Amphibians and Reptiles on roads in northeast Suffolk.

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AMPHIBIANS AND REPTILES ON ROADS IN NORTHEAST SUFFOLK J.M.R. BAKER Summary Sightings of amphibians and reptiles were recorded over 3195 km of travel in northeast Suffolk from March to September each year from 2013 to 2020. Two hundred and ninety carcasses were recorded (9 per 100 km), one live common toad and one live grass snake. Amphibian roadkill comprised almost exclusively common toad (n = 202), followed by common frog (n = 6) and great crested newt (n = 2). Slowworm was the most abundant reptile mortality (n = 49), followed by grass snake (n = 30) and adder (n = 1). The numbers of toads peaked with the spring breeding migration, but they were killed on roads throughout the year, with a second, smaller, peak in the autumn. Reptiles were found mostly during the warmer months, probably reflecting higher activity levels during this time. Although reptile carcasses were relatively scarce, they were found at rates comparable to that of the hedgehog, a more familiar roadkill victim.

Introduction Roads have a huge, mostly negative, impact on wildlife. For example, negative impacts on birds extend, on average, 700 m from roads, so that in an area with high road density such as Great Britain over 70% of land is affected (Cooke et al., 2020).

Among the negative impacts of roads on wildlife (direct mortality, habitat fragmentation, pollution run-off and noise) roadkill is the most immediately apparent.

Counts of wildlife killed on roads do not, however, necessarily translate into impacts on populations. In fact roadside habitat can favour some species (e.g. Cooke et al., 2020) so that in such cases high mortality may reflect greater local abundance.

Nevertheless, information gained from extensive and systematic monitoring of roadkill can be useful not only in quantifying its magnitude but also in determining population trends, species distribution mapping and studies of behaviour, contaminants and disease (Schwartz et al., 2020).

The current observations are primarily concerned with amphibians and reptiles (herpetofauna). These taxa are highly vulnerable to the negative effects of roads (Hels & Buchwald, 2001; Rytwinski & Fahrig, 2012) and impacts on amphibian populations extend as far as 500 m from the road (review in Beebee, 2013). With the exception of the common toad Bufo bufo, which crosses roads in large numbers during its springtime migration, resulting in mortalities concentrated at particular locations (e.g.

Gittins, 1983), there is little information on herpetofaunal roadkill in Britain. This may be, at least in part, because amphibians and reptiles are relatively small animals and more easily overlooked than larger vertebrates. Nevertheless, in one of the earliest studies of road mortality, conducted on a two-mile section of the A6003, on the western outskirts of Corby, in 1959 and 1960, Hodson (1966) found that an amphibian, the common frog Rana temporaria was the most frequent non-avian casualty. The only other herpetofauna he recorded was the grass snake Natrix helvetica but at a much lower frequency (one per year). The relative scarcity of reptile roadkill in Britain is probably a further reason for the lack of data for these species.

The current study is based on observations made during recreational travel (by Trans. Suffolk Nat. Soc. 56 (2020)

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bicycle or on foot) around rural roads in northeast Suffolk. As the initial motivations were recreation and exercise, the selection of routes travelled has not been systematic. Nevertheless, the observations span eight years, longer than most professional research projects, and therein lies their value as data representative of the locality.

Methodology I recorded herpetofauna during the months of March to September from 2013 to 2020. Observations were made along routes setting off from Halesworth, mostly within the parishes of Bramfield, Chediston, Holton, Linstead Parva, Spexhall, Walpole, Wenhaston and Wissett with occasional forays beyond to Blythburgh or Dunwich. Journeys were initially taken for purposes of recreation or exercise and were mostly by bicycle but also include some made during running along rural roads.

The distance surveyed ranged from 309 to 461 km per year, spread fairly evenly across each month of the survey season. No single roadkill observation was counted more than once. The routes travelled were measured using an online route planning tool (Mapometer.com) to allow the frequency of observations to be standardised as observations per 100 km of road travelled.

The life stage of toads was recorded as either adult, or juvenile, based on body size and judged by eye. Grass snakes were categorised as adult or female (in the case of large individuals), juvenile and hatchling, again based on size and judged by eye.

In 2014 I added hedgehogs to the survey for comparative purposes. Road casualty data for this species have been used to generate national mortality estimates (Wembridge et al., 2016).

