One thousand camera traps reveal that forest mammal communities have a consistent structure and similar vulnerability across the tropics
Submitted by editor on 16 October 2019.
Get the paper! By Francesco Rovero
That biological communities in similar environments but distant regions share forms and functions is a known pattern of diversity attributed to convergent evolution. For tropical forest mammals, which include some of the most iconic and threatened animals on earth, the existence of this pattern has been inferred by comparing assemblages of species from distribution maps at coarse scales (110 to 200 km resolution). However, we did not know how this manifested in ‘real’ communities detected on the ground, at the fine scale. A number of reasons may determine different patterns: for example, local factors such as habitat heterogeneity, climate and anthropogenic pressures may shape communities differently from those predicted from distribution maps.
We set out to address this question by using data from camera traps, remote and hidden ‘eyes’ attached to trees that detect passing animals 24/7. We used data from nearly 1000 camera traps set in 16 tropical forest protected areas forming the Tropical Ecology, Assessment and Monitoring (TEAM) Network, which is currently an initiative within the global, newly launched Wildlife Insights platform (https://www.wildlifeinsights.org). From a single season of standardized sampling (60 camera traps deployed for 30 days in each forest), these cameras yielded over 300 000 images of 171 species of mammals.
We focussed our analyses on two ‘functional traits’: body mass and trophic position (whether a species is a carnivore, insectivore, herbivore or omnivore) which together inform on life history and niche of species. We used a novel hierarchical model in occupancy framework that accounts for what we call 'imperfect detection’, species that may be present but not detected by camera traps, to compare communities in different areas and estimate both guild richness and the average body mass of species in the communities. We then related these metrics to a number of covariates of habitat extent, protection, productivity and of anthropogenic disturbance.
Our results generally support the hypotheses of convergent community structure, as the composition in trophic guild was broadly similar among communities. Interestingly however, we also found that vulnerability of these communities is also similar, as some trophic guilds tended to respond similarly, across communities, to covariates. In particular, insectivore richness increased significantly with protected area extent and decreased with surrounding human density. Moreover, the average body mass of species in the communities increased with distance to human settlements.
Map above: the 16 tropical forest protected areas in the TEAM Network where mammal communities were assessed with standardized surveys using camera traps. For each community, pie charts report the trophic guild composition as the mean estimates of the proportion of species in each trophic guild on the total species richness, representing their trophic structure. Numbers indicate the observed richness of mammals. Chart below: the relationship between region-specific mean community body mass and distance to settlements. Open circles indicate mean body mass for the observed species of the 16 communities.
These findings are important and bear conservation relevance. They suggest that similar habitats and selective pressures within the tropical forest biome shaped communities at the fine scale with similar trophic strategies and responses to drivers of change. These similarities provide a foundation for assessing communities under ever increasing anthropogenic threats, and suggest a potential for sharing conservation measures across different areas within the tropics. By determining forest mammals’ vulnerability and drivers of change at the fine scale, moreover, our results complements global prioritization analyses based on habitat suitability models, which is a highly recommended research avenue by the global mammal conservation strategy.
