Road-kills in New Zealand: long-term effects track population changes and reveal colour blindness

Abstract

Road-kills were recorded at random throughout New Zealand, on 96359 km of roads, avoiding towns and busy motorways, from 1963-2018. Traffic increase from 1.04 m to 4.33 million vehicles during the study had little effect on mortality, even at the greater traffic density in the North Island. Seasonal changes measured on 8435 km (151 trips) between Lower Hutt and Otaki from 1985-2015 showed lowest mortality in winter. Major differences in species identification between two independent observers on the same route, from 2009-2014, resulted from one being red/green colourblind. Possum (Trichosurus vulpecula) numbers dipped briefly in the 1970s, peaked in the 1990s, and have declined since then where there has been widespread poisoning to protect trees, birds, and limit bovine TB. Rabbits (Oryctolagus cuniculus) increased steadily after control was lifted in the 1980s and now dominate the road-kills; the effect of RHD, introduced in 1997, does not register, probably because it causes short-term local oscillations. Hedgehog (Erinaceus europaeus) numbers show no clear trend and, unlike the other species, North and South Island patterns differ; the lower numbers in the South may reflect the cooler climate. Brown hares (Lepus europaeus) remain relatively stable, with a doubling in numbers since the 1980s in parallel with rabbits. The predators, cats (Felis catus) and mustelids (Mustela furo, M.erminea, M. nivalis), followed their prey increase until the 1990s when extensive predator control began; they then declined, although rabbit and rat (Rattus rattus, R. norvegicus) numbers continued to rise. In the 1950-60s, far more live mammals were being seen on and from roads, and adaptations to traffic have evolved. These historical records may be useful to assess future changes in road-kill following the adoption of silent electric cars, driverless vehicles, and public transport.