Who prefers the dark? Daily activity of dung beetles from an Amazonian region

Temporal niche partitioning is an important mechanism for minimizing interspecific competition and promoting species coexistence in complex ecosystems. This allows sympatric competing species to exploit resources at different times of the day, minimizing direct competition, and provides insights into the evolutionary processes of circadian distribution of communities, resource availability, and intra- and interspecific competitive pressure. In this study, we examined how diel activity patterns (nocturnal and diurnal periods) of dung beetle assemblages (species richness, abundance, and assemblage structure) within a conserved region of Amazonian forest. To achieve this, we sampled dung beetles using pitfall traps baited with human feces during the day and night in a conserved Amazonian forest. Overall, we sampled a total of 430 individuals and 32 species. While dung beetle species richness and abundance did not differ significantly between day and night, a pronounced shift in species composition between dial activity patterns was observed, indicating clear temporal segregation of dung beetle species. Five species were classified as diurnal, four species were considered nocturnal, and one species was an activity period generalist. Our findings highlight the importance of temporal dynamics as drivers of dung beetle species distributions in tropical forests, directly influencing resource partitioning and promoting behavioral adaptations that facilitate species coexistence in the Amazon rainforest.

Methods
Study area

Fieldwork was carried out in June 2022 at the RAPELD module of the Instituto de Biodiversidade e Conservação da Amazônia (IBCAM), near Presidente Figueiredo, Amazonas, Brazil (1°45'S, 60°08'W). The site is a conserved terra-firme forest with minimal human disturbance and a tropical monsoon climate according to the Köppen classification.

Data collection

We performed one 24-h sampling cycle using 24 baited pitfall traps — 12 active during daytime (06:00–18:00) and 12 during nighttime (18:00–06:00). Traps were arranged as 12 sampling points (pairs ~5 m apart) spaced ≥ 100 m to standardize effort, and baited with ≈25 g fresh human feces (an appropriate sampling method to estimate dung beetle assemblages, see Mora-Aguilar et al., 2023 and Salomão et al., 2021). Specimens were identified to species or morphospecies using regional keys and the INPA entomological collection (Vaz-de-Mello et al. 2011).

Data analysis

Sample completeness was assessed with sample-coverage estimators (iNEXT; Chao et al., 2014) in R 4.3.0 (R Core Team, 2023). The effect of diurnal and nocturnal periods on species richness and abundancewase tested using generalized linear models (GLMs) with error families chosen after diagnostic checks (Crawley, 2013). Each sampling point was considered a replicate.

Species were classified as diurnal/nocturnal/generalist using a multinomial classification approach (Chazdon et al., 2011). The effect of diurnal and nocturnal periods on dung beetle assemblage structure was tested with permutation analysis of variance (PERMANOVA), and its multivariate dispersionwase tested with permutational multivariate analysis of dispersion (PERMDISP) (Anderson, 2001). To visualize how diurnal and nocturnal dung beetle assemblages were segregated, non-variate multidimensional scaling (NMDS) was used (Anderson & Willis, 2003). PERMANOVA, PERMDISP, and NMDS were performed based on the Bray-Curtis similarity index.