Tracking rodent‐dispersed large seeds with Passive Integrated Transponder (PIT) tags

Abstract

Summary Seed dispersal, a critical phase in the life history of many plants, is poorly understood due to the difficulty of tracking and monitoring dispersing seeds until they reach their ultimate fate. Scatter‐hoarding rodents play a substantial part in the seed dispersal process of many plant species, however, existing tracking methods do not allow seed monitoring without risk of influencing the hoarding process and seed fate. Here, we describe and test the use of Passive Integrated Transponders (PIT) tags inserted into seeds for the tracking and monitoring of large seeds dispersed by rodents. Unlike other tagging methods, PIT tagging combines the advantages of leaving no external cues and being readable without disturbance of caches. Rodents cannot remove these tags. We evaluated the performance of PIT tagging through a series of trials with Quercus acorns dispersed by rodents, both in North America and in Europe, with equipment from different manufacturers. We quantified effects of tagging on seed removal and caching, cache pilferage and seed germination, by comparison between PIT‐tagged and untagged acorns. We evaluated the detectability of buried tags to researchers. Minimal effects of PIT tagging on seed removal, caching, pilferage and germination were found. Buried PIT tags were retrieved with high reliability by naïve researchers, even at burial depths up to 30 cm. Identification codes could be read even when multiple tags were buried at a single location, as in larder hoarding. The method was successfully applied in two field studies of dispersal of Quercus palustris and Q. rubra acorns by Eastern grey squirrels Sciurus carolinensis in North America, and Q. robur acorns by Wood mice Apodemus sylvaticus in the Netherlands. The proportion of seeds recovered was comparable to that in studies using traditional thread tags. We conclude that PIT tagging is a particularly suitable method for tracking and monitoring of seeds dispersed by scatter‐hoarding rodents. PIT tagging solves most of the main problems generally encountered when following the fate of rodent‐dispersed seeds over time.