The population dynamics of Daphnia magna are observed under three different conditions; low, medium, and high density. The effects of different population densities on the survivorship and reproduction of Daphnia are observed over a two-week period within a lab environment. Over the two week period, the numbers of parent Daphnia alive and dead are recorded daily, along with the amount of offspring produced each day. From the main parameter investigated, the net reproductive rate, the results of the experiment support that higher densities result in less successful reproduction and decreased fecundity. Values for the instantaneous growth rate of the populations also suggests that low and medium density populations allow for greater growth rates than high density populations. The results indicate that greater resource availability is directly related to higher fecundity, but that medium density populations can have similar growth rates despite a much smaller birth rate.
In any population, birth rates, mortality rates, immigration, and emigration determine whether the population’s numbers will increase, decrease, or remain the same. Factors that can have great effects on both fecundity and mortality rates can include density-dependent mechanisms (those that have a greater influence on population dynamics as population densities remain high) and intraspecific competition (occurs when members of the same species compete for resources). The objective of our experiment is to determine the effect that population density has on both the survival and reproduction of lab-grown Daphnia magna. Because the experiment was done in a controlled lab environment, immigration and emigration of individuals can be ignored. This study will address how certain variables, such as net reproductive rate and instantaneous rate of growth, change with respect to three different population densities of Daphnia. The ecological hypothesis of this experiment is that higher densities will lead to more competition for food between Daphnia, resulting in lower rates of survival and reproduction. A similar experiment was carried out by Cox et al. (1992), who found that decreased food levels resulted in a smaller cumulative fecundity but larger offspring while relatively high food levels resulted in many offspring of smaller size. This illustrates how organisms can alter their life history strategies in the face of stressful conditions and limited resources. Yurista and O’Brien (2001) observed the population dynamics of Daphnia in the natural environment and found that both food availability and quality have the greatest effect on the population growth of Daphnia middendorffiana. They also observed that greater resources lead to greater survivorship and fecundity, and that predation (a density-dependent factor excluded from our experimentation) can play a significant role in altering the distribution of the Daphnia and decreasing the population size. According to Preuss et al. (2009), populations that have to cope with low densities of algae are limited by food availability, whereas populations that have access to a high food supply are limited by crowding effects. They describe “crowding” as a density-dependent mechanism because of a chemical substance that is released upon physical contact of two individuals (which would happen more frequently at higher densities). They also found that daphnia at higher densities produced much less offspring than those at lower population densities. According to Luerling et al. (2003), Daphnia pulex react to these crowding chemicals in the same way that they react to food limitations. Although growth and survivorship are unaffected by high population densities, reproduction is significantly decreased. As demonstrated in the experimentation of Cox et al. (1992), Luerling et al. (2003) suggest that, in times of low food availability, individuals change their life history...
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