Geology topics

Craig A. Stricker

Publications and source records attributed to Craig A. Stricker.

At least 37 records · Page 2Linked to original sources

USGS2021

Temporal influences on selenium partitioning, trophic transfer, and exposure in a major U.S. river

Hydrologic and irrigation regimes mediate the timing of selenium (Se) mobilization to rivers, but the extent to which patterns in Se uptake and trophic transfer through recipient food webs reflect the temporal variation in Se delivery is unknown. We investigated Se mobilization, partitioning, and trophic transfer along approximately 60 river miles of the selenium-impaired segment of the Lower Gunnison River (Colorado, USA) during six sampling trips between June 2015 and October 2016. We found temporal patterns in Se partitioning and trophic transfer to be independent of those in dissolved Se concentrations and that the recipient food web sustained elevated Se concentrations from earlier periods of high Se mobilization. Using an ecosystem-scale Se accumulation model tailored to the Lower Gunnison River, we predicted that the endangered Razorback Sucker ( Xyrauchen texanus ) and Colorado Pikeminnow ( Ptychocheilus lucius ) achieve whole-body Se concentrations exceeding aquatic life protection criteria during periods of high runoff and irrigation activity (April–August) that coincide with susceptible phases of reproduction and early-life development. The results of this study challenge assumptions about Se trophodynamics in fast-flowing waters and introduce important considerations for the management of Se risks for biota in river ecosystems.

Colorado↗

USGS2021

Breeding at higher latitude is associated with higher photoperiodic threshold and delayed reproductive development in a songbird

Many seasonally breeding animals exhibit a threshold day length (critical photoperiod; CPP) for gonadal growth, and populations breeding at higher latitudes typically have a higher CPP. Much less is known about latitudinal variation in CPP in migratory population that winter away from their breeding range and must time their reproduction to match favorable conditions at their destination. To address the relationship between migration, breeding latitude, and CPP, we held two closely related songbird populations in a common environment. One population is resident ( Junco hyemalis carolinensis ), the other winters in sympatry with the residents but migrates north to breed ( Junco hyemalis hyemalis ). We gradually increased photoperiod and measured indices of readiness to migrate (fat score, body mass) and breed (cloacal protuberance volume, baseline testosterone, and gonadotropin releasing hormone challenged testosterone). To estimate breeding latitude, we measured hydrogen isotopes in feathers grown the preceding year. As we predicted, we found a higher CPP in migrants than residents, and a higher CPP among migrants deriving from higher as opposed to lower latitudes. Migrants also terminated breeding earlier than residents, indicating a shorter breeding season. To our knowledge, this is a first demonstration of latitudinal variation in CPP-dependent reproductive timing in bird populations that co-exist in the non-breeding season but breed at different latitudes. We conclude that bird populations appear to exhibit local adaptation in reproductive timing by relying on differential CPP response that is predictive of future conditions on the breeding ground.

Indiana↗

USGS2020

Evidence of post-breeding prospecting in a long-distance migrant.

Organisms assess biotic and abiotic cues at multiple sites when deciding where to settle. However, due to temporal constraints on this prospecting, the suitability of available habitat may be difficult for an individual to assess when cues are most reliable, or at the time they are making settlement decisions. For migratory birds, the postbreeding season may be the optimal time to prospect and inform settlement decisions for future breeding seasons. We investigated the fall movements of flammulated owls ( Psiloscops flammeolus ) within breeding habitat after fledglings had gained independence and before adults left for migration. From 2013 to 2016, we trapped owls within a breeding population wherein all nesting owls and their young have been banded since 1981. We used stable isotopes in combination with mark–recapture data to identify local individuals and differentiate potential prospecting behavior from other seasonal movements such as migration or staging. We commonly captured owls in the fall—predominantly hatch-year owls—that were not known residents of the study area. Several of these nonresident owls were later found breeding within the study area. Stable isotope data suggested a local origin for virtually all owls captured during the fall. Our results suggest that hatch-year flammulated owls, but also some after-hatch-year owls, use the period between the breeding season and fall migration to prospect for future breeding sites. The timing of this behavior is likely driven by seasonally variable costs associated with prospecting. Determining the timing of prospecting and the specific cues that are being assessed will be important in helping predict the extent to which climate change and/or altered disturbance regimes will modify the ecology, behavior, and demographics associated with prospecting.

