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Invasive planktivores as mediators of organic matter exchanges within and across ecosystems


Abstract and Figures

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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Oecologia (2017) 184:521–530
DOI 10.1007/s00442-017-3872-x
Invasive planktivores as mediators of organic matter exchanges
within and across ecosystems
Scott F. Collins1 · David H. Wahl1
Received: 18 July 2016 / Accepted: 15 April 2017 / Published online: 27 April 2017
© Springer-Verlag Berlin Heidelberg 2017
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 com-
petitors for planktonic resources, to mediators and proces-
sors of nutrients and energy within and across ecosystems.
Keywords Asian carp · Invasive species · Cross-boundary
flux · Insect emergence · Indirect effect
Unraveling the complex processes that structure food webs
has historically been addressed under two epistemologi-
cal approaches: an individualistic and community-based
approach that emphasizes the organism, population, or com-
munity as the central foci and a holistic ecosystem-based
approach concerned with the fluxes and exchanges of energy
and materials (Pickett et al. 2007; Moore and de Ruiter 2012).
Bridging these sub-disciplines to address pressing ecological
issues such as biological invasions can be powerful, as dif-
fering perspectives can better inform questions of organisms
and their impacts on ecological processes (Jones and Law-
ton 1995; Loreau and Holt 2004). Food web ecologists have
expanded the traditional adage, who-eats-whom, to further
consider the flows and fluxes of materials through interacting
species and across trophic levels (Lindeman 1942; Moore and
de Ruiter 2012), which invites the question, what of the con-
sumed? For instance, when a predator consumes prey, there
is a reduced pool of prey, but the constituent materials of the
prey (i.e., nutrients, organic matter) are not immediately lost
to the ecosystem; rather, they are re-packaged as assimilated
consumer biomass, respired, or expelled as physiological
Abstract Bighead carp (Hypophthalmichthys nobilis)
are an invasive planktivore that can greatly deplete plank-
tonic 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 plank-
tonic and benthic habitats, and across ecosystem bounda-
ries. Here, we report evidence from a manipulative experi-
ment 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 produc-
tion of a native planktivorous fish within planktonic habi-
tats. Reduced herbivory indirectly increased phytoplankton
(chlorophyll a). Direct consumption of organic matter by
bighead carp supported high carp production and concomi-
tant 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
Communicated by Robert O. Hall.
Electronic supplementary material The online version of this
article (doi:10.1007/s00442-017-3872-x) contains supplementary
material, which is available to authorized users.
* Scott F. Collins
1 Illinois Natural History Survey, Kaskaskia Biological Station,
Sullivan, IL, USA
522 Oecologia (2017) 184:521–530
1 3
byproducts (i.e., egesta, excreta). With respect to the latter,
the inefficient acquisition of nutrients and energy by consum-
ers can have considerable impacts on food webs (e.g., Wotton
and Malmqvist 2001; Vanni 2002).
Biological invasions are stressors to ecosystem pro-
cesses (Crooks 2002), and are an integral driver of global
environmental change (Vitousek et al. 1997). Invasive spe-
cies can greatly alter their surroundings by exploiting lim-
iting resources (e.g., Kennedy and Hobbie 2004; Baxter
et al. 2004), by altering nutrient cycles (e.g., Collins et al.
2011), and changing organic matter budgets (e.g., Mineau
et al. 2012), to name a few. Although the negative effects
of exotic species have been described for individuals and
whole ecosystems alike, simultaneous investigations of
impacts across biological levels of organization are rare
(Simon and Townsend 2003). Understanding the mecha-
nisms through which biological invasions alter ecological
processes (e.g., trophic dynamics, energy flows) should
aid in predicting the range of direct, indirect, positive, and
negative effects that occur within invaded habitats as well as
in adjacent habitats connected through material exchanges
(Polis et al. 1997; Reiners and Driese 2001). Unfortunately,
few studies actually test the processes or pathways through
which impacts occur, which is a critical shortcoming of bio-
logical invasion research (reviewed in Levine et al. 2003).
Bigheaded carp (Hypophthalmichthys spp.) is a collec-
tive name designated to the species bighead carp (H. nobi-
lis) and silver carp (H. moltrix). These fishes are highly pro-
ductive planktivores whose northward expansion through
the Mississippi River basin poses a threat to the Laurentian
Great Lakes ecosystem (Chick and Pegg 2001). Extremely
high densities of bigheaded carp have been observed in
river–floodplain ecosystems, with estimates of 5500 kg wet
mass river km1 of silver carp in the Illinois River (Sass
et al. 2010) and nightly catches in some floodplain lakes
in excess of 1000 kg wet mass (Collins et al. 2015b). Con-
siderable quantities of organic matter must be consumed
to produce and sustain the observed levels of fish biomass.
Bigheaded carp efficiently filter plankton particles from the
water column (e.g., phyto-, zooplankton; Radke and Kahl
2002; Sass et al. 2014), which removes a food resource for
some native fishes (Irons et al. 2007; Sampson et al. 2009).
To date, the direct effects of these invasive fishes within
planktonic habitats are generally appreciated (i.e., who eats
whom?). What remains unclear are the attendant effects
of bigheaded carp in other aquatic habitats that are linked
through the exchange of organic matter (i.e., what of the
Migratory fishes are vectors and mediators of material
exchanges in river–floodplain ecosystems (e.g., Flecker
et al. 2010). The quantities of resources consumed by
mobile fishes and the fates of these resources (i.e., assim-
ilated as biomass, egested) may have impacts in adjacent
habitats. Consumed but unassimilated materials from fishes
like bigheaded carp can subsidize organisms in benthic
habitats via the sedimentation of egested particles (Polis
et al. 1997; Wotton and Malmqvist 2001; Schindler and
Scheuerell 2002). These effects can further propagate into
adjacent ecosystems as organisms mature or redistribute.
Other non-native fishes also alter the cross-boundary move-
ments of aquatic insects (Baxter et al. 2004; Epanchin et al.
