facilitate another can be difficult, especially when
negatively alter prey densities, behaviours, or distributions in com-
plex ways (Kerfoot & Sih, 1987; Werner & Peacor, 2003). In some
to other predators (e.g. Adams, Pearl, & Bruce Bury, 2003; Huss &
Nilsson, 2011; Thayer, Haas, Hunter, & Kushler, 1997). Here, blue-
gill indirectly increased the growth of invasive bighead carp at low
densities, but not at high densities. Interestingly, facilitated growth
of a small number of invasive fishes should shorten the duration they
arevulnerableto larger predators, which would be advantageous in
newly colonised habitats. Based on the responses of invertebrates,
themechanismsenhancing bigheadcarpgrowthappearstohaveoc-
Fishes that shift foraging to alternate food resources to lessen the
ment by recycling nutrients, which feedback to alter the food web (e.g.
bigheadcarpcanexploitverysmall foodparticles (e.g.rotifers),rapid
gillfacilitatedthe growth of bighead carp,wefound no evidence of
facilitation between common carp and bighead carp, suggesting
thatcharacteristicsofthefoodweb itselfalso mediatethesespecies
facilitation. For instance, seaweed (Ascophyllumn nodosum; Bertness,
Leonard, Levine, Schmidt, & Ingraham, 1999) and zebra mussels
(Dreissena polymorpha; Thayer etal., 1997) altered habitat structure
intheir respectiveenvironments,whichenhanced the fitnessof cer-
tain in situ taxa. Intraguild predation can also alleviate regulatory
constraintsand facilitatethe establishment ofother species,such as
whenbluegillconsumethepredators(dragonflies)oflarval bullfrogs
(Rana catesbeiana;Adams etal., 2003). Here, alterations to material
flowsviafishegestionandexcretionappear tohave stimulatedden-
sities of benthic macroinvertebrates via benthic-pelagic coupling, a
pattern observed byothers (Collins & Wahl, 2017). Byshunting or-
ganic matter to collector- gatherers, r- strategist Chironomidae larvae
effectswere undetected in mixed treatments suggesting that either
noeffectoccurredorstarved bluegillquicklymaskedanybottom-up
effects.Because juvenile bighead carp consumed the same foodre-
effects in a manner similar to other manipulative experiments (e.g.
Collins, Baxter,Marcarelli, & Wipfli, 2016). Comparison of this bot-
tom-up response between experimentsis difficult, as Chironomidae
densities were nearlyeight times greater in the bluegill experiment
relative to the common carp experiment (both pre and post).This
distinction highlights that the conditions of the food web are an im-
portantmediator offacilitation.Weacknowledgethat differencesin
fish feeding traits (bluegillvs. common carp), specifically their likeli-
hoodofexploitingbenthic orterrestrialprey,mayalsoinfluencethis
facilitation mechanism to some degree. Bluegill may have consumed
andinvertebrates(e.g.Brabrand,Faafeng,&Nilssen, 1990;Glaholt&
were shorter in duration than the growing season of juvenile fishes,
sustained exploitation of zooplankton and rotifers by bighead carp
wouldpresumablyhave similarimpacts onfish growththrough time.
Futurestudiesshouldaddresshowspecies interactionsandresource
partitioningmayalter ecosystemmetabolism,biomass turnover,and
energyflows among foodweb pathways(e.g.Collins etal., 2016) to
better characterise the bioenergetic basis that drives facilitation.
Functional Ecology
Response-surface designs can be logistically difficult; however, the
abilityto test for co-occurringpositiveandnegative mechanisms is
amajor strength. In a classic synthesis of interspecific competition
experiments, Connell (1983) described the prevalence of positive
interspecific responses in experiments otherwise focused on com-
petition. In many contexts, species thought to compete for limited
resources actually confer benefits to perceived competitors by al-
tering food web architecture. By accounting for the directionality of
species interactions within our experimental framework and track-
ingresponses of prey at lower trophic levels, we provide a clearer
understanding of how competitive forces act to influence facilita-
tionbetweenspecies.Bigheadcarp’s ability to filter small particles
fromfeedbackswithinthefoodweb.Yet,surpluspreywas shared
amongst bighead carp, and ultimately only low densities exhibited
enhanced growth. Ultimately, the magnitude of facilitation was con-
strainedbyfoodwebproductivityandthedensityofbighead carp
We specifically thank M. Diana, S. Butler, and M. Naninni for their
logistical, field, and laboratory assistance. We also thank members
oftheKaskaskia,RidgeLake,andSamParr Biologicalstations ofthe
Illinois Natural History Survey, as well as graduate students from the
University of Illinois for their intellectual discussions and feedback.
obtained before commencement of the study. All fishes were ac-
laws and regulations.