Colorado↗

USGS2020

Short- and long-term responses of riparian cottonwoods (Populus spp.) to flow diversion: Analysis of tree-ring radial growth and stable carbon isotopes

Long duration tree-ring records with annual precision allow for the reconstruction of past growing conditions. Investigations limited to the most common tree-ring proxy of ring width can be difficult to interpret, however, because radial growth is affected by multiple environmental processes. Furthermore, studies of living trees may miss important effects of drought on tree survival and forest changes. Stable carbon isotopes can help distinguish drought from other environmental factors that influence tree-ring width and forest stand condition. We quantified tree-ring radial expansion and stable carbon isotope ratios (δ 13 C) in riparian cottonwoods ( Populus angustifolia and P. angustifolia x P. trichocarpa) along Snake Creek in Nevada, USA. We investigated how hydrological drought affected tree growth and death at annual to half-century scales in a partially dewatered reach (DW) compared to reference reaches immediately upstream and downstream. A gradual decline in tree-ring basal area increment (BAI) began at DW concurrent to streamflow diversion in 1961. BAI at DW diverged from one reference reach immediately but not from the other until nearly 50 years later. In contrast, tree-ring δ 13 C had a rapid and sustained increase following diversion at DW only, providing the stronger and clearer drought signal. BAI and δ 13 C were not significantly correlated prior to diversion; after diversion they both reflected drought and were correlated for DW trees only. Cluster analyses distinguished all trees in DW from those in reference reaches based on δ 13 C, but BAI patterns left trees intermixed across reaches. Branch and tree mortality were also highest and canopy vigor was lowest in DW. Results indicate that water scarcity strongly limited cottonwood photosynthesis following flow diversion, thus reducing carbon assimilation, basal growth and survival. The dieback was not sudden, but occurred over decades as carbon deficits mounted and depleted streamflow left trees increasingly vulnerable to local meteorological drought.

Nevada↗

USGS2019

Migratory divides coincide with reproductive barriers across replicated avian hybrid zones above the Tibetan Plateau

Migratory divides are proposed to be catalysts for speciation across a diversity of taxa. However, it is difficult to test the relative contributions of migratory behaviour vs. other divergent traits to reproductive isolation. Comparing hybrid zones with and without migratory divides offers a rare opportunity to directly examine the contribution of divergent migratory behaviour to reproductive barriers. We show that across replicate sampling transects of two pairs of barn swallow ( Hirundo rustica ) subspecies, strong reproductive isolation coincided with a migratory divide spanning 20 degrees of latitude. A third subspecies pair exhibited no evidence for a migratory divide and hybridised extensively. Within migratory divides, overwintering habitats were associated with assortative mating, implicating a central contribution of divergent migratory behaviour to reproductive barriers. The remarkable geographic coincidence between migratory divides and genetic breaks supports a long-standing hypothesis that the Tibetan Plateau is a substantial barrier contributing to the diversity of Siberian avifauna.

Tibetan Plateau↗

USGS2019

Carbon chemistry of intact versus chronically drained peatlands in the southeastern USA

