Alien toxic toads suppress individual growth and phenotypic development of native predatory salamanders.

Biological invasions are key to understanding ecological processes that determine the formation of novel interactions. Alien species can negatively impact floral visitation to native species, but native species may also facilitate early establishment of closely related alien species by providing a preadapted pollinator community. We tested whether floral visitation to alien species depended on phylogenetic relatedness and floral similarity to native species. In a field experiment, we simulated the early stages of an invasion by adding potted alien plants into co‐flowering native communities. We paired each alien plant with a host native plant, and recorded floral visitation to them for 3,068 hr (totalling 84,814 visits). We used 34 alien and 20 native species in 151 species combinations. We tested whether the number of floral visits to alien plants, the proportion of visits to alien plant relative to visits to both alien and native plants, and the similarity in flower visitor compositions of alien and native plants depended on phylogenetic and floral trait distances between alien and native species. Floral visitation to alien species was highest when they had intermediate floral trait distances to native species, and either low or high phylogenetic distances. Alien species received more similar flower‐visitor groups to natives when they had low phylogenetic and either low or high floral trait distances to native species. Co‐flowering native species may facilitate floral visitation to closely related alien species, and distantly related alien plants seem to avoid competition for flower visitors with native plants. Alien species with similar floral traits to natives compete with them for flower visitors, and alien species with dissimilar floral traits may not share flower visitors with native species. Alien species with intermediate floral trait distances to natives are most likely to receive flower visitors, as they are not too dissimilar and may still share flower visitors with native species, but not too similar to compete for flower visitors with them. The non‐linear patterns between floral visitation and similarity of the alien and native species suggest that an interplay of facilitation and competition simultaneously drives the formation of novel plant‐pollinator interactions. Read the free Plain Language Summary for this article on the Journal blog.

Through artificial selection and inbreeding, strains of laboratory mice have been developed that vary in the expression of a single or suite of desired traits valuable to biomedical research. In addition to the selected trait(s), these strains also display variation in pelage color, body size, physiology, and life history. This article exploits the broad phenotypic variation across lab mouse strains to evaluate the relationships between life history and metabolism. Life history variation tends to exist along a fast-slow continuum. There has been considerable interest in understanding the ecological and evolutionary factors underlying life history variation and the physiological and metabolic processes that support them. Yet it remains unclear how these key traits scale across hierarchical levels, as ambiguous empirical support has been garnered at the intraspecific level. Within-species investigations have been thwarted by methodological constraints and environmental factors that obscure the genetic architecture underlying the hypothesized functional integration of life history and metabolic traits. In this analysis, we used the publicly available Mouse Phenome Database by the Jackson Laboratory to investigate the relationships among life history traits (e.g., body size, reproduction, and life span) and metabolic traits (e.g., daily energy expenditure and insulin-like growth factor 1 concentration). Our findings revealed significant variation in reproductive characteristics across strains of mice as well as relationships among life history and metabolic traits. We found evidence of variation along the fast-slow life history continuum, though the direction of some relationships among these traits deviated from interspecific predictions laid out in previous literature. Furthermore, our results suggest that the strength of these relationships are strongest earlier in life.

To understand the growth and nutritional strategies of two alien Asteraceae species in both nutrient‐rich and nutrient‐poor environments by measuring the stoichiometric characteristics of plants and soil nutrients, two alien species Bidens pilosa and Mikania micrantha from subtropical zone of southeast China was studied. The two species grow in P‐rich environments and/or have a strong capacity for absorption and utilization of P. The two invasive alien species studied used diverse nutrition and growth strategies under different habitats. For B. pilosa under low‐N environments, leaves showed lower N:P and C:P than in other habitats. In contrast, roots showed greater N:P, C:P and C:N ratios, suggesting a “resource conservative strategy,” whereas under high‐N and high‐P, or high‐N and low‐P environments, B. pilosa roots had lower N:P and C:P ratios, suggesting a “growth competitive strategy.” Conversely, M. micrantha did not match either strategy in high‐ or low‐resource habitats. The growth of B. pilosa is resource‐dependent, mainly driven by C and N content, and C:N ratio in the soil, as well as allocation of P within the plant. The growth of M. micrantha is less limited by soil resources, but mainly by tradeoffs between allocation to leaves versus roots. There are various growth and nutrient strategies under different resource environments (e.g., resource conservative strategy, and/or growth competitive strategy), as well as fundamental trade‐offs between life history traits, which all contribute to the successful invasion by alien species.

