Trachelipus rathkii (Brandt, 1833)

Boeraeve, Pepijn, Arijs, Gert, Segers, Stijn, Smedt, Pallieter De, Spinicornis & Utm, Belgium. Every grid cell of the, 1908, Habitat and seasonal activity patterns of the terrestrial isopods (Isopoda: Oniscidea) of Belgium, Belgian Journal of Entomology 116, pp. 1-95 : 84-86

36. Trachelipus rathkii (Brandt, 1833)

( Fig. 20 h , Fig. 40 , Map 36 , Table 38)

Trachelipus rathkii occurs in a wide range of habitats and is common across the country except in the south, more specifically in the ecological regions of the Ardennes and the Lorraine ( Map 36 ). T. rathkii withstands inundation very well ( OOMS et al., 2020) and appears frequently near streams and rivers in open landscapes, forests and anthropogenic habitats. A relatively high number of records (43.2%) are registered in open landscape habitats ( Table 38). In particularly for grasslands, T. rathkii can be considered as a characteristic species, next to the very common species Armadillidium vulgare and Philoscia muscorum . T. rathkii occurs in forest, but is mostly confined to forest edges; however, this can be different in other regions in Europe ( DE SMEDT et al., 2018a).

T. rathkii is less commonly found in open landscape and anthropogenic habitat during summer months ( Fig. 40 ). T. rathkii has a relatively low desiccation resistance ( DIAS et al., 2013) and this might explain the observed pattern. In forests is the corrected number of records remarkably higher in January–February, for which we do not have a clear explanation. Using pitfall traps, DE SMEDT et al. (2021) found however an activity peak in late summer compared to other seasons.

RESULTS OF ORDINATION ANALYSIS

The ordination of species according to the main habitat type ( Fig. 41 ) shows characteristic species for all three main habitat types. The first axis is mainly a forest gradient with forest species to the left and non-forest species to the right and represents most of the variation (68%). The second axis separates species with a large proportion of records in open landscape from species most frequently recorded in anthropogenic habitat. Species like A. pictum , P. affinis , A. pulchellum , P. conspersum , L. hypnorum and A. opacum are mostly observed in forest habitat. These species have deciduous forest as their main habitat type and are rare or even absent outside forests. Species like T. sarsi and T. rathkii are, compared to other species, more frequently recorded in open landscapes. A large share of species is located near the open landscape “corner” but in between two habitat types i.e. H. mengii , T. albidus and T. provisorius between open landscape and forest and P. hoffmannseggii , A. vulgare and M. leydigii between open landscape and anthropogenic habitat. Although these species have an important share of their observations in open landscapes, open landscape records never exceed 50% of the corrected number records ( Fig. 42 ). Anthropogenic habitat on the other hand has many characteristic species like P. spinicornis , P. pruinosus , A. nasatum , P. dilatatus and A. dentiger . In the middle of the ordination diagram we find a collection of species without a strong habitat preference. These are P. scaber , O. asellus , P. muscorum , H. danicus and H. riparius .

The ordination of species according to the two-month period of observation ( Fig. 43 ) show some clear preferences of species for different seasons. The two-month periods are ordinated clockwise indicating an overall seasonal pattern. The first two axis represent 77.9% of the variation in the species data. The first axis (56%) represents mainly the differences between winter (to the left) and summer (to the right). The second axis (21.9%) represents the difference between spring (lower part) and autumn (higher part). Small and drought-sensitive species like T. sarsi , T. albidus , H. danicus and T. pygmaeus are more commonly found during winter months while some large and drought-resistant species are more commonly found in summer such as A. nasatum and A. vulgare . Some species are most commonly found during spring and early summer like A. opacum and H. montivagus , while H. riparius is most commonly found in autumn. P. hoffmanseggii is most commonly found in summer and autumn and is the only species that prefers summer as well as autumn. Few species have no clear seasonal pattern.

Figures

Map 36. Distribution map for Trachelipus rathkii.

Fig. 20. a, Eluma caelata; b, Cylisticus convexus; c, Porcellio monticola; d, Porcellio scaber; e, Porcellio spinicornis; f, Porcellionides pruinosus; g, Porcellium conspersum; h, Trachelipus rathkii.

Fig. 40. Corrected number of observations per two-month period for Trachelipus rathkii (N = 189).

Fig. 41. Principal component analysis of species according to their relative abundance in the different main habitat types.

Fig. 42. Graphical representation of the species used in the PCA-ordination according to the species main habitat. Length of the coloured parts of the stacks represent the relative number of records for each species per habitat type.

Fig. 43. Principal component analysis of species according to their relative abundance in the different two-month periods.