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USGS2002

On the origin of brucellosis in bison of Yellowstone National Park: a review

Brucellosis caused by Brucella abortus occurs in the free-ranging bison ( Bison bison ) of Yellowstone and Wood Buffalo National Parks and in elk ( Cervus elaphus ) of the Greater Yellowstone Area. As a result of nationwide bovine brucellosis eradication programs, states and provinces proximate to the national parks are considered free of bovine brucellosis. Thus, increased attention has been focused on the wildlife within these areas as potential reservoirs for transmission to cattle. Because the national parks are mandated as natural areas, the question has been raised as to whether Brucella abortus is endogenous or exogenous to bison, particularly for Yellowstone National Park. We synthesized diverse lines of inquiry, including the evolutionary history of both bison and Brucella , wild animals as Brucella hosts, biochemical and genetic information, behavioral characteristics of host and organism, and area history to develop an evaluation of the question for the National Park Service. All lines of inquiry indicated that the organism was introduced to North America with cattle, and that the introduction into the Yellowstone bison probably was directly from cattle shortly before 1917. Fistulous withers of horses was a less likely possibility. Elk on winter feedgrounds south of Yellowstone National Park apparently acquired the disease directly from cattle. Bison presently using Grand Teton National Park probably acquired brucellosis from feedground elk.

Idaho, Montana, Wyoming↗

USGS2006

Behavioral profiles of the captive juvenile whooping crane as an indicator of post-release survival

Predation by bobcats (Lynx rufus) is the major cause of mortality in captive-reared whooping cranes (Grus americana) released into the wild to establish a nonmigratory flock in Florida. This study investigated whether rearing methods (parent-rearing, hand-rearing, or hand-rearing with exercise) of cranes, and behaviors observed in birds either before or shortly after release in the wild, are associated with survival after release. Rearing methods did not affect survival first year post-release, which was 55 ? 8% in 2 yr (1999 and 2000). Logistic regression revealed, however, that foraging bouts (+), walking bouts (-), and body weight (-) before release, and nonvigilant bouts (-) after release were significantly associated with survival. These results suggest that post-release survival of whooping cranes might be increased by rearing techniques that promote foraging.

Zoo Biology↗

USGS2020

Discrimination of biological scatterers in polarimetric weather radar data: Opportunities and challenges

For radar aeroecology studies, the identification of the type of scatterer is critically important. Here, we used a random forest (RF) algorithm to develop a variety of scatterer classification models based on the backscatter values in radar resolution volumes of six radar variables (reflectivity, radial velocity, spectrum width, differential reflectivity, correlation coefficient, and differential phase) from seven types of biological scatterers and one type of meteorological scatterer (rain). Models that discriminated among fewer classes and/or aggregated similar types into more inclusive classes classified with greater accuracy and higher probability. Bioscatterers that shared similarities in phenotype tended to misclassify against one another more frequently than against more dissimilar types, with the greatest degree of misclassification occurring among vertebrates. Polarimetric variables proved critical to classification performance and individual polarimetric variables played central roles in the discrimination of specific scatterers. Not surprisingly, purposely overfit RF models (in one case study) were our highest performing. Such models have a role to play in situations where the inclusion of natural history can play an outsized role in model performance. In the future, bioscatter classification will become more nuanced, pushing machine-learning model development to increasingly rely on independent validation of scatterer types and more precise knowledge of the physical and behavioral properties of the scatterer.

