Biomedical subjects

William F Fagan

Publications and source records attributed to William F Fagan.

16 recordsLinked to original sources

PubMed2006

Phylogenetic and growth form variation in the scaling of nitrogen and phosphorus in the seed plants.

Plant biomass and nutrient allocation explicitly links the evolved strategies of plant species to the material and energy cycles of ecosystems. Allocation of nitrogen (N) and phosphorus (P) is of particular interest because N and P play pivotal roles in many aspects of plant biology, and their availability frequently limits plant growth. Here we present a comparative scaling analysis of a global data compilation detailing the N and P contents of leaves, stems, roots, and reproductive structures of 1,287 species in 152 seed plant families. We find that P and N contents (as well as N : P) are generally highly correlated both within and across organs and that differences exist between woody and herbaceous taxa. Between plant organs, the quantitative form of the scaling relationship changes systematically, depending on whether the organs considered are primarily structural (i.e., stems, roots) or metabolically active (i.e., leaves, reproductive structures). While we find significant phylogenetic signals in the data, similar scaling relationships occur in independently evolving plant lineages, which implies that both the contingencies of evolutionary history and some degree of environmental convergence have led to a common set of rules that constrain the partitioning of nutrients among plant organs.

Analysis of Variance↗

PubMed2006

Signatures of ecological resource availability in the animal and plant proteomes.

Although substantial and ecologically significant differences in elemental composition are well documented for whole organisms, little is known about whether such differences extend to lower levels of biological organization, such as the elemental composition of major molecules. In a proteome-scale investigation of 9 plant genomes and 9 animal genomes, we find that the nitrogen (N) content of plant proteins is lower than that in animal proteins. Furthermore, protein N content declines with the intensity of gene expression for plants, whereas the N content of animal proteins shows no consistent pattern with expression. Additional analyses indicate that the differences in N content between plant and animal proteomes and in plant proteins as a function of gene expression cannot be attributed to protein size, GC content, gene function, or amino acid properties. These patterns suggest that ecophysiological selection has operated to conserve N in plants via decreased reliance on N-rich amino acids. This inference was supported by an analysis of conserved and variable sites indicating that the N content of plant amino acids coded by variable sites is similar to that of the sites conserved between plant and animal genomes and shows no association with expression level. In contrast, in animals, the N content of amino acids coded by variable sites is significantly higher than that for conserved sites, suggesting relaxation of selective constraints for N usage in the animal lineage. This constitutes the first evidence for an influence of environmental resource availability on proteomes of multicellular organisms.

Amino Acids↗

PubMed2006

Quantifying the extinction vortex.

We developed a database of 10 wild vertebrate populations whose declines to extinction were monitored over at least 12 years. We quantitatively characterized the final declines of these well-monitored populations and tested key theoretical predictions about the process of extinction, obtaining two primary results. First, we found evidence of logarithmic scaling of time-to-extinction as a function of population size for each of the 10 populations. Second, two lines of evidence suggested that these extinction-bound populations collectively exhibited dynamics akin to those theoretically proposed to occur in extinction vortices. Specifically, retrospective analyses suggested that a population size of n individuals within a decade of extinction was somehow less valuable to persistence than the same population size was earlier. Likewise, both year-to-year rates of decline and year-to-year variability increased as the time-to-extinction decreased. Together, these results provide key empirical insights into extinction dynamics, an important topic that has received extensive theoretical attention.

Animals↗

PubMed2006

Average dispersal success: linking home range, dispersal, and metapopulation dynamics to reserve design.

Spatially explicit models for populations are often difficult to tackle mathematically and, in addition, require detailed data on individual movement behavior that are not easily obtained. An approximation known as the "average dispersal success" provides a tool for converting complex models, which may include stage structure and a mechanistic description of dispersal, into a simple matrix model. This simpler matrix model has two key advantages. First, it is easier to parameterize from the types of empirical data typically available to conservation biologists, such as survivorship, fecundity, and the fraction of juveniles produced in a study area that also recruit within the study area. Second, it is more amenable to theoretical investigation. Here, we use the average dispersal success approximation to develop estimates of the critical reserve size for systems comprising single patches or simple metapopulations. The quantitative approach can be used for both plants and animals; however, to provide a concrete example of the technique's utility, we focus on a special case pertinent to animals. Specifically, for territorial animals, we can characterize such an estimate of minimum viable habitat area in terms of the number of home ranges that the reserve contains. Consequently, the average dispersal success framework provides a framework through which home range size, natal dispersal distances, and metapopulation dynamics can be linked to reserve design. We briefly illustrate the approach using empirical data for the swift fox (Vulpes velox).

Animals↗

PubMed2006

Using survival analysis to study translocation success in the Gila topminnow (Poeciliopsis occidentalis).

