One hundred and six years of change in a Sonoran Desert plant community: Impact of climate anomalies and trends in species sensitivities.

A major restriction in predicting plant community response to future climate change is a lack of long-term data needed to properly assess species and community response to climate and identify a baseline to detect climate anomalies. Here, we use a 106-year dataset on a Sonoran Desert plant community to test the role of extreme temperature and precipitation anomalies on community dynamics at the decadal scale and over time. Additionally, we tested the climate sensitivity of 39 desert plant species and whether sensitivity was associated with growth form, longevity, geographic range, or local dominance. We found that desert plant communities had shifted directionally over the 106 years, but the climate had little influence on this directional change primarily due to nonlinear shifts in precipitation anomalies. Decadal-scale climate had the largest impact on species richness, species relative density, and total plant cover, explaining up to 26%, 45%, and 55% of the variance in each, respectively. Drought and the interaction between the frequency of freeze events and above-average summer precipitation were among the most influential climate factors. Increased drought frequency and wetter periods with frequent freeze events led to larger reductions in total plant cover, species richness, and the relative densities of dominant subshrubs Ambrosia deltoidea and Encelia farinosa. More than 80% of the tested species were sensitive to climate, but sensitivity was not associated with a species' local dominance, longevity, geographic range, or growth form. Some species appear to exhibit demographic buffering, where when they have a higher sensitivity to drought, they also tend to have a higher sensitivity to favorable (i.e., wetter and hotter) conditions. Overall, our results suggest that, while decadal-scale climate variation substantially impacts these desert plant communities, directional change in temperature over the last century has had little impact due to the relative importance of precipitation and drought. With projections of increased drought in this region, we may see reductions in total vegetation cover and species richness due to the loss of species, possibly through a breakdown in their ability to demographically buffer climatic variation, potentially changing community dynamics through a change in facilitative and competitive processes.

Marine subsidies produce cactus forests on desert islands.

In island systems, nitrogen-rich seabird guano is a marine subsidy that can shape terrestrial plant communities. In zones of nutrient upwelling such as the Gulf of California, copious seabird guano is commonplace on bird islands. Several bird islands host regionally unique cactus forests, especially of the large columnar cactus, cardón (Pachycereus pringlei). We show that a chain of interactions across the land-sea interface yields an allochthonous input of nitrogen in the form of seabird guano, fueling the production of some of the densest cactus populations in the world. Fish, seabird, guano, soil, and cactus samples were taken from the representative seabird island of San Pedro Mártir for nitrogen stable isotope ratio measurements, which were compared to soil and cactus samples from other seabird and non-seabird Gulf islands and terrestrial ecosystems throughout the range of the cardón. Isla San Pedro Mártir δ15N values are distinctively high, ranging from fish + 17.7, seabird + 19.7, guano + 14.8, soil + 34.3 and cactus + 30.3 compared to average values across non-bird sites of + 13.0 (N = 213, S.D. = 3.7) for soil and + 9.8 (N = 212, S.D. = 3.4) for cactus. These δ15N values are among the highest ever reported for plants. Seabird island soil and cactus δ15N values were consistently significantly enriched relative to mainland and non-bird islands, a relationship expected due to the progressive volatilization of 14N rich ammonia from decomposing guano deposits. Our findings demonstrate that seabird-mediated marine nutrient deposits provide the source for solubilized nitrogen on desert islands, which stimulate terrestrial plant production in the cardón cactus beyond that seen in either mainland ecosystems or non-seabird islands.

Integrating Earth-life systems: a geogenomic approach.

For centuries, scientists have recognized and worked to understand how Earth's mutable landscape and climate shape the distribution and evolution of species. Here, we describe the emerging field of geogenomics, which uses the reciprocal and deep integration of geologic, climatic, and population genomic data to define and test cause-effect relationships between Earth and life at intermediate spatial and temporal scales (i.e., the mesoscale). Technological advances now power the detailed reconstruction of landscape and evolutionary histories, but transdisciplinary collaborations and new quantitative tools are needed to better integrate Earth-life data. Geogenomics can help build a more unified theory and characterize the boundary conditions under which geologic and climatic processes generate new biodiversity, how species' responses differ, and why.

Accurate dosimetric measurement of large extended SSD fields for comparison to TPS models.

