Evaluating Causes of Population Change in North American Insectivorous Songbirds

Although the North American Breeding Bird Survey (BBS) is a principal source of information regarding populations of most North American bird species, many features of the survey complicate analysis of population change. Correlation studies based on BBS data cannot be used to define cause and effect relationships unambiguously. Recently, Böhning-Gaese et al. (1993) presented an analysis of population trends in insectivorous songbirds using data from the BBS. They concluded that predation has played an important role in influencing population trends. We review aspects of the analysis methods for estimating population trends (e.g., observer effects, data subset) and for associating mean trends with species attributes (e.g., confounding of attributes). Using alternative analyses of the same BBS data, we demonstrate that the evidence that predation is associated with population declines is weaker than they suggested. Based on our analyses the only factor among those tested that was consistently associated with population trends was migration status (short-distance migrant/resident versus long-distance migrant) during the period 1978–1987. Also, we present evidence that the harsh winters of the mid-1970s severely depressed populations of short-distance migrant species and may have been responsible for the observed associations between migration status and population trends.     

Global population divergence of the sea star Hippasteria phrygiana corresponds to the onset of the last glacial period of the Pleistocene

Genetic structure and connectivity of populations of the globally distributed and eurybathic sea star Hippasteria phrygiana (Parelius 1768) were studied in 165 individuals sampled from three oceanic regions: the North Pacific Ocean, the South Pacific Ocean (considered to include the adjacent regions of the Southern Ocean and the southern Indian Ocean) and the North Atlantic Ocean. A nuclear gene region (ATP synthase subunit α intron #5, ATPSα) and a mitochondrial gene region (cytochrome oxidase subunit I, COI) were amplified and sequenced. Significant heterogeneity was detected in an AMOVA analysis among the three sampled oceanic regions for COI, but not for ATPSα. Neither gene showed significant genetic heterogeneity within the North Atlantic, as assessed by ΦST values. Significant heterogeneity was detected for COI (but not ATPSα) in the North Pacific, but the converse was true in the South Pacific. Coalescent simulations suggested that the three regions have been diverging with little or no gene flow for the past 50–75,000 years, a time frame that corresponds to the onset of the last glacial period of the Pleistocene. A possible genetic signature of recent population expansion (or non-neutrality) was detected for each gene in the North Pacific, but not in the other two oceanic regions.     

Song of the humpback whale — Population comparisons

Summary Humpback whale songs recorded on tropical calving grounds exhibit different dialects depending on the oceanic basin. Songs sampled simultaneously from two populations in the North Pacific (Hawaii and Mexico) were essentially identical. These North Pacific songs were clearly different from the song type shared by two populations in the North Atlantic (Cape Verde Islands and West Indies). Songs from the Southern Hemisphere (Tonga) represent a third distinct dialect. Our evidence shows that, despite annual change in song organization, significant differences in humpback song occur between isolated ocean basins, while only subtle differences exist within an oceanic population (Hawaii and Pacific Mexico).     

Extinction of Mammal Populations in Western North American National Parks

Patterns of local extinction of mammal populations in western North American parks were examined in relation to current biogeographic and population lifetime models. The analysis was based on species sighting records as of 1989. While western North American parks are obviously not true isolates, patterns of mammal extinction in them are nonetheless consistent with two predictions of the land-bridge island hypothesis. First, the number of extinctions has exceeded the number of colonizations since park establishment, and, second, the rate of extinction is inversely related to park area. Factors influencing the lifetime of mammal populations were evaluated using a stepwise multivariate survival analysis procedure for censored data. Survival time for mammal populations was positively related to estimated initial population size. After accounting for population size, species within the order Lagomorpha were particularly prone to extinction. Finally, after controlling for population size and taxon variation, survival time was positively related to age of maturity, indicating that species with longer generation times—age of maturity and generation time are highly correlated in mammals—persist longer in absolute time.     