2010; Collins et al. 2016a) and amphibians (Bradford 1989;
Knapp et al. 2001). Because studies have largely focused
on planktonic responses to bigheaded carp invasions, it
remains unclear how these fishes impact the coupling of
habitats (e.g., benthic-pelagic, aquatic-terrestrial).
Here, we present the results of a manipulative experi-
ment that examined how an invasive planktivore altered
pools and fluxes of organic matter between habitats (i.e.,
benthic, planktonic) and across ecosystem boundaries (i.e.,
aquatic to terrestrial ecosystems). We report evidence dem-
onstrating 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. Most notably, these
cross-boundary impacts indicate that the effects of big-
headed carp may not be constrained to aquatic ecosystems.
Hypotheses and study design
By consuming, assimilating, and egesting organic matter,
bigheaded carp may play an integral role influencing food
web dynamics. We tested the hypothesis that bighead carp
alter material exchanges between planktonic and benthic
habitats within aquatic ecosystems, with effects that prop-
agate from aquatic to terrestrial ecosystems. We predicted
strong and negative direct effects of bigheaded carp on
planktonic food resources, which would reduce the produc-
tion of native fishes. Furthermore, increased production of
bighead carp and corresponding contributions of egested
particles would subsidize benthic habitats. Thus, we pre-
dicted benthic Chironomidae larvae and channel catfish
would respond positively. Finally, emergence of aquatic
insects serves as a flux of material from aquatic to terres-
trial ecosystems. We predicted that egested material by big-
head carp indirectly increases export of organic matter from
aquatic to terrestrial ecosystems via adult insect emergence.
We tested these hypotheses with an additive experi-
ment, which is recommended for evaluating the effects of
non-native fishes (Fausch 1998). The experiment lasted
3 months and was conducted during June–September of
2014 in earthen ponds (0.04 ha. wetted area; clay-lined sed-
iment; 1.5–1.75 m water depths) at the Sam Parr Biological
523Oecologia (2017) 184:521–530
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Station, Kinmundy IL, USA. Ponds had vegetation that
extended 1.5 m from the wetted edges. Ten ponds were
filled 5 weeks prior to start of the experiment, at random,
with filtered water (300 μm sieve to remove larval fishes)
from Forbes Lake (UTM: 38.726099, 88.779831) to
ensure similar inoculums of plankton communities and
to allow for colonization by benthic macroinvertebrates.
Water temperatures averaged (±SD) 21.9 ± 1.1 °C dur-
ing the experiment and did not differ between treatments
(ANOVA, F1, 9 = 0.44, p = 0.76). The additive press design
sought to compare how a highly productive planktivore act-
ing as a stressor within an invaded ecosystem directly and
indirectly alters pools and fluxes of organic matter with an
aquatic environment and across ecosystem boundaries rela-
tive to a native fish community. Henceforth, we use a native
versus invaded dichotomy when describing the effects of
bighead carp, where native fish assemblages are controls
and ponds of native fishes plus bighead carp are invaded.
For this experiment, we distinguish benthic from planktonic
habitats within ponds, and refer to cross-boundary fluxes of
organic matter from aquatic to terrestrial ecosystems.
All ten ponds were stocked with juvenile native fishes
with functional traits representative of floodplain lake habi-
tats of large river ecosystems, and included representative
taxa of the Illinois River and the upper Mississippi River
ecosystem. Native fishes encompassed a range of func-
tional traits, including benthic predators (Channel catfish
Ictalurus punctatus), obligate planktivores (Golden shiner
Notemigonus crysoleucas), and taxa that forage on both
benthic and planktonic invertebrates (Largemouth bass
Micropterus salmoides; Red shiner Cyprinella lutrensis).
Field surveys by Illinois Natural History Survey biologists
determined that juvenile bigheaded carp can dominate (65–
98%) juvenile fish assemblages in floodplain lakes of the
Illinois River (Collins et al. 2017). Juvenile bighead carp
(average ± SD, 7.62 ± 1.78 g) were stocked at a density
of 0.42 fish m2 in five randomly selected ponds. Bighead
carp were obtained from a regional commercial hatchery
(Osage Catfisheries, Inc., Osage Beach, MO, USA). Big-
head carp abundance comprised 58% of the fish assem-
blage of invaded ponds.
Food web sampling and analyses
We quantified changes in dominant organic matter pools
within the water column over the duration of the experi-
ment. Algal communities were sampled monthly as float-
ing filamentous algae and phytoplankton. Filamentous
algae were sampled at random from the top 5 cm of the
water column within floating quadrats (0.5 × 0.5 m float-
ing frame, 2 subsamples per pond). Samples were dried
and weighed to the nearest 0.1 g. Strong winds pushed all
filamentous algae into the vegetated margins of the ponds
near the end of the experiment. Therefore, only a 2-month
response for filamentous algae is reported. Phytoplankton
was quantified monthly, based on chlorophyll a concentra-
tions from a depth-integrated water sample. Water was fil-
tered through 0.7 μm glass fiber filters. Chlorophyll a was
extracted with 90% acetone for 24 h, and then measured
via fluorometry (Turner Design, model TD700, Sunnyvale,
California, USA; APHA 2005). Turbidity (nephelometric
units, NTU; electronic turbidimeter) was assessed from 1-L
samples collected at the midpoint of the water column.
Zooplankton samples were collected monthly at three
random locations within each pond from a depth-integrated
water sample. Samples were filtered through a 20-μm
sieve, and then stored in Lugol’s solution. Zooplankton
samples were counted and identified to Family. Subsets of
50 individuals were measured using an optical micrometer
to determine body length. Taxa-specific body lengths were
used as input in length–mass regressions to obtain biomass
(dry mass; McCauley 1984). Daily zooplankton produc-
tion was estimated with a regression model for freshwater
invertebrates (Plante and Downing 1989; R2 = 0.79). We
estimated production as per unit area to make compari-
sons across trophic levels in similar units. Exploitation
efficiency was calculated as the amount of zooplankton
production consumed by fishes (see below) relative to the
amount produced, to examine the top-down influence of
fish predation.
To estimate how much bighead carp subsidized benthic
habitats, macroinvertebrate larvae were sampled monthly.