Results Two hundred and ninety-two amphibians and reptiles were seen over 3195 km of road surveyed (Table 1). All specimens recorded were dead, except for one toad and one grass snake both of which are included in the data in Table 1. The most abundant species was common toad (n = 203), followed by slow-worm Anguis fragilis (n = 49), and grass snake (n = 31). Very few common frogs (n = 6) and great crested newt Triturus cristatus (n = 2) were found and only a single adder Vipera berus. Two species present locally that were not found on roads were the smooth newt Lissotriton vulgaris and the viviparous lizard Zootoca vivipara.

Table 1. Numbers of different species on roads 2013-2020 Great crested newt Common frog Common toad Slow-worm Grass snake Adder Hedgehog km surveyed

2013

2014

2015

2016

2017

2018

2019

2020

Total

0 0 3 8 5 0 309

0 0 6 2 3 0 10 407

0 1 11 2 7 1 5 393

0 0 23 6 2 0 5 393

1 3 45 9 3 0 5 421

0 1 34 9 4 0 2 410

0 0 53 7 4 0 11 403

1 1 28 6 3 0 7 461

2 6 203 49 31 1 45 3195*

*Difference between total km and sum of annual values is due to rounding.

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Over the period of study the numbers of amphibians (comprising mainly common toads) increased while counts of reptiles and hedgehog fluctuated at relatively low numbers showing no obvious trends over time (Fig. 1).

Figure 1. Observations of amphibians, reptiles and hedgehog per 100 km surveyed annually from 2013 to 2020.

Amphibians (mostly common toads) peaked in numbers in March but could be found throughout the survey season (Fig. 2), especially during or immediately following periods of rain. For example, on 28 July 2019 six toads were found over a 10.9-km

Figure 2. Amphibian, reptile and hedgehog road sightings, by month, 2013-2020.

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journey (55 per 100 km) after rain following a hot, dry spell. All of the toads found during the springtime peak in March and April were adult (n = 112). Over the remaining months of May to September 16 of the sightings were juveniles, approximately half-grown, and 75 were adults.

Reptiles were found most frequently during the warmer months of the year (May to September), rising to a peak in August (Fig. 2). Hedgehogs were recorded throughout the season but more frequently during warmer months (June to September).

Examination of reptile sightings by species (Fig. 3) shows that slow-worms were found from May to September, with a peak in August, while most grass snakes were found from June to September.

Figure 3. Reptile and hedgehog road sightings, by month, 2013-2020.

The number of grass snakes found during August and September was elevated by hatchlings (n = 7) but other life stages were also found at this time of year (see Table 2).

Table 2: Life stages of grass snakes found over two-month periods.

Apr-May Jun-Jul Aug-Sep

Adult

Female

Juvenile

Hatchling

0 4 5

1 3 1

1 5 4

0 0 7

Note that it was not possible to determine the sex of adults in most cases, except for particularly large snakes which were identified as female.

In two instances remains of grass snakes were still present five and six weeks after their original discovery. In a third case, skeletal remains of a relatively large individual were still recognisable as a snake three months after first finding the carcass (Fig. 4).

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This interval spanned 30 June to 30 September 2019, a period of very low rainfall, which seems the likely cause of such prolonged persistence.

Discussion Use and limitations of roadkill count data Recording roadkill as in the present study does not in itself allow inferences to be made about the impact of roads on local herpetofauna populations. Neither can roadkill counts be used as direct comparisons of relative abundance because species differ in mobility and other factors such as the speed of movement crossing roads (Hels & Buchwald, 2001), willingness to move onto road surfaces, the persistence of carcasses and ease of detection of those remaining. Roadkill data do, however, provide information about species’ presence and their behavioural interactions with roads. To an extent trends within road mortality data (differences between species and seasonal variation within species) reflect patterns of activity. More mobile species, with greater dispersal or migratory distances are more likely to encounter and be killed on roads (Bonnet et al., 1999; Meek, 2009; Meek, 2012) and seasonal peaks in mortality often coincide with the timing of migratory behaviour.

