"Our research provides an essential missing piece in understanding the broader effects of warming on natural populations," said Rudolf, professor of biosciences. "Even when individual organisms seem to thrive at higher temperatures, the population as a whole may still suffer as competition for resources intensifies."
To reveal how temperature influences competition and population growth, the team focused on Daphnia pulex, a small zooplankton species that plays a vital role in freshwater food webs and water quality. By manipulating temperature and population density in a controlled laboratory setting, the researchers isolated the effects of rising temperatures on population dynamics. The results were both fascinating and troubling.
The experiment revealed that competition among the individuals became significantly stronger as temperatures rose. In fact, for every 7 degrees Celsius increase in temperature, competition effects doubled, causing a dramatic 50% population decline at the highest tested temperature. While moderate warming (12-19 C) initially boosted population growth by accelerating metabolism and reproduction, at higher temperatures these benefits vanished as the increase in competition took its toll, leading to sharp population declines even as individual organisms tolerated the higher temperatures.
"We know warming temperatures increase metabolism and reproduction in ectotherms, but we found that warmer temperatures also create competition that limits survival and reproduction," said Lillie Stockseth, alumna and first author of the study. Stockseth carried out the experiment as part of her undergraduate senior thesis in Rudolf's lab and is now working for the Houston Zoo. "As temperatures rose toward the physiological limit for these populations, increased competition began to outweigh those metabolic benefits and led to population declines. This is an important warning for ecosystems facing rising temperatures - populations could approach decline at less severe temperatures than we've thought."
The findings challenge the assumption that warming always benefits ectotherm populations by boosting individual growth. Instead, they show that rising temperatures can harm populations by intensified competition even before physiological stress becomes a critical factor. This increase in competition can also destabilize populations, further intensifying the risk of local extinctions, especially in environments with frequent temperature fluctuations.
"Our findings suggest that many species could face rapid population declines long before they reach their thermal tolerance limits," said Zoey Neale, alumna and former graduate student in Rudolf's lab who now works as a data scientist at the University of Texas MD Anderson Cancer Center. "This has major implications for conservation as it means that temperature-driven population collapses could occur at lower warming thresholds than previously expected and could affect species that were thought to be resilient to temperature changes."
As global temperatures continue to rise, research like this provides crucial insights that can help predict and mitigate biodiversity loss, a key step to ensuring that vulnerable species and ecosystems receive the protection they need before it's too late.
Research Report:Strengthening of negative density dependence mediates population decline at high temperatures
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