For each sample period, offshore habitats (subsam-
ples) >1 m deep were sampled within each pond using a
15 × 15 × 15 cm Ekman Bottom Grab (Wildco, Wildlife
Supply Co.). In the laboratory, larvae were separated from
detritus, identified to Order, and measured to the nearest
0.5 mm. Biomass was calculated from length to weight
relationships (Benke et al. 1999).
Survival and growth rates for each fish species were
used to estimate secondary production (g m2 day1) dur-
ing the experiment. Survival was determined as the num-
ber and percent of individuals remaining at the conclusion
of the experiment relative to the numbers stocked. Growth
rates (g day1) were quantified as the difference between
the initial and final mean weights of all individuals over
the duration of the experiment. Gut contents were sampled
monthly from a minimum of five individuals (collected
via seining) of each species via gastric lavage. Proportions
of fish gut contents were identified and grouped into four
dominant pools of organic matter: (1) amorphous materi-
als including phytoplankton cells, particulate matter, and
other unidentifiable materials; (2) filamentous algae; (3)
macroinvertebrates; (4) zooplankton. To quantify path-
ways of organic matter flow through fishes and how they
were affected by the presence of bighead carp, we used the
524 Oecologia (2017) 184:521–530
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trophic basis of production approach (Benke and Wallace
1980) that accounts for both the quality and quantity of a
diet item in its contribution to production. Fish production
and gut content proportions were used to quantify the pro-
duction attributable to diet items consumed, the total quan-
tity of each item consumed, and the total egested materials
(Benke and Wallace 1980; All equations in S1). Proportions
of diet items consumed during our experiment were aver-
aged for each sample period for each pond. The portion of
production attributed to a given diet item (Eq. 1; Fi) was
calculated as:
where Gi is the proportion of food type i in the consumers
diet, AEi is the assimilation efficiency of food type i, and
NPE is the net production efficiency (Cross et al. 2011).
Assimilation efficiencies for fishes were: amorphous detri-
tus 0.3; filamentous algae 0.33; macroinvertebrates 0.75;
zooplankton 0.95 (Benke and Wallace 1980; Chen et al.
1985). The net production efficiency of age-0 fish was 0.24,
which is consistent with other studies examining the con-
sumptive effects of juvenile fishes (Cross et al. 2011; Col-
lins et al. 2016b).
To determine the relative contribution of each diet item
to fish production (Eq. 2; PFij) for each sampling interval,
we used
where Pj is the total sum of production estimates for each
fish species. The total consumption of food resources by
fishes during the experiment for each food type i to species
j was calculated by dividing PFij by the product of AEi and
NPE. Finally, we estimated the flux of egested organic mat-
ter (Eq. 3; Ej) to benthic habitats by species, as:
These methods allowed us to isolate the contributions of
bighead carp egestion from other fishes, and also were use-
ful because shallow ponds are poorly suited for sediment
traps because of wind-driven re-suspension of materials
(Kozerski 1994).
Finally, to test the indirect effect of bighead carp on
cross-boundary fluxes of organic matter, we measured the
emergence rates (mg DM m2 week1) of adult aquatic
insects with cylindrical sticky traps suspended under a
plastic enclosure. The bottom edge of each enclosure
was submerged below the surface of the water to ensure
a closed environment and allowed us to quantify flux per
unit area. Transparency sheets were constructed into cylin-
ders (0.062 m2), and coated in Tanglefoot resin. Traps were
ij =
AEij ×NPEij
deployed for 7 days, and then were replaced, ensuring con-
tinuous sampling during the experiment. In the laboratory,
adult insects were counted. Subsets of individuals (20 per
pond per sample period) were measured to determine aver-
age length. Length–weight regressions were used to esti-
mate biomass (Sabo et al. 2002).
Direct and indirect effects of bighead carp were ana-
lyzed using analysis of variance (ANOVA), with treatment
as the fixed factor and fish production, benthic macroin-
vertebrate biomass, zooplankton biomass and production,
chlorophyll a, total egestion, and insect emergence as
response variables. Because ecological effects may require
time to manifest, the treatment × time interaction was used
to assess the responses of food web components to detect
any lagged effects of bighead carp. For all statistical tests, p
values <0.05 were considered significant. All response vari-
ables were log10 transformed to correct for any non-nor-
mality of residuals and heteroscedasticity. Analyses were
conducted using SAS v.9.3 (SAS Institute, Cary, North
Carolina, USA).
Reduced pools of organic matter within the water col-
umn coincided with high rates of bighead carp produc-
tion. Bighead carp accrued 4.35 ± 0.36 g DM m2 (aver-
age ± SE) of biomass during the experiment, with survival
averaging 90% ± 0.03. Bighead carp reduced standing
crop biomass of free-floating filamentous algae by 91%,
relative to controls after 2 months (Fig. 1a; F1, 9 = 95.76,
p < 0.001). Filamentous algae averaged about 26.8% of
the diets of bighead carp. Bighead carp consumed an esti-
mated 4.09 g DM m2 of filamentous algae throughout
the experiment (Table 1). Zooplankton constituted 22.1%
of bighead carp diets. Direct predation of zooplankton by
bighead carp resulted in an 88% reduction in zooplankton
standing crop biomass (F1, 9 = 147, p < 0.001) and a 61%
reduction in zooplankton secondary production (Fig. 1b;
F1, 9 = 13.87, p = 0.005), relative to controls. Bighead carp
consumed an estimated 0.05 ± 0.002 g DM m2 day1
of zooplankton, and a total of 5.31 ± 0.32 g DM m2 of
zooplankton production over the duration of the experi-
ment. Exploitation efficiency of zooplankton production by
bighead carp averaged 64.3 ± 5.1% (Table 2). Top-down
control of zooplankton by bighead carp was also associated
with increased chlorophyll a concentrations (Fig. 1c; F1,
9 = 4.43, p = 0.06). However, turbidity (abiotic and biotic
particles) did not differ between treatments (F1, 9 = 0.67,
p = 0.48).
