Removing invasive stream macrofauna shifts non-target invertebrate mesofauna through facilitation

Positive interactions among non-native species can drive invasional
meltdowns to the detriment of native biota. Here, we assessed whether
targeted control of aquatic invasive species (AIS) can benefit native
species by eliminating synergies among invaders. We did so by monitoring
changes in the abundance of native and non-native benthic invertebrates
following the targeted removal of non-native fish and crustaceans in 10
streams on the island of Oʻahu (Hawaiʻi, USA). Benthic invertebrate
sampling was conducted in paired control reaches and removal reaches in
each stream immediately following removals, one month following removals,
and then at two to three-month intervals for 16 months. Temporal variation
in the abundance, composition, and diversity of the native and non-native
invertebrates was compared among streams and between treatment reaches
using multivariate data visualization and mixed effects models. We
observed both seasonal shifts in overall community composition and
treatment-specific effects on the abundance of common taxa that were
mediated by the number of AIS fish removed. Most notably, as removal of
non-native poeciliid fish increased, we observed concordant decreases (-32
± SE 13%) in non-native caddisfly (Cheumatopsyche analis) and increases
(122 ± SE 69%) in partially native chironomid midges in the treatment
reaches relative to the control reaches. Our results provide experimental
evidence supporting the hypothesis that predation by introduced poeciliids
on midges indirectly facilitates non-native caddisfly populations via
competitive release. Our findings indicate that removal of poeciliids
allows midges to outcompete non-native caddisflies and increase their
abundance. Our study illustrates that targeted removal of non-native
species can have cascading negative effects on other introduced taxa.
Understanding trophic relationships among invasive and native species can
thus enhance management efforts by maximizing benefits to native species
relative to effort and cost.