Effects of an Invasive Plant on Population Dynamics in Toads

When populations decline in response to unfavorable environmental change, the dynamics of their population growth shift. In populations that normally exhibit high levels of variation in recruitment and abundance, as do many amphibians, declines may be difficult to identify from natural fluctuations in abundance. However, the onset of declines may be evident from changes in population growth rate in sufficiently long time series of population data. With data from 23 years of study of a population of Fowler's toad (Anaxyrus [ = Bufo] fowleri) at Long Point, Ontario (1989–2011), we sought to identify such a shift in dynamics. We tested for trends in abundance to detect a change point in population dynamics and then tested among competing population models to identify associated intrinsic and extrinsic factors. The most informative models of population growth included terms for toad abundance and the extent of an invasive marsh plant, the common reed (Phragmites australis), throughout the toads’ marshland breeding areas. Our results showed density‐dependent growth in the toad population from 1989 through 2002. After 2002, however, we found progressive population decline in the toads associated with the spread of common reeds and consequent loss of toad breeding habitat. This resulted in reduced recruitment and population growth despite the lack of significant loss of adult habitat. Our results underscore the value of using long‐term time series to identify shifts in population dynamics coincident with the advent of population decline. Efectos de una Planta Invasora sobre las Dinámica Poblacional de Sapos     

Building a better baseline to estimate 160 years of avian population change and create historically informed conservation targets

Globally, anthropogenic land-cover change has been dramatic over the last few centuries and is frequently invoked as a major cause of wildlife population declines. Baseline data currently used to assess population trends, however, began well after major changes to the landscape. In the United States and Canada, breeding bird population trends are assessed by the North American Breeding Bird Survey, which began in the 1960s. Estimates of distribution and abundance prior to major habitat alteration would add historical perspective to contemporary trends and allow for historically based conservation targets. We used a hindcasting framework to estimate change in distribution and abundance of 7 bird species in the Willamette Valley, Oregon (United States). After reconciling classification schemes of current and 1850s reconstructed land cover, we used multiscale species distribution models and hierarchical distance sampling models to predict spatially explicit densities in the modern and historical landscapes. We estimated that since the 1850s, White-breasted Nuthatch (Sitta carolinensis) and Western Meadowlark (Sturnella neglecta) populations, 2 species sensitive to fragmentation of oak woodlands and grasslands, declined by 93% and 97%, respectively. Five other species we estimated nearly stable or increasing populations, despite steep regional declines since the 1960s. Based on these estimates, we developed historically based conservation targets for amount of habitat, population, and density for each species. Hindcasted reconstructions provide historical perspective for assessing contemporary trends and allow for historically based conservation targets that can inform current management.     

Range-wide effects of breeding- and nonbreeding-season climate on the abundance of a Neotropical migrant songbird

Geographic variation in the population dynamics of a species can result from regional variability in climate and how it affects reproduction and survival. Identifying such effects for migratory birds requires the integration of population models with knowledge of migratory connectivity between breeding and nonbreeding areas. We used Bayesian hierarchical models with 26 years of Breeding Bird Survey data (1982-2007) to investigate the impacts of breeding- and nonbreeding-season climate on abundance of American Redstarts (Setophaga ruticilla) across the species range. We focused on 15 populations defined by Bird Conservation Regions, and we included variation across routes and observers as well as temporal trends and climate effects. American Redstart populations that breed in eastern North America showed increased abundance following winters with higher plant productivity in the Caribbean where they are expected to overwinter. In contrast, western breeding populations showed little response to conditions in their expected wintering areas in west Mexico, perhaps reflecting lower migratory connectivity or differential effects of winter rainfall on individuals across the species range. Unlike the case with winter climate, we found few effects of temperature prior to arrival in spring (March-April) or during the nesting period (May-June) on abundance the following year. Eight populations showed significant changes in abundance, with the steepest declines in the Atlantic Northern Forest (-3.4%/yr) and the greatest increases in the Prairie Hardwood Transition (4%/yr). This study emphasizes how the effects of climate on populations of migratory birds are context dependent and can vary depending on geographic location and the period of the annual cycle. Such knowledge is essential for predicting regional variation in how populations of a species might vary in their response to climate change.     

