Life Sciences Commons^™

Sexually Antagonistic Selection, Sexual Dimorphism, And The Resolution Of Intralocus Sexual Conflict, Robert M. Cox, Ryan Calsbeek

Dartmouth Scholarship

Males and females share most of their genomes and express many of the same traits, yet the sexes often have markedly different selective optima for these shared traits. This sexually antagonistic (SA) selection generates intralocus sexual conflict that is thought to be resolved through the evolution of sexual dimorphism. However, we currently know little about the prevalence of SA selection, the components of fitness that generate sexual antagonism, or the relationship between sexual dimorphism and current SA selection. We reviewed published studies to address these questions, using 424 selection estimates representing 89 traits from 34 species. Males and females often …

Parallel Shifts In Ecology And Natural Selection In An Island Lizard, Ryan Calsbeek, Wolfgang Buermann, Thomas B. Smith

Dartmouth Scholarship

Natural selection is a potent evolutionary force that shapes phenotypic variation to match ecological conditions. However, we know little about the year-to-year consistency of selection, or how inter-annual variation in ecology shapes adaptive landscapes and ultimately adaptive radiations. Here we combine remote sensing data, field experiments, and a four-year study of natural selection to show that changes in vegetation structure associated with a severe drought altered both habitat use and natural selection in the brown anole, Anolis sagrei.

Results: In natural populations, lizards increased their use of vegetation in wet years and this was correlated with selection on limb length …

The Role Of Causal Processes In The Neutral And Nearly Neutral Theories, Michael R. Dietrich, Roberta L. Millstein

Dartmouth Scholarship

The neutral and nearly neutral theories of molecular evolution are sometimes characterized as theories about drift alone, where drift is described solely as an outcome, rather than a process. We argue, however, that both selection and drift, as causal processes, are integral parts of both theories. However, the nearly neutral theory explicitly recognizes alleles and/or molecular substitutions that, while engaging in weakly selected causal processes, exhibit outcomes thought to be characteristic of random drift. A narrow focus on outcomes obscures the significant role of weakly selected causal processes in the nearly neutral theory.

Habitat-Mediated Foraging Limitations Drive Survival Bottlenecks For Juvenile Salmon, Brian P. Kennedy, Keith H. Nislow, Carol L. Folt

Dartmouth Scholarship

Realistic population models and effective conservation strategies require a thorough understanding of mechanisms driving stage-specific mortality. Mortality bottlenecks for many species occur in the juvenile stage and are thought to result from limitation on food or foraging habitat during a "critical period" for growth and survival. Without a way to account for maternal effects or to measure integrated consumption rates in the field, it has been virtually impossible to test these relationships directly. Hence uncertainties about mechanisms underlying such bottlenecks remain. In this study we randomize maternal effects across sites and apply a new method for measuring consumption integrated over …

Fish Distributions And Nutrient Cycling In Streams: Can Fish Create Biogeochemical Hotspots, Peter B. Mcintyre, Alexander S. Flecker, Michael J. Vanni, James M. Hood, Brad W. Taylor, Steven A. Thomas

Dartmouth Scholarship

Rates of biogeochemical processes often vary widely in space and time, and characterizing this variation is critical for understanding ecosystem functioning. In streams, spatial hotspots of nutrient transformations are generally attributed to physical and microbial processes. Here we examine the potential for heterogeneous distributions of fish to generate hotspots of nutrient recycling. We measured nitrogen (N) and phosphorus (P) excretion rates of 47 species of fish in an N-limited Neotropical stream, and we combined these data with population densities in each of 49 stream channel units to estimate unit- and reach-scale nutrient recycling. Species varied widely in rates of N …

The Tempo And Mode Of Three‐Dimensional Morphological Evolution In Male Reproductive Structures, Mark A. Mcpeek, Li Shen, John Z. Torrey, Hany Farid

