Weak intra-guild predation promotes strong effects of biodiversity on ecosystem function
A large number of biodiversity and ecosystem functioning experiments have shown that more diverse consumer communities are more efficient at capturing their prey 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 causes them to be less efficient at controlling prey resources. These conflicting results need to be resolved if we are to predict how consumer diversity influences prey populations in nature. To explore how resource partitioning and IGP jointly impact the functional role of consumer diversity, 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 impact 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 IGP and resource partitioning increase verses decrease impacts of consumer diversity.
Our model simulations demonstrate that resource partitioning by consumers and IGP jointly influence how consumer diversity impacts prey abundance. When consumers were specialists with high resource partitioning, we found that all consumer parameter combinations tended to weaken the effect of consumer diversity on resource consumption. These results are consistent with previous studies showing that IGP tends to negate effects of consumer diversity on prey suppression. However, and somewhat surprisingly, when consumers were generalists and parameter values allowed the IG prey to achieve higher equilibrium abundances than the IG predator, then weak to moderate levels of IGP tended to strengthen the functional impact of consumer diversity. The magnitude of these impacts were non-trivial, increasing consumption of prey by as much as 1.2x that of a one-consumer system. However, as IGP became increasingly strong, we found a ‘hump-shaped’ relationship where the effects of consumer diversity via resource partitioning was highest at intermediate levels of IGP. These results show that IGP does not always counteract biodiversity effects established by resource partitioning. Additionally they potentially provide a resolution to previous empirical results showing contrasting effects of multiple consumer on the functioning of systems.