K.A.N., G.G.S., and D.H.W. conceived the ideas and designed the
experiments; K.A.N. collected the data; K.A.N. and S.F.C. analysed
thedata; K.A.N.andS.F.C. led thewritingof themanuscript.All au-
thors contributed critically to the drafts and gave final approval for
Data for this study are available through the University of Illinois: (Nelson, Collins, Sass,
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How to cite this article: Nelson KA, Collins SF, Sass GG,
facilitation between native and invasive juvenile fishes. Funct
... Nevertheless, their consumptive effects on phyto-and zooplankton can be substantial. By efficiently exploiting multiple planktonic resources (Collins and Wahl 2017), and by outcompeting native fishes (e.g., Schrank et al. 2003;Nelson et al. 2017), bighead carps have accumulated tremendous population biomass and constitute a large pool of organic matter within invaded riverine ecosystems. Like all biological invasions, exotic species introduce new and sometimes strong interactions with native organisms (Paine 1980). ...
... For instance, evidence from our own work found that bighead carp caused strong reductions in pools of organic matter within planktonic habitats. In turn, bighead carp shunted large quantities of organic matter from pelagic to benthic habitats in the form of egested fish waste (Collins and Wahl 2017) (Fig. 2b), which increased densities of larval midges in benthic sediments (Chironomidae; Collins and Wahl 2017;Nelson et al. 2017). Interestingly, these increases were not constrained to the aquatic environment. ...
Pervasive environmental degradation has altered biodiversity at a global scale. At smaller scales, species extirpations, invasions, and replacements have greatly influenced how ecosystems function and interact by affecting the exchanges of energy, materials, and organisms. In this chapter, we examine how a variety of environmental stressors, and associated species losses and gains, change the exchange of resources (materials or organisms) within and among ecosystems. We specifically consider how changes that occur within an ecosystem may trigger effects that reverberate (e.g., directly, indirectly, and via feedbacks) back and forth across ecological boundaries and propagate to other ecosystems connected via exchanges of materials and organisms. Our synthesis provides cursory overviews of ecosystem “openness” as it has been addressed by community ecologists and the conceptual development of ecological frameworks used to examine resource exchanges between ecosystems. We then describe four case studies and examine how species losses and gains affect food web structure via resource exchanges between ecosystems, with particular emphasis on effects spanning land-water boundaries. Finally, we discuss the need for more complex conceptual treatment of the interconnectedness of food webs among ecosystems.
... Several studies indicate that invasive carp can strongly affect the growth or condition of native species when there is strong diet overlap (e.g. DeBoer et al., 2018;Irons, Sass, McClelland, & Stafford, 2007;Nelson, Collins, Sass, & Wahl, 2017;Pendleton, Schwinghamer, Solomon, & Casper, 2017;Sampson, Chick, & Pegg, 2009;Schrank, Guy, & Fairchild, 2003). Notably, the interactions of bigheaded carps with native fishes have, to our knowledge, exclusively assessed juvenile or adult stages, whereas no study has assessed competitive interactions with larval life stages. ...
... Because of reduced growth rates resulting from competition, larval bluegill may be vulnerable to direct predation by native piscivores for longer periods of time (Persson, Andersson, Wahlstrom, & Eklov, 1996;Reznick, Bryga, & Endler, 1990). If so, bighead carp could facilitate native predators through their effects on fish larvae (Collins, Nelson, DeBoom, & Wahl, 2017;Nelson et al., 2017). By affecting the coupling of habitats via organism movements during early ontogeny, bighead carp may also indirectly disrupt predator-prey interactions of native taxa. ...