Dartmouth Scholarship

Various evolutionary forces may shape the evolution of traits that influence the mating decisions of males and females. Phe- notypic traits that males and females use to judge the species identify of potential mates should evolve in a punctuated fashion, changing significantly at the time of speciation but changing little between speciation events. In contrast, traits experiencing sexual selection or sexually antagonistic interactions are generally expected to change continuously over time because of the directional selection pressures imposed on one sex by the actions of the other. To test these hy- potheses, we used spherical harmonic representations of the shapes …

Micrornas And The Advent Of Vertebrate Morphological Complexity, Alysha M. Heimberg, Lorenzo F. Sempere, Vanessa N. Moy, Phillip C. J. Donoghue, Kevin J. Peterson

Dartmouth Scholarship

The causal basis of vertebrate complexity has been sought in genome duplication events (GDEs) that occurred during the emergence of vertebrates, but evidence beyond coincidence is wanting. MicroRNAs (miRNAs) have recently been identified as a viable causal factor in increasing organismal complexity through the action of these ≈22-nt noncoding RNAs in regulating gene expression. Because miRNAs are continuously being added to animalian genomes, and, once integrated into a gene regulatory network, are strongly conserved in primary sequence and rarely secondarily lost, their evolutionary history can be accurately reconstructed. Here, using a combination of Northern analyses and genomic searches, we show …

Spatial Scaling Of Avian Population Dynamics: Population Abundance, Growth Rate, And Variability, Jason Jones, Patrick J. Doran, Richard T. Holmes

Dartmouth Scholarship

Synchrony in population fluctuations has been identified as an important component of population dynamics. In a previous study, we determined that local‐scale (<15‐km) spatial synchrony of bird populations in New England was correlated with synchronous fluctuations in lepidopteran larvae abundance and with the North Atlantic Oscillation. Here we address five questions that extend the scope of our earlier study using North American Breeding Bird Survey data. First, do bird populations in eastern North America exhibit spatial synchrony in abundances at scales beyond those we have documented previously? Second, does spatial synchrony depend on what population metric is analyzed (e.g., abundance, growth rate, or variability)? Third, is there geographic concordance in where species exhibit synchrony? Fourth, for those species that exhibit significant geographic concordance, are there landscape and habitat variables that contribute to the observed patterns? Fifth, is spatial synchrony affected by a species' life history traits? Significant spatial synchrony was common and its magnitude was dependent on the population metric analyzed. Twenty‐four of 29 species examined exhibited significant synchrony in population abundance: mean local autocorrelation (ρ) = 0.15; mean spatial extent (mean distance where ρ = 0) = 420.7 km. Five of the 29 species exhibited significant synchrony in annual population growth rate (mean local autocorrelation = 0.06, mean distance = 457.8 km). Ten of the 29 species exhibited significant synchrony in population abundance variability (mean local autocorrelation = 0.49, mean distance = 413.8 km). Analyses of landscape structure indicated that habitat variables were infrequent contributors to spatial synchrony. Likewise, we detected no effects of life history traits on synchrony in population abundance or growth rate. However, short‐distance migrants exhibited more spatially extensive synchrony in population variability than either year‐round residents or long‐distance migrants. The dissimilarity of the spatial extent of synchrony across species suggests that most populations are not regulated at similar spatial scales. The spatial scale of the population synchrony patterns we describe is likely larger than the actual scale of population regulation, and in turn, the scale of population regulation is undoubtedly larger than the scale of individual ecological requirements.

Changes In Nitrogen Cycling During The Past Century In A Northern Hardwood Forest, Kendra K. Mclauchlan, Joseph M. Craine, W. Wyatt Oswald, Peter R. Leavitt, Gene E. Likens

Dartmouth Scholarship

Nitrogen (N) availability, defined here as the supply of N to terrestrial plants and soil microorganisms relative to their N demands, limits the productivity of many temperate zone forests and in part determines ecosystem carbon (C) content. Despite multidecadal monitoring of N in streams, the long-term record of N availability in forests of the northeastern United States is largely unknown. Therefore, although these forests have been receiving anthropogenic N deposition for the past few decades, it is still uncertain whether terrestrial N availability has changed during this time and, subsequently, whether forest ecosystems have responded to increased N deposition. Here, …

