Complex Food Webs, Predation, and Competition

The paper “Complex food webs prevent competitive exclusion among producer species” looks at the effects that nutrient supply and predation have on the survival of multiple producer species. Brose did this by randomly assigning nutrient intake efficiency to five producers. The same producers were also put into a food web formulated based on the niche model. Then using a nutrient intake model that included two limiting nutrients a simulation was run to see what would happen to the network with just producers and what would happen to the food web including producers and consumers. There were also food web models that differed in terms of predators. Some webs only had predators that were generalist so they ate the producers in proportion to their biomass and other webs where predators are given random preference for a particular producer.

After the simulations were compared it was found that 99.7% of the time in the networks with just producers only one producer dominated and all others became extinct. This extinction is thought to be caused by competition between producers where the most efficient nutrient obtainer eventually excludes all other species. When the food webs with predators are simulated then a majority of the time more producers survive than in webs without predators. 86.7% of simulations with predators that had random preferences had more producers than number of limiting nutrients and 91.9% of simulations with predators, with random food preferences, had more producers than the number of limiting nutrients. This suggests that there is big reduction on the influence of competition when there are predators but that without predators, producers will engage in competition and the dominant competitors will drive the other producers to extinction.

As mentioned in the article these networks have assumptions built in or effects not taken into account. Although not mentioned the model also does not take the environment or disturbance. (It has been proposed by Connell (1980) that if there is intermediate harshness in environmental conditions the effectiveness of predators would be reduced allowing completion to occur. It has also been proposed in ecology that disturbances could allow inferior competitors to exist.) However the point of this model seems to have been to illustrate that just predation by itself can limit competition and it does illustrate the general concept of predation influencing competition.