1 | INTRODUCTION
No ecosystem has escaped human alteration and a poor understanding of
factors leading to species
decline can result in inefficient or ineffective species restoration
(Craig et al ., 2017; Roni et al ., 2018). For riverine
ecosystems, major anthropogenic alterations occurred in the 20th century
through dam construction, concomitant flow regulation, and establishment
of nonnative fishes; all which contributed to the imperilment of many
native fishes (Burkhead, 2012; Kominoski et al ., 2017; Lamothe
and Drake, 2018). In certain cases, the primary cause of imperilment can
be directly linked to nonnative fish predation (Vander Zanden et
al . 1999; Pelicice and Agostinho, 2009; David, 2017). In other cases
when there has a poor understanding of ecosystem dynamics prior to
severe habitat and species alteration, researchers may find it difficult
to identify initial causes of native species declines, determine the
extent to which they can be reversed, and assess trade-offs in recovery
efforts (Cochran-Biederman et al ., 2015; Thurstan, 2022). Such
knowledge gaps can also lead to extensive management efforts such as
habitat manipulation (e.g., increasing structural complexity, mimicking
variable flows, and reestablishment of floodplain connection) and
nonnative species control that may not necessarily restore ecosystem
processes needed to reestablish imperiled species if efforts are not
sufficient to address proximate causes of population declines (Booth,
2016; Albertson, 2018).
Limited trophic resource availability is a potential proximate cause of
some species’ declines but changes to food-web dynamics can be difficult
to isolate and quantify (Naiman et al ., 2012; Albertson et
al ., 2018; Booth, 2016; Thoms and DeLong, 2018). The difficulty arises
because changes in food-web dynamics can be mediated by synergistic
indirect abiotic (habitat homogenization and fragmentation) and biotic
(reduced prey diversity and abundance or competition with nonnative
species) effects from river regulation and fish community compositional
changes (Naiman et al ., 2012; Booth, 2016; Haubrock, 2018).
Nevertheless, trophic resource use studies have the capacity to increase
our understanding of changes in riverine food-web dynamics although
there is a scarcity of unaltered systems for comparison (Turner et
al ., 2015; McManamay et al ., 2018; Vander Zanden, 2003). In
certain instances, species or habitat compositional gradients can be
used to make comparisons but these studies are not able to detect a
departure from conditions prior to severe alterations, which might
identify trophic resources that supported a more stable ecological
system (Haubrock et al ., 2018; Rogosch and Olden, 2020; Pennocket al ., 2021). However, with the availability of museum
specimens, use of stable isotopes, and advances in Bayesian statistical
analyses, methods exist to use a hypothesis-driven framework to detect
departures from a historical condition and changes to food-web dynamics
that may underly the continued imperilment of native fishes (Turneret al. , 2015; Alp and Cucherousset, 2022).
The Colorado River Basin is a well-known example of a riverine ecosystem
whose 20th century alteration resulted in the
imperilment of many native fishes, including the system’s historical top
predator Colorado Pikeminnow (Ptychocheilus lucius ) (Tyuset al ., 1982; Minckley et al. 2003). In the San Juan River
subbasin, Colorado Pikeminnow was nearly extirpated by the 1990s (Ryden
and Ahlm, 1996; Ryden, 2003). However, 20 years of conservation
management that included mimicking a more-natural hydrograph from the
most upstream reservoir, intensive removal of large-bodied nonnative
fishes, and a hatchery augmentation program that stocked over 5 million
juvenile fish, reestablished Colorado Pikeminnow (Franssen et
al ., 2016). Yet, the current Colorado Pikeminnow adult population is
relatively small because age-specific survival rates of stocked fish are
low (Clark et al ., 2018). A significant contribution to these low
survival rates seems unlikely to be driven by nonnative fish predation
as neither Colorado Pikeminnow nor more common fishes, responded
positively to intensive nonnative fish removal (Franssen et al .,
2014); possibly because the most abundant large-bodied nonnative fish,
Channel Catfish (Ictalurus punctatus ), is not highly piscivorous
(Hedden et al ., 2021; Pennock et al ., 2021; Heddenet al ., 2022). Additionally, attempts to mimic a more-natural
hydrograph have been hampered by increased aridity in the San Juan River
basin (Pennock et al ., 2022).
Given piscivorous Colorado Pikeminnow likely benefit from a robust fish
prey base (Vanicek and Kramer, 1969), some of the slow and limited
reestablishment of the species to the San Juan River could be due to
trophic deficiencies caused by river regulation and species turnover
(Franssen et al ., 2007). To assess this hypothesis, we used
Bayesian methods to conduct stable isotope analyses of the river’s fish
community prior to and after river regulation that was coincident with
species compositional changes. To isolate the effects of species
turnover, we quantified changes to community-wide trophic structure
metrics (diversity of assimilated basal resources, food chain length, as
well as overall resource use diversity, similarity, and distribution)
with and without the inclusion of extirpated and new invasives. We then
calculated changes in trophic niche overlap for fish species present
before and after river regulation to explore potential changes in
resource competition. To better understand any changes in both
community-wide trophic structure metrics and niche overlap, we
quantified changes in trophic dispersion (niche breadth). Finally, we
inferred specific changes in trophic resource use of Colorado Pikeminnow
between time periods using an isotopic diet mixing model.