COS 109-9
Local extinction, unintentional rewilding, and the historical biogeography of islands in the Gulf of California
The processes that led to the formation of plant and animal communities encountered in modern landscapes are often cryptic. Yet, biogeographic patterns of extant species provide clues of their origin. This is especially the case on islands where isolation and vicariance distil evolutionary and historical patterns. Tiburón Island in the Gulf of California, the largest island in North America south of Canada, was recently separated (ca. 6,000 years ago) from mainland Mexico, but contains surprising and unresolved biogeographic mysteries. A half dozen disjunct temperate plant species are restricted on the high mountaintops of this desert island, several hundred kilometers south of the next nearest populations. Does the presence of these species represent recent long distance dispersal or ancient vicariance from past Ice Ages? Bighorn sheep (Ovis canadensis) were introduced to Tiburón Island in 1975 as a conservation measure. Surprisingly, no evidence suggested their past occurrence on this land-bridge island. Did bighorn sheep once occupy Tiburón, and if so what are the implications for future management of this iconic species? Genetic analyses, modern and ancient, provide an avenue to answer these specific questions to dig beyond observed distributions.
Results/Conclusions
Analysis of modern Chloroplast DNA sequences from throughout the range of the exemplar disjunct relict on Tiburón Island, Canotia holacantha(Celastraceae), has revealed significant genetic variation between island and northern populations. The results support the ancient vicariance origin hypothesis of the Tiburón population, suggesting the same for the other half-dozen disjunct temperate species on the island. Tiburón Island and the coast of the Gulf of California was likely not a refuge for desert environments in the Ice Ages, and the elevation shifts in vegetation known to occur further north in the Sonoran Desert extended to the shores of the Gulf.
Fossil dung morphologically similar to that of bighorn sheep was discovered in a dung mat deposit in the mountains of Tiburón Island. The fossil dung was 14C-dated to 1476–1632 calendar years before present and was confirmed as bighorn sheep by morphological and ancient DNA analysis of the 12S ribosomal RNA and control regions. Native desert bighorn sheep were extirpated from the island sometime in the last ~1500 years. This discovery refutes conventional wisdom that bighorn sheep are not native to Tiburón Island, and establishes its recent introduction as an example of unintentional rewilding – the introduction of a species without knowledge that it was once native and has since gone locally extinct.