Effects of Food Quality on Isopod Population Dynamics

Abstract

1 Growth and survival of juvenile Armadillidium vulgare (Latrielle) in the laboratory were greatest on a diet of an excess of freshly fallen leaves from dicotyledonous plants but decreased with decreasing supply of this food, even though alternative lower quality foods were provided in excess. Growth in the laboratory was positively related to temperature but mortality was not clearly related to temperature over the range observed in the field. 2 Growth and survivorship of a field population on a Breckland grass heath are described for a 2-year period during which density declined from 673m-2 to 102 m-2. The demand for high-quality food by the field population peaked during a period of rapid growth in spring and early summer when supply of leaf litter from dicotyledonous plants was at a minimum. Litter production peaked in autumn after isopods had finished breeding and when low temperatures reduced their consumption rates. 3 Competition for high-quality food in spring and early summer during this period could have reduced growth and survivorship sufficiently to account for the observed decline in population density. 4 A simulation model, based on laboratory observations, closely predicted the observed decline in the field population. Further simulations also predicted the higher population densities observed in the field before this study and also the lower equilibrium densities found on the site since the study. Key-words: Population dynamics, competition, food quality, woodlouse, Armadillidium vulgare, densitydependence, growth, survivorship, consumption * Present address: Dr S.P. Rushton, Department of Agricultural Biology, The University, Newcastle-uponTyne NEl 7RU, UK. Introduction The role of food quality in the population dynamics of herbivores has been intensively studied and its importance in limiting populations widely recognized (Lawton & McNeil, 1979; Crawley, 1983; Dempster, 1983). In contrast, the influence of food quality on the population dynamics of decomposers has received little attention. As most decomposers have a broad range of diet and dead organic material is relatively abundant in most ecosystems, it is often assumed that detrivores are surrounded by an abundance of suitable food. For this reason Warburg, Linsenmair & Bercovitz (1984), in a comprehensive review, suggest that food is not important in influencing population fluctuations of terrestrial isopods; instead they propose that the dynamics of isopod populations are primarily controlled by climatic variables, such as temperature and humidity, since these strongly affect growth, survivorship and reproduction of isopods. Growth and survivorship are also importantly influenced by food quality, as demonstrated for Armadillidium vulgare (Latreille) by Cromack (1967) and Merriam (1971). The quality of isopod foods in the field shows much variation (Hassall & Rushton, 1984) but this would not limit isopod populations if optimal quality foods were always present in excess. However, populations could be adversely affected by either a shortage of the best quality foods or by a temporal separation in supply and demand of optimal food as is the case for herbivores in the African savannah (Sinclair,