Effects of human disturbances on wildlife behaviour and consequences for predator-prey overlap in Southeast Asia

Human disturbances, affecting over 75% of Earth's land surface, impact wildlife community composition and behavior. Understanding the effects of these disturbances on species interactions, such as predation and competition, is crucial. One key area of impact is the alteration of diel activity patterns, or how species distribute their activity throughout the 24-hour cycle. Animals may adapt their activity patterns to avoid human encounters due to historical and ongoing hunting pressures, or competition for resources. The balance between resource acquisition, predation risk, and competition shapes these patterns. Anthropogenic disturbances, including forest edges, logging, and human presence, cause many animals to shift their activity patterns, potentially altering species interactions. Previous research suggests a shift towards nocturnality to avoid diurnal humans, but the extent and consistency of these shifts, particularly towards crepuscular activity (dawn and dusk), remain unclear. Crepuscular periods offer moderate light and thermal conditions, balancing foraging benefits and thermoregulatory challenges, whereas nocturnal periods provide protection from light-adapted predators but reduce foraging efficiency and introduce thermoregulatory costs. Interspecific variation in response to human disturbance is likely linked to species traits and the likelihood of negative human interactions. This study investigates these behavioral shifts in Southeast Asian wildlife communities, testing hypotheses related to the shift to nocturnality in disturbed habitats, the role of species replacement in driving community-level patterns, the influence of hunting pressure, and the impact on predator-prey and competitor overlap. Southeast Asia, with its hyperdiverse mammal assemblages and high rates of human disturbance, serves as an ideal model system. The region's deforestation and forest fragmentation, coupled with increased hunting access, create significant threats to its vertebrate communities.

Existing literature highlights the widespread impacts of anthropogenic disturbances on wildlife diel activity. Studies have shown that some terrestrial animals reduce daylight activity to avoid humans, but the extent to which this leads to crepuscular or nocturnal shifts remains unclear. Differences in guild-level responses have been reported, with large, hunted species showing stronger avoidance of humans compared to smaller animals or predators. The relationship between temporal overlap and species interactions is also complex, as co-occurrence doesn't necessarily imply interaction. Human activities, along with habitat loss and degradation, may increase spatial and temporal overlap, potentially influencing the likelihood of interactions.

This study uses camera trap data from 10 tropical forest landscapes in Southeast Asia (Thailand, Peninsular Malaysia, Sumatra, Singapore, and Malaysian Borneo) to investigate the impact of human disturbance on wildlife diel activity patterns and species interactions. A total of 1218 cameras were deployed across these landscapes, resulting in 58,608 trap nights and 31,138 independent detections of vertebrates >1 kg. The Forest Landscape Integrity Index (FLII) was used as a proxy for human disturbance, ranging from 0 (most disturbed) to 10 (intact). Circular kernel density functions were used to analyze activity patterns, comparing intact and degraded forests. Multinomial logit mixed models (MNLMMs) were employed to assess the influence of disturbance and species traits (body size, feeding guild) on diel activity, incorporating species- and landscape-level random effects. Temporal overlap (Δ) between species pairs was calculated to assess changes in potential interactions (predation and competition) between intact and disturbed forests. The study included 57 mammal species, four terrestrial bird species, and two reptiles. Species-level analyses focused on species with sufficient detections (≥15 in each diel category), while species pair analyses required ≥20 detections in both intact and disturbed forests. Guilds were defined based on diet (carnivore, herbivore, omnivore) and body size (small <4kg, medium 4-20kg, large >20kg). Analyses were conducted with and without the four most abundant species (pigs and macaques) to isolate the effects of species turnover from behavioral adaptation. The sensitivity of results to the disturbance classification was assessed by comparing different thresholds of the FLII.

Community-level analyses revealed a significant shift towards increased diurnal and crepuscular activity and decreased nocturnal activity in disturbed forests. This shift was primarily driven by species turnover, with a substantial increase in detections of diurnal generalist omnivores (pigs and macaques), rather than behavioral adaptation by individual species. When pigs and macaques were excluded, rarer specialist species showed an increase in nocturnal activity in disturbed forests. Guild-level responses were highly variable: large carnivores showed a reduction in early morning activity and became more cathemeral, medium carnivores shifted towards diurnality, and large and small herbivores remained crepuscular. Large and medium omnivores increased crepuscular activity, while small omnivores showed a non-significant increase in diurnal and crepuscular activity. Multinomial logit mixed models confirmed a three-way interaction between disturbance, body size, and feeding guild, with larger carnivores and omnivores showing significant shifts away from diurnality in disturbed habitats, while medium-sized carnivores and omnivores became more diurnal. Rarer specialist species exhibited a significantly higher probability of nocturnal activity in disturbed habitats compared to the abundant generalists. Importantly, analyses of temporal overlap between predator-prey and competitor pairs revealed no significant differences between intact and disturbed forests, suggesting that net species interactions might be conserved despite the shifts in diel activity.

This study demonstrates that human disturbance in Southeast Asian forests induces changes in wildlife diel activity patterns through both behavioral adaptation and species turnover. Rarer specialists become more nocturnal in disturbed areas, while the increase in diurnal generalists, especially pigs and macaques, shapes community-level patterns. The variation in guild-level responses, influenced by factors like hunting pressure and body size, highlights the complexity of human impacts on wildlife behavior. The lack of change in predator-prey and competitor overlap suggests that the overall structure of species interactions may be more resilient than individual species' activity patterns. Differences compared to some previous studies might be explained by pre-existing extinction filters in Asian communities, leaving behind more disturbance-tolerant species. The relatively limited behavioral changes observed might also reflect a trade-off between avoiding human activity and maximizing fitness under optimal light conditions.

This research underscores the complex interplay between human disturbance, wildlife behavior, and species interactions. While human activities drastically alter individual species’ diel activity, the overall structure of the community, in terms of predator-prey and competitive interactions, appears more resilient. Future research should focus on linking behavioral shifts to actual predation and competition rates, as well as exploring the consequences of specific species declines (e.g., due to hunting or disease) on the remaining community.

The study's use of the FLII as a proxy for human disturbance might not fully capture the nuanced effects of various disturbance types and histories. The binary classification of forests as intact or disturbed, while simplifying analysis, might mask subtle changes. Also, the focus on larger vertebrates (>1 kg) excluded smaller animals that may respond differently to human disturbance.