Factors related to the occurrence of hybrids between brown haresLepus europaeusand mountain haresL. timidusin Sweden

Abstract

Hybridization occurs among many species, and may have implications for conservation as well as for evolution. Interspecific gene flow between brown hares Lepus europaeus and mountain hares L. timidus has been documented in Sweden and in continental Europe, and has probably to some extent occurred throughout history in sympatric areas. What local factors or ecological relationships that correlate with or trigger hybridization between these species has however been unclear. We studied spatial distribution of hybrids between brown hares and mountain hares in Sweden in relation to characteristics of the sampled localities (hunting grounds). In a sample of 70 brown hares collected from 39 populations in south-central Sweden during 2003Á2005, 11 (16%) showed introgressed mtDNA from mountain hares. Among the brown hares from their northern range, i.e. in general the most recent establishments, the corresponding figure was 75% (9/12). The frequency of samples with hybrid ancestry increased significantly with latitude, altitude and hilliness, and were higher (pB0.1) in recently established populations and/or where the proportion of arable land was low. Several site-specific parameters were correlated, e.g. latitude as expected to hilliness, and no parameter explained the occurrence of hybrids exclusively. Instead, the appearance of mountain hare mtDNA among brown hares was associated with a conglomerate of parameters reflecting landscapes atypical for the brown hare, e.g. forest dominated and steep areas where the species quite recently was established. We suggest that these abiotic factors mirror the main aspect influencing hybridization frequency, namely the density or relative frequency of the two species. In atypical brown hare landscapes with recent establishment, mountain hares are probably relatively more common. When one species dominate in numbers, or when both species display low densities, increased frequency of hybridization is expected due to low availability of conspecific partners, a phenomenon referred to as Hubbs' principle.

Parapatric species and the implications for climate change studies: a case study on hares in Europe

“…However, in addition to resources, exclusion mediated by hybridization (see Rhymer & Simberloff, ) can occur when species differ in density, especially in highly unbalanced situations. This was also suggested as a potential factor mediating the L. europaeus / L. timidus interactions (Thulin, ; but see Jansson et al ., ), and even explaining the ancestral local extinction of L. timidus in the Iberian Peninsula due to displacement by L. granatensis (e.g. Melo‐Ferreira et al ., ).…”

“…This was also suggested as a potential factor mediating the L. europaeus/L. timidus interactions (Thulin, 2003; but see Jansson et al, 2007), and even explaining the ancestral local extinction of L. timidus in the Iberian Peninsula due to displacement by L. granatensis (e.g. Melo-Ferreira et al, 2007).…”

The roles of leporid species that have been translocated: a review of their ecosystem effects as native and exotic species

“…In Sweden, introduced Lepus europaeus hybridise with Lepus timidus, and, even when gene introgression appears to be low, behavioural differences between the males (where Lepus europaeus males guard females, preventing mating with their conspecifics) may cause the loss of species‐specific litters (Thulin , Jansson et al. ). Hybridisation in Ireland is particularly concerning, where the endemic subspecies Lepus timidus hibernicus has undergone long‐term population declines.…”

“…For instance, genetic evidence shows that Lepus corsicanus is suffering from the effects of hybridisation due to the introduction of Lepus europaeus and Lepus granatensis, which are deteriorating the gene pool of this Vulnerable species (Pietri et al 2011). In Sweden, introduced Lepus europaeus hybridise with Lepus timidus, and, even when gene introgression appears to be low, behavioural differences between the males (where Lepus europaeus males guard females, preventing mating with their conspecifics) may cause the loss of species-specific litters (Thulin 2003, Jansson et al 2007. Hybridisation in Ireland is particularly concerning, where the endemic subspecies Lepus timidus hibernicus has undergone long-term population declines.…”

Inferring responses to climate warming from latitudinal pattern of clonal hybridization

“…This may increase the probability of hybridization due to asymmetric availability of mates. Indeed, according to the Hubbs principle (Hubbs, 1955), a species decreasing in abundance is expected to breed with individuals of another more abundant species as conspecific individuals become scarce (Avise & Saunders, 1984;Grant & Grant, 1997;Jansson, Thulin, & Pehrson, 2007;Randler, 2002;Wirtz, 1999).…”