Exploitation of food resources in planktonic habitats
by bighead carp corresponded with reduced production of
the total native fish assemblage (F1, 9 = 4.76, p = 0.06);
525Oecologia (2017) 184:521–530
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a pattern driven by golden shiners, whose production
was 54% lower in ponds with bighead carp (Fig. 1d; F1,
9 = 10.01, p = 0.01). Strong reductions in zooplankton
by bighead carp resulted in fewer zooplankton being con-
sumed by golden shiners and ultimately less production
(Table 1). No effects of bighead carp were detected for
the production of largemouth bass (Fig. 1d; F1, 9 = 0.79,
p = 0.39). Red shiners experienced high mortality rates in
all ponds and accurate estimates of production could not be
Bighead carp shunted a sizeable portion of unassimi-
lated organic matter from planktonic to benthic habitats.
We estimated that total egestion by all fish species was
425% greater in ponds with ponds invaded by bighead
carp, relative to controls (Fig. 2a; F1, 9 = 97.2, p < 0.001).
Bighead carp egested 16.03 DM g m2 of organic matter
(i.e., sum of all diet categories) over the 3-month experi-
ment. Reduced production of golden shiners also resulted
in lower rates of egestion in ponds with bighead carp (F1,
9 = 9.57, p = 0.014). Though egestion by golden shiners
was reduced in ponds with bighead carp, the net impact
was positive, as contributions by bighead carp exceeded
losses by golden shiners. Egestion did not differ between
ponds for largemouth bass or channel catfish (p > 0.05).
Larval Chironomidae had 90% higher biomass in
ponds with bighead carp relative to controls (Fig. 2b, F1,
26 = 4.70, p = 0.04), which was consistent with our pre-
diction. However, no differences were observed for
Trichoptera, Odonata, or Ephemeroptera (p > 0.05). In con-
trast to our predictions, production of channel catfish (ben-
thic predators) did not respond to increased egested mate-
rial (Fig. 1d; F1, 9 = 2.13, p = 0.18).
Impacts of bighead carp on cross-boundary fluxes of
aquatic insects varied. Emergence of adult Chironomidae
midges across the aquatic-terrestrial boundary increased by
228% in ponds with bighead carp (Fig. 3a, F1, 9 = 49.73,
p < 0.001). In contrast, average weekly emergence of
Chaoboridae was reduced by 79% in ponds with bighead
carp (Fig. 3b, F1, 9 = 23.18, p < 0.001). No changes were
observed for Culicidae, nor for the Orders Trichoptera,
Ephemeroptera, or Odonata (p > 0.05; Fig. 3c–f).
Bighead carp were associated with reduced filamentous
algae, reduced biomass and production of zooplankton,
and reduced production of a native planktivore. In addi-
tion, emergence of adult Chaoboridae midges was reduced
whereas Chironomidae midges increased relative to con-
trols. In some cases, the effects of bighead carp were visu-
ally striking when compared to controls (Fig. 4). Reduc-
tions in pools of organic matter within planktonic habitats
were accompanied by a high degree of carp secondary pro-
duction and egestion of consumed materials. We detected
positive responses in benthic habitats which propagated
Fig. 1 Effects of bighead
carp on standing crop biomass
or secondary production of
major organic matter pools
within water columns of
experimental ponds. Values are
mean ± standard error (n = 5).
a Standing crop biomass of
filamentous algae on the surface
on ponds after 2 months. b
Secondary production of the
total zooplankton community. c
Phytoplankton responses based
on changes in chlorophyll a
concentrations within water col-
umn. d Secondary production
of native golden shiner (GOS),
largemouth bass (LMB), and
channel catfish (CCF) within
experimental ponds. Letters
above columns denote signifi-
cant treatment effects. Asterisk
on panel (d) indicates species-
specific significant differences
between treatment and control
1.2 a
Fish secondary production
(g DM
m-2 3-mo-1)
Control Invaded
Control Invaded
Chlorophyll a
(μg L-1)
Zooplankton secondary production
(g DM m-2 3-mo-1)
Filamentous algae biomass
(g DM m-2)
(a) (b)
526 Oecologia (2017) 184:521–530
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Table 1 Comparison of total consumption of dominant organic matter pools and corresponding contributions of diet items to fish secondary
production (assimilated) and egestion of unassimilated organic matter by fish assemblages in native controls vs. invaded ecosystems
Due to high mortality rates in treatment and controls, red shiners were not evaluated. All values are reported mean ± standard error, g m2 dry
Response Species Treatment Amorphous materials Filamentous algae Macroinvertebrates Zooplankton
Total consumption Bighead carp Control
Invaded 17.81 ± 2.81 4.09 ± 0.41 2.2 ± 0.34 5.31 ± 0.21
Golden shiner Control 0.13 ± 0.03 0.01 ± <0.01 <0.01 ± <0.01 0.61 ± 0.06
Invaded 0.06 ± 0.02 <0.01 ± <0.01 <0.01 ± <0.01 0.28 ± 0.08
Largemouth bass Control 0.78 ± 0.09 0.03 ± <0.01 1.25 ± 0.06 0.98 ± 0.04
Invaded 0.79 ± 0.1 0.02 ± <0.01 1.1 ± 0.24 0.87 ± 0.13
Channel catfish Control 2.24 ± 0.18 0.1 ± 0.01 5.89 ± 0.41 0 ± 0
Invaded 2.44 ± 0.35 0.09 ± 0.01 4.87 ± 0.4 0 ± 0
Contribution to fish production Bighead carp Control
Invaded 1.28 ± 0.2 0.32 ± 0.03 0.4 ± 0.06 1.21 ± 0.05
Golden shiner Control 0.01 ± <0.01 <.01 ± <0.01 0.01 ± <0.01 0.14 ± 0.01
Invaded 0.01 ± <0.01 0.01 ± <0.01 0.01 ± <0.01 0.06 ± 0.02
Largemouth bass Control 0.06 ± 0.01 <.01 ± <0.01 0.22 ± 0.01 0.22 ± 0.01
