Ecologia e biologia riproduttiva in una popolazione alloctona di Trachemys scripta elegans in Calabria

Abstract

This research was carried out from 2009 to 2011 in the Lake of Angitola on the reproductive biology of the slider turtle Trachemys scripta elegans, an alien species that in this site has been naturalized since the 1980’s. According to the International Union for Conservation of Nature (IUCN), invasive species are one of the most important causes of biodiversity decline on a global scale. As a consequence, the impact of introduced species on local ecosystems is important to evaluate. Pet voluntary introductions are done by private people and usually concern only the release of a few individuals. However, the number of releases is as numerous as the number of pet owners that don't want to keep their pet anymore and consequently decide to "release" it. Although only a small percentage of these alien species will become invasive, the environmental impact would be insidious and often irreversible. In fact on a global scale alien species may be as damaging to native species and ecosystems as the loss and degradation of habitats. We can describe 3 steps in the biological invasions processes, comprising the initial dispersal, when an organism moves long distances to areas outside its native range, for example through human assisted dispersal; establishment of self-sustaining populations within the non-native range; and invasion of the new range. However, when species are invasive, they have strongly positive demographic trends and are often numerous, therefore their management can be extremely difficult and expensive. Our experimental model was Trachemys scripta elegans, the most popular freshwater turtle in the pet trade in the world. It is a medium to large-sized turtle as females reach a carapace length of up to 24 cm and males up to 20 cm. This reptile lives in a wide variety of freshwater habitats and it is a habitat generalist. It prefers larg pools and ponds with soft bottoms, and with many aquatic plants and suitable basking sites for thermoregulation. Trachemys scripta elegans is an opportunistic omnivore subsisting on a wide variety of plant and animal foods. The food preferences change with age. Juveniles are highly carnivorous. Instead the adults, mainly eat vegetable food. In the late 1970s a several turtle farms were established in southern USA, where the hatchlings are packed, in hundreds in small boxes, and are shipped abroad. At least 80% of them die during the first year in captivity. If released terrapins survive in nature, they pose a threat to local wildlife. Only in 1996 the total exports of Trachemys scripta elegans from USA were 8 million individuals, of which 3 million individuals were imported by Europe. With effect from 1997 the EU implemented a ban on the import of this species, in fact already in 1975 the U.S. Food and Drug Administration banned the sale of turtles under 10 cm carapace length in the United States and Canada because they transmitted human salmonellosis. Trachemys scripta elegans distribution includes eastern USA, in particular the Mississipi valley from Illinois to the Gulf of Mexico. It now has a far greater distribution area that include a lot of countries in America, Europe and Asia, where it has naturalized in some cases. Trachemys scripta elegans released in seminatural conditions were first recorded in Campobasso in the early 1970s, but sightings only becoming more common in the mid-80s. It is presently known in all of Italy except for Valle d’Aosta, Sardinia and Campania. Reproduction is common in captivity, but little is known about its reproduction in nature. Some authors suggest that naturalization wouldn’t be a serious problem, but it is known that this species is certainly able to reproduce in Friuli Venezia Giulia, Latium, Emilia Romagna and in Calabria. This research was aimed to investigate reproductive biology of Trachemys scripta elegans in order: • to evaluate if reproduction is an effective phenomenon that could interest the study area; • to estimate the reproductive success in natural condition; • to determine preferences of Trachemys scripta elegans in the nest site selection; • to define the step of the invasion process of slider turtle in the Lake of Angitola, in order to suggest conservation measures. The research was conducted at Lake of Angitola, a 196 ha artificial lake located about 5 km from Pizzo Calabro. The lake became a WWF oasis in 1975, and ten years later was declared an International Ramsar area due to the large presence of migratory birds. Today the lake is included in the Regional Park of Serre and in 2005 the area was proposed as a Site of Community Importance (SIC) by ministerial decree. The climate is typically Mediterranean; the vegetation is very varied: the shrub layer is formed by aromatic herbs; the arboreal layer includes a forests of genus Quercus . The animal communities are very interesting, especially the bird population; in fact we can see more than 150 different species per year. The study was conducted from three years, and included 3 monitors per month, in particular from April to October, when the turtles were more active. The lake’s shores were divided into eight transects using environmental criteria. Data collection were sampling focused on visual census observations, on the nesting sites research and on trapping. Also to analyze the nest site selection we divided the whole lake perimeter by differing vegetative covers and differing soil types to study the use of these selected environments by nesting females. Vegetative cover was classified by five categories by a buffer of 50 m, and including shrub land, grass, fruit growing and reforestation; mixed category was used in cases where no one coverage was present in more than 75% of the subdivisions. Soil type was classified by using the methodology of Casanova (1991), yielding three main categories of dirt, sand and clay substrates and the possible combination of them. Collected variables used in this research were tested with InStat 3.0 software; in particular we used Chi-square goodness of fit, Kruskal-Wallis and Spearman Rank Correlation tests. The results of the visual census showed that Cavalcavia was the preferred basking area for Trachemys thermoregulation. In fact, in this area was observed the highest percentage of turtle presence that performed basking on the emerged deadwood. The heterogeneity χ² test, that was used to compare number of observation and the areas, was statistically significant (N=2015; χ²=893,03; d.f.