The molecular mechanisms of freezing tolerance are unresolved in the perennial trees that can survive under much lower freezing temperatures than annual herbs. Since natural conditions involve many factors and temperature usually cannot be controlled, field experiments alone cannot directly identify the effects of freezing stress. Lab experiments are insufficient for trees to complete cold acclimation and cannot reflect natural freezing-stress responses. In this study, a new method was proposed using field plus lab experiments to identify freezing tolerance and associated genes in subtropical evergreen broadleaf trees using Camellia oleifera as a case. Cultivated C. oleifera is the dominant woody oil crop in China. Wild C. oleifera at the high-elevation site in Lu Mountain could survive below -30°C, providing a valuable genetic resource for the breeding of freezing tolerance. In the field experiment, air temperature was monitored from autumn to winter on wild C. oleifera at the high-elevation site in Lu Mountain. Leave samples were taken from wild C. oleifera before cold acclimation, during cold acclimation and under freezing temperature. Leaf transcriptome analyses indicated that the gene functions and expression patterns were very different during cold acclimation and under freezing temperature. In the lab experiments, leaves samples from wild C. oleifera after cold acclimation were placed under -10°C in climate chambers. A cultivated C. oleifera variety "Ganwu 1" was used as a control. According to relative conductivity changes of leaves, wild C. oleifera showed more freezing-tolerant than cultivated C. oleifera. Leaf transcriptome analyses showed that the gene expression patterns were very different between wild and cultivated C. oleifera in the lab experiment. Combing transcriptome results in both of the field and lab experiments, the common genes associated with freezing-stress responses were identified. Key genes of the flg22, Ca2+ and gibberellin signal transduction pathways and the lignin biosynthesis pathway may be involved in the freezing-stress responses. Most of the genes had the highest expression levels under freezing temperature in the field experiment and showed higher expression in wild C. oleifera with stronger freezing tolerance in the lab experiment. Our study may help identify freezing tolerance and underlying molecular mechanisms in trees.

Life history traits and stress tolerance were studied in four domestic species of Drosophila–D. melanogaster, D. simulans, D. auraria and D. immigrans– to understand how they adapt to their environments. In all species, larval weight approximately doubled in 1 day. The relative egg weight (egg weight : pupal weight) was smaller and the larval period was longer in D. immigrans than in the other three species. The pupal period was the longest in D. auraria. However, the adaptive significance of these differences in larval and pupal periods was not clear. The pupal case was generally thicker in the larger species, probably to support the larger pupal body. The start of oviposition was earliest and reproductive effort was greatest in female D. simulans, followed by female D. melanogaster. In contrast, starvation tolerance and the increase in bodyweight after eclosion was greater in D. immigrans and D. auraria than in the other two species. Pupal desiccation tolerance was greatest in D. melanogaster and lowest in D. auraria, and the less tolerant species seemed to select more humid sites for pupation. Adult tolerance to desiccation was greatest in D. melanogaster and lowest in D. simulans. In contrast, adult cold tolerance was greater in D. auraria and adult heat tolerance was lower in D. immigrans than in the other species. These differences in life history traits and stress tolerance represent the Drosophila species differential adaptations, and are assumed to allow coexistence of the species.

One of the most dominant concepts in invasion ecology is the stage-based invasion model, consisting of transport, introduction, establishment and spread. Many species fail at one of the stages, with propagule pressure (i.e. number of introduced individuals) identified as a principal factor affecting establishment success. Population characteristics such as phenotypic plasticity and beneficial life history traits may facilitate successful transition of species through different stages of the process; however, studies on the latter are not so common and most of those studies focus on terrestrial taxa. In this study, we hypothesized seven life history traits that may be beneficial for invasion success of aquatic species, and determined those traits for established non-indigenous species (NIS) in the North and Baltic Seas (i.e. marine environment) and Great Lakes-St. Lawrence River regions (i.e. freshwater environment). This is the first study that examined certain life history traits of all NIS established in particular regions, as well as compared those traits between marine and freshwater habitats. Our study determined some differences in life history traits between NIS in the marine and freshwater habitats. Those differences were connected to different taxonomic groups that were dominant NIS in these two types of habitats. Furthermore, species originating from different donor regions had also different life history traits. The majority of NIS in both regions were r-strategists. There was a significantly higher number of NIS that were able to reproduce both asexually and sexually and to produce dormant stages in the freshwater than in marine habitat. Finally, as r-strategy, asexual reproduction and dormancy were dominant traits of NIS in the freshwater habitat, freshwater ecosystems may be under greater invasion risk than marine ones, as those traits reduce both demographic and environmental stochasticity during the invasion process.

