Anthropogenic thermal gradient in managed landscapes determines physiological performance and explains the edge-biased distribution of ectothermic arthropods

Abstract

The intensive management of tree plantations has replaced and fragmented native forests worldwide. A direct consequence of this activity is the creation of new edges, which generate changes in microclimatic conditions in the adjacent forest as well as in the neighboring clearcut stands left over after harvesting by clearcut logging. Thus, newly created anthropogenic thermal gradients could influence ectothermic responses regarding abundance and physiology of native species in fragmented landscapes. Interestingly, the consequences of these changes have not been studied in insects. To test the effects of the anthropogenic thermal gradient on the abundance and physiology of ectothermic species, we chose the fragmented Maulino forest and the ground-dwelling beetle Ceroglossus chilensis (Coleoptera: Carabidae) as our biological model, working with seven fragments of native forest surrounded by active and clearcut pine plantation stands. We measured temperature variables and beetle abundance in different months using transects perpendicular to the edge and used general linear modelling to explain abundance with temperature. In addition, we assessed the preferential body temperatures (Tb) and Rollover Speed (RoS) of C. chilensis at five different temperatures in controlled conditions to explain field abundance. We found that mean ambient temperatures were up to 42% higher in clearcut stands than in native forest fragments during summer. However, ambient temperatures in autumn were similar between native forest fragments and clearcut stands. The abundance of C. chilensis, is concentrated near the edge in the native forest (between 2 and 10 m to interior of native forest), regardless of month sampled. The abundance of C. chilensis was higher at the edge than inside native forest interiors or clearcut stands, regardless of the month. Ambient temperature surpasses preferential body temperature (14–15.99 °C) and physiological performance of C. chilensis (15 °C), particularly during summer. Ceroglossus chilensis abundance at the edge of clearcut stands could be favored by understory structural complexity in understory vegetation, creating suitable microclimatic conditions and increasing abundance of prey resource. In essence, we found that the higher temperatures found near the edge inside forest interiors might enable individuals of C. chilensis to display thermoregulatory behavior by concentrating activity at these particular regions, resulting in increased abundances and more concentrated occurrences, helping to explain the edge-biased distribution. Thus, we offer evidence that physiological performance might be a key functional trait to explain abundance of ectothermic species in fragmented forest landscapes. We submit to decision makers linked to the forestry industry, that managing the understory of young tree plantations and increasing the structural complexity at remnant forest edges to cushion high temperatures in clearcut stands would enable more effective conservation planning of thermosensitive ectotherms in disturbed forest landscapes.