Apparent climate-mediated loss and fragmentation of core habitat of the American pika in the Northern Sierra Nevada, California, USA.

Contemporary climate change has been widely documented as the apparent cause of range contraction at the edge of many species distributions but documentation of climate change as a cause of extirpation and fragmentation of the interior of a species' core habitat has been lacking. Here, we report the extirpation of the American pika (Ochotona princeps), a temperature-sensitive small mammal, from a 165-km2 area located within its core habitat in California's Sierra Nevada mountains. While sites surrounding the area still maintain pikas, radiocarbon analyses of pika fecal pellets recovered within this area indicate that former patch occupancy ranges from before 1955, the beginning of the atmospheric spike in radiocarbon associated with above ground atomic bomb testing, to c. 1991. Despite an abundance of suitable rocky habitat climate warming appears to have precipitated their demise. Weather station data reveal a 1.9°C rise in local temperature and a significant decline in snowpack over the period of record, 1910-2015, pushing pika habitat into increasingly tenuous climate conditions during the period of extirpation. This is among the first accounts of an apparently climate-mediated, modern extirpation of a species from an interior portion of its geographic distribution, resulting in habitat fragmentation, and is the largest area yet reported for a modern-era pika extirpation. Our finding provides empirical support to model projections, indicating that even core areas of species habitat are vulnerable to climate change within a timeframe of decades.

Microbiologic effectiveness of time- or concentration-based dosing strategies in Streptococcus pneumoniae.

This in vitro study evaluated the pharmacodynamic performance of levofloxacin using different dosing strategies against both a levofloxacin-sensitive (MIC = 1 mg/liter) and -resistant (MIC = 16 mg/liter) strain of Streptococcus pneumoniae. The strain was genotypically characterized by a mutation in gyrA and two mutations in parE; resistance was shown not to be efflux-mediated. The purpose of this study was to determine if simulated levofloxacin dosing strategies focused either on time or concentration would affect microbiologic outcome. Differing peak concentration/MIC ratios (1,2, and 10), T>MIC (3.6,9.6,15.6, and 24 h corresponding to 15, 40, 65, and 100% of the 24-h dosing interval), and AUC/MIC ratios (13-180) were generated by varying dosing strategies. Initial bacterial inocula were decreased by 99.9% in each experiment conducted. Despite the wide variation in exposure levels, in terms of AUC/MIC, Cp-max/MIC, and T>MIC, the kill portions of the bacterial density curves were super-imposable between all permutations of antibiotic exposure. However, there appeared to be an AUC/MIC breakpoint (35-40) defining bacterial regrowth. Over a 10-fold concentration range, levofloxacin appeared to kill S. pneumoniae in a concentration-independent fashion. When given in concentrations suitable to achieve specified pharmacodynamic endpoints (AUC/MIC >/=35), levofloxacin demonstrated the ability to eradicate both a levofloxacin-resistant and levofloxacin-sensitive strain of S. pneumoniae in the in vitro model.

Comparative pharmacodynamics of three newer fluoroquinolones versus six strains of staphylococci in an in vitro model under aerobic and anaerobic conditions.

Six strains of staphylococci were exposed to levofloxacin, moxifloxacin, or trovafloxacin in an in vitro pharmacodynamic model under both aerobic and anaerobic conditions. Each agent demonstrated a rapid 3-log(10) kill versus susceptible isolates regardless of condition. Against clinical isolates with reduced susceptibility, regrowth occurred by 24 h and was frequently associated with further increases in MICs.

A comparative analysis of nested subset patterns of species composition.

We present a broad comparative assessment of nested subsets in species composition among ecological communities. We assembled presence-absence data from a broad range of taxa, geographic regions, and spatial scales; and subjected this collection of datasets to common analyses, including a variety of metrics for measuring nestedness and null hypotheses against which to evaluate them. Here we identify ecological patterns in the prevalence and strength of nested subset structure, and assess differences and biases among the available methodologies. In all, we compiled 279 presence-absence matrices, of which 163 do not overlap in their coverage of species and sites. The survey includes studies on vertebrates, arthropods, mollusks, plants, and other taxa; from north temperate, tropical, and south temperate latitudes. Our results were as follows. Statistically significant nestedness was common. Assemblages from landbridge archipelagos were strongly nested, and immigration experiments were least nested. This adds further empirical support to the hypothesis that extinction plays a major role in producing nested structure. Nestedness was positively correlated with the ratio of the areas of the largest and smallest sites, suggesting that the range in area of sites affects nestedness. Taxonomic differences in nestedness were weak. Higher taxonomic levels showed stronger nesting than their constituent lower taxa. We observed no effect of distance of isolation on nestedness; nor any effects of latitude. With regard to methodology, the metrics Nc and Ut yielded similar results, although Nc proved slightly more flexible in use, and deals differently with tied sites. Similarities also exist in the behavior of N0 ("N") and Up, and between N1 and Ua. Standardized nestedness metrics were mostly insensitive to matrix size, and were useful in comparative analyses among presence-absence matrices. Most metrics were affected by the proportion of presences in the matrix. All analyses of nestedness, therefore, should test for bias due to matrix fill. We suggest that the factors controlling nested subset structure can be thought of as four filters that species pass to occur at a site: a sampling filter, a distance filter, a habitat filter, and an area filter - and three constraints on community homogeneity: evolutionary history, recent history, and spatial variation in the environment. The scale of examination can also have important effects on the degree of nestedness observed.

On the meaning and measurement of nestedness of species assemblages.

Nestedness of species assemblages occurs when thebiotas of sites with lower numbers of species tend to be subsets of the biotas at richer sites. We develop new quantitative and statistical techniques for measuring, testing, and comparing nestedness, and apply these methods to data from the literature. Significantly nonrandom nestedness was present in all 27 assemblages examined, and tended to be stronger in systems dominated by extinction, such as landbridge islands. Sets of assemblages that were very strongly nested were more likely to have greater species richness on one or a few large sites than on several smaller sites of equivalent total area - that is, to fall toward the "single large" side of the "Single Large Or Several Small" (SLOSS) continuum. Our analysis indicates that nestedness, when quantified as a single number for a presence-absence matrix, measures community-wide differences in incidence (the frequency of occurrence or "distribution" of species). Factors that lead to consistent differences among species in immigration or extinction rates cause strong patterns of nestedness of species assemblages. Nestedness is negatively related to beta diversity: nestedness is low when beta diversity is high, and vice versa. Conservation managers will thus seek to minimize nestedness and the development of nested structure in systems of nature reserves.