The Great Dismal Swamp (GDS) is a large temperate swamp in Virginia/North Carolina with peat soils historically resistant to microbial decomposition. However, this peatland has been subject to ~200 years of disturbance during which extensive drainage, fire suppression, and wide-spread logging have increased decomposition and dramatically decreased the distribution of Atlantic white cedar (AWC). The purpose of this study was to determine the impact of long-term drainage and AWC loss on the carbon chemistry of GDS peats. Peat cores were collected from three drained GDS vegetation communities (pocosin, AWC, and red maple-black gum) and compared to cores collected from an intact, undrained AWC peatland at the Alligator River National Wildlife Refuge (AR) in North Carolina, USA. The AR peats had higher lignin content in the deeper peat intervals, and lignin content and % organic carbon were largely invariant with depth compared to the GDS peats. The concentrations of syringyl group phenols were greater in the surface layers of GDS peats, likely reflecting the selective removal of AWC and transition from gymnosperms to angiosperms. Acid to aldehyde ratios for vanillyl and syringyl group phenols indicated that the GDS peats were more decomposed, particularly at depth, and that this occurred under aerobic conditions. Moreover, solid state 13C NMR confirmed a coincident loss of carbohydrates and increase in recalcitrant byproducts of carbohydrate degradation with depth. These data indicate that long-term drainage has accelerated the decomposition of peat at the GDS, reducing the capacity and stability of the carbon sink.

Virginia, North Carolina↗

USGS2019

Benthic algal (Periphyton) growth rates in response to nitrogen and phosphorus: Parameter estimation for water quality models

Nitrogen (N) and phosphorus (P) are important pollutants that can stimulate nuisance blooms of algae. Water-quality models (e.g., WASP, CE-QUAL-R1, CE-QUAL-ICM, QUAL2k) are valuable and widely used management tools for algal accrual because of excess nutrients in the presence of other limiting factors. These models utilize the Monod and Droop equations to associate algal growth rate with dissolved nutrient concentration and intra-cellular nutrient concentration. Having accurate parameter values is essential to model performance; however, published values for model parameterization are limited, particularly for benthic (periphyton) algae. We conducted a 10-day mesocosm experiment and measured diatom-dominated periphyton biomass accrual through time as chlorophyll a (chl a) and ash-free dry mass (AFDM) in response to additions of N (range 5-12,390 µg NO3-N/L) and P (range 0.89-59.51 µg SRP/L). Resulting half saturation coefficients and growth rates are similar to other published values, but minimum intra-cellular nutrient concentration (quota, Qmin) are higher than those previously reported. Saturation concentration for N ranged from 150 to 2450 µg NO3-N/L based on chl a and from 8.5 to 60 µg NO3-N/L when based on AFDM. Similarly, the saturation concentration for P ranged from 12 to 29 µg-P/L based on chl a, and from 2.5 to 6.1 µg-P/L based on AFDM. These saturation concentrations provide an upper limit for streams where diatom growth can be expected to respond to nutrient levels and a benchmark for reducing nutrient concentrations to a point where benthic algal growth will be limited.

Journal of the American Water Resources Associatio↗

USGS2019

Migratory strategy explains differences in timing of female reproductive development in seasonally sympatric songbirds.

1. Divergent migratory strategies among populations can result in population-level differences in timing of reproduction (allochrony) and local adaptation. However, the mechanisms underlying among-population variation in timing are insufficiently understood, particularly in females. 2. We studied differences in reproductive development and its related mechanisms along the hypothalamic-pituitary-gonadal axis (HPG) in closely related migratory and sedentary (i.e., resident) female dark-eyed juncos (Junco hyemalis) living together in sympatry during early spring. We predicted that residents would be more reproductively developed than migrants. Alternatively, we predicted that females might exhibit similar reproductive development in response to the same environmental cue despite differences in migratory behavior. To compare their degree of reproductive development during seasonal sympatry and the underlying mechanisms of these differences, we collected ovarian and liver tissue in early spring prior to migration, and compared abundance of gene transcripts associated with reproduction using quantitative PCR. We also used stable hydrogen isotopes to infer relative breeding and wintering latitude of migrants. 3. We found more abundant transcripts for luteinizing hormone receptor and aromatase in addition to significantly heavier ovaries in residents than in migrants. Together these results suggest greater sensitivity and response to upstream endocrine stimulation in resident females. Transcript abundance for other receptors associated with reproduction, however, did not differ between populations. When comparing ovarian development within migrants, females with lower hydrogen isotopes (indicating higher breeding latitudes) had smaller ovaries, suggesting that longer-distance migrations may further delay reproductive development. 4. Based on differences in ovary mass and transcript abundance, we conclude that females that differ in migratory strategy also differ in timing of reproductive development. Differences in timing of reproductive development between migrant and resident females could drive reproductive isolation between populations. This study also reveals mechanisms that could drive allochrony and enable us to predict how seasonal organisms will respond to a changing world.

