Changes to the trophic structure of a desert fish community following
river regulation and species turnover: implications for an endangered
top predator
Abstract
A poor understanding of factors leading to species decline can result in
inefficient or ineffective species restoration. Endangered Colorado
Pikeminnow ( Ptychocheilus lucius) was nearly extirpated from the
San Juan River, NM, USA and recent efforts to reestablish the species
via hatchery augmentation of juveniles has yet to reach the targeted
number of adults in the system. To assess how changes to the river’s
food web could be limiting reestablishment of this top predator, we used
stable isotope (δ 13C and δ 15N)
signatures of the fish community pre- (museum specimens) and post river
regulation with coincident extirpations and invasions. Following river
regulation, four of five community-wide trophic structure metrics used
to quantify resource use diversity were reduced and species turnover
likely had little effect per se. For species sampled in both time
periods, shared trophic resources generally increased (mean niche
overlap = +35.7%). However, Colorado Pikeminnow experienced a large
decrease in niche breadth (-72%) and diet mixing models suggested a
shift from piscivory towards insectivory in contemporary collections.
Our results suggested an overall reduction in basal resource
availability after river regulation. We suggest increased reliance on
similar resources may be limiting fish prey for Colorado Pikeminnow,
ultimately contributing to the slow and limited reestablishment in the
San Juan River. This study adds to the growing body of knowledge
provided by museum specimens and stable isotope analyses to identify
food-web dynamics that are a departure from historical conditions which
can provide novel information critical to imperiled species management
in modified systems.