Dominic Henry, EWT Conservation Science Unit, Ecological Modelling Specialist, 

Road networks form a substantial part of current and future infrastructure development and are a major hazard for local wildlife populations. Due to high mortality rates, wildlife-vehicle collisions are a threat not only to individual animals but to broader animal communities, populations, and species. The threat of wildlife-vehicle collisions is set to increase globally as an additional 25 million kilometres of road are expected to be added to the global road network by 2050.

We need to estimate how many wildlife deaths result from vehicle collisions if we want to understand this impact on wildlife populations and develop strategies to prevent or reduce the collisions, so we need to get out there and do surveys.

However, conducting roadkill surveys is a resource-intensive exercise. Labour costs, fuel, mileage, and vehicle maintenance are just some of the costs incurred for this type of research. Depending on the study site and survey durations, these costs can escalate quickly. Research budgets are often limited, and it is important to make each rand go as far as possible. In light of financial resource limitations, the question is, is there an alternative to conducting daily surveys of roadkill while still having a robust study design?

When designing roadkill surveys, there is a trade-off between survey frequency (and all the logistical effort and costs incurred) and roadkill carcass detection. In this regard, carcass persistence (the period a carcass remains detectable before it is removed by decomposition or scavengers) is important; the longer a carcass persists, the greater the likelihood it will be detected with lower survey effort by conducting more infrequent surveys.

Part of our research, published recently in the Journal of Environmental Management, looked at this question. The study was based on Wendy Collinson-Jonker’s roadkill dataset collected in the Greater Mapungubwe Transfrontier Conservation Area. An observer drove a standardised 120 km transect daily for a month, recording all carcasses as well as how long they remained on the road. Over 340 carcasses were recorded, comprising 73 species ranging from the smallest frogs to a Brown Hyaena. We used these data to simulate what would happen if surveys were conducted with different intervals from 2–10 days.

We found that the proportion of carcasses detected decreased sharply when survey intervals were longer than three days. However, we showed that survey costs could be reduced by up to 80% by conducting non-daily surveys. By carefully considering the objectives of the survey and characteristics of the focal species, researchers can substantially reduce the costs of their studies. We developed a web app ( that can be used by researchers to compare survey costs across a variety of survey characteristics. This web app allows researchers to assess the trade-off between carcass detection and cost easily.

Details of the journal article:

Citation: Henry, D.A.W., Collinson-Jonker, W.J., Davies-Mostert, H.T., Nicholson, S.K., Roxburgh, L. and Parker, D.M. 2021. Optimising the cost of roadkill surveys based on an analysis of carcass persistence. Journal of Environmental Management 291:112664

Link to paper:

Link to app:

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A word from the CEO March 2023

When Clive Walker, Neville Anderson, and James Clarke registered the Endangered Wildlife Trust in 1973, They had no idea where it would go or what it would do for species and habitat conservation in the region. This year the Endangered Wildlife Trust commemorates 50 years of conservation excellence. The EWT has achieved remarkable gains for many species,

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