Testing the ghost of competition present: Community assembly in phytoplankton communities

Background/Question/Methods

Understanding the mechanisms of coexistence, diversity and invasibility in communities are among the major goals of ecology. Species assemblages are based, in part, on the traits of species relative to environmental factors (nutrients, temperature, light). Community susceptibility to invasions may depend not only on the traits of resident species, but also on the traits of the invading species and interactions between resident and invader traits. However, interactions between invader species may also be important. Studies that only examine final assembled communities might miss the effects of transient invader interactions, which can be important for the invasion process and the final community assemblage. Model predictions suggest that species that are unsuccessful in invading a community can also play a significant role in excluding an otherwise successful invader during the invasion process. To empirically test the indirect effects of these “ghost species”, we used phytoplankton species to conduct a community assembly experiment at different nutrient levels with high and low invasion rate by a pool of invader species.

Results/Conclusions

Results showed that the effects of invasion are strongly dependent on nutrient level. At both nutrient levels, high and low invasion rate did not result in successful invasion. However, despite the lack of invasion success, at low phosphorus levels invasion highly influenced diversity and functional group composition of the resident community. Invasion maintained lower species and functional group diversity, relative to controls. Before the invasion, resident communities were dominated by green algae and only a few cyanobacteria. Later in the experiment, cyanobacteria increased in abundance and were co-dominant with green algae in the control communities (without attempted invasion). However, in communities that underwent invasion, green algae remained dominant, with few cyanobacteria present. Although the invading species did not establish in the resident community, they may have altered the resident communities, acting as “ghost species”. Theory suggests that such effects may be common in natural communities, but have rarely been examined.