Remote Sensing↗

USGS2016

Spatially explicit modeling in ecology: A review

The use of spatially explicit models (SEMs) in ecology has grown enormously in the past two decades. One major advancement has been that fine-scale details of landscapes, and of spatially dependent biological processes, such as dispersal and invasion, can now be simulated with great precision, due to improvements in computer technology. Many areas of modeling have shifted toward a focus on capturing these fine-scale details, to improve mechanistic understanding of ecosystems. However, spatially implicit models (SIMs) have played a dominant role in ecology, and arguments have been made that SIMs, which account for the effects of space without specifying spatial positions, have an advantage of being simpler and more broadly applicable, perhaps contributing more to understanding. We address this debate by comparing SEMs and SIMs in examples from the past few decades of modeling research. We argue that, although SIMs have been the dominant approach in the incorporation of space in theoretical ecology, SEMs have unique advantages for addressing pragmatic questions concerning species populations or communities in specific places, because local conditions, such as spatial heterogeneities, organism behaviors, and other contingencies, produce dynamics and patterns that usually cannot be incorporated into simpler SIMs. SEMs are also able to describe mechanisms at the local scale that can create amplifying positive feedbacks at that scale, creating emergent patterns at larger scales, and therefore are important to basic ecological theory. We review the use of SEMs at the level of populations, interacting populations, food webs, and ecosystems and argue that SEMs are not only essential in pragmatic issues, but must play a role in the understanding of causal relationships on landscapes.

Ecosystems↗

USGS2006

Breeding biology and success of a reintroduced population of the critically endangered Puaiohi ( Myadestes palmeri )

The ultimate success of reintroduction programs for endangered species depends on the ability of reintroduced animals to breed in the wild. We studied the nesting success and breeding biology of a reintroduced population of Puaiohi ( Myadestes palmeri ) on the island of Kaua'i, Hawaii. Thirty-four captive-bred Puaiohi were released into the Alaka'i Swamp in 1999-2001 and monitored using radiotelemetry. Ten females and two males paired with wild and other released birds, including one polygynous trio. From March to September, 31 nests were built. Mean clutch size was 2.0 eggs, daily nest survival was 0.97 ± 0.01 (mean ± SE) and overall nest success was 0.40 ± 0.02. We confirmed predation, most probably by rats ( Rattus spp.), as the greatest cause of nest failure, occurring at 38% of active nests with known fates, and causing the death of two nesting adult females. Ground-based rodent control proved ineffective at protecting nest attempts. Successful nests fledged an average of 1.4 young each ( n = 10), and 85% of fledglings survived at least two weeks. Importantly, breeding behavior and success were comparable to those of wild Puaiohi. This is the first record of breeding in the wild from captive-bred endangered Hawaiian passerines. The ability of captive-bred Puaiohi to survive and breed successfully in the wild bodes well for future releases of this and other endangered passerines, but high predation rates on nests and nesting females highlights the importance of maintaining and restoring safe habitat for recovery.

The Auk↗

USGS2009

Lead in the environment

Anthropogenic uses of lead have probably altered its availability and environmental distribution more than any other toxic element. Consequently, lead concentrations in many living organisms may be approaching thresholds of toxicity for the adverse effects of lead. Such thresholds are difficult to define, as they vary with the chemical and physical form of lead, exposure regime, other elements present and also vary both within and between species. The technological capability to accurately quantify low lead concentrations has increased over the last decade, and physiological and behavioral effects have been measured in wildlife with tissue lead concentrations below those previously considered safe for humans.s.236 Consequently. lead criteria for the protection of wildlife and human health are frequently under review, and 'thresholds' of lead toxicity are being reconsidered. Proposed lead criteria for the protection of natural resources have been reviewed by Eisler. Uptake of lead by plants is limited by its generally low availability in soils and sediments, and toxicity may be limited by storage mechanisms and its apparently limited translocation within most plants. Lead does not generally accumulate within the foliar parts of plants, which limits its transfer to higher trophic levels. Although lead may concentrate in plant and animal tissues, no evidence of biomagnification exists. Acid deposition onto surface waters and soils with low buffering capacity may influence the availability of lead for uptake by plants and animals, and this may merit investigation at susceptible sites. The biological significance of chronic low-level lead exposure to wildlife is sometimes difficult to quantify. Animals living in urban environments or near point sources of lead emission are inevitably subject to greater exposure to lead and enhanced risk of lead poisoning. Increasingly strict controls on lead emissions in many countries have reduced exposure to lead from some sources. and the .reduction of lead in gasoline has resulted in lower tissue lead concentrations in humans and wildlife from many, particularly urban, locations. However, it has been suggested that increasing use of organic lead compounds as catalysts for the production of plastics and as wood preservatives and biocides could adversely affect wildlife. The most significant source of direct wildlife mortality from lead is spent gunshot and fishing sinkers. Elevated mortality from shot ingestion in avian species resulted in the introduction of nontoxic (steel) shot zones along certain flyways in the United States in the mid-1970s and a total ban on the use of lead for waterfowl and coot hunting nationwide by 1992. Several other countries are now following suit and have either banned or are in the process of restricting the use of lead shot for waterfowl hunting. In the United States it has been estimated that since the 1986 hunting season. when the use of nontoxic shot became widespread. over 6 million ducks have not been lost to lead poisoning. Raptors, especially eagles, have also apparently benefited. although lead poisoning from ingestion of bullet fragments remains a problem for the critically threatened California condor. Quantifying reductions in lead mortality rates would be difficult since eagle populations throughout North America are rapidly recovering from other anthropogenic perturbations, especially organochlorine pesticides.

