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Initial estimation of lizard population densities on the selected islands of the Lastovo Archipelago, Croatia

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Abstract

On several islands in the Croatian archipelago of Lastovo, it has been observed that introduced mammal species might have an impact on native species populations. One of the invasive species found on the islands is the Black rat. Rats are predators, and evidence from other parts of the world are conflicting, some research indicates that rat predation may be the cause of the population decline of lizards and others that rats do not have influence on lizards. Rat eradication is planned on some islands of Lastovo Archipelago as part of LIFE Artina - "Seabird Conservation Network in the Adriatic" project (LIFE17 NAT/HR/000594) and with this study we wanted to make an initial assessment of the lizard population densities on them before eradication. Two field trips to the Lastovo Archipelagos were made in May and June 2019 on selected 14 islands. Islands range in size from 0,35 to 47,42 hectares. During three field visits to each island, a linear transect with a length of 500–1000 m was established, where lizards were observed twice in a single day. Distance sampling was used to estimate the density, and each specimen's perpendicular distance was measured. Each island's data from the transects was analyzed separately with the program Distance 7.3. A total of 4164 adult individuals, including 2287 Dalmatian Wall Lizard, (Podarcis melisellensis) individuals and 1877 Sharp-snouted Rock Lizard (Dalmatolacerta oxycephala) individuals, were observed. Both species were recorded on all the islands visited. The density of the D. oxycephala, is 22 - 282 individuals per hectare, while the P. melisellensis is 28 - 251 individuals per hectare. These numbers represent initial values, and this assessment must be repeated following eradication to compare the situation before and after so it can be determined what kind of influence will rat eradication have on lizard populations on islands of Lastovo Archipelago.
Nature Park Lastovo islands is located in the Croatian part of the Adriatic
Sea and consist of the southernmost Croatian populated island of Lastovo
with unpoupulated adjoining islands. On several islands in the archipelago of
Lastovo, it has been observed that introduced mammal species might have an
impact on native species populations. One of the invasive species found on
the islands is the Black rat. Rats are predators, and evidence from other parts
of the world are conflicting, some research indicates that rat predation may
be the cause of the population decline of lizards, and others that rats do not
have an influence on lizards. Rat eradication is planned on some islands of
Lastovo Archipelago as part of LIFE Artina - "Seabird Conservation
Network in the Adriatic" project (LIFE17 NAT/HR/000594) and with this
study we wanted to make an initial assessment of the lizard population
densities on them before eradication. Assessment was conducted on two
species: Dalmatian Wall Lizard, Podarcis melisellensis (Fig. 1) and Sharp-
snouted Rock Lizard, Dalmatolacerta oxycephala (Fig. 2).
Introduction
Methodology
Results and discussion
A total of 4164 adult individuals, including 2287 Dalmatian Wall Lizard
individuals and 1877 Sharp-snouted Rock Lizard individuals, were observed.
Both species were recorded on all the islands visited.
The density of the D. oxycephala,is 22 -282 individuals per hectare, while
the P. melisellensis is 28 -251 individuals per hectare. The highest density of
the D. oxycephala was recorded on the island Mali Maslovnjak and the
smallest on Bratin (Fig. 3). On the island Srednji Vlašnik was recorded the
highest density of the P. melisellensis, and the smallest on the Zaklopatica
(Fig. 4). P. melisellensis is more numerous on the islands where habitat type
A and B prevails, but on islands where habitat type C and D prevails a
greater number of D. oxycephala was recorded.
Conclusions
These numbers represent initial values, and this assessment must be repeated
following eradication to compare the situation before and after so it can be
determined what kind od influence will rat eradication have on lizard
populations on islands of Lastovo Archipelago.
Acknowledgements
We are very grateful to the Association BIOM for financing this survey as
part of the LIFE Artina - "Seabird Conservation Network in the Adriatic"
project (LIFE17 NAT/HR/000594) and help with the research. We want to
thank Ivan Knezović for help with field work.
Two field trips to the Lastovo Archipelago were made in May and June 2019
on selected 14 islands where the Black rats are present (Fig. 3). Islands range
in size from 0,35 to 47,42 hectares. During three field visits to each island, a
linear transect with a length of 5001000 m was established, where lizards
were observed twice in a single day. Distance sampling was used to estimate
the density, and each specimen's perpendicular distance was measured. We
recognized four main habitat categories on the islands. The first habitat type
A, was characterised by a vegetation higher than 50 cm, the second one ’,
was characterised by vegetation less than 50 cm, the third C, were rock
formations, and the fourth named D, was rocky island edge. Each island's
data from the transects were analysed separately with the program Distance
7.3.
¹Association Hyla, I. Lipovac 7, HR-10 000 Zagreb
Daria Kranželić¹, Ivona Burić¹, Bruno Schmidt¹
Initial estimation of lizard population densities on the
selected islands of the Lastovo Archipelago, Croatia
Figure 1. Adult Dalmatian Wall Lizard, Podarcis melisellensis from
Lastovo Archipelago.
Figure 2. Adult Sharp-snouted Rock Lizard, Dalmatolacerta
oxycephala from Lastovo Archipelago.
Figure 4. The density of the D. oxycephala and P. melisellensis per
hectare on individual islands.
Figure 3. Island Smokvica in the Lastovo Archipelago.
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