Foundation species in food webs

Background/Question/Methods

Foundation species are basal species that structure ecological communities by creating physical structure and modulating ecosystem processes. While foundation species may have trophic interactions within a community, they exert their influence largely through non-trophic interactions.  Foundation species also play an important role in maintaining biodiversity but have not been explored in a food web context. One challenge to incorporating foundation species into classical food web theory is including non-trophic interactions in food web models.

Here, we include non-trophic interactions in a non-linear bioenergetic predator- prey model to explore how the presence of a foundation species influences food web assembly and how subsequent foundation species removal influences food web collapse. We test four different foundation species models; 1) the basal model, where foundation species reduce the metabolic rates of only basal species, 2) consumer model, where foundation species reduce the metabolic rates of only consumers, 3) the total model, where foundation species reduce the metabolic rates of all species, and 4) a control model, where foundation species have only trophic interactions. We tested each of these models across six metabolic rate treatments ranging from one-tenth to ten times the allometric baseline metabolic rate. 

Results/Conclusions

The consumer model consistently retained fewer species and exhibited simplified food web structure (e.g. low connectance, linkage density, clustering coefficient) during community assembly.  After foundation species removal, consumer model food webs experienced the greatest number of cascading extinctions whereas basal model webs retained the greatest number of species and had the most complex food web structure. The highest metabolic rate treatment resulted in the greatest species loss including scenarios where the food web completely collapsed to zero species.

Our results suggest that when a foundation species lowers the metabolic rates of only consumer species, food webs are species poor, simplified, and susceptible to collapse when foundation species removal. Lowering the metabolic rates of consumers may result in stronger feeding interactions that can destabilize predator-prey interactions and lead to species losses. On the other hand, when a foundation species lowers metabolic rates of only basal species, resultant food webs are species rich, complex, and robust to foundation species removal. Lowering the metabolic rates of basal species may allow them to withstand initial transient dynamics or low intial population sizes. Thus, basal species can establish in a community creating more paths of energy to higher trophic levels leading to species-rich robust webs.