Certain aspects of animal mating behavior can significantly influence food chains and ecosystem functioning. Studies on North American fish have shown that aggressive mating behavior in males can actually promote the survival of zooplankton.

Eastern Mosquitofish and Their Impact on Ecosystems

The eastern mosquitofish (Gambusia holbrooki) originates from North America and was artificially introduced into various bodies of water to control mosquito larvae. Over time, it became clear that this invasive species causes considerable harm to freshwater ecosystems, as it preys not only on mosquitoes but also on other invertebrates, amphibian larvae, and zooplankton.

The Role of Mating Behavior in Ecosystems

The impact of mosquitofish on ecosystems has been studied for a long time, but the influence of mating behavior has often been overlooked. This species exhibits pronounced sexual dimorphism: females are larger than males and differ in several behavioral traits.

An international team of researchers investigated how differences in mating behavior within eastern mosquitofish populations affect their prey communities. To do this, they conducted experiments in controlled environments using large tanks (mesocosms) that simulated freshwater ecosystems. Each tank was seeded with sediment and plankton from a nearby pond to recreate a natural environment. The fish were sorted by sex and placed in tanks with varying male-to-female ratios—from no males at all to tanks dominated by males, as well as intermediate ratios of 25%, 50%, and 75% males. There was also a control tank with no fish.

Aggressive Male Behavior and Its Consequences

Male mosquitofish display aggressive and persistent behavior toward females, constantly pursuing them for mating and using coercion rather than courtship. In biology, this is referred to as sexual harassment.

In tanks with a high proportion of aggressive males, researchers observed a significant decline in the number and average size of large zooplankton, along with an increase in algae due to reduced grazing pressure from zooplankton.

In tanks with fewer aggressive males, these effects were less pronounced: zooplankton numbers and size remained higher. This is because persistent harassment forces females to spend more energy avoiding males rather than foraging, which allows the fish’s prey to survive better. Additionally, microorganisms that are not constantly harassed can avoid hiding as deeply and devote more attention to reproduction and feeding.

Impact on the Food Chain

As a result, females consume less zooplankton, and their behavior indirectly affects the entire food chain. Meanwhile, the males themselves, preoccupied with chasing females, also hunt invertebrates less frequently.