The effects of food and density of the movement ofArion ater andAriolimax columbianus (Pulmonata: Stylommatophora) between habitats

One town’s invasion by the pest slug Arion vulgaris (Gastropoda: Arionidae): microsatellites reveal little introgression from Arion ater and limited gene flow between infraspecific races in both species

The terrestrial slug Arion vulgaris has recently spread across most of Europe, often causing the local extinction of resident populations of Arion ater s.l. The species hybridize, which leads to the prediction of massive introgression of A. ater genes into A. vulgaris. To test this, we used 16 microsatellite markers applied to samples of both species collected around Görlitz, Germany, during the invasion. Amongst A. vulgaris individuals with typical genitalia, an analysis using structure suggested that only 6% were appreciably admixed with local A. ater; admixture did not increase over the course of the invasion. Amongst the ~4% of slugs with intermediate genitalia, microsatellites confirmed that they were often hybrids, their anatomy correlating with the estimated share of ancestry from each species. The microsatellites also distinguished the three subspecies of A. ater previously recognized on the basis of genital anatomy and mitochondrial DNA. The subspecies were not well mixed spatially, with A. a. ater in wilder places and A. a. rufus never found in the Polish part of the town; nevertheless, hybridization between them was occurring. Unexpectedly, the microsatellites indicated three genetic races amongst A. vulgaris; these occurred in different districts and are mixing spatially and genetically only slowly.

Effect of density-dependent individual movement on emerging spatial population distribution: Brownian motion vs Levy flights

Individual animal movement has been a focus of intense research and considerable controversy over the last two decades, however the understanding of wider ecological implications of various movement behaviours is lacking. In this paper, we consider this issue in the context of pattern formation. Using an individual-based modelling approach and computer simulations, we first show that density dependence ("auto-taxis") of the individual movement in a population of random walkers typically results in the formation of a strongly heterogeneous population distribution consisting of clearly defined animal clusters or patches. We then show that, when the movement takes place in a large spatial domain, the properties of the clusters are significantly different in the populations of Brownian and non-Brownian walkers. Whilst clusters tend to be stable in the case of Brownian motion, in the population of Levy walkers clusters are dynamical so that the number of clusters fluctuates in the course of time. We also show that the population dynamics of non-Brownian walkers exhibits two different time scales: a short time scale of the relaxation of the initial condition and a long time scale when one type of dynamics is replaced by another. Finally, we show that the distribution of sample values in the populations of Brownian and non-Brownian walkers is significantly different.

Biology of slugs (Agriolimacidae and Arionidae: Mollusca) in New Zealand hill country pastures

Life cycles of the slugs Deroceras reticulatum and Arion intermedius were studied over a 2.5-year period in a sheep-grazed pasture in the Kaimai Range, New Zealand. D. reticulatum approximated a bivoltine phenology, with intervals between consecutive generations ranging from 4 to 7 months and maximum life span from 8 to 12 months. Egg laying occurred in autumn and spring-early summer. Periods of egg hatching were followed by a phase of juvenile growth characterised by linear increase in the logarithm of population mean body weights. This was followed by a phase of reproductive activity. A. intermedius had an annual life cycle. Eggs laid during late summer and autumn hatched during autumn-winter. Immature slugs were characterised by a period of low growth rate during winter followed by a period of rapid growth culminating in reproductive maturity. In both species, the phase of rapid growth rate was associated with enlargement of the hermaphrodite gland. The size of the hermaphrodite gland relative to body weight reached a maximum during the spermatozoon stage, generally coincident with maximum body weight. During the reproductive period of D. reticulatum and A. intermedius the body weight remained relatively stable but the hermaphrodite gland became progressively smaller as the slugs approached the post-reproductive stage. The albumen gland attained maximum weight at the oocyte stage of the hermaphrodite gland, at the onset of reproductive activity. The pasture, strongly dominated by grasses, was heterogenous in slope, vegetation and treading/grazing influences from sheep due to stratification of the habitat into contour tracks, associated with sheep movement around the slope contours, and intertrack areas. High population densities of D. reticulatum were associated with the track edges (kerbs) and the upper intertrack slopes, while densities of A. intermedius were highest for the intertrack slopes. Low densities of both species occurred on the tracks and at sheep campsites. Analysis of cohort life tables indicated that these dispersion patterns were important in the dynamics of the populations. The stage mortality contributing most to the variance in reproductive populations of both species was that between hatching and onset of reproduction; most of the mortality occurred shortly after hatching on south-facing slopes, but tended to occur later in the life cycle on north slopes. For D. reticulatum this mortality was inversely related to initial density and associated, at least in part, with predation by carabids and birds and with treading by sheep. Density relationships, both at whole plot and plot stratum level, indicated that variations in natality had a stabilizing influence on D. reticulatum populations.