Consumer functional response and competition in consumer-resource systems

Abstract

This study investigates the effect of the functional response of resource consumers on the relationship between resource overlap and competition for some two-consumer, two-resource models. Two measures of competition are examined: α, the competition coefficient, and β, an index of the ease of invasion by the second consumer species when the first is at its carrying capacity. A comparison of systems with linear (type-1) and decelerating (type-2) functional responses shows that: (1) Competition coefficients are functions of the population densities of consumers or resources in systems with type-2 responses. (2) Competition coefficients may differ substantially in magnitude between systems with type-1 and type-2 functional responses. (3) The relative handling time of different resources is important in determining the relationship between overlap and competition. Positive correlations between capture rates (per unit resource) and handling times cause the system with type-2 functional responses to exhibit a higher level of competition for a given level of overlap than for the case of negative correlation. (4) If the functional response is type-2 it may be possible to obtain a priority effect in which either consumer species can exclude the other. (5) Invasion may be easier in a system with type-1 functional responses than in a similar system with type-2 functional responses, even when competition coefficients are larger in the former. Accelerating functional responses also affect the relationship between overlap and competition, but realistic models of such responses are likely to be very complex. Several currently accepted ideas in competition theory depend upon the assumption of a linear functional response, and are unlikely to be generally valid.