Translocation, the intentional release of captive-propagated and/or wild-caught animals into the wild in an attempt to establish, reestablish, or augment a population, is a commonly used approach to species conservation. Despite the frequent mention of translocation as an aid in threatened or endangered species recovery plans, translocations have resulted in the establishment of few sustainable populations. To improve the effectiveness of translocation efforts, it is essential to identify and adopt features that contribute to successful translocations. This study analyzed 148 translocations of the endangered Gila topminnow (Poeciliopsis occidentalis) to identify various factors that have significantly influenced translocation success. We quantified success as the "persistence time" of translocated populations and used survival analysis to interpret the role of several factors. The following factors affected persistence times of translocated populations: season in which the fish were translocated, habitat type of the translocation site, and genetic origin of the fish stocked. In general, factors associated with stocking, the population stocked, and the site of translocation can significantly affect the persistence of translocated populations and thus increase the probability of translocation success. For Gila topminnow, future translocations should be undertaken in late summer or fall (not early summer), should occur into ponds (not streams, wells, or tanks), and should generally utilize individuals from genetic lineages other than Monkey Spring. For other species, a key lesson emerging from this work is that life history attributes for each translocated species need to be considered carefully.

Animals↗

PubMed2005

When can herbivores slow or reverse the spread of an invading plant? A test case from Mount St. Helens.

Here we study the spatial dynamics of a coinvading consumer-resource pair. We present a theoretical treatment with extensive empirical data from a long-studied field system in which native herbivorous insects attack a population of lupine plants recolonizing a primary successional landscape created by the 1980 volcanic eruption of Mount St. Helens. Using detailed data on the life history and interaction strengths of the lupine and one of its herbivores, we develop a system of integrodifference equations to study plant-herbivore invasion dynamics. Our analyses yield several new insights into the spatial dynamics of coinvasions. In particular, we demonstrate that aspects of plant population growth and the intensity of herbivory under low-density conditions can determine whether the plant population spreads across a landscape or is prevented from doing so by the herbivore. In addition, we characterize the existence of threshold levels of spatial extent and/or temporal advantage for the plant that together define critical values of "invasion momentum," beyond which herbivores are unable to reverse a plant invasion. We conclude by discussing the implications of our findings for successional dynamics and the use of biological control agents to limit the spread of pest species.

Animal Feed↗

PubMed2005

Nonrandom larval dispersal can steepen marine clines.

Sharp and stable clinal variation is enigmatic when found in species with high gene flow. Classical population genetic models treat gene flow as a random homogenizing force countering local adaptation across habitat discontinuities. Under this view, dispersal over large spatial scales will lower the effectiveness of adaptation by natural selection at finer spatial scales. Thus, random gene flow will create a shallow phenotypic cline across an ecotone in response to a steep selection gradient. In sedentary marine species that disperse primarily as larvae, nonrandom dispersal patterns are expected due to coastal hydrodynamics. Surprisingly sharp phenotypic and genotypic clines have been documented in marine species with high gene flow. We are interested in the extent to which nonrandom dispersal could accentuate such clines. We model a linear species range in which populations have stable and uniform densities along a selection gradient; in contrast to random dispersal, convergent advection of larvae can amplify phenotypic differentiation if coupled with a semipermeable dispersal barrier in the convergence zone. The migration load caused by directional dispersal pushes the phenotypic mean away from the local trait optimum in downstream populations, that is, near the convergence zone. A dispersal barrier is possible as a result of colliding currents if the water and larvae are mostly displaced offshore, away from suitable settlement habitat. Disjunctions in a quantitative trait were enlarged in the convergence zone by faster current flows or a more complete dispersal barrier. With advection of larvae per generation one-third as far as the average dispersal distance by diffusion, convergence on a dispersal barrier with 40% permeability generated a trait disjunction across the convergence zone of two phenotypic standard deviations. Without directional dispersal, similar clines also developed across a habitat gap, where population density was low, or across dispersal barriers with less than 1% permeability. These findings suggest that the types of hydrographic phenomena often associated with marine transition zones can strongly affect the balance between gene flow and selection and generate surprisingly steep clines given the large-scale gene flow expected from larvae.

Animal Migration↗

PubMed2004

Lost in time, lonely, and single: reproductive asynchrony and the Allee effect.

