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Can Amphibians Help Conserve Native Fishes?


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Native fish populations have continued to decline worldwide despite advances in management practices. As such, new approaches are needed to complement the old. In many flowing and standing waters, larval amphibians are the dominant vertebrate taxa. This can be important to fisheries due to amphibians’ ability to influence macroinvertebrate communities, alter benthic habitat, and supply nutrients in aquatic systems. These changes can, in turn, affect the ecology and fitness of other aquatic organisms such as fishes. Due to their large effects in some systems, it is suggested that fisheries managers carefully consider actions that may affect amphibian populations and actively conserve them in some cases. Preservation of riparian areas and amphibian-associated microhabitats may even be used as a tool to positively impact freshwater fisheries by conserving amphibians that help maintain aquatic systems. Therefore, knowledge of local amphibian life histories and behaviors may be important in conserving associated freshwater fisheries.
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Can Amphibians
Help Conserve Native
Fisheries | 327
Niall G. Clancy | Department of Ecology, Montana State University, 310 Lewis Hall, Bozeman, Montana 59717.
Bull frog at the John Heinz National Wildlife Refuge in Philadelphia, Pennsylvania.
Photo credit: Bill Buchanan/USFWS.
328 Fisheries | Vol. 42 • No. 6 • June 2017
Native sh populations have continued to decline worldwide despite advances in management practices. As such, new approach-
es are needed to complement the old. In many owing and standing waters, larval amphibians are the dominant vertebrate taxa.
This can be important to sheries due to amphibians’ ability to inuence macroinvertebrate communities, alter benthic habitat,
and supply nutrients in aquatic systems. These changes can, in turn, aect the ecology and tness of other aquatic organisms
such as shes. Due to their large eects in some systems, it is suggested that sheries managers carefully consider actions that
may aect amphibian populations and actively conserve them in some cases. Preservation of riparian areas and amphibian-
associated microhabitats may even be used as a tool to positively impact freshwater sheries by conserving amphibians that help
maintain aquatic systems. Therefore, knowledge of local amphibian life histories and behaviors may be important in conserving
associated freshwater sheries.
Population management of shes has historically employed
a diverse array of techniques, including habitat management,
hatchery-reared sh stocking, species conservation, and harvest
regulation (Cowx and Gerdeaux 2004). Despite the many suc-
cesses of these techniques, the overall abundance and distribu-
tion of native North American shes steadily declined throughout
the 20th century (Williams et al. 1989), and climate change is
predicted to further impact freshwater shes in the 21st century
(Heino et al. 2009). As such, a complementary suite of techniques
and approaches is needed if management is to prevent further
losses. One such complementary approach is the preservation of
organisms that maintain ecosystem processes and geomorphic
functions (Mills et al. 1993). Indeed, freshwater organisms that
are particularly dominant or have a high biomass can exert a sig-
nicant inuence on sympatric species (Vanni 2002). In many
ponds, wetlands, and stream headwaters, larval amphibians of the
orders Anura (frogs and toads) and Urodela (salamanders) are the
dominant vertebrate taxa (Davic and Welsh 2004; Ranvestel et
al. 2004; Gibbons et al. 2006). Fisheries management plans that
incorporate amphibians will likely be benecial to much of the
aquatic community.
Due to their high biomass in some systems, amphibians can
have measurable effects on lotic and lentic habitats (Seale 1980;
Rantala et al. 2015) and food webs in aquatic systems (Burton
and Likens 1975; Pough 1980; Unrine et al. 2007). These effects
can be divided into three general categories: (1) trophic interac-
tions, (2) direct habitat alteration, and (3) nutrient redistribution
(Figure 1). Here, I describe the three primary roles of amphibians
in freshwater ecosystems and provide direction for future con-
servation of native sh and amphibian populations, a common
management objective. Additionally, I give some suggestions for
incorporating amphibians into sheries management plans fol-
lowing the precedent of Knapp et al. (2001).
