Alternative final states when Daphnia magna invades a system with Keratella cochlearis

The effect of Daphnia magna invasion on a system with Keratella cochlearis was studied with respect to the competition outcomes between these two planktonic species in cultures lasting for 75 days. Four Daphnia invasion events were carried out at different moments of Keratella population growth curve. We found that D. magna always displaced to + K. cochlearis when it was inoculated during the first period of growth curve (minimum rate was 1 individual of D. magna invading 1000 individuals of K. cochlearis). The ratio of competitors was decisive on the competition outcomes, because when Keratella could develop a large population before the onset of competition, this fact could tip the balance in favour of its persistence. Thus, when D. magna was inoculated during K. cochlearis stationary period (minimum rate in the invasion moment was 1 D: 6000 K in all these treatments) different alternative competitive results were observed: D. magna was excluded or K. cochlearis was excluded or they co-existed, reaching a ratio of 1 D: 300 K. Keratella exhibited two distinct models of growth dynamics and, depending on these dynamics, the competition result varied. We did not find statistically significant differences between population features (growth rate, fecundity, etc.) when comparing them among the different trajectories which promoted different competition outcomes. Although the implied competition mechanisms were known, their effect on competitors, and therefore on the competition outcomes, depended on population history (self-dynamics).