Introduction
Since its development, thermal imaging
technology was predominantly used in industrial applications. It was
regularly used in construction fields to measure heat flow, identify
structural defects (Grinzato, Vavilov et al. 1998, Balaras and Argiriou
2002), detect gas leaks, for powerline maintenance and for the
management of forest fires. More recently, applications ranged from
determining the structural features of volcanoes (Harris 2013), to
medical applications such as detection of breast cancer (Mostovoy 2008,
Ng 2009) and monitoring of pain (Agarwal, Spyra et al. 2008), (see
(Vollmer and Möllmann 2018) for a full review of the use of thermal
imaging across study areas).
Thermal imaging technologies were
increasingly used in wildlife surveys from the 1960s (see (Croon,
McCullough et al. 1968, Parker and Driscoll 1972)) although their
widespread use has been limited due to the cost of the equipment and a
lack of exposure of this type of equipment to biologists. The majority
of wildlife survey work undertaken with thermal technology was in the
detection of large wild animals such as pigs and ungulates using fixed
or rotary wing aerial survey techniques (and occasionally comparing
these results to visual surveys over the same area) (Parker and Driscoll
1972, Havens and Sharp 1998, Focardi, De Marinis et al. 2001). Limited
work has been done on abundance estimates of smaller animals and the
detection of fossorial animals and/or their burrows with thermal imagers
(Boonstra, Krebs et al. 1994). Burrows of fossorial animals can be
difficult to detect with more traditional white-light spotlighting used
during ground surveys, particularly where vegetation is present.
Additionally, those burrows that can be found often give little and
subjective indication as to whether that burrow is occupied.
The detection of occupied burrows is particularly important when the
burrowing animal is a pest, such as rabbits (Oryctolagus
cuniculus ) in Australia. Rabbits are a significant agricultural and
environmental pest and are listed as a key threatening process
(Commonwealth of Australia 2016). Rabbits were estimated to cause
>AUD$206 million per annum in agricultural losses (Gong,
Sinden et al. 2009). Additionally, it was estimated that private and
public landholders spend approximately AUD$6 million per annum
controlling rabbits. They are listed as a direct threat for 321 species
of Australian plants and animals and 75 endangered ecological
communities (Commonwealth of Australia 2016). The most effective
long-term method of controlling rabbits is the removal of their harbour.
Usually this means the destruction of their burrow (hereafter referred
to as a warren) through ripping programs, however, the success of
ripping programs is greatly influenced by the presence of surrounding
active warrens (McPhee and Butler 2010). Reopening of ripped warrens can
occur if any nearby warrens remain intact, therefore it is essential
that all warrens and warren entrances within the treatment area are
located. Where rabbit numbers are high (>5 rabbits/Ha),
warrens can be easy to locate due to the lack of vegetative cover.
However, where numbers are lower, or non-palatable plants are abundant,
warren entrances can be obscured and difficult to find.
Thermal imagers may provide a way
to detect these obscured rabbit warrens. Boonstra et al (1994) used
thermal imagers to differentiate between occupied and unoccupied arctic
ground squirrel (Urocitellus parryii ) burrows (where the location
of the burrow was known). They also identified that the presence of
dense vegetation was a limiting factor in thermal surveys. Technological
development of thermal imagers has progressed rapidly over the last 10
years, and there is a proliferation of thermal imagers available for
consumers. Therefore, we felt it time to re-evaluate thermal imagers for
the detection of animal burrows. We investigated whether consumer
thermal imagers could be used as a tool to assist land managers with
identifying rabbit warrens, particularly if obscured by vegetation. Here
we investigate the use of thermal imagers to 1) determine whether active
and inactive rabbit warrens could be detected with a thermal imager, and
(if so) 2) to evaluate the efficacy of consumer imagers to professional
imagers and visual inspection.