Data from: Life history evolution in guppies VIII: the demographics of density regulation in guppies (Poecilia reticulata)

In prior research, we found the way guppy life histories evolve in response to living in environments with a high or low risk of predation is consistent with life history theory that assumes no density-dependence. We later found that guppies from high predation environments experience higher mortality rates than those from low predation environments, but the increased risk was evenly distributed across all age/size classes. Life history theory that assumes density-independent population growth predicts that life histories will not evolve under such circumstances, yet we have shown with field introduction experiments that they do evolve. However, theory that incorporates density regulation predicts this pattern of mortality can result in the patterns of life history evolution we had observed. Here we report on density manipulation experiments performed in populations of guppies from low predation environments to ask whether natural populations normally experience density regulation and, if so, to characterize the short term demographic changes that underlie density regulation. Our experiments reveal that these populations are density regulated. Decreased density resulted in higher juvenile growth, decreased juvenile mortality rates and increased reproductive investment by adult females. Increased density causes reduced offspring size, increased fat storage by adult females and increased adult mortality.

既往研究中，我们发现花鳉（guppy）的生活史在应对高或低捕食风险环境时的演化模式，与不考虑密度依赖的生活史理论相符。后续我们进一步发现，来自高捕食风险环境的花鳉种群死亡率高于低捕食风险环境的种群，但死亡率的升高在所有年龄/体型组中均匀分布。假设种群增长与密度无关的生活史理论预测，在这类情境下生活史不会发生演化，但我们通过野外引种实验已证实，花鳉的生活史确实发生了演化。然而，纳入密度调控的生活史理论则预测，这种死亡率模式可导致我们观测到的生活史演化格局。本研究针对来自低捕食风险环境的花鳉种群开展密度操控实验，旨在探究自然种群是否普遍存在密度调控现象，若存在则进一步阐明支撑密度调控的短期种群动态变化特征。实验结果表明，此类花鳉种群确实受密度调控：种群密度降低会促使幼体生长速率提升、幼体死亡率下降，同时增加成年雌性的繁殖投入；而种群密度升高则会导致后代体型减小、成年雌性的脂肪储备增加，同时提升成体死亡率。