Using Age Structure to Detect Impacts on Threatened Populations: a Case Study with Steller Sea Lions

Abstract:   A delayed response to change is often a characteristic of long-lived species and presents a major challenge to monitoring their status. However, rapid shifts in age structure can occur even while population size remains relatively static. We used time-varying matrix models to study age-structure information as a tool for improving detection of survivorship and fecundity change and status. We applied the methods to Steller sea lions (  Eumetopias jubatus ), a long-lived endangered marine mammal found throughout the North Pacific Rim. Population and newborn counts were supplemented with information on the fraction of the population that was juvenile, obtained by measuring animals in aerial photographs taken during range-wide censuses. By fitting the model to 1976–1998 data, we obtained maximum-likelihood estimates and 95% confidence intervals for juvenile survivorship, adult survivorship, and adult fecundity in the mid-1980s, late 1980s, and 1990s. We used a series of nested models to test whether the data were best fit by a model with one, two, or three temporal changes in demographic rates, and we fit the models to different lengths of data to test the number of years of data needed to detect a demographic change. The declines in the early 1980s were associated with severely low juvenile survivorship, whereas declines in the 1990s were associated with disproportionately low fecundity. We repeated these analyses, fitting only to the count data without the juvenile-fraction information, to determine whether the age-structure information changed the conclusions and/or changed the certainty and speed with which demographic-rate changes could be detected. The juvenile-fraction data substantially improved the degree to which estimates from the model were consistent with field data and significantly improved the speed and certainty with which changes in demographic rates were detected.     

Structure and migration corridors in Pacific populations of the Swordfish Xiphius gladius, as inferred through analyses of mitochondrial DNA

 The swordfish Xiphias gladius is a migratory oceanic species distributed in sub-tropical and temperate waters worldwide. Studies utilizing mitochondrial DNA (mtDNA) have demonstrated genetic subdivision between ocean basins, as well as within the Atlantic basin. However, there has been no support of population subdivision within the Pacific. We sequenced 629 base pairs of the control region for 281 swordfish collected in the Pacific. A rate heterogeneity parameter, alpha, was found to be 0.201, indicating substantial variation in mutation rate within the control region of swordfish. Hierarchical analysis of molecular variance supported significant genetic structuring among Pacific populations. Northern and southern populations in the western Pacific were significantly divergent, while populations in the east appeared to be genetically continuous. Regression analysis supported a correlation of genetic differentiation with geographic distance along a U-shaped corridor of gene flow. These results reveal a pelagic biogeographic pattern heretofore unrecognized in the Pacific, and reject the null hypothesis that Pacific populations of swordfish are unstructured and comprise only a single homogeneous stock. Received: 10 November 1998 / Accepted: 4 February 2000     

Population Trends in Pacific Oceanic Sharks and the Utility of Regulations on Shark Finning

Accurate assessment of shark population status is essential for conservation but is often constrained by limited and unreliable data. To provide a basis for improved management of shark resources, we analyzed a long‐term record of species‐specific catches, sizes, and sexes of sharks collected by onboard observers in the western and central Pacific Ocean from 1995 to 2010. Using generalized linear models, we estimated population‐status indicators on the basis of catch rate and biological indicators of fishing pressure on the basis of median size to identify trends for blue (Prionace glauca), mako (Isurus spp.), oceanic whitetip (Carcharhinus longimanus), and silky (Carcharhinus falciformis) sharks. Standardized catch rates of longline fleets declined significantly for blue sharks in the North Pacific (by 5% per year [CI 2% to 8%]), for mako sharks in the North Pacific (by 7% per year [CI 3% to 11%]), and for oceanic whitetip sharks in tropical waters (by 17% per year [CI 14% to 20%]). Median lengths of silky and oceanic whitetip sharks decreased significantly in their core habitat, and almost all sampled silky sharks were immature. Our results are consistent with results of analyses of similar data sets. Combined, these results and evidence of targeted fishing for sharks in some regional fisheries heighten concerns for sustainable utilization, particularly for oceanic whitetip and North Pacific blue sharks. Regional regulations that prohibit shark finning (removal of fins and discarding of the carcass) were enacted in 2007 and are in many cases the only form of control on shark catches. However, there is little evidence of a reduction of finning in longline fisheries. In addition, silky and oceanic whitetip sharks are more frequently retained than finned, which suggests that even full implementation of and adherence to a finning prohibition may not substantially reduce mortality rates for these species. We argue that finning prohibitions divert attention from assessing whether catch levels are sustainable and that the need for management of sharks should not be addressed by measures that are simple to implement but complex to enforce and evaluate. Tendencias Poblacionales de Tiburones del Océano Pacífico y la Utilidad de Regulaciones sobre Cercenamiento de Aletas     