Invaded 0.06 ± 0.01 <.01 ± <0.01 0.2 ± 0.04 0.2 ± 0.03
Channel catfish Control 0.16 ± 0.01 0.01 ± <0.01 1.06 ± 0.07 0 ± 0
Invaded 0.18 ± 0.03 0.01 ± <0.01 0.88 ± 0.07 0 ± 0
Total egestion Bighead carp Control
Invaded 12.47 ± 1.97 2.74 ± 0.28 0.55 ± 0.09 0.27 ± 0.01
Golden shiner Control 0.09 ± 0.02 0 ± 0 <.01 ± <0.01 0.03 ± <0.01
Invaded 0.04 ± 0.01 0 ± 0 <.01 ± <0.01 0.01 ± <0.01
Largemouth bass Control 0.55 ± 0.06 0.02 ± <0.01 0.31 ± 0.02 0.05 ± <0.01
Invaded 0.56 ± 0.07 0.01 ± <0.01 0.27 ± 0.06 0.04 ± 0.01
Channel catfish Control 1.57 ± 0.13 0.07 ± 0.01 1.47 ± 0.1 0 ± 0
Invaded 1.71 ± 0.24 0.06 ± 0.01 1.22 ± 0.1 0 ± 0
Table 2 Estimated production (g m2) and consumption (g m2) of zooplankton by bighead carp and the native fish assemblage over the dura-
tion of the 12-week experiment during summer 2014
SE = standard error. Exploitation efficiency = consumption of prey: prey production
Treatment Pond Estimated zooplank-
ton production
Consumption of zoo-
plankton by bighead
Exploitation effi-
ciency by bighead
carp (%)
Consumption of
zooplankton by all
Exploitation efficiency
by all fishes (%)
Control 2 23.40 1.33 5.7
3 23.96 1.51 6.3
4 19.45 1.68 8.6
8 19.75 1.73 8.8
9 23.09 1.71 7.4
Mean ± SE 21.9 ± 1.0 1.6 ± 0.1 7.4 ± 0.6
Invaded 1 7.34 5.25 71.5 6.25 85.1
5 7.44 4.75 63.9 5.38 72.4
6 7.04 5.58 79.3 7.03 99.8
7 10.84 5.94 54.8 7.23 66.7
10 9.66 5.01 51.9 6.39 66.1
Mean ± SE 8.5 ± 0.8 5.3 ± 0.2 64.3 ± 5.1 6.5 ± 0.3 78.0 ± 6.4
527Oecologia (2017) 184:521–530
1 3
into the terrestrial environment through the emergence of
Chironomidae midges. Such patterns are similar to other
riverine invaders like zebra mussels (Dreissena polymor-
pha), which greatly transformed large river food webs by
altering planktonic resources for some taxa while facilitat-
ing others (Thayer et al. 1997; Strayer et al. 1999).
Bighead carp’s exploitation of planktonic resources
propagated effects across trophic levels and ecological
boundaries. By reducing zooplankton, bighead carp indi-
rectly increased the phytoplankton community via trophic
cascade. Moreover, by consuming a shared prey resource
and reducing golden shiner production, bighead carp indi-
rectly reduced the quantity of materials egested by shiners,
thus impacting material exchanges between planktonic and
benthic habitats. These reductions were offset by the strong
contributions of egested materials by bighead carp. As pre-
dicted, egested materials subsidized benthic habitats, which
increased larval Chironomidae, and resulted in a substan-
tial increase in the emergence of their adult life stage.
These findings indicate that large river planktivores can
affect organisms in adjacent habitats by altering benthic–
planktonic coupling, with effects that can propagate across
ecosystems boundaries (Flecker et al. 2010). In contrast,
Chaoboridae emergence decreased in the presence of big-
head carp. The mechanism is unclear but may have resulted
because Chaoboridae feed on zooplankton or because
they were directly consumed by fishes. Increased emer-
gence of Chironomidae offset reductions of Chaoboridae
midges by nearly an order of magnitude. Overall, the net
effect of bighead carp resulted in a higher export of aquatic
insects, which is opposite of other non-native fishes (c.f.,
Baxter et al. 2004; Epanchin et al. 2010). Theoretically, by
increasing these cross-boundary exchanges, bighead carp
could indirectly subsidize terrestrial insectivores, thereby
altering terrestrial community dynamics (e.g., Murakami
and Nakano 2002). Because Chironomidae can disperse
considerable distances (Muehlbauer et al. 2014), the effects
of bighead carp may extend into the terrestrial landscape.
Such impacts warrant further exploration.
Reconciling the observed and sometimes counter-intu-
itive responses in our experiment required knowledge of
the composition (i.e., who eats whom?), quantities (i.e.,
organic matter flows), and fates (i.e., what of the con-
sumed?) of materials consumed by bighead carp. For
instance, zooplankton constituted only 22% of bighead carp
diets, yet accounted for 39% of their production because
they are a high quality food source. Complementary esti-
mates of daily and total zooplankton production indicated
reduced energy flows through that food web component,
and allowed us to further quantify exploitation efficiency
(e.g., the ratio of the consumption of prey to their produc-
tion) of these fishes—a metric seldom estimated. Bighead
carp alone consumed 64% of zooplankton production
over the experiment (78% when including native fishes).
Strong exploitation by an invader is not unprecedented,
as non-native brown trout (Salmo trutta) also exploit a
greater proportion of benthic invertebrate production rela-
tive to native fishes (Huryn 1998). Estimates of consump-
tion of zooplankton by fishes in our experiment were close
to estimates of zooplankton production, despite using two
differing analytic methods. By estimating secondary pro-
duction of zooplankton in response to bighead carp, our
experiment empirically supports what many have suspected
and inferred through changes in density and standing crop
biomass—bighead carp substantially reduce energy flow
through zooplankton assemblages.
Most fisheries management agencies neglect to sam-
ple or calculate ecosystem processes, relying instead on
changes in densities or standing crop biomass (e.g., Meyer
1997; Collins et al. 2015a; Skern-Mauritzen et al. 2015).
