Species Translocation Menaces Iberian Waterfrogs

Abstract

The numerous reports on declining amphibian populations throughout world led to creation in December 1990 of Declining Amphibian Populations Task Force (DAPTF), activated by Species Survival Commission of International Union for Conservation of Nature and Natural Resources, one of whose main goals is the identification of target populations, species, and regions which merit immediate attention. We want to highlight one such target region, Iberian peninsula, and a particular species, Rana perezi. Despite major efforts on behalf of Spanish conservation authorities to preserve local and endemic species and fact that Iberian peninsula is considered one of redoubts of biodiversity in Europe, locally adapted populations are menaced by potentially fatal effects of events such as species translocations (Dodd & Seigel 1991; Reinert 1991). If a unique reproductive mode such as hybridogenesis is added to problem of species translocation, difficulties of preserving local species increase. Hybridogenesis is widely prevalent in European water frogs, having originated in Rana esculenta complex, which includes seven species, and a series of hybridogenetic lineages stemming from interspecific hybridizations. Several systems, consisting of a host species (or parental species) and a hybridogenetic lineage in each, are found throughout Europe (Graf & Polls-Pelaz 1989). During gametogenesis in such systems hybrid premeiotically excludes one of its parental genomes by producing only gametes containing other parental genome. This remaining genome is transmitted clonally because premeiotic exclusion implies prevention of recombination through crossing over. In most cases genome of one of species, generally R ridibunda, is clonally transmitted, whereas other parental genome is excluded. In general, lost genome corresponds to that of species living in sympatry with hybrid. Because of its special way of reproduction, hybridogenesis may present new problems for conservation of natural populations. The voluntary or fortuitous introduction of samples from other species of Rana capable of provoking hybridogenesis can cause alterations that can modify genetic structure of local parental populations. The hybrids may act as transmitters of allelic variants that could profoundly modify gene frequencies in original populations. A highly successful hybrid is expected because survival of hybrids has been demonstrated to be superior to that of parentals in experimental nonhybridogenetic hybrid populations of other animal species (Howard et al. 1993). Hybrid individuals also tend to have other advantages, such as higher growth rates, lower metabolic demand, and resistance to diseases. In addition, their developmental periods are often faster and more stable (Mitton & Grant 1984; Mitton et al 1986; Allendorf & Leary 1986; Ledig 1986). Nevertheless, due to sterility, these hybrids would present a reduced fitness compared with hybridogenetic populations. One of these hybridogenetic systems corresponds to hybridization between R perezi and R ridibunda, found in southern France and northeastern Spain. The distribution of hybrid populations in strictly delimited, and, in recent studies on genetic variability of parental syntopical species R perezi (262 individuals, 29 populations), we found significant differences in rate of polymorphism between those populations where hybridization occurs and remaining ones in central and western Spain where hybridogenetic hybrids were not originally found (Mann-Whitney U-test, U = 0.014, p < 0.02). During course of our study, an introduction of bullfrog (Rana catesbeiana) and of water frogs from northern Italy occurred in Sierra de Gata (western Spain) from a nearby, unsuccessful frog farm (GarciaPaper submitted February 28, 1994; revised manuscript accepted June 2, 1994.