Surveying Smooth Newts at Riseholme (2007)
Dr Charles
Deeming
Department of Biological Sciences, University of Lincoln
cdeeming@lincoln.ac.uk
The Riseholme
Park campus
of the
University of
Lincoln
is home to a small,
oval-shaped pond
(with an area of ~18 m˛) originally created as a wildlife
habitat. It was know that there was a population of smooth newts
(Lissotriton vulgaris)
breeding in the pond but it was unclear how large a
population used this habitat. During the spring and summer of
2007 newts were
captured on average every 8 days in order to gain an estimate of
the numbers of newts breeding in the pond.
Two methods
were used for the adult newts.
Firstly,
with
assistance of conservation
biology
students a
survey of the pond was
made at dusk
with
torches
to
locate
the
animals, which were then caught in
nets and transferred to a bucket for temporary holding
overnight. The second method
involved setting
bottle traps (made from empty drinks
bottles) in
the water at the edge of the pond, which were left overnight and
collected early the next morning. The numbers of male
and female
newts were recorded together with the belly markings of
individuals. This was achieved by carefully placing the each
newt in a 30 ml universal tube. This system meant that handling
of the newts was minimal but it also allowed a digital
photograph to be taken of the ventral surface spots on the belly
and throat. Once a clear digital image was obtained the newt was
returned to the pond. On each capture day the identity of each
newt was determined by eye with reference to printed photographs
of newts previously captured and recorded. Any newts that had
not been previously
captured were added to the database.
Over the four-month survey period (mid-March to mid-July) 172 individual newts were identified with more males (N = 110) being caught than females (N = 62). Eighty-six of the identified newts were recaptured at least once. The time between initial capture and the first capture varied 5 to 107 days with a median of 27 days. More male newts were recaptured (61 versus 25 females) but they showed no significant difference in the time between capture and first recapture (medians = 27 days for both genders). Median number of captures per newt was 2 with thirty-eight newts caught twice, 26 caught three times, 14 newts were caught four times, 6 were caught five times, 3 were caught six times and 2 newts were caught seven times (see Figure below for images of male number 32). Over the survey period the proportion marked in each sample showed a significant positive correlation with successive trapping occasions (Spearman’s rank correlation coefficient, rs = 0.900, P < 0.001) and so the population was considered to be effectively closed.
|
30th March 2007 |
10th April 2007 |
28th April 2007 |
9th June 2007 |
17th June 2007 |
|
Male number 32 was captured seven times – as the breeding season progressed this individual lost condition and the intensity of its belly spots decreased. |
||||
Population estimates for the breeding population was determined using four methods. Firstly, population size was estimated after each capture event using the two-sample method of Greenwood & Robinson (2006): Population estimate = {([number caught + 1] x [total number caught previously + 1]) / [number of individuals recaptured+ 1]} – 1. Except for the first and last values the population estimates (see Figure below left) were comparable averaging 203 newts (SE = 6 and 95% confidence limits for the mean of ± 12 newts). Three models (Greenwood & Robinson, 2006) were tested for estimating the population size of newts using data over the entire survey period. The pseudo-removal method (population = 204; 95% C.I. = 173 – 215) allows for behavioural responses to being trapped, the Schnabel method (population = 202; 95% C.I. = 190 – 215) allows for temporal differences between capture occasions, and the Burnham and Overton method (population = 210; 95% C.I. = 168 – 252) allows for heterogeneity in catchability amongst individuals.
|
|
|
|
Cumulative population of newts captured (dotted line) and estimated population size (solid line) against Julian day (90 = March 31st) |
Number of newts trapped per survey day against Julian day (90 = March 31st) |
Capture of the newts exhibited a clumped dispersal pattern and individual newts were moving to and from the pond at different times of the season. The population estimates contrast with the number of newts caught in each trapping event – this averaged at 20-25 individuals (see Figure above right). It was clear that unless some attempt is made to identify individual newts in a sustained survey then the population of newts breeding in any pond will be grossly underestimated. This will have impact on the accuracy of any assessment of a breeding habitat for newts. The use of plastic tubes to hold newts and digital photography could assist in improving survey techniques for this and other species of newt.
Future studies in 2008 will: 1) examine whether the same newts return to the pond; 2) investigate whether the pond is able to sustain a comparable population of newts; and 3) determine where the newts spend their time in the periods that they are not present in the water.
Many thanks to Rosie Blackman, Jamie Glossop, Hannah Hind, Sophie Bennett, Delphine Suty, Clare Barnard, Stuart Brown together with Katherine, Emily and Roslyn Deeming for their kind assistance in catching and recording the newts.
References
Greenwood, J. J. D. & Robinson, R. A. (2006). General census methods. In Ecological Census Techniques, 2nd Edition, 87-185. Sutherland, W. J. (Ed.). Cambridge: Cambridge University Press.