Book chapter↗

USGS2016

Coupled downscaled climate models and ecophysiological metrics forecast habitat compression for an endangered estuarine fish

Climate change is driving rapid changes in environmental conditions and affecting population and species’ persistence across spatial and temporal scales. Integrating climate change assessments into biological resource management, such as conserving endangered species, is a substantial challenge, partly due to a mismatch between global climate forecasts and local or regional conservation planning. Here, we demonstrate how outputs of global climate change models can be downscaled to the watershed scale, and then coupled with ecophysiological metrics to assess climate change effects on organisms of conservation concern. We employed models to estimate future water temperatures (2010–2099) under several climate change scenarios within the large heterogeneous San Francisco Estuary. We then assessed the warming effects on the endangered, endemic Delta Smelt, Hypomesus transpacificus , by integrating localized projected water temperatures with thermal sensitivity metrics (tolerance, spawning and maturation windows, and sublethal stress thresholds) across life stages. Lethal temperatures occurred under several scenarios, but sublethal effects resulting from chronic stressful temperatures were more common across the estuary (median >60 days above threshold for >50% locations by the end of the century). Behavioral avoidance of such stressful temperatures would make a large portion of the potential range of Delta Smelt unavailable during the summer and fall. Since Delta Smelt are not likely to migrate to other estuaries, these changes are likely to result in substantial habitat compression. Additionally, the Delta Smelt maturation window was shortened by 18–85 days, revealing cumulative effects of stressful summer and fall temperatures with early initiation of spring spawning that may negatively impact fitness. Our findings highlight the value of integrating sublethal thresholds, life history, and in situ thermal heterogeneity into global change impact assessments. As downscaled climate models are becoming widely available, we conclude that similar assessments at management-relevant scales will improve the scientific basis for resource management decisions.

California↗

USGS2024

A systematic review of the effects of climate variability and change on black and brown bear ecology and interactions with humans

Climate change poses a pervasive threat to humans and wildlife by altering resource availability, changing co-occurrences, and directly or indirectly influencing human-wildlife interactions. For many wildlife agencies in North America, managing bears ( Ursus spp.) and human-bear interactions is a priority, yet the direct and indirect effects of climate change are exacerbating management challenges. Understanding the underlying ecological drivers of bear responses to climate variability and change, and the implications for conflict, will be critical for maintaining human-bear coexistence in North America. We synthesized 120 articles that identified direct and indirect mechanisms by which climate variability and change affect brown bears ( Ursus arctos ) and American black bears ( Ursus americanus ) in North America. The literature focused on examining climate impacts on bear diet, body size, habitat selection, space use, activity, denning chronology, and population demographics and dynamics. Across these categories, we summarized the documented and projected bear responses and resulting implications for human-bear interactions. Climate-driven changes in natural food availability were frequently implicated in influencing bear behavior and demography, and creating conditions under which interactions with humans are likely to increase. Bears in North America may face increased challenges as habitat and natural food availability continue to be altered by climate change. Our review provides a foundation upon which to identify climate drivers of bear ecology, conditions conducive to human-bear interactions, and adaptive management strategies. Given substantial evidence of climate impacts to bears, incorporating climate considerations into bear management can help managers strategically allocate resources and promote human-bear coexistence.