PURPOSE: There is little evidence in the literature which quantifies the accuracy of Treatment Planning Systems (TPSs) using large fields at extended SSD (eSSD). This paper introduces the approach taken at Christchurch Hospital, New Zealand to validate the use of the Monaco TPS for Total Body Irradiation (TBI) treatments. METHODS: A purpose-built device for allowing precise movements of block-like phantoms called a Phantom Mobility Device (PMD) was used for collecting measurements at eSSD. These measurements were used for determining the ability of the Monaco TPS (originally validated for SSDs between 80 and 110 cm) to accurately model dose distributions for TBI treatments at Christchurch Hospital on either treatment machine one (T1) or two (T2) with SSD values of 341 and 432.6 and clinically useful field sizes of 120 and 170 cm, respectively. RESULTS: We found that within the limits of measurement uncertainty the PMD contributed no determinable scatter to the measurements and proved a reliable approach for eSSD dose measurements. Additionally, by applying depth and off-axis distance constraints of use for TPS information it is possible to use the existing Monaco CCC model at eSSD for block phantom geometries. Dose Difference (DD) analysis showed a clinically acceptable agreement between the CCC model and measured data over a range of depths and off-axis distances. CONCLUSIONS: The PMD was determined to be a useful tool for accurate measurement of extended SSD treatment fields. Monaco TPS CCC model agreed well for block phantoms so future comparisons to anthropomorphic phantoms or patient data are feasible.

Controls of plant diversity and composition on a desert archipelago.

AIM: With the most robust floristic data set for any arid archipelago, we use statistical modeling to determine the underlying controls of plant diversity and species composition. LOCATION: The study was undertaken in the Midriff Islands of the Gulf of California, Mexico. METHODS: Using the area-diversity relationship we estimate the power coefficient z with generalized linear models (GLM). We tested eight predictors (area, human presence, habitat diversity, topography, distance to mainland, island type, precipitation, and seabird dynamics) using a step-wise process on the same GLM procedure. Plant species composition was assessed by conducting a non-standardized principal component analysis on a presence-absence matrix of the 476 (plant species) × 14 (islands). Finally, families were tested for over or under representation with a X 2 analysis subjected to a Bonferroni correction. RESULTS: The classic species-area model explained 85% of the variance in island plant diversity and yielded a slope (z) of 0.303 (±0.01). When the effect of area is removed, four additional factors were shown to account for observed variation; habitat diversity (34%), seabird dynamics (23%), island type (21%), topography (14%). Human presence and distance to mainland were not predictors of species richness. Species composition varies significantly with island area; small islands have a particular flora where certain families are overrepresented, such as Cactaceae, while the flora of larger islands is strongly dependent on the continental source. MAIN CONCLUSIONS: The factors that control diversity levels are expressions of geology, landscape heterogeneity, and land-sea connections. Species assemblages in small islands are governed by copious marine nutrients in the form of guano that depress species diversity. Distance to mainland and human presence hold no predictive power on diversity. The results show these islands to be isolated arid ecosystems with functioning ecological networks.

Local extinction and unintentional rewilding of bighorn sheep (Ovis canadensis) on a desert island.

Bighorn sheep (Ovis canadensis) were not known to live on Tiburón Island, the largest island in the Gulf of California and Mexico, prior to the surprisingly successful introduction of 20 individuals as a conservation measure in 1975. Today, a stable island population of ∼500 sheep supports limited big game hunting and restocking of depleted areas on the Mexican mainland. We discovered fossil dung morphologically similar to that of bighorn sheep in a dung mat deposit from Mojet Cave, in the mountains of Tiburón Island. To determine the origin of this cave deposit we compared pellet shape to fecal pellets of other large mammals, and extracted DNA to sequence mitochondrial DNA fragments at the 12S ribosomal RNA and control regions. The fossil dung was 14C-dated to 1476-1632 calendar years before present and was confirmed as bighorn sheep by morphological and ancient DNA (aDNA) analysis. 12S sequences closely or exactly matched known bighorn sheep sequences; control region sequences exactly matched a haplotype described in desert bighorn sheep populations in southwest Arizona and southern California and showed subtle differentiation from the extant Tiburón population. Native desert bighorn sheep previously colonized this land-bridge island, most likely during the Pleistocene, when lower sea levels connected Tiburón to the mainland. They were extirpated sometime in the last ∼1500 years, probably due to inherent dynamics of isolated populations, prolonged drought, and (or) human overkill. The reintroduced population is vulnerable to similar extinction risks. The discovery presented here refutes conventional wisdom that bighorn sheep are not native to Tiburón Island, and establishes its recent introduction as an example of unintentional rewilding, defined here as the introduction of a species without knowledge that it was once native and has since gone locally extinct.