Physiological tolerances account for range limits and abundance structure in an invasive slug

Abstract

Despite the importance of understanding the mechanisms underlying range limits and abundance structure, few studies have sought to do so. Here we use a terrestrial slug species, Deroceras panormitanum, that has invaded a remote, largely predator-free, Southern Ocean island as a model system to do so. Across Marion Island, slug density does not conform to an abundant centre distribution. Rather, abundance structure is characterized by patches and gaps. These are associated with this desiccation-sensitive species' preference for biotic and drainage line habitats that share few characteristics except for their high humidity below the vegetation surface. The coastal range margin has a threshold form, rapidly rising from zero to high density. Slugs do not occur where soil-exchangeable Na values are higher than 3000 mg kg(-1), and in laboratory experiments, survival is high below this value but negligible above it. Upper elevation range margins are a function of the inability of this species to survive temperatures below an absolute limit of -6.4 degrees C, which is regularly exceeded at 200 m altitude, above which slug density declines to zero. However, the linear decline in density from the coastal peak is probably also a function of a decline in performance or time available for activity. This is probably associated with an altitudinal decline in mean annual soil temperature. These findings support previous predictions made regarding the form of density change when substrate or climatic factors set range limits.

Figure 1

Deroceras panormitanum density plotted against altitude for all 1109 quadrats sampled on Marion Island. The open triangles indicate Trypot Beach on the east coast and the open squares the high altitude site above Micked Pickle Cove. The inset shows the natural neighbour interpolation of slug density (numbers m⁻²) across the island.

Figure 2

(a) Weighted marginal means (±95% CI) of slug density in each of the seven habitat types investigated, obtained from a generalized linear model including spatial terms, altitude and habitat. (b) Weighted marginal means (±95% CI) of slug density in the biotic (circles) and drainage line (squares) habitats, obtained from a generalized linear model including spatial terms, altitude, aspect and the two habitats.

Figure 3

Frequency distributions of soil surface temperature (filled bars) and the supercooling points (SCP) of D. panormitanum (hatched bars) at (a) sea level (ca 10 m.a.s.l.) and at (b) 200 m.a.s.l. Note that the lowest soil temperatures at 200 m lie beyond the frequency distribution of the SCPs.

Figure 4

(a) Mean (±95% CI) survival of D. panormitanum exposed to different seawater concentrations (converted to Na equivalents in mg kg⁻¹). (b) Deroceras panormitanum density in 56 coastal quadrats plotted against soil-exchangeable Na concentration.