Biological Conservation↗

USGS2021

Divergent species‐specific impacts of whole ecosystem warming and elevated CO2 on vegetation water relations in an ombrotrophic peatland

Boreal peatland forests have relatively low species diversity and thus impacts of climate change on one or more dominant species could shift ecosystem function. Despite abundant soil water availability, shallowly rooted vascular plants within peatlands may not be able to meet foliar demand for water under drought or heat events that increase vapor pressure deficits while reducing near surface water availability, although concurrent increases in atmospheric CO 2 could buffer resultant hydraulic stress. We assessed plant water relations of co‐occurring shrub (primarily Rhododendron groenlandicum and Chamaedaphne calyculata ) and tree ( Picea mariana and Larix laricina ) species prior to, and in response to whole ecosystem warming (0 to +9°C) and elevated CO 2 using 12.8‐m diameter open‐top enclosures installed within an ombrotrophic bog. Water relations (water potential [Ψ], turgor loss point, foliar and root hydraulic conductivity) were assessed prior to treatment initiation, then Ψ and peak sap flow (trees only) assessed after 1 or 2 years of treatments. Under the higher temperature treatments, L. laricina Ψ exceeded its turgor loss point, increased its peak sap flow, and was not able to recover Ψ overnight. In contrast, P. mariana operated below its turgor loss point and maintained constant Ψ and sap flow across warming treatments. Similarly, C. calyculata Ψ stress increased with temperature while R. groenlandicum Ψ remained at pretreatment levels. The more anisohydric behavior of L. laricina and C. calyculata may provide greater net C uptake with warming, while the more conservative P. mariana and R. groenlandicum maintained greater hydraulic safety. These latter species also responded to elevated CO 2 by reduced Ψ stress, which may also help limit hydraulic failure during periods of extreme drought or heat in the future. Along with Sphagnum moss, the species‐specific responses of peatland vascular communities to drier or hotter conditions will shape boreal peatland composition and function in the future.

Minnesota↗

USGS2006

Landscape attributes and life history variability shape genetic structure of trout populations in a stream network

Spatial and temporal landscape patterns have long been recognized to influence biological processes, but these processes often operate at scales that are difficult to study by conventional means. Inferences from genetic markers can overcome some of these limitations. We used a landscape genetics approach to test hypotheses concerning landscape processes influencing the demography of Lahontan cutthroat trout in a complex stream network in the Great Basin desert of the western US. Predictions were tested with population- and individual-based analyses of microsatellite DNA variation, reflecting patterns of dispersal, population stability, and local effective population sizes. Complementary genetic inferences suggested samples from migratory corridors housed a mixture of fish from tributaries, as predicted based on assumed migratory life histories in those habitats. Also as predicted, populations presumed to have greater proportions of migratory fish or from physically connected, large, or high quality habitats had higher genetic variability and reduced genetic differentiation from other populations. Populations thought to contain largely non-migratory individuals generally showed the opposite pattern, suggesting behavioral isolation. Estimated effective sizes were small, and we identified significant and severe genetic bottlenecks in several populations that were isolated, recently founded, or that inhabit streams that desiccate frequently. Overall, this work suggested that Lahontan cutthroat trout populations in stream networks are affected by a combination of landscape and metapopulation processes. Results also demonstrated that genetic patterns can reveal unexpected processes, even within a system that is well studied from a conventional ecological perspective. ?? Springer 2006.

Landscape Ecology↗

USGS2026

Life-history stages and behavior influence demographic classification of moose captured on remote cameras

