Pollution Inhibits Predation: Differential Impacts of Metal Contaminants on the Functional Responses of Native and Invasive Gammaridae
This study demonstrates that exposure to realistic metal pollutant mixtures suppresses predation in both native and invasive amphipods, but the invasive species *Dikerogammarus haemobaphes* exhibits greater resilience than its counterparts, suggesting that pollution may differentially alter invasion impacts by providing a relative advantage to tolerant invaders.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Freshwater rivers and streams are under siege from two distinct but often overlapping threats: the arrival of invasive species and the slow accumulation of chemical pollution. Invasive species are animals that have moved into a new environment, often outcompeting or eating the native life that belongs there. Pollution, particularly from metals released by industry and agriculture, coats the water in substances that can harm the health and behavior of the creatures living within it. Scientists have long studied how these two problems work separately, but they have struggled to understand what happens when they collide. Does a polluted river make it harder for an invasive predator to hunt, potentially giving native species a fighting chance? Or does the pollution weaken the native species so much that the invader gains an even stronger advantage? To answer this, researchers look at the "functional response," a concept that simply describes how a predator's eating rate changes as the number of available prey increases. It reveals whether a predator is a relentless hunter that keeps eating as food gets plentiful, or if it slows down because it gets tired or full.
In a recent study, researchers set out to test how a mixture of common metal contaminants affects the hunting skills of three different types of freshwater shrimp-like creatures known as amphipods. Two of these species are notorious invaders from the Ponto-Caspian region, while the third is a native species found in the United Kingdom. The team placed these animals in water containing realistic, low-level mixtures of copper, aluminium, iron, and manganese, mimicking the conditions found in many polluted European waterways. They then offered the amphipods two different types of prey: small, swimming water fleas and the larvae of non-biting midges, often called bloodworms. By counting how many prey were eaten at different densities, the researchers could measure exactly how the pollution altered the predators' ability to find, catch, and consume their food.
The results showed that pollution acts as a powerful brake on predation for all the creatures involved. When the water was clean, the invasive species generally proved to be more efficient hunters than the native one, consuming more prey as the density increased. However, once the metals were introduced, the hunting success of every species dropped significantly. The animals were less likely to start eating at all, and when they did, they consumed far fewer prey items. The pollution seemed to disrupt their senses or their energy, making the act of hunting much more difficult. This suppression was so severe under high levels of contamination that the native species essentially stopped showing any pattern of hunting based on how much food was available; they simply did not eat enough to establish a rhythm.
Yet, the story was not the same for every invader. While the native species and one of the invaders suffered a sharp decline in their hunting ability, the other invasive species displayed a surprising resilience. Under low levels of pollution, this particular invader maintained a higher feeding rate than the others, continuing to hunt effectively even as its competitors struggled. It managed to keep its predatory impact relatively high, whereas the native species and the other invader saw their hunting efficiency collapse. This suggests that not all invaders are affected equally by pollution. While the presence of metals generally weakens the food web, it may inadvertently favor specific invasive species that are better at tolerating these chemical stresses.
The study highlights that the interaction between pollution and invasion is complex and depends heavily on the specific traits of the species involved. It is not a simple case where pollution helps the invader or helps the native; rather, it reshuffles the deck based on who can withstand the toxic environment best. The findings suggest that in polluted waters, the native species may lose its ability to compete for food not just because of the invader's aggression, but because the chemical environment has dulled its senses and slowed its movements. Meanwhile, a pollution-tolerant invader might continue to dominate, altering the ecosystem in ways that a clean-water scenario would not predict. This research provides a clearer picture of how human-made contaminants can subtly change the rules of nature, potentially allowing certain invasive species to thrive where others would fail, ultimately changing the balance of life in our freshwater systems.
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