Landpissebedden van de Ooijpolder: deel 2. Ecologie (Crustacea: Isopoda: Oniscidea)

Terrestrial isopods of the Ooijpolder: part 2. ecology (Crustacea: Isopoda: Oniscidea) From 1991 up to 1998 the isopod fauna of 1464 squares of 100x100 m was sampled in the Ooijpolder. This area is situated along the river Waal, east of Nijmegen (the Netherlands), close to the German border. In part 1 (Wijnhoven 2000) the distributions and habitats of seventeen species of woodlice were discussed. In that paper the basic information on topography, methods, species list and list of first and second order habitats can be found. In this second and last part the ecological information is summarised. First the zoogeographical position of the species is indicated. This proved to be important in understanding the distribution patterns and habitats of the species in the Ooijpolder. Two species originate from the alpine region: Trichoniscoides helveticus and Hyloniscus riparius. They are rare in the western parts of our country but common and widespread in Germany. These species represent the Central-European influence on the eastern part of the country. In contrast two other species occurring in the Ooijpolder are of Western-Atlantic origin: Metatrichoniscoides leydigii and Trichoniscoides albidus. Both are regularly found in the western parts of the Netherlands, but very rarely in Germany. In figures 3-7 a cluster analysis is presented for each first order habitat. It is clear that each habitat has its own species composition. Some species were often found together, like the soil-dwelling H. mengii and T. helveticus, O. asellus and P. scaber, and T. pusillus, L. hypnorum and P. muscorum. The woodlice fauna of each first and second order habitat is discussed, represented by the figures 8 to 12 and 13 to 20. On the regularly flooded area along the river Waal only two species were found: T. rathkii and H. riparius. The clay soils were richest in species, being especially important for the soil-dwelling Trichoniscids. Roadside verges, arable fields and grasslands were poor in species, whereas woodlands on clay soils, water edges, ditches and shrubland could contain up to twelve species. The sex ratio’s of the species are given in table 1. In H. mengii and T. helveticus the sex ratio of the subadults seemed to be 50% (table 2). Tables 4 and 5 demonstrate that most females of H. mengii and T. helveticus moved to the deeper levels of the soil when breeding. Especially in H. mengii these vertical migrations of the breeding females lead to a greater relative number of males near the surface layers of the soil. This may also explain the low incidence of gravid females reported for several soil-dwelling species collected by hand. Gravid females are probably particularly vulnerable to fluctuations in air humidity. For this reason they move to microsites having the most stable microclimatological conditions. Probably this behaviour can be found for other species of woodlice too. It is likely that breeding females of the soil-dwelling species as well as other species tend to stay in their shelters (table 6) for a long time. Woodlice have remarkably few natural enemies. During the Ooijpolder-survey some interesting observations were made. Numerous specimens were found infested with yeast-like organisms (fig. 31; P. scaber, O. asellus and H. danicus). Ten species were found infested with iridovirus (Wijnhoven & Berg 1999). Six species of woodlouse-flies (Diptera: Rhinophoridae) were recorded: Rhinophora lepida was the most common and widespread (fig. 21, 32). Larvae of Megaselia rufipes (Diptera: Phoridae) were regularly found scavenging on O. asellus. Records of the phoretic mite Bakerdania elliptica associated with O. asellus are presented in table 7. In March/April and November/December the percentage of O. asellus which were infested seemed higher, as well as the mean number of mites per carrier. All phoretic mites were adult females carrying eggs. It is suggested B. elliptica’s phoretic behaviour is a means to find suitable microsites for its offspring. Table 9 illustrates that the distribution of woodlice largely depends on the groundwater level of the soil. Some species show a preference for the more calcareous habitats, like buildings, walls and stones: P. spinicornis and A. vulgare (fig. 38). Yet this did not coincide with the amount of calciumcarbonate in the soil (table 10). Probably T. rathkii and H. riparius prefer the more calcareous soil types. The ecology of T. rathkii is rather complex. Also in literature the ecological data on T. rathkii are somewhat confusing. The figures 3, 5 and 6 show T. rathkii is associated with different species in different habitats. On the flood plains T. rathkii was often found in shrubs and woodlands, but in the polder it avoided these habitats (fig. 38, 39), occurring in the open fields mainly. It is suggested here that the species competes with P. scaber and to some extent with O. asellus. The distribution of T. rathkii in the Ooijpolder shows a strong vicariant pattern with those of the other two species (fig. 41, 42). In figure 43 their similarity is given. Trachelipus rathkii often seems to fill in the niche left by one of the ‘missing’ species, but being absent where both P. scaber and O. asellus occur.