Species presence The single adder carcass was a particularly significant record. It was found in August 2015 on Blackheath Road, adjacent to Blackheath Common to the south of Wenhaston. Adders disappeared from Blackheath Common in the 1960s, probably due to lack of favourable management and repeated burning in conjunction with November 5th celebrations (Mark Jones, pers. comm.). More recently the local commons, which are small parcels of heathland, have been managed sympathetically for wildlife by the Wenhaston Commons Group. Nationally the adder is a particular conservation concern (Julian & Hodges, 2019) and recolonisation of former habitat seems to be relatively unusual. Reptile surveys carried out within the Commons failed to detect adder, although in 2007 a slough was found by a local resident (Mark Jones, pers. comm.). The road-killed adder in the present survey provides further hope of recolonisation of Blackheath Common, presumably by individuals moving along the Blyth river valley which provides a potential link to populations on the coast.

Amphibians Amphibian mortalities were predominantly common toads. This is consistent with the findings of a meta-analysis of central European amphibian road mortality (Elzanowski et al., 2009) which found that common toad was the most frequently recorded species. The variation in numbers of amphibians (mostly toads) over the year (Fig. 2) is consistent with behaviour. In the spring toads migrate to breeding ponds and in doing so may encounter roads. Males may even sit on roads, where the flat surface gives a good vantage point to locate migrating females (Gittins, 1983). The peak in numbers of toads in March is consistent with the timing of breeding migration. The smaller peak in numbers at the end of the season may also correspond with migratory behaviour as observed elsewhere where toads start their breeding migration in the autumn (e.g. Sinsch, 1988). Although half of the annual toad mortality was recorded

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during springtime migration, (50% in March, or 56% from March to April) it is not solely confined to conspicuous road crossings at this time.

Roadkill counts cannot directly be used as indicators of relative abundance, but nevertheless the magnitude of difference between toads and other amphibians in the present study is striking. In a study of amphibian roadkill in rural Denmark, Hels and Buchwald (2001) calculated ‘efficiency’ and concluded that their routine surveys on foot identified roughly half of the actual number of toads killed (53%), about a third of the common frogs (32%) and only about one fifteenth (7%) of the smooth and great crested newts. Applying these values to the current roadkill count data still leaves a huge bias towards numbers of toads (adjusted counts: great crested newt = 30, common frog = 19, common toad = 379).

Another factor potentially contributing towards higher common toad mortality is this species’ greater breeding migratory distance. In contrast, common frogs are more likely to breed in the nearest available water body and hence travel less during the breeding migration. Both common toad and common frog occur within farmland habitats (Swan & Oldham, 1993; Baker & Halliday, 1999), the predominant land use locally, but the current roadkill data hint at a scarcity of the latter in rural areas locally.

Reptiles The seasonal pattern of road mortality for slow-worms (Fig. 3) corresponds with the warmer months of the year. Slow-worms are not known to undertake seasonal migrations (Beebee & Griffiths, 2000) but they inhabit a broad range of habitats including rough grassland, hedgerow and embankments (Edgar et al., 2010) as found bordering roads. Hence roadkill during the warmer months may reflect greater activity within relatively small home ranges (from several hundred [Edgar et al., 2010] to several thousand [Schmidt et al., 2017] square metres), immediately adjacent to roads. The annual pattern of road mortality recorded is broadly similar to that observed for other lizard species (wall lizard Podarcis muralis and western green lizard Lacerta bilineata) in western France where numbers peak during late summer and early autumn (Meek, 2014). Even so, slow-worms are mostly fossorial and struggle to gain purchase when moving over paved surfaces (pers. obs.) so is not readily apparent why they should move onto roads.

The majority of grass snakes were found from June to September, coincident with egg-laying and hatching, which occur in June/July and August/September respectively (Beebee & Griffiths, 2000). The timing of grass snake road mortality in continental Europe (France) is variable. Similar to the current study Bonnet et al., (1999) found that mortality was highest in the warmer months of June to October whereas Meek (2009) found grass snake mortality throughout most of the active season from April to October. In the former study hatchling mortality was greatest in June-July, in the latter in June. In the present study newly hatched grass snakes were found only later in the season, during August and September. This difference in timing of hatchlings may be due to earlier breeding in France. In the present study, although hatchlings increased the number of grass snakes during the late season, other stages were also

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found at this time. There was no apparent increase in numbers of grass snakes during the breeding period, which occurs from late March to June (Beebee & Griffiths, 2000) suggesting that grass snakes do not travel great distances during this activity.

