With increased globalization, our world has seen unprecedented levels of connectivity resulting in human-caused spread and introduction of organisms. This rapid dispersal of species at both temporal and spatial scales can result in invasive species which have severe negative consequences for the natural function of our ecosystems (Cardinale et al. 2012, Ricciardi et al. 2013). In fact, invasive species are considered a leading threat to global biodiversity, ecosystem function, and services important to humanity including clean water and food production (Bellard et al. 2016, Russell and Blackburn 2017).
Invasive species disrupt invaded ecosystems by severely altering food webs. This occurs as invaders are often better competitors for resources within invaded environments. In particular, freshwater ecosystems, such as lakes, rivers, and streams have been threatened by the spread and subsequent food web effects of invasive species. One such invader, the “Asian carp” which includes two species of fish: Silver Carp (Hypophthalmichthys molitrix) and Bighead carp (Hypophthalmichthys nobilis), have invaded much of the central U.S. and are currently threatening to invade the Great Lakes (Chick and Pegg 2001, Zhang et al. 2016).
Asian carp influence invaded ecosystems by consuming high volumes of both phytoplankton (primary producer) and zooplankton (primary consumer) organisms which reduces critical resources from the base of the food web (Chick and Pegg 2001). The overall effect of these food web alterations has been linked to negative effects on native fishes which rely on these food resources (Irons et al. 2007). Increased stresses on fisheries have important implications for local economies relying on fishing as both an industry and recreational activity.
Furthermore, food web modifications have been shown to influence resources spanning ecosystem boundaries onto the land through aquatic insect emergence (Collins and Wahl 2017) and could hypothetically influence predatory birds relying on a fish based diet. Despite these clear consequences of invasive Asian carp, there are disputes as to the underlying mechanisms driving food web alterations of invaded ecosystems. For example, is the suppression of phytoplankton (or primary producers) by Asian carp limiting food for primary consumers such as zooplankton, which then cascades upwards to fish? Or is it that Asian carp suppress both phytoplankton and zooplankton through their feeding which reduces both levels of the food web for other fish?
In this study, we use 25 years of long-term ecosystem monitoring to address whether invasive Asian carp have suppressed the primary producers (phytoplankton) of a large-river reservoir ecosystem since their invasion. In addition, we wanted to know if invasive carp affected certain habitats within this reservoir ecosystem differently than others. We detected decreases in phytoplankton biomass (measured as chlorophyll-a) within certain reservoir habitats since the establishment of Asian carp. Importantly, both environmental and biological drivers of phytoplankton showed no differences before and after invasion suggesting changes detected were the result of invasive Asian carp and not changes in climate or other extraneous factors. Results of our study suggest habitat specific effects of invasive Asian carp on an important food web resource within a large-river reservoir.
Specifically, reductions in phytoplankton were greater in slower flow habitats such as embayments compared to faster flow main channel habitats, suggesting that Asian carp have greater food web effects in certain river and lake habitats. These results are important because they may represent a generalizable pattern helpful in predicting Asian carp effects throughout the Mississippi River Basin and Great Lakes region. Our analysis suggests ecosystem changes from invasive Asian carp, however, the direct and indirect effects on higher trophic levels, such as fish and neighboring terrestrial ecosystems remain unknown. Species introductions are occurring at unprecedented rates, thus understanding and combatting the influences of invasive species on ecosystem functions remains a challenge for ecologists and managers (Vander Zanden et al. 1999, Ricciardi et al. 2013). Our work suggests an ecosystem-level change resulting from species invasion and highlights the value of long-term data in better understanding the importance of invasive species to a changing planet.
- Bellard, C., Cassey, P., & Blackburn, T. M. (2016). Alien species as a driver of recent extinctions. Biology Letters, 12(2), 2015.
- Cardinale, B. J., Duffy, J. E., Gonzalez, A., Hooper, D. U., Perrings, C., Venail, P., … & Kinzig, A. P. (2012). Biodiversity loss and its impact on humanity. Nature, 486(7401), 59-67.
- Chick JH, Pegg MA, (2001) Invasive carp in the Mississippi River Basin. Science 292: 2250-2251
- Collins, S. F., & Wahl, D. H. (2017). Invasive planktivores as mediators of organic matter exchanges within and across ecosystems. Oecologia, 1-10.
- Irons KS, Sass GG, McClelland MA, Stafford JD (2007) Reduced condition factor of two native fish species coincident with invasion of non‐native Asian carps in the Illinois River, USA Is this evidence for competition and reduced fitness?. J Fish Biol 71: 258-273
- Ricciardi A, Hoopes MF, Marchetti MP, Lockwood JL (2013) Progress toward understanding the ecological impacts of nonnative species. Ecol Monogr 83: 263-282
- Russell, J. C., & Blackburn, T. M. (2017). The rise of invasive species denialism. Trends in ecology & evolution, 32(1), 3-6.
- Vander Zanden MJ, Casselman JM, Rasmussen JB (1999) Stable isotope evidence for the food web consequences of species invasions in lakes. Nature 401: 464-467
These findings are described in the article entitled Top-down effects of an invasive omnivore: detection in long-term monitoring of large-river reservoir chlorophyll-a, published in the journal Oecologia. This work was led by Benjamin B. Tumolo from Montana State University.