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Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

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79 records · Page 5Linked to original sources
USGS2007

Has climatic warming altered spring flowering date of Sonoran Desert shrubs?

With global warming, flowering at many locations has shifted toward earlier dates of bloom. A steady increase in average annual temperature since the late 1890s makes it likely that flowering also has advanced in the northern Sonoran Desert of the southwestern United States and northwestern Mexico. In this study, phenological models were used to predict annual date of spring bloom in the northern Sonoran Desert from 1894 to 2004; then, herbarium specimens were assessed for objective evidence of the predicted shift in flowering time. The phenological models were derived from known flowering requirements (triggers and heat sums) of Sonoran Desert shrubs. According to the models, flowering might have advanced by 20-41 d from 1894 to 2004. Analysis of herbarium specimens collected during the 20th century supported the model predictions. Over time, there was a significant increase in the proportion of shrub specimens collected in flower in March and a significant decrease in the proportion collected in May. Thus, the flowering curve - the proportion of individuals in flower in each spring month - shifted toward the start of the calendar year between 1900 and 1999. This shift could not be explained by collection activity: collectors showed no tendency to be active earlier in the year as time went on, nor did activity toward the end of spring decline in recent decades. Earlier bloom eventually could have substantial impacts on plant and animal communities in the Sonoran Desert, especially on migratory hummingbirds and population dynamics of shrubs.

Southwestern Naturalist↗
USGS2007

Changes in a northwestern Florida gulf coast herpetofaunal community over a 28-y period

Population declines of amphibians and reptiles throughout the world have led to the initiation of projects to monitor their status and trends. Historical collections give an indication of which species occurred in an area at one time, although the ambiguity surrounding locations and environmental conditions associated with collection decreases the value of this information source. Resampling using the same general protocols can give valuable insights to changes in community structure. However, this is only feasible when sampling methodology and exact site locations are known. From 2002-2005 we resampled 12 sites in St. Marks National Wildlife Refuge in Florida's panhandle, an area in which intensive herpetological surveys were conducted in 1977-1979. We documented a general decrease in species richness among the diversely managed sites, changes in dominant species and diversity and an increasing trend toward homogeneity of the herpetofaunal community among habitats. Changes were attributed to four causes: 28-y of forest community succession, changes in management practices, non-detection of species due to variation in sampling conditions and a decrease in occupancy by four amphibians and three reptiles. The use of population and habitat-related indexes helped define possible influences on community change and can be used to target species for monitoring. Declines of these seven species are of concern, especially considering the protected status of the refuge and its increasing isolation as surrounding landscapes are converted to urbanized settings.

American Midland Naturalist↗
USGS2016

Evidence for range contraction of snowshoe hare in Pennsylvania

In Pennsylvania, Lepus americanus (Snowshoe Hare) is near the southern limits of its range and at risk of range contraction because of loss of early-successional forest and impacts of climate change. We used hunter-harvest data to investigate changes in the distribution of Snowshoe Hare in Pennsylvania (1983–2011), forest inventory and land-use data to assess changes in amount and distribution of early-successional forest (1988–2011), and occupancy modeling (2004) to identify habitat and climate variables that explain the current distribution of Snowshoe Hare. We determined presence of Snowshoe Hare based on visual sightings, observations of tracks, and DNA analysis of fecal pellets, and used repeated visits to sampling sites and occupancy models to estimate occupancy rates (Ψ). Hunter-harvest data indicated the range of Snowshoe Hare in Pennsylvania contracted towards northwestern and northeastern portions of the state. Based on occupancy modeling, Snowshoe Hare were most likely to occupy early-successional and mixed deciduous-coniferous forest types and areas with colder winter temperatures, which coincided with the distribution of hunter harvests. Among the 4 forest types, we estimated Ψ = 0.52-0.79 and Ψ = 0.10-0.32 where winter temperatures were coldest and warmest, respectively. Total forest loss was <1% during 1988-2011, and the loss of early-successional forest in the current and former range of Snowshoe Hares was similar as were mean patch size and a fragmentation metric of early-successional habitat. Thus, changes in forest characteristics did not explain the range contraction we observed. We used climate-model predictions and our occupancy model to predict that average occupancy probability across northern Pennsylvania may decline from 0.27 in 2004 to 0.10–0.18 by 2050–2059, depending on the climate model. The range of Snowshoe Hare in Pennsylvania has contracted to regions of Pennsylvania with the coldest winter temperatures and most persistent snowpack, and based on projected climate change, our results suggest further range contraction of Snowshoe Hare in Pennsylvania.

Pennsylvania↗
USGS2003

Ord's kangaroo rats living in floodplain habitats: Factors contributing to habitat attraction

High densities of an aridland granivore, Ord's kangaroo rat (Dipodomys ordii), have been documented in floodplain habitats along the Yampa River in northwestern Colorado. Despite a high probability of inundation and attendant high mortality during the spring flood period, the habitat is consistently recolonized. To understand factors that potentially make riparian habitats attractive to D. ordii, we compared density and spatial pattern of seeds, density of a competitor (western harvester ant, Pogonomyrmex occidentalis), and digging energetics within floodplain habitats and between floodplain and adjacent upland habitats. Seed density within the floodplain was greatest in the topographically high (rarely flooded) floodplain and lowest immediately after a spring flood in the topographically low (frequently flooded) floodplain. Seed densities in adjacent upland habitat that never floods were higher than the lowest floodplain habitat. In the low floodplain prior to flooding, seeds had a clumped spatial pattern, which D. ordii is adept at exploiting; after spring flooding, a more random pattern resulted. Populations of the western harvester ant were low in the floodplain relative to the upland. Digging by D. ordii was energetically less expensive in floodplain areas than in upland areas. Despite the potential for mortality due to annual spring flooding, the combination of less competition from harvester ants and lower energetic costs of digging might promote the use of floodplain habitat by D. ordii.