Dramatic declines in mussel bed community diversity: response to climate change?

Mussel beds along the wave-exposed coast of the eastern North Pacific Ocean serve as an important habitat, harboring a high diversity of species. A comparison of California mussel bed community diversity in 2002 to historical data (1960s to 1970s) revealed large declines (mean loss 58.9%), including some declines >141 species (approximately 80% loss). Concurrent work revealed inconsistent changes in mussel populations (biomass and bed thickness) along the California coast, suggesting that diversity declines may be related to large-scale processes rather than local habitat destruction. Potential factors causing declines in mussel community diversity are discussed, with regional climate change associated with the Pacific Decadal Oscillation and climate change induced alterations of ecological interactions and biological processes suggested as likely causes. Although extensive literature has predicted the potential effects of climate change on global diversity, this study is one of the few examples of declines attributed to climate change.     

Mitochondrial DNA sequence variation and phylogeography of oceanic insects (Hemiptera: Gerridae: Halobates spp.)

Relatively few insects have invaded the marine environment, and only five species of sea skaters, Halobates Eschscholtz (Hemiptera: Gerridae), have successfully colonized the surface of the open ocean. All five species occur in the Pacific Ocean, H. germanus White also occurs in the Indian Ocean, whereas H. micans Esch- scholtz is the only species found in the Atlantic Ocean. We sequenced a 780 bp long region of the mitochondrial cytochrome oxidase subunit I gene (COI) for a total of 66 specimens of the five oceanic Halobates species. Our purpose was to investigate the genetic variation within species and estimate the amount of gene flow between populations. We defined 27 haplotypes for H. micans and found that haplotype lineages from each of the major oceans occupied by this species are significantly different, having sequences containing five to seven unique base substitutions. We conclude that gene flow between populations of H. micans inhabiting the Atlantic, Pacific, and Indian Ocean is limited and hypothesize that these populations have been separated for 1 to 3 million years. Similarly, there may be limited gene flow between H. germanus populations found in the Pacific and Indian Ocean and between H. sericeus populations inhabiting the northern and southern parts of the Pacific Ocean. Finally, we discuss our findings in relation to recent hypotheses about the influence of oceanic diffusion on the distribution and population structure of oceanic Halobates spp. Received: 29 July 1999 / Accepted: 23 November 1999     

Short-term stability of genetic structure in populations of the sea anemone Metridium senile

Populations of the sea anemone Metridium senile (L.) were sampled from several locations in eastern North America in two series, one collected from 1977–1979 and the other from 1981–1985. Fourteen populations were sampled twice at one- to six-year intervals. Samples were analyzed for temporal differences in genetic composition at both the single locus and multiple locus levels. Overall patterns of geographic variation in allele frequency did not change between series. Regressions describing clines did not differ significantly, and loci not showing clinal variation in the first series remained similar in the second. Analysis of populations sampled twice produced no systematic evidence of change in allele frequency with time for any of four polymorphic loci. Comparisons with computer simulations of repeated sampling of multiple locus genotypes from panmictic populations with free recombination also revealed little temporal change at this level. One population showed possible evidence of recruitment from a different gene pool. Other significant departures from expectations reflected more reproducibility of genotype distributions between samples than expected for sexual populations. This excess stability likely results jointly from clonal reproduction and little sexual recruitment. Despite these indications of genetic stability in adult populations, newly settled juveniles were genetically different from resistant adults in one population, demonstrating the potential for genetic change by immigration. Successful sexual recruitment seems to be rare, even though larvae regularly settle from the plankton. Although interpretation of these results is somewhat limited by lack of knowledge of longevities and generation times, to the extent that they reflect longer-term trends, they suggest that at least some of the observed patterns of geographic variation in allele frequency probably result from natural selection.     

