Density Estimates of Unmarked Large Mammals at Camera Traps Vary among
Models, Species, and Years, Signalling Importance of Model Assumptions
Abstract
Density estimation is a key goal in ecology but accurate estimates
remain elusive, especially for unmarked animals. Data from camera-trap
networks combined with new density estimation models can bridge this gap
but recent research has shown marked variability in accuracy, precision,
and concordance among estimators. We extend this work by comparing
estimates from two different classes of models: unmarked spatial
capture-recapture (spatial count, SC) models, and Time In Front of
Camera (TIFC) models, a class of random encounter model. We estimated
density for four large mammal species with different movement rates,
behaviours, and sociality, as these traits directly relate to model
assumptions. TIFC density estimates were typically higher than SC model
estimates for all species. Black bear TIFC estimates were
~ 10-fold greater than SC estimates. Caribou TIFC
estimates were 2-10 fold greater than SC estimates. White-tailed deer
TIFC estimates were up to 100-fold greater than SC estimates.
Differences of 2-5 fold were common for other species in other years. SC
estimates were annually stable except for one social species; TIFC
estimates were highly annually variable in some cases and consistent in
others. Tests against densities obtained from DNA surveys and aerial
surveys also showed variable concordance and divergence. For gregarious
animals TIFC may outperform SC due to the latter model’s assumption of
independent activity centres. For curious animals likely to investigate
camera traps, SC may outperform TIFC, which assumes animal behavior is
unaffected by cameras. Unmarked models offer great possibilities, but a
pragmatic approach employs multiple estimators where possible, considers
the ecological plausibility of assumptions, and uses an informed
multi-inference approach to seek estimates from models with assumptions
best fitting a species’ biology.