Boreal ground-beetle (Coleoptera: Carabidae) assemblages of the mainland and islands in Lac la Ronge, Saskatchewan, Canada

Abstract

We tested the applicability of the “passive sampling” hypothesis and theory of island biogeography (TIB) for explaining the diversity of forest-dwelling carabid assemblages (Carabidae: Coleoptera) on 30 forested islands (0.2–980.7 ha) in Lac la Ronge and the adjacent mainland in Saskatchewan, Canada. Species richness per unit area increased with distance to mainland with diversity being highest on the most isolated islands. We detected neither a positive species-area relationship, nor significant differences in species richness among island size classes, or between islands and the mainland. Nonetheless, carabid assemblages distinctly differed on islands <1 ha in area and gradually approached the structure of mainland assemblages as island area increased. Small islands were characterised by abundant populations of small-bodied, winged species and few if any large-bodied, flightless species like Carabus taedatus Fabricius. Our findings suggest that neither the “passive sampling” hypothesis nor the theory of island biogeography adequately explain carabid beetle diversity patterns observed among islands in Lac la Ronge. Instead, we hypothesise that population processes such as higher extinction rates of large-bodied, flightless species and the associated release of smaller-bodied, flying species from intra-guild predation on small islands contribute to observed differences in the structure of carabid assemblages between islands.

Species traits modify the species-area relationship in ground-beetle (Coleoptera: Carabidae) assemblages on islands in a boreal lake

“…Our observation of negative abundance-area and species-area relationships for small-bodied species is also at odds with predictions of the theory of island biogeography. In a related study [ 43 ], we showed that the majority of negative co-occurrences between carabids on these islands involved large and small-bodied carabids. It is generally though that competition does not significantly influence carabid assemblages except through intra-guild predation [ 84 ] or at high densities [ 85 ] as may be expected on true islands.…”

“…Furthermore, the strong interaction between species traits and island area revealed opposing patterns for two of the three traits examined: species richness and abundance of small-bodied, macropterous species was negatively related to island area, while the opposite pattern was observed for large-bodied, flightless species. In fact, we did not detect a positive SAR if species traits were not considered, likely because of the associated divergent response in richness and abundance (see [ 43 ]). Thus, inclusion of species traits in SAR modelling can help reveal complex interactions between species traits and area, as well as reconcile both niche theory and neutral theory in a single framework [ 10 ].…”

“…In contrast, rugged igneous and metamorphic bedrock [ 42 ] is exposed in the northern and central reaches, giving rise to a collection of >1,300 islands. These islands are covered mainly by mixed and conifer-dominated forest, including some gaps surrounded by forest (see [ 43 ]), with sand beaches, bogs, and meadows comprising small areas on some islands. Except for a few small cabins, islands are not impacted by human activity.…”

“…A minimum of 20 males and 20 females of each species, chosen at random from each island, were measured, except in cases where fewer specimens in good condition were available. In an effort to better understand the island effect on body size, we also measured a sample for each of the above listed carabid species collected with pitfall traps from mainland sites near Lac la Ronge (see [ 43 ]).…”

“…We then assessed the interaction between life-history traits and island area using results from that model. We included the number of trap days as a covariate to account for minor differences in trapping effort due to lost traps (see [ 43 ]). Residuals for models assessing species richness by island area met the assumptions of normality and equal variance.…”