Chapter 19 - Life histories in groundwater organisms

Primates show life history traits similar to those of cetaceans, such as small litter size, long gestation, long lactation, and long lifespan, in spite of striking contrasts in habitats, diet, mobility, and range size between them. Ecological factors (food and predation) may influence their life history traits in various ways, but social factors (social structure and reproductive strategies) may be more important for the life history of primates, in which both sexes live together even outside the breeding season. Group-living primates are classified into female-bonded species and female-dispersal species, based on the patterns of female dispersal after maturity. A comparison of life history parameters shows that female-dispersal species have a slower life history (gestation length, weaning age, age at first reproduction, and interbirth interval) than the female-bonded species, except for neonatal weight and weaning weight, which may be determined in relationship to female body weight. To elucidate factors promoting the slow life history, we focus on Atelinae and Hominidae (female-dispersal species) and examine their interspecific and intraspecific variation in social structure and male reproductive tactics in relationship to life history traits. Most Atelinae species form multimale and multifemale groups, and variation in their life history features may reflect relationships among males and their reproductive tactics. In howler monkeys, both males and females disperse, and infanticide by males may lead to a fast life history. In other Atelines, infanticide rarely occurs, although it has the effect of reducing interbirth interval. Forcible copulation by males occasionally occurs in spider monkeys. Variations in grouping among females reflecting their flexible foraging efforts according to distribution of high-quality foods may have some effects on the fast–slow continuum in the life history features of female Atelinae. Hominidae exhibit larger variations in life history features than Atelinae, probably because of their diverse social structure. Solitary nature and male reproductive tactics may have great influences on the life history of female great apes. Female orangutans, who usually live a solitary life, show the slowest life history. Maturing female orangutans need a longer time to establish their own home range and relationships with reproductive mates than female chimpanzees and gorillas, who transfer into other groups immediately after emigration. Female gorillas show the lowest age at first reproduction and the shortest interbirth interval. Intensive caretaking of the immature by male gorillas may facilitate early weaning, and infanticide by males may promote a prolonged bonding between a protector male and females to shorten the interbirth interval. Similar life history traits have been found in four long-term study sites of chimpanzees. Only females at Bossou show a fast life history, probably the result of high-quality foods and single male group composition under isolated conditions. The more frequent and stable association between females and males and more promiscuous mating in bonobos may facilitate the search for mating partners and lead to a shorter interbirth interval than chimpanzees. Frugivorous orangutans and chimpanzees may suffer more costs of female dispersal through decreased foraging efficiency than folivorous gorillas, and chimpanzees with fission–fusion grouping may suffer more social stress than gorillas in highly cohesive groups. Such differences may generally shape the fast–slow continuum of life history in female-dispersal primate species.KeywordsAge at first reproductionAtelinaeFemale-dispersal speciesHominidaeInterbirth intervalLife history

Terrestrial plant communities often become invaded by alien species, which may benefit from high growth rates, strong phenotypic plasticity and reduced negative impacts from local soil communities. At the same time, terrestrial communities are increasingly more often exposed to periods of drought. However, how drought affects the competition between alien and native plants directly, and indirectly, through changing impacts of soil communities on plant performance, remains poorly understood. Here, we performed a greenhouse pot experiment in which we examined biomass responses of five native and five naturalized alien species (all occurring in mesic grasslands) to drought and benign soil moisture conditions, while growing in interspecific, intraspecific or absence of competition, in the presence or absence of native soil biota. We expected that alien plant species are less negatively affected by soil biota, but more negatively affected by drought than native species, and that drought indirectly weakens soil‐community‐driven competitive benefits of alien plant species over native ones. On average, soil‐community effects on plant biomass were positive, but native performance was less positively affected by soil communities than alien performance, suggesting reduced impacts of soil‐borne enemies on alien plants. Drought more negatively affected alien‐ than native plant performance. Drought impacts on plant biomass did not depend on soil community presence, but in the presence of soil biota, plants overall invested more in root biomass when exposed to drought. The effects of competition were subtle and species‐specific. To better understand the observed positive soil‐community effects on plant performance in our study, we examined mycorrhizal root colonization of plants grown in absence of competition. Among‐species variation in mycorrhizal colonization explained plant performance differences between soils with and without live soil communities, indicating a key role for arbuscular mycorrhizal fungi as driver of plant performance. However, mycorrhizal colonization did not differ between alien and native plants and was unaffected by drought. Overall, our study suggests that drought may weaken alien plant invasions through stronger direct negative impacts on alien than on native plant performance, but that drought does not affect soil‐biota‐driven differences in plant performance between alien and native plants. Read the free Plain Language Summary for this article on the Journal blog.