Trophic interactions in aquatic ecology are a well-established
phenomenon in which changes in the abundance of one species
may alter the structure of the entire food web (Vanni 2002). In
freshwater habitats, larval salamanders and anurans typically oc-
cupy different trophic levels, because salamanders tend to be ob-
ligate carnivores (Davic and Welsh 2004), whereas tadpoles are
generally herbivores (Altig et al. 2007). A number of studies have
found that salamanders decrease the densities of their aquatic
Figure 1. The three general eects of larval amphibians on freshwater sheries: eects on other animals through trophic
interactions, eects on aquatic habitat through grazing and bioturbation, and eects on water chemistry through nutrient
redistribution. Figure by T. David Ritter.
Fisheries | 329
invertebrate prey through direct predation and nonconsumptive
effects (Huang and Sih 1991). Members of the genera Ambysto-
ma, Amphiuma, Cryptobranchus, Desmognathus, Dicamptodon,
Notophthalmus, Siren, and Taricha have all been implicated in
altering densities of freshwater invertebrates (Petranka 2010). In
some cases, predation by larval salamanders may be so extensive
that responses may cascade through multiple trophic levels and
regulate algal production and detritus–litter food webs (Davic
and Welsh 2004). Indeed, the large effect of some salamanders
on invertebrate populations has led some authors to label them
as “keystone species” (Paine 1969; Davic and Welsh 2004). Im-
pacts on invertebrate populations are likely to affect insectivorous
shes that are sympatric with salamanders.
In contrast to larval salamanders, anuran tadpoles are largely
herbivorous (see Altig et al. 2007) and therefore can change in-
vertebrate communities by inuencing the biomass or productiv-
ity of primary producers; for example, tadpole losses in a Pana-
manian stream led to decreases in macroinvertebrate biomass and
diversity, most likely through the consumption of biolm and
changes in benthic algal communities (Rantala et al. 2015). An
additional study in four Panamanian streams found no difference
in macroinvertebrate biomass before and after the loss of its anu-
ran populations, but it did report shifts in the functional feeding
groups of the invertebrate community from shredder to scraper
dominance (Colon-Gaud et al. 2008). However, tadpoles of some
taxa, such as the American bullfrog Rana catesbeiana, can even
directly prey upon sh eggs and juveniles, which can have critical
management implications where such sh are endangered (Muel-
ler et al. 2006).
Larval amphibians also constitute a food resource for other
animals, both aquatic and terrestrial (Rundio and Olson 2003;
Petranka 2010). A study in a New Hampshire forest found that
metamorphosed salamanders were a more nutritious food source
than birds, mice, and shrews and comprised a greater biomass
than that of all breeding birds and was at least equal to that of all
small mammals (Burton and Likens 1975). Additionally, depos-
ited eggs and carcasses of larval anurans and salamanders can
be a terrestrial-derived, seasonal food source for aquatic organ-
isms (Seale 1980; Capps et al. 2015). Where sh are introduced
into previously shless lakes and ponds, amphibian populations
often decline, and subsequent removal of these sh can lead to
population recovery (Knapp et al. 2007). In addition, where sh
and larval salamanders co-occur, salamanders can incur noncon-
sumptive effects such as size reduction and reduced likelihood
of metamorphosis (Kenison et al. 2016). As such, management
actions that prioritize presence of nonnative shes or overabun-
dance of native shes over larval amphibian conservation may
inadvertently impact an important part of a sheries food web
(Knapp et al. 2001). In short, larval amphibians of both orders can
inuence macroinvertebrate communities. Accordingly, sheries
professionals should consider how local amphibian populations
inuence the invertebrate food resources of a shery and where
amphibians act as a food resource themselves.
Habitat management is one of the most widely appreciated
and accepted tenets of sheries conservation. The term “habitat”
generally includes both physical and biological variables, such
as water depth and quality, substrate type, amount of cover, and
macrophyte abundance (Fisher et al. 2012). Tadpoles can alter
their surrounding biotic and abiotic habitats, in streams and still
waters, through two mechanisms: (1) grazing and (2) the mechan-
ical disturbance of benthic sediment from swimming (Flecker et
al. 1999). These two mechanisms, though different, are insepara-
ble. Several studies have found large decreases in benthic sedi-
ment and suspended particulate concentration with increasing
tadpole abundance (Seale 1980; Flecker et al. 1999; Ranvestel et
al. 2004). In addition to affecting primary producers, decreases in
sediment can affect invertebrates and small shes that are reliant
on certain benthic conditions (Wood and Armitage 1997; Angradi
1999). Sediment can smother both primary and secondary pro-
ducers (Power 1990); therefore, sediment removal may be one of
the most important impacts of tadpoles in freshwater. Succinctly,
tadpole foraging can change the benthic habitat of primary pro-
ducers, invertebrates, and small shes in both lotic and lentic hab-
itats. In turn, this may affect sh species of management concern.
