Interactions among competing nematode species affect population growth rates

Investigations of the interplay of organisms in an ecological community are a prerequisite to understanding the processes that shape the structures of those communities. Among several types of interactions, interest in the positive interactions of species that compete for the same resource has grown, as they may provide a mechanism enabling coexistence. In the laboratory experiment described herein, the effects of interspecific interaction on the population growth of two bacterial-feeding nematode species, Panagrolaimus cf. thienemanni and Poikilolaimus cf. regenfussi, were investigated. Specifically, we asked: (1) whether there is an interspecific interaction between organisms competing for a mutual resource and (2) whether these interactions are altered by the competitors' initial densities and (3) their variable growth rates (induced by different food supplies). Each treatment initially contained 48 nematode individuals, but at different species ratios (48:0; 32:16; 24:24; 16:32; 0:48). The populations were provided with three different bacterial densities (10⁸, 10⁹, and 10¹⁰ cells ml^-1) as food. The data were analyzed using a generalized linear mixed model. The best-fitting model revealed a significant decline in population growth rates with an increasing species ratio, but depending on the food density and species. These results provide strong evidence for positive interspecific interactions that vary with both species density and food-supply level. They also suggest important roles for positive interspecific interactions in habitat colonization and in maintaining the coexistence of species in the same trophic group.