Snapshots of standing crops inadequately account for turno-
ver of new biomass, leading to disparities between biomass
and production. Consider the sampling metrics and treat-
ment effect sizes between larval (standing crop biomass)
and adult Chironomidae (rate of flux). Standing crop bio-
mass of Chironomidae larvae was 90% greater in invaded
Bighead carp
Native fishes
Chironomidae biomass
(mg DM m-2)
Total egested material
(g DM m-2)
Control Invaded
Control Invaded
Fig. 2 Effects of bighead carp on organisms associated with ben-
thic habitats. a Estimated flux of egested material from native fishes
and bighead carp. b Standing crop biomass (n = 5; mean ± standard
error) of Chironomidae larvae
528 Oecologia (2017) 184:521–530
1 3
ponds. In contrast, the flux of adult Chironomidae biomass
emerging from invaded ponds increased by 228%. In our
case, inferences based solely on standing crop biomass
would underestimate the impacts of bighead carp, when
in reality effects were far greater. Production estimates
allowed us to calculate organic matter flows and exploita-
tion efficiencies based on relatively simple calculations.
Without these metrics, we would not have been able to
estimate bighead carps ability to exploit zooplankton
resources. For the trophic basis of production, reliable esti-
mates are dependent on accurate diet and secondary pro-
duction data (Cross et al. 2011). Fortunately, the ponds had
a simple food web structure, which is advantageous from
an experimental perspective. Moreover, juvenile bigheaded
carp exhibit schooling behavior, so they forage in similar
locations, which likely contributed to homogeneity of diets.
The use of regression models to estimate zooplankton pro-
duction introduces some biases based on the predictive
capabilities of the model (Morin et al. 1987). For instance,
estimates of larger bodied Copepods can be inflated when
compared to other production approaches (i.e., egg ratio
method; Stockwell and Johannsson 1997). These large
bodied taxa are also more susceptible to predation by big-
headed carp (Sass et al. 2014). Nevertheless, we reason any
biases would be consistent across controls and treatments.
Animals play a crucial role in altering movements of
nutrients and energy (Polis et al. 1997). Recycling and
translocation of materials by fishes have largely focused
on nutrient contributions via excretion and responses by
primary producers (e.g., Vanni 2002), yet coupling of
habitats via sedimentation of particulates is widely recog-
nized as ecologically important (Polis et al. 1997; Wotton
and Malmqvist 2001; Schindler and Scheuerell 2002). We
Time (week)
12345678910 11 12
12345678910 11 12
(e) Trichoptera
Emergence (mg DM m-2 week-1)
(d) Odonata
(f) Ephemeroptera
(a) Chironomidae
(b) Chaoboridae
(c) Culicidae
Invaded Control
Fig. 3 Emergence (mg DM m2 week1; mean ± standard error) of adult aquatic insects from ponds with (invaded; n = 5) and without (native
controls; n = 5) bighead carp. All responses are in dry mass
Fig. 4 Ponds adjacent to one another stocked with native fishes (top)
and an invaded pond, stocked with the same native fish assemblage
plus bighead carp (bottom). Visual differences were observed in free-
floating filamentous algal mats after 2 months. Photo credit: Scott F.
529Oecologia (2017) 184:521–530
1 3
detected strong associations between bighead carp growth,
egestion, and Chironomidae biomass in benthic habi-
tats, which was consistent with our predictions. Egested
material subsidized benthic habitats and appears to have
increased the standing crop biomass and productivity
(inferred via flux of emergence) of Chironomidae (r-strate-
gists). In addition, sedimentation of other materials such as
phytoplankton cells (also indirectly enhanced by bighead
carp) may have also contributed to observed responses. The
lack of response by channel catfish (benthic predator) may
be due several factors including insufficient quantities of
egested material relative to their body size, poor resource
quality relative to other food resources, and availability of
other food resources. Our study focused primarily on larger
animals, but additional steps are needed to examine how
these subsidies alter other ecosystem processes including
decomposition rates, ecosystem metabolism, and biogeo-
chemical transformations.
Additive designs are useful for studying the effects of
invasive species because they hold all factors equal except
for the invader (Fausch 1998). A salient critique of additive
designs is the confounding effect of adding more individu-
als, irrespective of the invader. Thus, it can be difficult to
determine whether an effect was due to the invader or add-
ing more individuals. For instance, would similar effects
occur had we added equivalent numbers of native plankti-
vores such as gizzard shad (Dorosoma cepedianum), big-
mouth buffalo (Ictiobus cyprinellus), American paddlefish
(Polyodon spathula), river carpsucker (Carpiodes carpio),
etc.? Presumably so; but to do so would be artificial and
would not reflect patterns observed in nature. Populations
of these native large river fishes pale in comparison to the
numbers of bigheaded carp within the Mississippi River
and its tributaries (Irons et al. 2007; Collins et al. 2015b).
With the substantial numbers of bigheaded carp inhabit-
ing large river (Sass et al. 2010) and floodplain lakes (Col-
lins et al. 2015b) of the Mississippi River and tributaries,
it is logical to further consider how population dynamics
of these invaders are shaping river–floodplain ecosystems.
The large numbers of bigheaded carp are undoubtedly
contributing large quantities of egested material to these
ecosystems. Our experiment indicated that these contri-
butions could have substantial food web effects, but these
patterns need to be evaluated in more hydrologically com-
plex ecosystems (e.g., variable flows among river–flood-
plain habitats). Although bighead carp were restricted to
the confines of the pond environment, in river–floodplain
ecosystems, the movements of these mobile consumers are
dynamic and will dictate where these egested particles are
delivered. Hydrologic conditions (e.g., velocity) should fur-
ther mediate the area over which these particles ultimately
settle. Consideration of bigheaded carp as mediators of
organic matter exchanges provides a clearer framework
for predicting the impacts of these invasive planktivores
in river–floodplain ecosystems. If the observed ecological
processes from our experiment scale to river–floodplain
ecosystems, our perception of bighead and silver carp
should evolve beyond competitors for planktonic resources,
to facilitators, mediators, and processors of nutrients and
energy within and across ecosystems.
Acknowledgements We specifically thank M. Diana, S. Butler, M.