=5; P‹0,0001). Monthly monitoring activities of basking showed an increasing of turtle presence from April to July, and it is possible to observe a subsequent decreasing of basking activities from August to October. During the research we found 229 nests. The greatest distance of a nest from the shoreline was 100 m, but the greatest number of nests was found between 0 and 20 m from the shoreline. Kruskal-Wallis non-parametric test association between number of eggs and months of the year was statistically significant (KW=14,318; P=0,0137). “Movrella” and “Ceramida” were the main nesting areas; in these transects there are a lot of emerged spaces that female turtles can use for nesting activities. The correlation between the number of eggs and the depth of the nest was statistically extremely significant (N=229; r=0,2105; P<0,0001). More than 85% of nests were found in mixed soil type habitats, particurarly in dirt-sand and dirt-clay soil types. Chi squared test between number of nests and soil type categories was statistically significant (N=229; χ²=116,22; d.f.=3; P<0,0001). More than 65% of nests were found in environments with mixed vegetative cover (Grass+Shrubland). Chi squared test associations between number of nests and the 4 vegetative cover types were statistically significant (N=229; χ²=116,22; d.f.=3; P‹0,0001). We collected by trapping, and marked 26 specimens and in particular 6 juveniles; the more effective trapping method was represented by “basking trap”, that we used on the water surface. In Europe reproduction under natural conditions has been reported only for Spain and France. As we said before, little is known about Trachemys scripta elegans reproduction out of captivity in Italy, but in 2002 Ficetola recorded the first reproduction case of Trachemys scripta elegans on the Po Delta on sympatric condition with an Emys orbicularis population. The population of Angitola represents one of the largest reproductive population of Europe and the most important for our country. In Italy Trachemys scripta elegans deposition in seminatural conditions takes place once a year at the end of June. We recorded for the first time that in our study area deposition takes place twice a year, as reported for the American populations: we think that the first event in April-May, with hatching in June and July, and the second event in July- August, with hatching in September-October. Some tropical Trachemys species tend to nest in open areas that receive direct sunlight for at least part of each day. Analysis of the use of vegetative cover in our study area revealed that Trachemys scripta elegans nests were not randomly distributed within the general area used for nesting, and it suggesting that females were actively selecting nesting sites. In fact the largest number of nests around the lake was found in areas with mixed vegetative cover, as reported for Trachemys callirostris in Colombia. Mixed vegetative cover might reduce thermal stress to the nesting females, as well as lower the detectability of nests to natural predators and human hunters. It has been reported that Trachemys nesting females prefer only dirt soil, but in our study area the largest number of nests in our study area were found in habitat with mixed soil type categories, these zones were characterized by heterogeneous granulometry that increase the ventilation of the nest and the humidity degree for hatching success. We investigated experimentally the occupation of basking sites thus providing a better knowledge on this behavioral pattern. During thermoregulation turtles of lake Angitola mainly bask along the emergent deadwood. Floating (thermoregulation on the water surface) is showed especially during the summer season. This population performs basking from April to October at different times of the day: for example during the cooler months the time of sunlight exposure increases and the basking behavior occurs during the hottest hours of the day. These results are also important for further evaluations on competition for occupation of basking places between this allocthonous specie and Emys orbicularis, our endangered European pond turtle. In fact as reported in literature that Trachemys scripta elegans requires less exposure time for thermoregulation than Emys orbicularis, and for this reason American slider turtle is the winning species in competition for basking sites in sympatric conditions. The invasion process of the Trachemys population inhabiting the lake of Angitola is at the second step, in fact we are in front of a self-sustaining and growing population. Moreover the local population of Emys orbicularis, historically present in the lake until the early 2000's, nowadays is probably extinct. In conclusion it is very important to complete echo-ethological investigations for better understanding the biology and ecology of this invader, and to provide a monitoring and controlling plans, and to define translocation and eradication plans, but this operations could be extremely difficult and more expansive. The results of this research were objects of a scientific paper and of same participation for congresses. Two more papers are in preparation.