Symbiosis between bacteria and oceanic clonal sea star larvae in the western North Atlantic Ocean

Bacteria were present in the majority of clonal sea star bipinnariae (Luidia sp.) collected between June 1987 and August 1990 from oceanic populations in the subtropical western North Atlantic Ocean. Light and electron microscopy revealed dense aggregations of bacteria within lobes of the epidermal cuticle in the gastric region of larvae. Gram-negative rods and less-common elongate spiral bacteria were observed, always in close association with branching epidermal microvilli. Intact and partially digested bacteria occurred in vesicles and phagosomes within epidermal cells, a probable indication of phagocytotic activity by the host. The association of larvae of Luidia sp. and bacteria ranged widely on both geographical and temporal scales. During July and August 1988, larvae were abundant (1.6 to 11.1 m³) in the mixed surface layer (<10 to 100 m) of stations in the Gulf Stream, the Sargasso Sea and the North Equatorial Drift, and 96% (mean of 5 stations, n=8 to 10 larvae in each station) of the individuals surveyed harbored bacteria beneath their cuticle. In a single station on the Florida Current sampled during winter, spring, and summer months between 1989 and 1991, 79 to 90% of the larvae harbored bacteria. The incidence of symbiosis was higher in actively cloning larvae than in non-cloning larvae. This is the first documentation of a symbiosis in field populations of echinoderm larvae.     

Patterns of mtDNA diversity in North Atlantic populations of the mussel Mytilus edulis

Patterns of (female) mitochondrial DNA diversity were investigated in the blue mussel Mytilus edulis. Mytilus edulis is a ubiquitous member of contemporary North Atlantic hard-substrate communities and well represented in studies of this region. Mytilus edulis was surveyed in North America and Europe, as well as mid-Atlantic sites in Greenland, Iceland, and the Faroe Islands. Mitochondrial DNA sequences revealed considerable population structure but no monophyly of haplotypes between any major regions. Coalescent analyses suggest that migration across the Atlantic Ocean has prominently been from North American source populations and that Greenland was colonized recently and exclusively from North America. In North America, there was support for two regional groups along the North American coastline. Surprisingly, we also found evidence of recombination between some Mytilus edulis and Mytilus galloprovincialis female mtDNA sequences, particularly in northern Europe. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. An erratum to this article can be found at     

Mitochondrial DNA phylogeography of the harbour porpoise Phocoena phocoena in the North Pacific

Genetic structure and phylogeography of the harbour porpoise Phocoena phocoena in the North Pacific were examined using 358 bps sequences from the 5′ end of the mitochondrial DNA control region including those reported previously and newly obtained from the west Pacific. AMOVA and pairwise population φ _st estimates clearly revealed genetic differentiation between an east/south and a north/northwest group with the break along the Pacific Rim at British Columbia. In addition, nested clade phylogeographical analysis, neutrality tests, mismatch distribution analysis, genetic diversities and Mantel test, suggested that the observed genetic structure might have been influenced by contiguous range expansion with restricted gene flow in the direction from south to north along the North American coasts and east to west along the Pacific Rim in the middle to late Pleistocene.     

Analysis of population genetic structure of the green sea urchin (<Emphasis Type="Italic">Strongylocentrotus droebachiensis</Emphasis>) using microsatellites

We measured within- and among-population genetic variation in the green sea urchin (Strongylocentrotus droebachiensis) at 11 sites in the north Atlantic and northeast Pacific by using four-locus microsatellite genotypes. We found no differentiation among populations from Atlantic Canada, but strong differentiation across the north Atlantic and between the Atlantic and Pacific samples. High inbreeding coefficients at three loci are consistent with high variance in reproductive success. One population that was recently decimated by disease was strongly differentiated from some others, but there was little differentiation otherwise among populations in Atlantic Canada. On a larger scale, populations in Atlantic Canada were more similar to a population from the north Pacific than to populations in the northwest Atlantic. Differentiation among populations at this large spatial scale is consistent with biogeographical hypotheses of: (1) Pleistocene population reduction and isolation in the northeast Atlantic, but (2) extinction in the northwest Atlantic followed by extensive recolonization from the Pacific. In contrast to other recent studies of trans-Atlantic organisms, we found no evidence of extensive gene flow across the north Atlantic.Communicated by R.J. Thompson, St. John's     

Are Declines in North American Insectivorous Songbirds Due to Causes on the Breeding Range?