Naninni, B. Van Ee, M. Stanton, B. Smith 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.
Author contribution statement SFC and DHW conceived and
designed the experiment. SFC performed the experiment. SFC and
DHW analyzed the data. SFC wrote the manuscript.
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... A quantification of the diet of omnivores is central to understanding the important food web interactions of a species. Fluctuations in fish biomass or food quantity and/or quality, may greatly alter the food habits of silver carp, and thus alter their role in phytoplankton dynamics (Collins & Wahl, 2017;Tumolo & Flinn, 2017;Lin et al., 2020). Our results suggest that it is also important to consider the variation in diet composition that can occur ontogenetically when stocking silver carp for phytoplankton biomass control. ...
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Stocking of silver carp [Hypophthalmichthys molitrix (Val.)] is a common tool used in eutrophic aquatic systems to control phytoplankton. As the mesh size of gill rakers undergoes change as growth proceeds, the particle selectivity and dietary importance of phytoplankton in younger fish may be different than those in older individuals. We conducted an enclosure experiment to assess how a gradient of fish size affected grazing pressure. The presence of fish strongly reduced zooplankton biomass and phosphorus (P) excretion, and P sequestered by fish could largely account for the lower total phosphorus (TP) concentrations observed in the presence of fish than in their absence. Moreover, the nutritional importance of phytoplankton in fish decreased with increasing fish size. Both the overall P excretion and grazing pressure decreased, whereas phytoplankton biomass and the yield of chlorophyll a per TP increased with increasing fish size. This suggested that phytoplankton might become more limited by P availability but less controlled by grazing with increasing fish size. Stocking with large silver carp would strongly reduce grazing pressure and increase the yield of phytoplankton per TP.
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Artificial habitat structures are used to mitigate habitat loss within aquatic ecosystems and to increase angler catch rates; however, the potential trophic outcomes of concentrating fish at structure additions are rarely evaluated. We compared fish and fish prey assemblages between reservoir coves with and without newly added offshore habitat structures (PVC cubes enclosing 2.25 m3). Further, we tested whether offshore habitat additions changed fish assemblage and abundance in littoral habitats of treated coves. Coves with offshore structures had higher concentrations of fish and an assemblage structure dominated by Pomoxis and Lepomis species at offshore sites compared to coves without offshore structures. There was no evidence for an effect of offshore structures on fish relative abundance and assemblage structure in nearby littoral habitats. Zooplankton and benthic macroinvertebrate abundances did not change because of the addition of habitat structures. Lack of a response of lower trophic organisms may be due to either low predation mortality, or losses to predation are replaced by increased prey production or immigration at the offshore structures. Our results indicated that artificial structures effectively concentrated fish but increases in fish density may be partitioned among newly available structure in offshore habitats and existing structure within littoral habitats. Future studies are needed to test for a relationship between foraging space and food resources provided by artificial habitats, which may be useful for optimizing the number, size, and material composition of structures used in habitat enhancement programs.
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The current paper aims to diagnose the state of fisheries of three Chinese carp species whish have been introduced in Tunisian reservoirs since 1981: (silver carp Hypophthalmichthys molitrix, herbivorous carp Ctenopharyngodon idella and bighead carp Aristichthys nobilis) and seeks to study the benefits and risks associated with their introduction. Chinese carps cannot reproduce naturally in freshwater reservoirs. Eventually, artificial breeding operations and seeding of the dams with farm-produced fry are carried out by the Technical Centre of Aquaculture every year. Statistical analyses have shown a strong correlation between the landed quantity and the number of fries stocked each year. The impact assessment showed that the risks and benefits associated with the introduction of the three species are variable. Regarding their benefits, it was clear that the herbivorous carp has provided effective and sustainable control of the extensive development of aquatic vegetation in the eutrophic reservoirs and canal systems. The value of the other two species, though, remains less obvious, particularly for the bighead carp. The consequences of their introduction on ecosystems and native species seem to be negligible, especially when the densities are low. Eventually, it seems judicious to increase the stocking of the herbivorous carp, silver carp and bighead carp in Tunisian reservoirs.
... Common carp can influence water quality and nutrient dynamics and can reduce aquatic macrophytes and macroinvertebrate abundance (Parkos et al. 2003, Matsuzaki et al. 2007). Through their benthic feeding, carp affect bottom-up processes, modifying nutrient and turbidity concentrations and primary producer abundance and diversity, but may also exert top-down effects by preying upon invertebrates or changing the foraging condition for other fishes (Weber and Brown 2009), as has been shown in the context of invasive Asian carps (e.g., Collins and Wahl 2017). Additionally, carp can have strong direct and indirect impacts on nutrient dynamics and littoral community structure through their bioturbation and excretion (Matsuzaki et al. 2007). ...
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Abstract Multiple invasive species may interact, influencing one another and generating synergistic effects on food webs and ecosystem processes. We investigated the interaction between two non‐native species widespread in the western USA: common carp (Cyprinus carpio) and Russian olive (Elaeagnus angustifolia), an invasive riparian tree associated with di‐nitrogen fixation. Deep Creek, Idaho, was an International Biological Program site in the early 1970s; at that time, carp were rare and Russian olive was absent. Subsequently, Russian olive was introduced and established a dense stand, increasing allochthonous inputs and nitrogen‐rich benthic organic matter. Since 1971, carp density has increased ~4× (an increase our bioenergetic analysis suggests could not have been sustained in the absence of Russian olive). Carp gut contents in 2013–2014 revealed, on average, ~40% olives, and, similarly, stable isotope analyses revealed ~58% of carp tissues were derived from olives. A small‐scale, short‐term experimental exclusion of these subsidized carp caused ~3× increases in macrophytes and chlorophyll‐a, suggesting they may limit algae and macrophyte biomass. Moreover, carp that consumed olives excreted more nitrogen (~2× more ammonium, ~2× more total dissolved nitrogen, and ~3× more total nitrogen) compared to those that had not, which may amplify recycling and export from streams invaded by both species. This scenario is characteristic of an “invasional meltdown,” with attendant changes in food webs and ecosystem processes.