Understanding the environmental and anthropogenic drivers of spatial patterns in life history variation for exploited fish populations is important when making management decisions and designating stock boundaries. These considerations are especially germane for stocks that are overfished or recently rebuilt, such as lingcodOphiodon elongatus, a commercially and recreationally valuable species of groundfish along the West Coast of North America. Between 2015 and 2017, we collected 2189 lingcod from 24 port locations, spanning 28° of latitude from southeast Alaska (60°N) to southern California (32°N), to investigate latitudinal patterns in size- and age-structure, growth, timing of maturity, condition, and mortality, as well as to identify biologically relevant population breakpoints along the coast. We found strong latitudinal patterns in these life history and demographic traits consistent with Bergmann’s rule: lingcod from colder, northern waters were larger-at-age, lived longer, matured at larger sizes, and had lower natural mortality rates than lingcod from warmer, southern waters. Female lingcod were larger-at-age, lived longer, and matured at larger sizes compared to males within each examined region. In addition, we found evidence for strong associations between lingcod life history traits and the oceanographic variables of sea surface temperature and chlorophylla. Cluster analysis using life history traits indicated that central Oregon is a biologically relevant breakpoint for lingcod along the US West Coast. This breakpoint based on life history traits, in conjunction with a recently identified population genetic breakpoint between central and northern California, highlights the need for future lingcod stock assessments to consider multiple sources of information to best inform management of this trans-boundary stock.

Some aspects of human life history are unique among primates. Most notably, humans have a younger weaning age, a later age at first parturition, a shorter female reproductive period, and a longer lifespan than other living hominoid species. Obtaining a better understanding of when and how life history changed during human evolution is important to those studying the evolutionary developmental biology of extinct hominins, as life history traits pace developmental processes. Life history traits are thought to be linked via tradeoffs, such that changes in early life history traits directly affect those that follow later in life, and vice versa. However, it is also worth considering how changes to a single life history trait may indirectly affect other traits by way of modifying selective pressures acting on individuals and groups. For example, because they affect the size and demographic structure of a group, late life history traits (e.g., lifespan) may also affect the evolution of life history traits that occur earlier in life, but by modifying selective pressures acting on juveniles rather than by triggering a physiological tradeoff. This review marks an effort to begin to disentangle the ways in which early and late life history traits may affect each other both directly and indirectly. We concentrate on female life history characteristics. First, we review previous research on the evolution of the postmenopausal lifespan in women. Next we discuss recent findings concerning the relationship between the optimal length of the female reproductive period, mortality, and weaning age that show that selection favors a shorter female reproductive period in the presence of a younger weaning age. We discuss the implications this finding holds for understanding the evolution of life history traits that are of particular interest to developmental biologists.