Obtaining accurate information on demographic states, such as the age and sex classes of animals, is an important step for monitoring wildlife populations. Traditionally, demographic data are collected from harvest, aerial surveys and telemetry studies. However, these methods can be expensive, limited to small spatial scales, or biased due to human behavior. Remote cameras have become a mainstay for studying and monitoring wildlife as they are relatively inexpensive, can be deployed over large spatial scales, and effort can be accounted for during surveys. For some species, a variety of demographic information, such as sex and age classes, can be obtained from pictures. Moose Alces alces are a photogenic species found across boreal and semi-boreal forests of the Northern Hemisphere. Previous studies have used demographic data from remote cameras to estimate demographic parameters and population dynamics. A primary assumption is that these age and sex classes are accurately classified. However, numerous factors can influence the ability of observers to identify age and sex classes of moose captured on cameras. We used data from 84 cameras from a 3-year period (2021–2024) in northern Maine, USA, to evaluate how temporal, environmental, site-level, and endogenous factors influence observers' ability to classify age and sex classes of moose. Using Bayesian categorical regression models, we found that temporal variability, position and proximity of moose from cameras, and the behavior of moose influenced our ability to identify age and sex classes. This information can be used to decide which periods to use data for population modeling and how to design studies to reduce the amount of uncertainty associated with different age and sex classes. We anticipate that our approach could also be used for other species whose age and sex classes can be differentiated using remote cameras.

Maine↗

USGS2005

Crane reproductive physiology and conservation

Some unique features of crane reproduction, management, and conservation are described. Because cranes are sexually monomorphic, sexing is difficult and must be accomplished using behavior, laparoscopy, cloacal examination, genetic techniques, or fecal steroid analysis. Although husbandry techniques for cranes are similar to those used with other nondomestic birds, a number of basic characteristics, such as extreme aggressiveness, imprinting by the crane chick on man, a delayed molt in the immature crane, delayed sexual maturity, and infertility, pose special problems for the propagator. Artificial insemination is a practical solution to crane infertility. Vigorous captive management and propagation efforts must become increasingly important if several endangered crane species are to survive the continuing decline in wild populations. The ultimate goal is the restoration of suitable habitat and sustainable native populations.

Zoo Biology↗

USGS1998

An individual-based, spatially-explicit simulation model of the population dynamics of the endangered red-cockaded woodpecker, Picoides borealis

Spatially-explicit population models allow a link between demography and the landscape. We developed a spatially-explicit simulation model for the red-cockaded woodpecker, Picoides borealis , an endangered and territorial cooperative breeder endemic to the southeastern United States. This kind of model is especially appropriate for this species because it can incorporate the spatial constraints on dispersal of helpers, and because territory locations are predictable. The model combines demographic data from a long-term study with a description of the spatial location of territories. Sensitivity analysis of demographic parameters revealed that population stability was most sensitive to changes in female breeder mortality, mortality of female dispersers and the number of fledglings produced per brood. Population behavior was insensitive to initial stage distribution; reducing the initial number of birds by one-half had a negligible effect. Most importantly, we found that the spatial distribution of territories had as strong an effect on response to demographic stochasticity as territory number. Populations were stable when territories were highly aggregated, with as few as 49 territories. When territories were highly dispersed, more than 169 territories were required to achieve stability. Model results indicate the importance of considering the spatial distribution of territories in management plans, and suggest that this approach is worthy of further development.

North Carolina↗

USGS1987

Bird behavior and mortality in relation to power lines in prairie habitats

Research was conducted to determine the magnitude of avian mortality caused by power transmission lines in prairie habitats during the two spring and two fall migration periods between July 1980 and May 1982. Searches for dead birds were made at least twice weekly during each migration period. Study sites were selected to include 'worst-case' situations involving potentially large concentrations of birds.In total, 633 dead birds were found beneath 9.6 km of power lines. About 81% of the birds were found during fall migration. Removal of birds by scavengers was of minor, although local, importance, and observer error in finding birds was greatest in areas of dense vegetation. Total kill was estimated at 1,332 birds.Data were gathered on more than 7,000 bird flights observed in the vicinity of the power lines. Sixty-eight percent of the birds did not respond to the presence of the power lines. Flaring and climbing over the conductor or overhead ground wire occurred in about 25% of the flights. One hundred nine birds in 82 flocks were observed to collide with a power line. Of these birds, 87% flared to climb over the power line before colliding. The overhead ground wire was responsible for most deaths, as 102 of 109 birds collided with it.Whereas none of the mortality observed was considered to be biologically significant at the particular sites examined, the cumulative effect of mortality sustained from collisions with power lines may be important, particularly to populations of rare or endangered birds.

