Pace Michael L, Porter Karen, Feig Yvette S
Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Road, 96822, Honolulu, Hawaii, USA.
Department of Zoology and Institute of Ecology, University of Georgia, 30602, Athens, Georgia, USA.
Oecologia. 1984 Jul;63(1):43-51. doi: 10.1007/BF00379783.
Evidence for genetically determined life history variability within a population or a species is rare. In this three year experimental examination of a parthenogenetically reproducing population of the planktonic crustacean Daphnia parvula, we found evidence for a succession of clones or groups of clones that exhibited distinctive body size and reproductive differences that were maintained after numerous generations under standardized conditions in the laboratory. The D. parvula population reached maximum density in the fall and maintained relatively high densities through the winter and spring. Isolates from this fall-winter-spring period all had a larger body size at death and higher fecundity when compared with summer isolates under natural food and temperature conditions. These differences could not be accounted for by differences in temperature and food abundance among the seasons. An additional difference in these experiments was a shift in reproductive effort by the summer isolate which produced a higher proportion of its offspring in the first two broods. The shift in life history characteristics and a summer decline of the Daphnia parvula population was correlated with both an increase in inedible and perhaps toxic blue-green algae and an increase in a dipteran predator Chaoborus. Comparison of the survivorship curves for all of the seasonal life history experiments indicated that D. parvula survivorship was not lower during the summer discounting a toxic effect from blue-green algae. Positive population growth on natural food in the laboratory at this time indicated food was not limiting and that predation was the probable cause of the population decline.Laboratory life history experiments under standardized food and temperature conditions were run with D. parvula isolates from the spring and summer plankton. Genetically based differences as determined in these experiments were smaller body size, lower fecundity, smaller brood size, and shorter life span for the summer animals relative to spring animals. Thirty seven percent of the summer animals also reproduced at an earlier age under standardized conditions. The shift in reproductive effort to earlier broods by summer animals rnder natural conditions appeared to be a phenotypic response as the summer isolate did not produce a higher proportion of its offspring in early broods under standardized conditions.When estimates of predatory mortality were added to the life tables of the standardized experiments, the earlier reproduction of some of the summer animals allowed a population increase under a regime of intense predation. Life tables for the spring animals predicted a population decline under these circumstances. Predictable seasonal changes in biotic factors such as predation suggest a mechanism whereby diverse life history patterns with corresponding differences in r may be maintained within a population.
在一个种群或物种内，由基因决定的生活史变异性的证据很罕见。在对浮游甲壳类动物小型水蚤孤雌生殖种群进行的为期三年的实验研究中，我们发现了一系列克隆体或克隆体群体的证据，它们表现出独特的体型和繁殖差异，在实验室标准化条件下经过多代后仍能保持。小型水蚤种群在秋季达到最大密度，并在冬季和春季保持相对较高的密度。与自然食物和温度条件下的夏季分离株相比，从这个秋冬春时期分离出的个体在死亡时体型更大，繁殖力更高。这些差异不能用季节间温度和食物丰度的差异来解释。这些实验中的另一个差异是夏季分离株的繁殖努力发生了转变，它在前两窝中产生的后代比例更高。小型水蚤种群生活史特征的转变和夏季数量的下降与不可食用甚至可能有毒的蓝藻数量增加以及双翅目捕食者摇蚊数量增加有关。对所有季节性生活史实验的生存曲线进行比较表明，小型水蚤在夏季的存活率并不低，排除了蓝藻的毒性影响。此时在实验室中以天然食物为食时种群的正增长表明食物并不受限，捕食可能是种群数量下降的原因。在标准化食物和温度条件下，对春季和夏季浮游生物中的小型水蚤分离株进行了实验室生活史实验。在这些实验中确定的基于基因的差异包括，与春季动物相比，夏季动物体型更小、繁殖力更低、窝卵数更小、寿命更短。在标准化条件下，37% 的夏季动物也在更早的年龄开始繁殖。在自然条件下，夏季动物将繁殖努力转移到更早的窝中，这似乎是一种表型反应，因为在标准化条件下，夏季分离株在前几窝中并没有产生更高比例的后代。当将捕食死亡率的估计值添加到标准化实验的生命表中时，一些夏季动物更早的繁殖使得在强烈捕食的情况下种群数量增加。在这些情况下，春季动物的生命表预测种群数量会下降。诸如捕食等生物因素中可预测的季节性变化表明了一种机制，通过这种机制，种群内可能维持具有相应r差异的多种生活史模式。