Impact Of Minimum Winter Temperatures On The Population Dynamics Of Dendroctonus Frontalis, J. KhảI TrầN, Tiina Ylioja, Ronald F. Billings, Jacques Régnière, Matthew P. Ayres

Dartmouth Scholarship

Predicting population dynamics is a fundamental problem in applied ecology. Temperature is a potential driver of short-term population dynamics, and temperature data are widely available, but we generally lack validated models to predict dynamics based upon temperatures. A generalized approach involves estimating the temperatures experienced by a population, characterizing the demographic consequences of physiological responses to temperature, and testing for predicted effects on abundance. We employed this approach to test whether minimum winter temperatures are a meaningful driver of pestilence from Dendroctonus frontalis (the southern pine beetle) across the southeastern United States. A distance-weighted interpolation model provided good, spatially explicit, …

Integration Without Unification: An Argument For Pluralism In The Biological Sciences, Sandra D. Mitchell, Michael R. Dietrich

Dartmouth Scholarship

In this article, we consider the tension between unification and pluralism in biological theory. We begin with a consideration of historical efforts to establish a unified understanding of evolution in the neo‐Darwinian synthesis. The fragmentation of the evolutionary synthesis by molecular evolution suggests the limitations of the general unificationist ideal for biology but not necessarily for integrating explanations. In the second half of this article, we defend a specific variety of pluralism that allows for the integration required for explanations of complex phenomena without unification on a large scale.

Evaluating The Long-Term Metacommunity Dynamics Of Tree Hole Mosquitoes, Alicia M. Ellis, L. Philip Lounibos, Marcel Holyoak

Dartmouth Scholarship

Four different conceptual models of metacommunities have been proposed, termed “patch dynamics,” “species sorting,” “mass effect,” and “neutral.” These models simplify thinking about metacommunities and improve our understanding of the role of spatial dynamics both in structuring communities and in determining local and regional diversity. We tested whether mosquito communities inhabiting water-filled tree holes in southeastern Florida, USA, displayed any of the characteristics and dynamics predicted by the four models. The densities of the five most common species in 3–8 tree holes were monitored every two weeks during 1978–2003. We tested relationships between habitat variables and species densities, spatial synchrony, …

Genetic And Maternal Determinants Of Effective Dispersal: The Effect Of Sire Genotype And Size At Birth In Side-Blotched Lizards, Barry Sinervo, Ryan Calsbeek, Tosha Comendant, Christiaan Both

Dartmouth Scholarship

We assessed genetic factors on progeny dispersal due to sire color morph genotypes in a field pedigree and lab crosses, and we measured maternal effects by studying both natural and experimentally induced egg size variation. Progeny were released into nature upon hatching, but we recorded dispersal distance at maturity, which reflects effective dispersal after viability selection has run its course. Progeny dispersal was significantly affected by sire genotype. Progeny from orange sires dispersed the farthest. Progeny from blue sires dispersed intermediate distances. Progeny from yellow sires were the most philopatric. Sire genotype effects interacted with egg size. In particular, enlarged …

Coexistence Of The Niche And Neutral Perspectives In Community Ecology, Mathew A. Leibold, Mark A. Mcpeek

Dartmouth Scholarship

The neutral theory for community structure and biodiversity is dependent on the assumption that species are equivalent to each other in all important ecological respects. We explore what this concept of equivalence means in ecological communities, how such species may arise evolutionarily, and how the possibility of ecological equivalents relates to previous ideas about niche differentiation. We also show that the co-occurrence of ecologically similar or equivalent species is not incompatible with niche theory as has been supposed, because niche relations can sometimes favor coexistence of similar species. We argue that both evolutionary and ecological processes operate to promote the …

Time Constraints Mediate Predator-Induced Plasticity In Immune Function, Condition, And Life History, Robby Stoks, Marjan De Block, Stefanie Slos, Wendy Van Doorslaer, Jens Rolff

