Quantifying search effort of moving animals at several spatial scales using first-passage time analysis: effect of the structure of environment and tracking systems
Résumé
1. How and at what spatial scale(s) animals change their movements in relation to their environment is central to several topics in ecology and conservation, including foraging ecology, habitat selection and dispersal. A method (first-passage time analysis, FPT) has recently been proposed to measure changes in movements through the landscape, as an index of search effort at the pertinent spatial scales. This method seems largely applicable to an increasing number of studies using satellite, radio-tracking or global positioning system (GPS), but its limits have not yet been assessed. 2. Here I used several movement simulations to examine the ability of FPT analysis to detect arearestricted search (ARS) according to different changes in movements, different patch structures and tracking accuracy. 3. FPT analysis was able to detect changes in movements when both speed and sinuosity changed, or when the animal reacted to patch boundaries. It was also able to detect ARS within the same path at several spatial scales in patches (nested or not) of different sizes. 4. Tracking accuracy affected the detection of ARS by FPT analysis. With the widely used Argos system, a minimum of 13 locations in effective ARS was necessary to detect this behaviour; seven when velocity filtering was applied. Similarly, spatial error in location affected the estimation of the ARS scale value, but the application of velocity filtering reduced this effect. 5. Comparisons between a real GPS track and pathways simulating the Argos error showed that the time-sampling rate of locations (due to satellite-pass frequency) decreased the probability of detecting ARS at small scales (<10 km), while the spatial error decreased this probability by >50% across the whole range of scales. A velocity filter enabled significant reductions in this effect. 6. Synthesis and application. Within limits, FPT analysis is highly suitable for animal movement analysis, either to quantify habitat use, or to determine the scale most relevant for describing an ecological system or factors affecting movement decisions. In anticipation of increasing applications of FPT analysis in applied ecology, I provide recommendations for the use of the technique with several tracking methods.