Functional Ecology↗

USGS2019

Bridging the gap between salmon spawner abundance and marine nutrient assimilation by juvenile salmon: Seasonal cycles and landscape effects at the watershed scale

Anadromous Pacific salmon are semelparous, and resource subsidies from spawning adults (marine-derived nutrients, or MDN) benefit juvenile salmonids rearing in freshwater. However, it remains unclear how MDN assimilation relates to spawner abundance within a watershed. To address this, we examined seasonal, watershed-scale patterns of MDN assimilation in rearing coho ( Oncorhynchus kisutch ) and Chinook ( O. tshawytscha ) salmon and compared it with spawner biomass and landscape features in a western Alaska watershed with contrasting structural complexity in two sub-drainages. Adult salmon biomass density was estimated from escapement and spawner distribution data, and MDN assimilation in juvenile salmon was estimated via stable isotopes. In the North River, MDN assimilation was lowest in early summer, prior to annual spawning migrations, increased after spawning, and peaked in late winter. In the more complex mainstem Unalakleet River, MDN assimilation was higher but varied minimally from summer through fall before increasing in late fall and winter. Summer MDN assimilation, prior to salmon spawning, was primarily a function of habitat complexity, where MDN was highest in sloughs and the more complex mainstem river. After salmon spawned, fall MDN assimilation was a function of adult pink and Chinook salmon biomass as well as MDN assimilation that occurred prior to spawning (that is, summer MDN), but unrelated to total summer biomass (all salmon species biomass combined). Thus, MDN assimilation by juvenile salmon in the fall was a function of species-specific adult spawner abundance but seasonal patterns of MDN assimilation were masked in complex habitat where summer MDN assimilation remained high.

Alaska↗

USGS2019

Effects of age and environment on stable carbon isotope ratios in tree rings of riparian Populus

Stable carbon isotopes of riparian tree rings are enabling improved reconstruction of past climate variability, but this advance is limited by difficulty distinguishing the effects of tree age from those of climate. We investigated relative influence of age and climate trends in genus Populus, which dominates floodplain forests in Europe, Asia and North America. We related precipitation and river flow to annual variation in stable carbon isotope ratio (δ13C) in trees with a broad distribution of ages spanning two hundred years. On the floodplain of the lightly regulated Little Missouri River, North Dakota, USA, we examined a total of 845 rings from seven specimens of cottonwood tree (Populus deltoides W. Bartram ex Marshall ssp. monilifera [Aiton] Eckenwalder). Cottonwood δ13C decreased from pith to bark in whole wood, but this trend was almost completely eliminated in purified cellulose. The δ13C offset between whole wood and cellulose increased from pith to bark, consistent with trends of decreasing cellulose and increasing hemicellulose as a proportion of whole wood. While annual ring width was correlated with monthly precipitation from April through June, δ13C showed strong correlations only in June and July, suggesting that these complementary proxies allow resolution of seasonal variation in water availability. We conclude that past climate can be reconstructed from δ13C of purified cellulose from cottonwood without detrending for tree age.

