Distribution and abundance of birds in the White Mountains, California

This paper quantifies the distribution and abundance of birds in the White Mountains, Inyo and Mono counties, California, during spring-summer 1989-91, to establish a baseline for monitoring the area's avifauna. Overall 58 species were encountered in the single-leaf pinyon-Utah juniper ( Pinus monophylla-Juniperus osteosperma ) zone, and 61 species in the bristlecone-limber pine ( P. longaeva-P. flexilis ) zone. The bristlecone-limber pine zone had a significantly greater overall bird abundance relative to the pinyon-juniper. Both zones were characterized by few very abundant species, a few moderately abundant species, and numerous rare species. The Black-throated Gray Warbler (scientific names in tables), Gray Flycatcher, and Pinyon Jay were the most abundant species in the pinyon-juniper whereas the Clark's Nutcracker, Mountain Chickadee, and Cassin's Finch were the most abundant species in the bristlecone-limber pine. There were few ecological or taxonomic replacements of species between zones, with the differences in distribution and abundance related primarily to the interaction between elevation and vegetation. Significant inter-year variation in abundance was found for about 20 species in each zone—more species showed declining rather than increasing trends. The Mountain Chickadee and White-breasted Nuthatch declined, whereas the Gray Flycatcher and Rock Wren increased across years in both zones. Reasons for declines in some species might be the severe drought that continued throughout this study.     

Phylogenetic divergence in a desert fish: differentiation of speckled dace within the Bonneville, Lahontan, and upper Snake river basins

Historical events have had a great impact on the biogeography of fishes of western North America. We examined the genetic variation of the speckled dace ( Rhinichthys osculus ) to determine the effects on this species of extensive hydrological changes during the last 10 million years in the Bonneville and Lahontan basins of the Great Basin and the upper Snake River Basin. Eight hundred sixty-nine base pairs of the mitochondrial gene cytochrome b were sequenced from 97 individuals representing 22 populations within these 3 basins, as well as from 2 individuals of longnose dace ( Rhinichthys cataractae ) that served as outgroups. Additionally, 13 speckled dace sequences representing 3 Bonneville populations were used from GenBank. Phylogenetic relationships were reconstructed using maximum parsimony and maximum likelihood criteria. Analysis of molecular variance was used to determine population structure and to estimate the amount of gene flow across the community boundaries. Three distinct clades were reconstructed representing the Lahontan Basin, the northern Bonneville and upper Snake River basins, and the southern Bonneville Basin. Additionally, most of the population structuring was explained by variation among basins (65.33%). Speckled dace demonstrated high genetic variation. As hypothesized, the northern and southern Bonneville specimens formed separate clades; however, the southern Bonneville clade was basal to a sister clade formed by the northern Bonneville/upper Snake River and Lahontan clades. These relationships indicate that Pliocene connections between the Snake, Lahontan, and Bonneville drainages, rather than more recent Pleistocene connections, best explain population structuring in speckled dace.     

Nitrogen limitation constrains sustainability of ecosystem response to CO2

Enhanced plant biomass accumulation in response to elevated atmospheric CO2 concentration could dampen the future rate of increase in CO2 levels and associated climate warming. However, it is unknown whether CO2-induced stimulation of plant growth and biomass accumulation will be sustained or whether limited nitrogen (N) availability constrains greater plant growth in a CO2-enriched world. Here we show, after a six-year field study of perennial grassland species grown under ambient and elevated levels of CO2 and N, that low availability of N progressively suppresses the positive response of plant biomass to elevated CO2. Initially, the stimulation of total plant biomass by elevated CO2 was no greater at enriched than at ambient N supply. After four to six years, however, elevated CO2 stimulated plant biomass much less under ambient than enriched N supply. This response was consistent with the temporally divergent effects of elevated CO2 on soil and plant N dynamics at differing levels of N supply. Our results indicate that variability in availability of soil N and deposition of atmospheric N are both likely to influence the response of plant biomass accumulation to elevated atmospheric CO2. Given that limitations to productivity resulting from the insufficient availability of N are widespread in both unmanaged and managed vegetation, soil N supply is probably an important constraint on global terrestrial responses to elevated CO2.