Previous studies have reported a recent decline in breeding populations of migratory songbirds in eastern and central North America. Several explanations have been suggested: deforestation on the wintering grounds in the tropics and habitat loss, increased predation pressure, and increased cowbirds parasitism on the breeding range. We used these factors to assign 47 species of insectivorous passerines to groups with contrasting vulnerability, and then used the North America Breeding Bird Survey to analyze population trends in these groups on a large continental scale. Variables indexing susceptibility to predation on the breeding ground were most strongly correlated with population trends form 1968 to 1987. During the period from 1978 to 1987, migratory status was also significantly associated with population trends long-distance migrants to the neotropics exhibited a small, nonsignificant decreasing trend, whereas residents and short-distance migrants increased strongly. During the same time period, the group of species with low nest location, open nest, and high cowbird parasitism declined significantly. Although it is difficult to separate the effects of multiple factors, our analyses suggest that predation on the breeding ground in North America has played a larger role in the decline of migratory songbirds than deforestation on the wintering grounds in the tropics.     

Application of Genetic Bottleneck Testing to the Investigation of Amphibian Declines: a Case Study with Natterjack Toads

Abstract: Declines of amphibians are causing widespread concern and in some cases have been explained by factors such as climate change and the spread of microbial pathogens. A problem remains, however, in the unequivocal demonstration of decline in situations where populations may be undergoing natural fluctuations in abundance. We describe the application of a genetic test for bottlenecks ( Cornuet & Luikart 1996 ) that should distinguish between natural oscillations and true population declines. British natterjack toads (   Bufo calamita  ) provide examples of populations that have not declined in recent decades, populations that have declined, and populations in which trends are unclear. Microsatellite allele frequency data from these populations were tested for heterozygote excess and shifts in allele frequency distributions, and inferences from these computations about bottlenecks (i.e., persistently smaller population sizes than the recent means) were compared with demographic information. The genetic test correctly identified bottlenecks and should therefore prove useful in demonstrating whether amphibian declines have occurred where long-term demographic time series are not available.     

Rapid declines of large mammal populations after the collapse of the Soviet Union

Anecdotal evidence suggests that socioeconomic shocks strongly affect wildlife populations, but quantitative evidence is sparse. The collapse of socialism in Russia in 1991 caused a major socioeconomic shock, including a sharp increase in poverty. We analyzed population trends of 8 large mammals in Russia from 1981 to 2010 (i.e., before and after the collapse). We hypothesized that the collapse would first cause population declines, primarily due to overexploitation, and then population increases due to adaptation of wildlife to new environments following the collapse. The long‐term Database of the Russian Federal Agency of Game Mammal Monitoring, consisting of up to 50,000 transects that are monitored annually, provided an exceptional data set for investigating these population trends. Three species showed strong declines in population growth rates in the decade following the collapse, while grey wolf (Canis lupus) increased by more than 150%. After 2000 some trends reversed. For example, roe deer (Capreolus spp.) abundance in 2010 was the highest of any period in our study. Likely reasons for the population declines in the 1990s include poaching and the erosion of wildlife protection enforcement. The rapid increase of the grey wolf populations is likely due to the cessation of governmental population control. In general, the widespread declines in wildlife populations after the collapse of the Soviet Union highlight the magnitude of the effects that socioeconomic shocks can have on wildlife populations and the possible need for special conservation efforts during such times. Declinación Rápida de las Poblaciones de Mamíferos Mayores después del Colapso de la Unión Soviética