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Cost effective drift fences for toads and newts
J.W. Arntzen1, R.S. Oldham, D.M. Latham
Department of Biological
Sciences,
De Montfort University, Scraptoft Campus, Leicester
LE7 9SU, UK
1
present address: School of Biological
Sciences,
Brambell Building,
University of Wales, Bangor LL57
2UW, UK
Abstract. The construction,
cost and performance
of toad and newt drift fences and associated
traps used
at three study sites is described. Fence efficiency,
in terms of the capture of animals immigrating to
breeding sites, was calculated to lie between 39 and 63% of the estimated breeding population for the
common toad (Bufo bufo),
and between 45 and 61% for the crested newt (Triturus
cristatus).
Fences were
consistently
less efficient
at intercepting emigrating
animals;
less than 10% of toads and between 34 and
40% of newts were captured. Cost and performance of the systems
are compared with data from the
literature.
Introduction
Drift fences, usually associated with pitfall traps, have been used extensively in studies of
amphibian ecology (e.g. Cummins, 1920; Storm and Pimentel, 1954; Oldham, 1966;
Gibbons and Bennett, 1974; Campbell and Christman, 1982). They are used to inter-
cept animals travelling overland and are especially suitable for species exhibiting
seasonal and directional movement to and from breeding sites. Barriers claimed to be
fully amphibian proof have been devised but these tend to be expensive both in terms of
material and labour, for example aluminium flashing (Gibbons and Semlitsch, 1981) at
3 European Community Units (ECU) per metre (1978 price) or at 2.5 ECU per m (Dodd
and Scott, 1994), "Aco" plastic drift fence at about 30 ECU per metre (installed, 1992
price; Brehm, 1989 and Aco Company, pers. comm.), vehicle crash barriers at about 20
ECU per metre (installed, 1989 price; Podloucky, 1989) and paving slabs at 15 ECU per
metre (installed, 1993 price; Oldham and Cameron, in prep.). Other materials are
mentioned (but not costed) in the literature, for example plastic fabric and 4 x 4 mm
mesh (Meinig, 1989). Economy of labour and finance normally dictate the use of
barriers that are less than totally effective, but most papers pay little attention to the
details of fence construction and the efficiency achieved. The purpose of the present
paper is to describe the fences we have used for two species, the common toad, Bufo bufo,
and the crested newt, Triturus cristatus. In particular, we will i) provide construction
138
details, not commonly provided elsewhere, ii) provide information on the relative
efficiency of different systems and report upon the error associated with these estimates,
and iii) provide information on the relative costs.
Fence construction and maintenance
Toad fences. In our studies we used netting of hexagonal wire mesh ("chicken wire")
which provides a highly durable, easily shaped barrier requiring relatively little support
and which comfortably survives a three year study. A gauge of 0.5 inch (1.3 cm) mesh is
suitable to obstruct all but the smallest adult toads. Toads readily scale a vertical fence
and it is necessary to bend the wire to provide an overhang of about 10 cm at the top. At
the base the wire is either dug into the ground to a depth of about 10 cm, or bent by 10 0
cm, to allow for the addition of weights which keep the wire flush with the ground. We
have no evidence of toads purposefully burrowing under fences, although they occa-
sionally bury themselves beside the fence, which could result in escape if there is no
basal extension or lip. For toads, fence height is relatively unimportant since they have a
weak jump. For economy we used chicken wire of 61 cm width, which provides an
overall fence height of about 40 cm. If interception of movement in only one direction
was of concern, then an asymmetrical fence with upper and lower overhangs pointing in
a direction opposite to the toad migration was appropriate. If two-way movement was to
be monitored then higher wire (width 76 or 91 cm) bent in the shape of an "S" in cross
section, may be used (Swan, 1986). A more effective two-way barrier was provided by
grafting additional wire on to the top of the fence to provide a "T" cross-sectional
shape.
Wooden supports (I m x 5 cm x 5 cm) were provided at 10 m centres and pitfall
traps, in the form of 10 1
plastic buckets, at approximately the same spacing. The traps
were set flush with the ground, tangential to the fence (fig. 1), and filled with water to a
depth of 5 to 10 cm. The water prevents desiccation, helps to disguise captures from
predators and hinders the escape of frogs, which might otherwise jump out. A tight fit
between the fence and trap is essential and this can be achieved by securing the wire
beneath the lip of the bucket. In dry soil, drain holes, punched in the side of the bucket
about 10 cm from the base, were used to prevent flooding, but in waterlogged soil a
small embankment was built around each trap. Alternatively, skewers or weights were
used to stop the traps floating. At 1993 prices the materials for this fence/trap system
cost approximately 1.5 ECU per metre and took about eight man-hours per 100 m to
install.
