Steven E. Travis

Ecological and Demographic Effects on Intraspecific Variation in the Social System of Prairie Dogs

Gunnisons prairie dogs (Cynomys gunnisoni) are social North American ground squirrels whose social system has been shown to vary with food resource distributions, as predicted by the habitat variability–mating system model. We expanded this model to include the effects of variations in population densities, in addition to resource distributions, on both the social system and the individual mating strategies of Gunnisons prairie dogs. Specifically, we predicted that monogamy would be associated with uniform resources, regardless of population density, giving way to polygyny with increasing resource patchiness at intermediate densities, and to multiple males and females at high population densities. In addition, we predicted inbreeding at low densities and outbreeding at high densities on the basis of naturally low levels of genetic eity in this species. We tested these predictions by comparing social systems and mating strategies at two prairie dog colonies near Flagstaff, Arizona, during a 3—yr period o...

Species-specific SSR alleles for studies of hybrid cattails (Typha latifolia × T. angustifolia; Typhaceae) in North America

UNLABELLED PREMISE Studies of hybridizing species are facilitated by the availability of species-specific molecular markers for identifying early- and later-generation hybrids. Cattails are a dominant feature of wetland communities, and a better understanding of the prevalence of hybrids is needed to assess the ecological and evolutionary effects of hybridization. Hybridization between Typha angustifolia and T. latifolia produce long-lived clones, known as Typha ×glauca, which are considered to be invasive. Although morphological variation in cattails makes it difficult to recognize early- and later-generation hybrids, several dominant, species-specific RAPD markers are available. Our goal was to find codominant, species-specific markers with greater polymorphism than RAPDs, to identify later-generation hybrids more efficiently. • METHODS We screened nine SSR (simple sequence repeat) loci that were described from populations in Ukraine, and we surveyed 31 cattail populations from the upper Midwest and eastern USA. • KEY RESULTS Seven SSR loci distinguished the parent taxa and were consistent with known species-specific RAPD markers, allowing easier detection of backcrossing. We used linear discriminant analysis to show that F(1) hybrid phenotypes were intermediate between the parent taxa, while those of backcrossed plants overlapped with the hybrids and their parents. Log(leaf length/leaf width), spike gap length, spike length, and stem diameter explained much of the variation among groups. • CONCLUSIONS We provide the first documentation of backcrossed plants in hybridizing cattail populations in Michigan. The diagnostic SSR loci we identified should be extremely useful for examining the evolutionary and ecology interactions of hybridizing cattails in North America.

DNA fingerprinting reveals low genetic diversity in Gunnison's prairie dog (Cynomys gunnisoni)

The use of molecular techniques for the assessment of familial relationships among social species of mammals has become relatively commonplace. However, some species represent poor candidates for such studies due to naturally low levels of genetic diversity, leading to unacceptably large standard errors associated with estimates of relatedness. Here, we report on a preliminary study of genetic diversity within two populations of a social species of ground squirrel, Gunnisons prairie dog ( Cynomys gunnisoni ) using DNA fingerprinting. We observed low levels of diversity in the form of large mean coefficients of genetic similarity among individuals occupying the same population. Overall similarity, determined from the combined data, yielded by three minisatellte probes, ranged from 55 to 61%. These values place Gunnisons prairie dog at the extreme upper end of the range of similarity values reported for outbred species of mammals (ca. 0.20-0.50). As a partial means of explaining these results, and as a means of comparing our results to those of similar studies using allozymes, we determined the level of differentiation between our two study colonies in the form of an F -statistic analog. A value of 0.11 (± 2.26 × 10−3) was obtained and is similar to values reported from allozyme studies (0.07–0.12). A significance test of this value yielded a positive result (D = 5.63, d.f. = 1, P < 0.025), demonstrating that gene flow between populations is limited, a factor that may help to maintain low levels of diversity.

