Monitoring the status and trends of tropical forest terrestrial vertebrate communities from camera trap data: a tool for conservation

Abstract

Reducing the loss of biodiversity is key to ensure the future well being of the planet. Indicators to measure the state of biodiversity should come from primary data that are collected using consistent field methods across several sites, longitudinal, and derived using sound statistical methods that correct for observation/detection bias. In this paper we analyze camera trap data collected between 2008 and 2012 at a site in Costa Rica (Volcan Barva transect) as part of an ongoing tropical forest global monitoring network (Tropical Ecology Assessment and Monitoring Network). We estimated occupancy dynamics for 13 species of mammals, using a hierarchical modeling approach. We calculated detection-corrected species richness and the Wildlife Picture Index, a promising new indicator derived from camera trap data that measures changes in biodiversity from the occupancy estimates of individual species. Our results show that 3 out of 13 species showed significant declines in occupancy over 5 years (lowland paca, Central American agouti, nine-banded armadillo). We hypothesize that hunting, competition and/or increased predation for paca and agouti might explain these patterns. Species richness and the Wildlife Picture Index are relatively stable at the site, but small herbivores that are hunted showed a decline in diversity of about 25%. We demonstrate the usefulness of longitudinal camera trap deployments coupled with modern statistical methods and advocate for the use of this approach in monitoring and developing global and national indicators for biodiversity change.

Figure 1. Maps showing the sampling design and modeled occupancy for species where occupancy depended on covariates.

A. Distribution of camera trap sampling points along the Volcan Barva transect, Costa Rica. B,C,D,E: Modeled occupancy for Ocelot, Baird's tapir, Central American red brocket and Central American agouti at the baseline year (2008).

Figure 2. Temporal dynamics in occupancy for selected species.

Occupancy dynamics for Lowland paca (A), Nine-banded armadillo (B), Baird's tapir (C), Collared peccary (D), Jaguar (E) and all cat species combined –Ocelot, Margay, Jaguar, Puma and Jaguarundi– (F). Observed occupancies (naive) are shown as points, and modeled occupancy is shown as a solid line (median or mode of the posterior distribution). The shaded gray region spans the highest posterior density intervals for occupancy. The small inset graphs in A and B symbolize the statistical signal of λ between any two particular years. Values of λ significantly smaller than 1 are symbolized in red; values not significantly different from 1 are symbolized by grey. These are not shown for panels C,D,E and F because λ was not significantly different from 1 between years for any of these species.

Figure 3. Comparison of different metrics of community dynamics.

A. Modeled changes in mammal species richness at Volcan Barva transect, Costa Rica. The solid line shows the estimated value of richness and the shaded grey region spans the 95% confidence limits of the estimated distribution. B. Temporal changes in the Wildlife Picture Index (WPI) of 13 mammal species at Volcan Barva. The solid line is the mode of the posterior distribution of the WPI for each year and the shaded area spans the highest posterior density intervals for the WPI. Temporal changes in the WPI when it is disaggregated for species that are coveted by hunters (C) –Lowland paca, Central American agouti, Collared peccari– and species that are not normally hunted in the area (D) –all other species. E. Temporal changes in the WPI when it is disaggregated by functional groups of species; high posterior densities for each group are not shown for simplicity. The only group with a WPI significantly smaller than 1 in 2012 is the small herbivore group.