Fluctuating food availability may permit coexistence in bacterivorous nematodes

The population growth of two bacterivorous nematodes, Panagrolaimus sp. and Poikilolaimus sp., isolated from Movile Cave, Romania, was studied at 20 degrees C at seven different densities of Escherichia coli as food source (10(7)-10(10) cells ml(-1)). By using semi-fluid media in a simple discharge system, we examined nematode and bacterial abundances over 56 days with high temporal resolution. At high nematode population densities, grazing severely reduced bacterial abundance-decreasing cell numbers by more than one order of magnitude. The two nematode species showed distinct differences in their reaction to different levels of food supply. Panagrolaimus realized its maximum population growth rate (r = 0.262 d(-1)) at the highest provided food density, i.e., 10(10) cells ml(-1). At lower food concentrations, its population growth was reduced, and at densities below 5 x 10(8) cells ml(-1) reproduction could no longer compensate the experimentally induced mortality rate of 0.068 d(-1), thus resulting in extinction of the experimental populations. In contrast, Poikilolaimus thrived best at a bacterial density of 5 x 10(8) cells ml(-1) (r = 0.203 d(-1)). Higher bacterial densities did not result in higher reproduction, but even somewhat hampered population growth of Poikilolaimus. Our investigation implies that, in a combined population, Poikilolaimus would be a superior competitor at low food availability whereas Panagrolaimus would benefit from high food densities. In an environment with regular fluctuations of food availability, these traits may prevent competitive exclusion and explain the observed long-term coexistence of the two species in their natural habitat.