I assessed the value of life history studies as an approach to evaluating the impact of recent clearfell logging on pond breeding frogs. I conducted a number of distinct studies investigating different aspects of amphibian life history in response to logging using Crinia signifera and Litoria ewingii as my model species. Life history studies were successfully able to: ‚Äöassess breeding site suitability for L. ewingii-by correlating habitat components with life history traits and fitness. Breeding ponds that maximised the fitness of L. ewingii were higher elevation ponds with reduced shading, steeper bank slopes and reduced pond isolation. The outcomes of the study enabled me to provide management guidelines for the construction of ponds designed to maximise the fitness of target species. ‚Äödetermine the consequences of decreased shading associated with vegetation removal around permanent and ephemeral ponds for larval fitness in L. ewingii. Increased shading around permanent ponds resulted in reduced survival. Increased shading in ephemeral ponds resulted in decreased developmental rate and a higher variation in size at metamorphosis. The outcome of the study provided guidelines for vegetation management around breeding sites. ‚Äöinvestigate landscape use of the commercially managed forest by Crinia signifera in order to determine the fraction of the forest used as habitat, the effect of logging upon movement through the forest, and the consequences for reproductive output. Crinia signifera was distributed widely throughout the forest landscape for up to 500 m around each permanent breeding site. Ponds surrounded by unlogged forest were colonized almost two times faster than ponds surrounded by logged forest. AS a result, total reproductive output at unlogged ponds was double that of ponds surrounded by recently logged coupes. ‚Äö demonstrate that loggingi can affect maternal reproductive investment and offspring fitness in both Crinia signifera and Litoria ewingii. Eggs were collected from ponds located in either logged or tmlogged forest and raised to metamorphosis under common laboratory conditions. Egg size was greater at unlogged than logged sites for both species. For L. ewingii, size at hatching was also greater at unlogged sites; but for C. signifera size at hatching was independent of logging treatment. For L. ewingii, survival to metamorphosis was greater at logged sites. For C. signifera survival at metamorphosis did not differ with logging treatment despite greater survival at hatching in eggs from logged sites. confirm that fluctuating asymmetry has limitations as an indicator of ecosystem stress and the population health of C. signifera in response to logging. The findings indicate a decrease in fluctuating asymmetry (and implied environmental stress) but also a decrease in body size and body condition (i.e. fitness) as a result of logging. There was no significant relationship between the levels of subtle asymmetry in individuals and the following life history traits: clutch size, clutch dry mass, average egg dry mass, oviduct dry mass, testes size, age, body size and body condition. My research confirms that logging significantly influences the life history of L. ewingii and C. signifera. I argue that, by linking changes in life history traits with assessments of fitness, life history studies can be used to understand why species respond to logging in the ways they do. In this way, life history studies can provide data on which to base constructive, defensible recommendations for the ecologically sustainable management of commercial forests with regard to the frog species that use them. Life history studies are a valid experimental approach that can be used to redress the data deficit relating to the impact of logging on amphibians in Australia.

The topic of this thesis is the latitudinal diversity gradient: the general increase in numbers of species from high latitudes towards the equator. Although it is a well-documented and very general pattern, there is little consensus among biologists as to the causes of high tropical species richness. In Chapter 1 I give an overview of the latitudinal diversity gradient and hypotheses proposed to explain it, and outline a framework for developing and testing hypotheses. The emphasis in this thesis is on non-equilibrium hypotheses, and in particular on the idea that rates of species diversification are higher at low latitudes. In Chapter 2 I directly test the hypothesis that rates of species diversification increase towards the equator, using the phylogenetic method of sister-group comparisons. For birds and butterflies, 1 show that there does indeed appear to be an increase in rates of diversification towards lower latitudes. One possible explanation for this pattern links climatic conditions at low latitudes to faster speciation, via a causal chain which includes a higher rate of molecular evolution. In Chapter 3 I test the prediction that rates of molecular evolution are faster at low latitudes, using phylogenies reconstructed from DNA sequence data for birds. Although the results provide no evidence for a latitudinal effect on rates of cytb and ND2 evolution in birds, this chapter demonstrates a way in which further tests on a wider range of genes and taxa can be carried out as more sequence data becomes available. If rates of diversification are higher in the tropics, this may result from latitudinal gradients in life history traits which could influence rates of speciation or extinction. However, previous attempts to test for such patterns have rarely controlled for phylogenetic relationships among species, and have never controlled for geographic range overlap. In Chapter 4 I present a method for analyzing latitudinal variation in life history traits which takes both of these problems into account. For birds, this method indicates strong latitudinal variation in geographic range size and clutch size, less strong variation in body size and niche width, and no variation in the strength of sexual selection. While this does not prove that life history plays a part in generating the latitudinal diversity gradient, a robust demonstration of latitudinal variation in life history traits is an important prerequisite for any hypotheses which link life history with high tropical species richness. Speciation and extinction rates are believed to vary with species' life history and ecology. If speciation or extinction rates also vary with latitude, we should expect species with certain traits to show stronger latitudinal diversity gradients than others. We should also expect to see latitudinal variation in the macroecological structure of species assemblages. Chapters 5 and 6 confirm these expectations. In Chapter 5 I show that smaller-bodied bird species have steeper latitudinal diversity gradients than larger species. This is reflected in systematic latitudinal variation in the shape of body size frequency distributions, usually thought to be relatively consistent across regions. In Chapter 6 I show that body size - abundance relationships in the Australian marsupial fauna differ considerably between temperate and tropical subsets of the fauna. Whereas the temperate species display the typical negative relationship, the tropical species show no significant relationship. This pattern is consistent with the explanation that extinction rates vary with respect to body size, abundance and the latitude in which a species occurs. Together, the results of the analyses presented in this thesis provide evidence in favour of the non-equilibrium view that there are more species in the tropics because rates of species diversification are higher. It seems likely that this is due to latitudinal variation in environmental conditions, leading to latitudinal variation in species' life histories and the structure of species assemblages, and to latitudinal variation in rates of speciation or extinction. However, it is still difficult to judge whether it is variation in speciation rate (cradle model) or extinction rate (museum model) which is of primary importance, or if both are equally important. Further work on this question, applying the large-scale comparative methodology used here, will be necessary to progress towards a full understanding of the high species richness of the tropics.