Nutrients such as nitrogen and phosphorus are extremely
important to the growth and survival of all aquatic organisms
(Sterner and Elser 2002). Freshwater animals can increase the
concentration of nutrients through release of urea and solid waste.
Larger animals, such as sh, can have similar or even greater total
excretion rates than small, abundant animals, such as zooplankton
(Vanni 2002). In habitats with and without sh, many amphibians
also can substantially affect ecosystems due to nutrient redistribu-
tion (Connelly et al. 2011).
In some systems, both anuran and urodelan larvae can supply
signicant amounts of nutrients in streams, which often are im-
portant to primary producers and eventually other animals via nu-
trient ow through food webs (Vanni 2002; Connelly et al. 2011).
However, nutrient inputs that contribute substantially on a local-
ized scale may contribute more modestly over larger scales, and
because amphibians leave freshwater following metamorphosis,
aquatic nutrient subsidies are seasonal and depend on their spe-
cic life stage (Keitzer and Goforth 2013). Additionally, where
temperature-related declines of sh cause a subsequent loss of
nutrients supplied to a stream, the effect may be partially buffered
where large populations of larval salamanders (Munshaw et al.
2013) and tadpoles are found. However, more research is needed
to fully understand the effects of amphibian nutrient redistribu-
tion on freshwater systems.
As freshwater animal populations experience large global de-
clines (World Wildlife Fund 2016), sheries management must
embrace new approaches and techniques to conserve native spe-
cies. Larval frogs, toads, and salamanders can be important in
maintaining the structure and function of freshwater ecosystems
through trophic interactions, direct habitat alteration, and nutrient
redistribution (Figure 1). Because of the abundance and subse-
quent effects of these vertebrates on the structure and function of
some aquatic ecosystems, managers should incorporate amphib-
ians into native sh conservation plans. Despite many manage-
ment plans incorporating sh effects on amphibian populations,
to the author’s knowledge, few if any management plans have
incorporated the reverse. Approaches for doing this can be broken
into organismal and land management-based approaches.
Organismal approaches are often what sheries managers are
responsible for directly. Such approaches for incorporating am-
phibians include removal of invasive shes where native amphib-
ians are abundant, ceasing hatchery stocking of naturally shless
lakes (Knapp et al. 2001), and recording the types and numbers
of amphibians observed during eldwork. Recording amphibian
sightings takes minimal effort and can be helpful to those at-
tempting to compile information on anuran and urodelan popula-
330 Fisheries | Vol. 42 • No. 6 • June 2017
tions and may aid in conservation efforts. Invasive amphibians,
such as the American toad Bufo americanus, should also be care-
fully monitored and controlled if necessary. Additionally, popula-
tion modeling efforts should determine whether incorporation of
amphibian abundances can increase model accuracy.
In contrast to the manipulation of organisms, sheries man-
agers may not be directly responsible for management of ripar-
ian and aquatic habitats but may ll more of an advisory role.
Therefore, land management approaches for conserving amphib-
ians and sh must be tailored to specic land managers’ needs.
In areas of high amphibian abundance, these approaches should
include limiting human impacts in headwater, pond, and wetland
habitats; maintaining a riparian buffer zone (Petranka and Smith
2005); reducing pesticide application near waterways (Davidson
and Knapp 2007); and maintaining or improving important am-
phibian microhabitats in both aquatic areas and the surrounding
riparian areas during restoration activities. Some microhabitats
that are especially important to amphibians include dense tree
stands, rotting logs, leaf litter, backwaters, and wetlands (Sem-
litsch and Bodie 2003). In some cases, it may be useful to con-
sider published thermal niches and habitat preferences for am-
phibian species of interest (Welsh 2011).
