SANTA CRUZ, Calif. – For decades, ecologists have known that how a species looks or eats affects its environment. But a new study by researchers at the University of California, Santa Cruz, shows that social behavior related to mating can be an equally powerful ecological force.
The research, published March 11 in Proceedings of the Royal Society B , demonstrates that the reproductive behaviors of a single species can trigger a “threshold response” that dictates the size and abundance of prey communities—a finding that challenges long-held assumptions about what drives ecological change.
In their study, the team focused on the western mosquitofish, a species referred to as the “plague minnow” due to its aggressive impact on native habitats when introduced. Male mosquitofish are persistent and coercive in their sexual pursuits, relentlessly chasing females for mating opportunities. By manipulating male harassment levels in a lab setting that simulated freshwater ponds, the researchers discovered a stark difference in environmental outcomes depending solely on the intensity of this social behavior.
“Researchers have generally not tested the influence of mating behaviors on ecosystems, largely because we assumed that only traits shaped by natural selection would impact ecology,” said the study’s senior author, Eric Palkovacs, a professor of ecology and evolutionary biology. “At the most general level, the major breakthrough of this work is that we show that sexual selection matters for ecology,” said co-author Suzanne Alonzo, also a professor of ecology and evolutionary biology, in whose research group the study originated.
Mentored by Alonzo and Palkovacs, then-Ph.D. student Doriane Weiler developed the original idea for and led the project, supported by a National Science Foundation Graduate Research Fellowship that funded her experimental work. As part of the study, she transformed a portion of UC Santa Cruz’s Coastal Science Campus into a laboratory of 52 miniature pond ecosystems, or “mesocosms.”
Inside each simulated pond, the team meticulously recreated a natural freshwater environment. They placed a hollow concrete cinder block at the center of each tank to provide essential shelter for the fish, filled them with water, and seeded them with phytoplankton and nutrients to stimulate growth of microscopic algae that form the base of the food web.
To ensure a robust community of prey, they then introduced a mix of zooplankton collected from a local pond and supplemented them with tiny lab-grown crustaceans commonly known as water fleas. The entire array was covered with mesh netting to protect the fish from predators such as herons, while still exposing the water to natural sunlight and temperature fluctuations.
The centerpiece of the experiment, however, was the manipulation of the male mosquitofish’s social “type”—how persistently males pursued females to mate. Before being introduced to the ponds, the fish were tattooed with tiny, colorful tags near their dorsal fins that allowed researchers to track individual behavior.
To create the “high-harassment” and “low-harassment” groups, the team relied on a behavioral quirk: Males kept away from females for a period become significantly more persistent in their mating attempts once reunited. So they formed the high-harassment group by isolating them from females for just over three weeks.
Weiler said the inspiration for this manipulation was the result of a lucky accident. While counting fish for a different project, she temporarily separated males and females. “One of the males who was kept without females jumped into a tank with females, and I noticed his mating behavior was very exaggerated,” Weiler said. “When I looked into the literature, I found that other researchers had observed the same effect—isolating males can alter their mating behavior.”
The low-harassment males spent the same time period in larger tanks with a 1-to-2 male-to-female ratio, allowing them to engage in normal social and mating interactions. Once the preparation was complete, the researchers introduced 14 fish—seven males and seven females—into each pond mesocosm. By selecting one male of each tag color for every tank, the team ensured a consistent and identifiable social environment, setting the stage to observe how the “pre-conditioned” harassment levels of the males would eventually reshape the entire aquatic community.
The study’s most novel finding is that high levels of male harassment significantly reduced the abundance of dominant zooplankton and decreased the average body size of the water fleas. These ecological changes were not observed in tanks with low-harassment fish, suggesting that a specific behavioral threshold must be crossed before social interactions reshape an entire food web.
“Harassment is metabolically costly for both males and females,” noted Alonzo, explaining that the constant energy expended in chasing or evading mating attempts likely increases the fishes’ hunger. To sustain these high activity levels, the fish—particularly the harassed females, which are much larger than males—may consume more prey or target larger, more nutrient-dense organisms.
Under high-harassment conditions, the mosquitofish essentially expanded their “dietary footprint,” selecting against larger prey and driving down the population of preferred zooplankton. In contrast, when harassment was low, the fish had a negligible impact on the zooplankton community’s composition.
“This demonstrates that the impact of a species introduction is not just about how many fish are added to a pond,” said Palkovacs, “but the specific mating dynamics they bring with them.”
Weiler added that mosquitofish are notorious for their ecosystem effects when they invade new habitats, as one of the most widespread invasive freshwater fish species. “What surprised us is that those impacts weren’t fixed,” Weiler said, “they depended on how intensely males were harassing females.”
While the study utilized the mosquitofish as a model, the implications are far-reaching. Palkovacs—also director of UC Santa Cruz’s Institute of Marine Sciences and its Fisheries Collaborative Program —emphasized that this dynamic is likely common across other ecosystems.
“Sexually selected traits, such as mating behaviors, are just as, if not more, likely to have ecosystem consequences as traits involved in survival,” Alonzo said. “We are only just beginning to study how mating interactions such as these affect ecosystems, let alone how they affect responses to environmental change.”
Proceedings of the Royal Society B Biological Sciences
Experimental study
Animals
Intraspecific variation in mating behaviour modulates the effects of mosquitofish introduction on prey communities
11-Mar-2026
Authors declare no competing interests.