Shiny predators of the forest floor

My PhD may be primarily about birds, but during the two years and a half to this point I’ve ended up often putting on the shoes of an entomologist. After all, ConFoBi is all about crossing the borders between scientific disciplines, and a major component of my project is to understand interactions between birds, their insect prey and the forest environment. Coming from the ornithological perspective, birds are not too picky eaters, and whether they are choosing species X or species Y of beetle on the menu does not matter too much for most European forest birds. Beetles are also far from being the exclusive source of food, and more often that not the favourite snacks for birds are instead caterpillars and spiders, which are nutritionally very valuable for nestlings.

So, understandably, my first priority when sorting pitfall trap catches from the ConFoBi plots, which I had set up in spring 2020, was not so much getting into the nitty-gritty of species identification, but instead to count all invertebrates and sort them at the wider order or even class level (beetles, wasps, flies, snails, among many others). The resulting abundance data (or, more correctly said, activity-density) should reflect the overall availability of invertebrate resources for birds foraging at the ground level, such as robins, wrens or various thrush species. What I realized in the meantime is that these samples also provide a valuable insight into the insect world itself, in particular of the charismatic ground beetle family (Carabidae), and I have started working jointly with another PhD student, Sebastian Schwegmann, so we can get species ID’s of all carabid beetles we obtained in the pitfall traps.

Carabids are often large, shiny, easy to catch in pitfall traps, well-studied in terms of basic ecology and not too complicated to identify (saying „easy“ would be a stretch for someone not so experienced in insect ID). In other words, they are attractive study objects for pretty much the same reasons that birds are. A majority of carabids are predatory, chasing and catching invertebrate prey (and even small vertebrates!) across the forest floor, fulfilling an important ecological role. They are adapted to this lifestyle with strong mandibles, long and agile legs, nocturnal or crepuscular habits, and many have lost the ability to fly. Some species (e.g. Cychrus) even developed an especially slender thorax, head and mandibles, so they can enter snail shells and prey on the animal inside.

The carabid family contains many species which favour forests, others which prefer open land and others which are habitat generalists. The literature on Central European forests tells us that forests which are younger, more intensively managed, with more conifer trees or surrounded by structurally simple croplands often see an „invasion“ of generalist and open-land species in the forest assemblage, actually leading to an increase in richness despite loss of forest-specialized beetles. That is why, when looking at carabid diversity and its conservation in forests, it is often more informative to look at the ratio of forest to open land species‘ abundances, instead of simple species richness. The ConFoBi area, covering a landscape forest cover gradient, and a largely mountainous area, offers a good opportunity to look at the interplay between topography and landscape in determining carabid assemblages, besides finer-scale forest structure.

So far, we have identified 37 species of carabids, including 5 species which are classified as either vulnerable or near-threatened in national and regional red lists: Carabus intricatus, Carabus irregularis, Licinus hoffmannseggi, Harpalus laevipes and Pterostichus diligens. All except the last (which prefers wetlands and bogs) are forest specialists. C. irregularis is closely linked to shady moist forests on steep slopes and canyons, while the other three are linked to early-successional forest stages or openings in mature forest, habitats which are not favoured by the current forest management.

Finally, the photographs above also show three other species from the genus Carabus which are relatively widespread but exemplify distinct habitat preferences - Carabus auratus is a classic species of open cropland, Carabus coriaceus (the largest carabid in Germany, with one of our specimens reaching a length of 4 cm) prefers forests and in particular their edges, while Carabus violaceus is a generalist species.

There is still a lot of work ahead, but I am definitely looking forward to see what we can uncover by looking at these shiny predators of the forest floor.

by João Manuel Cordeiro Vale Pereira (B6)