When does the timing of species arrivals matter for the eventual composition of a community? If, because of its early arrival, a species contributes more to the eventual community than a late arriving species, a priority effect has occurred. Priority effects make community composition less predictable, because the arrival times of species are hard to predict. They can result from many types of interspecific interactions, including resource competition, intraguild predation and interference competition. Previous studies of such interactions have shown that the occurrence of priority effects depends largely on system productivity. These types of interactions are often studied in the absence of other species, however, whether they lead to priority effects depends on the larger food web they are part of. It is thus important to assess the effects of additional trophic levels on the occurrence of priority effects.
In the work presented here, we develop a mechanistic model of two consumers competing for a single resource and sharing a predator, with one of the consumers interfering with the resource uptake of the other. By varying both resource supply and predation pressure, we study the combined effects of enrichment and predation on the occurrence of priority effects.
For a priority effect to occur in our model, the interfering species has to be the weaker resource exploiter of the two consumers, or it has to be more susceptible to predation. When it is the weaker resource exploiter, a minimum level of nutrient input is required for interference to be strong enough to cause a priority effect. When the interfering species is more susceptible to predation, a priority effect actually requires predation, which in itself also requires a minimum level of nutrient inflow. However, the priority effect disappears when predation pressure becomes too strong, also when the two competitors are equally preferred by the predator. This is so because predation reduces population densities and thereby the strength of interference. Our results show how the effects of resources and predation combine to result in the absence or presence of priority effects during community assembly.