CHAPTER 1 INTRODUCTION
5. FAUNA
4.2 Field trials
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as possible may be because traffic volumes are usually lower at dawn due to general working hours (pers.obs.). Consequently, there is likely to be less damage to and/or removal of the roadkill carcasses. With fewer vehicles on the road, it is likely to be safer for the observers to be stopping/starting during their transects (Clevenger et al.
2003).
There is little data to support reasons for selecting the time of day for roadkill transects (Clevenger et al. 2003; Erritzøe et al. 2003; Mackinnon et al. 2005; Ramp et al. 2005). Consequently, it would appear that there is no single ‘best fit’
recommended method to detect multi-vertebrate roadkill surveys. However, the timing of some species-specific transect sampling seem to be based on the activity budgets of the target species, rather than because of the angle of the sun. For example, Jackson (2003) surveyed at night to examine the impact of roads on nightjars (Caprimulgidae) which are nocturnal, and Russell et al. (2009) surveyed at dawn and dusk when surveying a number of bat species. By comparison, Hels &
Buchwald (2000) started sampling amphibian roadkill at dawn as they believed that this would minimise the removal of carcasses by daytime scavengers.
Whilst data in my study did not show any significant differences in the time of day selected for surveying, there were fewer detection errors when the sun was higher.
Therefore, multi-species roadkill sampling should ideally be conducted between 1.5 hours after dawn and 1.5 hours before dusk.
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who sampled medium to large-sized mammals weekly over two years. Bright et al.
(2005) conducted monthly mammal surveys based on decomposition estimates of roadkill carcasses and as a result, volunteers were instructed not to repeat journeys within a 30-day period in case carcasses were ‘double-counted’. In contrast, Sutherland et al. (2010) sampled daily for two months over two years for amphibians whilst Hels & Buchwald (2000) sampled for seven months over three years (with the assumption that there was daily sampling). Both these sampling periods were when the amphibians they were targeting were most active. Quintero-Angel et al. (2012) sampled snake roadkill once every two weeks and this was based on estimates of how long snakes remained on the road after they had been hit by a vehicle.
Of the studies that have sampled all vertebrates and were therefore most similar to mine, some collected daily data (Smit & Meijer 1999; Clevenger et al 2003;
Ciesiolkiewicz et al. 2006; Santos et al. 2011), some sampled weekly (Taylor &
Goldingay 2004; Barthelmess & Brooks 2010; Bager & da Rosa 2011), some sampled bi-monthly (Barrientos & Bolonio 2009; Carvalho & Mira 2011; Quintero- Angel et al. 2012), whilst others sampled monthly (Vestjens 1973; Coelho et al.
2008). Bager & da Rosa (2011) who sampled weekly over two years, for vertebrates stated that weekly sampling did not attain sampling sufficiency when all classes were considered together, but was adequate for reptiles and medium-sized mammals.
Further sampling to twice a week, showed that birds still had not been adequately sampled (Bager & da Rosa 2011) due to the high richness of bird species in the area. Santos et al. (2011) state that based on the higher removal rate of roadkill, surveys of vertebrate roadkill should be conducted daily, even for larger species (>10 kg).
The huge variation in time frames, sampling frequency and consequently fragmentation of the data makes it difficult to assess and therefore to make comparisons between sampling frequencies (Erritzøe et al. 2003).The results of my study align with the recommendation of daily sampling to detect all vertebrate roadkill (Bager & da Rosa 2011; Santos et al. 2011). The data show that 40 days is adequate to sample all four taxa, with birds being the most diverse group, and therefore requiring the greatest sampling frequency.
57 4.2.2 Transect length
It is apparent from some roadkill surveys that shorter distances were likely selected for either species specific reasons or because of localised conditions. Hels &
Buchwald (2000) sampled a distance of 0.6 km for amphibians, whilst Gerht (2002) conducted a 41.8 km roadkill survey to obtain indices for raccoon (Procyon lotor) populations. This formed part of a larger study of monitoring raccoon population demographics that covered an area of 32.39 km2. Loughry & McDonough (1996) sampled a 5 km stretch of road to measure Armadillo (Darypus novemcinctus) roadkill and compared this population with a live population at another site. Further, Snow et al. (2011) sampled six segments of road totaling 32.2 km which were sampled 4-7 times per week over 29 months. The study site was on an island measuring approximately 34 km long and 6.5 km wide, which would suggest that the road transect length was selected based on the size of the island. However, the criteria used for selecting the number of sampling days were not clear. In contrast, Haikonen and Summala’s (2001) study in Finland examined the impacts of roads on the country’s population of Moose (Alces alces) and White-tailed deer (Odocoileus virginianus), covering all roads and a greater distance (300,000 km2).
Of the studies that sampled all vertebrates and were therefore most similar to mine, three sampled distances less than 40 km (34 km; Antworth et al. 2005; 32 km; Hell et al. 2005; 26 km; Carvalho & Mira 2011), two sampled over 100 km (195 km; Coelho et al. 2008; 117 km; Bager & da Rosa 2011). Clevenger et al. (2003) sampled two transects totalling 248.1 km. Dreyer (1935) and Dickerson (1939) recorded vertebrate roadkill whilst travelling in America and covered distances of 75,000 and 1,500 km respectively. Malo et al. (2004) used data collected from a traffic collision database on a 3,253 km stretch of highway over 13 years, whilst Smit & Meijer (1999) used data collected by traffic inspectors on unspecified distances on Dutch highways.
As with sampling frequency, the huge variation in sampling distance of existing methods (e.g. Bager & da Rosa 2011; Carvalho & Mira 2011) made them difficult to compare (Erritzøe et al. 2003). My study proposes a sampling distance of 100 km to adequately sample the four vertebrate taxa, with birds being the most diverse group, and therefore requiring the greatest sampling distance. For data to be comparable in
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future global roadkill detection research, and for surveys sampling all vertebrate taxa, further modelling covering various distances is recommended.