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First assessment of the population of Hermann's tortoise in Croatia (NP Mljet)

Staff of Croatian herpetological society – Hyla spent a field research for the project „Monitoring of Hermann's tortoise (Testudo hermanni) and native species of snakes in the Mljet National Park", during the summer of 2015. Goals of this project were to establish a continuous monitoring of the tortoise and to record snake species within the national park. Project was carried out in collaboration with the public institution of the Mljet National Park. We monitored the tortoise using the method of linearly transects. This method includes a researcher who walks on the previously defined line in the field, which is exactly one hundred meters long. During the walk, researcher is looking in front of him, left of him and right of him and records an individual tortoise which he sees, along with the distance between the spotted animal and the transect line (Image 1).

 

PodlogaImage 1. – Recording individuals on the transect line

We walked 20 000 meters in total, and after we collected the data, we made a graph (Image 2). On the x-axis are the distances on which we recorded the individuals while walking on the transect line. In this case, the greatest distance on which the tortoise has been recorded was four meters. On the y-axis are the number of the recorded individuals on the field. We generated the appropriate detection curve which gave us an assessment of the maximum number of individuals on the transect line (in ideal case). In this case that is 12 individuals. The density of the individuals is the same on all parts of the transect line (0, 1, 2, 3, 4 meters) so we can claim that the maximum average number of individuals (on every transect line) is 12. So we can calculate how much tortoises we didn't see on each transect line.

 

Graf

Image 2. Graph of a curve which estimates the population density (numbers are for example)


Surface area beneath the curve represents the individuals which were recorded during the transect line walks and the surface area above the curve represents the individuals which weren't recorded but live there. Ratio between these surfaces is 1:1 which means that for every recorded individual there is one that we didn't record. If we recorded 25 tortoises (numbers are for example), that means we didn't see 25 tortoises and the total number of individuals is 50. If we divide that number with the total surface area of the transect line (for example 800 m2) we'll get a population density of 6,25 individuals on 100 m2 (625/ha).
The results from this field research showed us that the population density of Hermann's tortoise in the Mljet National Park is 3-4 individuals/ha. In the whole park area there are 10 000 individuals of the Hermann's tortoise.

Source:
Jelić D. i Baškiera S. (2015): Monitoring (praćenje stanja) kopnene kornjače (Testudo hermanni) i autohtonih vrsta zmija na području NP Mljet. Terenski izvještaj. HHD – Hyla. 34 str.
Sutherland W. J. (2006): Ecological Census Techniques. Cambridge University Press

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