Identifying linkages between life-history traits and small population processes is essential to effective multispecies conservation. Reproductive asynchrony, which occurs when individuals are reproductively active for only a portion of the population-level breeding period, may provide one such link. Traditionally, reproductive asynchrony has been considered from evolutionary perspectives as an advantageous bet-hedging strategy in temporally unpredictable environments. Here, we explore the dynamic consequences of reproductive asynchrony as a density-dependent life-history trait. To examine how asynchrony affects population growth rate and extinction risk, we used a general model of reproductive timing to quantify the temporal overlap of opposite-sex individuals and to simulate population dynamics over a range of initial densities and empirical estimates of reproductive asynchrony. We also considered how protandry, a sexually selected life-history strategy that often accompanies asynchrony, modulates the population-level effects of reproductive asynchrony. We found that asynchrony decreases the number of males a female overlaps with, decreases the average probability of mating per male/female pair that does overlap, and leaves some females completely isolated in time. This loss of reproductive potential, which is exacerbated by protandry, reduces population growth rate at low density and can lead to extinction via an Allee effect. Thus reproductive asynchrony and protandry, both of which can be evolutionarily advantageous at higher population densities, may prove detrimental when population density declines.

Animals↗

PubMed2004

Modeling and analysis of stoichiometric two-patch consumer-resource systems.

Ecological stoichiometry studies the balance of energy and multiple chemical elements in ecological interactions to establish how the laws of thermodynamics affect food-web dynamics and nutrient cycling in ecosystems. In this paper, we incorporate stoichiometric principles in a model with habitat heterogeneity and dispersal in order to better understand population growth dynamics. This model describes a situation where a resource is separated into two patches by a barrier. Growth of the resource in each patch is limited by soil fertility and self-crowding. The consumer disperses between the two patches and is not affected by the barrier. The consumer's growth is potentially limited by the phosphorus content of the acquired resource. Mathematical analysis of this model and simulations are performed. Several biological implications, including an observed "stoichiometric extinction effect," are demonstrated with simulation where the stoichiometric mechanism appears to promote extinction in a patchy environment. This is in sharp contrast to the notion that stoichiometry mechanism promotes diversity in spatially homogeneous settings. Another important result is the rediscovery of a simple and plausible biological mechanism that generates local and global extinction. In this setting, which can be considered a spatially mediated form of apparent competition, the dispersal of the consumer from the rich patch can des the growth of the resource in the poor patch and in some situations can lead to the extinction of the resource in the poor patch.

Animals↗

PubMed2004

Competition and stoichiometry: coexistence of two predators on one prey.

The competitive exclusion principle (CEP) states that no equilibrium is possible if n species exploit fewer than n resources. This principle does not appear to hold in nature, where high biodiversity is commonly observed, even in seemingly homogenous habitats. Although various mechanisms, such as spatial heterogeneity or chaotic fluctuations, have been proposed to explain this coexistence, none of them invalidates this principle. Here we evaluate whether principles of ecological stoichiometry can contribute to the stable maintenance of biodiverse communities. Stoichiometric analysis recognizes that each organism is a mixture of multiple chemical elements such as carbon (C), nitrogen (N), and phosphorus (P) that are present in various proportions in organisms. We incorporate these principles into a standard predator-prey model to analyze competition between two predators on one autotrophic prey. The model tracks two essential elements, C and P, in each species. We show that a stable equilibrium is possible with two predators on this single prey. At this equilibrium both predators can be limited by the P content of the prey. The analysis suggests that chemical heterogeneity within and among species provides new mechanisms that can support species coexistence and that may be important in maintaining biodiversity.

Animals↗

PubMed2003

Socioeconomics drive urban plant diversity.

Spatial variation in plant diversity has been attributed to heterogeneity in resource availability for many ecosystems. However, urbanization has resulted in entire landscapes that are now occupied by plant communities wholly created by humans, in which diversity may reflect social, economic, and cultural influences in addition to those recognized by traditional ecological theory. Here we use data from a probability-based survey to explore the variation in plant diversity across a large metropolitan area using spatial statistical analyses that incorporate biotic, abiotic, and human variables. Our prediction for the city was that land use, along with distance from urban center, would replace the dominantly geomorphic controls on spatial variation in plant diversity in the surrounding undeveloped Sonoran desert. However, in addition to elevation and current and former land use, family income and housing age best explained the observed variation in plant diversity across the city. We conclude that a functional relationship, which we term the "luxury effect," may link human resource abundance (wealth) and plant diversity in urban ecosystems. This connection may be influenced by education, institutional control, and culture, and merits further study.

Arizona↗

PubMed2003

Biodiversity, habitat area, resource growth rate and interference competition.