Dartmouth Scholarship

The simultaneous presence of predators and a limited time for development imposes a conflict: accelerating growth under time constraints comes at the cost of higher predation risk mediated by increased foraging. The few studies that have addressed this trade-off have dealt only with life history traits such as age and size at maturity. Physiological traits have largely been ignored in studies assessing the impact of environmental stressors, and it is largely unknown whether they respond independently of life history traits. Here, we studied the simultaneous effects of time constraints, i.e., as imposed by seasonality, and predation risk on immune defense, …

Bacteriophage Migration Via Nematode Vectors: Host-Parasite-Consumer Interactions In Laboratory Microcosms, John J. Dennehy, Nicholas A. Friedenberg, Yul W. Yang, Paul E. Turner

Dartmouth Scholarship

Pathogens vectored by nematodes pose serious agricultural, economic, and health threats; however, little is known of the ecological and evolutionary aspects of pathogen transmission by nematodes. Here we describe a novel model system with two trophic levels, bacteriophages and nematodes, each of which competes for bacteria. We demonstrate for the first time that nematodes are capable of transmitting phages between spatially distinct patches of bacteria. This model system has considerable advantages, including the ease of maintenance and manipulation at the laboratory bench, the ability to observe many generations in short periods, and the capacity to freeze evolved strains for later …

Linking Direct And Indirect Data On Dispersal: Isolation By Slope In A Headwater Stream Salamander, Winsor H. Lowe, Gene E. Likens, Mark A. Mcpeek, Don C. Buso

Dartmouth Scholarship

There is growing recognition of the need to incorporate information on movement behavior in landscape-scale studies of dispersal. One way to do this is by using indirect indices of dispersal (e.g., genetic differentiation) to test predictions derived from direct data on movement behavior. Mark–recapture studies documented upstream-biased movement in the salamander Gyrinophilus porphyriticus (Plethodontidae). Based on this information, we hypothesized that gene flow in G. porphyriticus is affected by the slope of the stream. Specifically, because the energy required for upstream dispersal is positively related to slope, we predicted gene flow to be negatively related to change in elevation between …

Viral Ecology And The Maintenance Of Novel Host Use, John Dennehy, Nicholas Friedenberg, Robert Holt, Paul Turner

Dartmouth Scholarship

Viruses can occasionally emerge by infecting new host species. However, the early phases of emergence can hinge upon ecological sustainability of the virus population, which is a product of both within‐host population growth and between‐host transmission. Insufficient growth or transmission can force virus extinction before the latter phases of emergence, where genetic adaptations that improve host use may occur. We examined the early phase of emergence by studying the population dynamics of RNA phages in replicated laboratory environments containing native and novel host bacteria. To predict the breadth of transmission rates allowing viral persistence on each species, we developed a …

Ecological Costs And Benefits Of Defenses In Nectar, Lynn S. Adler, Rebecca E. Irwin

Dartmouth Scholarship

The nectar of many plant species contains defensive compounds that have been hypothesized to benefit plants through a variety of mechanisms. However, the relationship between nectar defenses and plant fitness has not been established for any species. We experimentally manipulated gelsemine, the principal alkaloid of Carolina jessamine (Gelsemium sempervirens), in nectar to determine its effect on pollinator visitation, nectar robber visitation, and male and female plant reproduction. We found that nectar robbers and most pollinators probed fewer flowers and spent less time per flower on plants with high compared to low nectar alkaloids. High alkaloids decreased the donation of fluorescent …

Origin Of The Eumetazoa: Testing Ecological Predictions Of Molecular Clocks Against The Proterozoic Fossil Record, Kevin J. Peterson, Nicholas J. Butterfield

Dartmouth Scholarship

Molecular clocks have the potential to shed light on the timing of early metazoan divergences, but differing algorithms and calibration points yield conspicuously discordant results. We argue here that competing molecular clock hypotheses should be testable in the fossil record, on the principle that fundamentally new grades of animal organization will have ecosystem-wide impacts. Using a set of seven nuclear-encoded protein sequences, we demonstrate the paraphyly of Porifera and calculate sponge/eumetazoan and cnidarian/bilaterian divergence times by using both distance [minimum evolution (ME)] and maximum likelihood (ML) molecular clocks; ME brackets the appearance of Eumetazoa between 634 and 604 Ma, whereas …

