Zooplankton studies reveal how rising temperatures can lead to population crashes

Researchers at Rice University reveal important links between rising and falling populations of species, shedding new light on how global warming threatens natural ecosystems. The study, published in Ecology and led by Volker Rudolf, revealed that rising temperatures within the population exacerbate competition and ultimately leads to population crashes at higher temperatures. It provides one of the first clear experimental confirmations that temperature rise changes the forces controlling the dynamics of natural populations.

“Our research provides an essential and lacking part in understanding the broader impact of warming on natural populations,” said Rudolf, a professor at Biosciences. “Even if individual organisms appear to thrive at higher temperatures, the entire population can still suffer as competition for resources grows.”

To clarify how temperature affects competition and population growth, the team focused on Daphnierex, a small zooplankton species that plays an important role in freshwater food networks and water quality. By manipulating temperature and population density in a controlled laboratory setting, the researchers isolated the effects of temperature rise on population dynamics. The outcome was both attractive and troublesome.

This experiment revealed that individual competition becomes significantly stronger as temperatures rise. In fact, with every 7 degrees Celsius, the effect of competition doubled, with the highest temperature tested, dramatically reducing the population of 50%.

Moderate warming (12–19 c) initially boosted population growth by accelerating metabolism and reproduction, but at higher temperatures these benefits disappeared as increased competition was sacrificed, leading to a sudden population decline, even if individual organisms allowed higher temperatures.

“We know that temperature increases the metabolism and reproduction of external heat, but we find that warm temperatures also create competition to limit survival and reproduction.” Stockseth conducted the experiment as part of an undergraduate senior paper at Rudolf’s Lab and is currently working at the Houston Zoo.

“As temperatures rise towards the physiological limits of these populations, increasing competition has begun to outweigh these metabolic benefits, leading to population decline, an important warning for ecosystems facing rising temperatures.

The findings challenge the assumption that warming always benefits the skin population by promoting individual growth. Instead, they show that even before physiological stress becomes a key factor, competition intensifying temperatures can harm the population. This increased competition could also destabilize the population, further increasing the risk of local extinction, especially in environments with frequent temperature fluctuations.

“Our findings suggest that many species may have a rapid decline in populations before they reach heat tolerance limits,” said Zoe Neal, a graduate student at Rudolfravo, who is currently a data scientist at the University of Texas MD Anderson Cancer Center. “This has a significant conservation effect as this means that temperature-driven population collapse can occur at a lower threshold previously than expected and can affect species that were thought to be resilient in temperature changes.”

As global temperatures continue to rise, such studies provide important insights to help predict and mitigate biodiversity losses. This is an important step to ensure that vulnerable species and ecosystems receive the necessary protection before it is too late.