A large number of biodiversity and ecosystem functioning experiments have shown that total consumer biomass tends to be higher in diverse consumer communities whenever consumers exhibit resource partitioning. In contrast, studies on predator-prey dynamics have shown that multi-consumer assemblages often exhibit intra-guild predation (IGP), which leads to lower total consumer biomass. These conflicting results need to be resolved if we are to predict the functional role of consumer diversity in ecosystems.
To explore how resource partitioning and IGP jointly impact total consumer biomass, we built a simple model with 2 resources and 2 consumer species. For the resource species, we held the parameters governing their dynamics constant so that differences among resource species did not influence results. For each consumer, we held the total consumption rate constant, but varied the resource partitioning by the two consumers on the two resource species. We also varied the strength of IGP by changing the consumption of one consumer on another. Lastly, we varied parameters governing dynamics of the consumer species (e.g., assimilation efficiencies, mortality rates, etc.) to determine conditions for which resource partitioning and IGP increase verses decrease total consumer biomass.
Results/Conclusions
Simultaneously considering resource partitioning and IGP advances our understanding of consumer coexistence and function derived from the classic IGP model and the simple Lotka-Volterra competition model. First, resource partitioning always promotes coexistence even when the IG predator is the superior competitor, which does not happen in classic IGP model. When the IG predator is the inferior competitor, weak IGP further promotes consumer coexistence. Second, we found a “hump-shaped” relationship between total consumer biomass (Ctot) and the strength of IGP when the IG predator has the lower competition-free invasion growth rate (Rmax). Rmax is the growth rate of each consumer alone when the resources are at their carrying capacity. This hump-shape is more prominent when resource partitioning is low. Last, and more interestingly, consumer coexistence and Ctot respond independently to the synergistic effects of IGP and resource partitioning. This means that species coexistence and ecological function can be decoupled, which differs from predictions of simple Lotka-Volterra competition models. In conclusion, weak IGP not only can promote consumer coexistence but also can increase their total biomass, although these effects can be independent.