Predicting performance for ecological restoration: a case study using Spartina alterniflora

The success of population-based ecological restoration relies on the growth and reproductive performance of selected donor materials, whether consisting of whole plants or seed. Accurately predicting performance requires an understanding of a variety of underlying processes, particularly gene flow and selection, which can be measured, at least in part, using surrogates such as neutral marker genetic distances and simple latitudinal effects. Here we apply a structural equation modeling approach to understanding and predicting performance in a widespread salt marsh grass, Spartina alterniflora, commonly used for ecological restoration throughout its native range in North America. We collected source materials from throughout this range, consisting of eight clones each from 23 populations, for transplantation to a common garden site in coastal Louisiana and monitored their performance. We modeled performance as a latent process described by multiple indicator variables (e.g., clone diameter, stem number) and estimated direct and indirect influences of geographic and genetic distances on performance. Genetic distances were determined by comparison of neutral molecular markers with those from a local population at the common garden site. Geographic distance metrics included dispersal distance (the minimum distance over water between donor and experimental sites) and latitude. Model results indicate direct effects of genetic distance and latitude on performance variation among the donor sites. Standardized effect strengths indicate that performance was roughly twice as sensitive to variation in genetic distance as to latitudinal variation. Dispersal distance had an indirect influence on performance through effects on genetic distance, indicating a typical pattern of genetic isolation by distance. Latitude also had an indirect effect on genetic distance through its linear relationship with dispersal distance. Three performance indicators had significant loadings on performance alone (mean clone diameter, mean number of stems, mean number of inflorescences), while the performance indicators mean stem height and mean stem width were also influenced by latitude. We suggest that dispersal distance and latitude should provide an adequate means of predicting performance in future S. alterniflora restorations and propose a maximum sampling distance of 300 km (holding latitude constant) to avoid the sampling of inappropriate ecotypes.

Differentiating individuals and populations of mule deer using DNA

We developed individual- and population-specific genetic markers for mule deer (Odocoileus hemionus hemionus) to provide wildlife managers with an efficient means of prosecuting poaching cases and analyzing population structure at the genetic level. We used DNA fingerprinting and analyses of mitochondrial DNA (mtDNA) to investigate genetic variation within and among mule deer populations occupying geographic regions adjacent to the North and South rims of the Grand Canyon, Arizona. No 2 deer shared the same DNA banding pattern (i.e., DNA fingerprints were unique to individuals). Fingerprinting further revealed fixation indices (F-statistics) indicative of little genetic differentiation between populations on the North and South rims, but analyses of single-strand conformation polymorphisms (SSCP) of mtDNA in the control region, or D-loop, revealed differentiation (P < 0.001). We identified 7 mtDNA genotypes with ≥1 mitochondrial type characteristic of subpopulations found on the North and South rims. DNA fingerprinting and SSCP provided useful tools for identifying individuals and populations of mule deer from tissue samples.

Red mangrove life history variables along latitudinal and anthropogenic stress gradients

Mangroves migrate northward in Florida and colonize marshes historically dominated by salt marsh species. In theory, this migration should be facilitated by greater numbers of propagules stemming from increased reproductive activity and greater genetic variability caused by outcrossing. We aimed to determine if stand reproduction and % outcrossing were affected by cold stress (stress increases with latitude), anthropogenic stress (human population density as a proxy), and years since a major hurricane. Further, we wished to determine if mutation rate varied with the stressors and if that affected stand reproduction. Both coasts of Florida from the southern Florida Keys to Tampa Bay on the Gulf of Mexico coast, and Merritt Island on the Atlantic coast. We conducted field surveys of frequency of reproducing trees (104,211 trees surveyed in 102 forested stands), incidence of trees showing albinism in propagules, and% outcrossing estimated from the ratio of albino:normal propagules. Structural equation modeling (SEM) was used to test a conceptual model that served as a multivariate hypothesis. Reproductive frequencies varied by site and increased with latitude although more strongly on the Gulf coast. Our SEM results indicate that outcrossing increases in this predominately selfing species under conditions of cold and anthropogenic stress, and that this increases reproductive output in the population. Further, we find that increased mutation rates suppress stand reproductive output but there is no significant relationship between outcrossing and mutation rate. Tree size responded to stressors but did not affect stand reproduction. Reproduction increased with years since major hurricane. Potential for colonization of northern Florida salt marshes by mangroves is enhanced by increased reproductive rates that provides more propagules and outcrossing that should enhance genetic variation thereby promoting adaptation to novel environmental conditions. Natural (cold) stress reduced mutation rate and increased stand reproductive output but anthropogenic stress did the opposite.