Full-text available
• Early life stages of fishes are sensitive to ecological and environmental disturbances and experience very high mortality rates. During early ontogeny, the growth and survival of larval fish often depends on food availability. Because habitat and diet shifts are strongly tied to individual body size, factors that influence the growth rates of individuals (e.g. resource limitation, competition) also affect other aspects of ontogeny including the timing of habitat or diet shifts. In the context of biological invasions, non‐native species can potentially disrupt the interaction of larval fish with their food via competition for shared prey, reducing growth and survival during a vulnerable period of an organism's life history. • We hypothesised that invasive planktivores negatively affect native species through the vulnerable larval life stage via competition for zooplankton resources. To test this hypothesis, we conducted a series of experiments to assess and contrast the effects of invasive (bighead carp, Hypophthalmichthys nobilis) and native (golden shiners, Notemigonus crysoleucas) planktivores on zooplankton densities, and their effects on the growth, survival, abundance, and habitat use of larval bluegill (Lepomis macrochirus). • Overall, the effects of the invasive planktivore were consistently greater than the native planktivore in terms of reduced prey densities, reduced bluegill growth rates, and delays to the timing of ontogenetic habitat shifts. Growth rates of bluegill larvae were reduced by 58–87% in the presence of bighead carp and 37% in the presence of golden shiners (relative to controls), but such reductions did not consistently lead to reduced survival (in mesocosm experiment) or relative abundance (in pond experiment). However, we estimated that bighead carp and golden shiners delayed ontogenetic habitat shifts in bluegill by 9–24 and 1–3 days, respectively. • Although we did not detect an effect of planktivores on the numbers of larval bluegill, our findings suggest that bighead carp may still affect bluegill ontogeny and freshwater food webs by disrupting the timing of ontogenetic habitat shifts. By affecting the coupling of habitats via organism movements during early ontogeny, bighead carp may indirectly disrupt predator–prey interactions of native taxa.
... The successful establishment and subsequent dominance and expansion of Bighead Carp Hypophthalmichthys nobilis and Silver Carp H. molitrix populations in the USA is an example of a biological invasion where little is known about interactions with potential predators (Zhang et al. 2016;Lampo et al. 2017). Competition between Hypophthalmichthys populations and native planktivores (Kolar et al. 2007;Sampson et al. 2009;Collins and Wahl 2017;Nelson et al. 2017) could reorganize prey communities available to native piscivores. Previous assessments of interspecific interactions between Hypophthalmichthys carps and native fishes have focused on competition for limited prey resources (e.g., Kolar et al. 2007;Sampson et al. 2009;Nelson et al. 2017). ...
... Competition between Hypophthalmichthys populations and native planktivores (Kolar et al. 2007;Sampson et al. 2009;Collins and Wahl 2017;Nelson et al. 2017) could reorganize prey communities available to native piscivores. Previous assessments of interspecific interactions between Hypophthalmichthys carps and native fishes have focused on competition for limited prey resources (e.g., Kolar et al. 2007;Sampson et al. 2009;Nelson et al. 2017). Few studies have focused on the susceptibility of Hypophthalmichthys to predation (Negonovskaya 1980;Wolf and Phelps 2017), and none, to our knowledge, have quantified predator foraging efficiency for either Silver or Bighead carps. ...
The establishment of Bighead Carp Hypophthalmichthys nobilis and Silver Carp Hypophthalmichthys molitrix throughout the Mississippi River basin potentially expands the prey base for native predators. A mechanistic understanding of interactions between non‐native prey and native predators is needed to assess the potential for predator regulation of Hypophthalmichthys carp populations and impacts on native predator assemblages. We conducted a series of experiments to quantify the selectivity and efficiency of Largemouth Bass Micropterus salmoides predation on juveniles of both of these Hypophthalmichthys species, and behaviors potentially influencing selectivity and efficiency. Selectivity was measured over 24 hours in 2‐m diameter pools with one of two prey assemblages consisting of three individuals each of three species: (1) Bighead Carp with native littoral (Bluegill Lepomis macrochirus) and pelagic prey (Golden Shiner Notemigonus crysoleucas) or (2) Bighead Carp, Silver Carp, and a morphologically similar native prey (Gizzard Shad Dorosoma cepedianum). Foraging efficiency and predator‐prey behaviors were quantified in 45‐minute trials where Largemouth Bass foraged on 10 individuals of a single prey species inside a 750‐L observation tank. All prey species were readily attacked and consumed by Largemouth Bass, with Silver Carp selected less than Gizzard Shad and Bighead Carp selected at a higher rate than any of the other prey species. Of the species tested, Bighead Carp formed the tightest schools and were captured most efficiently by Largemouth Bass. Overall, Hypophthalmichthys carps were similar to native prey in their vulnerability to Largemouth Bass; therefore, factors affecting Hypophthalmichthys carp availability relative to native prey may shape post‐invasion predator‐prey interactions. This article is protected by copyright. All rights reserved.
