Ecological Modelling

Exploitative competition and ecological effective abundance

Summary

1. This study analyzes the effect of exploitative competition and density dependence by using an individual-based simulation model.

2. There are two components of exploitative competition (leftovers and resource-recovery competition). In leftovers competition, individuals compete for food resources that have been left by others. In resource-recovery competition, the consumption of foods by individuals affects growth rates of food populations.

3. The individual-based simulation was conducted to examine how these two types of competition affect density-dependent processes. In the simulation, individual organisms search for food organisms within their home range. Food organisms reproduce in three different ways. (1) Replacement. All food organisms die and are replaced with a new food population. (2) Recruitment. Food organisms do not disappear during simulation unless they are eaten by individuals, and new food organisms are recruited based on density-dependence. (3) Neighborhood recruitment. Food organisms reproduce depending on their abundance within the neighborhood area. The simulation was conducted by varying the number of individuals. home range size of individuals, area of simulation, reproductive rate of food, and reproductive mode of food organisms. The number of food organisms taken by individuals, and the number of individuals over several different scales, were measured. Then multiple regression was conducted to examine the relationships between several different estimations of abundance and the number of food organisms taken by an individual.

4. The effects of different types of ecploitative competition have varying significance depending upon the spatial scale or estimated population density. The number of individuals within the home range of an individual negatively related to individual food gain. This suggests that leftovers competition can be detected within the home range of an individual.

5. In the case in which a food population was regulated over the whole simulation area, the number of individuals in the whole area were positively related to individuals food gain in the case of the case of recruitment and neighborhood recruitment of reproduction of food, but positively in replacement. In the case in which a food population was regulated in smaller parts of the whole area, the estimated number of individuals within a food population were related to individual food gain but the total population size does not affect individual food gain.

6. Individual-based simulation models that do not a priori assume population level phenomena are useful to predict population level phenomena. This kind of model can be applied to other problems for analyzing relationships between individuals and groups such as populations and communities.

Key-words: individual-based simulations, scale, competition, multi-levels, density-dependence.