Of olives and carp: interactive effects of an aquatic and a terrestrial invader on a stream‐riparian ecosystem

Abstract

Multiple invasive species may interact, influencing one another and generating synergistic effects on food webs and ecosystem processes. We investigated the interaction between two non‐native species widespread in the western USA: common carp (Cyprinus carpio) and Russian olive (Elaeagnus angustifolia), an invasive riparian tree associated with di‐nitrogen fixation. Deep Creek, Idaho, was an International Biological Program site in the early 1970s; at that time, carp were rare and Russian olive was absent. Subsequently, Russian olive was introduced and established a dense stand, increasing allochthonous inputs and nitrogen‐rich benthic organic matter. Since 1971, carp density has increased ~4× (an increase our bioenergetic analysis suggests could not have been sustained in the absence of Russian olive). Carp gut contents in 2013–2014 revealed, on average, ~40% olives, and, similarly, stable isotope analyses revealed ~58% of carp tissues were derived from olives. A small‐scale, short‐term experimental exclusion of these subsidized carp caused ~3× increases in macrophytes and chlorophyll‐a, suggesting they may limit algae and macrophyte biomass. Moreover, carp that consumed olives excreted more nitrogen (~2× more ammonium, ~2× more total dissolved nitrogen, and ~3× more total nitrogen) compared to those that had not, which may amplify recycling and export from streams invaded by both species. This scenario is characteristic of an “invasional meltdown,” with attendant changes in food webs and ecosystem processes.

Diets of invasive channel catfish are subsidized by invasive riparian trees

“…the subterminal mouths of suckers are inefficient for surface feeding) because they did not evolve with fruiting riparian trees. Olive availability may provide a disproportionate advantage to nonnative catfish and carp, which have evolutionary histories of frugivory (Chick et al, 2003; Heinrich et al, 2021). However, the net effects of olive subsidies likely depend on consumer traits in the recipient community.…”

“…Specifically, Heinrich et al (2021) found that ~58% of common carp tissues were derived from olive fruit in a small Idaho stream. Using bioenergetic modelling, Heinrich et al (2021) also reported that olive fruit provided a trophic subsidy that facilitated the invasion of nonnative common carp and that the subsidy resulted in higher carp biomass after the invasion. A similar relationship may exist between olive and catfish, but catfish are less adapted for consuming seeds (e.g.…”

“…Invasive species can act synergistically to increase the rate and success of additional species introductions (Simberloff, 2006; Simberloff & Von Holle, 1999). For example, monocultures of invasive riparian trees can have particularly acute effects on both habitat and nonnative species introductions in riverine ecosystems (Heinrich et al, 2021; Katz & Shafroth, 2003; Mahoney et al, 2019; Mineau et al, 2012). Specifically, riparian invasion can alter sediment transport and retention, causing changes in channel morphology (West et al, 2020).…”

“…Olive fruit may provide an important energetic subsidy for catfish in the San Juan River similar to that reported for common carp ( Cyprinus carpio ) elsewhere. Specifically, Heinrich et al (2021) found that ~58% of common carp tissues were derived from olive fruit in a small Idaho stream. Using bioenergetic modelling, Heinrich et al (2021) also reported that olive fruit provided a trophic subsidy that facilitated the invasion of nonnative common carp and that the subsidy resulted in higher carp biomass after the invasion.…”

Food web perspectives and methods for riverine fish conservation

“…Invasive species can significantly alter riverine food webs by rerouting basal resources that support consumers (Moore et al, 2012), altering nutrient cycling (Atkinson et al, 2010; Capps & Flecker, 2013; Heinrich et al, 2021), and competing with and preying upon native organisms (Baxter et al, 2004; Koel et al, 2019). Understanding the impacts of invasive species—and if/how they can be controlled—often necessitates a food web approach.…”