Knowing When To Draw The Line: Designing More Informative Ecological Experiments, Kathryn L. Cottingham, Jay T. Lennon, Bryan L. Brown

Dartmouth Scholarship

Linear regression and analysis of variance (ANOVA) are two of the most widely used statistical techniques in ecology. Regression quantitatively describes the relationship between a response variable and one or more continuous independent variables, while ANOVA determines whether a response variable differs among discrete values of the independent variable(s). Designing experiments with discrete factors is straightforward because ANOVA is the only option, but what is the best way to design experiments involving continuous factors? Should ecologists prefer experiments with few treatments and many replicates analyzed with ANOVA, or experiments with many treatments and few replicates per treatment analyzed with regression? …

Estimating Metazoan Divergence Times With A Molecular Clock, Kevin J. Peterson, Jessica B. Lyons, Kristin S. Nowak, Carter M. Takacs, Matthew J. Wargo, Mark A. Mcpeek

Dartmouth Scholarship

Accurately dating when the first bilaterally symmetrical animals arose is crucial to our understanding of early animal evolution. The earliest unequivocally bilaterian fossils are 555 million years old. In contrast, molecular-clock analyses calibrated by using the fossil record of vertebrates estimate that vertebrates split from dipterans (Drosophila) 900 million years ago (Ma). Nonetheless, comparative genomic analyses suggest that a significant rate difference exists between vertebrates and dipterans, because the percentage difference between the genomes of mosquito and fly is greater than between fish and mouse, even though the vertebrate divergence is almost twice that of the dipteran. Here we show …

Stage-Specific And Interactive Effects Of Sedimentation And Trout On A Headwater Stream Salamander, Winsor H. Lowe, Keith H. Nislow, Douglas T. Bolger

Dartmouth Scholarship

In species with complex life cycles, stage-specific effects of environmental conditions combine with factors regulating stage-specific recruitment to determine population-level response to habitat disturbance. The abundance of the stream salamander Gyrinophilus porphyriticus(Plethodontidae) is negatively related to both logging-associated sedimentation and brook trout (Salvelinus fontinalis) in headwater streams throughout New Hampshire, USA. To understand the mechanisms underlying these patterns, we investigated stage-specific and interactive effects of sedimentation and brook trout on G. porphyriticus. We conducted quantitative surveys of salamanders, brook trout, and substrate embeddedness in 15 first-order streams and used a controlled experiment to test the direct and interactive effects of …

Antipredator Behavior And Physiology Determine Lestes Species Turnover Along The Pond-Permanence Gradient, Robby Stoks, Mark A. Mcpeek

Dartmouth Scholarship

Identifying key traits that shape trade-offs that restrict species to only a subset of environmental gradients is crucial to understanding and predicting species turnover. Previous field experiments have shown that larvae of Lestes damselfly species segregate along the entire gradient of pond permanence and predator presence and that differential predation risk and life history constraints together shape their distribution. Here, we report laboratory experiments that identify key differences in behavior and physiology among species that structure their distributions along this gradient. The absence of adaptive antipredator behavioral responses against large dragonfly larvae and fish of Lestes dryas, the only species …

Estimating Community Stability And Ecological Interactions From Time-Series Data, A. R. Ives, B. Dennis, K. L. Cottingham, S. R. Carpenter

Dartmouth Scholarship

Natural ecological communities are continuously buffeted by a varying environment, often making it difficult to measure the stability of communities using concepts requiring the existence of an equilibrium point. Instead of an equilibrium point, the equilibrial state of communities subject to environmental stochasticity is a stationary distribution, which is characterized by means, variances, and other statistical moments. Here, we derive three properties of stochastic multispecies communities that measure different characteristics associated with community stability. These properties can be estimated from multispecies time-series data using first-order multivariate autoregressive (MAR(1)) models. We demonstrate how to estimate the parameters of MAR(1) models and …

