Stock-Recruit Relationships and Regional Coexistence in a Lottery Competitive System: A Simulation Study

Abstract

A lottery competition occurs among species when individuals compete for access to units of resource without which they cannot join the adult (breeding) population. It is a form of interference competition in which the chance of an individual''s winning or losing is largely determined by priority of arrival at a vacant unit of resource. Resources involved may be such things as living sites for sessile or sedentary species, or nest sites, or spawning or display sites. Such resources become available for reoccupancy on the death or departure of the current owner. Lottery competition may be particularly important in marine demersal forms which produce large numbers of pelagic larvae which disperse prior to settling into adult habitat. Lottery competitive systems are inherently unstable but can be persistent under a wide range of conditions. A simulation model is developed to explore the effects on local persistence of differences among the competing species in their survivorship after gaining a unit of resource, their fecundity, the age, after gaining the resource unit, at which individuals become sexually mature, and the shape of their stock-recruitment curves (which relate number of recruits in a cohort to the size of the adult population, and thus to the number of eggs produced initially). In addition, the effect on persistence of the number of species involved in the competition, and of some resource units remaining unused was also examined. The shape of the stock-recruitment curve is very important in determining local persistence. Linear curves prevented persistence for all except ecologically very similar groups of species. When the relationship had moderate curvature (with number of recruits becoming asymptotic as population size increased) persistence for ecologically realistic periods of time was possible for groups of species which differed substantially in the other attributes affecting their competitive abilities. Such curved stock-recruitment relationships are usual for species such as most fish in which there is production of large numbers of larvae which suffer a heavy mortality prior to recruiting to the juvenile and adult habitat. A wide range of species of terrestrial as well as marine organism may possess attributes predisposing them to enter into lottery competitive systems. In such cases, the effectiveness of competitive exclusion seems likely to be much less than previously expected.