Southwestern Naturalist↗
USGS2007

Aquatic vertebrate assemblages of the upper Clear Creek Watershed, California

We sampled streams in the Upper Clear Creek Watershed in northwestern California in fall 2004 and fall 2005 to document assemblages of aquatic vertebrates and to provide resource managers with information on the importance of these assemblages in terms of regional biodiversity. We used single-pass backpack electrofishing to sample 15 sites in fall 2004 and the same 15 sites plus 4 new sites in fall 2005. We captured 10 fish taxa and 2 species of larval amphibians. Seven of the fish taxa were native species. Of the exotic species, only brook trout (Salvelinus fontinalis) occurred at more than 1 site. Ordinations by nonmetric multidimensional scaling indicated a gradient from sites with rainbow trout (Oncorhynchus mykiss), Pacific giant salamander (Dicamptodon tenebrosus), and tailed frog (Ascaphus truei) to sites dominated by riffle sculpin (Cottus gulosus), California roach (Hesperoleucas symmetricus), and Sacramento sucker (Catostomus occidentalis). The gradient in species composition was associated with changes in elevation, gradient, discharge, and substrate. The Upper Clear Creek Watershed represents a unique area of overlap between the North Coast California amphibian fauna and the Central Valley fish fauna with a notable paucity of exotic fishes and amphibians. Preservation of the integrity of native aquatic assemblages is an important goal for aquatic resource management in the region; our results provide a critcial baseline to gauge future management actions.

California↗
USGS2012

Associations of benthic macroinvertebrate assemblages with environmental variables in the upper Clear Creek watershed, California

Benthic macroinvertebrates are integral components of stream ecosystems and are often used to assess the ecological integrity of streams. We sampled streams in the upper Clear Creek drainage in the Klamath—Siskiyou Ecoregion of northwestern California in fall 2004 (17 sites) and 2005 (original 17 plus 4 new sites) with the objectives of documenting the benthic macroinvertebrate assemblages supported by the streams in the area, determining how those assemblages respond to environmental variables, assessing the biological condition of the streams using a benthic index of biotic integrity (IBI), and understanding the assemblages in the context of biodiversity of the ecoregion. We collected both reach-wide (RW) and targeted-riffle (TR) macroinvertebrate samples at each site. The macroinvertebrate assemblages were diverse, with over 150 genera collected for each sampling protocol. The macroinvertebrate assemblages appeared to be most responsive to a general habitat gradient based on stream size, gradient, flow, and dominance of riffles. A second important habitat gradient was based on elevation and dominance of riffles. A gradient in water quality based on concentrations of dissolved ions and metals was also important. Models based on these 3 gradients had Spearman's rank correlations with macroinvertebrate taxonomic composition of 0.60 and 0.50 for the TR and RW samples, respectively. The majority (>50%) of the sites were in good or very good biological condition based on IBI scores. The diversity of macroinvertebrate assemblages is associated with the diversity of habitats available in the Klamath—Siskiyou Ecoregion. Maintaining the aquatic habitats in good condition is important in itself but is also vital to maintaining biodiversity in this diverse and unique ecoregion.

California↗
USGS2003

Effects of river flow regime on cottonwood leaf litter dynamics in semi-arid northwestern Colorado

We compared production and breakdown of Fremont cottonwood ( Populus deltoides wislizenii ) leaf litter at matched floodplain sites on the regulated Green River and unregulated Yampa River in semi-arid northwestern Colorado. Litter production under trees was similar at sites in 1999 (250 g/m 2 , oven-dry) but lower in 2000 (215 and 130 g/m 2 ), a drought year that also featured an outbreak of defoliating beetles at the Yampa River site. Our production values were similar to the few others reported for riparian forests within semi-arid or arid areas. Leaf litter in portions of the floodplain not inundated during the spring flood lost organic matter at the same rate as leaves placed in upland sites in 1998 and 2000: 35 to 50% of organic matter during an approximately 160-day spring and summer period. Inundated litter lost 55 to 90% of its organic matter during the same period. Organic matter loss from inundated leaves increased with duration of inundation and with deposition of fine sediment. Pooled across locations, leafpack data suggested that nitrogen concentration (mg N/kg organic matter) increased until about 65% of the initial organic matter was lost. This increase likely reflected the buildup of microbial decomposer populations. The role of insects and other macroinvertebrates in litter breakdown apparently was minor at both sites. Large spatial and temporal variation in litter dynamics in aridland floodplain settings is ensured by microtopographic variation in the alluvial surface coupled with year-to-year variation associated with most natural flood regimes. Factors reducing flood flow frequency or magnitude will reduce overall breakdown rates on the floodplain towards those found in drier upland environments.

Colorado↗