| Predator-prey interactions play major and direct roles in the structuring of zooplankton communities.Predation of Asplanchna may drive not only the ecological,but also the evolutionary dynamics of prey populations.In the presence of predators,many planktonic invertebrates develop various inducible defenses that reduce the risk of predation,such as morphology,behavior or life history.With Brachionus angularis and B.calyciflorus as prey species,and A.brightwelli and A.sieboldi as predator species,this study investigated the life history traits of the two species of predators at different temperatures and prey(Brachionus)densities,and inducible defenses of the two species of preys in ecology and morphology,and their rapid evolutionary changes under different indirect predation pressure of Asplanchna.The results of these researches will help us further understand the mechanism of rapid evolution,population dynamics and coexistence of predator and prey.The main research contents and results were presented as following:The life-table demography and population growth of A.brightwelli were investigated at four temperatures(16,20,24 and 28°C)using B.angularis as prey at four densities(10,20,30 and 40 ind./mL).The results showed that temperature affected significantly all the life-table demographic parameters(age-specific survivorship and fecundity,average lifespan,life expectancy at hatching,generation time,net reproductive rate and intrinsic rate of population increase)and population growth rate obtained from the population growth studies,prey density affected the generation time,net reproductive rate,intrinsic rate of population increase and population growth rate,and the interaction between temperature and prey density affected the generation time and population growth rate.Both the average lifespan and life expectancy at hatching were the longest at 16°C,the generation times were longer at lower temperatures(16 and 20°C)and higher prey densities(30 and 40 ind./mL),the net reproductive rates were higher at lower temperatures(16 and 20°C)and 20-40 ind./mL of B.angularis,and the population growth rates were higher at 20°C under 20-40 ind./mL of B.angularis.The life-table demography and population growth of A.sieboldi cultured at three temperatures(16,20 and 24°C)employing B.calyciflorus as prey at four densities(5,10,15and 20 ind./mL)were investigated,and found that temperature affected significantly the average lifespan,life expectancy at birth,generation time,net reproductive rate and intrinsic rate of population increase of A.sieboldi;the interaction between temperature and prey density affected markedly the generation time;both temperature and prey density,and their interaction affected the peak population density;prey density,and the interaction between temperature and prey density affected the population growth rate.Regardless of the prey density,the average lifespan,life expectancy at birth and generation time of A.sieboldi decreased with increasing temperature.The net reproductive rate of A.sieboldi at 20°C was the highest,but at 24°C the lowest.The intrinsic rate of population increase of A.sieboldi was higher at 20°C than at 16 and 24°C.The peak population abundance and population growth rate of A.sieboldi at 16°C and fed 20 ind./mL of B.calyciflorus was the highest.Parental B.angularis females cultured at 1.0×10~6 and 2.0×10~6 cells/mL of Scenedesmus obliquus were indirectly exposed to 0,40,80 and 160 ind./L of A.brightwelli using mesh enclosure,and their life-table demographic parameters,population growth rates and morphological characters were calculated and measured.The results showed that Asplanchna-released kairomone significantly decreased the average lifespan,life expectancy at hatching,generation time and net reproduction rate,but increased the proportion of sexual offspring of parental B.angularis females.The threshold Asplanchna density required for significant effects varied with food level.Kairomone released by Asplanchna at 80 ind./L increased significantly the intrinsic rate of population increase of B.angularis cultured at2.0×10~6 cells/mL of S.obliquus,which would offset the mortality of exposed females from predation.The accumulation of kairomone in aquatic environments enhanced the indirect effect of Asplanchna on the population growth of B.angularis.The present results indicated that rapid population growth of B.angularis induced by Asplanchna kairomone might facilitate the coexistence of preys with predators,and higher proportion of sexual offspring and then resting egg production might help the preys avoid the predators in time instead of facing the enemy through defenses.The life-table demographic parameters,population growth rate and morphological parameters of B.calyciflorus in response to various densities of A.sieboldi(0,40,80 and 160ind./L)at two food levels(1.0×10~6 and 2.0×10~6 cells/mL of S.obliquus)were investigated.To resist predators,B.calyciflorus developed long postero-lateral spines.The life-table demographic results showed that predation only significantly shortened the generation time of B.calyciflorus at the algal level of 2.0×10~6 cells/mL,but did not affect other demographic parameters.The population growth tests revealed that predator density,algal level and their interactions all significantly influenced the population growth rate of prey rotifers.At 1.0×10~6cells/mL of S.obliquus,treatments with 40,80 and 160 ind./L of A.sieboldi significantly reduced the population growth rate of B.calyciflorus.In contrast,the population growth rate was not affected by 40 ind./L of A.sieboldi at 2.0×10~6 cells/mL of S.obliquus.These results suggested that algal level probably affected the inhibitory effects of predation on the population growth of prey rotifers,and the underlying mechanisms should be further investigated.The rapid evolution of B.angularis population under different indirect predation pressure of A.brightwelli was investigated.After 75 days of selection,the evolutionary changes of the net reproductive rate,intrinsic rate of population increase,population growth rate,maximum population density,egg ratio,mictic rate,lorica length and width,body size and egg volume of B.angularis were observed under low predation pressure of A.brightwelli(at 8 ind./150mL),but those of the intrinsic rate of population increase,maximum population density,egg ratio,mictic rate of B.angularis were observed under heavy predation pressure of A.brightwelli(at 16 ind./150mL).The net reproductive rate,intrinsic rate of population increase,population growth rate and maximum population density of B.angularis under low predation pressure of A.brightwelli were higher than the control,but the intrinsic rate of population increase,population growth rate and maximum population density of B.angularis under high predation pressure of A.brightwelli were lower than the control.The mictic rates of B.angularis under predation pressures of A.brightwelli were higher than the control.The results indicated that a certain predation pressure of Asplanchna increased the population growth rate and the short-term fitness,predation pressure induced higher levels of sexual reproduction and resting egg formation which may result in a higher long-term fitness,and low predation pressure facilitated B.angularis population evolve faster.The rapid evolution of B.calyciflorus population under different indirect predation pressure of A.sieboldi was investigated.After 60 days of selection,the life expectancy,average lifespan,generation time,net reproductive rate,intrinsic rate of population increase,population growth rate,maximum population density,egg ratio,and length of antero-lateral and postero-lateral spines in B.calyciflorus under low predation pressure were significantly higher than control.Under heavy predation pressure,the life expectancy,average lifespan,mictic rate,and length of antero-lateral and postero-lateral spines of B.calyciflorus were observable higher than control,but the reverse was also true for the net reproductive rate,population growth rate,maximum population density and egg ratio.The intrinsic rate of population increase,population growth rate,mictic rate and length of postero-lateral spines were the suitable fitness parameters.Low predation pressure dramatically increased the short-term fitness;high predation pressure decreased the short-term fitness,but contributed to the higher long-term fitness through increasing the mictic rate and length of postero-lateral spines.Decrease in reproduction caused by the induced defences of B.calyciflorus might be related to the Asplanchna predation pressure.The sustainable population growth of B.calyciflorus under severe predation pressure might be attributed to rapid adaptation to variable habitat. |
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