Thermal niche helps to explain the ability of dung beetles to exploit disturbed habitats

Abstract

In terrestrial ecosystems, insects face a wide range of temperatures among habitats and time; consequently, the thermal niche is one of the main determinants of habitat selection and temporal patterns of activity. The replacement of native forests changes micro-climatic conditions and reduces the diversity of dung beetles; however, the physiological mechanisms behind these changes are not clear. We explore the role of the thermal niche in dung beetles to explain the ability of native species to exploit human-created habitats. Using infrared thermography, we measured variables associated with the thermal niche in 17 native species and used linear mixed-effects model and ANOVAs to compare disturbed habitats and the native forest. Endothermy and body mass explained the ability of dung beetles to exploit human-created open habitats. Small and diurnal species with very low endothermy were able to exploit deforested open habitats; evening/nocturnal/crepuscular species showed similar body mass and high endothermy in all habitats. Regarding thermoregulation mechanisms, none of the species (except one) showed defined or efficient mechanisms of physiological thermoregulation. In view of the accelerated process of forest replacement and climate change, a more profound understanding of the physiological requirements of species is essential to predict and mitigate future extinctions.

Figure 1

Mean temperature values measured during 12 full days in three habitats of the Atlantic forest of Argentina: native forest (green line), agroforestry parklands (blue line) and open pastures (red line). The full day (24 h) was divided in two periods based on changes in temperature and the time of sunrise and sunset in spring: (1) from 7:00 to 18:00 h (light blue period) and (2) from 18:00 to 7:00 h (gray period).

Figure 2

Endothermy and thermoregulation analysis in dung beetles. (a) Red circle = thorax temperature (T_th), blue circle = abdomen temperature (T_abd) and green circle = environmental temperature (T_env). During flight, the differences between the mean value of T_th and T_env represent endothermy (excess temperature) and the difference between the slopes of T_th and T_abd, T_abd and T_env, and their signs represent the physiological thermoregulation mechanisms. The colour scale in the left column and in the insect represents a gradient of body temperature, from high (white and yellow shades) to low (violet and black shades). (b) Physiological thermoregulation mechanism: (i) abdominal passive heat transfer (different T_th and T_abd slopes, positive T_th slope, and similar T_abd and T_env slopes) and (ii) abdominal active heat transfer (similar T_th and T_abd slopes, positive T_th and T_abd slopes, different T_abd and T_env slopes). In all graphs, the vertical dotted line indicates the time at which slopes were considered.

Figure 3

Analysis of variance testing the significance of the linear mixed-effects model for endothermy (whiskers, median and outliers) values among different dung beetles species according to their activity (diurnal = light blue blocks, evening/nocturnal/crepuscular = gray blocks) from the native forests (NF), agroforestry parklands (AP) and open pastures (OP) of the Atlantic forest of Argentina. Common letter are not significantly different (P > 0.05, Tukey post-hoc comparison tests). Black letters compare endothermy among species with the same activity and among environments. Red letters = difference in endothermy among species according to their activity (diurnal and evening/nocturnal/crepuscular) in the same environment (e.g. NF).

Figure 4

Body mass (whiskers, median and outliers) in diurnal (light blue blocks) and evening/nocturnal/crepuscular (gray blocks) dung beetles from the native forests (NF), agroforestry parklands (AP) and open pastures (OP) of the Atlantic forest of Argentina. Common letters are not significantly different with P > 0.05 (Conover post-hoc comparison tests, Kruskal–Wallis H tests). Black letters compare endothermy among species with the same activity and among environments. Red letters = difference in endothermy among species according to their activity (diurnal and evening/nocturnal/crepuscular) in the same environment (e.g. NF).

Figure 5

Results of flight thermoregulation experiments in dung beetle species (Kruskal–Wallis H test). Red line = thorax temperature (T_th), blue line = abdomen temperature (T_abd) and green line = environmental temperature (T_env). Each graph shows, as an example, an individual from each of the following species: (a) Canthon conformis, (b) Canthon curvodilatus, (c) Canthon histrio, (d) Canthon podagricus, (e) Canthon quinquemaculatus, (f) Canthon smaragdulus, (g) Coprophanaeus cyanescens, (h) Coprophanaeus saphirinus, (i) Deltochilum brasiliensis, (j) Deltochilum furcatum, (k) Deltochilum komareki, (l) Deltochilum morbillosum, (m) Dichotomius carbonarius, (n) Dichotomius mormon, (ñ) Dichotomius nisus, (o) Dichotomius sericeus and (p) Ontherus sulcator. In all graphs, Y axe = Temperature (°C), X axe = Time (seconds) (as it is represented in the graph a), and the vertical dotted line indicates the time at which slopes were considered.