Life history responses to environmental conditions include a combination of fecundity-survival schedules and behavioral strategies that yield the highest fitness in a given environment. In this study, we examined the pattern of covariation in avian life history strategies along an elevational gradient by comparing variation in life history traits, including most components of parental care, between phylogenetically paired taxa from low- and high-elevation sites. We found that high-elevation species had significantly lower annual fecundity but provided greater parental care to their offspring. However, a strong negative relationship between offspring number and duration of parental care along the elevational gradient suggested that high-elevation species were shifting investment from offspring number toward offspring quality. Although adult survival did not differ between high- and low-elevation species, higher juvenile survival may have compensated for lower annual fecundity in high-elevation species. The elevation at which breeding occurred strong- ly influenced the partitioning of parental behavior between sexes. Male participation in nestling provisioning was significantly greater in high-elevation species. In turn, altitudinal variation in the frequency of biparental care closely covaries with the intensity of sexual selection, ultimately resulting in the strong elevational pattern of sexual dimorphism. More- over, elevational variation in costs of development and maintenance of secondary sexual traits constitutes an additional effect on fecundity-survival schedules along elevational gradients. Thus, a trade-off between fecundity and parental care, and associated interactions among morphological, life history, and behavioral traits play important roles in the evolution of life history strategies in birds.

This study concerns the longitudinal dispersion of fluid particles which are initially distributed uninformly over one cross section of a uniform, steady, turbulent open channel flow. The primary focus is on developing a method to predict the rate of dispersion in a natural stream. Taylor's method of determining a dispersion coefficient, previously applied to flow in pipes and two-dimensional open channels, is extended to a class of three-dimensional flows which have large width-to-depth ratios, and in which the velocity varies continuously with lateral cross-sectional position. Most natural streams are included. The dispersion coefficient for a natural stream may be predicted from measurements of the channel cross-sectional geometry, the cross-sectional distribution of velocity, and the overall channel shear velocity. Tracer experiments are not required. Large values of the dimensionless dispersion coefficient D/rU* are explained by lateral variations in downstream velocity. In effect, the characteristic length of the cross section is shown to be proportional to the width, rather than the hydraulic radius. The dimensionless dispersion coefficient depends approximately on the square of the width to depth ratio. A numerical program is given which is capable of generating the entire dispersion pattern downstream from an instantaneous point or plane source of pollutant. The program is verified by the theory for two-dimensional flow, and gives results in good agreement with laboratory and field experiments. Both laboratory and field experiments are described. Twenty-one laboratory experiments were conducted: thirteen in two-dimensional flows, over both smooth and roughened bottoms; and eight in three-dimensional flows, formed by adding extreme side roughness to produce lateral velocity variations. Four field experiments were conducted in the Green-Duwamish River, Washington. Both laboratory and flume experiments prove that in three-dimensional flow the dominant mechanism for dispersion is lateral velocity variation. For instance, in one laboratory experiment the dimensionless dispersion coefficient D/rU* (where r is the hydraulic radius and U* the shear velocity) was increased by a factory of ten by roughening the channel banks. In three-dimensional laboratory flow, D/rU* varied from 190 to 640, a typical range for natural streams. For each experiment, the measured dispersion coefficient agreed with that predicted by the extension of Taylor's analysis within a maximum error of 15%. For the Green-Duwamish River, the average experimentally measured dispersion coefficient was within 5% of the prediction.