So close so different: what makes the difference?

Dario Ottonello, Department of Environmental Sciences, Informatics and Statistics, Cà Foscari University of Venice

Stefania D’Angelo, WWF Italia

Fabrizio Oneto, Centro Studi Bionaturalistici

Stefano Malavasi, Department of Environmental Sciences, Informatics and Statistics, Cà Foscari University of Venice

Marco Alberto Luca Zuffi, Museum of Natural History, University of Pisa

Filippo Spadola, Department of Veterinary Sciences, University of Messina

Phenotypic plasticity is an evolutionary adaptation to environmental variation often used to explain the intraspecific difference observed within various taxa, that could be influenced by local variation in abiotic and biotic factors associated with habitat type (Lubcke and Wilson, 2007). Nowadays, the introduction of alien species is one of the main stressor of freshwater ecosystem biodiversity (Genovesi, 2007), and it could have a great impact on freshwater communities causing cascading impacts on food webs and complex interactions among species (Ricciardi and MacIsaac, 2011). In the last decades, many species of fishes have been deliberately introduced worldwide to provide food or sport leisure, but also released from aquaria, bait buckets, and water gardens, as contaminants of fish intended for stocking, or in ballast water (Strayer, 2010).

Despite fish introduction has been noticed as one of the principal cause of fresh-water extinctions (Dextrase and Mandrak, 2006), eco-system processes alteration (Pitcher and Hart, 1995) and change in aquatic community assemblage (Parkos et al., 2003), very few data about impact on freshwater reptiles are available.Within this group, freshwater turtles are known to vary in different natural history traits among conspecific populations both over broad geographic area (Lovich et al., 1998; Joos et al., 2017) as well as among adjacent sites (Tucker at al., 1998). They usually follow a growth pat-tern that involves rapid growth from hatching to sexual maturity, followed by little or no growth once maturity is attained (e.g., Wilbur, 1975; Bury, 1979).

Growth can be influenced both by different ecological factors, such as hatching size, food availability and habitat suit-ability (Mahmoud, 1969; Bury, 1979). Moreover, different growth patterns can lead to the same size (Bury, 1979; Andrews, 1982) or to different size at the maturity (Stearns and Koella, 1986). These differences can have also a direct impact on reproductive output in a taxonwhere clutch size often increases with maternal body size (Zuffi et al., 2004). In particular, Ottonello et al. (2017a) showed how the presence of two alien fishes, Cyprinus carpio Linnaeus, 1758 and Gambusia holbrooki Girard, 1859, can alter the abundance and the assemblage of the invertebrate community with a considerable impact on niche width and diet composition of an endangered turtle.

Local food availability and quality may indeed play a key role to regulate feeding strategy with consequences on growth, reproductive output and population density (Dunham and Gibbons, 1990; Parmenter and Avery, 1990). For these reasons we have aimed at investigating if the presence of alien fishes can have an impact at local scale on a population of an endangered turtle using as case study two neighbour wild sub-populations under similar climatic and environmental conditions. As model species we selected Emys trinacris Fritz et al., 2005, the only native freshwater turtle of Sicily, a large island off the coast of Italy. The Sicilian pond turtle lives in wetlands and slow-moving water bodies (e.g., lagoons, deltas, inland waters and mountain lakes) with soft bottoms and abundant aquatic vegetation, especially on the banks, from the sea level up to 1036 m a.s.l. (Marrone et al., 2016).

However, the presence of the species at about 1250 m a.s.l. was recently observed (F. Marrone and R. Scardino, pers. obs.). In view of the low number of sicilian indigenous fish (Zerunian, 2004) and the absence of specialized aquatic vertebrate predators (Aa. Vv., 2008), it is likely to assume that Emys trinacris is one of the top aquatic predators in many of these environments in Sicily, with an opportunistic and generalist pattern oriented mainly towards aquatic invertebrates (Ottonello et al., 2017a). Because of the introduction of non-native fish species, that are now the most represented non-indigenous taxon occurring in Sicily (Marrone and Naselli-Flores, 2015), new dynamics that can significantly alter and threaten the structure of the native biota have been established (Naselli-Flores and Barone,2012) and top predators are likely to be valuable indicators of eco-system health (Landres et al., 1988).

Although some data and experimental studies highlighted the poorly competitive abilities of the genus Emys against some alien species, like Trachemys scripta (Cadi and Joly, 2003, 2004) and Micropterus salmoides Lacepède, 1802(Lacomba and Sancho, 2004; Ayres and Cordero, 2007), robust evidence of interactions in the wild is still lack-ing (but see Polo-Cavia et al., 2010; Lambert et al., 2019). Moreover, Rakausas et al. (2016), studying the predator-prey interactions between a recent invader in Lituania, the Chinese sleeper (Perccottus glenii Dybows-ki, 1877) and the European pond turtle stated that this fish does not directly contribute to the decline of E. orbicularis because hatchlings turtles are resistant to P. glenii predation and, adults of E. orbicularis consumed juvenile P. glenii, but no data about indirect impacts (e.g., trophic resource competition) have been evaluated. Therefore, we assumed that Sicilian pond turtles in a habitat without alien fishes would have access to wider food availability than would individuals in a habitat with introduced alien fishes. We therefore hypothesized that this would affect their rates of energy intake and we predicted that turtles in a fish-less habitat would: (1) exhibit different patterns of growth, (2) have higher annual reproductive output, and (3) have higher abundance.

To test this hypothesis in the wild, we compared the parameters of two Emys trinacris subpopulations inhabiting a fish-inhabited and a fish-less lakes during a multi-year study. The evidence of the negative impact of the presence of introduced fish fauna can provide additional information to further understand the effect of alien species on native fauna as well as promote adequate management plans.


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