Experimental and modeling approaches for understanding the boom and bust population dynamics of the pollution indicator, Capitella teleta

Background/Question/Methods

Compared to marine hard-bottom communities, the exact mechanisms of density dependence in marine soft-bottom communities are less well understood. Deposit feeders are key components of soft-bottom communities. Though it is widely accepted that populations of deposit-feeders are food limited, other factors than exploitation competition, such as competition for space and inter-individual interference, may also affect population density.

We explored the mechanisms of density dependence in the widespread deposit feeder, Capitella teleta (formerly Capitella sp. I), which dominates in polluted sediments and which shows highly variable population dynamics. There are at least two mechanisms by which worms may compete: through competition for ingestible particles and through direct physical interference among individuals. We designed our study to distinguish between these two mechanisms of density dependence by independently manipulating the amount and quality of sediment available to worm populations. The experiment was set up with 5 treatments: a control treatment (100% natural sediment; low worm density), two treatments with reduced organic content (75 % natural sediment and 50 % natural sediment) and two treatments with increased initial worm densities (manipulated by reducing the amount of natural sediment per worm to achieve a medium density and a high density compared to the control treatment).

Results/Conclusions

For all treatments there was an initial increase in population size measured both as worm biomass and abundance. Population size peaked after a number of weeks, and thereafter declined as the sediment was depleted of food. Populations in the 100 % natural sediment (low density) seemed to grow to a higher total biomass and abundance compared to both populations with less space (i.e., higher initial density) and populations that lived in sediment of a lower food quality. Whereas time to reach peak population size did not differ among treatments, peak size (i.e., total biomass) did differ. In addition, there was a clear trend of decreasing peak size with increasing initial population density or decreasing sediment quality (sediment organic content). These results provide insights into the relative importance of food limitation and inter-individual interactions in limiting worm population dynamics.