Ecological studies of isopoda and diplopoda in dune grassland

Abstract

Distribution, population dynamics, biomass and population production of macrodecomposers have been studied in order to clarify the role of four species in the processes of energy transfer in a dune grassland ecosystem. The-species worked on were Armadillidiun vulgare, Porcellio scaber, Philoscia muscorum (Isopoda) and Cylindroiulus latestriatus (Diplopoda). Population data were available for the species from 1968 to 1975. Preliminary investigations indicated little difference in the diet of each species and in their interactions with predators; however there were significant differences in vertical and horizontal distribution. The vertical distribution of each species changed with season, mainly in response to variations in temperature in winter and relative humidity-in summer. Despite these movements there was a consistent-difference in the mean depth of each species. The millipede showed a particularly marked difference from the woodlice because it was mainly a root layer inhabitant whereas they were most abundant in the litter layer. Horizontal distribution studies indicated that P. muscorum stable predominated in the stable dune while A. vulgare. and P. scaber were more abundant towards the dune crest. The last two species showed evidence of migration between different areas of the dune system in early summer 1975. Changes in abundance and aggregation were, also. observed. P. muscorum declined from a maximum density of 103.7 0.1m-2 in August 1970 to zero in October 1974 whereas A. vulgare increased from a minimum of 1.7 0.1m 2 in January 1969 to a maximum of 73.9 in September 1973. P. scaber and C. latestriatus exhibited smaller changes; maximum and minimum estimates were 27.1 and 0.9 0.1 m 2, and 16.0 and 1.9 0.1m 2 respectively, All species showed changes in aggregation during the course of the study. Aggregation increased in the woodlice in 1974 and 1975 and decreased in the millipede. It is suggested that the changes in density and aggregation were related to changing conditions on the study site, particularly those caused by rabbit disturbance and sand accumulation. There were seasonal density fluctuations also. Changes in size structure, the breeding biology and cohort dynamics of the A. vulgare and P. scaber populations were examined to obtain insight into the importance of natality, mortality and migration in determining these fluctuations. The density of each species peaked in late summer when brood release occurred and declined at other times due to continuous losses. Immigration caused spring and summer density increases and appeared to be the main determinant of breeding population size. Population size structure, breeding biology and biomass of C. latestriatus was studied. The rate of development of C. latestriatus at Spurn is intermediate between that of populations to the North and South. It is suggested that the more northerly populations take longer to mature due to lower mean annual temperatures. Comparison of biomass and population production for the three isopod species indicated large fluctuations in individual species but the total values were fairly steady with a maximum mean annual standing crop of 744 mg live weight 0.1m 2 during 1973/74 and a minimum of 295 during 1968/69. Maximum and minimum mean annual population production figures for the three isopods -combined were 366.6 mg dry weight 0.1n 2 during 1971/72 and 147.4 during 1968/69. Finally the results are discussed in relation to niche separation, iv life history strategies and the role of macrodecomposers in the dune grassland ecosystem