Overall, conservation of native amphibian populations and
control of invasive populations may be an effective tool for man-
aging freshwater sheries. These actions will be most effective
where larval amphibian populations are large. It should be noted
that some actions may possibly have unforeseen consequences
for sh populations of interest due to the inherent complexity of
aquatic systems. As such, the aforementioned approaches should
be applied in an adaptive management framework. Where am-
phibians and shes coexist, the stream corridor can be thought
of as a mosaic of amphibian and sh habitats. Maintenance of
this mosaic is likely important for all organisms within, and it is
possible that ignoring amphibian species may lead to unintended
degradation of aquatic ecosystems.
First and foremost I thank my two mentors for this article,
Wyatt Cross and Andrea Litt. I also thank Chris Clancy, Tom
McMahon, Eric Scholl, David Schmetterling, Jeff Schaeffer, and
two anonymous reviewers for their thoughtful comments on this
article. Additionally, I thank the very talented T. David Ritter for
creating the incredible gure seen in this article. More of his work
can be seen at Last, thank you to my family
and friends who gave me so much support during the preparation
of this work.
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Abstract Predators can influence prey directly through consumption or indirectly through nonconsumptive effects (NCEs) by altering prey behavior, morphology, and life history. We investigated whether predator‐avoidance behaviors by larval long‐toed salamanders (Ambystoma macrodactylum) in lakes with nonnative trout result in NCEs on morphology and development. Field studies in lakes with and without trout were corroborated by experimental enclosures, where prey were exposed only to visual and chemical cues of predators. We found that salamanders in lakes with trout were consistently smaller than in lakes without trout: 38% lower weight, 24% shorter body length, and 29% shorter tail length. Similarly, salamanders in protective enclosures grew 2.9 times slower when exposed to visual and olfactory trout cues than when no trout cues were present. Salamanders in trout‐free lakes and enclosures were 22.7 times and 1.48 times, respectively, more likely to metamorphose during the summer season than those exposed to trout in lakes and/or their cues. Observed changes in larval growth rate and development likely resulted from a facultative response to predator‐avoidance behavior and demonstrate NCEs occurred even when predation risk was only perceived. Reduced body size and growth, as well as delayed metamorphosis, could have ecological consequences for salamander populations existing with fish if those effects carry‐over into lower recruitment, survival, and fecundity.
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Migrations of animals can transfer energy and nutrients through and among terrestrial and aquatic habitats. Pool-breeding amphibians, such as the wood frog (Lithobates sylvaticus), make annual breeding migrations to ephemeral wetlands in forest habitats in the eastern and midwestern United States and Canada.To model the influence of wood frogs on nutrient transport and transformation through time, we coupled long-term population monitoring data (1985–2005) from a wood frog population with estimates of the elemental composition of wood frog egg masses and emerging juveniles.Over the 21-year study period, 8.8 kg carbon (C), 2.0 kg nitrogen (N) and 0.20 kg phosphorus (P) were transported from the terrestrial to the aquatic habitat and approximately 21 kg C, 5.5 kg N and 1.2 kg P were exported to the surrounding terrestrial habitat by wood frogs.During the study period, the average net flux of C, N and P was from aquatic to terrestrial habitats, but the magnitude and direction of the net flux was element dependent. Thus, the net flux of C, N and P did not always flow in the same direction.Predicting long-term trends in nutrient and energy flux by organisms with biphasic life cycles should rely on long-term population data to account for temporal variability. This is especially true for organisms that are sensitive to long-term shifts in temperature and precipitation patterns, such as amphibians that breed in ephemeral pools.
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Salamanders are abundant consumers in many temperate streams and may be important recyclers of biologically essential nutrients, but their ecological role is poorly understood. The ecological significance of nutrient recycling by salamanders may vary spatially and seasonally because of their potentially patchy distribution in streams and the dynamic nature of stream hydrology and other nutrient fluxes. We examined the spatial and seasonal variation of salamander-driven nutrient recycling in 3 headwater streams in the southern Appalachian Mountains. We quantified the aggregate areal excretion rates of N (NH4+-N) for the larvae of the 2 most abundant salamander species in these steams before and after leaf fall to examine spatial and seasonal variation in the supply of nutrients from salamanders. We used short-term nutrient additions in each stream to examine temporal heterogeneity in the ecosystem demand for NH4+-N. Before leaf fall, salamanders were capable of meeting 10% of the ecosystem demand for NH4+-N and could turn over the ambient nutrient pool in 3 km. The significance of this contribution declined to 3% after leaf fall and the turnover length increased 7×. The ecological significance of salamander nutrient excretion varied by as much as 17× within streams and was as high as 30% of the nutrient demand in some stream sections, a result suggesting that salamanders may create biogeochemical hotspots in these nutrient-limited ecosystems. Thus, salamanders appear to be capable of contributing substantially to stream nutrient cycles through the excretion of limiting nutrients and may be underappreciated members of headwater stream ecosystems, particularly at small spatial scales. However, this contribution varied substantially seasonally and spatially.