Akwt fences. Chicken wire is unsuitable as a barrier since the smallest mesh readily
available (1.3 cm) will allow newts to pass through. Instead, chicken wire (1.3 or 2.5 cm
mesh) was used as a support and a combination of 500 gauge polythene and "Netlon"
greenhouse shading (0.3
cm mesh) as the barrier to an overall fence height of 50 cm (fig.
2). Newts are good climbers and some trouble was taken to provide a lip on each side of
139
Figure 1. Schematic
representation
of a chicken wire fence/pitfall system
for toads.
the barrier. These lips were supported by rot-proof twine. The posts, at 5 or 6 m centres,
were set closer together than for the toad barrier and a span of metal wire between the
posts carried the additional load of the sandwich of materials. Netlon was preferred to
polythene on the lower part of the barrier because of: (i) its greater durability (it easily
withstood a three year project and was not damaged by burying); (ii) it allows air
circulation; this reduces the possibility of interference with the newts' olfactory migra-
tory cues and also makes the fence less prone to wind damage. Polythene was used for
the upper part of the barrier because it would be harder for the newts to climb if they did
succeed in passing the lip. The fence might be further improved by the addition of two-
sided ':\verhang in the polythene at the top of the fence (fig. 2). If damaged the fence was
repaired using strong adhesive "Post Office packing tape". In places exposed to wind
and sunshine the polythene did not last the three year study period and had to be
replaced. The use of UV-resistant polythene, at twice the normal price, might be
recommended. Pitfall traps were as described for the toad fence. At 1993 prices the
materials for this fence/trap system cost approximately 3.5 ECU per metre and took
about 30 man-hours per 100 m to install.
An alternative material used on some occasions was "Netlon cladding". This was self
supporting, very durable, easier to erect (except on uneven ground) and less easily
damaged by the growth of vegetation. Again the base of the fence was buried by about
10 cm and the top was provided with a two-sided polythene overhang of 10 cm each
side. The cost of this system was about 6 ECU per metre.
140
Figure 2. Schematic
representation
of a fence/pitfall
system
for newts. The actual fence
consists of a sandwich
of polythene
and "Netlon" greenhouse
shading
and is supported by chicken
wire (see
text for details).
141
Fence maintenance. Both kinds of fence require frequent checks, both to ensure their
efficacy (to remove debris which might fall into the traps and enable escape, top up
water in the pitfall traps or bale it out, etc.) and the well-being of the trapped animals. A
disadvantage of the method is that rodents, insects and other small animals sometimes
drown in the traps. This can be avoided by using bottomless containers as traps
(Oldham 1966), especially appropriate where desiccation is not a problem, as in moist
woodland.
Fences and pitfall traps decline in efficiency as vegetation grows around them during
the spring and summer. They can also be damaged by extreme wet and dry conditions
as a result of flooding and cracked soil respectively. Furthermore, as with any field
equipment they may be damaged by animals, vandals or thieves. Each of these condi-
tions was countered by appropriate maintenance. We have found "Stockholm tar"
(available from agricultural suppliers) to be a useful deterrent against theft.
Fences were susceptible to damage by the growth of vegetation adjacent to the fence,
especially the newt fences. This was cleared manually when necessary. An alternative to
mechanical removal of encroaching vegetation are herbicides such as "Round up" (P.S.
Franklin pers. comm.).
Study sites and monitoring.
Fences, as described, were constructed in 1990 at three sites in
Leicestershire to monitor amphibian movements during three successive breeding sea-
sons. At study site A (Charnwood) a 800 m long fence was set around a 1.2 hectare lake.
The barrier crossed several streams and habitats. The fence, with an overhang pointing
outwards, was intended primarily to intercept immigrating toads. Newt barriers were
constructed around ponds at site B (C?addesby, 300 m2) and site C (Corn Close, 800 m2)
respectively 100 m and 150 m in circumference. They were intended to intercept
animals on both inward and outward migration. Inspection of the traps at sites was daily
or twice daily during periods of peak movement. In off-peak periods inspection was
carried out every third day at a minimum. It appeared that most pitfall captures
occurred at night so that morning collection minimised exposure to predators. We
report on the functioning of the fences in the first year they were operational.
Fence efficiency
Population size has been estimated using a single capture-mark-recapture determina-
tion (Lincoln Index). All captures at the outside pitfalls and fence during the breeding
season were marked and released over the fence. The sum of these animals provided the
first sample. All captures at the inside pitfalls, after breeding, provided the second
sample. The recaptures were toads and newts caught on both occasions. This method
does not take into account animals present within the perimeter before the fence was
erected or mortality during the breeding season. It also assumes that animals are not
deterred by the fence into retreating from the site and that their behaviour is the same
on approach and exodus. Fence efficiency (FE) is defined as the percentage of the
population approaching the fence which was caught in the pitfalls.