For the majority of species, per capita growth rate correlates negatively with population density. Although the popular logistic equation for the growth of a single species incorporates this intraspecific competition, multi-trophic models often ignore self-limitation of the consumers. Instead, these models often assume that the predator-prey interactions are purely exploitative, employing simple Lotka-Volterra forms in which consumer species lack intraspecific competition terms. Here we show that intraspecific interference competition can account for the stable coexistence of many consumer species on a single resource in a homogeneous environment. In addition, our work suggests a potential mechanism for field observations demonstrating that habitat area and resource productivity strongly positively correlate to biodiversity. In the special case of a modified Lotka-Volterra model describing multiple predators competing for a single resource, we present an ordering procedure that determines the deterministic fate of each specific consumer. Moreover, we find that the growth rate of a resource species is proportional to the maximum number of consumer species that resource can support. In the limiting case, when the resource growth rate is infinite, a model with intraspecific interference reduces to the conventional Lotka-Volterra competition model where there can be an unlimited number of coexisting consumers. This highlights the crucial role that resource growth rates may play in promoting coexistence of consumer species.

Animals↗

PubMed2002

Habitat edges and predator-prey interactions: effects on critical patch size.

We use partial differential equation models to examine the effects of cross-edge incursions by a predator on the persistence or extinction of a patch-resident prey species. For each of two predator-incursion profiles (namely, a constant incursion distance and a constant loss rate for predators during incursions), we examine the conditions under which the predator can (and cannot) influence the critical patch size of a prey species.

Animals↗

PubMed2000

Trophic Interactions during Primary Succession: Herbivores Slow a Plant Reinvasion at Mount St. Helens.

Lupines (Lupinus lepidus var. lobbii), the earliest plant colonists of primary successional habitats at Mount St. Helens, were expected to strongly affect successional trajectories through facilitative effects. However, their effects remain localized because initially high rates of reinvasive spread were short lived, despite widespread habitat availability. We experimentally tested whether insect herbivores, by reducing plant growth and fecundity at the edge of the expanding lupine population, could curtail the rate of reinvasion and whether those herbivores had comparable impacts in the older, more successionally advanced core region. We found that removing insect herbivores increased both the areal growth of individual lupine plants and the production of new plants in the edge region, thereby accelerating the lupine's intrinsic rate of increase at the front of the lupine reinvasion. We found no such impacts of herbivory in the core region, where low plant quality or a complex of recently arrived natural enemies may hold herbivores in check. In the context of invasion theory, herbivore-mediated decreases in lupine population growth rate in the edge region translate into decreased rates of lupine spread, which we quantify here using diffusion models. In the Mount St. Helens system, decreased rate of lupine reinvasion will result in reductions in rates of soil formation, nitrogen input, and entrapment of seeds and detritus that are likely to postpone or alter trajectories of primary succession. If the type of spatial subtleties in herbivore effects we found here are common, with herbivory focused on the edge of an expanding plant population and suppressed or ineffective in the larger, denser central region (where the plants might be more readily noticed and studied), then insect herbivores may have stronger impacts on the dynamics of primary succession and plant invasions than previously recognized.

Mount St. Helens↗

PubMed2000

Dispersal, Environmental Correlation, and Spatial Synchrony in Population Dynamics.

Many species exhibit widespread spatial synchrony in population fluctuations. This pattern is of great ecological interest and can be a source of concern when the species is rare or endangered. Both dispersal and spatial correlations in the environment have been implicated as possible causes of this pattern, but these two factors have rarely been studied in combination. We develop a spatially structured population model, simple enough to obtain analytic solutions for the population correlation, that incorporates both dispersal and environmental correlation. We ask whether these two synchronizing factors contribute additively to the total spatial population covariance. We find that there is always an interaction between these two factors and that this interaction is small only when one or both of the environmental correlation and the dispersal rate are small. The interaction is opposite in sign to the environmental correlation; so, in the normal case of positive environmental correlation across sites, the population synchrony will be lower than predicted by simply adding the effects of dispersal and environmental correlation. We also find that population synchrony declines as the strength of population regulation increases. These results indicate that dispersal and environmental correlation need to be considered in combination as explanations for observed patterns of population synchrony.

Moran effect↗

PubMed1999

Variation Thresholds for Extinction and Their Implications for Conservation Strategies.

We examine the degree to which fitting simple dynamic models to time series of population counts can predict extinction probabilities. This is both an active branch of ecological theory and an important practical topic for resource managers. We introduce an approach that is complementary to recently developed techniques for estimating extinction risks (e.g., diffusion approximations) and, like them, requires only count data rather than the detailed ecological information available for traditional population viability analyses. Assuming process error, we use four different models of population growth to generate snapshots of population dynamics via time series of the lengths commonly available to ecologists. We then ask to what extent we can identify which of several broad classes of population dynamics is evident in the time series snapshot. Along the way, we introduce the idea of "variation thresholds," which are the maximum amount of process error that a population may withstand and still have a specified probability of surviving for a given length of time. We then show how these thresholds may be useful to both ecologists and resource managers, particularly when dealing with large numbers of poorly understood species, a common problem faced by those designing biodiversity reserves.

extinction risk↗