Resource availability is a dominant structuring force of ecological communities, varying across short and long temporal scales and in response to environmental disturbance. As availability varies then so, too, do the expected outcomes of species competing for shared resources. Resource-Ratio Theory (R-R) offers a framework to predict the effects of altered resource availability on community structure but, as often as it may be cited, it is one of the least-tested concepts underpinning current ecological thought.

R-R proposes that every population has an intrinsic characteristic with respect to any two essential resources known as its optimum [consumption] proportion (OP), which can be graphically represented as its optimum consumption line (OCL). Given the OCLs for two populations competing under limitation by two resources, R-R permits graphical analysis of coexistence, competitive displacement, and the outcome of invasion.

Application of R-R for prediction of ecological scenarios is hampered, however, by the availability of methods to determine populations’ OPs, OCLs, and corresponding intrinsic growth rates. Application is further impeded by the complexity of analysis of relative growth rates influenced by extrinsic mortality and depletion of resources by consumption.

We have developed an experimental system and mathematical technique for determining and validating a population’s OP, OCL, and intrinsic growth rate for two essential resources.

Results/Conclusions

We show that, based on our experimentally determined characteristics, populations can be distinguished by their OCLs. Applying these determined characteristics to our model has allowed us to predict competitive outcomes in constant resource supply scenarios and demonstrate the sensitivity of competitive outcomes to variable-supply scenarios.

Our data show that simple dual-limitation, without interactive co-limitation, is sufficient to describe a population’s OCL. Our model suggests that the outcomes of competition for two shared, essential resources is dependent not only on the supply ratio but also the absolute availability of those resources. Specifically, at high resource availabilities differences in growth rates between populations are likely not great enough to be deterministic in light of any environmental stochasticity. In contrast, at low resource availabilities we observe rapid, deterministic outcomes of competition. If our modeling analysis applies to real-world systems then this suggests that changes in the absolute availability of resources may be a more important factor structuring communities than changes in their relative availabilities. Our findings may also help to explain the variable success observed in reported attempts to apply R-R Theory to predict competitive outcomes in extant communities.