142
Fence
efficiencies
at breeding
sites. The results obtained with both target species are shown in
table 1. A clear distinction was observed between the proportions of the populations
caught immigrating (FEi) and emigrating (FEo). This was expected for the toads since
the overhang of the fence pointed outwards and could be easily scaled from the inside.
The marked difference at the newt sites cannot be explained in the same way. Imperfec-
tions in the fence and pitfall traps, which increase later in the season as vegetation
grows, may account for some of the difference. Other contributory factors could be a
tendency for newts to remain inside the pond perimeter until late in the season, or
changes in newt behaviour, such as a reduced tendency to rush headlong into the traps
after the breeding season or greater persistence in attempting to pass the barrier. Fence
efficiency results were similar at sites B and C, and, at site B, higher for females than for
males (table 1).
For toads there was a marked sexual difference in FEi, the value for females being
estimated at more than 1.6 times the value for males. The, on average, substantially
larger females may find climbing more difficult, especially if in amplexus, or the smaller
males may have been better able to penetrate the fence. Again, there may be behav-
ioural differences, such as a greater imperative on the part of the females to achieve the
pond whereas males are known to spend time on the approach searching for females.
Fence efficiencies
recorded in the literature. Swan (1986) recorded toad immigration efficien-
cies similar or marginally higher than ours. She worked on three fenced ponds (table 1), ),
each of the two smaller ones for two succesive years each. FEi varied between 62 and
71
%, FEo between 21 and 31 %. Her values for FEo are higher than ours, probably the
result of using wire bent in the shape of an "S" in an attempt to prevent toads passing
the barrier ('fence trespassing') in either direction. Gittins (1983) used a plastic barrier
and recorded relatively high values for both FEi and FEo, with little difference between
the sexes. The difference between the values for FEi and FEo was ascribed to mortality
within the enclosure. The high efficiencies recorded by Oldham (1966) for Bufo ameri-
canus were obtained by visiting the fence three times in each 24 hours. Two of these visits
were made during the hours of darkness and about half the captures were made at the
fence itself, rather than in the traps. This suggests that the lower efficiencies in the other
studies, which usually involved only one collection per day, during daylight, may, in
part, have been caused by failure of toads to enter the pitfall traps.
For newt the estimated values revolve around 50% for FEi. Outlying values at 23%
were obtained for an incomplete fence constructed for studing T. carnifex (Andreone and
Giacoma, 1989) and for a low fence (Amtkjaer, 1981). The implied significance of fence
height is not supported by Verrell's (1985) result (69%) using a relatively low fence. The
only fence approaching full efficiency in intercepting migrating T. dobrogicus,
combines
height and depth with an especially solid construction (Schramm, 1992). Values for FEo
are mostly 10 to 20% lower than FEi. The outlying (low) value for FEo obtained by
Bouton (1986) for T. marmoratus is probably due to finishing observations early in the
year. The two studies (Swan, 1986 and Franklin, 1993) involving the same ponds in
143
144
successive years showed wide variations in fence efficiency changes from one year to the
next. For FEi the change ranged from a 9% increase to a 29% decrease and for FEo
from a 9% increase to a 9% decrease.
Discussion
A wide variation in fence efficiencies is illustrated in table 1, but it is clear that most
systems do not achieve the complete sampling suggested as possible in some studies (e.g.
Bell, 1979, for the crested newt). As pointed out by Dodd (1991) few amphibian studies
address the importance of fence trespass and his observations suggest that many species
are able to circumvent a drift-fence pitfall trap enclosure, partly by using holes beneath
the fence made by invertebrates or mammals. On the other hand, a 100% proof fence
may not result in a maximum FE score due to mortality and other factors referred to
earlier.
Most systems involve a compromise between the rival requirements of cost reduction
and efficiency. It must be admitted, in the present study, that the pains taken to improve
the design of newt fences were not clearly rewarded by improved catch efficiencies (table
1) in comparison with other systems. Anything less than 100% efficiency carries with it
the likelihood of sampling bias. Bias in terms of sex and size are illustrated in the present
work. A species bias and increased mortality are also possible. Potentially even more
significant, and harder to control for, is temporal bias. Estimated efficiencies during
emigration were consistently and significantly lower than during immigration, both in
our studies and in the literature.
Acknowledgements. Our gratitude goes
to those people-too many to name individually-who helped
in the construction and removal of the fences, to the Leicestershire and Rutland Trust for Nature
Conservation (M. Walpole),
the Co-operative
Wholesale
Society Ltd. (K. Preston), N. Whait and G.F.
Collings for access to their land and to Chr. Arntzen for drawing the figures. Financial support was
provided by NERC through its Joint Agricultural
and Environment
Programme'.
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