Palaeogeography, Palaeoclimatology, Palaeoecology↗

USGS2018

Temporal records of diet diversity dynamics in individual adult female Steller sea lion (Eumetopias jubatus) vibrissae

Detailed information on the nutrition of free-ranging mammals contributes to the understanding of life history requirements, yet is often quite limited temporally for most species. Reliable dietary inferences can be made by analyzing the stable carbon (C) and nitrogen (N) isotopic values ( δ 13 C and δ 15 N) of some consumer tissues; exactly which tissue is utilized dictates the inferential scope. Steller sea lion (SSL) vibrissae are grown continuously without shedding and thus provide a continuous multi-year record of dietary consumption. We applied a novel kernel density approach to compare the δ 13 C and δ 15 N values along the length of SSL vibrissae with δ 13 C and δ 15 N distributions of potential prey species. This resulted in time-series of proportion estimates of dietary consumption for individual SSL. Substantial overlap in δ 13 C and δ 15 N distributions for prey species prevented a discrete species-scale assessment of SSL diets; however, a post hoc correlational analysis of diet proportion estimates revealed grouping by trophic level. Our findings suggest that adult female SSL diets in the western and central Aleutian Islands shift significantly according to season: diets contain a higher proportion of lower trophic level species (Pacific Ocean perch, northern rockfish, Atka mackerel and walleye pollock) in the summer, whereas in the winter SSL consume a much more diverse diet which includes a greater proportion of higher trophic level species (arrowtooth flounder, Kamchatka flounder, darkfin sculpin, Pacific cod, Pacific octopus, rock sole, snailfish, and yellow Irish lord).

Oecologia↗

USGS2018

Determination of δ13C, δ15N, or δ34S by isotope-ratio-monitoring mass spectrometry using an elemental analyzer

This report describes procedures used in the Geology, Geophysics, and Geochemistry Science Center of the U.S. Geological Survey in Denver, Colorado, to determine the stable-isotope ratios 13 C/ 12 C, 15 N/ 14 N, and 34 S/ 32 S in solid materials. The procedures use elemental analyzers connected directly to gas-source isotope-ratio mass spectrometers. A different elemental–analyzer–mass-spectrometer system is used for 13 C/ 12 C and 15 N/ 14 N than is used for 34 S/ 32 S to accommodate differences in reagents, catalysts, and instrument settings.

Techniques and Methods↗

USGS2017

Mechanisms associated with an advance in the timing of seasonal reproduction in an urban songbird

The colonization of urban environments by animals is often accompanied by earlier breeding and associated changes in seasonal schedules. Accelerated timing of seasonal reproduction in derived urban populations is a potential cause of evolutionary divergence from ancestral populations if differences in physiological processes that regulate reproductive timing become fixed over time. We compared reproductive development in free-living and captive male dark-eyed juncos deriving from a population that recently colonized a city (~35 years) and ceased migrating to that of conspecifics that live in sympatry with the urban population during winter and spring but migrate elsewhere to breed. We predicted that the earlier breeding sedentary urban birds would exhibit accelerated reproductive development in the spring along the hypothalamic-pituitary-gonadal (HPG) axis as compared to migrants. We found that free-living sedentary urban and migrant juncos differed at the level of the pituitary when measured as baseline luteinizing hormone (LH) levels, but not in increased LH when challenged with Gonadotropin-Releasing Hormone (GnRH). Among captives held in a common garden, and at the level of the gonad, we found that sedentary urban birds produced more testosterone in response to GnRH than migrants living in the same common environment, suggesting greater gonadal sensitivity in the derived urban population. Greater gonadal sensitivity could arise from greater upstream activation by LH or FSH or from reduced suppression of gonadal development by the adrenal axis. We compared abundance of gonadal transcripts for LH receptor (LHR), follicle stimulating hormone receptor (FSHR), glucocorticoid receptor (GR), and mineralocorticoid receptor (MR) in the common-garden, predicting either more abundant transcripts for LHR and FSHR or fewer transcripts for GR and MR in the earlier breeding sedentary urban breeders, as compared to the migrants. We found no difference in the expression of these genes. Together these data suggest that advanced timing of reproduction in a recently derived urban population is facilitated by earlier increase in upstream baseline activity of the HPG and earlier release from gonadal suppression by yet-to-be-discovered mechanisms. Evolutionarily, our results suggest that potential for gene flow between seasonally sympatric populations may be limited due to urban-induced advances in the timing of reproduction and resulting allochrony with ancestral forms.