Technical Report↗

USGS2003

Western crevice and cavity-roosting bats

Among the 45 species of bats that occur in the United States (U.S.), 34 species regularly occur in western regions of the country. Many of these “western” species choose roost sites in crevices or cavities. Herein we provide an introduction to the biology of bats that roost in cavities and crevices and assess the challenges and opportunities associated with monitoring their populations. We reviewed recent studies and examined the U.S. Geological Survey Bat Population Database (BPD) for records of western bats using crevice and cavity roosts. We found records of 25 species of western bats that use crevice or cavity roosts for at least part of their annual cycle. There were relatively few (n = 92) observations or counts for these species in the BPD, representing only 6% of the observations in the database. This paucity of records likely reflects the difficulty of observing bats in such situations rather than actual use. We found no long-term data adequate for population trend analysis among this group of bats. Since the development of miniaturized radio transmitters, our knowledge about bats that roost in cavities and crevices has increased. Future challenges associated with monitoring these species will include understanding variability in the types of roosts used as well as the roost-switching behavior exhibited by many species.

Report↗

USGS2003

Western crevice and cavity-roosting bats

Book chapter↗

USGS2018

Pre‐fire drought and competition mediate post‐fire conifer mortality in western U.S. National Parks

Tree mortality is an important outcome of many forest fires. Extensive tree injuries from fire may lead directly to mortality, but environmental and biological stressors may also contribute to tree death. However, there is little evidence showing how the combined effects of two common stressors, drought and competition, influence post‐fire mortality. Geographically broad observations of three common western coniferous trees subjected to prescribed fire showed the likelihood of post‐fire mortality was related to intermediate‐term (10 yr) pre‐fire average radial growth, an important component of tree vigor. Path analysis showed that indices of competition and drought stress prior to fire can be described in terms of joint effects on growth, indirectly affecting post‐fire mortality. Our results suggest that the conditions that govern the relationship between growth and mortality in unburned stands may also apply to post‐fire environments. Thus, biotic and abiotic changes that affect growth negatively (e.g., drought stress) or positively (e.g., growth releases following thinning treatments) prior to fire may influence expressed fire severity, independent of fire intensity (e.g., heat flux, residence time). These relationships suggest that tree mortality may increase under stressful climatic or stand conditions even if fire behavior remains constant.

Ecological Applications↗

USGS2016

Ecological requirements for pallid sturgeon reproduction and recruitment in the Missouri River—A synthesis of science, 2005 to 2012

