Specialist-generalist competition in variable environments; The consequences of competition between resources

Consumer-resource models have long been employed to help understand competition between consumers, but the resources have usually been assumed to be independent of each other. The impact of competition between resources is explored for a case in which two specialists and one generalist compete for two nutritionally substitutable resources. Coexistence of all three consumers requires that the relative abundance of the two resources fluctuates over time. The conditions allowing coexistence of all three consumer types is generally broadened considerably by the presence of competition between resources. In addition, competition between resources usually increases the density of the generalist consumer relative to the specialists. These results are due to the fact that the generalist persists by "consuming" negative covariance between resources, and competition between the resources increases the negative covariance in the absence of the generalist. The densities of the competing consumers often respond in counterintuitive ways to mortality imposed on one of the consumers. Frequently, the mean population size of one consumer is very insensitive to its increasing mortality over a broad range of mortality rates, but abruptly goes extinct when mortality surpasses a threshold value.