... Coarse woody habitat structures are a tool that is being employed to increase habitat complexity in impoverished or naturally structure-less areas, and serve as a nutrient source in the littoral zones (Czanercka 2015). Coarse woody habitat serves as a growing surface for periphyton and degrading CWH provides a direct nutrient input in its vicinity for macrophyte and invertebrate communities and ultimately the whole lake through habitat coupling and food web pathways (Nelson et al. 2017;Smokorowski et al. 2006;Vadeboncoeur & Lodge 2000;Ziegler et al. 2017). Thus, given the structural and nutrient effects of CWH structures, it would be expected that CWH would have an impact on macrophyte and benthic macroinvertebrate communities (Czarnecka 2015;Helmus & Sass 2008;Sass et al. 2019;Smokorowski et al. 2006). ...
Full-text available
Structural habitat enhancement has been long established as a popular tool to counter habitat loss due to land-use and development. One enhancement approach is the introduction of Coarse Woody Habitat (CWH) to improve the establishment of macrophyte, macroinvertebrate, and fish communities. Here we test for the effects of CWH in Northern boreal lakes in the context of mitigation projects. We constructed Coarse Woody Habitat structures in a structure-less littoral zone of Lake Steepbank within the Oil Sands Region of Alberta, Canada. Enhancement structures featured increased macrophyte and invertebrate richness and biomass compared to reference sites and pre-treatment assessments over three years. Enhanced sites also retained improved richness (macrophytes), diversity (macroinvertebrates), and biomass (both), despite STIN loss and degradation of enhancement structures over time. Using beta diversity components, constituting richness agreement, community differentiation, and site relationships, and testing their relative importance revealed that replacement was more dominant for invertebrates and increasing similarity more important for macrophyte communities post-enhancement. Our study shows the value of CWH addition for macroinvertebrate and macrophyte communities in what is otherwise a structure-less environment. Community changes over time showcase how beta diversity should be more strongly incorporated in restoration and enhancement studies to quantify community shifts that otherwise would not be captured in alternative diversity measures.
... In particular freshwater ecosystems are described as having strong trophic linkages and are uniquely susceptible to modification by invasive consumers (Carpenter et al., 1985;Strong et al., 1992;Gallardo et al., 2016). However, there is a growing body of work that presents a more balanced view of consumptive effects of aquatic invasive species, one in which there are both winners and losers following an invasion Nelson et al., 2017;Marcarelli et al., 2020;Albertson et al., 2021). Planktivorous filter feeding fish can have important consumptive effects when introduced outside of native ranges by disrupting recipient ecosystem food web dynamics (Lin et al., 2014;Gallardo et al., 2016). ...
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The ability of organisms to survive ingestion and digestion by their predators, or endozoochory, is a fascinating ecological phenomenon that can facilitate predator-mediated dispersal of prey and alter interaction strengths within ecological networks. However, the role of endozoochory in the context of invasive species is considered less often. Throughout the United States, Silver Carp (Hypophthalmichthys molitrix) are prolific invaders that often alter food web structure of recipient ecosystems through the consumption of basal resources. Despite the biogeochemical and food web effects of Silver Carp, there is limited understanding of plankton prey survival after Silver Carp consumption and digestion, and even less known about the ecological effects of selective diets and potential survival. In this study, we quantify hindgut contents of Silver Carp collected from Kentucky Lake, Kentucky, Tennessee River Valley, United States. We found the majority (83%) of phytoplankters within hindguts of Silver Carp showed little digestion prior to egestion. Our study suggests digestion limitations of Silver Carp may have important ecological implications for invaded environments. These results may be applicable in understanding how this rapidly spreading invasive fish can influence food web dynamics and biogeochemical cycles pertinent to toxic algal blooms within recently invaded ecosystems, and forecasting invasion in the near future.
... While GRP models are intended to describe relative habitat quality instead of predicting realized growth (Tyler and Brandt 2001)-which requires consideration of other ecological factors such as habitat selection, inter-and intraspecific competition, and predation (e.g. Nelson et al. 2017;Coulter et al. 2018a, b)-contextualizing BHC habitat quality with reported growth rates can better communicate the implications of increased nutrient loads on Lake Michigan's vulnerability to BHC. Furthermore, these results highlight the tight link between BHC habitat quality, primary production, and nutrient loads in phosphorus-limited systems like Lake Michigan (Shimoda et al. 2011;Warner and Lesht 2015;Rowe et al. 2017). ...