Linking Dispersal To Local Population Dynamics: A Case Study Using A Headwater Salamander System, Winsor H. Lowe

Dartmouth Scholarship

Dispersal can strongly influence local population dynamics and may be critical to species persistence in fragmented landscapes. Theory predicts that dispersal by resident stream organisms is necessary to offset the loss of individuals to downstream drift. However, there is a lack of empirical data linking dispersal and drift to local population dynamics in streams, leading to uncertainty regarding the general demographic significance of these processes and the power of drift to explain observed dispersal patterns. I assessed the contribution of dispersal along a first-order stream to population dynamics of the headwater salamander Gyrinophilus porphyriticus (Plethodontidae). I conducted mark–recapture surveys of …

Interactions Among Environmental Drivers: Community Responses To Changing Nutrients And Dissolved Organic Carbon, Jennifer L. Klug, Kathryn L. Cottingham

Dartmouth Scholarship

Biological communities are frequently exposed to environmental changes that cause measurable responses in properties of the community (hereafter called environmental drivers). Predicting how communities respond to changing environmental drivers is a fundamental goal of ecology. Making predictions, however, can be very difficult, particularly when multiple environmental drivers change simultaneously and there are interactions among the drivers. We investigated the effects of the interaction between changes in nutrient loading and changes in colored dissolved organic matter (measured as dissolved organic carbon, DOC) on the dynamics of phytoplankton communities over a 7‐yr period. In 1991, Long Lake, a small seepage lake in …

Trophic Cascades, Nutrients, And Lake Productivity: Whole-Lake Experiments, Stephen R. Carpenter, Jonathan J. Cole, James R. Hodgson, James F. Kitchell, Michael L. Pace, Darren Bade, Kathryn L. Cottingham

Dartmouth Scholarship

Responses of zooplankton, pelagic primary producers, planktonic bacteria, and CO2 exchange with the atmosphere were measured in four lakes with contrasting food webs under a range of nutrient enrichments during a seven-year period. Prior to enrichment, food webs were manipulated to create contrasts between piscivore dominance and planktivore dominance. Nutrient enrichments of inorganic nitrogen and phosphorus exhibited ratios of N:P > 17:1, by atoms, to maintain P limitation. An unmanipulated reference lake, Paul Lake, revealed baseline variability but showed no trends that could confound the interpretation of changes in the nearby manipulated lakes. Herbivorous zooplankton of West Long Lake (piscivorous fishes) …

Consequences Of Dominance-Mediated Habitat Segregation In American Redstarts During The Nonbreeding Season, Peter P. Marra, Richard T. Holmes

Dartmouth Scholarship

Several species of migratory songbirds exhibit a distinct form of habitat segregation while on their Neotropical wintering grounds in which males and females occupy different habitat types. In the American Redstart (Setophaga ruticilla), that sexual habitat segregation is a result of behavioral dominance of older males. In that study, we examined whether such dominance behavior and the resulting differential habitat segregation has consequences for the condition or survival of excluded individuals. We quantified the physical condition and survival of redstarts (both males and females) occupying two habitat types that differed in the proportion of males and females present …

Host-Driven Population Dynamics In An Herbivorous Insect, Tiina Ylioja, Heikki Roininen, Matthew P. Ayres, Matti Rousi, Peter W. Price

Dartmouth Scholarship

Understanding the nature and relative importance of endogenous (density-dependent) and exogenous (density-independent) effects on population dynamics remains a central problem in ecology. Evaluation of these forces has been constrained by the lack of long time series of population densities and largely limited to populations chosen for their unique dynamics (e.g., outbreak insects). Especially in herbivore populations, the relative contributions of bottom-up and top-down effects (resources and natural enemies, respectively) have been difficult to compare because population data have rarely been combined with resource measurements. The feeding scars of a wood-mining herbivorous insect (Phytobia betulae Kangas; Diptera: Agromyzidae) of birch …