California↗

USGS2017

Opinion: Why we need a centralized repository for isotopic data

Stable isotopes encode and integrate the origin of matter; thus, their analysis offers tremendous potential to address questions across diverse scientific disciplines ( 1 , 2 ). Indeed, the broad applicability of stable isotopes, coupled with advancements in high-throughput analysis, have created a scientific field that is growing exponentially, and generating data at a rate paralleling the explosive rise of DNA sequencing and genomics ( 3 ). Centralized data repositories, such as GenBank, have become increasingly important as a means for archiving information, and “Big Data” analytics of these resources are revolutionizing science and everyday life.

Proceedings of the National Academy of Sciences of↗

USGS2017

Metamorphosis affects metal concentrations and isotopic signatures in a mayfly ( Baetis tricaudatus ): Implications for the aquatic-terrestrial transfer of metals

Insect metamorphosis often results in substantial chemical changes that can alter contaminant concentrations and fractionate isotopes. We exposed larval mayflies ( Baetis tricaudatus ) and their food (periphyton) to an aqueous zinc gradient (3-340 µg Zn/l) and measured zinc concentrations at different stages of metamorphosis: larval, subimago, and imago. We also measured changes in stable isotopes (δ 15 N and δ 13 C) in unexposed mayflies. Larval zinc concentrations were positively related to aqueous zinc, increasing 9-fold across the exposure gradient. Adult zinc concentrations were also positively related to aqueous zinc, but were 7-fold lower than larvae. This relationship varied according to adult substage and sex. Tissue concentrations in female imagoes were not related to exposure concentrations, but the converse was true for all other stage-by-sex combinations. Metamorphosis also increased δ 15 N by ~0.8‰, but not δ 13 C. Thus, the main effects of metamorphosis on insect chemistry were large declines in zinc concentrations coupled with increased δ 15 N signatures. For zinc, this change was largely consistent across the aqueous exposure gradient. However, differences among sexes and stages suggest that caution is warranted when using nitrogen isotopes or metal concentrations measured in one insect stage (e.g. larvae) to assess risk to wildlife that feed on subsequent life stages (e.g. adults).

Environmental Science & Technology↗

USGS2017

In vivo isotopic fractionation of zinc and biodynamic modeling yield insights into detoxification mechanisms in the mayfly Neocloeon triangulifer

Diversity and biomass of aquatic insects decline in metal-rich aquatic environments, but the mechanisms by which insects from such environments cope with potentially toxic metal concentrations to survive through adulthood are less well understood. In this study, we measured Zn concentrations and isotopes in laboratory-reared diatoms and mayflies ( Neocloeon triangulifer ) from larval through adult stages. The larvae were fed Zn-enriched diatoms, and bio-concentrated Zn by a factor of 2.5–5 relative to the diatoms but maintained the same Zn-isotopic ratio. These results reflect the importance of dietary uptake and the greater rate of uptake relative to excretion or growth. Upon metamorphosis to subimago, Zn concentrations declined by > 70%, but isotopically heavy Zn remained in the subimago bodies. We surmised that the loss of isotopically light Zn during metamorphosis was due to the loss of detoxified Zn and retention of metabolically useful Zn. Through the transition from subimago to imago, Zn concentrations and isotope ratios were virtually unchanged. Because the decrease in Zn body concentration and increase in heavier Zn are seen in the subimagos relative to the larvae, the compartmentalization of Zn must be occurring within the larvae. A biodynamic model was constructed, allowing for isotopic fractionation and partitioning of Zn between metabolically essential and detoxified Zn reservoirs within larvae. The model provides a consistent set of rate and fractionation constants that successfully describe the experimental observations. Specifically, metabolically essential Zn is isotopically heavier and is tightly held once assimilated, and excess, isotopically light Zn is sequestered, detoxified, and ultimately lost during the metamorphosis of larvae to subimagos. To our knowledge, this is the first documentation of in vivo isotopic fractionation in insects, offering an improved understanding of the mechanisms and rates by which the N. triangulifer larvae regulate excess Zn in their bodies.

