英文摘要: | Climate change will decrease worldwide biodiversity through a number of potential pathways1, including invasive hybridization2 (cross-breeding between invasive and native species). How climate warming influences the spread of hybridization and loss of native genomes poses difficult ecological and evolutionary questions with little empirical information to guide conservation management decisions3. Here we combine long-term genetic monitoring data with high-resolution climate and stream temperature predictions to evaluate how recent climate warming has influenced the spatio-temporal spread of human-mediated hybridization between threatened native westslope cutthroat trout (Oncorhynchus clarkii lewisi) and non-native rainbow trout (Oncorhynchus mykiss), the world’s most widely introduced invasive fish4. Despite widespread release of millions of rainbow trout over the past century within the Flathead River system5, a large relatively pristine watershed in western North America, historical samples revealed that hybridization was prevalent only in one (source) population. During a subsequent 30-year period of accelerated warming, hybridization spread rapidly and was strongly linked to interactions between climatic drivers—precipitation and temperature—and distance to the source population. Specifically, decreases in spring precipitation and increases in summer stream temperature probably promoted upstream expansion of hybridization throughout the system. This study shows that rapid climate warming can exacerbate interactions between native and non-native species through invasive hybridization, which could spell genomic extinction for many species.
Changes in species ecology associated with climate change have been documented for a broad range of organisms1, 6, yet empirical understanding of how climate change influences evolutionary processes and resulting patterns of biodiversity is limited. One consequence of climate-induced range shifts is increased sympatry between previously isolated species, potentially resulting in introgressive hybridization (genes from an invasive species spread into a native species)7, 8. Climate-induced expansions of introgression have been predicted for many terrestrial and aquatic species, especially species that are sensitive to temperature and streamflow conditions2, 6, 9. Although hybridization can increase the adaptive potential of closely related species through periods of climate change7, hybridization driven by human activities, such as translocation of species, tends to occur quickly and reduce fitness10, genomic integrity11, and ultimately native species diversity12. Despite predictions that interspecific hybridization may increase as a result of species range shifts and human impacts, empirical evidence linking such evolutionary changes to recent climatic change is extremely scarce3, 13. Salmonids—a group of fishes of enormous ecological and socio-economic value—are ideal organisms for examining how climate change facilitates hybridization between native and non-native species. Hybridization and introgression are particularly common between salmonids and other fish because there are limited pre- or post-zygotic barriers to introgression12, and widespread introductions have created sympatry between many previously allopatric species11. Moreover, the distribution, abundance and phenology of salmonid fishes are strongly influenced by climatic conditions through species-specific adaptations to water temperature and the timing and magnitude of streamflow14. Thus, ongoing climate change is expected to differentially affect salmonid species, possibly expanding zones of introgressive hybridization as some species expand their distribution and increase in abundance during periods of warming and shifting hydrologic regimes14, 15. Quantifying spatial and temporal genetic changes in wild populations, including introgression, provides strong support for climate-induced evolutionary change13, 16. Such data, however, are limited among vertebrates, especially for rare and endangered species. Here we use long-term genetic monitoring data (1978–2008) to test the prediction that climatic variation has affected the spread of introgressive hybridization between threatened native westslope cutthroat trout and non-native rainbow trout. Cutthroat trout (Oncorhynchus clarkii) and introduced rainbow trout can overlap in time and space during spring-spawning and produce fertile offspring when they interbreed17. Introgression often continues until a hybrid swarm (a randomly mating population containing only hybrid individuals) is formed and all the native genomes are lost18. Introgression poses a serious threat to all subspecies of inland cutthroat trout in western North America as a result of widespread stocking of rainbow trout into historical cutthroat trout habitats; two subspecies are now extinct and five are listed as threatened under the US Endangered Species Act. The westslope cutthroat trout is the most widely distributed subspecies, and hybridization is the leading threat to the persistence of genetically pure populations; known non-hybridized populations occupy less than 10% of their historic range19. Relative to cutthroat trout, rainbow trout prefer warmer temperatures, lower spring flows, earlier spring runoff, and tolerate greater environmental disturbance15, 17, 20, 21. Therefore, we tested the prediction that increased summer stream temperatures, decreased spring precipitation, and wildfire disturbance have influenced the spatiotemporal spread of hybridization throughout the Flathead River system (USA and Canada; Supplementary Fig. 1 and Supplementary Table 1). The drainage presents an ideal location to examine this prediction because it is one of America’s most pristine river systems22 and does not contain other introduced species that may influence interactions between these species. However, the basin has warmed considerably over the past century, with annual average temperatures warming at two times the global average23. From 1948 to 2008, mean annual air temperatures increased by 0.82 °C, a rate of 0.14 °C/decade, yet from 1978 to 2008 warming nearly tripled to 0.36 °C/decade24. Increasing air temperatures are contributing to a decrease in spring snowpack and a shift towards an earlier spring season, resulting in peak spring runoff two to three weeks earlier than the historic average and lower spring and summer flows23, 24, 25, which are directly contributing to increased summer stream temperatures26, 27. Changes in spring temperature and precipitation correspond strongly to these observed changes and significantly influence the timing and magnitude of streamflow in the basin25, 28. Furthermore, wildfire has burned approximately 16% of the basin from 1984 to 2008, potentially increasing the rate of stream warming by removing canopy cover24. Such changes are hypothesized to benefit rainbow trout, as spawning and recruitment are limited by high spring flows and cold water temperatures15. Over 20 million rainbow trout were stocked throughout the Flathead River system, primarily in low elevation areas, beginning in the late 1800s and ending in 19695. Despite massive stocking efforts, genetic samples collected from the late 1970s and early 1980s detected low levels (<2%) of hybridization in just 2 of 20 sites, and a hybrid swarm (representing multiple generations of hybridization) with a predominant (92%) genetic contribution from rainbow trout was discovered in the lower valley in 1994 (Fig. 1a and Supplementary Table 1)5, 29. Conversely, samples from the 2000s showed introgression in 9 of the 18 previously non-hybridized sites, demonstrating that introgression increased rapidly over a 30-year period (from 10% to 52% of all sites; Fig. 1b). In the most extreme example, introgression in one population increased tenfold from 3% to 33%.
| http://www.nature.com/nclimate/journal/v4/n7/full/nclimate2252.html
|