Although sedimentation is a naturally occurring phenomenon in rivers, land-use changes have resulted in an increase in anthropogenically induced fine sediment deposi-tion. Poorly managed agricultural practices, mineral extraction , and construction can result in an increase in suspended solids and sedimentation in rivers and streams, leading to a decline in habitat quality. The nature and origins of fine sediments in the lotic environment are reviewed in relation to channel and nonchannel sources and the impact of human activity. Fine sediment transport and deposition are outlined in relation to variations in streamflow and particle size characteristics. A holistic approach to the problems associated with fine sediment is outlined to aid in the identification of sediment sources, transport, and deposition processes in the river catchment. The multiple causes and deleterious impacts associated with fine sediments on river-ine habitats, primary producers, macroinvertebrates, and fisheries are identified and reviewed to provide river managers with a guide to source material. The restoration of rivers with fine sediment problems are discussed in relation to a holistic management framework to aid in the planning and undertaking of mitigation measures within both the river channel and surrounding catchment area.
In a pond ecosystem near St. Louis, Missouri, natural variations in tadpole biomass during 1971-1972 were accompanied by shifts in patterns of nutrient cycling and primary production, particularly when metamorphoses caused abrupt removal of these transient consumers. In the field, increased tadpole biomass was associated with: (1) reduced standing crop of suspended particles, including phytoplankton, the tadpoles' major food source; (2) a shift in the state of nitrogen from largely particulate to largely dissolved; (3) reduced rates of primary production, from both H^1^4CO"3 uptake and diurnal oxygen methods; (4) a nonliner effect on phytoplankton specific growth rates; (5) a shift in phytoplankton community structure away from filamentous blue-green algae; and (6) a reduced proportion of active chlorophyll @a in the photosynthetic pigments of phytoplankton. From laboratory experiments, the potential impact of tadpoles on nitrogen flux, through feeding and nutrient release, was estimated. Several conclusions were made: (1) Suspensions feeding by tadpoles reduced concentrations of suspended particles. Under conditions of low particles: high tadpoles, the specific growth rates of tadpoles reduced. Recruitment was absent except under conditions of high particles: low tadpoles; the most diverse community of new tadpole recruits (four species) was observed under such conditions. All four species (three genera) had similarly sized particles in their guts. These field observations are consistent with an hypothesis of competition among the tadpoles. (2) Tadpoles apparently were regulatory consumers; they became a large component with respect to phytoplankton. Nitrogen flux through tadpoles was within the same order of magnitude, and sometimes exceeded the estimated N uptake by phytoplankton. (3) Tadpoles probably regulated primary production by both reducing standing crop and altering specific growth rates of algae. At maximum tadpole biomass, suspended particle concentrations were stabilized near the laboratory-determined threshold concentration for feeding by these Rana tadpoles. When metamorphosis removed these transient consumers, rates of primary production increased dramatically. (4) Interactions within the pond ecosystem apparently determined aquatic-terrestrial nutrient balances for the amphibian communities. Some species deposited more nutrient in their eggs than was assimilated by larvae, but the community as a whole extracted nutrient from the ecosystem. Nutrient input in eggs was much less than that assimilated by autotrophs.