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Anthropogenic stressors that affect ecological processes in the Laurentian Great Lakes can impact their susceptibility to bioinvasions. Bighead Hypophthalmichthys nobilis and Silver Carp H. molitrix, collectively ‘bigheaded carps’ (BHC), are planktivorous fishes threatening to invade Lake Michigan. While previous studies indicate the lake contains habitat suitable for BHC growth, there is a need to understand how anthropogenic-driven changes to the abiotic and biotic environment could alter its vulnerability to BHC. We applied a spatially explicit model of BHC growth rate potential (GRP; g g−1 d−1) to nine biophysical model scenarios to evaluate changes in habitat suitability in Lake Michigan. Scenarios differed in meteorology (cool, reference, warm), annual tributary phosphorus loads (0, 3300, and 5600 MTA), and the presence/absence of invasive dreissenid mussels. Mussel effects on BHC GRP relied on their contact with the surface mixed layer (SML), the depth of which was affected by meteorology. The warm year advanced the expansion of Bighead Carp habitat by increasing temperature-dependent foraging rates and lessening the time of competitive interaction with mussels due to earlier stratification separating mussels from the SML. Phosphorus loads were the most influential driver of the lake’s suitability. Compared to present conditions, we estimate BHC could have grown an additional 8–40% annually in the 1980s when mussels were not in the lake and phosphorus loads were higher. Our study demonstrates how climate change and nutrient enrichment can increase Lake Michigan’s vulnerability to BHC by affecting thermal regime and productivity, thereby limiting negative effects of dreissenid mussels on BHC growth.
... It is plausible that the more recent invasion of silver and bighead carp increased the resilience of an invasive-dominant state through mutualistic interactions with common carp. For example, recent experiments have shown that bighead carp couple benthic-planktonic habitats via high egestion rates, which effectively directs more resources to benthic sediments to the apparent benefit of some benthic macroinvertebrates Nelson et al., 2017). If habitat coupling is strong, the forage base of common carp could increase (Fig. 5 -F4). ...
Regime shifts-persistent changes in the structure and function of an ecosystem-are well-documented for some ecosystems and have informed research and management of these ecosystems. In floodplain-river ecosystems, there is growing interest from restoration practitioners in ecological resilience, yet regime shifts remain poorly understood in these ecosystems. To understand how regime shifts may apply to floodplain-river ecosystems, we synthesize our understanding of ecosystem dynamics using an alternate regimes conceptual framework. We present three plausible sets of alternate regimes relevant to natural resource management interests within the Upper Mississippi River and Illinois River. These alternate regimes include: 1) a clear water and abundant vegetation regime vs. a turbid water and sparse vegetation regime in lentic, off-channel areas, 2) a diverse native fish community regime vs. an invasive-dominated fish community regime, and 3) a regime characterized by a diverse and dynamic mosaic of floodplain vegetation types vs. one characterized as a persistent invasive wet meadow monoculture. For each set of potential alternate regimes, we review available literature to synthesize known or hypothesized feedback mechanisms that reinforce regimes, controlling variables that drive regime transitions, and current restoration pathways. Our conceptual models provide preliminary support for the existence of alternate regimes in floodplain-river ecosystems. Quantitatively testing hypotheses contained within the conceptual model are important next steps in evaluating the model. Ultimately, the synthesis and evaluation of alternate regimes can inform the utility of resilience concepts in restoration and management on the Upper Mississippi River and Illinois River and improve our understanding of ecosystem dynamics in other large, heavily managed floodplain-river ecosystems.
... Bigheaded carps were introduced to southern portions of the Mississippi River, USA watershed in the 1970s and now threaten to invade the Laurentian Great Lakes via the Illinois River, as well as the headwaters of the Mississippi River. These fishes threaten aquatic food web pathways [26,27], negatively affect commercially-and recreationally-harvested native fishes [28,29], dominate fish communities [30], and negatively affect recreation [31]. Intensive management and control efforts are being directed against these species, including the use of eDNA detection rates as a surveillance tool in uninvaded areas [32]. ...
