Can non-invasive methods replace radiocollar-based winter counts in a 50-year wolf study? Lessons learned from a three-winter trial

Context: Monitoring low-density, elusive predators such as grey wolves (Canis lupus) has often been undertaken via live-capture and radio-collaring. Recent advances in non-invasive methods suggest live-captures may not be necessary for adequate monitoring. Further, non-invasive methods are considered best practice when possible.

Aims: I evaluated whether a suite of non-invasive methods could replace aerial radiotelemetry to census resident pack wolves.

Methods: I employed aerial snow-tracking, ground snow-tracking, camera-trapping, non-invasive genetic surveys, and community-scientist reports during three winters (2019–2021) in north-eastern Minnesota, USA to census pack wolves in a 2060 km² area. I attempted to enumerate individual pack sizes as has been historically undertaken to compile the census. Traditional aerial radiotelemetry methods were also conducted for comparison.

Key results: Ground snow-tracking and camera-trapping provided the most similar information to radiotelemetry for determining pack counts and territory information, and, in some cases, documented higher pack counts than those obtained by aerial radiotelemetry. Radiotelemetry was the best method for determining pack territories, but was limited to radioed packs. A staggered application of both approaches resulted in increased precision and additional pack-level information without greatly increasing overall field effort. Non-invasive methods allowed trapping for radio-collaring to be reduced to every other year (a 50% reduction), but depending on trapping success, survival of animals, and radio-collar battery life, might even be reduced to every third year.

Conclusions: In this 3-year trial, non-invasive methods were not sufficient to completely replace radio-collaring. Nevertheless, non-invasive methods allowed for a 50% reduction in trapping, increased the annual wolf-count precision, and increased community involvement. Anticipated technological improvements in non-invasive methods should reduce some issues encountered – but others will likely persist, in part, because of the fundamental nature of non-invasive methods.

Implications: Less reliance on captures, enhanced pack information, and increased public involvement are all successful outcomes of this 3-year trial of non-invasive methods for monitoring wolf populations. Non-invasive methods continue to broaden and improve technologically, and information from trials such as this will help guide others as they increasingly implement non-invasive methods as partial or complete replacements for traditional capture-based methods.