Armored catfish (Loricariidae) are the major grazers of attached algae in pools of the Rio Frijoles, Panama (9@?9' N, 79@?44' W). In the dry season, sunny pools were inhabited by @?6 individuals loricariids per square metre of grazeable substratum. At these densities, armored catfish depleted algae and cleared sediment from bedrock substrata, leaving sparse standing crops of small, adnate diatoms (primarily Achnanthes spp.). To study the effects of armored catfish at 1/6 their natural density, I stocked four 6-7 cm (SL), 10-g Ancistrus (the most common size class of the most common species in stream pools) in each of five stream pens. Pens enclosed 4 m^2 of bedrock substratum, and were alternately stocked or left empty during three consecutive periods of 29, 11, and 11 d. At the end of each period, standing crops of sediment and attached algae, and rates of photosynthesis by attached algae, were measured. The attached algae that developed with sparse Ancistrus had higher standing corps with larger cells or colonies, and higher primary productivity, than did periphyton subjected to heavy grazing by unconfined armored catfish. Even heavy grazing, however, was less deleterious to attached algae than prolonged sedimentation on substrata in enclosures left empty for 11 or 29 d. The net effect of Ancistrus on their algal food changed from depletion at high grazer densities to enhancement at low grazer densities, as sedimentation became more limiting to algae than grazing.
Taxonomic and functional diversity in freshwater habitats is rapidly declining, but we know little about how such declines will ultimately affect ecosystems. Neotropical streams are currently experiencing massive losses of amphibians, with many losses linked to the chytrid fungus, Batrachochytrium dendrobatidis (Bd).We examined the ecological consequences of the disease-driven loss of amphibians from a Panamanian stream. We quantified basal resources, macroinvertebrates, N uptake and fluxes through food-web components and ecosystem metabolism in 2012 and 2014 and compared them to pre-decline (2006) and 2 year post-decline (2008) values from a prior study.Epilithon biomass accrued after the decline, more than doubling between 2006 and 2012, but then decreased fivefold from 2012 to 2014. In contrast, suspended particulate organic matter (SPOM) concentrations declined continuously after the amphibian decline through 2014.Biomass of filter-feeding, grazing and shredding macroinvertebrates decreased from 2006 to 2014, while collector–gatherers increased during the same time period. Macroinvertebrate taxa richness decreased from 2006 (52 taxa) to 2012 (30 taxa), with a subsequent increase to 51 taxa in 2014.Community respiration, which initially decreased after the amphibian decline, remained lower than pre-decline in 2012 but was greater than pre-decline values in 2014. Gross primary production remained low and similar among years, while uptake length in both 2012 and 2014 was longer than pre-decline. Nitrogen flux to epilithon increased after the decline and continued to do so through 2014, but N fluxes to fine particulate organic matter and SPOM decreased and remained low.Our findings underscore the importance of studying the ecological consequences of declining biodiversity in natural systems over relatively long time periods. There was no evidence of functional redundancy or compensation by other taxa after the loss of amphibians, even after 8 years.
Tadpoles of the web-footed frog, Rana palmipes, are epibenthic consumers that are widely distributed in the Neotropics. Rana tadpoles feed on algae and sediments and potentially act as ecosystem engineers by modifying habitat structure via their foraging activities. We conducted two experiments in the Andean piedmont of Venezuela to examine the following questions: (1) Can Rana tadpoles influence benthic sediment distributions? (2) Are growth rates of sediment-feeding tadpoles density dependent? and (3) Are sediments viable nutritional sources for tadpoles? A field enclosure experiment was conducted in which sediment accrual and daily growth rates were compared among four tadpole density treatments ranging from 1 to 10 individuals per m2. Tadpoles had highly significant effects on stream sediment accrual that were inversely related to tadpole density; thus, benthic sediments rapidly accumulated when tadpole density was low (one tadpole per m2), whereas substrata were thoroughly cleared of sediments when tadpole densities were high (10 tadpoles per m2). Furthermore, over the course of the experiment, daily growth rates of tadpoles were strongly affected by tadpole density, and individuals from low-density treatments displayed greater than five times the daily growth of tadpoles from high-density treatments. A second experiment was conducted in wading pools to assess the importance of benthic stream sediments as a nutritional source for tadpoles. Tadpole daily growth and development were measured in pools in which diets were supplemented with stream sediments and compared to tadpoles from pools with no sediment addition. Tadpoles displayed net positive growth and significantly higher development rates when stream sediments were added to wading pools. In contrast, tadpoles lost weight in the absence of sediment supplements. Our results suggest that tadpoles can act as ecosystem engineers by reducing sediment accrual rates; however, low availability of sediments can negatively feed back on tadpoles by reducing their daily growth rates.