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Although environmental DNA (eDNA) is increasingly being used to survey for the presence of rare and/or invasive fishes in aquatic systems, the utility of this technique has been limited by a poor understanding of whether and how eDNA concentrations relate to fish density, especially in rivers. We conducted a field study to systematically test whether the eDNA released by a model invasive fish, Silver Carp (Hypophthalmichthys molitrix), was related to the density of this species in a large river. We quantified fish density throughout the 460 km long Illinois River using hydroacoustic surveys at 23 sites while concurrently collecting 192 surface water samples for eDNA analysis. We found that Silver Carp numerical density and biomass density were positively and non-linearly related to eDNA concentration and detection rate. Both eDNA concentration (copy number) and detection rate increased rapidly as Silver Carp density increased but plateaued at moderate densities. These relationships could prove useful for estimating Silver Carp relative abundance in newly invaded locations where population numbers are low to moderate. Future studies should explore the causes of this nonlinear relationship as it would ultimately benefit aquatic species monitoring and management programs.
... In the sympatric treatments, however, the smallest isotopic niche sizes occurred when conspecifics were at n = 6, not at n = 4, contrary to theory (Svanbäck & Bolnick, 2006). Correspondingly, the interaction of reduced intra-and interspecific competition in the all-species treatment might have been positively interacting to facilitate the niche expansions (Nelson, Collins, Sass, & Wahl, 2017). Alternatively, in the all-species treatment, the species-pair direct effects that were apparent in the species-pair sympatric treatments might have been buffered by indirect effects (Calizza et al., 2017;David et al., 2017). ...
1.Ecological theory on the trophic impacts of invasive fauna on native competitors is equivocal. While increased inter‐specific competition can result in coexisting species having constricted and diverged trophic niches, the competing species might instead increase their niche sizes to maintain energy intakes. Empirical experiments can test invasion theory on competitive interactions and niche sizes across different spatial scales and complexity. 2.The consequences of increased inter‐specific competition from a model alien fish Leuciscus idus were tested on two taxonomically and trophically similar native fishes, Squalius cephalus and Barbus barbus. Competitive interactions were tested in tank aquaria using comparative functional responses (CFRs) and cohabitation trials. The consequences of these competitive interactions for the trophic niche sizes and positions of the fishes were tested in pond mesocosms. 3.CFRs revealed that compared to B. barbus, L. idus had significantly higher attack and consumption rates; cohabitation trials revealed B. barbus growth rates were depressed in sympatry with L. idus. For L. idus and S. cephalus, differences in their functional response parameters and growth rates were not significant. 4.Pond mesocosms used stable isotope metrics to quantify shifts in the trophic niche sizes of the fishes between allopatry and sympatry using a substitutive experimental design. Isotopic niches were smaller and more divergent in sympatric paired species than predicted by their allopatric treatments, suggesting trophic impacts from inter‐specific competition. However, an all‐species sympatric treatment revealed similar niche sizes with allopatry. This maintenance of niche sizes in the presence of all species potentially resulted from the buffering of direct competitive effects of the species‐pairs by indirect effects. 5.Experimental predictions from tank aquaria assisted the interpretation of the constricted and diverged trophic niches detected in the paired‐species sympatric treatments of the pond mesocosms. However, the all‐species sympatric treatment of this experiment revealed greater complexity in the outcomes of the competitive interactions within and between the species. These results have important implications for understanding how alien species integrate into food webs and influence the trophic relationships between native species. This article is protected by copyright. All rights reserved.
... Silver Carp may have negative impacts on native fishes , DeBoer et al. 2018, Irons et al. 2007, Pendleton et al. 2017, Varble et al. 2007) indirectly through bottom-up cascading effects of phytoplanktivory on the composition and/ or biomass of the zooplankton community, or by direct consumption of zooplankton prey. Results of mesocosm experiments (e.g., Collins and Wahl 2017, Nelson et al. 2017, Schrank et al. 2003, long-term monitoring (De-Boer et al. 2018, Phelps et al. 2017, Sass et al. 2014, Tumolo and Flinn 2017, and observations by anglers (Upholt 2017) showed that invasive carp may catalyze reorganization of river foodwebs. ...
