Print PageIndirect interactions between species have long been of interest to ecologists. One such interaction type takes place when energy or materials flow via one or more intermediate species between two species with a direct predator-prey relationship. Previous work has shown that, although each such flow is small, their great number makes them important in ecosystems.
A new network analysis method, dynamic environ approximation, was used to quantify the fraction of energy flowing from prey to predator over paths of length greater than 1 (indirect flow fraction) in a commonly studied food web model. Web structure was created using the niche model and dynamics followed the Yodzis-Innes model. The effect of food web size (10 to 40 species) and connectance (0.1 to 0.48) on the indirect flow fraction was examined. Because the niche model is stochastic, 250 model realizations were run for each pair of size and connectance values. For each realization, the pairwise indirect flow fraction for every predator-prey interaction in the model was computed and then averaged over the whole network.
Results/Conclusions
The mean indirect flow fraction of the model food webs is 0.092, with a standard deviation of 0.0279. It tends to increase with system size; however, it peaks at intermediate connectance levels. The median indirect flow fraction shows a similar pattern, but with a mean of 0.051 and a standard deviation of 0.0284. The maximum ranges from 0.57 to 1.0, with a mean of 0.93 and a standard deviation of 0.075; the minimum was always zero.
These results help us build a bridge between contemporary community ecology and systems ecology, while providing a new way of looking at ecosystem complexity. In the future, it will be important to examine the effects of other system attributes on the indirect flow fraction and find out whether this quantity is linked to the vulnerability of food webs to species loss.
