The Horseshoe whip snake (Hemorrhois hippocrepis) on Ibiza: predator release in an invasive population

Abstract

The key to fighting a biological invasion may lie in understanding every variable that can explain its success. The Enemy Release Hypothesis (ERH) states that when an invader arrives to a new environment, the absence of its common enemies (predators, parasites and competitors) facilitates the invasion success. The Horseshoe whip snake (Hemorrhois hippocrepis) has been recently introduced from the Iberian Peninsula to the island of Ibiza, and it is currently threatening the only endemic vertebrate, the Ibiza wall lizard (Podarcis pityusensis). We hypothesized that the snake invasion success is caused by the absence of natural predators, and we checked the ERH by relating the tail breakage rate to predation pressure. The invasive population showed a much lower incidence of tail breakage than the source population, which is in agreement with the almost absence of snake predators among the Ibizan reduced and naïve native vertebrate community. These results confirm the ERH, and support the prolongation of invasive snake trapping campaigns.

Demographic monitoring of the invasive ladder snake on Formentera (Balearic Islands, Spain)

“…Snakes' nocturnality, broad movements, low recapture rates, crypsis, and periods of inactivity (Durso & Seigel, 2015; Steen, 2010; Ward et al., 2017) may have prevented the study of their demography in the past. However, in recent years, the number of studies assessing snake populations introduced out of their native range has substantially grown (e.g., Cabrera‐Pérez et al., 2012; Capinha et al., 2017; Christy et al., 2010; Guzy et al., 2023; Montes et al., 2020). Demographic studies are essential to determine the effect of environmental variables on life history (Blouin‐Demers et al., 2002; Gregory & Larsen, 1996) and to manage invasive species (Park, 2004).…”

“…Snake introductions out of their native range have led to over 100 established populations worldwide (Capinha et al., 2017). Invasive snakes are one of the main drivers of species extinction worldwide (Rodda et al., 1997; Rodda & Fritts, 1992) and are extremely damaging in insular ecosystems (Fritts & Rodda, 1998) given the lack of pathogens, parasites, and predators in the new territory, which is commonly termed as the enemy release hypothesis (Montes et al., 2020; Roy et al., 2011). These impacts are exemplified by the brown tree snake Boiga irregularis (Bechstein, 1802) in Guam, which has caused the loss of native species (Rodda et al., 1997; Rodda & Fritts, 1992; Rodda & Savidge, 2007; Wiles et al., 2003).…”

Prey naïveté rather than enemy release dominates the relation of an invasive spider toward a native predator

“…As one of the most predominant and intuitive, the enemy release hypothesis posits that nonindigenous species are released from the pressure of predators and parasites once introduced to their exotic ranges (Elton, 1958 ; Keane & Crawley, 2002 ). Introduced species might have advantages over resident species against native enemies by, for example, not being recognized as prey or hosts for parasites in the exotic range (e.g., Cottrell & Shapiro‐Ilan, 2003 ; Gozzi et al., 2020 ; Montes et al., 2020 ; Tierney et al., 2020 ). However, Elton ( 1958 ) mentions that species leaving coevolved predators and parasites from the native areas immediately meet novel potential enemies once introduced in the exotic range.…”

“…Therefore, the enemy release hypothesis as a driving force behind successful invasions should be tested for already established invasive species that are in their spreading phase (i.e., abundant or dominant) in the exotic range. Furthermore, studies investigating the role of enemy release as a causal mechanism of invasiveness are mainly based on invasive plant and vertebrate species (e.g., Carpenter & Cappuccino, 2005 ; Gozzi et al., 2020 ; Hawkes, 2007 ; Hierro et al., 2005 ; van Kleunen et al., 2010 ; Lankau et al., 2004 ; Liu & Stiling, 2006 ; Meijer et al., 2016 ; Montes et al., 2020 ; Schultheis et al., 2015 ; Tierney et al., 2020 ), whereas only a limited number is focused on arthropods (e.g., Aliabadi & Juliano, 2002 ; Juliano et al., 2010 ; Paula et al., 2021 ; Zuharah & Lester, 2010 ).…”

“…might have advantages over resident species against native enemies by, for example, not being recognized as prey or hosts for parasites in the exotic range (e.g., Cottrell & Shapiro-Ilan, 2003;Gozzi et al, 2020;Montes et al, 2020;Tierney et al, 2020). However, Elton (1958) mentions that species leaving coevolved predators and parasites from the native areas immediately meet novel potential enemies once introduced in the exotic range.…”