Beautiful Killers—The Goldfish Effect

The resistance and resilience of ecosystems to disturbance and change are astonishing; however, all good things eventually have an end. Depending on the severity of the disturbance, the ecosystems' ability to balance these disturbances dwindles, and well-established systems can crash. Introductions of foreign animals into ecosystems happen around the world, in every environment, and can lead to drastic consequences—though not all negative—within the recipient ecosystem (Chaffin et al. 2016). So-called “alien species” or “invasive alien species” (IAS) can stem from virtually all organism groups, and while not all newly introduced species become problematic, some of them establish functioning populations, can dominate over native species, and lead to major disruptions of ecosystem functioning, local species extinction, or act as vectors of diseases (Colautti and MacIsaac 2004; Crowl et al. 2008).

Among the groups which can have a great impact on recipient ecosystems are fish, with almost 200 recorded introduced species in Europe as of today (Carpio et al. 2019). Although animal migrations can lead to natural species displacements, human actions are responsible for many modern-day introduction events. Relevant vectors for fish introductions can include fishery or release of ornamental pet fish (Carpio et al. 2019). Goldfish, Carassius auratus, are among the most used and beloved pet fish, being sold around the globe to be kept in fish bowls, aquariums, and garden ponds. The consequences of the release of these ornamental pets which to many people, are just “small colorful fish” in natural systems can be drastic. There are multiple reasons why goldfish are the ideal candidate for being a “successful” IAS. Goldfish, stemming from the same family (Cyprinidae) as the well-known carp, are extremely resilient to adverse environmental conditions, and, as omnivorous fish, eat virtually anything in their vicinity such as aquatic plants, or insect larvae. What seems so harmless to our eyes when swimming in our indoor aquarium is indeed a perfect killing machine when left to do as they please.

In their original research article, combining an eco-functional approach and food web modelling based on taxonomic, functional, and stable isotopes analysis, Lejeune, Lepoint, and Denoël (2024) delve into the devastating effects of introduced goldfish as predators in the food webs of naturally fishless permanent ponds in France. Historically, these ponds harbored palmate newts as their dominant amphibian predator, which suffered subsequent exclusion from the ponds after fish introduction. In ponds that feature goldfish, only a third of the consumer communities' richness persevered, which makes the extent of simplification of the entire food web evident. Additionally, reduced food chain length and the prevalence of detritus-feeding groups in ponds with introduced fish, severe habitat and biodiversity loss by reduction or complete eradication of macrophytes, the loss of larger-bodied cladocerans in favor of smaller ones, and eradication of all amphibian tadpoles except those of bufonids in the goldfish ponds paint a dire picture of an aquatic wasteland.

Although the interaction pathways of the introduced goldfish and their surrounding are undoubtedly complex, the end result is rather straightforward: an ecological catastrophe.

Usually, permanent ponds harbor substantial aquatic biodiversity, and their unique characteristics such as pronounced habitat heterogeneity, clear water conditions, and well-established aquatic vegetation make them true gems in our landscapes in terms of their ecological value. Aquatic communities in ponds exist in a comparatively small space together, and species are highly adapted to the conditions, including native predators such as newts or predatory macroinvertebrates. Once introduced fish occupy key roles in the local food web, such as top predator, and exert unprecedented top-down pressure, the entire structure gets reshaped, with far-reaching implications for pond trophic state, community composition, and ecosystem functioning. As Lejeune, Lepoint, and Denoël (2024) also point to in their concluding remarks, besides the dramatic effects on the pond itself, further effects of food web simplification become even more striking when considering the linkages that exist between water bodies and their surrounding terrestrial matrix. Many consumers rely on aquatic resources to supplement their diet, and aquatic insect communities are important sources of essential nutrients for the terrestrial food web (Epanchin, Knapp, and Lawler 2010). Those subsidies cannot be supplemented by non-aquatic species; thus, if those species are eradicated by hungry goldfish, the associated nutrients are also lost. This perspective shows that seemingly harmless organisms can create negative ripple effects across ecosystem boundaries, culminating in impacts on regional biodiversity and ecosystem services that at first sight might seem unrelated—yet, everything is connected.

It is safe to say, that IAS constitute a stressor, leaving the recipient ecosystems and their communities more vulnerable and less resilient to further deterioration, for example by climate change (Crowl et al. 2008; Lejeune, Lepoint, and Denoël 2024). In the aquatic realm, climate change-related conditions that are expected in the future such as increasing eutrophication or rising water temperatures, will benefit highly resilient species such as goldfish, and, as previously shown, goldfish can indeed accelerate regime shifts and thus co-create and exacerbate unfavorable conditions for many organisms. The importance of (permanent) ponds as biodiversity havens and small ecosystems that deliver a multitude of valuable ecosystem services to their environment and our societies is by now a well-researched topic. Considering the ongoing climate change and anthropogenic pressures influencing our landscapes, it is of utmost importance to adequately protect and manage the water bodies we have—including the management of introduced species.

For IAS, as with many other issues, the best medicine is prevention. Although it is virtually impossible to create ideal conditions for every group of organisms simultaneously, and reality shows that compromises are the currency of life, conservation efforts should focus on preserving conditions that allow the existence of diverse communities. This entails the adequate policy framework and regulations set in place to have legal means when dealing with IAS, and requires enhanced monitoring and reporting on the IAS situation in our water bodies, to have accurate estimates of economic damage and impact on local ecosystems (Epanchin-Niell, 2017; Cuthbert et al. 2021).

As the removal of IAS is a tedious, time, and resource-consuming task, and the complete eradication of IAS from all affected systems can feel near impossible, the prevention of such situations has to be at the center of decision-making processes. IAS management has to be system-appropriate (Oertli and Parris 2019), as introductions of fish in small ponds might be easier to handle (e.g., via electrofishing), compared with larger water bodies where native organisms may have more shelter options and the effects of IAS are delayed. As mentioned by the authors (Lejeune, Lepoint, and Denoël 2024), education measures are an important tool. Enabling people—local citizens, site managers, and affected stakeholders—to understand firstly the potential impact some non-native animals might have in the local ecosystems, but also the “goldfish effect” of tipping the scale in favor of one particular species and the cascade of ensuing biodiversity loss and all its implications, including the loss of organisms exporting essential nutrients, pollinator species, and so forth. Increased awareness of the inter-connectedness of ecosystems and the impact even the smallest creatures can have will enable informed management strategies, including the conservation of fishless ponds in our landscapes, within which aquatic diversity thrives.

Lena Fehlinger: conceptualization, writing – original draft, writing – review and editing.

The author declares no conflicts of interest.