There are few other reptile road mortality data for Britain. Hodson (1966) recorded a single grass snake each year of a two-year study, equivalent to 0.145 per 100 km (over the distance he surveyed between March and September). The encounter rate for dead grass snakes in the current study is greater, but a similar order of magnitude, 0.939 per 100 km.

No viviparous lizards were seen although they occur within the locality, for example on the heathland commons in Wenhaston. The absence of road killed viviparous lizards could be due to their relatively small home ranges, or avoidance of roads, or because their relatively small carcasses are difficult to detect on a road surface, or a combination of these factors. One of the very early wildlife/road studies (Pickles, 1942) produced a similar result in that a survey by bicycle revealed no viviparous lizards, even though they were present locally.

30 June 2019

2 September 2019

30 September 2019

Figure 4. Persistence of grass snake remains over a three-month period.

Carcass persistence Persistence of roadkill remains creates the potential for ‘double-counting’ – the same carcass may be counted more than once if the road in question is searched repeatedly at intervals shorter than the persistence time of carcasses. Carcass persistence on roads varies between species (Meek, 2012) and is affected by scavenging (Slater, 2002) and weather conditions (Meek, 2012). Cooke (1995) noted that carcasses of common toad could dry and mummify, persisting for weeks. During the current study it was noted that the remains of grass snakes could also be highly persistent under dry conditions, being recognisable for several weeks or months. This is in contrast to

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Meek (2009) who recorded a mean carcass persistence time for reptiles in western France of 7.9 days (SD = 10.4). The current observations suggest that reptile carcass persistence is variable and, exceptionally, much longer than previously recorded. In some studies carcasses are removed from the road which avoids the possibility of repeat counts. In the current study they were left in place but the interval between surveys of the same route was usually sufficient that amphibian carcasses were unlikely to persist that long and reptiles were so infrequent that individuals/locations could be remembered. Hence it is unlikely that carcasses were counted more than once in the current study.

Could road sightings be used to investigate herpetofauna population trends?

Systematically collected roadkill data can be used to examine population trends (Schwartz et al. 2020) as has been done for an amphibian (agile frog Rana dalmatina) and reptiles in western France (Meek 2012 and 2020). In Britain roadkill data for common toads have been used to identify population declines driven by road mortality (Cooke, 2011). Heigl et al. (2017) concluded that citizen science monitoring of herpetofaunal roadkill in Austria was more cost-efficient than using paid workers but it is questionable whether this activity would be suitable to extend to volunteers in Britain. For toads, the most abundant herpetofaunal roadkill, the most effective use of survey effort is to focus on the spring migration at specific road crossing sites, and data from such crossings have already been used to determine national population declines (Carrier & Beebee, 2003; Petrovan & Schmidt, 2016).

The incidence rates for slow-worms and grass snakes in the current study were of a similar order of magnitude to those of hedgehogs. Collation of mortality data for the latter has been used to better understand spatial and temporal trends (Wembridge et al., 2016; Wright et al., 2020) and as a source of population trend data in the People’s Trust for Endangered Species’ Mammals on Roads survey. The latter scheme, though, gathers data from journeys travelled by car, which cover long distances rapidly, whereas most amphibian and reptile carcasses are too small to be readily detected this way. Further, reptiles are relatively abundant in east Suffolk, whereas incidence rates in some parts of Britain (e.g. the Midlands or the Northwest) are likely to be lower and probably insufficient to engage volunteer interest. Herpetofaunal roadkill data for Britain (other than for common toad) are likely to remain hard-won.

Although they probably do not justify a dedicated recording scheme, data from generic recording schemes, for example Cardiff University’s Project Splatter, could improve our understanding of herpetofaunal behaviour on roads and could perhaps contribute to other areas. For example roadkill could be a source of specimens for studying snake fungal disease (Franklinos et al., 2017). Data on the persistence of herpetofaunal roadkill and the detection rates of carcasses would also be welcomed.

Such information has been used to estimate mortality rates from roadkill counts in the hedgehog (Wembridge et al., 2016) and such an approach may also be useful for herpetofauna.

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Acknowledgements I am grateful to Mark Jones for his input to this note and to Arnie Cooke, Roger Meek and David Wembridge for helpful comments, support and inspiration.

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