Is competition an interaction as relevant as predation for tropical planktonic cladocerans?

Abstract Aim To test the relevance of competition and predation for the two most abundant and frequent cladoceran populations, the medium-sized Ceriodaphnia richardi Sars, 1901 and the large-sized Daphnia gessneri Herbst, 1967, in a tropical shallow lake. Methods Laboratory experiments and weekly samplings in the lake for a year were performed to evaluate: density fluctuations and reproduction; predation on juveniles and adults by larvae of Chaoborus brasiliensis Theobald, 1901; competitive interactions and possible implications of competition and predation on the fluctuations of populations in the lake. Juveniles and adults of both cladocerans were offered to larvae instar IV of chaoborid, for testing selective predation in laboratory experiment. Competitive interaction between the two cladocerans was tested in experiments, with high (0.70 mg C.L-1) and low (0.25 mg C.L-1) concentrations of the edible chlorophycean Desmodesmus spinosus Chodat. Results C. richardi and D. gessneri were more abundant in the cool season, when their densities were inversely correlated, and when invertebrate predators, C. brasiliensis and the water mite Krendowskia sp., were less abundant. Despite higher concentrations of food and larger number of offsprings produced by the cladocerans in the warm season in the lake, their densities were lower, coincident with the increase of invertebrate predators, mainly chaoborid larvae. C. brasiliensis preyed on adult C. richardi and on young D. gessneri. In the competition experiments, the densities and the population growth rates of C. richardi were higher than those of D. gessneri. The competition results support the “small body size” and the “rmax” hypotheses. The exclusion of D. gessneri by C. richardi occurred only in experimental conditions, while they coexist in the lake, but without reaching their carrying capacity. Conclusions Predation is the key factor influencing the populations of cladocerans, whereas competition seems to play a secondary role, probably restricted to some periods of lower food concentration in the cool season.

摘要 研究目的：为探究竞争与捕食作用对热带浅湖中两种最为丰富且常见的枝角类（Cladocera）种群的影响，本研究选取中型物种光滑角盘蚤（Ceriodaphnia richardi Sars, 1901）与大型物种盖瑟水蚤（Daphnia gessneri Herbst, 1967）作为研究对象。
研究方法：本研究通过室内受控实验与为期一年的湖泊周度采样，对以下内容进行评估：种群密度波动与繁殖情况、巴西幽蚊（Chaoborus brasiliensis Theobald, 1901）幼虫对两种枝角类成体与幼体的捕食作用、种间竞争相互作用，以及竞争与捕食对湖泊中种群波动的潜在影响。室内捕食选择性实验中，向第4龄期的幽蚊幼虫投喂两种枝角类的成体与幼体，以检验其捕食选择性。种间竞争实验设置可食用绿藻刺被集星藻（Desmodesmus spinosus Chodat）的高浓度组（0.70 mg C·L⁻¹）与低浓度组（0.25 mg C·L⁻¹），以此探究两种枝角类间的竞争关系。
研究结果：光滑角盘蚤与盖瑟水蚤在凉季的丰度更高，此时二者的种群密度呈负相关，且无脊椎捕食者巴西幽蚊与克伦水螨属（Krendowskia sp.）的丰度较低。尽管暖季湖泊中的食物浓度更高，两种枝角类的繁殖后代数量更多，但其种群密度却更低，这与无脊椎捕食者（主要为幽蚊幼虫）的丰度升高相吻合。巴西幽蚊会捕食光滑角盘蚤成体与盖瑟水蚤幼体。竞争实验结果显示，光滑角盘蚤的种群密度与种群增长率均高于盖瑟水蚤，该结果支持“小型个体优势”与“内禀增长率（rmax）”假说。仅在实验条件下，光滑角盘蚤会排挤盖瑟水蚤；而在自然湖泊中二者可共存，但均未达到环境容纳量。
研究结论：捕食作用是影响枝角类种群动态的关键因素，而竞争仅发挥次要作用，其影响可能仅局限于凉季食物浓度较低的部分时段。