Use of Morphometric Measurements to Differentiate between Species and Sex of King and Clapper Rails

Abstract. King Rails (Rallus elegans) and Clapper Rails (Rallus longirostris) are large, secretive waterbirds whose ranges overlap in brackish marshes along the Atlantic and Gulf Coasts. King and Clapper Rails are difficult to separate by physical appearance and there is currently no reliable method to distinguish between the two species. Here, the relative effectiveness of using discriminant analysis of morphometric measurements to identify and sex King and Clapper Rails was examined. Mean measurements of wing, tarsus, and weight were different between male King and Clapper Rails and between female King and Clapper Rails. However, for all measurements except culmen, male Clapper Rails and female King Rails were not different. Discriminate analysis of morphometric measurements revealed that wing, tarsus, and culmen measurements differentiated between King and Clapper Rails, but cross-validation results for male Clapper Rails were only 73%. Male King Rails were larger than female King Rails for all morphometric measurements and male Clapper Rails were larger than female Clapper Rails for all morphometric measurements except for the tail. Wing and tarsus measurements differentiated between male and female King Rails and wing, tarsus, and culmen measurements differentiated between male and female Clapper Rails.

Nestling sex ratios do not support long-term parity in two species with different life-history strategies

ABSTRACT To maximize fitness, breeding adults may respond to environmental processes by adjusting their progenys sex ratios. R. A. Fisher in 1930 hypothesized that frequency-dependent selection would result in equal investment in sons and daughters over the long term, yielding a balanced sex ratio if the costs of raising a son and daughter are equal. Diverse hypotheses have tried to explain population and brood-by-brood deviations from this mean as well as annual variation by focusing on adult sex ratios, resources, abiotic conditions, and female and male quality. We collected data in 2002–2010 to explore population-level variation in nestling sex ratios in 2 migratory grassland songbird species: the Bobolink (Dolichonyx oryzivorus) and Savannah Sparrow (Passerculus sandwichensis). These species differ in migratory strategy (long-distance vs. short-distance), and morphological dimorphism. Fishers hypothesis was rejected for Savannah Sparrows (n = 684 nestlings; 39% male) but not rejected for Bobolinks (n = 390 nestlings; 53.8% male). No relationship was found between nestling and adult sex ratios measured in the same year. In descriptive analyses at the brood level, male and female body size and age, and ecological conditions (temperature and precipitation) failed to predict nestling sex ratios. Although male nestlings were heavier than female nestlings and resource availability changed through the season, these factors did not influence sex ratios relative to female body size or seasonality. For Savannah Sparrows, larger broods tended to be male-biased. While we were otherwise not able to explain deviation in offspring sex ratio for Savannah Sparrows, our results suggest that the ecological and evolutionary pressures that affect sex ratios may be both species- and population-specific.

Competitive interactions between native Spartina alterniflora and non-native Phragmites australis depend on nutrient loading and temperature

We explored the nature and impact of competitive interactions between the salt marsh foundational plant Spartina alterniflora and invasive Phragmites australis in New England under varying levels of anthropogenic influence from nutrient loading and temperature warming. Plants were grown with and without competition in mesocosms over a four-month growing season. Mesocosms were split evenly among three levels of nutrient additions and two temperatures varying by an average of ~3° C, manipulated using small greenhouses. We measured aboveground productivity as total biomass, numbers of new stems, and mean stem height. Nutrient enrichment increased all growth parameters, while competition generally reduced aboveground biomass and the production of new stems in both species. Most importantly, smooth cordgrass suffered no negative consequences of competition when no nutrients were added and temperature was elevated. The results of this study suggest that minimizing nutrient loading into coastal marshes could be an important factor in slowing the spread of common reed into the low marsh zone of New England salt marshes as global temperatures continue to warm.

Strong associations between plant genotypes and bacterial communities in a natural salt marsh

Abstract Although microbial communities have been shown to vary among plant genotypes in a number of experiments in terrestrial ecosystems, relatively little is known about this relationship under natural conditions and outside of select model systems. We reasoned that a salt marsh ecosystem, which is characterized by twice‐daily flooding by tides, would serve as a particularly conservative test of the strength of plant–microbial associations, given the high degree of abiotic regulation of microbial community assembly resulting from alternating periods of inundation and exposure. Within a salt marsh in the northeastern United States, we characterized genotypes of the foundational plant Spartina alterniflora using microsatellite markers, and bacterial metagenomes within marsh soil based on pyrosequencing. We found significant differences in bacterial community composition and diversity between bulk and rhizosphere soil, and that the structure of rhizosphere communities varied depending on the growth form of, and genetic variation within, the foundational plant S. alterniflora. Our results indicate that there are strong plant–microbial associations within a natural salt marsh, thereby contributing to a growing body of evidence for a relationship between plant genotypes and microbial communities from terrestrial ecosystems and suggest that principles of community genetics apply to this wetland type.