Evaluation of the role of small mammals in grassland ecosystems: A modelling approach

Abstract

Development of a grassland ecosystem simulation model quantifying mammal interaction with other ecosystem components indicates their influence on community structure and function. The approach has been to: (1) identify potential areas of small mammal interaction with system function; (2) quantify the interactions; (3) construct a model representing these interactions; (4) synthesize quantitative information on small mammal interactions with auxiliary information to obtain a mathematical representation of the major functional aspects of an ecosystem; and (5) simulate the ecosystem to estimate overall influence of small mammals. To facilitate the identification of potential influence, six small mammal functional groups and three major ecosystem components were specified. Small mammal groups include three feeding categories: (1) herbivore, (2) carnivore, and (3) granivore; and three behavioral categories: (1) soil mover, (2) food storer, and (3) builder. The three ecosystem components include: (1) abiotic, (2) primary producer, and (3) consumer-decomposer. A six by three matrix was formed with rows representing the six small mammal groups and columns representing the three ecosystem components. Each compartment of the matrix indicates potential influences of the small mammal group on the ecosystem component. Simulations indicate that two small mammal activities have greatest impact on other ecosystem components. Upward translocation of soil has a significant positive effect on both percent bare surface soil and quantity of nitrogen in the top soil layer; and consumption of arthropods has a significant effect on arthropod biomass dynamics. Even with extremely high small mammal densities direct consumption has an insignificant effect on primary production. Based on indices quantifying total impact of small mammals, the hypothesis that grassland ecosystem structure can be fundamentally altered by changes in the small mammal community is tentatively rejected. Ecosystem function appears to be affected by relatively small fluctuations in small mammal density and by certain changes in composition of the small mammal community. Routes and rates of material flow within an ecosystem are influenced by small mammals, producing subtle but quantitative changes in ecosystem dynamics which are directly or indirectly a function of small mammal population dynamics. Therefore, the hypothesis that grassland ecosystem function is responsive to changes in the small mammal community cannot be rejected.