Science of the Total Environment↗

USGS2016

Wood decay in desert riverine environments

Floodplain forests and the woody debris they produce are major components of riverine ecosystems in many arid and semiarid regions (drylands). We monitored breakdown and nitrogen dynamics in wood and bark from a native riparian tree, Fremont cottonwood ( Populus deltoides subsp. wislizeni ), along four North American desert streams. We placed locally-obtained, fresh, coarse material [disks or cylinders (&sim;500–2000 cm 3 )] along two cold-desert and two warm-desert rivers in the Colorado River Basin. Material was placed in both floodplain and aquatic environments, and left in situ for up to 12 years. We tested the hypothesis that breakdown would be fastest in relatively warm and moist aerobic environments by comparing the time required for 50% loss of initial ash-free dry matter ( T 50 ) calculated using exponential decay models incorporating a lag term. In cold-desert sites (Green and Yampa rivers, Colorado), disks of wood with bark attached exposed for up to 12 years in locations rarely inundated lost mass at a slower rate ( T 50 = 34 yr) than in locations inundated during most spring floods ( T 50 = 12 yr). At the latter locations, bark alone loss mass at a rate initially similar to whole disks ( T 50 = 13 yr), but which subsequently slowed. In warm-desert sites monitored for 3 years, cylinders of wood with bark removed lost mass very slowly ( T 50 = 60 yr) at a location never inundated (Bill Williams River, Arizona), whereas decay rate varied among aquatic locations ( T 50 = 20 yr in Bill Williams River; T 50 = 3 yr in Las Vegas Wash, an effluent-dominated stream warmed by treated wastewater inflows). Invertebrates had a minor role in wood breakdown except at in-stream locations in Las Vegas Wash. The presence and form of change in nitrogen content during exposure varied among riverine environments. Our results suggest woody debris breakdown in desert riverine ecosystems is primarily a microbial process with rates determined by landscape position, local weather, and especially the regional climate through its effect on the flow regime. The increased warmth and aridity expected to accompany climate change in the North American southwest will likely retard the already slow wood decay process on naturally functioning desert river floodplains. Our results have implications for designing environmental flows to manage floodplain forest wood budgets, carbon storage, and nutrient cycling along regulated dryland rivers.

Arizona, Colorado, Nevada↗

USGS2016

Reproductive allochrony in seasonally sympatric populations maintained by differential response to photoperiod: Implications for population divergence and response to climate change

Reproductive allochrony presents a potential barrier to gene flow and is common in seasonally sympatric migratory and sedentary birds. Mechanisms mediating reproductive allochrony can influence population divergence and the capacity of populations to respond to environmental change. We asked whether reproductive allochrony in seasonally sympatric birds results from a difference in response to supplementary or photoperiodic cues and whether the response varies in relation to the distance separating breeding and wintering locations as measured by stable isotopes. We held seasonally sympatric migratory and sedentary male dark-eyed juncos ( Junco hyemalis ) in a common garden in early spring under simulated natural changes in photoperiod and made measurements of reproductive and migratory physiology. On the same dates and photoperiods, sedentary juncos had higher testosterone (initial and gonadotropin-releasing hormone induced), more developed cloacal protuberances, and larger testes than migrants. In contrast, migratory juncos had larger fat reserves (fuel for migration). We found a negative relationship between testis mass and feather hydrogen isotope ratios, indicating that testis growth was more delayed in migrants making longer migrations. We conclude that reproductive allochrony in seasonally sympatric migratory and sedentary birds can result from a differential response to photoperiodic cues in a common garden, and as a result, gene flow between migrants and residents may be reduced by photoperiodic control of reproductive development. Further, earlier breeding in response to future climate change may currently be constrained by differential response to photoperiodic cues.

American Naturalist↗