Can Amphibians Help Conserve Native Fishes?

Abstract

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.

Full text

Can Amphibians
Help Conserve Native
Fishes?
Fisheries | www.fisheries.org 327
Niall G. Clancy | Department of Ecology, Montana State University, 310 Lewis Hall, Bozeman, Montana 59717.
E-mail: niallgc66@gmail.com
FEATURE
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.
INTRODUCTION
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
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 | www.fisheries.org 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.
DIRECT HABITAT ALTERATION
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.
NUTRIENT REDISTRIBUTION
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.
MANAGEMENT IMPLICATIONS
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.
ACKNOWLEDGMENTS
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 www.rittercraft.com. Last, thank you to my family
and friends who gave me so much support during the preparation
of this work.
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