Large riverine systems are diverse and dynamic and are made up of multiple habitat types of lentic and lotic water. They are also heavily modified by humans and today nearly all habitats in many large rivers have been drastically altered. These modifications often include disconnecting lentic habitats either permanently or intermittently from the main channel. The Merwin Preserve at Spunky Bottoms (Merwin) began as a connected backwater that was leveed and drained for agriculture in the 1920s and restored in 1999, with restoration allowing it to become a disconnected backwater habitat. This status changed in 2013 when record flooding on the adjacent Illinois River overtopped and breached the levee creating an unmanaged and intermittent connection allowing the river access at moderately high river stages. During the past 20 years, the fish community at Merwin has undergone several changes that follow three drought events pre-breach, the exchange of fishes from the mainstem following the breach in 2013, and subsequent low water conditions of much of the area as river levels drop. Long term data and more intensive sampling efforts during the drought of 2012 showed relative abundance of sport fishes declined during, or immediately following, pre-breach drought events and post-breach low water conditions while relative abundance of non-sport and non-native fishes remained stable. The unique story of Merwin can provide a case study for other large river restoration projects on the effects of drought, climate change, and impacts of an unmanaged connection of a previously disconnected habitat to an adjacent large river.
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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.
The invasion of silver carp (Hypophthalmichthys molitrix) and bighead carp (H. nobilis) or “bigheaded carps” has caused extensive ecological and economic harm throughout the Mississippi River and its tributaries. To prevent their continued spread upstream toward the Great Lakes, intense commercial harvest was implemented on the Illinois River, a large tributary that connects the Mississippi River to Lake Michigan. Since implementation, harvest has reduced densities at the invasion front while also presenting an opportunity to generate a synthesis on ecosystem resilience in the face of accelerating invasion. Resilience, the ability of an ecosystem to recover after perturbation, was observed at local scales and within some taxa but has yet to manifest at a river-wide scale and often co-varied with abiotic environmental or seasonal factors. Thus, while intensive harvest has limited further spread of bigheaded carps, and evidence of additional secondary ecosystem benefits exists, opportunities remain to identify potential pathways that could spread such ecosystem benefits even farther.
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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 investigated whether spawning by non-native Chinook salmon influenced aerial insect abundance in the riparian zone of Thompson Creek, a tributary of Lake Michigan, located in Michigan, USA. Specifically, we evaluated whether decades of salmon disturbance affected patterns of aquatic insect emergence, and how both live salmon and salmon carcasses influenced the abundance of terrestrial carrion flies. Retaining wall timbers from a low-head dam on Thompson Creek were removed, providing a unique opportunity to compare stream reaches that were exposed to the immediate ecological impacts of salmon (i.e., disturbance, subsidy effects) with reaches experiencing decades of spawning activity. Using sticky traps to collect aerial insects, we observed fewer adult aquatic insects in downstream reaches conditioned to decades of salmon disturbance in comparison to naïve upstream reaches. Reduced abundance in downstream reaches was primarily driven by taxa more susceptible to disturbance in the larval life stage (e.g., Diptera: Simuliidae, Ephemeroptera). A greater abundance of adult Chironomidae midges were detected in upstream reaches with higher numbers of spawning salmon and carcasses. Though abundance of adults differed between upstream and downstream reaches, we observed no evidence of early emergence. In addition, carrion fly abundance was greatest at reaches with more live and dead salmon. Evidence from our study suggests that non-native salmon have the potential to influence patterns of aerial insect abundance in riparian zones. Our findings suggest that non-native Chinook salmon can affect aerial insect assemblages; however, the propagating effects of these changes through riparian food webs warrant further investigation.
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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.
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We critically evaluate some of the key ecological assumptions underpinning the use of nutrient replacement as a means of recovering salmon populations and a range of other organisms thought to be linked to productive salmon runs. These assumptions include: (1) nutrient mitigation mimics the ecological roles of salmon, (2) mitigation is needed to replace salmon-derived nutrients and stimulate primary and invertebrate production in streams, and (3) food resources in rearing habitats limit populations of salmon and resident fishes. First, we call into question assumption one because an array of evidence points to the multi-faceted role played by spawning salmon, including disturbance via redd-building, nutrient recycling by live fish, and consumption by terrestrial consumers. Second, we show that assumption two may require qualification based upon a more complete understanding of nutrient cycling and productivity in streams. Third, we evaluate the empirical evidence supporting food limitation of fish populations and conclude it has been only weakly tested. On the basis of this assessment, we urge caution in the application of nutrient mitigation as a management tool. Although applications of nutrients and other materials intended to mitigate for lost or diminished runs of Pacific salmon may trigger ecological responses within treated ecosystems, contributions of these activities toward actual mitigation may be limited.
Successful conservation efforts require a consideration of ecosystem function. In this chapter, I present seven principles from ecosystem science to serve as a foundation for ecosystem conservation: 1. Ecosystems are open, which should lead to an emphasis on conserving the fluxes across ecosystem boundaries and linkages with surrounding ecosystems. 2. Ecosystems are temporally variable and continuously changing; the present bears the legacies of past disturbances. 3. Ecosystems are spatially heterogeneous on a range of scales, and essential processes depend on that heterogeneity. 4. Indirect effects are the rule rather than the exception in most ecosystems. 5. Ecosystem function depends on its biological structure. 6. Although several species perform the same function in ecosystems, they respond differently to variations in their biotic and abiotic environment, thereby reducing variation in ecosystem function in a changing environment. 7. Humans are a part of all ecosystems. A river restoration project provides an example of the use of ecosystem principles to design effective conservation strategies; yet we have little experience with the use of measures of ecosystem structure or function to measure the efficacy of conservation efforts. Research is needed to find appropriate metrics and to determine their range in regional reference systems. A critical first step in using such an approach is to identify and preserve a wide range of ecosystems that could be used for regional references. Unanswered questions remain that limit our ability to use measures of ecosystem structure and function to guide conservation practices. Finding answers to these questions is critical because the public values the goods and services that are a consequence of ecosystem function.