UC Riverside

While the processes that led to the formation of modern plant communities are often cryptic, biogeographic patterns of extant species can provide clues to their origin. The Midriff Islands, an archipelago in the Gulf of California at the center of the Sonoran Desert, provide an opportunity to investigate the origins of the desert. This research uses three case studies at three different time scales to better understand the factors responsible for modern biodiversity.

Chapter 1 revisits the theory of island biogeography and incorporates the long history of humans on the Midriff Islands to determine factors controlling plant species richness. Area, habitat diversity, island type, and seabird dynamics explain 98% of the variability in species richness across this archipelago. Interestingly, human presence is not predictive, suggesting an island system with ancient human interactions that functions in a pre-Anthropocene state.

Chapter 2 investigates Holocene extinctions. In 1975, bighorn sheep (Ovis canadensis) were introduced as a novel element to Isla Tiburón as a conservation measure. Fossil dung found on Isla Tiburón was 14C-dated to 1476-1632 years before present and identified as Ovis canadensis by morphological and ancient DNA analysis. Bighorn sheep went locally extinct on the island sometime in the last ~1500 years prior to their "unintentional rewilding." This discovery questions the definition of a non-native species and extends an ecological and conservation baseline.

Disjunct long-lived plant taxa on Isla Tiburón suggests climate and vegetation change on the Midriff Islands in the Pleistocene. Chapter 3 is a phylogeographic study of the desert edge species Canotia holacantha (Celastraceae) that tests whether Canotia on Isla Tiburón is a Pleistocene relict or a recent dispersal event. Results suggest long isolation and divergence of Canotia on Tiburón but recent arrival in the core of its modern day distribution in Arizona. In contradiction to an expected temperate origin, Canotia seems to have tracked the northward shift of the desert's edge at the end of the last Ice Age from glacial refugia in Sonora or Chihuahua.

Collectively, this research helps illuminate the history of the desert and establishes baselines to support management decisions of the world's best-preserved archipelago.