This report is intended to synthesize the state of the scientific understanding of pallid sturgeon ecological requirements to provide recommendations for future science directions and context for Missouri River restoration and management decisions. Recruitment of pallid sturgeon has been low to non-existent throughout its range. Emerging understanding of the genetic structure of pallid sturgeon populations sets a broad framework for species and river management decisions, including decisions about managing the future genetic diversity of the species, but also decisions about where and what type of river restoration actions will be effective for subpopulations of this highly migratory species. Adult pallid sturgeon may migrate hundreds of kilometers (km) to spawn and their progeny may disperse even greater distances downstream as drifting free embryos. As a result of their complex life history pallid sturgeon naturally exploit a wide range of habitats during their life cycles. The construction of dams and reservoirs has fragmented habitats and may have shifted Missouri River subpopulations downstream. Research has not identified one primary biological or ecological constraint that appears to limit populations of the pallid sturgeon. With the present (2013) state of knowledge many life stages and life-stage transitions cannot be ruled out as contributing to recruitment failure. Biological opinions in 2000 (and amended in 2003) presented the dominant hypotheses for recruitment failure that existed at that time. Emphasis was on the role of the flow regime, specifically spring flow pulses (“spring rises”), to condition spawning substrate and cue reproductive aggregations and migrations, and on low flows and additional slow, shallow-water area to serve as rearing habitat for age-0 to juvenile pallid sturgeon. Studies on spawning habitat dynamics have documented that habitat patches selected for spawning by fish in the Lower Missouri River (Missouri River downstream from Gavins Point Dam to the confluence with the Mississippi River) are dominantly on outside, revetted bends in the deepest, fastest, and most turbulent water. Studies in more natural habitat on the Yellowstone River have documented spawning in convergent flow in the middle of the channel on discrete patches of gravel within a sand-dominated channel, an arrangement that may be more effective in attracting aggregations of reproductive fish compared to the nearly continuous revetment on the Lower Missouri River. Pallid sturgeon spawn in the spring and early summer during periods of increasing day length. Water temperature consistently exerts a threshold effect for spawning at 16–18 °C. In addition, the role of water temperature is indicated by pauses and reversals in upstream migrations that have been associated with cold weather fronts that create a transient decrease in water temperature. From 2005 to 2012 on the Lower Missouri River, no obvious relations between flow pulses and fish movements and spawning behaviors have been apparent. However, pallid sturgeon tracking at the Upper Missouri–Yellowstone confluence indicates that in most years, most telemetered pallid sturgeon migrate out of the Missouri River and into the Yellowstone River in the June–July timeframe in association with the spring pulse. This pattern was disrupted in 2011 when a high flow pulse with warm temperatures and high turbidity emanated from the Milk River, followed by record releases from Fort Peck Dam, and 36–39 percent of the telemetered population migrated up the Upper Missouri. This result supports the hypothesis that sufficiently large flow pulses may trigger migration and aggregation but it is not clear that functional pulses are within reservoir management authorities. Notably, a pallid sturgeon free embryo was captured on the Upper Missouri River in 2011 and another single, genetically confirmed embryo was captured on the Yellowstone River in 2012. Research on free-embryo drift has indicated the potential for hundreds of miles of downstream dispersal. Lack of distance to accommodate the extended downstream dispersal period of free embryos on the Upper Missouri and Yellowstone Rivers is the predominant hypothesis for recruitment failure in the upper basin. Long drift distances in the Lower Missouri River may be responsible for shifting Lower Missouri River sub-populations further into the Middle Mississippi River. Physical understanding of drift processes indicates that mean velocities could be slowed through decreased discharges or increased channel hydraulic radius (width and topographic diversity) to reduce free-embryo dispersal distances. In addition, the probability that free embryos are transported into and retained in channel-margin habitats is theoretically amenable to channel re-engineering that would increase cross-channel secondary currents in bends or channel expansions. Considerable uncertainty persists, however, about whether extended drift of Lower Missouri River larvae is responsible for recruitment failure. If drift distance is limiting, it is important to discern whether it would be advisable to retain larvae within the Missouri River, and where along the river restoration projects should be placed to optimize survival and growth of age-0 and juvenile sturgeon. Longitudinal differences in female pallid sturgeon fecundity lend support to the hypothesis that recruitment failure may be due, in part, to fish having insufficient nutrition to produce the numbers of gametes needed for the population to grow, perhaps because of simultaneous declines in prey-fish populations and their habitats. Establishing a chain of causality from habitat decline, to prey-fish populations, to sturgeon diets, to sturgeon fecundity, and to pallid sturgeon population growth presents a considerable scientific challenge. In addition to the dominant hypotheses relating pallid sturgeon populations to changes in flow regime and channel morphology, other factors have been identified that might be sources of stress and contribute to recruitment failure. Among these are water quality and contaminants. Ambient water-quality monitoring on the Missouri River has demonstrated summer episodes when dissolved oxygen dips below 5 milligrams per liter, a threshold that may be stressful especially to age-0 and juvenile sturgeon. Documented cases of intersex in shovelnose and pallid sturgeon indicate that agricultural and municipal sources of endocrine disrupting chemicals also may have a role in pallid sturgeon recruitment failure. Scientific understanding of the ecological requirements of pallid sturgeon has increased almost exponentially in the last two decades, and efforts are now turning from understanding fundamental biology of the species to quantifying how population dynamics relate to potential management actions. Progress in developing the science needed to inform management actions on the Missouri River may benefit from continuation of monitoring of reproductive cycles, reproductive movements, growth, and survival of telemetry tagged adults, increased emphasis on focused, complementary field and laboratory studies of factors influencing early life history, implementation of studies to resolve the role of food limitations in growth, survival, and reproductive condition, and implementation of studies designed specifically to parameterize models linking management to populations.

Missouri River↗