Comparative spatial behaviour and longevity in cicadas in unburnt vs. burnt forest areas with different management

Abstract

Recently burnt habitats are a challenge for the persistence of animal populations. Insects that survive the fire, for example belowground, may sometimes show behavioural plasticity and manage to find adequate resources. But the disturbed habitat is usually not favourable to their survival and reproduction, because of increased predation and limiting resources. Whether insects survive, die or emigrate from burnt areas depends on species-specific traits. Cicadas live belowground as nymphs for several years, appearing aboveground as adults for just a few days. They often emerge in severely burnt forests that may have been logged afterwards, where the habitat structure and composition is totally different from the green forest where the eggs were laid. In order to quantify the movements and survival of cicadas in different disturbance contexts, we radio-tagged and followed, in July 2017, 63 males of Lyristes plebejus, a common European cicada species. We used a wildfire of 1235 ha that took place two years before the study, and its vicinity, to set up six study zones (of around 37 ha each) of Mediterranean-climate Aleppo pine forest, with two zones per disturbance treatment (unburnt forest, burnt-unlogged forest and burnt-logged forest, with 19, 21 and 23 radio-tracked cicadas, respectively). A detailed photo-cartography of the zones with the help of a drone was used to locate points of interest such as unburnt forest patches, standing dead trees across the landscape or general suitable habitat. Preliminary results show considerable dispersal distance, with a maximum of 890 m of accumulated movement for one individual. In burnt zones, the majority of long movements (>200 m) ended up in vegetation patches with unburnt canopy. Although we found little dispersal differences between burnt-unlogged and burnt-logged zones, there were smaller Minimum Convex Polygons (MCP) of cicada locations in unburnt than in burnt-unlogged and burnt-logged zones (average of 1.1, 5.8 and 3.0 ha, respectively). Moreover there was higher mean individual longevity in unburnt (4.8 days) than in burnt zones (4.0 days in burnt-unlogged and 3.0 days in burnt-logged). In conclusion, burnt areas appear as a less suitable habitat for Lyristes plebejus because of lower canopy cover. Accordingly, individuals may be more exposed to predation and have lower resource availability. In burnt areas, individuals with good fitness can look for better habitat such as unburnt vegetation patches or external unburnt forest. These results emphasize the importance of unburnt patches, that should be excluded from salvage logging, for insects.