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Bighead carp (Hypophthalmichthys nobilis) are an invasive planktivore that can greatly deplete planktonic resources. Due to the inefficient conversion of food into fish tissue, large portions of consumed materials are egested and shunted to benthic habitats. We explored how bighead carp alter pools of organic matter between planktonic and benthic habitats, and across ecosystem boundaries. Here, we report evidence from a manipulative experiment demonstrating that bighead carp greatly reapportion pools of organic matter from planktonic to benthic habitats to such a degree that additional effects propagated across ecological boundaries into terrestrial ecosystems. Strong direct consumption by bighead carp reduced filamentous algae, biomass and production of zooplankton, and production of a native planktivorous fish within planktonic habitats. Reduced herbivory indirectly increased phytoplankton (chlorophyll a). Direct consumption of organic matter by bighead carp supported high carp production and concomitant losses of materials due to egestion. Perhaps in response to organic matter subsidies provided by fish egestion, ponds having bighead carp had higher standing crop biomass of Chironomidae larvae, as well as cross-boundary fluxes of their adult life stage. In contrast, we detected reduced cross-boundary fluxes of adult Chaoboridae midges in ponds having bighead carp. Consideration of bighead carp as mediators of organic matter exchanges provides a clearer framework for predicting the direct and extended impacts of these invasive planktivores in freshwater ecosystems. The perception of bighead carp must evolve beyond competitors for planktonic resources, to mediators and processors of nutrients and energy within and across ecosystems.
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Effective management and monitoring programs require confidence regarding basic biological sampling. Gear comparisons are often required to determine the most effective techniques. Such is the case for populations of invasive Asian carps Hypophthalmichthys spp., which have recently occurred in large numbers throughout sections of the Mississippi River basin. We tested five gears (mini-fyke nets, beach seine, purse seine, pulsed-DC electrofishing, and gill net) that targeted juvenile (age 0) Silver Carp H. molitrix at sites along the Illinois River during 2014 and 2015 to determine the most effective ones for age-0 Silver Carp. We considered the most cost-effective gear to be the one that provided the largest catch at a minimal expenditure of labor. Mini-fyke nets were the most effective at collecting large numbers of age-0 Silver Carp, followed in decreasing order by beach seines, pulsed-DC electrofishing, purse seines, and gill nets. The smallest Silver Carp were caught in beach seines and the largest were caught in gill nets, and there was considerable variation in size distributions among gears. However, when we considered cost-effectiveness in terms of labor hours for each gear, both beach seines and mini-fyke nets had similar and overlapping labor expenditures. Gill nets and purse seines were not cost-effective, as they required more labor and had lower overall catch rates. Received May 23, 2016; accepted September 19, 2016
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Abstract Artificial additions of nutrients of differing forms such as salmon carcasses and analog pellets (i.e. pasteurized fishmeal) have been proposed as a means of stimulating aquatic productivity and enhancing populations of anadromous and resident fishes. Nutrient mitigation to enhance fish production in stream ecosystems assumes that the central pathway by which effects occur is bottom‐up, through aquatic primary and secondary production, with little consideration of reciprocal aquatic‐terrestrial pathways. The net outcome (i.e. bottom‐up vs. top‐down) of adding salmon‐derived materials to streams depend on whether or not these subsidies indirectly intensify predation on in situ prey via increases in a shared predator or alleviate such predation pressure. We conducted a 3‐year experiment across nine tributaries of the N. Fork Boise River, Idaho, USA, consisting of 500‐m stream reaches treated with salmon carcasses (n = 3), salmon carcass analog (n = 3), and untreated control reaches (n = 3). We observed 2–8 fold increases in streambed biofilms in the 2–6 weeks following additions of both salmon subsidy treatments in years 1 and 2 and a 1.5‐fold increase in standing crop biomass of aquatic invertebrates to carcass additions in the second year of our experiment. The consumption of benthic invertebrates by stream fishes increased 110–140% and 44–66% in carcass and analog streams in the same time frame, which may have masked invertebrate standing crop responses in years 3 and 4. Resident trout directly consumed 10.0–24.0 g·m−2·yr−1 of salmon carcass and
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We compared three entrapment gears to determine which method was the most effective for capturing invasive Bighead Carp Hypophthalmichthys nobilis and Silver Carp H. molitrix in terms of numbers of fish captured and labor invested. Gears were deployed concurrently in two backwater lakes of the Illinois River during the summers of 2012–2014. Overall, the nightly catch rates of all fishes, Bighead Carp, and Silver Carp were one to three orders of magnitude greater in pound nets than in either fyke nets or hoop nets. Pound nets collected larger Bighead Carp than hoop nets and fyke nets. Hoop nets were ineffective at catching Asian carp in backwater lakes. Estimation of the effort required to deploy, maintain, and remove each gear type indicated that pound nets were the most cost effective gear due to their high catch rates of Asian carp relative to the labor hours invested to collect the catch. Pound nets appear to be an effective means of removing Asian carp in backwater lake habitats of the Illinois River.